Ultrastructural changes in the guinea-pig placenta, with special reference to organelles associated with steroidogenesis
Burgess, S.M.; Tam, W.H.
Journal of Anatomy 126(Pt 2): 319-327
1978
ISSN/ISBN: 0021-8782 PMID: 670066 Document Number: 139146
The ultrastructure of the day 18 to term guinea pig placenta was studied. A scanty amount of agranular endoplasmic reticulum and a few mitochondria with tubular cristae, which are usually associated with steroidogenesis, were found in the spongy zone syncytiotrophoblast as early as day 18 of gestation. The number of mitochondria with tubular cristae increases after day 30. The parietal endodermal cells also possess a small amount of agranular endoplasmic reticulum before day 30, but the amount of this reticulum increases dramatically from day 30 to term. Mitochondria with tubular cristae are never seen in these cells. Information from the literature suggests that the enzymes for the synthesis of cholesterol are found in the agranular endoplasmic reticulum while the conversion of cholesterol to pregnenolone, an immediate precursor of progesterone, occurs in the mitochondria. Traces of progesterone are secreted by the placenta on day 14, but this secretory activity increases 83-fold after day 30. A functional zonation may exist in the guinea pig placenta whereby most of the cholesterol is synthesized in the parietal endoderm and the progesterone is produced in the spongy zone. Other changes in the spongy zone include the decrease in amount and the disorganization of the granular endoplasmic reticulum, and the progressive appearance of autophagic vacuoles with granular endoplasmic reticulum inclusion, after day 30. The precise nature of these changes is not clear, although they may signal the re-organization of this zone for steroidogenesis. No steroid producing organelles can be observed in the labyrinthine syncytiotrophoblast and the chorionic giant cells. These 2 placental regions apparently are not capable of steroid metabolism.