Physiology of Reproduction

Function of the female reproductive system.

At puberty the ova begins to mature. At the follicular phase, an ovum matures within a cyst called “Graafian follicle” until it reaches the surface of the ovary where rupture occurs. The ovum is discharged into the peritoneal cavity. This periodic liberation of matured ovum into the peritoneal cavity is referred to as ovulation. This ovum finds its way into the fimbriated end of the fallopian tube. On its way to the uterus, if it meets a spermatozoan, the male gamete and union occurs and conception or fertilization takes place. The empty Graafian follicle, after ovulation is called as corpus luteum (yellow body), which secretes progesterone, a hormone that prepares the uterus for receiving the fertilised ovum.

Female hormones:

The ovaries produce steroid hormones, estrogen and progesterone.

Oestrogen:

It is responsible for development and maintenance of the female reproductive organs and the secondary sexual characteristics associated with the adult female. Estrogen also plays an important role in breast development and in monthly cyclic changes (menstrual cycle) in the uterus.

Progesterone:

Progesterone regulates the changes that occur in the uterus during the menstrual cycle. It is secreted by the corpus luteum. Progesterone is important for conditioning the endometrium in preparation for implantation of the fertilised ovum. If the pregnancy occurs, progesterone is essential for maintaining a normal pregnancy. In addition, it works along with oestrogen in preparing the breast for secretion of milk.

Physiology of reproduction:

Menstrual cycle or uterine cycle:

It is a series of changes in the uterus resulting in the discharge of blood from the vagina each month. Menstruation can be defined as, “sloughing and discharge of the lining of the uterus if conception does not take place.” This time varies in different women and also from time to time-in same woman. The first day of the cycle is the first day when bleeding begins. The ovarian hormones control the menstrual cycle. There are three main phases and they affect the tissue structures of the endometrium. The average time of menstrual cycle is 28 days and recurs regularly from puberty to menopause except in pregnancy. The three phases are:

1. Proliferative phase: Follicular stimulating hormonal level increases in blood, stimulating oestrogen secretion, which causes the endometrium to thicken and become more vascular. This phase follows menstruation and lasts until ovulation.

2. Secretary phase: The secretary phase follows ovulation and is under the influence of progesterone and oestrogen from the corpus luteum. Leutinising hormone level increases in blood. Under the combined stimulus of estrogen and progesterone, the endometrium reaches the peak of its thickening and vascularisation.

3. Menstrual phase: It is characterised by vaginal bleeding, lasts for 3 – 5 days. On absence of fertilization, the thickened endometrium is shedded. Two Gonadotrophic hormones are released by the anterior pituitary gland. They are:

-Follicular stimulating hormone:FSH is primarily responsible for stimulating the ovaries to secrete oestrogen and for maturation of ovum.

-Luteinising Hormone (LH): LH is primarily responsible for stimulating the corpus luteum for productoin of progesterone.

3. Puberty: This is the period in which, the reproductive organs develop and reach maturity. The first signs are breast development and appearance of pubic hair. The body grows considerably and takes on the female contour. Puberty culminates in the onset of menstruation, the first period being called menarche. The first few cycles are not accompanied by ovulation. Puberty usually occurs between 12 and 14 years.

Menopause: It is the end of a woman’ s reproductive life, characterised by the gradual cessation of menstruation. The period first becomes irregular and then ceases altogether. This occurs between the ages of 45 to 50. It is the normal part of aging and maturation. Menstruation ceases because the ovaries are no longer active. No more ovarian hormones are produced. The reproductive organs become atrophied.

Fertilization: Following ovulation, the ovum about (0.15 mm) in diameter passes into the fallopian tube and moves towards uterus. If coitus takes place at this time, the alkaline mucus attracts the spermatozoa. About 300 million sperms are deposited in the posterior fornix of the vagina. Those which are propelled by the cervical mucus reach the fallopian tube and others are destroyed by the acid medium of the vagina.

The matured sperm is capable of producing the enzyme hyaluronidase, which allows the sperm to penetrate the cell membrane, surrounding the ovum. Many sperm are needed for this, but only one will enter into the ovum and fertilisation occurs. After this, the membrane is sealed to prevent the entry of any further sperm and the nuclei of the two cells fuse. The sperm and the ovum each contribute half the complement of chromosomes to make a total of 46. The sperm and ovum are known as the male and female gametes. The fertilized ovum is known as the zygote. Implantation of the fertilised ovum (embedding) into the uterine cavity (endometrium) is called as nidation or nesting. Normally this occurs by the 11th day after ovulation and the endometrium closes over it completely.

Development of the fertilised ovum:

Fertilised ovum reaches the uterus by 3-4 days. Cell division takes place as 2 into 4,8,16, etc, till a cluster of cells formed known as morula (mulberry). Next a fluid filled cavity, a blastocele appears in the morula and it is known as blastocyst. Outside of blastocyst there is a single layer of cells known as trophoblast, while the remaining cells are clumped together forming an inner cell mass. The trophablast forms the placenta and chorion while the inner cell mass become fetus and amnion.

Formation of fetal membrane and placenta:

The trophoblast has two layers,

· Outer syncitiotrophoblast, which erodes the endometrium in the process of embedding.

· The inner cytotrophoblast produces a hormone called human chorionic gonadotrophin (HCG) which reacts on corpus lutuem to continue the pregnancy by producing oestrogen and progesterone.The trophoblast develops as placenta which will nourish the fetus until delivery.

The inner cell mass differentiates into three layers.

• From the ectoderm skin and nervous system are formed.
• From the mesoderm bones and muscles, heart and blood vessels and certain internal organs are formed.
• From the endoderm mucous membranes and glands are formed.

During the first three weeks following conceptual the fertilised ovum is termed as zygote. From 3-8 weeks, it is termed as embryo. The organs and systems are developed by 7th week. After 8 weeks, till birth it is termed as fetus.