Phases of the Ovarian Cycle

Transcription

1 OVARIAN CYCLE An ovary contains many follicles, and each one contains an immature egg called an oocyte. A female is born with as many as 2 million follicles, but the number is reduced to 300,000 to 400,000 by the time of puberty. Only a small number of follicles (about 400) ever mature because a female usually produces only one egg per month during her reproductive years. As the follicle matures during the ovarian cycle, it changes from a primary to a secondary to a vesicular (Graafi an) follicle (Fig. 16.8). Epithelial cells of a primary follicle surround a primary oocyte. Pools of follicular fl uid bathe the oocyte in a secondary follicle. In a vesicular follicle, the fl uid-fi lled cavity increases to the point that the follicle wall balloons out on the surface of the ovary. When appropriate, the vesicular follicle bursts, releasing the oocyte (often called an egg) surrounded by a clear membrane. This process is referred to as ovulation. Once a vesicular follicle has lost the oocyte, it develops into a corpus luteum, a glandlike structure. If the egg is not fertilized, the corpus luteum disintegrates. As mentioned previously, the ovaries produce eggs and also the female sex hormones estrogen and progesterone. A primary follicle produces estrogen, and a secondary follicle produces estrogen and some progesterone. The corpus luteum produces progesterone and some estrogen. Phases of the Ovarian Cycle

2 follicular phase, FSH promotes the development of follicles that primarily secrete estrogen. As the estrogen level in the blood rises, it exerts negative feedback control over the anterior pituitary secretion of FSH. The follicular phase now comes to an end. The estrogen spike at the end of the follicular phase has a positive feedback effect on the hypothalamus and pituitary gland. As a result, GnRH from the hypothalamus increases. There is a corresponding surge of LH released from the anterior pituitary. The LH surge triggers ovulation at about day 14 of a 28-day cycle. luteal phase :LH promotes the development of the corpus luteum. The corpus luteum secretes high levels of progesterone and some estrogen. When pregnancy does not occur, the corpus luteum regresses and a new cycle begins with menstruation

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4 Uterine Cycle During days 1 5, a low level of estrogen and progesterone in the body causes the endometrium to disintegrate and its blood vessels to rupture. On day 1 of the cycle, a fl ow of blood and tissues, known as the menses, passes out of the vagina during menstruation, also called the.menstrual period During days 6 13, increased production of estrogen by a new ovarian follicle in the ovary causes the endometrium to thicken and become vascular and

5 glandular. This is called the proliferative phase of the.uterine cycle.on day 14 of a 28-day cycle, ovulation usually occurs During days 15 28, increased production of progesterone by the corpus luteum in the ovary causes the endometrium of the uterus to double or triple in thickness (from 1 mmto 2 3 mm). The uterine glands mature and produce a thick mucoid secretion in response to increased progesterone. This is called the secretory phase of the uterine cycle. The endometrium is now prepared to receive the developing embryo. If this does not occur, the corpus luteum in the ovary regresses. The low level of progesterone in the female body results in the endometrium breaking down during menstruation

6 Steps of fertilization

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8 Infertility Infertility is the failure of a couple to achieve pregnancy after one year of regular, unprotected intercourse Causes of Infertility 1-The most frequent cause of infertility in males is low sperm count and/or a large proportion of abnormal sperm, which can be due to environmental infl uences 2-Body weight appears to be the most signifi cant factor in causing female infertility. 3-Other causes of infertility in females are blocked oviducts due to pelvic infl ammatory disease and endometriosis. Assisted Reproductive Technologies

9 Assisted reproductive technologies (ART) consist of techniques used to increase the chances of pregnancy. Often, sperm and/or eggs are retrieved from the testes and ovaries, and fertilization takes place in a clinical or laboratory setting. 1-intrauterine insemination (IUI). In IUI, fertility drugs are given to stimulate the ovaries. Then the donor s sperm are placed in the uterus If the prospective parents wish, sperm can be sorted into those believed to be X- bearing or Y-bearing to increase the chances of having a child of the desired sex. Fertilization of an egg with an X-bearing sperm results in a female child. Fertilization by a Y-bearing sperm yields a male child 2- In Vitro Fertilization (IVF) During IVF, conception occurs in laboratory glassware. After about two to four days, the embryos are ready to be transferred to the uterus of the woman, who is now in the secretory phase of her uterine cycle. If desired, the embryos can be tested for a genetic disease, and only those found to be free of disease will be used. 3-Gamete Intrafallopian Transfer (GIFT) The term gamete refers to a sex cell, either a sperm or an egg. Gamete intrafallopian transfer (GIFT) was devised to overcome the low success rate (15 20%) of in vitro fertilization. The method is exactly the same as in vitro fertilization, except the eggs and the sperm are placed in the oviducts immediately after they have been brought together Processes of Development As a human being develops, these processes occur: 1-Cleavage. Immediately after fertilization, the zygote begins to divide so that there are fi rst 2; then 4, 8, 16,

10 and 32 cells; and so forth. Increase in size does not accompany these divisions (Fig. 17.2). Cell division during cleavage is mitotic, and each cell receives a full complement of chromosomes and genes. 2-Growth. During embryonic development, cell division is accompanied by an increase in size of the daughter cells. 3Morphogenesis. Morphogenesis refers to the shaping of the embryo and is fi rst evident when certain cells are seen to move, or migrate, in relation to other cells. By these movements, the embryo begins to assume various shapes. 4-Differentiation. When cells take on a specifi c structure and function, differentiation occurs. The fi rst system to become visibly differentiated is the nervous system.