The ovarian interstitium

The stroma of the ovary is a vascularized soft tissue that consists of interstitial cells (Wikipedia). This comprises the compartment that forms the body of the ovary. Interstitial cells pack the areas between the functional units within the ovary, oocyte-containing follicles, and corpora lutea. Interstitial cells are designated as primary or secondary. Primary interstitial cells originate from fibroblastlike stromal cells. Generally, in species with long gestational periods, they differentiate during fetal development. This is the case in human ovaries where their development is maximal at 18 weeks of gestation. Whereas, in species with short gestational periods, they differentiate during the postnatal period. They were even found to develop on different postnatal days (pnd) between different strains of rats and mice. Postnatal day 12 was the time of their appearance in C57Bl/6J mice, whereas it was pnd 10 in C57Bl/6Pa mice. They were first observed on pnd 7 in Swiss albino mice. Primary interstitial cells that have developed in the fetal guinea pig ovary, begin to regress during postnatal life. However, factors that regulate this regression are not known. Primary interstitial cells in a number of species possess cytological and ultrastructural characteristics of steroidigenic cells. These characteristics include sudanophilic lipid droplets (staining positive with Sudan black) and orangelles related to steroidogenesis.