Structural and functional reserve of steroidogenic membranes in stimulated luteal cells of pregnant rats.

A comparison was made of the ultrastructural features of the remaining corpora lutea (CL) of five unilaterally ovariectomised and five control pregnant rats. Unilateral ovariectomy (ULO) was carried out on Day 8 of gestation; this treatment has been shown to double the normal rate of progesterone secretion by the remaining ovary within 8 days. On Day 16, the ovaries from both ULO and control rats were examined stereologically. The mass of luteal tissue remaining in ULO rats was only 44% of that in the controls, but their plasma progesterone concentration was 81% and not significantly different from the control value. The CL were 10% heavier in ULO rats and the amount of luteal cell cytoplasm per CL was 21% greater. The percentage of the CL occupied by luteal cells was 15% greater in ULO rats but the interstitial space was 50% less. There was no significant change in the percentage of the luteal cell cytoplasm occupied by cytosol, agranular endoplasmic reticulum (AER), mitochondria and electron-dense granules. The surface area per unit cytoplasmic volume of AER and outer and inner mitochondrial membranes (OMM and IMM, respectively) was not affected. However, expressed per CL, both the AER and IMM membrane surface areas were increased by around 30%. These morphometric changes would only account for about 30% of the reported increase in hormone secretion by the CL of ULO rats, and they suggest that the CL of Day 16 pregnant rats may have reached their optimal structural condition.