INITIAL VASCULARIZATION IN THE PIG PLACENTA .2. DEMONSTRATION OF GLAND AND AREOLA-GLAND SUBUNITS BY HISTOLOGY AND CORROSION CASTS

Tissues from 10 pregnant pigs between 91/2 and 43 days post coitum (p. c.) were prepared for histology and vascular corrosion casts to examin e the vascularisation of the gland and areola-gland subunits of the ea rly pig placenta. The endometrial vascular networks shown by casts wer e equal in both glandular and interglandular fields from days 91/2 to 14-15 p.c. This represents a cyclic stage uninfluenced by the early im planting embryo on day 13 p.c. By day 15 p.c., the first areola format ion was observed both histologically and on casts. The maternal areola r and glandular capillary network developed into a widely meshed loose type, in contrast to the interareolar type which had a dense and para llel architecture. In some cases extremely large, crooked, and highly anastomosed maternal capillaries developed between these two networks. This specialized transition is thought to form the base for the ring- like seal formation (RSF) of the areolar periphery. A firm materno-fet al anchor therefore ''seals'' the areola and its glandular secretory c ontents from its interareolar surroundings. Therefore, the glandular a nd maternal areolar vasculatures are clearly discernable from that of the fetal areolar vasculature. The former are associated with the know n materno-fetal substance transfer of the porcine areola-gland subunit . This association, however, seems to remain unspecialized in early pl acentation, as only the regular roundish areola-not their regular areo la-can be distinctly detected. (C) 1994 Wiley-Liss, Inc.