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Write an essay on the hormonal control of the menstrual cycle. What occurs in the uterine and ovarian cycles?
The menstrual cycle which begins at puberty and ends at menopause, describes the cyclic changes in the uterus and the ovary. The main function of the ovary is to produce a haploid oocyte whilst the main function of the uterus is to accept the fertilized ovum which then impants into the endometrium and receives nourishment from blood vessles of the uterus. In humans, the menstrual cycle lasts 28 days and is determined by follicle growth and the corpus luteum lifespan. The ovarian cycle has two distinct phases; in the follicular phase, oocyte and its surrounding follicle mature and ends with the oocyte being ovulated, in the luteal phase, the remaining follicle develops into an endocrine corpus luteum and if there is no implantation there is sloughing of the uterine endometrium and involution of the corpus luteum into a corpus albicans. In regards to the uterus, the main stages are menstruation, proliferative and secretory phase. Structure of the ovary The cyclic nature of the menstrual cycle was reflected in the ovarian cycles which was seen when an ovary is inspected under a microscope. The outercortex of the ovary has germ cells within follicles that are at different stages of development. Each of these follicles contains a primary oocyte, surrounded by zona pellucida and a variable number of granulose cells depending on what stage of development they are at. During late stages of development there are also thecal cells surrounding the granulosa cells. Between the follicles, the stroma contains corpora lutea, which is a temporary endocrine structure that secretes high levels of progesterone and moderate levels of estradiol and inhibin A. If there is no implantation the corpous luteus is broken down by macrophages and is replaced by type 1 collagen formed by fibroblasts; this forms the corpus albicans. The inner medulla of the ovary conists of vessels, nerves and steroid secreting cells. Formation of primary follicles During fetal development there are a pool of stem cells called oogonia which increase by proliferation. Unlike in males, meiosis starts during fetal development but is arrested in prophase of meiosis 1 to form a primary
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