Age-related Microscopic Changes in the Thyroid Gland of West African Dwarf Goat During Foetal and Post-natal Periods of Development
The histometric and histologic standard techniques were used to provide comprehensive information on the morphological changes in the thyroid gland of West African Dwarf goat during the foetal and postnatal development stages. The foetal age was determined using crown rump length and the pre-pubertal and pubertal age of animals was determined by dentition. The early foetal thyroid, surrounded by a thin capsule of dense irregular connective tissue, was continuous with well vascularised loose connective tissue septa. The parenchyma was composed of solid cell clusters of follicular epithelial cells surrounding small lumina. Follicular arrangement was indistinct in foetal thyroid by 50-70 days of gestational age and many follicles of different sizes were present by 95-125 days onwards. Significant variations in the mean follicular diameter, mean capsule thickness and mean epithelial cell height of the thyroids were observed in all stages of development. The mean large follicular diameter in the foetal, pre-pubertal and pubertal ages were 17.70 ± 0.09 Âµm, 54.41 ± 0.28 Âµm,142.77 ± 0.51 Âµm respectively. The follicular cells were of low cuboidal shape in foetuses, assumed high cuboidal or columnar form in pre-pubertal group and squamous in older pubertal age. Ultimobranchial follicles were encountered in early foetal goat thyroids, while focal areas of follicular cell hyperplasia were frequently seen in the older pubertal thyroids. The strong PAS-positive reaction increased strikingly from the 95-125 days foetal age, and by pubertal age all follicles were fully distended with colloid. Colloid vacuolation (colloid droplets) were encountered frequently by the age of 95-125 days, indicating the ability to synthesize hormones towards the last trimester of gestation. Parafollicular cells were distinguished by its pale cytoplasm and large nucleus at 75-90 days onwards. They were located basally and as clusters in the interfollicular tissue. This finding suggested possible high prenatal function for the thyroids of goats in the synthesis of thyroid hormones.
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