Endocrinology of Reproduction Lecture 8 part 3
Cards
list the cells of the ovary.
Oogonia (germ cell), Theca cells (interstitial cells), and Granulosa cells (follicular cells; surround and nurse developing oocytes)
What is the oogonia?
The oogonia is the germ cells of the ovary. It is the counterpart of the spermatogonia in the testis. oogonia exist only during fetal life and turn in to a fixed number of primary oocytes by birth.
What do theca cells do?
Theca cells are interstitial cells. The Steroidogenic counterpart of Leydig cells in the testis. They are stimulated by LH to produce androstenedione.
What are granulosa cells? What do they do?
Granulosa cells are follicular cells; they surround and nurse developing oocytes. They are the counterparts of the Sertoli cells. Granulosa cells are stimulated by FSH to produce abundant aromatase and thus can convert theca cell androstenedione to estrogen.
List all the phases in the menstrual cycle in order.
Early follicular, Mid-follicular, Late follicular, Ovulation, Early luteal, Mid-luteal, Late luteal.
What happens during the Early follicular phase?
The 1st phase of 7. LH and FSH levels rise due the the fall of their inhibitors of progesterone, estrogen, and inhibin A . The uterine lining degenerates due to lack of progesterone and causes menstruation. Stimulated by rising FSH, a new group of follicles begins to develop. Proliferating granulosa cells now secrete inhibin B.
What happens during the Mid-follicular phase?
Rising inhibin B begins to suppress FSH release. As FSH decreases, one follicle become dominant while others undergo atresia. Estrogen levels increase as a result of LH-stimulated theca cell production of androgens and subsequent granulosa cell conversion of androgens to estrogen by FSH stimulated aromatase. Estrogen stimulates further growth of the granulosa and theca cells by autocrine/paracrine actions.
What happens during the late follicular phase?
Estrogen induced growth of the follicle results in rapidly rising estrogen levels, which stimulate further follicular growth. Rising estrogen renders the anterior pituitary increasingly more sensitive to GnRH by inducing more GnRH receptors on the gonadotropes, which is a rare positive feedback mechanism seen in the body. Due to the stronger GnRH secretion, LH release increases dramatically. Estrogen (+FSH) induces LH receptors on the granulosa cells that encompass the oocyte.
What happens during the ovulation phase?
This is the fourth of the seven phases. The rapid increase in LH triggers ovulation through action of the now LH-sensitive granulosa cells, which release lytic enzymes and prostaglandins to expel the oocyte. There is a passive release (co-secretion) of FSH and a subsequent inhibin B response. The LH surge also induces completion of the oocyte's first meiotic division, preparing it for fertilization.
What happens during the early luteal phase?
This is the 5th of 7 phases. The LH surge and the loss of oocytes singaling to the follicle result in a metabolic shift where the remaining luteal cells produce less estrogen and inhibin B but increasing amounts of progesterone and inhibin A. (These are the theca and granulosa cells in the corpus luteum/on the ovary)
What happens during the mid-luteal phase?
This is the 6th of 7 phases. As the corpus luteum grows under LH stimulation, (progesterone, estrogen, and inhibin A) rise maximally. The resulting high progesterone and estrogen inhibit both GnRH and LH release while inhibin A inhibits FSH release. If implantation occurs, the trophoblasts (outer layer) of the embryo produce hCG (=LH), which maintains the corpus luteum through the first two months of pregnancy. If not, we move on to the late luteal phase.
What happens during the late luteal phase?
This is the 7th of 7 phases. Afterwards, the cycle repeats where we go back to the early follicular phase. In this phase, the corpus luteum, lacking LH degenerates. Consequently, progesterone, estrogen, and inhibin A levels fall sharply releasing both the hypothalamus and the anterior pituitary from inhibition.
