Development of the endocrine system.

Development of the hypophysis (pituitary gland.)

The hypophysis arises from two separate sources. One portion arises as an evagination of the ectoderm of the primitive buccal cavity which extends as Rathke’s pouch toward the floor of the diencephalon. The pouch comes in close relationship with a downgrowth, the infundibulum, from the floor of the diencephalon. The pouch comes in close relationship with a downgrowth, the infundibulum, from the floor of the diencephalon. The infundibulum gives rise to the neural stalk and the pars nervosa. Rathke’s pouch loses its attachment to the pharyngeal roof by rupture of its original stalk and develops into the adenohypophysis. The cells of its anterior wall proliferate actively and reduce the lumen of the pouch to a narrow cleft. This proliferation forms the pars distalis and from its upper part cells extend around the neural stalk to form the pars tuberalis. The posterior wall of the original pouch, lying between the residual cleft and the pars nervosa, remains thin and becomes the pars intermedia.

Development of the thyroid gland.

The thyroid gland develops as a median downgrowth of the base of the tongue. The foramen cecum of the adult tongue is a vestige that indicates the site of origin. The thyroglossal duct, which connects the developing gland with the foramen cecum, usually becomes obliterated. Remnants of the duct may give rise to cysts or to the pyramidal lobe, a cranial extension of the isthmus. The thyroid gland may include also derivatives of the branchial pouches, such as the ultimobranchial body.

The primordium of the gland initially is a compact mass of epithelial cells which later splits to form a network of solid cords or plates. These break up into small cellular groups, in each of which a lumen develops. As the lumen enlarges, the cells surrounding it become arranged into a single layer to form the definitive follicle.

Development of the parathyroid gland.

The parathyroid glands develop from the endoderm of the pharyngeal pouches, the superior parathyroids from the third pouch. In their development, the inferior parathyroids are closely associated with the developing thymus and are drawn down with it during its caudal migration. Normally, they migrate only as far as the lower poles of the thyroid gland.

Не знайшли потрібну інформацію? Скористайтесь пошуком google:

Генерація сторінки за: 0.003 сек.