Investigation of ultrastructure of the adenohypophysis in the domestic pig (Sus scrofa domestica). Part III. Capillary vessels of the pars anterior.

The investigations were carried out on 34 hypophyses of pig embryo aged c. 46--79 days of fetal life. Capillary vessels of the glandular part have the three-layer structure, and this structure is in general similar to the one of the capillaries of adult vertebrates. In the investigated stages of the fetal life the structure undergoes certain modifications; pores with septa (so called fenestra) occur in the cytoplasms of the endothelial cells, microvilli protruding to the lumen of the vessels disappear, continuous basement membrane is developed and the number of reticular fibres in the outer layer increases. Because of the continuity of the endothelial cells and the occurrence of the fenestrae--we may classify these vessels in the group of capillaries of the visceral type. It seems that the observed endothelial cells can mediate in the transport of fluids and some substances between the gland cells and blood, and also can take part in the transport of fragments of the degenerated cells of the pars anterior of the hypophysis to the lumen of blood vessels.