Human Fertilisation and Development Essay Example

Liquid secretions from various glands combine with sperm to form the semen, or seminal fluid.

Ejaculated semen may contain as many as 400 million sperm. Oocytes are also haploid, and since they are formed only by females, the 23rd chromosome can only be an X chromosome. Each oocyte is protected by several layers of granulosa cells called the corona radiata. Beneath the corona radiata lays the zona pellucida which is involved in binding the sperm cell, and through which the sperm cell must also penetrate prior to fertilization. The nucleus in an oocyte is called the germinal vessicel, and the nucleolus is referred to as the germinal spot. Eggs are produced in the ovaries, oval-shaped organs in the groin that also generate sex hormones.

At birth, a female's ovaries contain hundreds of thousands of undeveloped eggs, each surrounded by a group of cells to form a follicle, or sac; however, only about 360-480 follicles reach full maturity. During puberty the action of hormones causes several follicles to develop each month. Normally, just one follicle fully matures, rupturing and releasing an ovum through the ovary wall in a process called ovulation. The mature egg enters one of the paired fallopian tubes, where it may be fertilized by a sperm and move on to the uterus to develop into a fetus.

The lining of the uterus, called the endometrium, prepares for pregnancy each month by thickening, but if fertilization does not take place, the endometrium is shed during menstruation. The development of a human begins with fertilization, a process by which the spermatozoan from the male and the oocyte from the female unite to

give rise to a new organism, the zygote. During sexual intercourse, a man releases approximately 300 million sperm into a woman's vagina, but only one of the sperm can fertilize the ovum.

The successful sperm cell must enter the uterus, swim up the fallopian tube to meet the ovum, and then pass through a thick coating, known as the zona pellucida, that surrounds the egg. The head of the sperm cell contains enzymes that break through the zona pellucida and allow the sperm to penetrate the egg. Once the head of the sperm is inside the egg, the tail falls off, and the outside of the egg thickens to prevent another sperm from entering, and the fertilized egg (zygote) develops into an embryo. The embryo is now a hollow sphere of cells called a blastocyst.

The blastocyst implants itself in the uterine wall. Gastrulation occurs in which cells migrate inward and form a rudimentary digestive cavity. The resulting gastrula has three layers of cells. After gastrulation the three embryonic tissue layers give rise to specific organ systems. Tissues and organs take shape in a developing embryo as a result of cell shape changes, cell migration and programmed cell death. In a process called induction, adjacent cells and cell layers influence each other’s differentiation via chemical signals.

Pattern formation, the emergence of the parts of a structure in their correct relative positions, involves the response of genes to spatial variations of chemicals in the embryo. Meanwhile, the four extra embryonic membranes develop: the amnion, the chorion the yolk sac, and the allantois. The embryo floats in the fluid-filled amniotic cavity, while the chorion

and embryonic mesoderm form the embryo’s part of the placenta. The placenta’s chorionic villi absorb food and oxygen from the mother’s blood.

Human embryonic development is divided into three trimesters of about 3 months each. During the first trimester the embryo begins to develop all vital organs of the baby and by 9 weeks shows a fetus. The fetus looks like a miniature human, although its head is still oversized for the rest of the body. During the second trimester, the fetus continues growing and developing. Its arms, legs, fingers and toes have lengthened. It has the face of an infant with eyebrows and eyelashes. It develops fingernails and toenails and is covered with fine hair.

And also it begins to move, kick and open and closes its eye and its teeth are forming. During the third trimester, the fetus gains the strength it will need to survive outside the protective environment of the uterus. The fetus circulatory system undergoes changes that will allow the switch to air breathing and it continues to grow adding layers of fat, bones begin to harden and its muscles thicken. It also loses much of its fine hairs, except on its head and the head changes its proportion. Finishing development is underway preparing for birth.

Hormonal changes induce birth; estrogen makes the uterus more sensitive to oxytocins, which acts the prostagland into initiate labor. The cervix dilates, the baby is expelled by strong muscular contractions, and the placenta follows. To conclude, life begins when male and female gametes unite during fertilisation. The embryo develops into a fetus in the mother’s womb and after the three trimesters

of growing and developing, the mother cuddles her new born baby boy or girl in her arms which puts a smile on her face.