Abstract
The thyroid is one of the largest endocrine glands that regulate metabolism, growth and development, thermogenesis as well as calcium homeostasis. Nevertheless, less information exists regarding comparative morphological development of the thyroid gland with age in male goats (Capra hircus). The objective of the present study was to establish histological, histochemical, histomorphometric and ultrastructural changes in the thyroid gland of neonatal and adult male goats. This was examined in sixteen clinically healthy male goats, including 8 neonatal (7-day-old) and 8 adult (2- to 3-year-old) goats. Thyroid were collected and prepared for histological and histochemical examination by hematoxylin and eosin, Masson's trichrome, and periodic acid–Schiff staining. Parameters of histomorphometry: Using ImageJ software, the diameter of follicle, epithelial height (the distance from the base to the apex of a single epithelial cell that borders on follicles), and area occupied by colloid and density of parafollicular (C) cells. Transmission electron microscopy was used to study ultrastructural features. The thyroid glands of neonatal goats had small- to medium-sized follicles lined mostly with high cuboidal follicular cells with moderate staining in the colloid and many prominent peripheral vacuolations indicating active colloid resorption. In contrast, ovaries of adult goats had larger follicles lined primarily with cuboidal follicular cells low in height, containing densely eosinophilic and relatively homogenous colloid and fewer resorption vacuoles. PAS staining showed strong positivity in the colloid and apical cytoplasm of follicular cells of neonate. Ultrastructurally, neonatal follicular cells contained large amounts of rough endoplasmic reticulum and Golgi apparatus that was well developed as well as secretory vesicles, microvilli and colloid droplets while adult follicular cells had fewer cytoplasmic organelles and less vesiculation. Adult goats had a greater number (P < 0.1) of mature large parfollicular (C) cells than young goats. These results collectively illustrate an age-dependent remodeling of the male goat thyroid gland from one that is structurally active and secretory during neonatal development in males to one that is mostly storage oriented and relatively stable during adulthood. These results give significant baseline information on morphological and ultrastructural data for investigation of thyroid gland development and age-related functional adaptation in domesticated goats.

Key words: Thyroid gland, Capra hircus histology, developmental morphology, calcitonin