FETAL DEVELOPMENT OF THE PITUITARY-GLAND IN THE BEAGLE

Background: The pituitary gland of the dog is different from all anima ls and is described as ''typical'' for mammals. How might this complex pituitary gland of the dog be formed in fetal life? The current study examined the fetal development of the complex and unique dog pituitar y and the ontogeny of specific cell types in the pars distalis. Method s: Adenohypophysis of the beagle, from 25 to 60 days of gestation and at 2 days of age, was studied by immunocytochemical and histological s taining. Results: At 25 days gestation, the primordium (Rathke's pouch ) of the adenohypophysis began to form by an upward evagination from t he epithelium of the primary oral cavity. At 38 days, the pituitary gl and showed the same morphology as in adult dogs, being merely smaller. Five walls of Rathke's pouch (anterior wall [A], lateral walls [L], p osterior wall [P], and upper wall [U]) were found at 25 days, and by 3 8 days they had specialized into specific regions of the adenohypophys is through complex and unique processes. The pars intermedia was deriv ed from the U and the dorsal area of the A. The pars tuberalis was der ived from the dorsal area of the A. The pars distalis was formed by mo re singular processes: the peripheral areas of the pars distalis were first formed by A and P; then the ventral lumen of the extensive Rathk e's lumen surrounded by these areas was filled up by proliferation of cells, although the dorsal lumen remains as Rathke's lumen after 38 da ys. The blood capillaries may play an important role in the developmen t of parencymal cells in the Rathke's pouch during canine fetal life. At 30 days gestation, the first adrenocorticotropic hormone cells were found in the anterior- and posterior-ventral regions (derived from mi ddle and ventral areas of the A and the P) of the pars distalis anlage , and blood capillaries invaded the parenchymal cells from the mesency me surrounding the anlage. At 38 days, portal vessels without capillar y loops in the median eminence had appeared, and growth hormone and lu teinizing hormone cells appeared in the ventral areas of A and P in th e pars distalis. By 52 days, when capillary loops were seen in the por tal vessels in the median eminence, these types of cells spread throug h the whole pars distalis. Conclusion: These areas in the epithelium o f Rathke's pouch at 25 days may differentiate into specific regions of the pars distalis during subsequent fetal life, through complex proce sses that are characteristic to the canine species. (C) 1998 Wiley-Lis s, Inc.