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The phases of the menstrual cycle that denote changes in the uterine wall are:
Proliferative phase 10 days
Secretory phase 14 days
Menstrual phase 4 days
At this stage an ovarian follicle, stimulated by FSH, grows towards maturity and produces oestrogen. Oestrogen stimulates the proliferation of the endometrium in preparation for the reception of a fertilised ovum. The endometrium becomes thicker by rapid cell multiplication accompanied by an increase in the numbers of mucous secreting glands and blood capillaries. This phase ends when ovulation occurs and oestrogen production stops.
Immediately after ovulation, the lining cells of the ovarian follicle are stimulated by LH to develop the corpus luteum which produces progesterone. Under the influence of progesterone the endometrium becomes oedematous and the secretory glands produce increased amounts of watery mucous. This is believed to assist the passage of the spermatozoa through the uterus to the uterine tubes where the ovum is usually fertilised. There is a similar increase in the secretion of watery mucous by the glands of the tubes and into the vagina by cervical glands.
If the ovum is not fertilized menstruation occurs.
When the ovum is not fertilized the corpus luteum begins to degenerate. Progesterone and oestrogen levels fall. The menstrual flow consists of secretions from the endometrial glands, endometrial cells, blood and the unfertilized ovum.
At about the age of 50 years a woman enters menopause - the time when ova are no longer produced and the woman is longer fertile. The amount of estrogen and progesterone decreases.
This is the fusion of the sperm and the egg. Fertilisation and the establishment of pregnancy together are referred to as conception. The fusion of sperm and ovum nuclei together form a Zygote. This zygote gives rise to a new individual.
The embryo after two months of fertilization comes to resemble the human being and from then to birth is referred to as the foetus. The development of the foetus continues until it is ready to be born after 280 days or 9 months. The period of intra-uterine development of the foetus is known as the gestation period. This is followed by birth or parturition. During the seventh week of gestation, the embryo has both male and female primordial genital ducts. The gonad develops a cortex and a medulla. Until the sixth week of development, the structures are identical in both sexes. In genetic male, the medulla develops during the seventh, eighth weeks into a testes, and the cortex regresses. Leydig cells appear and androgen is secreted. In genetic females, the cortex develops into an ovary and the medulla regresses.
During the foetal growth fetal membranes such as the amnion and chorion help protect, nourish and support the foetus. The amnion forms a sac around the foetus that is filled with amniotic fluid. The placenta is the organ of exchange of nutrients between the mother and the embryo. During pregnancy the corpus luteum and placenta secrete progesterone.
High levels of estrogen and progesterone during pregnancy cause an increase in the size of the breasts. The hormone prolactin secreted.
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