Which of the following is seen in the ovulatory phase:
Stimulation of continuation of reduction division of oocytes Ref: Speroff Clinical Gynecologic Endocrinology & Infeility, 7/e chapter 6; Guyton 11/e p1012 The ovarian cycle can be divided into three phases: the follicular phase, ovulation, and the intent phase. The ovarian cycle depends completely on the gonadotropic hormones FSH and LH, secreted by the anterior pituitary gland. During each cycle there is a cyclical increase and decrease of both FSH and LH, as shown in the figure which causes cyclical ovarian changes. The ovaries in a newborn child contain lakhs of primordial follicles (approx 10 to 20 lakhs at the time of bih and no new primordial follicle is formed after bih). Each primordial follicle consists of an oocyte (primary oocyte), surrounded by a single layer of granulosa cells. The primary oocyte in primordial follicle is arrested in the diplotene stage of meiotic prophase (of the first meiotic division). Brief review of Ougenesis: Each primary oocyte has a diploid set of chromosomes and must undergo 2 Meiotic divisions: Meiosis 1 and Meiosis II to give rise to an Ovum with a haploid set of chromosomes, necessary for feilization. Primary Oocyte --(Meiosis I) --) First Polar Body (Discarded afterward) + Secondary oocyte -- (Meiosis II)) Secondary Polar Body (Discarded afterward) + Ovum Follicular phase After pubey, in each cycle, when FSH and LH from the anterior pituitary begin to be secreted in significant amount. some of the primordial follicles (approx 6 to 12 in each cycle) begin to grow. In earliest stage of follicular phase the primary oocyte grows in size. The single layer of granulosa cells grow to form additional layers. These follicles are known as primary follicles. During the first few days of each monthly female cycle the concentration of both FSH and LH increase slightly to moderately, with the increase in FSH slightly greater than that of LH and preceeding it by a few days. These hormones especially FSH, cause accelerated growth of 6 to 12 primary follicles each month. Initially there is rapid proliferation of the granulosa cells giving rise to additional layers. In addition cells derived from the ovarian interstitium collect in several layers outside the granulosa cells, giving rise to a second mass of cells- theca. Theca is divided into 2 layers: theca interna which have the ability of secrete additional steroid sex hormones similar to the granulosa cells and theca externa which forms the capsule of the developing follicle. After the early proliferative phase of growth, lasting for a few days, the mass of granulosa cells secrets follicular fluid that contains a high concentration of estrogens. Accumulation of this fluid causes an antrum to appear within the mass of granulosa cells. FSH acts on the granulosa cells to cause secretion of estrogens. This estrogen in turn causes the granulosa cells to form increasing numbers of FSH receptors; this causes a positive feedback effect, making the granulosa cells more sensitive to FSH. The early growth of the primary follicle up to the antral stage is stimulated mainly by FSH alone. Then greatly accelerated growth occurs, leading to still larger follicles called vesicular follicles. This accelerated growth is caused by - positive feedback effect of the estrogens - FSH and estrogens combine to promote LH receptors on the granulosa cells, thus allowing LH stimulation to occur in addition to FSH stimulation and creating an even more rapid increase in the follicular secretion. After the early growth in the follicular phase, one of the follicle begins to outgrow other growing follicle Ida dominant follicle. The remaining 5 to 11 developing follicles undergo atresia. This ensures that only one follicle ruptures to liberate only one ovum for feilization. Ovulation Just before ovulation, ceain changes occur which are necessary for the ovulation. About 2 days before ovulation, the rate of secretion of LH by the anterior pituitary gland is markedly increased rising 6 to 10 folds and peaking about 16 hrs before ovulation (LH surge). FSH also increases about 2 to 3 folds at the same time and both FSH and LH act synergistically to cause rapid increase in the follicle size. The LH acts on the theca and the granulosa cells conveing them mainly to progesterone-secreting cells. Thus the estrogen level begins to fall about I day before ovulation, while progesterone level increases. Progesterone enhances the activity of proteolytic enzymes responsible, together with prostaglandins, for digestion and rupture of the follicular wall leading to ovulation. Luteal phase After expulsion of the ovum from the follicle, the remaining granulosa and theca interna cells change rapidly into lutein cells, under the influence of LH. Total mass of the remaining follicle is now termed corpus luteum. The granulosa cells secrete large amounts of female sex hormone (more progesterone than estrogens). The theca cells secrete mainly the androgens rather than the female sex hormones, but these androgens are conveed by the granulosa cells into the the female hormones. Estrogens in paicular and progesterone to a lesser extent, secreted by the corpus luteum have a strong feedback effect on the anterior pituitary gland to decrease secretions of both LH and FSH. In addition the tutein cells secrete another hormone Inhihin A. This hormone inhibits secretion of FSH by the anterior pituitary gland. Decreasing blood concentrations of both FSH and LH result in degeneration of the corpus luteum. Involution of corpus luteum results in sudden cessation of secretion of estrogen, progesterone, and inhibin A. This removes the inhibitory feedback on the anterior pituitary gland, allowing it to begin the secretion of FSH and LH again which initiate the growth of new group of follicles, thus beginning a new ovarian cycle. The paucity of secretion of progesterone and estrogens by the corpus luteum leads to menstruation by the uterus. In the background of the ovarian cycle now let's see the options given in the question. Continuation of reduction division (meiosis) occurs in the ovulatory phase As already discussed, the primary oocyte is arrested in the prophase stage of meiosis 1. Just before ovulation, the LH surge initiates the continuation of meiosis, giving rise to secondary oocyte and a polar body. The secondary oocyte enters meiosis II but gets arrested in metaphase approx. 3 hrs before ovulation. It is the secondary oocyte which is released at the time of ovulation. Meiosis II is completed only if the oocyte is feilized by a sperm, otherwise the cell degenerates. Inhibin A is increased in the Luteal phase There are 2 forms of inhihin: Inhibin A and Inhibin B Inhibin B is predominantly secreted by the granulosa cells in the follicular phase of the ovarian cycle. Inhibin A secretions begin to rise in the late follicular phase and reach a peak level in the midluteal phase. Inhibin-A, causes suppression of FSH levels during the luteal phase, which contribute to transition from luteal to follicular phase (of the next cycle). The role of Inhibin B is in securing the dominance of the dominant follicle and leading to atresia of the other growing follicles. FSH increases steroid synthesis in granulosa cells FSH acts on the granulosa cells to cause production of female sex hormones- both estrogens and progesterone. Although :production of these steroids are seen in all 3 phases of the ovarian cycle; it occurs mainly in the follicular phase. Activin causes FSH to act on the granulosa cells Activin is one of the peptides secreted by granulosa cells under the influence of FSH. Activin in turn augments FSH action on the granulosa cells and also increases FSH release from the pituitary. Thus it has a positive feedback effect. Role of activin is mainly seen in the follicular phase.
Stimulation of continuation of reduction division of oocytes
Inhibin A is increased
FSH increases steroid synthesis in granulosa cells
Activin causes FSH to act on granulosa cells