A) When does the Graafian Follicle develop and what hormones and products does it release?
B) What is the role of the Graafian Follicle cells in the second half of the cycle (yes they have a different name) and what do they produce and to what effect?
a. The Graafian follicle is mature ovarian follicle that has a size of around 2.5 cm in diameter and contains the mature egg. It is made up of theca cells that enclose the granulosa cells. The granulosa cells have a central lumen that contains the ovum. Graafian follicle is formed from secondary follicle after first meiotic division is completed by the ovum and the second meiotic division is formed but ovulation is not occurred. Graafian follicle development is under the influence of follicle stimulating hormone secreted by the pituitary. It occurs in the follicular phase of the ovarian cycle. The Graafian follicle contains the secondary oocyte which is released upon ovulation.
The Graafian follicle contains granulosa cells that secrete estrogen in response to FSH. Estrogen induces mitosis in endometrial cells and causes their proliferation. Estrogen levels also trigger the mid-cycle rise in luteinizing hormone (LH), which then triggers the rupture of the follicle during ovulation releasing the oocyte. Inhibin B and activin is also secreted by the Graafian follicle which inhibits FSH secretion by the pituitary. This inhibits development of neighboring antral follicles.
The theca interna can secrete androgens in response to LH in vivo. The follicular fluid that surrounds the developing oocyte contains follistatin, a glycoprotein that is secreted by granulosa cells. Theca cells will also secrete progesterone after ovulation. Hyaluronic and lytic enzymes are produced by the granulosa cells that are required for rupture of the follicle during ovulation.
b. The Graafian follicle is converted to the corpus luteum after ovulation. It contains the outermost thecal layer being converted to luteal cells upon ovulation. The luteal cells are yellowish in color due to pigmentation. The lutein cells or luteal cells produce progesterone, which is required for differentiation of the endometrial cells for embryo implantation. The residual granulosa cells also secrete some estrogen, which is involved in the selection of the dominant follicle.
The progesterone has several functions as its production is required for luteal cell survival. It prepares the luminal epithelium and stroma for embryo attachment and implantation. It inhibits uterine contraction that expels the egg. It is involved in decidualization of endometrial stromal cells that provide nutrition to implanted embryo. It takes over the function of the placental hCG till the placenta is formed. In livestock, it is required for regulate the timing of ovulation. It also targets the mammary gland and stimulates its growth. Progesterone also prevents lactation.
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