THE DEVELOPMENT AND FATE OF THE CORPUS LUTEUM IN THE VERTEBRATE SERIES

Summary 1. Published descriptions of the development of the corpus luteum show that the majority of the luteal cells are derived from the granulosa cells. 2. In the vertebrates, other than mammals, the development of a corpus luteum does not appear to be associated solely with viviparity. 3. The fate of the theca interna cells varies greatly, even in closely related species. 4. The theca interna cells may remain in groups at the periphery of the gland, or they may invade the developing corpus luteum and either: (a) disappear entirely a few days after ovulation, (b) become indistinguishable from the granulosa cells, or (c) possibly revert to fibroblasts. 5. The function of the theca interna or theca-luteal cells in the mature corpus luteum is unknown. It is, however, possible that they secrete oestrogen. 6. The blood vessels of the corpus luteum are derived from the theca interna plexus of capillaries; the corpus luteum is well vascularized by the time that implantation occurs. 7. The connective tissue of the corpus luteum is believed by many authors to be derived from theca interna cells that have reverted to fibroblasts. In the Artiodactyla and in Macaca, and possibly in other animals, there is evidence that the endothelial cells give rise to the reticular framework. 8. The corpus luteum is generally fully developed by the time of implantation of the blastocyst. 9. The appearance of the vacuoles in the luteal cells of pregnant mammals is not necessarily indicative of impending cessation of function or of degeneration. Such vacuolation may mark a ‘resting period’ as in those animals in which implantation is delayed, or a ‘transition period’ during which the placenta is becoming established structurally and functionally. 10. The time of appearance of retrogressive changes in the corpus luteum of pregnancy in any species is probably related to the degree of activity of the placenta in that species and to the prolactin level in the anterior pituitary. 11. Accessory, aberrant and other atypical forms of corpora lutea have been reported in the macaque, the tree porcupine and the mare.