Changes After Fertilization

Changes after Fertilization

Implantation: The fertilized egg (zygote) moves down the Fallopian tube and continuously undergoes cell division. Thus it forms a hollow ball of cells, called embryo. The embryo gets embedded in the wall of the uterus, which is thick and has muscles, glands and a large number of blood capillaries. This process is called implantation.

Pregnancy: The developing embryo at first derives nourishment directly from the mother’s blood flowing in the vessels lining the uterine wall. In about three weeks, it starts absorbing food and oxygen through an organ called placenta. The placenta is a disc like organ in the lining of the uterine wall. It has numerous villi, which are in direct contact with the mother’s blood flowing in the uterine wall. These villi provide a large surface area for glucose and oxygen to pass from the mother to the embryo and for wastes produced by the embryo to be passed into the mother’s blood. The embryo is connected to the placenta by a tube called the umbilical cord. By eight weeks, the embryo starts showing human features and is referred to as foetus. The total period of embryonic development, from the time of fertilization to birth, is called gestation period. It is around 280 days, or 9 months, in humans.

Birth: The wall of the uterus develops a thick layer of muscles during pregnancy. At the time of birth, the uterine muscles contract rhythmically and powerfully, causing labour pains to the mother. Finally, the baby is expelled by the contraction of the uterine muscles. This is called birth or parturition.

What happens when the egg is not fertilized: If the ovum is not fertilized in the upper part of the oviduct, it keeps on descending and is finally passed out through the vagina. It remains in the body for about 24 - 72 hours. As an egg is released for fertilization every month, the uterus also prepares itself every month for the implantation of a fertilized egg. The uterus becomes thick-walled and spongy in order to nourish the embryo. If no fertilization takes place, the thick uterine wall is no longer needed. So, it gradually begins to shrink. This shrinkage ruptures its blood vessels. As a result, blood and mucus ooze out of the vagina. This period, which lasts for 3—5 days, is called the menstrual period, and the process is called menstruation. If the ovum is fertilized, it gets implanted in the uterus wall and embryonic development starts. In this case, the uterus continues to develop in order to hold the embryo. And in this case, there is no question of its shrinkage resulting in menstruation.

Development of Embryo

The zygote (or fertilised egg) divides repeatedly to make a ball of hundreds of cells. This is called an embryo. The embryo moves down the oviduct into the uterus. The embryo gets embedded in the soft and thick lining of the uterus (see Figure 8). This is called implantation. When the embryo settles down in the uterus, the woman is said to have become pregnant (or said to have conceived). The embryo starts growing into a baby. The embryo gets food and oxygen from the blood vessels in the lining of the uterus. The cells of embryo begins to form specialised groups that develop into different tissues and organs of the baby. Embryo is an early stage in a developing baby. Thus, an unborn baby at an early stage of development in the uterus is called an embryo. The body features of the unborn baby are not much developed at the embryo stage. The unborn baby remains an embryo in the first eight weeks of pregnancy. A human embryo at 4 to 5 week's development is shown in Figure 9. We cannot identify any body features (hands, legs, head, eyes and ears, etc.) of the developing baby in this embryo.

The embryo continues to grow and develop in the uterus to form a baby. The embryo gradually develops body parts such as hands, legs, head, eyes and ears, etc. An unborn baby in the uterus at the stage when all the body parts can be identified, is called a foetus (The word 'foctus' is pronounced as 'fetus'). A human embryo becomes a foetus after about eight weeks of pregnancy. From about eight weeks until birth, the unborn baby is called foetus (see Figure 10). It takes about 38 weeks (about nine months) from the fertilisation of egg to. the formation of fully developed development of the foetus into a baby is complete, the mother gives birth to the baby. The fully formed baby comes out of the mother's body through vagina. And we say that a baby is born. This is how humans reproduce by giving birth to babies. All of us were born in this way.