(SC) in sensory ganglia - Springer Link

Zeitschrift fiir Zellforschung 52, 567--597 (1960)

Institute of Human Anatomy, University of Milan (Director: Prof. A. BAIRATI) O B S E R V A T I O N S ON T H E M O R P H O L O G Y , SUBMICROSCOPIC S T R U C T U R E AND B I O L O G I C A L P R O P E R T I E S OF S A T E L L I T E CELLS (S.C.) I N S E N S O R Y G A N G L I A OF MAMMALS* By E~io

PA~s~

With 16 Figures in the Text

(Received March 29, 1960) I. Introduction

First VALENTIN (1836) described in sensory ganglia a peculiar type of cells laying on the neuron inside its connective capsule. Later on these cells were given various names : "capsular cells" ( QUADE 1939), "capsule cells" (KuNTZ and SULKI~ 1947), "cellules satellites" (CAJAL 1909 and m a n y others), "gliocitos" (Dr, CASTRO 1921, 1946), "ganglionar neuroglia" (HoRTEOA, POLAK and PRADO 1942), "periphere Glia" (H~.RZOG 1954), "Hiillzellen", "Hiillplasmodium", "Nebenzellenplasmodium" (STSHR jr. 1928, 1939, 1941, 1943), "intracapsul/~re Zellen" (HoLMGRE~ 1901, 1902), "subcapsular cells" (PE~FIELD 1932), "Mantelzellen" (LEsHOSS~K 1907), "Polarkerne" {CouI~voIsI]~R 1868), "Randzellen" (KoHN 1907), "ScheidenzeUen" (Ko~N 1907), "Scheidenplasmodium" (RIEGELE 1932). Among these different names we selected t h a t of satellite cells (s.c.), because it has been more frequently used and does not imply a judgement as to the nature of such elements. To avoid confusion, we shall call capsule the connective envelope encircling the nerve cell, and sheath the covering formed b y the s.c. A thorough analysis of the data from a large literature shows t h a t our present knowledges on the s.c. are rather uncertain. E v e n basic informations on the very shape of the s.c. are wanting. The lack of exact informations depends on the inadequacy of the methods employed; most observations were made with the optical microscope, which is insufficient for the analysis of structures lying below the resolving power of the optical microscope itself. I n fact, some long unsolved problems were readily clarified b y the first researches with the electron microscope. I shall give a brief s u m m a r y of our present knowledges on the s.c. mainly acquired through the use of modern technics. II. Present knowledges on the s . e .

Only the following items will be dealt with, viz. : 1. shape and mutual connections of s. c. ; 2. structure; 3. connections with the neurons and function; 4. origin and nature. 1. Three main schemes of the morphology of s.c. m a y be advanced: a) Each ganglion nerve cell is surrounded b y on outer sheath of endothelial-like elements a n d an inner one of star-like and spindle-shaped cells (CAJAL 1909; CAJAL and * Research supported by a C.N.R. Grant. Z. Zellforsch., B d . 52

38

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E~.~Io PANNESE :

OLORIZ 1897). b) S.c. form, in the whole, a covering quite similar to a monolayered squamous epithelium (KEY and RETZIUS 1873; HANNOVER 1844; FRi~NTZEL 1867; PENTA 1934; PALUMBI 1944; STRAMIONO~I 1953). This covering shows a total (ST6HR jr. 1928, 1939; RIEGELE 1932; PASTOUI 1929), or partial sync:~r texture (ORTIz-PIc6N 1949, 1955; KUBOTA and HIOKI 1943; PALUMBI 1944). C) According to the Authors which made use of silver staining methods, the s. c. bear more or less long branching processes. The more complete description of this kind was offered by HORTEGA, t)OLAK and PRADO (1942). The latter Authors maintain t h a t perisomatic elements are sheet- or star-shaped with branching processes which intertwine and build an intricate plexus. The periaxonic elements enwrap the axon directly with the whole sheet-like cytoplasm, or with spirabshaped processes ("cspirocitos"), or else by means of a pair of processes like branches of pincers or finally with some branching processes. According to HORTEGA, POLAK and P~ADO (1942) the s.c. are discrete elements: DE CASTRO (1946), SCI~ARF (1958) and STRAMm~ONI (1953) agree with the latter view. Partial descriptions similar to those given b y HORTEGA, POLAK and PRADO (1942) m a y be found in the reports of BERTRAND and GUILLAIN (1933), DELLA PIETRA (1937), ORTIz-PIC6N (1949, 1955), SCHARENBERO (1952) and HERZOr (1954). Also HOLMGREN'S (1901) description m a y fit into this scheme: s.c. bear branching processes, which establish special relationships with the nerve cells. One more problem is of interest in connection with the shape of s.c., viz., whether the sheath formed by these cells is continuous or not. Many Authors, which maintain that the s.c. with their processes build a more or less dense plexus, favour the view t h a t interruptions exist in the cell sheath. Even STOHR jr. (1939, 1941) agrees t h a t the "Hfillplasmodium" is a sponge-like texture, whose meshes could enclose collagen fibers. On the contrary, DE CASTRO (1946) and STRAMIGNONI (1953) hold that s.c. form a continuous sheet. I t must be stressed t h a t in all the descriptions given so far a basic aspect has been overlooked; t.i., which is the equivalence to the living state of the images observed with the different technics. The possibility that technical artifacts m a y take place has largely been disregarded. With the electron microscope it has been readily shown that each s.c. is a discrete element, bounded by its own membrane, and that s.c. form a continuous layer around the ganglionic nerve cell (HEss 1955; WYBURN 1958). 2. In the cytoplasm of the s.c. mitochondria (KUBOTA and HIO~:I 1943), the centriole (KUBOTA and HIOKI 1943), the Golgi complex (KuBoTA and HIOKI 1943; KUNTZ and SULK~ 1947) and large granules similar to gliosomata (HoRTEaA, POLAK and PRAI)O 1942; STRAMIGNONI1953) have been demonstrated; however, the existance of the latter structures has been questioned by ORTIz-PIc6N (1949, 1955). Sometimes a spongy structure, or at least some vacuoles have been described (HORTEGA, I:)OLAKand PRADO 1942; 0RTIz-PIc(f)N 1949, 1955; STRAMIGNON~ 1953). According to HORTEGA, POLAK and PRADO (1942) cytoplasmic fibrils are apparent, while ST6HR jr. (1939, 1941), ORTIz-PIc6N (1949, 1955) and STRAMIGNONI (1953) were unable to observe them. Basophilic material is stained with the Nissl method (BERTRAND and GUILLAIN 1933; ST6HR jr. 1939, 1941; KUBOTA and HIOKI 1943; PALUMBI 1944).

Satellite cells in sensory ganglia

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With the electron microscope, I)E ROBERTIS and :BENNETT (1954), have shown in the cytoplasm of s.c. as in the cytoplasm of the neurons small vesicles (mean diameter 680 A); the latter might serve in the transport of fluids, tIEss (1955) described mitochondria, small granules and "scattered ergastoplasmic filaments", TAXI (1957) found a various content, generally scarce, of osmiophilous material. 0 n l y mitochondria and small diffuse granules, but no Nissl bodies, Golgi complex and endoplasmic reticulum, were reported b y WYBU~N (1958) in the cytoplasm of S.C.

Generally, s.c. are described as elements with a light cytoplasm lacking any structural specialisation, viz. scarcely differentiated. However, it m a y be open to question whether extraction of soluble materials might not play some role in this respect. 3. ST6HR jr. (1939, 1943) and RI~GEL~ (1932) hold t h a t between the s.c. sheath and the neuron a cytoplasmic continuity exists. But this view was disposed off b y KUBOTA and HIOKI (1943), PALU~BI (1944), DW CASTRO (1946), SCHAlCF (1958). HOLMGREN (1901, 1902) maintains t h a t s.c. processes penetrate the neuron's cytoplasm and build a network of filaments ("trophospongium"). Under some functional conditions, the filaments would act as minute canaliculi, thus favouring the exchanges between nerve and s.e. STSnR jr. (1939, 1943) and HOLMGREN (1901, 1902) views have now merely a historical interest. I t has been shown with the electron microscope t h a t s.c. and nerve cells are separated from each other b y means of their own limiting membranes (H]~ss 1955, WYBURN 1958); however a close contiguity exists between these two elements (LENHOSS]~K 1907, LEVI 1907, KUNTZ and SuL](IN 1947, SCHARV,NBERG 1952) and the s.c. adhere to the irregular surface of the nerve cell. To m y knowledge the ratio of the s. c. to the nerve cells number has never been properly studied. F r o m the few available data, s.c. seem to be relatively more scarce in young subjects and they increase with age (CAJAL 1909, STSHR jr. 1928). The functional relationships between nerve and s.c. have been interpreted in different ways. The s.c. would display protective (O~TIz-PIc6N 1949), isolating (HoRTEGA, POL~K and P~A])o 1942; DE CASTRO 1946), trophic (NAGEOTTE 1907, LEvi 1907, ORT~z-Pm6N 1949, DE CASTRO 1946) or secretory functions (LENnOSS~]~ 1907; ttORTEGA, POLAK and PRADO 1942; PENTA 1934; MOLL]~I~ 1939). KORNMOLLER (1950) is of the opinion t h a t s.c. produce acetylcholine and thus m a y influence the excitability of the adjacent neurons. I t has been held t h a t s.c. in the sympathetic ganglia represent the boundary of sinaptic areas (HoRTEGA and PRAI)O 1942), or b y intervening between preganglionic fibers and nerve cells m a y check the synaptic contact (DE CASTRO 1946; OI~TIz-Pm6N 1949, 1955). The latter view did not find its morphological basis in the observations with the electron microscope. 4. The majority of the Authors maintain t h a t s.c. originate from the ectoderm (BALFOUR 1877 ; DOHI~N 1891 ; KO~N 1907 ; K(~LLIKER 1905 ; STl~EETER 1905, 1911 ; LEN~OSS]~K 1907; LEW 1907; YN~rEMA 1937), but others claim a mesenchymal origin (Mo~Pu~Go and TIRELLI 1896). Various opinions were reported on the nature of the s.c. According to SCHRAMM (1864) and DOGIEL (1896), they are connective elements, or epithelial (FRXN~ZEL 38*

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1867), endothelial (KEY a n d RETZIUS 1873), glial (CAJALa n d OLORIZ 1897). The l a t t e r view was more recently held b y several Authors (t/ORTEGA, POLAK a n d PRADO 1942; PASTORI 1929; DELLA t)IETRA 1937; DE CASTRO 1946; ORTIz-PIc6~ 1949, 1955; KUNTZ a n d SULKIN 1947; SCHARENBERG 1952; STRAMIGNONI 1953). This hypothesis is m a i n l y based a) on the morphological similarities between s.c. a n d glial cells, as shown b y silver s t a i n i n g methods, a n d b) on the i n t i m a t e connections existing between s.c. a n d glial cells on one h a n d a n d the n e u r o n s on the other h a n d . On a c c o u n t of their scarce processes, s.c. were considered oligodendrocytes (ORTIZ-PIc6~ 1949, 1955; KUNTZ a n d SULKIN 1947; DELLA PIETRA 1937). According to HORTEGA, POLAK a n d PRADO (1942), however, both the cell shape a n d the relationships with the nerve fibers are characteristic features of oligodendrocytes; for this reason only periaxonic s.c. could correspond to the oligodendrocytes, while perisomatic cells should correspond to the astrocytcs. SCItARENBERG (1952) shared this opinion. HORTEGA a n d his collaborators (1942) p o i n t o u t t h a t the morphological correspondence between s.c. a n d glia cells is far from absolute; the two types of cells represent the result of a n a d a p t a t i o n to two different milieus. I t should be stressed, however, t h a t the cell shape is n o t a valid c r i t e r i u m i n the m a t t e r of the n a t u r e of a given cell.

III. Materials and methods The electron microscope pictures have been studied throughout in parallel with those obtained by phase contrast microscopy, from paraffin and methacrylates embedded material. The phase contrast method is in my opinion the more suited for a comparison with the electron microscope analysis: that is especially true when studying by phase contrast sections adjacent to the ultrathin sections used for the electron microscope observations. Spinal ganglia of the cervical and thoracic level of newborn and adult rat, of adult guinea pig, rabbit, cat, horse and ox were studied as well as the ganglion of the 5th nerve of adult cat. Small pieces of the ganglia from horse and ox were fixed by immersion. All the other ganglia were fixed in the living animal by injecting the fixative in the ascending aorta through the left ventricle; the descending aorta was tied directly under the diaphragm. 2% osmicacid, Zenker, Bouin, 10% formalin, Carnoy, potassium bromide-formalin at PH 1--2 (Ho~TEGA'S fluid) were used. The pieces were then embedded in paraffin; the specimens fixed in Palade fluid were embedded in methacrylates. One micron thick sections of the material embedded in paraffin were cut with the Young mod. HE microtome and examined by phase contrast. The material embedded in methacrylates was sectioned with the Porter-Bloom ultramicrotome. The ultrathin sections were examined at the Siemens Elmiskop I and II electron microscopesl; thicker sections mounted in glycerine were studied by phase contrast microscopy. Some ultrathin sections were stained with uranyl-acetate (WATSON 1958). Sections of ganglia fixed in formalin or in Carnoy fluid were treated at 370 or 55 o C. for 60 min with 1/5000 ribonuclease 2 dissolved in distilled water or in a buffered solution at PH 7, stained with 0,5% toluidin bleu (at pH 4,7 for 90 min), then kept for 5 min in an ammoniummolybdate solution, according to the technic suggested by LISON (1953). Other sections were stained with the Feulgen method. The Cajal silver staining (third formula) was applied for the ganglia of rat to examine the characters of axonic glomerulus (cf. chapt. VII). The technic for quantitative determinations will be reported in chapter VII. The Siemens Elmiskop II electron microscope belongs to the Institute of human anatomy of the University of Milan. The Elmiskop I electron microscope belongs to the "Inail Laboratory of electron microscopy" at the Clinic for occupational diseases of the University of Milan. The author expresses his thanks to dr. phis. S. DE PETRIS for the technical assistance given in obtaining electron photograms with the Siemens Elmiskop I microscope. 2 Salt and protease free crystalline ribonuclease (General Biochemical Inc.).

Satellite cells in sensory ganglia

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IV. Phase contrast observations Remarkable structural differences were apparent in specimens treated with different fixatives. Thus the fixative is an important agent in the genesis of some structural features. Pcrisomatie s.c. In the ganglia fixed in the Hortega fluid or 10% formalin or Bouin fluid, a cleft was nearly always apparent between the connective capsule and the nerve cell (Fig. 1 a - - d ) ; in this cleft lie, more or less scattered, the perisomatic s.c. Their shape is various. At phase contrast all the cell types described after silver staining are readily recognized, viz. : polygonal cells, with short processes (Fig. l d), star and spindle-shaped cells bearing sometimes long and branching processes (Fig. 1 a--c). The length, thickness of the processes as well as their smooth or granular surface m a y vary considerably. The cytoplasm appears at times spongy (Fig. 1 a), as described by I-[ORTlSGA,1DOLAKand PRADO (1942). I n osmic fixed preparations, no cleft appears inside the connective capsule; the perisomatic cells lie in close contact with the capsule and the perikaryon surface (Fig. 1 e--f). The shape of s. c. is rather uniform : the cytoplasm is laminar and its outline irregular for the existance of short and broad expansions by which adjacent elements are interlocked. The cytoplasm is never spongy, but encloses often granules, mainly perinuclear. Periaxonie s.c. A full analysis of such cells in the thin sections used for phase contrast observations is hard because of the tortous course of the axon which they enwrap and of their intimate contact with the connective envelope accompanying the axon. I n the preparations fixed in the Hortega fluid, in formalin and Bouin fluid, the periaxonic s.c. look similar to the elements described by HO~T~GA, POLAK and PRADO (1942) : spiral processes encircling the axon (Fig. 2b) and cells with a scarce cytoplasm enwrapping the axon with numerous, thin processes are apparent. After osmic fixation, the periaxonic cells look like laminar elements lying on the axon and coiled around it: sometimes the narrow and long cytoplasmic sheet winds itself spirally once or twice on the axon (Fig. 2d) building a sort of short but continuous sleeve around the latter. On the base of the mentioned data the problem of the equivalence between the appearance of the cells in the living state and respectively after the action of the various fixatives, m a y be stated as follows: 1. The well known images of the s.c. as they appear in silver stained preparations are undoubtedly the result of artifacts depending on the action of the fixatives: the same images m a y be obtained without the use of impregnation or staining methods. 2. Perisomatic and periaxonic s.c. are sheet-like elements deprived of branched processes and bearing only short expansions. This statement finds some support in the observations made by POM]~RAT (1952) in glia cells cultured in vitro: by addition of formalin a shrinkage of the laminar cytoplasm and its transformation into filiform processes seems to take place. The distortion of the cell morphology due to the fixative is particularly obvious in very thin and spread-out cells, as cells cultured in vitro and s.c. are. I n the case reported here it must be emphasized that the fixative does not determine only a direct shrinkage of the s.c., but also shrinkage of the adjacent nerve

F i g . 1 a - - f . P h a s e c o n t r a s t p h o t o m i c r o g r a p h s of p e r i s o m a t i c s . c . (1050 • ). a S p i n a l g a n g l i o n of a d u l t o x ; B o u i n f i x . , p a r a f f i n , b S p i n a l g a n g l i o n of a d u l t o x ; H o r t e g a f i x . , p a r a f f i n , c a n d d S p i n a l g a n g l i o n of a d u l t h o r s e ; B o u i n f i x . , p a r a f f i n , e a n d f S p i n a l g a n g l i o n of a d u l t h o r s e ; P a l a d e f i x . , e m b e d d e d i n methacrylatcs. C c o n n e c t i v e c a p s u l c ; N e n e r v e cell; S p e r i s o m a t i c s . c . (cf. t e x t , p g . 571)

ENNIO PANNESE: satellite cells in sensory ganglia

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F i g . 2 a - - d . P e r i a x o n i c s . c . w i t h a s p i r a l c o u r s e , a S p i n a l g a n g [ i o n of a d u l t r a t , P a l a d e f i x . ; e l e c t r o n m i c r o g r a p h (16 000 x ). T h e l i m i t s of t h e p e r i a x o i n c cell coils a r e c l o s e l y a d j a c e n t , b S p i n a l g a n g l i o n of a d u l t r a b b i t ; f o r m a l i n f i x . , p a r a f f i n ; p h a s e c o n t r a s t (1 800 • ). e a n d d S p i n a l g a n g l i o n of a d u l t g u i n e a p i g ; P a l a d c f i x . , e m b e d d e d i n m e t h a c r y l a t e s ; p h a s e c o n t r a s t (1 800 • ). N e n e r v e cell; a a x o n ; S n u c l e u s of p e r i a x o n i c s. c. ; c s coils of p e r i a x o n i c s . e . c y t o p l a s m ; c c o l l a g e n

cell. The p o r t i o n s of t h e s.c. c y t o p l a s m which stick m o r e closely to t h e n e r v e cell surface, become s t r e t c h e d as a consequence of t h e shrinkage of t h e n e u r o n a n d m a y a p p e a r as t r u e c y t o p l a s m a t i c processes. Moreover, t h e a r t i f a c t is r e n d e r e d m o r e d r a s t i c b y t h e s i m u l t a n e o u s s t a i n i n g of t h e connective envelope a n d t h e s.c.

574

ENNIO

PAl~NESE :

F i g . 3. S p i n a l g a n g l i o n o f a d u l t r a t . E l e c t r o n m i c r o g r a p h of a nerve cell, completely enwrapped t h e s . c . s h e a t h . N c n e r v e c e l l n u c l e u s ; nu n u e l c o l u s ; m m i t o e h o n d r i a ; S s.e. nuclei ; a axon ; e ergastoplasm i n s . e . c y t o p l a s m ; c c o l l a g e n (5 8 0 0 x )

by

with the silver methods, which have no specificity whatsoever. It must be recalled that in 1873 KrY and RETZIUS suggested that the s.c. processes are artificial formations due to shrinkage of the ganglionic cell.

Satellite cells in sensory ganglia

575

u Itistoehemieal tests The cytoplasm of some s.c. homogeneously stains with toluidin blue, but not as deeply as the neurons' cytoplasm. No staining is apparent after ribonuclease treatment. Not all the s.c. show this cytoplasmic basophilia; what m a y depend on a different R.N.A. cytoplasmic content from cell to cell, according to various phases of cell activity, or on a variable R.N.A. extraction, due to technical reasons. The ribonuclease test and the Feulgen reaction are considerably different in the nuclei of the s.c. and respectively of the ganglionic cells : the nucleus of the s.c. is more similar to t h a t of m a n y other kinds of cells.

u

Observations with the electron microscope

General structure of s. c. sheath in ganglia o/ adult animals I n all the animal species examined the cell sheath is built of discrete elements : no syncytial structure, or binucleated s.c. have been found. Each s. c. is bounded b y a membrane : the membranes of adjacent cells are separated b y an interval of 200 ~_ ca. in thickness. S.c. form a continuous sheath around the perikaryon (Fig. 3) and its axon (Fig. 2a, 7) whose thickness varies from area to area. I t is formed b y one single very flat layer or b y two to three layers of ceils. The thickness of the s.c. sheath m a y at places be lower t h a n the resolving power of optic microscope. This fact m a y explain why it m a y have appeared discontinuous to several Authors. Perisomatic s. c. m a y often be followed till the region in which the axon sprouts (Fig. 7, 8) to be replaced therefrom b y periaxonic elements. The replacement m a y take place at various levels: viz., perisomatic cells m a y enwrap the axon for a very short lenght, or periaxonic cells m a y lie over perisomatic cells. The axon m a y lack the myelin sheath for some extent and gives rise to a glomerulus (Fig. 8): its loops lie very close to the perikaryon. E v e n in these cases, the periaxonic s.c. enwrapping the glomerulus loops do not get in contact with the perikaryon; periaxonic s.c. and perikaryon are always separated b y an intervening layer of perisomatic s.c. (Fig. 8). Therefore, no s.c. encircles simultaneously the perikaryon and the axon. The s.c. lie around the perikaryon or the axon: at birth-time the axon which leaves the perikaryon does not form a glomerulus; at this time the perikaryon is already completely enveloped b y perisomatie s.c. During the postnatal growth, the axon lengthens and coils around the perikaryon; then, the surrounding periaxonic s.c. lie close to the outer surface of the already formed perisomatic cell sheath. An amorphous layer, opaque to the electrons, is apparent on the outer aspect of the peripheral membrane of s.c. (Fig. 6). This layer is continuous between two adjacent s.c., with no connections with the cell membranes; the latter, on the contrary, fold back on the nerve cell. This layer is in general thicker t h a n the cell membrane and not so neatly bounded. I t is distinctly separated from the membrane of the s.e. b y a zone of low electronic density from 200 to 400 A in thickness (in the average, 300 A). Rather t h a n a true membrane, the mentioned layer represents a plane in which an unknown connective material is gathered. This dense layer was described also by WYBvn~ as a basement membrane. A similar layer was observed between epidermis and dermis, separated from epidermal cell membrane b y a space 300 A (SELBY 1955, OnLA~D 1958) or

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E ~ - I o PAI;NESE :

F i g . 4. S p i n a l g a n g l i o n of a d u l t r a t . ]Electron m i c r o g r a p h of a p a r t of a s. e., c n w r a p p i n g t h e p e r i k a r y o n . N e n e r v e c e l l ; N b N i s s l b o d y ; m ~ n i t o c h o n d r i a ; S s.c. n u c l e u s ; l o s m i o p h i I i c d r o p s ( p i g m e n t ?); e e n d o p l a s m i c r e t i c u l u m ; ~; s m a l l " p a r a f i t a ' " c u t n e a r i t s e m e r g e n c e ; c c o l l a g e n (~0 090 x )

S a t e l l i t e cells in s e n s o r y g a n g l i a

577

Fig. 5. Spinal g a n g l i o n of a d u l t r a t . E l e c t r o n m i c r o g r a p h of a p e r i s o m a t i c s.c., e n w r a p p i n g t h e perik a r y o n . N e n e r v e cell; G Golgi coraplex; m m i t o e h o n d r i a ; S s.c. nucleus; e e n d o p l a s m i c r e t i c u i n m ; p s m a l l " p a r a f i t i " ; c collagen; me m y e l i n . (24 0O0 x )

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E~o

PANNESE:

Fig. 6

Satellite cells in sensory ganglia

579

400--500/~ in thickness (FAssKw and THEMAN~ 1959). I t was called "dermal m e m b r a n e " b y SELBY (1955), "Dermallamelle" b y FASSKE and THEMA~N (1959), "basement m e m b r a n e " b y ODLAN~ (1958). This submicroscopic layer should not be called "basement m e m b r a n e " , a name which has long been used to indicate a connective membrane built of collagen fibers. However, the existence of such a structure between the derivates of ectoderm and of the mesenchyme must be stressed. Externally to this layer, collagen fibers are continuous with the interstitial connective tissue of the ganglion; they do not appear very dense near the s.c. The collagen fibers are embedded in a structureless material of low density. Shape o[ 8. c. Perisomatic s.c. The sections which cut tangentially these ceils confirm the phase contrast observations b y showing their laminar shape; the surface is often irregular and contiguous cells m a y be connected through interdigitating processes (Fig. 6). Periaxonic s.e. I n sections tangential to the axons surface, periaxonic s.c. appear laminar; t h e y envelope a segment of the axon and build a tubular sheath to the latter. They m a y lie on the axon and enwrap it completely, or, less frequently (when t h e y are longer and narrower), they take a spiral course around the axon (Fig. 2a). The latter cells correspond to the "espirocitos" of the classic histology. However, the various turns of the spiral are in close contact to each other (Fig. 2a). The well known aspect of a spiral with videly spaced turns (Fig. 2b) revealed b y silver staining methods is due to the shrinkage of the thin and soft cytoplasm of the s.c. While in r a t the spiral's turns show in general closely adjacent limits, in guinea pig the cell boundaries m a y overlap to some extent ; in the latter case, the cell sheath of the axon is more complicated, and the cell m a y be quite spread in surface. The external outline of periaxonic s.e. is often very irregular. Structure o/8. c. Our knowledge on the structure of s.e. as well as of glia cell is still rather uncomplete, especially for technical reasons. Sometimes the electron microscope shows well preserved nerve cells, while the contiguous s.e. m a y appear e m p t y as a consequence of extraction of cytoplasmic materials. Here the structure of s.c. will be considered taking into account only preparations in which cell structure and mutual connections between cells are well preserved. The submicroscopic structure of perisomatic and periaxonic s.e. is not substantially different in the various species which we examined; therefore a description valid for the two cell types and for all the species examined m a y be given. The nucleus (Fig. 3, 4, 5), bounded by a double membrane and sometimes irregularly indented, encloses granular material and one nucleolus with a granular F i g . 6. S p i n a l g a n g l i o n of a d u l t r a b b i t . E l e c t r o n m i e r o g r a p h of a w e d g e , f o r m e d b y s.c., i n t e r v e n i n g b e t w e e n t w o c o n t i g u o u s n e r v e cells. T h e s.c. s u r f a c e is v e r y i r r e g u l a r ; a d j a c e n t c e l l s a r e c o n n e c t e d t h r m t g h t h e i n t e r d i g i t a t i o n of t h e i r r e g u l a r s u r f a c e s . N e n e r v e c e l l ; S s.e. n u c l e u s ; m m i t o c h o n d r i a ; v v e s i c l e s i n s.e. c y t o p l a s m , m o r e n u m e r o u s n e a r t o t h e m e m b r a n e s ; d m b o u n d a r y b e t w e e n s . e . a n d c o n n e c t i v e of t h e g a n g l i o n ( t h e i n n e r l i n e is t h e s.c. m e m b r a n e , t h e o u t e r l i n e is a t h i c k l a y e r of c o n n e c tive material). 20000 x

]~'ig. 7. S p i n a l g a n g l i o n of a d u l t r a b b i t . E l e c t r o n m i e r o g r a p h of a n e r v e cell, s e c t i o n e d a t t h e o r i g i n of t h e a x o n ; t h e l a t t e r is c o m p l e t e l y e n w r a p p e d b y s.c. N e n e r v e cell; a a x o n ; c s s a t e l l i t e cells; ~1~l n i t o chondria; c collagen (20000 • )

EN~IO PANNESE: Satellite cells i1~ sensory ganglia

581

F i g . 8. S p i n a l g a n g l i o n of a d u l t r a b b i t . E l e c t r o n m i e r o g q ' a p h of a n e r v e c e l l ; t h e a x o n l e a v e s t h e p e r i k a r y o n a n d g i v e s rise t o t h e g l o m e r u l u s . P e r i s o m a t i c s. e. lie d i r e c t l y o n t h e p e r i k a r y o n ; p e r i a x o n i c s.c. are s t r a t i f i e d e x t e r n a l l y t o t h e f o r m e r . N e n e r v e c e l l ; ao a x o n ' s o r i g i n ; a a x o n ; S n u c l e i of s.c. of t h e g l o m e l ~ l u s ; i p e r i s o m a t i e s.c. s h e a t h . (7 000 • )

texture of different electronic density from area to area. Sometimes the nucleolus lies close to the nuclear membrane. In s.c. cytoplasm round or club-shaped mitochondria, with typical cristae were found. The Golgi complex is represented b y flattened and smooth cisternae

582

EN~[O PANNSSE:

and clusters of small vesicles (Fig. 5, 12). Drops strongly and homogenously osmiophilous (pigment ?) (Fig. 4) and particles of medium density whose diameter ranges between 0,1 to 0,5/~, enclosing osmiophilic granules (Fig. 4, 5) are apparent. The hyaloplasm is characterized by an endoplasmic reticulum (corresponding to PALADE'S classic description) and by a thin network connecting the membranous structures. The endoplasmic reticulum shows a great variety of arrangements: vesicles and tubuli are widely dispersed in some cells (Fig. 4, 5), while in others they m a y be packed together (Fig. 10). Sometimes the cistcrnae are arranged in parallel rows at regular intervals (ergastoplasm; fig. 3, 9 b, c) : this aspect is more frequent in periaxonic s.c. The profiles of the endoplasmic reticulum nearly always are covered by small dense particles rich in R.N.A. as shown by PALADE (1956). The amount of these particles varies from cell to cell: sometimes these differences m a y depend on a partial extraction of materials as a consequence of the technical treatment, but in other cases the mentioned differences are not the result of technical artifacts. In fact, analogous differences m a y be found after histochemical tests, in ganglionic s.c. fixed in C A ~ o v ' s fluid, which does not extract the bulk of the ribonucleoproteins. Given the various amount of the endoplasmic reticulum and of the small particles, "light" and respectively "opaque" s.c. m a y be distinguished even in preparations treated with the most suited technic. The network is built of tiny filaments, less than 100 A, running in all directions (Fig. 9 a). The network is easily seen in sections stained with uranyl-acetate, but sometimes it can be shown even without this treatment; its absence, therefore, could depend on its low electron absorption. This network is not evident in all kinds of cells; therefore, it is peculiar of some cells, or else it depends on artifacts determined on a very delicate and hydrated cytoplasm, unsufficiently preserved by the osmic acid fixation. In the latter case, the dehydration would cause the clotting of a dispersed material and its precipitation as a network. I t seems clear, however, t h a t this network does not correspond to the glial filaments described in fibrous and cpendimal gliocytes (BAIRATI et al. 1956, FLEISCHHAYER 1957, GRAY 1959), nor to the s.c. fibrils revealed by the optic microscope after silver impregnation (HORTEGA, POLAK and I~ADO 1942; KU]3OTA and HIOKI 1943; HERZOO 1954). The appearance of fibrils in the s.c. could easily occur as a consequence of a clotting of hyaloplasmic structures (canaliculi of the endoplasmic reticulum and network of filaments) which could be covered by a silver mantle. The cytoplasm of s.c. shows also vesicles of various size (from 400 to 700 A), containing material of low density (Fig. 6, 10). These vesicles are similar to those described by DE ROBERTIS and BENNETT (1954) ; their number varies from cell to cell. The vesicles are in general more numerous near the cell membrane adjacent to the nerve cell (Fig. 10): some m a y lie against the membrane. As also the neuron contains similar vesicles, DE ROBERTIS and BENNETT (1954) suggest that they are due to a passage of materials from s.c. to nerve cells. This hypothesis m a y be correct; however, it is hardly possible to give the demonstration of vesicles going through the membrane of nerve and respectively of s.c.

Satellite

cells in sensory

ganglia

583

b

c F i g . 9 a - - c . C y t o p l a s m i c s t r u c t u r e s of s . e . ; e l e c t r o n m i c r o g r a p h s , a S p i n a l g a n g l i o n of a d u l t r a t . Section stained with nranyl-acetate. N e n e r v e cell; cs s a t e l l i t e cell; m b c o m p l i c a t e d p a t t e r n of t h e l i m i t i n g m e m b r a n e s of n e r v e a n d s . c . ; m m i t o e h o n d r i o n ; c c o l l a g e n . I n s . c . c y t o p l a s m a t h i n s u b m i c r o s c o p i c n e t w o r k of f i l a m e n t s is a p p a r e n t . (48 000 • ). b S p i n a l g a n g l i o n of n e w b o r n r a t . N e n e r v e cell; cs s a t e l l i t e cells; S n u c l e u s of a s . c . ; e e r g a s t o p l a s m . I n t h e c y t o p l a s m of t h e s . c . o n t h e l e f t , e r g a s t o p l a s m a n d s m a l l v e s i c l e s , n e a r t o t h e cell m e m b r a n e . (32 000 • ). e S p i n a l g a n g l i o n of a d u l t r a t . E r g a s t o p l a s m (e) i n t h e c y t o p l a s m of a s . c . (32 009 • ) Z, Z e l l f o r s c h . , B d . 52

39

584

ENNIo PAN~ES~: Relationship between s.c. and nerve cells

HEss (1955) and WYBURN (1958) observed that s.c. and nerve cells are clearly bounded by their own limiting membranes : the space in between is in the average from 150 to 200 ~. The surface of the perikaryon is made often irregular, by the presence of submicroscopic or microscopic digit-like processes (Fig. 4, 5). In newborn animaN, the indentation of the nerve cell surface is scarcely apparent; the cell processes form in later life. Possibly they correspond to the microscopic processes, called "parafiti" by NAGEOTTE (1906) and LEvi (1907, 1908). Submicroscopic protoplasmic processes are more or less frequent and complicated in different species; among the rodents, they seem particularly frequent in rabbits. Perisomatic s.c. are perfectly adapted to the irregular outline of the perikaryon (Fig. 9a). Therefore, the surface of contact between s. and nerve cell is nearly always greater than it appears in the microscopical preparations; a fact which must have a functional meaning. In the ganglionic chain of Periplaneta americana, HEss (1958) found intimate and complicated junctions between s. and nerve ceils. Also s.c. are intimately ~nterdigitated with each other; in some species the whole surface of the s.c. membrane is so large that in the electron microscope pictures the cell membranes may appear complicately superimposed (Fig. 6). WYBURN (1958) claims that in rabbit ganglia this appearance is due to a rich plexus of unmyelinated axons which surround each perikaryon. However, this interpretation does not seems to correspond always to the reality; a interdigitation of the irregular surface of s. and nerve ceils readily accounts for the aspects described by WrBUR~r (1958). The s.c. in the newborn rat

Each nerve cell in the spinal ganglia is surrounded by a continuous sheath, formed by discrete s.c. This sheath is thinner in the newborn rat than in the adult, because s.c. are always very thin and arranged in one layer. As a matter of fact, s.c. seem often stretched possibly as the result of an adaptation to the increasing nerve cell surface. In some areas the s.c. sheath seems to be built only of two membranes, with a very thin layer of cytoplasm in between (Fig. 11). The cell sheath is in general not thicker than 2000 A and may be less than 500 A. This m a y explain why it has been maintained that s.c. may lack around small neurons. The axon does not from a glomerulus : in general, it leaves the perikaryon as a straight process. Therefore the periaxonic s.c. do not lay over perisomatic s.c. I t appears from the mentioned data, that the s.c. sheath is more simple in the newborn than in the adult rat. At birth, collagen fibers are scarce; perhaps, the connective tissue of the ganglion develops more slowly than nerve and s.c. do. The neurons appear closely packed (Fig. l 1), because the s.c. sheath is very thin and the connective tissue scarce. In the adult the neurons are separated by thicker cell sheaths and connec. tivc envelopes. The ultrastructure of the s. c. is the same in the newborn and in the adult rat : mitochondria, Golgi complex, endoplasmic reticulum and small vesicles (4--700 ~x

S a t e l l i t e cells in s e n s o r y g a n g l i a in diameter) is apparent

can be seen since birth4ime

( F i g . 9 b , 10, 12).

with its parallel canaliculi, whose mean

585 Sometimes

width is 200/~

a centriole

ca.

Fig. 10. Spinal ganglion of newborn rat. Parts of two nerve ceils and the adjacent s.c.; electron mierograph. Ne nerve cell; N c nerve cell nucleus; S s.c. nucleus; m mitochondria; e endoplasmic reticulum in the cytoplasm of a s.c. The arrows point to s mall vesicles, close to the limiting membranes of nerve and s.c. (24000 • ) 39*

F i g . 1 l . S p i n a l g a n g l i o n of n e w b o r n r a t . E l e c t r o n m i c r o g r a p h . P a r t s of 4 n e r v e cells ( N e ) ; betweeI~ t h e l a t t e r v e r y s t r e t c h e d s a t e l l i t e ceils (cs). N o c o n n e c t i v e e n v e l o p e is ~ p p a r e n t . m m i t o c h o n d r i a ; N b N i s s l b o d y (24 000 ~ )

ENNIO PANNESE: Satellite cells in sensory ganglia

587

VII. Quantitative relationship between satellite and nerve cells Technic. The 3rd a n d t h e 4 t h spinal ganglion (of b o t h sides) of 4 a d u l t r a t s (6 m o n t h s of age, m e a n weight 250 gr.) a n d of 4 n e w b o r n rats. Z e n k e r fluid was i n t r o d u c e d in t h e a o r t a of t h e l i v i n g a n i m a l s t h r o u g h t h e left ventricle; t h e n t h e g a n g l i a were dissected, e m b e d d e d in p a r a f f i n a n d sections of 4 tt cut w i t h t h e Young He microtome. I n t h e sections a nerve cell was chosen which were easily recognizable for its location, close t o a b l o o d vessel or to a small b u n d l e of nerve fibers; t h e two o r t h o g o n a l d i a m e t e r s were m e a s u r e d b y m e a n s of a m i c r o m c t r i c eye-piece in t h e successive sections of t h e cell. T h e value of t h e t h i r d d i a m e t e r , o r t h o g o n a l t o t h e o t h e r two, was o b t a i n e d f r o m t h e n u m b e r of t h e sections in which t h e cell b o d y was found. T h e n in each section t h e nuclei of s.c. close to t h e p e r i k a r y o n were c o u n t e d ; these nuclei are r e a d i l y d i s t i n g u i s h e d f r o m those of f i b r o c y t e s on a c c o u n t of t h e i r s t r u c t u r e a n d position. The n e r v e cell v o l u m e was c a l c u l a t e d f r o m t h e t h r e e d i a m e t e r s , b y a p p l y i n g t h e f o r m u l a of t h e ellipsoid, which is t h e solid whose shape is m o r e s i m i l a r to t h a t of g a n g l i o n cells. H o w e v e r , as one nucleus m a y a p p e a r in t w o or m o r e a d j a cent sections, t h e n u m b e r of nuclei of s.c. r e s u l t i n g f r o m t h e count was c o r r e c t e d a c c o r d i n g t o ABERCROMBIE'S (1946) m e t h o d . Because t h e surface of t h e ellipsoid is n o t o b t a i n a b l e b y m e a n s of e l e m e n t a r y m a t h e m a t i c a l expressions, t h e surface of each nerve cell was considered like t h a t of a sphere, h a v i n g t h e s a m e v o l u m e of t h e ellipsoid. The e r r o r i n t r o d u c e d in excess in t h e c a l c u l a t i o n of t h e v o l u m e a n d in defect in e v a l u a t i n g t h e surface of each n e r v e cell was negligible; in fact i t is less t h a n t h e 10% a n d i t is e v e n l y d i s t r i b u t e d in all t h e size classes. Two h u n d r e d neurons were t h u s a n a l y z e d in t h e a d u l t a n d r e s p e c t i v e l y in t h e n e w b o r n rats. Results. T h e d i a g r a m s of F i g . 13 a n d 14 s u m m a r i z e t h e d a t a o b t a i n e d . The n e r v e cells were g r o u p e d in classes of progressive v o l u m e a n d surface, t a k i n g into a c c o u n t t h e m e a n error d e p e n d i n g on t h e m e a s u r e m e n t s . A n error of one degree in t h e r e a d i n g on t h e m i c r o m e t r i c eye-piece d e t e r m i n e s in t h e e v a l u a t i o n of t h e v o l u m e a n d surface of t h e n e r v e cell a c o m p a r a t i v e l y g r e a t e r error t h e g r e a t e r is t h e cell d i a m e t e r ; therefore t h e smaller n e u r o n s were g r o u p e d in classes of smaller a m p l i t u d e a n d t h e bigger neurons in classes of g r e a t e r a m p l i t u d e . F r o m t h e d i a g r a m s of Fig. 13 i t m a y be d e d u c e d : a) t h a t t h e n u m b e r of s.c. e n w r a p p i n g each n e u r o n increases in p a r a l l e l w i t h t h e increase of t h e v o l u m e a n d surface of t h e p e r i k a r y o n ; b) t h a t each s. c. corresponds to 370 ( • 30}# 2 of t h e p e r i k a r y o n surface; this v a l u e is n e a r l y c o n s t a n t for s m a l l a n d large n e r v e cells; c) each s.c. corresponds, on t h e average, to 2000 #3 of n e r v e cell v o l u m e ; however, t h i s value is lower for s m a l l nerve cells (e.g., for n e u r o n s of 4000 #3 1500/~3 ca.) t h a n for big ones (for n e u r o n s of 4 4 0 0 0 / z 3 2500 #3 ca.). The d i a g r a m s of Fig. 14, in which t h e d a t a o b t a i n e d in t h e n e w b o r n r a t are p l o t t e d , show t h a t t h e r a t i o b e t w e e n s.e. n u m b e r a n d p e r i k a r y o n v o l u m e a n d surface are f u n d a m e n t a l l y s i m i l a r to t h o s e f o u n d in t h e a d u l t . I n t h e n e w b o r n e a c h s.e. corresponds to 390 ( -4- 25)# 2 of t h e nerve cell surface {the same value as in t h e adult) and, in t h e average, to 1000 #3 ca. of t h e nerve cell v o l u m e (one half of t h e value f o u n d in t h e adult). As a m a t t e r of fact, ganglion cells in t h e n e w b o r n are v e r y small (from a few h u n d r e d s #3 to a little m o r e t h a n 4000 #3) ;

588

EsN~o PA~NESE:

therefore, t h e y can be only compared to the smallest nerve cells of the adult. F r o m the comparison of n e u r o n s of the same volume, f u n d a m e n t a l l y similar ratios result :

F i g . 12. S p i n a l g a n g l i o n of n e w b o r n r a t . A s a t e l l i t e cell (cs) i n t e r v e n i n g b e t w e e n a b l o o d v e s s e l (V) a n d a n e r v e cell (Ne); e l e c t r o n m i c r o g r a p h , ce e n d o t h e l i a l c e l l ; G G o l g i c o m p l e x ; m m i t o c h o n d r i a ; a a x o n (24.000 •

each s.e. corresponds in fact to 1500/~a i n the adult, to 1300 #a in the n e w b o r n i n the case of n e u r o n s of 4000 Ha.

Satellite cells in sensory ganglia

589

F r o m t h e d a t a of t h e a d u l t a n d t h e n e w b o r n i t is a p p a r e n t t h a t s . c . n u m b e r i n c r e a s e s i n p a r a l l e l w i t h n e u r o n ' s v o l u m e a n d s u r f a c e . T h e r e l a t i o n b e t w e e n s.c. n u m b e r a n d n e r v e cell s u r f a c e is n e a r l y c o n s t a n t f o r n e u r o n s of v a r i o u s size. I t m a y b e c o n c l u d e d , t h e r e f o r e , f o r t h e e x i s t e n c e of a d i r e c t p r o p o r t i o n a l i t y b e t w e e n s. c. n u m b e r a n d n e u r o n ' s s u r f a c e , r a t h e r t h a n b e t w e e n s. c. n u m b e r a n d n e u r o n ' s v o l u m e . I t is k n o w n t h a t Na S c h w a n n ' s cells a l s o p r e s e n t a q u i t e g0 s imilar relationship. /8 18 /r

Na

/2

8

10 8

2

G

,

2

12 io

l 52

2g z oo

Na

/s 1# 12 10

~

Na

-

--

J

8 @

0

x

!

/

2

3

~

5

6"

s nlooos.

o

?

q

5

s

7

i

8

g

I

ID 7] 12 /3_ b

s F,,10o.

Fig. 13 Fig. 14 Fig. 13. a Relationships between volume (V) of spinal ganglion cells (on the abscissa) and number (Na) of s.c. (on the ordinate), in the adult rat. b Relationships between surface (S) of spinal ganglion ceils (on the abscissa) and number (Na) of s.c. (on the ordinate), in adult rat. The a has been plotted as a vertical line for each class; the 13 ~ class includes only two nerve cells; therefore the vertical line is discontinuous Fig. 14. a Relationships between volume (V) of spinal ganglion cells (on the abscissa) and nmnber (Na) of s.c. (on the ordinate), in the newborn rat. b Relationships between surface (S) of spinal ganglion cells (on the abscissa) and number (Na) of s.c. (on the ordinate), in the newborn rat. For each class the ~ is plotted as a vertical line It has been mentioned before that sometimes the axon may encircle the perik a r y o n w i t h a g r o u p of coils ( a x o n i c g l o m e r u l u s ) : i n t h e s e cases, i t m a y b e h a r d t o d e c i d e w h e t h e r a s . c . lies o n t h e p e r i k a r y o n o r o n t h e a x o n . F o r t h i s r e a s o n , e v e n t u a l l y p e r i a x o n i c s.c. h a v e b e e n e r r o n e o u s l y c o n s i d e r e d p e r i s o m a t i c . T h i s e r r o r is r a t h e r i m p r o b a b l e w h e n s t u d y i n g n e w b o r n r a t s , b e c a u s e t h e g l o m e r u l u s is f o r m e d o n l y a f t e r b i r t h (cfr. I~ENttOSSI~K 1907, CAJAL 1909, a n d t h e p r e s e n t o b s e r v a t i o n s ) . I n t h e a d u l t t h e g l o m e r u l u s is m o r e c o n s p i c u o u s i n t h e l a r g e n e r v e cells (CAJAL 1 9 0 9 ) ; i t s f e a t u r e s a n d e x t e n s i o n a r e c h a r a c t e r i s t i c f o r e a c h s p e c i e s (LENttOSSl~K 1907). I n C a j a l s t a i n e d p r e p a r a t i o n s , I w a s a b l e t o s h o w t h a t t h e g l o m e r u l u s is r a t h e r u n c o m m u n i n t h e r a t a n d , i n case, s m a l l a n d w i t h a f e w coils ( t h e s a m e c o n d i t i o n w a s f o u n d i n t h e m o u s e b y CAJAL 1909). F o r t h i s r e a s o n

590

E~NIO PANNESE:

the r a t was chosen for the determinations reported above. On the other hand, the regular course of the diagrams indicates t h a t the various errors which possibly

a

b

c

c

f

g F i g . 15a-- g. S p i n a l g a n g l i a of n e w b o r n r a t ( a - - e ) a n d of a d u l t r a t ( f - - g ) ; Z e n k e r f i x . , h e m a t . - e o s i n (1.000 • ). P e r i s o m a t i c s . c . u n d e r g o i n g m i t o s i s ( a r r o w s ) ; i n d e n t a t i o n s of t h e p e r i k a r y o n s u r f a c e ( F i g . a, b, e, f, g ) (ef. t e x t , p g . 591)

were made in the evaluations are in the whole negligible. This consideration holds true also when comparing the diagram of the newborn (no axonic glomerulus present) and the adult rat (glomerulus sometimes present); the two diagrams are very similar in outline.

Satellite ceils in sensory ganglia

591

The comparison of the diagrams in Fig. 13 and 14 shows that in the spinal ganglia of the rat throughout the body's growth period the nerve cell volume increases (as shown by HATAI 1902, LEVI et al. 1908, OLIVO et al. 1932) and also the s.c. number increases presumably trough mitotic division. I n the first days after birth, in fact, mitotic figures m a y be often found in the cells adjacent to the neurons (Fig. 15a--e). The cells undergoing mitosis look somehow swollen and the corresponding area of the perikaryon surface looks indented: the latter is a further proof that the dividing cells are s. c. proper. I n the adult, the s.c. entering mitosis are very scarce; one over 50000 s.c., according to m y observations.

VIII. Considerations and conclusions The purpose of the present study has been the analysis of the microscopical and submicroscopical structure of the s. c. in the spinal ganglia. This research has also afforded a somehow deeper insight into the biological and functional peculiarities of s.c. l~Iorphological characters of the spinal neurons sheath and shape of the s.c. First, new evidence has been summoned that the s.c. build a continuous layer, which surrounds the nerve cell and isolates it from the connective constituents of the ganglion (Fig. 3, 12). The sheath is not a syncytial structure, but it is built of discrete cells, lying close to the neuron surface and adapting itself to the uneven outline of the latter. The well known description of the s.c., viz. of elements with branching processes is the result of a technical artifact. I n well preserved preparations, s.c. appear as laminar elements with more or less irregular boundaries deprived of branching processes. Structure. The electron microscope study reveals that the s.c. cytoplasm encloses structures similar to those found in all the other cells (Golgi complex, mitochondria, endoplasmic reticulum and sometimes ergastoplasm). I n sections stained with nranyl-acetate, a thin network among the tubuli of the endoplasmic reticulum is apparent. However, it is not yet possible to establish whether this structure has its equivalent in the living cell: it might depend, partially at least, on a clotting due to the dehydration of materials not well preserved by osmic acid treatment. Biological characteristics. During body's growth s.c. maintain their close contacts with the nerve cells and adapt themselves to the changes of the nerve cell volume and outline. The growth of the sheath's volume parallels the increment of the nerve cell volume and surface : such growth is due rather to an increase in the number of the s.c. through mitosis than of their volume. The volume of s.c. seems not to undergo remarkable changes; it is much smaller than that of the neurons and seems to follow the law of Driesch. An attempt can be made to range the s.c. in the classification of types proposed by G. BIzzozERo (1893). S.c. are not true perennial elements, as their number increases after birth while their volume does not increases in proportion to body's growth, but they are not labile elements. Personally, I never could find any evidence of s.c. death, while very seldom I noticed mytosis of s.c. in the adult. Therefore, s.c. could be classified as stable elements according to BIzzoZEl~O (1893). S.c. could then be representatives of the group of intermitotic differentiated cells, with long intermitotic period, or of the group of reversible postmitotic cells of COWDRY'S (1942) classification.

F i g . 16. A s c h e n l a t i c d r a w i n g of t h e g e n e r a l a r r a n g e m e n t a n d s t r u c t u r e of s.c. s h e a t h . O n e h a l f of t h e p e r i s o m a t i c s h e a t h , s e e n f r o m i t s i n n e r s u r f a c e , a f t e r r e m o v a l of t h e p e r i k a r y o n , a n d t h e w h o l e p e r i a x o n i c s h e a t h are r e p r e s e n t e d . I n t h i s case t h e a x o n f o r m s ~ g l o m e r u l u s . T h e p e r i a x o n i e s h e a t h h a s b e e n i n t e r r u p t e d t w i c e t o r e n d e r v i s i b l e t h e a x o n (a). S n u c l e u s of p e r i s o m a t i c a n d p e r i a x o n i c s. e. ; c e r g a s t o p l a s m ; er e n d o p l a s m i c r e t i c u l u m ; G G o l g i c o m p l e x ; m m i t o c h o n d r i a ; p s h e a t h ' s i n d e n t a t i o n s , w h e r c t h e " p a r a f i t i " c o m e i n ; v s m a l l vesicles, m o r e n m n e r o u s n e a r t o t h e l i m i t i n g m e l n b r a n e s

E ~ i o PAYLESS: Satellite cells in sensory ganglia

593

Function. I t seems rather doubtful that s.c. m a y display a mechanical function, because their cytoplasm does not show, even at the submicroscopic level, any "mechanical" structures; moreover the supporting function seems to depend on the connective stroma of the ganglion. I t seems more probable that s.c. play a trophic role toward the neuron. As a matter of fact, s.c. build around each neuron a continuous sheath, which intervenes between blood vessels and nerve cell (Fig. 12). The metabolic materials reaching the nerve cell from the vessels must therefore diffuse through the s.c. envelope. As these cells are closely adherent to each other, it does not seem likely that the materials m a y diffuse through the thin intervals between contiguous cells; more probable they diffuse through the s.c. cytoplasm. Open to mere speculation is at present the problem related to this trophic activity, viz. : whether s.c. are merely filtrating elements which select the metabolic materials that must reach the neurons, or they play a true metabolic role, by transforming the circulating raw materials in substances more easily available to the neurons. I t has been shown that the s.c. cytoplasm encloses sometimes an ergastoplasm. Obviously this finding is not a sufficient indication of a secreting function proper of s.c., but it might help to suggest that synthetic processes take place in the cytoplasm of s.c. Hypothetically it could be advanced that s.c. take part into the chemical transformation of materials diffusing from blood vessels. The possibility should not be discarded that neurons m a y need the collaboration of less differentiated cells able to performe a preliminary metabolic work. S.e. nature. The morphologic approach is not suitable for the solution of this hard problem, which has not been so far much clarified by experimentation. I t m a y be discussed, however, the problem of the likeness existing between s. c. and g]ia cells of the C.N.S. This view has been supported by m a n y Authors (cf. pag. 570). I n the past the similarities of shape in these two kinds of cells have been emphasized: branching processes were in fact described in glia and in s.c. Now we know that this is not true of s.c. and perhaps not even of all the glia elements. A~DR]~W and ASHWORTg (1944) maintained that in the C.N.S. some glia cells lack cytoplasmatic processes; the few data of electron microscopy at hand seem to support this contention. I t must be stressed, however, that the differences of the milieu surrounding the cells in the C.N.S. and respectively in the ganglia are such as to make understandable the existence of morphologic differences between cells of similar nature. Under these conditions, the mere similarity in the shape of the cells is not a valuable criterium to decide on their nature. More important similarities between s. and glia cells are to be found in their submicroscopic structure and in their biological characteristics. The cytoplasm of glia cells examined at the electron microscope is not highly differentiated as it is the cytoplasm of s.c. The cytoplasm of both cells under technical treatments shows similar features with labile structures, high water content; the enclosed materials are easily extractable from both. Also the physical relationship between protoplasmatic glia cells and neurons are similar to those which exist between s. and nerve cells. As to their function, there is a quite general agreement on the trophic function of protoplasmatic glia cells. The present researches seem to support the view that s.c. are stable elements: BAIRATI (1950) drew the same conclusions from his observations on glia cells.

594

E~nio PANNESE:

In m y opinion these data are in favour of the view that s. c. and protoplasmatic glia cells have a similar nature. However, this view calls for experimental evidence. On the basis of the data already at hand the problem of the individuality or the affinity of perisomatic s.c., periaxonic s.c., and S e a w A l l ' s cells seems not so uncertain. The observations reported in this paper show that all these cells belong to only one group of s. elements; a distinction seems possible only on the basis of their respective location. The hypothesis on the similarity of these cells, already supported by RANVIER (1888), KOLLIKER (1905), LENHOSS~K (1907) etc., finds further support in the recent evidences deriving from electron microscopical findings. Also the quantitative data reported at pg. 587--9 support this hypothesis. I t seems therefore possible to include in a large but rather homogeneous group various ceils of central and peripheral nervous organs, whose commun characteristics are a close relationship with the neurons and analogous biological properties. Zusammenfassung Die Untersuehung der perisomatischen und periaxonalen Satelliten in sensiblen Ganglien verschiedener S/iuger hat folgende Ergebnisse: Es wird naehgewiesen, dab die Satelliten nm das Neuron eine ununterbroehene Hfille bilden, die es yon den Bindegewebsstrukturen des Ganglions vollstiindig trennt. Jeder Satellit ist von seiner eigenen Zellmembran scharf begrenzt; die Membranen der anliegenden Zellen sind dureh Zwischenr/iume von etwa 200 ~ getrcnnt. Die Form der Satelliten ist im wesentlichen lamin/~r: die Abbildungen yon Zellen mit feinen verzweigten Forts~tzen, die haupts/ichlich durch Silberimpr/~gnation gewonnen wurden, geben meistens Artefakte wieder. Die Satelliten habcn innige Beziehungen zum Neuron, yon dem sie durch einen d/innen Zwischenraum (etwa 200 A), yon den entsprechenden Zellmembranen abgegrenzt, getrennt sind: die Satelliten passen sich jeder Unregelm~Bigkeit der Neuronenoberfl/iche an, die durch kleine Paraphyten hervorgerufen wird. Wo der Neurit erscheint, stellen sieh die perisomatischen Satelliten ein. Sie werden von den periaxonalen Satelliten ersetzt und diese ihrerseits von den Schwannschen Zellen. Die Satelliten enthalten manehmal ergastoplasmisehe Bildungen. I m grogen und ganzen ist die Struktur dieser Zellen derjenigen der Sehwannsehen Zellen und vieler protoplasmatisehen Glioeyten des Zentralnervensystems /ihnlieh. W/~hrend des k6rperliehen Waehstums erfahren die Satelliten eine bedeutend geringere Volumen-Zunahme als die Neurone, aber sie vermehren sieh h/iufig durch mitotisehe Teilung. Beim Erwaehsenen sind die Mitosen dagegen sehr selten. I)as endgfiltige Volumen der Satelliten ist eher gleichm/iBig, es entsprieht dem Driesehsehen-Gesetz. Auf Grund der gewonnenen Daten kann man diese Zellen als stabile Elemente im Sinne BIZZOZERO's betrachten. l~ber den funktionellen Wert der Satelliten/iugcrt sieh der Verfasser auf Grund der morphologisch und biologiseh gesammelten Daten. Da diese Zellen immer zwisehen den Blutgef/iBen und den Neuronen liegen, mug ihre T/itigkeit trophiseher Art sein. Die morphologisehen Untersuehungen k6nnen allerdings nieht feststellen, ob diese trophische Funktion nur in einer Filtrierung der von den t31utgef/iBen herkommenden Substanzen oder aueh in ihrer Verarbeitung besteht.

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SchlieBlieh b e h a u p t e t d e r Verfasser, da{] d i e p e r i s o m a t i s c h e n u n d p e r i a x o n a l e n S a t e l l i t e n e i n e r s e i t s eine grofle ~ h n l i c h k e i t m i t d e n p e r i n e u r o n a l e n p r o t o p l a s m a t i s c h e n G l i o c y t e n des Z e n t r a l n e r v e n s y s t e m s a u f w e i s e n , a n d e r e r s e i t s m i t d e n S e h w a n n s e h e n Zellen. E s ist v i e l l e i c h t m6glieh, i n e i n e r K a t e g o r i e viele Z e l l e n z u s a m m e n z u f a s s e n , die i n e n g e r B e z i e h u n g zu d e n N e u r o n e n s t e h e n u n d /~hnliehe f u n k t i o n e l l e E i g e n s e h a f t e n b e s i t z e n , Zellen, die s o w o h l d e m z e n t r a l e n als a u e h d e m p e r i p h e r e n N e r v e n s y s t e m a n g e h 6 r e n . References

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