The distribution and origin of interstitial tissue has been studied in various mammalian ovaries with histochemcal techniques for lipids. The interstitial tissue contains sudanophilic droplets of different sizes consisting of phospholipids, triglycerides and cholesterol and its esters; the lipids are comparatively unsaturated.
The granulosa of normal follicles contains heterogeneous lipid bodies of various sizes composed of unsaturated phospholipids. With the start of atresia these lipid bodies also develop neutral fats (triglycerides). Besides the heterogeneous lipid bodies, another type of sudanophilic lipid droplets of atresia, consisting of triglycerides, cholesterol and its ester and little phospholopids, are formed in the granulosa cells. When the number of these lipid droplets is sufficiently increased, the heterogeneous lipid bodies disappear from the atretic follicles. The theca of such atretic follicles hypertrophies and persists to form the interstitial tissue whereas the granulosa cells, along with the lipid droplets, regress and disappear.
In the rat ovary, the interstitial tissue occurs as scattered patches of various sizes that are derived from the theca of atretic preantral and antral follicles. Most of the lipid droplets of the interstitial tissue are mobilized from the preovulatory rat ovary. All of the bat ovary is occupied by interstitial tissue except for the cortex; the tissue is formed in the same way as in the rat. In the cat ovary, the interstitial tissue is sparsely scattered between the follicles and originates from the theca and adjacent stromal cells of normal and atretic follicles. In the dog ovary the interstitial tissue has a similar origin and also arises from invaginations of germinal epithelium. In the opossum ovary the interstitial tissue is present in the form of sparsely scattered patches of cells which arise from the theca of large preantral follicles. A few lipid bodies, consisting of unsaturated phospholipids, appear in the interstitial tissue.