Fetal development of the pituitary gland in the beagle.

Background: The pituitary gland of the dog is different from all animals 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 pituitary and the ontogeny of specific cell types in the pars distalis.

Methods: Adenohypophysis of the beagle, from 25 to 60 days of gestation and at 2 days of age, was studied by immunocytochemical and histological staining.

Results: At 25 days gestation, the primordium (Rathke's pouch) of the adenohypophysis began to form by an upward evagination from the epithelium of the primary oral cavity. At 38 days, the pituitary gland showed the same morphology as in adult dogs, being merely smaller. Five walls of Rathke's pouch (anterior wall [A], lateral walls [L], posterior wall [P], and upper wall [U] were found at 25 days, and by 38 days they had specialized into specific regions of the adenohypophysis through complex and unique processes. The pars intermedia was derived from the U and the dorsal area of the A. The pars tuberalis was derived from the dorsal area of the A. The pars distalis was formed by more singular processes: the peripheral areas of the pars distalis were first formed by A and P; then the ventral lumen of the extensive Rathke's lumen surrounded by these areas was filled up by proliferation of cells, although the dorsal lumen remains as Rathke's lumen after 38 days. The blood capillaries may play an important role in the development 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 middle and ventral areas of the A and the P) of the pars distalis anlage, and blood capillaries invaded the parenchymal cells from the mesencyme surrounding the anlage. At 38 days, portal vessels without capillary loops in the median eminence had appeared, and growth hormone and luteinizing hormone cells appeared in the ventral areas of A and P in the pars distalis. By 52 days, when capillary loops were seen in the portal vessels in the median eminence, these types of cells spread through the whole pars distalis.

Conclusion: These areas in the epithelium of Rathke's pouch at 25 days may differentiate into specific regions of the pars distalis during subsequent fetal life, through complex processes that are characteristics to the canine species.