Pregnant mouse corpora lutea: immunocytochemical localization of relaxin and ultrastructure

Relaxin was localized in corpora lutea of pregnant mouse ovaries by using the unlabeled antibody peroxidase-antiperoxidase technique and a highly specific rabbit antirat relaxin serum. Relaxin immunostaining was first observed in luteal cells located at the periphery of corpora lutea on Day 10 of gestation. The number of relaxin immunostained cells and the intensity of the stain gradually increased to reach a maximum between Days 16 and 18 of gestation. While a few luteal cells were specifically stained for relaxin on Day 1 postpartum, no luteal cells were stained on Day 2 postpartum. Ultrastructural studies of luteal cells from pregnant mouse ovaries revealed the presence of a distinct electron-dense, membrane-bound granule population, which was first observed on Day 12 of gestation. The granules increased in number to reach a maximum between Days 16 and 18 of gestation, and were absent by Day 2 postpartum. The appearance and disappearance of this granule population closely paralleled the relaxin immunostaining in the luteal cells. We suggest that the granules may be the subcellular sites of relaxin storage in the pregnant mouse ovary.