The female reproductive life differs than that of the male by being of specific limited period of time usually between the ages of 12 and 45. While the male reproductive years continue through out life with little decline in his fertility. The reproductive life in females stops between the ages of 40 and 45. At this time the monthly sexual cycle starts to cease working.
The female reproductive life begins approximately at the age of 12 years with the beginning of the monthly sexual cycle which takes place every 28 days. The main site of reproduction in females is the ovaries. Before the age of 12 years, a period of time which is called childhood the female child does not have active reproductive life. Namely, there is no monthly sexual cycle in childhood.
In this period of time, the two hormones that are secreted by the pituitary gland and which are responsible for the initiation of the monthly sexual cycle are not secreted to the blood circulation in significant amount to initiate a monthly sexual cycle. These two hormones are called: follicle stimulating hormone and luteinizing hormone.
After the age of 12 years these two hormones begin to be secreted by the anterior pituitary gland in significant amounts. Thus causing an initiation of the monthly cycle. These two hormones stimulate the synthesis and secretion of the steroid hormones estrogen and progesterone by the ovaries.
These two pituitary hormones are under the positive feedback control from the hormone gonadotropin releasing hormone which is secreted by the hypothalamus. FSH and LH are under negative feedback control from the hormones estrogen and inhibin which is secreted by the ovaries and the corpus luteum after ovulation occurs.
The sexual monthly cycle in females is characterized by rhythmical changes in the amount of FSH and LH that are secreted by the pituitary gland to the blood circulation. These changes are accompanied with changes in the follicles of the ovaries. Usually, every month there are few follicles in the ovaries which begin to mature with the progress of the monthly cycle by the effect of the hormones FSH and LH.
By the mid time of the monthly cycle only one follicle succeed to become mature so that it ovulates (release an ovum) by rupturing the follicular membrane releasing the ovum from the follicle. This ovum can then be fertilized by a sperm from the male. If the fertilization process is successful a fetus is then generated and pregnancy occurs.
In the normal process of ovulation in the non-pregnant female the remaining part of the follicle that was ruptured and ovulated becomes the corpus luteum. In the normal female monthly sexual cycle the LH secreted by the pituitary gland is important for the process of ovulation and rupturing of the follicular membrane which contains the mature ovum.
An experimental fact that was observed is that the concentration of LH in the blood increases drastically one or two days before ovulation. The corpus luteum secretes large amount of the hormones estrogen and progesterone to the blood circulation. The corpus luteum sustains for few days if there is no pregnancy and which after that period it is shed in the menstruum. This is true if the process of ovulation does not lead to pregnancy in which case the life of the corpus luteum is extended by two other months before it is shed in the menstruum.
The large amount of estrogen and progesterone that are secreted by the corpus luteum inhibit the secretion of the FSH and LH by the pituitary gland. by the end of the monthly cycle when the corpus luteum sheds the amount of estrogen and progesterone in the blood decrease drastically due to the absence of the corpus luteum. As a result the negative feedback of these two hormones on the FSH and LH is lifted which in turn begin to be secreted normally by the pituitary gland. This leads to the initiation of a new monthly cycle.
At this stage new follicles begin to be formed and mature and only one of them succeeds to ovulate by the mid-time of the monthly cycle. This process continues all over the period of the reproductive life of the female. At the end of the monthly cycle not only the corpus luteum sheds but also a menstruation occurs in which the endometrial lining of the uterus is shed out of the body.
