ABSTRACT
The pituitary gland has two embryonic origins. The neurohypophysis differentiates from the neural ectoderm of the infundibular floor of the third ventricle of the brain; the adenohypophysis forms from Rathke’s pouch, an oral ectodermal invagination from the stomodeum. The pars intermedia (PI) varies considerably in mammals; it may exist as only an epithelium of only two or three cells lining the hypophysial cleft or even be completely absent as in cetaceans. The term “fish” covers several groups of vertebrates; within this group is the lamprey, a cyclostome, whose PI is posterior to the pars distalis (PD) and fused with the overlying neural lobe. With the anuran adenohypophysis, the PD is flattened, elongated, and dorsally continuous with the PI and there is no hypophysial cleft between the two areas. When describing the vascularity of the PI parenchyma in relation to other areas of the pituitary, the phrase “relatively avascular” is commonly used.
