Oogenesis

Hormone Control in Oogenesis

Early hormone control helps the follicle to develop and forces oogenesis to occur in a cycle in a certain time period. The control begins in the hypothalamus which produces gonadotropin-releasing hormone (GnRH). GnRH is received by receptors in the anterior pituitary gland, which responds by releasing Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH). At the beginning of development, the granulosa cells express FSH receptors, which stimulate growth of the follicle. Theca cells express receptors for LH, which stimulates growth of the corpus luteum. Theca cells also produce androgens, which the granulosa cells convert to estrogen. Estrogen act back on the anterior pituitary gland to further FSH and LH surges, and also supports the growth of the endometrium. At some point, the dominate follicle begins to secrete inhibin, which acts back on the anterior pituitary gland to stop producing FSH. Only the dominant follicle, which is now FSH independent, will continue to grow.

During further development, the granulosa cells increase their FSH receptors and express LH receptors, while the Theca cells increase their LH receptors. This surge in hormone reception results in ovulation. After ovulation, if fertilization occurs, the corpus luteum secretes progesterone that supports the further growth of the endometrium. If, however, fertilization does not take place, then the hormone levels drop, the corpus luteum breaks down, no longer secreting progesterone, so that the endometrium sloughs off producing menstruation. (38)

Oogenesis
(36) Process of Oogenesis	Gametogenesis is the general process of gamete formation in both males and females. Oogenesis is the process of meiosis in female organisms from an oogonium to a primary oocyte, to a secondary oocyte, and then to an ovum. Spermatogenesis is a similar process of forming sperm by meiosis in males. Although both are important processes, we will focus on mammalian oogenesis in this project, because it directly relates to the processes of germ-line gene therapy.
	Process of Oogenesis Oogenesis begins soon after fertilization, as primordial germ cells travel from the yolk sac to the gonads, where they begin to proliferate mitotically. The germ cells multiply from only a few thousand to almost 7 million. They become oocytes once they enter the stages of meiosis several months after birth. Now called primordial follicles, they are made up of oogenic cells from the primordial germ cells surrounded by follicle cells from the somatic line. The oocyte is then arrested in the first meiotic prophase until puberty. At puberty, between 4 to 10 follicles begin to develop, although only 1-2 are actually released. Surrounding each oocyte is a zona pellucida, membrana granulosa, and theca cell layer. Each oocyte finishes its first meiotic division, creating a secondary oocyte and polar body, which serves no further function. It begins the next meiosis cycle and is arrested in its second metaphase, at which point it is released from the ovary in ovulation. It will not finish the meiosis cycle until it encounters the stimuli of a sperm. (38)

Hormone Control in Oogenesis Early hormone control helps the follicle to develop and forces oogenesis to occur in a cycle in a certain time period. The control begins in the hypothalamus which produces gonadotropin-releasing hormone (GnRH). GnRH is received by receptors in the anterior pituitary gland, which responds by releasing Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH). At the beginning of development, the granulosa cells express FSH receptors, which stimulate growth of the follicle. Theca cells express receptors for LH, which stimulates growth of the corpus luteum. Theca cells also produce androgens, which the granulosa cells convert to estrogen. Estrogen act back on the anterior pituitary gland to further FSH and LH surges, and also supports the growth of the endometrium. At some point, the dominate follicle begins to secrete inhibin, which acts back on the anterior pituitary gland to stop producing FSH. Only the dominant follicle, which is now FSH independent, will continue to grow. During further development, the granulosa cells increase their FSH receptors and express LH receptors, while the Theca cells increase their LH receptors. This surge in hormone reception results in ovulation. After ovulation, if fertilization occurs, the corpus luteum secretes progesterone that supports the further growth of the endometrium. If, however, fertilization does not take place, then the hormone levels drop, the corpus luteum breaks down, no longer secreting progesterone, so that the endometrium sloughs off producing menstruation. (38)
		(37)
		Hormone Levels During Oogenesis

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