An updated investigation on the antenatal development of the thyroid gland in white New Zealand rabbit with morphometric analysis

Abstract

The prenatal development of the thyroid gland was studied on 30 rabbit embryos and fetuses of both sexes. Their ages ranged between 9^th days old till the day of birth. The thyroid anlage appeared as an endodermal thickening in the floor of the primitive pharynx at 9^th days of gestation. At 11^th days old embryos, the thyroid bud was connected to the pharyngeal endoderm by the thyroglossal duct which began to disintegrate at 12^th days of gestation. The bilobation of primitive thyroid gland was exhibited with the beginning of isthmus organization at 16^th days of gestation. The first evidence of follicular organization appeared at 20^th days of gestation with many small follicles disseminated within the gland. The first morphological sign of functional differentiation of the thyroid gland appeared at 22^nd days of gestation as PAS positive thick rim of pre colloid material in apical parts of the follicular cells in some follicles. Very few parafollicular cells began to scatter among follicular cells at 22^nd days old fetuses. The definitive thyroid follicles storing vacuolated typical colloid organized at 28^th days of gestation and stained somewhat strongly with Eosin and PAS.