The histogenesis of the endocrine organs of the viviparous lizard, Xantusia vigilis

Abstract

The embryology and histogenesis of the pituitary, thyroid, parathyroid, thymus and ultimobranchial glands, the pancreatic islets, adrenal glands, and the gonads were studied in the viviparous lizard, Xantusia vigilis. The epithelial portion of the pituitary gland arises as a single diverticulum (Rathke's pouch) which subdivides into oral and aboral lobes. The aboral lobe develops into the pars intermedia and a very small portion of the pars distalis, and the oral lobe forms the greater part of the anterior pituitary. Basophil cells differentiate in the pars distalis at the 10-mm stage (about halfway through development), and acidophil cells at the 21 mm (about 2 weeks before birth). The follicles of the thyroid contain thin colloid at the 7–8-mm stage (about 1 week prior to the differentiation of pituitary basophil cells). The follicular cells appear most active during the last third of gestation. The parathyroid arises from the entoderm of the third visceral pouch and develops in the concavity of the bifurcating carotid artery. Fine acidophilic granules appear in the cytoplasm at the 8-mm stage. The thymus develops from the second and third pouches, and the constituent cells appear thymocytelike by the 7-mm stage. The ultimobranchial body arises unilaterally from a left diverticulum posterior to the fourth arch and develops into a large mass of epithelioid cells containing abundant acidophilic granules. The pancreas arises from a single dorsal and two ventral diverticula of the foregut, the dorsal forming the splenic portion and the left ventral most of the head. Cells with amphophilic granules (“muddy” cells) appear by the 3–4-mm stage among the cells of the cords derived from the dorsal rudiment. Alpha and beta cells appear by the 7–8-mm stage. Both coelomic epithelium and underlying mesenchymal cells proliferate to form the gonadal ridge in the 2–3 mm embryo. Primitive germ cells at first are scattered throughout the prospective gonadal ridge and adjacent tissues; later they become incorporated within the stroma of the enlarging gonad. A single medullary tubule is formed in the developing gonad of each sex at about the 5–6-mm stage. Sexual differentiation occurs during the 6.5–7.0-mm stage. The ovary only briefly retains rudimentary medullary tubule material, but the testis retains a small patch of cortical gonocytes until birth. Typical interstitial cells are not observed in either gonad. The Sertoli cell cytoplasm in the testis appears secretory and may be the source of fetal androgen. Interrenal tissue is derived from the region of the intermediate mesoderm, and forms a large mass of cells containing osmiophilic droplets by the 3-mm stage. Medullary elements form a dorsal cap over the interrenal cells and contain fine osmiophilic granules by the 8-mm stage.