Mechanisms associated with corpus luteum development.

The transition of a preovulatory follicle into a corpus luteum is a complex process involving mechanisms similar to wound healing and tumor formation. The objective of this review is to focus on mechanisms associated with corpus luteum development with specific attention to the follicular lineage of luteal cells, mechanisms associated with luteinization, and neovascular changes during luteal development. Corpora lutea are a continuation of follicular maturation and form from granulosal and theca interna cells. There is morphological and immunological evidence in ruminant species for the differentiation of granulosal and theca interna cells into large and small steroidogenic luteal cells, respectively. Different morphological, physiological, and biochemical characteristics of large and small luteal cells may reflect different follicular lineages with separate embryological origins. Following the preovulatory gonadotropin surge, follicular cells begin morphological, endocrinological, and biochemical changes associated with luteinization. Luteinization involves the transition of a preovulatory follicle into a highly vascular corpus luteum capable of secreting large quantities of progesterone. In addition, various cell types undergo hyperplasia, hypertrophy, and(or) migration during corpus luteum formation. An essential component of corpus luteum development is the recruitment of a blood supply. The development of a new microcirculatory bed involves breakdown of the follicular basement membrane, endothelial cell migration, endothelial cell proliferation, and development of capillary lumina. This process is regulated by the interaction of angiogenic and antiangiogenic substances. Further clarification of the preceding mechanisms may result in the development of improved methodologies for controlling the time of ovulation and(or) increasing pregnancy rates.