Ultrastructural changes of the developing blood vessels in the chick embryo adenohypophysis.

A study has been made of the ultrastructural changes undergone by the capillaries of the blood vessel network during development of the pars distalis of the adenohypophysis in the chicken embryo. The adenohypophysial sinusoids attain their full set-up during the last third of embryonic life, and the main features of the differentiation process of the capillary wall concern: (1) a progressive thinning of the endothelium commencing at the 14th i.d., accompanied by a reduction of the cytoplasmic organelles of its cells from the 16th i.d.; (2) a gradual disappearance of the largely overlapping cytoplasmic projections of adjoining endothelial cells, and a progressive increment of the pores, closed by diaphragms, in their thinnest peripheral extensions; (3) a marked narrowing, from the 14th i.d. on, of the perivascular space, which attains its definitive characteristics only at hatching, when the endothelial basement lamina forms. The significance of endothelial porous vacuoles throughout maturation of the vessel wall is discussed, and some remarks on the relationships between histogenesis and vessel development in the chicken pars distalis of the adenohypophysis are advanced.