The corpus luteum of the guinea pig. Fine structure at the time of maximum progesterone secretion and during regression

This report describes the fine structure of guinea pig luteal cells during the period of maximum progesterone secretion and throughout involution, in ovaries that have been fixed by perfusion, a technique that provides optimal preservation of steroid‐secreting tissues. Smooth endoplasmic reticulum (smooth ER), a prominent organelle in these cells, is particularly well preserved by this method of fixation. During the time of maximal progesterone secretion, luteal cells contain abundant tubules and cisternae of smooth ER, many mitochondria, a well‐developed Golgi complex and some lipid droplets. This fine structural picture is consistent with active steroidogenesis. Autophagy plays an important role in the regression of luteal cells in the corpus luteum. The onset of luteolysis is marked by the appearance of structurally complex autophagic vacuoles, one of which has not been described previously in luteal cells. This autophagic vacuole seems to originate from GERL (Golgi‐endoplasmic reticulum‐lysosomes) as a cup‐shaped structure, which subsequently increases in size and complexity. Regressing luteal cells also contain increased numbers of both dense bodies (lysosomes) and lipid droplets, and exhibit changes in nuclear and mitochondrial morphology. In contrast to previous reports in the literature, changes in the morphology of smooth ER were not observed as a characteristic feature of involution in corpora lutea of guinea pigs in the present study. Advanced regression of luteal cells is characterized by multiple fusion of lipid droplets and a decrease in the amount of smooth ER. Another mechanism active in the breakdown of the corpus luteum is the phagocytosis of luteal cells by macrophages. Although present at all stages, macrophages are most abundant in older corpora lutea, where they often surround dead or dying luteal cells.