The profiles of small and large follicles at low magnification contained colloids in follicular lumen surrounded by follicular cells that varied in shape and size as mentioned in Bakerwali goat
(Dar et al., 2018). The cuboidal follicular cells (Fig 1) contained nucleus with heterochromatin concentrated marginally on the nuclear membrane as an irregular layer and was also present as scattered clumps in the nucleus as reported earlier in West African Dwarf goat
(Igbokwe et al., 2015) and Bakerwali goat
(Dar et al., 2018).
The euchromatin was well dispersed in the nucleus with well-marked nucleoli (Fig 1). The nuclei were irregular circular or elliptical shape with some indentations as reported in Bakerwali goat
(Dar et al., 2018). It has been indicated that the shape of the nucleus in the follicular cell was influenced by the shape of the follicular cell as well as the various cytoplasmic structures in the cell. The thyroid follicular cells were squamous to cuboidal and contained a rounded nucleus as reported in White Fulani cattle
(Igbokwe and Ezeasor, 2015); however, the nucleus was round and flattened in the Bakerwali goat
(Dar et al., 2018).
The apical regions of the plasma membrane consisted of microvilli (Fig 2). The microvilli on the cuboidal follicular cells were numerous, thinner and finger-like, whereas the microvilli were sparse and short in flat follicular cells in Bakerwali goat
(Dar et al., 2018). The microvilli of thyroid follicular cells phagocytose colloid from follicular lumen so that thyroid hormones formed on the scaffold of thyroglobulin could be processed intracellularly and released into circulation
(French and Hodges, 1977). The cytoplasm of follicular cells showed mitochondria with varied shapes that were mostly localized on the apical cytoplasm abutting the colloid. The follicular cell mitochondria appeared as round, oval, rod-shaped and dumbbell-shaped (Fig 3) profiles as reported earlier in camel
(Mubarak and Sayed, 2005) and Bakerwali goat
(Dar et al., 2018).
The rough endoplasmic reticulum (Fig 4) was visible as elongated, irregular, elliptical cisterns in the cytoplasm of thyroid follicular epithelium in camel as earlier reported in White Fulani cattle
(Igbokwe and Ezeasor, 2015) and Bakerwali goat
(Dar et al., 2018). The cisternae of the rough endoplasmic reticulum were highly dilated in the thyroid glands of camel. These organelles varied in number and size in camel as earlier reported in some mammals
(Fujita, 1975) and Bakerwali goat
(Dar et al., 2018). These profiles of the rough endoplasmic reticulum were more localized in the basal and lateral aspects of the cytoplasm than in the apical cytoplasm as reported in Bakerwali goat
(Dar et al., 2018).
The Golgi complex (Fig 1) was well marked in thyroid sections of camel and consisted of flattened sacs, vacuoles and small vesicles as mentioned in Bakerwali goat
(Dar et al., 2018). The large Golgi complex and colloid droplets were commonly found in thyroid sections of camel that might be an indication of active thyroid. The presence of Golgi complexes, RER and secretory vesicles indicated the activity of follicular cells in the synthesis and secretion of thyroglobulin towards the follicular lumen as mentioned earlier in camel
(Abdel-Magied et al., 2000).
The close association of mitochondria to cisternae of RER as mentioned earlier in Bakerwali goat
(Dar et al., 2018). The small round and somewhat less dense vesicles were found in the subapical region and large colloid droplets were observed in the thyroid follicle. In the present study, small, highly electron-dense granules were primary lysosome (Fig 1) as reported earlier in West African Dwarf goat
(Igbokwe et al., 2015) and Bakerwali goat
(Dar et al., 2018).
The secretory granules (Fig 3) containing thyroglobulin were produced in the Golgi complex and moved towards the apical plasma membrane, where they release their contents by exocytosis into the follicular lumen
(Kameda et al., 1986). Electron dense granules were seen in the apical cytoplasm of thyroid follicular cells in camel as reported earlier in Bakerwali goat
(Dar et al., 2018). The fusion of colloid droplets and lysosomes had indicated the functional role of lysosomes in the release of thyroid hormones from thyroglobulin in the colloid droplets.
Few parafollicular cells or “C” cells (Fig 3) were observed in thyroid sections of camel were positioned basally between two follicular cells, close to the basement membrane but away from the follicular lumen as reported earlier in Bakerwali goat
(Dar et al., 2018). These parafollicular cells were mostly oval and round as in the thyroid gland of adult pig, cattle
(Igbokwe, 2013), West African Dwarf goats
(Igbokwe et al., 2015) and Bakerwali goat
(Dar et al., 2018). Mostly one cell was observed per thyroid follicle in camel; however, the parafollicular cells were numerous in mammals such as cat, dog, rabbit, rat
(Lupulescu and Petrovici, 1968) and one to two in Bakerwali goat
(Dar et al., 2018). The parafollicular cells were scarce in the thyroid gland of humans
(Nunez and Gershon, 1978) and deer
(Pantic, 1967). The parafollicular cells played a crucial role in calcium metabolism through calcitonin as reported in White Fulani cattle
(Igbokwe and Ezeasor, 2015). It may be concluded that the ultrastructure of the thyroid gland of the camel does not differ from that of other mammalian species.
