Developement, function and regression of the corpus luteum in the marsupial Trichosurus vulpecula

Abstract

Following ovulation, which usually takes place on day 1 of the 26-day oestrous cycle in T. culpecula, a single corpus luteum begins to develop. During the period before the uterine luteal phase begins on day 8 of the cycle, the granulosa lutein cells are active in the synthesis of what appears to be a cholesterol-like substance at a time when the ovarian secretion rates of progesterone are re!atively low. The uterine luteal phase extends from day 8 to day 15 of the cycle. During this phase the ovarian secretion rates of progesterone are relatively high, and reach maximal levels on days 12-1 3. This high secretory activity is manifested in the granulosa lutein cells by an apparent increase in the number of mitochondria, thought to be involved with the conversion of intracellularly stored cholesterol to pregnenolone, and the development of whorl-like configurations of agranular reticulum which are reported to be concerned with the conversion of pregnenolone to progesterone. A drop in progesterone concentration commences in pregnant and non-pregnant females on day 17 of the cycle, when early involution of the corpus luteunl is apparent. By day 20, the corpus luteum regresses to an early corpus albicans, and the ovarian secretion rate of progesterone falls progressively to levels similar to those in anoestrous females.