V Vol. 14(2), pp. 156-166, 14 Ja anuary, 2015 D DOI: 10.5897/A AJB2014.13678 8 A Article Number: 2F478A649625 ISSSN 1684-5315 5 C Copyright © 20 015 A Author(s) retain n the copyrigh ht of this article e h http://www.ac cademicjournals.org/AJB
Africcan Journal of Bioteechnology
Fu ull Length Research h Paper
Histo ologic and ultrastru uctura al obse ervatio ons on the thyrroid gland off the White W F Fulani ((Zebu) cattle e in Northe ern Nig geria Cas smir Onwua aso Igbokw we* and Da aniel Nwag gbo Ezeaso or Depa artment of Ve eterinary Anattomy, Faculty of Veterinaryy Medicine, U University of N Nigeria, Nsukkka. Receiv ved 30 January, 2014; 2 Accepted 22 December, 2 014
The histology and ultrastructure of thyroid glan nd of White F Fulani (Zebu)) cattle were examined d during the al period to show if varriations in m postnatal development d morphology exist with age and in re elation to tropical clim mate. Eight prepubertal p and ten pub bertal cattle o of abattoir o origin were u used to carry y out this study. Histologically, th he thyroid consisted c of well-develo oped capsule e, interlobula ar connectiv ve tissue, follicular an nd parafollic cular cells frrom prepube ertal to pube ertal age of 5 to 7 years s. The follicu ular cells were cuboidal in prepu ubertal period d with some e variations, became pre edominantly columnar in n pubertal age of 2 to 4 years and highly flattened cells we ere common in the older pubertal aniimals of 5 to o 7 years. The colloid d was obserrved in the lumen at all age and w was PAS-po ositive with abundant p peripheral vacuoles in n the puberta al age of 2 to o 4 years. Significant va ariations (p˂ ˂0.05) in stru ucture of the e thyroids were obserrved at all ag ge and the fo ollicular size and numbe r of follicles increased a as the cattle matured. Follicular height h was re educed significantly in th he older pub bertal cattle of 5 to 7 yea ars. Ultrastructurally, the follicula ar cells show wed highly dilated ciste ernae of rou ugh endoplas smic reticulum which decreased with age. Fllattened follicular cells seen s in the ollder puberta l cattle conta ained few cy ytoplasmic organelles and microv villi. Apically placed pseu udopods and d blebs were e commonly o observed in the pubertal age of 2 to 4 years s. Lysosoma al bodies in ncreased with age of d development. The para afollicular ce ells were e older catttle. They con encountere ed frequently y in the prep pubertal age e than in the nsisted of n numerous dense secre etory granules which inc creased in nu umber with a age. The pre esent results suggest an optimum thyroid function during g the puberttal. Our findiings would be useful in n interpreting g changes in thyroid morphology y during exp perimental an nd pathologic cal condition ns. Key words: Cattle, postnatal thyroid, histology, ulttrastructure. TRODUCTION N INT e thyroid is the largest and the firs st recognizable The end docrine gland during de evelopment in vertebrate es. Marked variatio ons in locatiion, gross and a histological
d have been observed in n featurres of the tthyroid gland differe ent vertebra ates (Dyce et al., 20 002). Thyroid d respo onses to envvironmental and nutrition nal influences
*C Corresponding author. a E-mail: casmir.igbokw
[email protected]. Tel: +234-60 034930393. Au uthor(s) agree that t this article remains permanently open access a under th he terms of the e Creative Com mmons Attribution License 4.0 0 Intternational Lice ense
Igbokwe and Ezeasor
differ across the phylum. In mammalian development, the thyroid hormones exercise significant effects on cell proliferation, differentiation and migration. The effects of thyroid hormones on growth and metabolism in all stages of mammalian development are well documented (Legrand, 1986). Calcitonin secreted by parafollicular cells lowers blood calcium levels by suppressing the resorptive action of osteoclasts and promotes calcium deposition in bones by increasing the rate of osteoid calcification (Mescher, 2010). The thyroid follicular and parafollicular cells undergo significant alteration during prenatal and postnatal development. The alterations pertain to the number of cells, their ultrastructure, as well as their hormone storage in the cells (Kameda et al., 1984; Fujita, 1975). The parafollicular cells produce mainly calcitonin that regulates calcium metabolism and it also produces few other regulatory peptides of the thyroid such as somatostatin, chromogranin A and neuron specific enolase (NSE) that are probably involved in intrathyroidal regulation of follicular cells (Sawicki, 1995). It is generally accepted that variations in the organelle content of the thyroid follicular cell reflect variations its hormone synthesis, secretion and absorption (Harrison and Young, 1970; Gorbman et al., 1983). Environmental factors such as temperature and photoperiods may influence follicular cell activity through nervous endocrine agents (Norris, 2007). Available literature on the histological, ultrastructural studies on the development of the thyroid gland in several exotic wild and domestic animal species including cattle are abundant (Fujita, 1975; Fujita, 1988; Hernandez et al., 1972; Schafie and Mashaly, 1974; Roy et al., 1978; Pardehi, 1981; Baishya et al., 1985; Sawicki and Zabel, 1997; Sawicki and Zabel, 1999; Hajovska, 2002; Jelinek et al., 2003; Peksa et al., 2011). Although, these studies exist on thyroid morphology, there is scant published literature on morphology of thyroid of our indigenous domestic animals of the humid tropical environment. However, many of these were on exotic breeds of cattle in the Mediterranean and temperate climate. The white Fulani (zebu) cattle unlike some of the exotic breeds are reared in the semi-arid and arid regions of the country by pastoralists that move from place to place in search of water and pasture. It is reasonable to hypothesize that there should be variation in structure and function of the thyroid in tropical breeds due to variation in genome, environmental condition and husbandry methods. The present study was therefore undertaken to investigate such possible variations and to relate the findings to function in a humid tropical climate. This would facilitate the accurate diagnosis of congenital, and acquired pathological abnormalities and iodinerelated disorders of the thyroid that may occur during rearing. MATERIALS AND METHODS Thyroid glands were obtained from eight prepubertal (less than 1
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year) and ten pubertal (3 to 6 years) White Fulani (Zebu) cattle slaughtered at the Nsukka market abattoir after being certified free of disease. The age was estimated by dentition (Torrel, 1998; Pace et al., 2003). Following death, thin slices taken from different parts of each thyroid lobe and isthmus were fixed in 10% neutral buffered formalin and processed routinely for light microscopy. Some of the sections were stained with haematoxylin and eosin and others with periodic acid Schiff (PAS). An ocular micrometer gauge calibrated with a stage micrometer was used to obtain values of histological parameters of the thyroid gland from selected sections under a light microscope with × 40 objective lens. Data were analyzed statistically using analysis of variance. Duncan’s multiple range tests was used to separate variant means, and significance was accepted at p < 0.05. For electron microscopy, small pieces of the organ were diced into 1 mm3 cubes, fixed in 2.5% glutaraldehyde in 0.12 M Millonig’s phosphate buffer at pH 7.4. They were post-fixed in 1% osmium tetroxide after rinsing in phosphate buffer for electron microscopy. The fixed pieces of the thyroid gland were dehydrated in graded ethanol, cleared in propylene oxide and embedded in epoxy resin. Ultra-thin sections (60 to 80 nm) were collected on copper grids, stained with uranyl acetate, and counterstained with Reynold’s lead citrate and they were examined under Philips CM10 transmission electron microscope accelerating at 80 KV (FEI, Eindhoven, The Netherlands).
RESULTS Histology Thyroid gland of prepubertal cattle of 10 to 11 months showed a developed thyroid capsule with collagenous fibers, moderate fibroblasts and profiles of vascular and nervous tissues. The inner part of the capsule penetrated the parenchyma through the trabeculae septa carrying enormous connective tissue elements, vascular structures and lymphatics. The trabeculae divided the organ into very distinct lobule (Figure 1a and b). These follicular cells were columnar in most follicles, but occasionally cuboidal follicular cells were seen in some colloiddistended large follicles. Generally, follicular lumina predominated over cellular and vascular element and medium- sized follicles were predominant, followed by large follicles and few small follicles. Small follicles were mainly in the periphery. All follicles contained a homogenously stained colloid with colloid droplets apparent in the periphery of the colloid (Figure 1b). The colloid was PAS- positive (Figure 1c). Parafollicular cells were present as in the previous age group. The thyroid tissue of about 2 to 4 years showed similar histological structure in the maturation of the capsule, trabeculae and their various connective tissue components. Larger vascular tissue and nerves bundle were encountered at this age than the prepubertal. The predominance of medium-sized follicles, followed by large follicles in the centre of the lobules was maintained (Figure 2a). Colloid droplets seen in the prepubertal also persisted and the colloid was intensely PAS- positive (Figure 2b and c). The cells were cuboidal to columnar with little variation in height. Parafollicular cells were observed as in the previous age. In cattle of 5 to 7 years focal areas of cell hyperplasia were present. Large irregular follicles were greatly
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Figure 1. a and b, Histological se ection of prepub bertal thyroid sh howing large fo ollicles(L), mediu um follicles and d sma all follicles (S) and trabeculae e (arrow) and colloid c droplets at higher ma agnification. C. A PAS-positive e colloid lumen (P) with w colloid droplets (arrow).
Figure F 2. a. His stological sectio ons of thyroid of o 2 to 4 years cattle showing profiles of irreg gular large and d mediumm sized follicles, some with colloid (C C) and others e empty (E). b. F Follicles at this age containing g copious c peripherral vacuole (arro ow). C. The co olloid was PAS-p positive (P) collo oid vacuoles (arrows).
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Figure 3a. Thy yroid histologica al section of 5--7 years cattle with highly disttended profiles of irregular large follicles (F), and increased lobulation off the parenchym ma (arrow). b. H Higher magnificcation of the large follicles showing areas off cellular hyperp plasia (X) and a accumulation of fibrous tissue (a arrow).
Table 1. Histomorphometric of thyroid follicle of postnata al white Fulani (z (zebu) cattle with h increasing devvelopment age..
Dev. Age e d/year Prepuberrtal Pubertal 2 to 4 years Pubertal 5 to 7 years
SFD (μm) 53.36d ± 0.30 0 61.43e ± 0.48 8 73.37f ± 0.84 4
MFD (μm)) 106.91d ± 0.65 109.55e ± 0.55 125.01f ± 1.13
LFD ((μm) 237.9 97d ± 0.75 264.8 89e ± 1.04 284.0 00f ± 0.98
C CT (μm) 9 90.35d ± 0.33 9 93.44e ± 0.38 1 107.08f ± 1.21
CH (μm)) 7.75b ± 0 0.05 7.45c ± 0 0.04 5.53d ± 0 0.08
a,b,c,d,e.F
Means in the same column c with diffe erent superscript are significantly different from ea ach other. SFD, small follicle dia ameter; eter; LFD, large follicular diamete er; CT, thicknesss of thyroid capsu ule; CH, follicular cell height. MFD, Medium follicle diame
disttended with colloid that was less intensely PA ASpossitive and fo ollicular cells presented flattened f nuc clei (Fig gure 3a and b). Evidence e of fibrous tissue t formation wass observed in the interffollicular con nnective tissu ue. Parrafollicular cells c were present sing gly. Significa ant varriations (p˂0.0 05) in the histtomorphomettry of the thyro oid gland were obse erved amongst the age groups during the t posstnatal developmental pe eriod. All his stological pa arameters (Table 1)) increased as s the thyroid matured in ag ge. Ultrastructure In prepubertal thyroids t of about a 10 to 11 months old o catttle, the follicu ular cells appe eared well diffferentiated and a varried in size an nd shape, particularly in th heir vertical and a late eral dimensio ons (Figure 4a). 4 Some where w column nar with h indented nucleus and contained c mo ore cytoplasm mic organelles and others were e cuboidal with w irregularrlyaped nucleus s with few cytoplasmic organelles. In sha
were evident in n additiion, very few flattened follicular cells w some e follicles. The e basement membrane w was well diffe erentiated and d the in nterfollicular sspace contain ned connectivve fibroblasts s, fibroccytes and ccollagen fibriils. Perifollicular vascula ar endotthelium was well develop ped. The cubo oidal follicula ar cells contained nu ucleus with he eterochromatiin (condensed d chrom matin) was cconcentrated marginally on the nuclea ar memb egular layer and was alsso present as brane as irre scatte ered clumps in the nucle eus. The eucchromatin was s well-d dispersed in tthe nucleus (F Figure 4b). Profiles of wellldevelloped rough e endoplasmic reticulum were apparent in n the cytoplasm and displayyed well-ma arked dilated d n some cells s. cisterrnae containiing flocculentt materials in Golgii complex was present in tthe supranucclear or atimes paran nuclear posittion and wass relatively e extensive with h lots of small-me edium sized vesicles a and secretory y granu ules in the cytoplasm. The apical part of the e cytop plasm showe ed numerouss small den nse secretory y vesiccles which were presumed to be primarry lysosomes s,
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F
F
Figu ure 4. a. Ele ectron microgra aph section off thyroid follicu ular epithelium showing g varying size and shape of thy yroid follicular cells asement memb brane (arrow) and a (F): Note the well delineated ba interfollicular conne ective tissue (X X), perifollicula ar capillary (C).. b. EM section of thyroid follicular cell (F) of prepubertal thyrroid show wing highly developed d dilatted RER with some floccullent matterials (arrow). Some S small vesicles (V) and de ense lysosome-like secrretory granules s are present (L L). c. EM photo ographs of thyrroid folliccular cell of pre epubertal showiing highly dilate ed RER(R), den nse secrretory granules s; probably ly ysosomes (L) that engaged in phagocytosis of large l colloid droplets d (C) to o release thyrroid horm mones through the basal partt of the epithelium (B).The apical cyto oplasm studded with microvilli faced the colloiid of lumen (X). d. EM photo graph off apical cytopla asm of follicularr cell of thyroid d of pubertal cattle, showing large e secretory vesicles that may be prep collo oid droplets (C) ( and dense secretory granules g proba ably lyso osomes (L), microvilli(arrow), cilium (broken arrow), nucleii of folliccular cells (N): Note the well delineated d juncttional complex (X). ( e. E EM photograph h of parafollicu ular cell (P)of prepubertal ca attle thyrroid interposed between two fo ollicular cells (F)).The parafollicu ular cellss showed numerous dense granules(D) in their t cytoplasm, in addition into some e mitochondria,, but lacked diilated cisternae e of R found in follicu ular cells, colloid d (C). RER
also llarge less dense vesicles with same re esemblance to o the lu uminal colloid were identified as colloid droplets. Few w lysos omal bodies were observe ed to have en ngulfed colloid d ets in the a drople apical cytopla asm. Moderate number of o mitocchondria of vvaried shape was presentt in the apica al positiion and few w were scattere ed in the cytoplasm (Figure e 4c). Als o, in the apiccal cytoplasm m, profiles of sshort microvilli were present am mongst single e long cilium m (Figure 4d d) close e to a well-d developed ju unctional com mplex. But in n some e columnar fo ollicular cells the microvilli appeared to o be mo ore than thosse in the cubo oidal cells. Th he elements of o the ccomplex were e identified be etween two ccolumnar cells and i ncluded the ttight junction,, intermediate e junction and d desm mosome and these followe ed each othe er in the orde er given n in an apical--basal directio on. Few para afollicular cells s with large oval nuclei were identified byy their basa al locatiion between follicular cellss. They also did not make e conta act with the ffollicular lum men (Figure 4 4e). Howeverr, these e relatively llarge, lightlyy stained ce ells contained d strikin ngly numerou us electron de ense secretory granules in n the ccytoplasm. T These cells also show wed abundan nt mitocchondria in the cytoplasm, but distende ed cisternae of o rough h endoplasm mic is scant. The surfacce of closes st conta act between tthe follicular cell and the e parafollicula ar cell la acked complex interdigita ations or speccialized areas of pla asma membra anes attachem ment. In tthe thyroid off pubertal ca attle of about 2 to 4 years s, the m matured folliccular structu ure displayed d in the pre epuberrtal was maintained during this adultt age (Figure e 5a). T The shape and size of th hese cells alsso varied from m e to follicle follicle e, such that cuboidal a and columna ar epithe elial cells we ere even pre esent within a follicle, as s previo ously noted d. But columnar cells were more e frequ ently encoun ntered. In the follicular epitthelial cells at a this a age, the arrangement sho owed well forrmed follicula ar cells with plasm ma membrane, basement membrane e, nucle eus and vario ous profiles cytoplasmic organelles as seen in the pube ertal age. A d definite interffolllicular con nnectivve tissue co ontaining perifollicular ca apillary endo othelia al cells, collag gen fibrils and fibroblast a amongst othe er conne ective tissue e elements were copiously presentt. These e componentts appeared tto more abun ndant unlike in n the p prepubertal ag ge. Junctiona al complexes were typical. Large e, small de ense secreta ary vesicles and colloid d drople ets were alsso prominent. In distincttion from the e prepu ubertal age, the follicular ccells of puberttal age of 2 to o 4 yea ars, in additiion to the numerous miccrovilli on the e apica al membrane e, also show wed moderatte number of o ciliate ed follicular epithelial ce ells in all of the sections exam mined at this a age (Figure 5b b and 5c). Usu ually, one ccilium per ce ell was present, although h occassionally two cilia togethe er have been n observed in n sectio ons amongst numerous m microvilli in thyyroid sections of th his age. Pse eudopods (a apocrine prottrusions) with h dome e-shaped or balloon-like sshape where observed on n the ap n pical surface,, it contained fine granular matrix often
Igbo okwe and Eze easor
Figu ure 5. a. EM photographs of 2 to 4 years thy yroid showing well w diffe erentiated epitthelium of fo ollicular cells (F) and hig ghly vasccularised interrfollicular conn nective tissue (V) with so ome endothelial cells (a arrow).Connectiv ve tissue fibroblasts and collag gen fibrils (C) and lumiinal colloid (L) are shown, b: EM photograph h of thyrroid of 2 to 4 ye ears thyroid showing thyroid follicular cell (F) with w app parent increase in microvilli (M),, with single ciliu um (box) occurrring amo ongst them on the apical cyttoplasm, c: Ma agnification of the apiccal cytoplasm showing s a sing gle cilium (brok ken arrow) and its basal bodies (b) and microtubu ule core. d. EM E photograph of cuboidal follicular cell c of thyroid of 2 to 4 years cattle showing apical pseudopods (arrow w) and numerous colloid drop plets (C). Note the high hly abundant co ollagen fibrils (R R) in the inter fo ollicular connecttive tissu ue. e. EM phottograph of thyrroid follicular ce ell showing dom meshaped apocrine activity a (broken arrow) in the apical cytoplas sm, note e the some collo oid vacuoles(C) and Golgi complex (arrow). f. EM pho otograph of para afollicular cells (P) of thyroid of 2 to 4 years ca attle wing increased accumulation of o dense granule es (D). These cells show were not frequently y encountered at this age as in the prepuberrtal, folliccular cell (F).
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with ccolloid drople et (Figure 5d a and e). These e pseudopods sugge ested evidencce of phagocyytosis of collo oid droplets by y the ffollicular cell. The intraccellular organ nelles of the e follicu ular cell app peared to be e quantitative ely similar to o those e of the previo ous age. The e nucleus wass also located d basallly, mitochond dria occurred in all parts o of the cell and d some e were close ely associated d with rough endoplasmic reticu ulum (RER). T The RER wa as largely loccated between n the n nucleus unliike in the prepubertal groups. The e cisterrnae of RER R maintained highly diste ended profiles s obserrved in the previous ag ge groups. S Several apica al vesiccles were pressent between n the apical m membrane and d the n nucleus, and d large vesiccles were re ecognized as colloi d droplets. D Dense lysosom me-like bodies were rare in n the a pical position n. The apical vvesicles could d be confused d with dilated bits of RER exxcept for the e absence of o riboso omes on the e surface of the apical ve esicles. Some e paraffollicular cellss were how wever encoun ntered in the e sectio ons examined and they a appeared to display more e dense e secretory g granules in th he cytoplasm m unlike in the e previo ous age (Figu ure 5f). In ccomparison to o the prepubertal age, the e adult thyroid d differe ed in the ssense that m microvilli obsserved in the e prepu ubertal age in ncreased in nu umber in the adult age and d prom inent single cilium was always pressent amongs st them . Also, the latteral surface exhibited inte erdigitations in n the prepubertal age, but th hese were only seldom m obserrved on the b basal half of the adult folllicular cell bu ut the ju unctional com mplex was similar in both h groups, bu ut more desmosome es were prese ent amongst ffollicular cells s. Paraffollicular cell were encountered frequently in the e prepu ubertal age unlike in this p present puberrtal thyroids of o 2 to 4 years cattle. In the puberrtal cattle thyrroid gland of 5 to 7 years the follicular arrrangement o of cells were e mainttained, seve eral of these follicularr cells were e squam mous cells th hat intermingle ed with some e cuboidal and d even few column nar cells in ssome follicless (Figure 6a)). Colla gen fibrils w with some con nnective tissu ue cells were e rema rkably incre eased in tthe interfolliccular space e. Gene erally, the follicular cells decreased in tthe profiles of o organ nelles present. Some of th he apical mod difications like e pseud dopds were rrare in sectio ons, even tho ough microvilli were present am mongst a sin ngle cilium a as previously y obserrved. Highly d dilated cisterna of RER co ommon in the e earlie er age groupss was highly rreduced and G Golgi complex x was not common nly present. The prepon nderant apica al vesiccles, including g colloid drop plets and dense secretory y granu ules were no ot a consisten nt finding in sseveral of the e flatten ned follicular cells (Figure 6b). Lysosom me-like bodies s ared to have appea e increased in the apical cytoplasm of o the cu uboidal cells in some folliccles. DISC CUSSION The present stud dy used the prepubertal and puberta al White e Fulani cattle e to provide information o on the compa arative e developme ent, morpho ology and h histochemistry y
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Figure 6. a. a EM photograph of thyroid follicles of puberta al cattle of 5 to 7 years cattle showing follicular cells c that were e mostly squa amous cells (a arrows) and abundant interfo ollicular connective e tissue (CT). b. b EM photogra aph of follicular cell of thyroid of 5-7 years showing flattened cells c devoid of dilated and the nucleus n (N) appe eared pyknotic.
of tthe thyroid gland. It is welll-established that the role e of the thyroid gland in vertebra ates is crucial to the gene eral metabolism of the t organism and the exp pression of this role e is dependent on the age, nutrition and a envvironmental, especially e climatic factors s. It is therefo ore rea asonable to expect e certain structural and functional varriations which modulate the e function of the t thyroid. The T histtologically components of the thyrroids remain ned esssentially the same s in the po ostnatal age group has be een desscribed in several mammalian thyro oids (Tomona ari, 195 58; Fujita, 1975; 1 Fujita, 1988). Ch hanges in the t histtometry of the thyroid com mponents we ere also notic ced durring developm ment and thes se were relatted to activity y of the thyroid durin ng postnatal developmentt. Ultrastructu ural features of the thyroid gland were essentia ally the same e in all age groups s as have been descrribed in som me mammals (Klinc ck et al., 197 70; Fujita, 19 975; Atoji et al., a 199 99; Abdel-Ma agied et al., 2000). How wever, extrem me ultrrastructural va ariability of the shape and size of follicu ular cellls in postnattal thyroids of o all animals s were notice ed, eve en amongst cells c of same follicle. Also, variable during devvelopment, we ere colloid drroplets (resorption vacuole es), lyso osomes and microvilli. This T sugges sts that thyro oid function is highly y sensitive to o several facttors that inclu ude e, climate, fe eeding and environmenta al factors. The T age find ding is supp ported by th he study of fine structu ural flucctuations in ra at thyroid durring 24 h by Uchiyama U et al. (19 986) and is similar s to the e variability observed o in the t thyroid structure e and hormon nal profile in some domes stic animals (Barnes s et al., 1958 8; Fisher et al., 1977; Alwa an, 200 09). As devellopment prog gressed from pre-pubertal to
ased structura al puberrtal age in this study, therre was increa and ffunctional diffferentiation of the follicu ular cells with h appea arance of abundant cyto oplasmic org ganelles, sub bapica al vesicles, lyssosomes and d colloid droplets. Howeverr, the p presence of pseudopods a and dome-sha aped apocrine e secre etion in the ap pical cytoplassm was remarrkable in the 2 to 4 years puberrtal cattle. A An increase in the dense e secre etory granule es of the parafollicula ar cells was s encou untered in the e older puberttal cattle of 5 to 7 years. The e thyroid fo ollicles varie ed histologiccally in size e (folliccular diamete er, capsular tthickness and cell heightt) and sshape during developmentt. This presen nt finding is in n agree ement with h histological findings of the thyroids by y Blum enthal (1955 5) in Europea an cattle, Matthur (1971) in n Asiat ic water bufffalo, Roy ett al. (1975) in goat and d Sawiccki et al. (199 92) in Europe ean bison. It w was observed d variab bility in size and shape o of follicles in n young bison n which h increased ssignificantly in older anim mals and large e follicle es were present in relatively great n number in the e very o old bison. How wever, Ranja an et al. (2011) observed no change in n the fo ollicular lining epithelium during histogenesis of foeta al thyro id of buffalo. In our op pinion, these variations in n follicu ular size amo ongst develop pmental stage es could be as resultt of the inffluence of tthe several internal and d enviro onmental facctors influenccing thyroid developmen nt and ffunction. Con ncerning the existence of low and high h cuboiidal or colum mnar cells in same follicle e, the presen nt resultt showed th hat low cubo oidal cell height had few w organ nelles, indicatting low activvity and high cuboidal cells s have more cellula ar organelles and were assumed to be e
Igbokwe and Ezeasor
active; but, this opinion is contrary to that of Atoji et al. (1999), who observed no differences in the organelles content of low and high cuboidal cells in adult camel but showed the existence of low cuboidal and high cuboidal/columnar cell height in different follicles in this specie and concluded that there is no relationship between follicular cell height and function. However, the present result is supported by (Roy et al., 1978), with observation of changes in epithelial height of thyroids of different age groups in goat kids and buffalo calves (Marthur, 1971). In addition Young (1966), Klinck et al. (1970), Fujita (1975) and Uchiyama et al. (1986) noted in dogs, pigs, human and rats that tall follicular cells are highly active, whereas flat (squamous) follicular cells were hypoactive. The dissimilar observation in camel in relation to organelle content and functionality unlike in the present study may be due to the harsh environment exposed to camels unlike the cattle used in this study. The thyroid glands in the camel may have to remain very active with several organelles abundant in squamous cuboidal and columnar follicular cells to carry out its functions in the harsh, dry arid environment. It must be pointed that our study showed that follicular cells of varied height sometimes existed amongst follicles of within the same thyroid lobe in all the species. This common observations showed that the functional activity of each follicular cells were not always the same in each thyroid lobe during development, but when follicles with columnar cells or cuboidal predominate in the thyroid lobe, it is said to be active. However, low (squamous) or flat cells in several follicles seen in older pubertal cattle suggest low thyroid activity. The hyperplastic follicular epithelium seen in older pubertal cattle of 5 to 7 years is part of the degenerative changes seen in old thyroids and can give rise to some form of neoplasia of thyroid, as seen in ageing human thyroids (Kondo et al., 2006). The follicular colloid showed a PAS-positive reaction in all of postnatal cattle with varying intensity. This histological finding indicated evidence of functional activity in the synthesis of thyroglobulin into follicular lumen to form colloid in the postnatal period. The presence of colloid was even reported in prenatal and postnatal cattle by Schafie and Mashaly (1974) and adult buffaloes (Roy and Yadava, 1975; Ranjan et al., 2011) with varying intensity as well. PAS-positive colloid was detected in human foetal thyroid at 13th to 14th week of gestation (Gaikwad et al., 2012). It therefore shows that the appearance of colloid and development of follicles in cattle occurs in the prenatal period. This opinion is supported by reports of Mitskavitch (1957) and Badawy et al. (1978). There were significant variations (p
