Menstrual cycle
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
Proliferative phase
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.
Secretory phase
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.
Menstrual Phase
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.
Menopause
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.
Fertilisation
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.
Pregnancy
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.
Lactation
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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