The Oogenesis Process : Diagram showing maturation of the ovum. The menstrual cycle is the physiological process that fertile women undergo for the purposes of reproduction and fertilization. The menstrual cycle is the scientific term for the physiological changes that occur in fertile women for the purpose of sexual reproduction.
The menstrual cycle is controlled by the endocrine system and commonly divided into three phases: the follicular phase, ovulation, and the luteal phase. However, some sources define these phases as menstruation, proliferative phase, and secretory phase. Menstrual cycles are counted from the first day of menstrual bleeding. The follicular phase or proliferative phase is the phase of the menstrual cycle in humans and great apes during which follicles in the ovary mature, ending with ovulation.
The main hormone controlling this stage is estradiol. During the follicular phase, follicle-stimulating hormone FSH is secreted by the anterior pituitary gland. FSH levels begin to rise in the last few days of the previous menstrual cycle and peak during the first week of the follicular phase. The rise in FSH levels recruits five to seven tertiary-stage ovarian follicles also known as Graafian or antral follicles for entry into the menstrual cycle. These follicles compete with each other for dominance.
FSH induces the proliferation of granulosa cells in the developing follicles and the expression of luteinizing hormone LH receptors on these granulosa cells. Two or three days before LH levels begin to increase, usually by day seven of the cycle, one or occasionally two of the recruited follicles emerges as dominant. Many endocrinologists believe that estrogen secretion of the dominant follicle increases to a level that indirectly lowers the levels of LH and FSH.
This slowdown in LH and FSH production leads to the atresia death of most of the recruited follicles, though the dominant follicle continues to mature. These high estrogen levels initiate the formation of a new layer of endometrium in the uterus. Crypts in the cervix are also stimulated to produce fertile cervical mucus that reduces the acidity of the vagina, creating a more hospitable environment for sperm.
In addition, basal body temperature may lower slightly under the influence of high estrogen levels. Ovulation is the phase in which a mature ovarian follicle ruptures and discharges an ovum also known as an oocyte, female gamete, or egg.
Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle. The time immediately surrounding ovulation is referred to as the ovulatory phase or the periovulatory period. The luteal phase or secretory phase is the latter part of the menstrual or estrous cycle. It begins with the formation of the corpus luteum and ends in either pregnancy or luteolysis.
The main hormone associated with this stage is progesterone, which is significantly higher during the luteal phase than in other phases of the cycle. Some sources define the end of the luteal phase as a distinct ischemic phase.
After ovulation, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum. It continues to grow for some time after ovulation and produces significant amounts of hormones, particularly progesterone, and to a lesser extent, estrogen. Progesterone plays a vital role in making the endometrium receptive to implantation of the blastocyst and supportive of the early pregnancy.
The hormones produced by the corpus luteum suppress production of the FSH and LH, causing the corpus luteum will atrophy. The death of the corpus luteum results in falling levels of progesterone and estrogen. This in turn causes increased levels of FSH, leading to recruitment of follicles for the next cycle. Continued drops in estrogen and progesterone levels trigger the end of the luteal phase, menstruation, and the beginning of the next cycle. The loss of the corpus luteum can be prevented by implantation of an embryo.
After implantation, human embryos produce human chorionic gonadotropin hCG , which is structurally similar to LH and can preserve the corpus luteum. Because the hormone is unique to the embryo, most pregnancy tests look for the presence of hCG. If implantation occurs, the corpus luteum will continue to produce progesterone and maintain high basal body temperatures for eight to twelve weeks, after which the placenta takes over this function. The ovarian cycle : The ovarian cycle is the series of changes that occur in the ovary during the menstrual cycle that cause maturation of a follicle, ovulation, and development of the corpus luteum.
The uterine cycle describes a series of changes that occur to the lining of the uterus, or endometrium, during a typical menstrual cycle. Several changes to the uterine lining endometrium occur during the menstrual cycle, also called the uterine cycle. The endometrium is the innermost glandular layer of the uterus. During the menstrual cycle, the endometrium grows to a thick, blood vessel-rich tissue lining, representing an optimal environment for the implantation of a blastocyst upon its arrival in the uterus.
Menstrual cycles are counted from the first day of menstrual bleeding and are typically 28 days long. During menstruation, the body begins to prepare for ovulation again. The levels of estrogen gradually rise, signalling the start of the follicular, or proliferation, phase of the menstrual cycle.
The discharge of blood slows and then stops in response to rising hormone levels and the lining of the uterus thickens, or proliferates.
Ovulation is triggered by a surge in luteinizing hormone. The sudden change in hormones at the time of ovulation sometimes causes minor changes in the endometrium and light midcycle blood flow.
After ovulation, under the influence of progesterone, the endometrium changes to a secretory lining in preparation for the potential implantation of an embryo to establish a pregnancy. If a blastocyst implants, then the lining remains as the decidua. This becomes part of the placenta and provides support and protection for the embryo during gestation. If implantation does not occur within approximately two weeks, the progesterone-producing corpus luteum in the ovary will recede, causing sharp drops in levels of both progesterone and estrogen.
This hormone decrease causes the uterus to shed its lining and the egg in menstruation. The average age of menopause in women is 52 years, but it can occur anytime between 45 and The Uterine Cycle : High estrogen and progesterone levels stimulate increased endometrial thickness, but following their decline from a lack of implantation, the endometrium is shed and menstruation occurs. The rest are reabsorbed into the ovary. The remaining follicle, called the dominant follicle, is responsible for producing large amounts of estrogen during the late follicular phase.
On day 14 of the cycle, a luteinizing hormone surge is triggered by the positive feedback of estrogen. This causes the secondary follicle to develop into a tertiary follicle, which then leaves the ovary 24—36 hours later. An important event in the development of the tertiary follicle occurs when the primary oocyte completes the first meiotic division, resulting in the formation of a polar body and a secondary oocyte.
The empty follicle then forms a corpus luteum which later releases progesterone to maintain a potential pregnancy. Consequently, androgens produced by theca cells diffuse into the granulosa cells which contain aromatase but lack the enzymes for the initial steps in estrogen synthesis.
The theca cells are in a better position to catalyze the initial steps in estrogen synthesis because they are closer to blood vessels and can take up LDL to obtain cholesterol. The Graafian follicle is the stage after the first meiotic division has completed but before ovulation. The oocyte is now a 2N haploid. The follicle is characterized by a large follicular antrum that makes up most of the follicle. The secondary oocyte, having undergone the first meiotic division, is located eccentrically.
It is surrounded by the zona pellucida and a layer of several cells known as the corona radiata. When released from the Graafian follicle and into the oviduct, the ovum will consist of three structures: oocyte, zona pellucida and corona radiata. After release of the ovum, the remaining cells of the granulosa and theca interna form the corpus luteum. The center contains the remains of the blood clot that formed after ovulation.
Surrounding the clot are glanulosa lutein cells and on the outside theca lutein cells. These cells produce progesterone and to a lesser extent cholesterol.
The granulosa lutein cells have an appearance characteristic of steroid-producing cells, with pale cytoplasm indicating the presence of lipid droplets. Theca lutein cells are smaller and more deeply stained. Blood vessels penetrate into region of the granulosa lutein cells allowing them to take up cholesterol to be used to synthesize progesterone. Each daughter cell receives one chromatid from the original chromosome pair to form the gamete producing cells that only have a single copy of each chromosome.
Again one of the daughter cells forms a polar body. Thus only one gamete is formed from one primary oocyte compare this to male gametogenesis. For more information compare normal mitotic division with meiotic division. Development of the follicles is stimulated by production of follicle stimulating hormone FSH by the pituitary gland. Ripening of the follicles then results in an increase in oestrogen levels, as oestrogen is secreted by follicular cells.
This increase in oestrogen levels feeds back to the pituitary, and suppresses further release of FSH negative feedback. The follicles also release a second hormone called inhibin , which also suppresses further production of FHS. As the oestrogen levels rise, this triggers a a mid cycle surge in a second pituitary hormone called Lutenising hormone LH , which causes the follicle to rupture ovulation.
LH also causes ruptured follicles to lutenise, forming a transitory endocrine organ called the corpus luteum.
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