Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete. When the two gametes combine, they merge the two sets of chromossome to create a cell with the total number of chromosomes needed to develop, known as a diploid cell. In humans when the haploid sperm and egg cell join in fertilisation the resulting zygote has a total of 46 chromosomes the correct number to develop. By having gametes which are haploid, when the gametes combine, diploid cells are maintained.
Also, the mixing of chromosomes in fertilisation is a source of genetic variation. Fertilisation produces a zygote , which will mature into an embryo. The number of cells increases by mitosis, and as the embryo develops, the cells begin to differentiate or specialise. Sexual reproduction, meiosis and gamete formation Sexual reproduction Two parents are needed in sexual reproduction. All primary oocytes are formed by the fifth month of fetal life and remain dormant in prophase of meiosis I until puberty.
After ovulation the oocyte is arrested in metaphase of meiosis II until fertilization. At fertilization , the secondary oocyte completes meiosis II to form a mature oocyte 23,1N and a second polar body. In males, gametogenesis begins at puberty and continues into advanced age. Primordial germ cells 46,2N migrate into the testes at week 4 of development and remain dormant.
At puberty, primordial germ cells differentiate into type A spermatogonia 46,2N. Type A spermatogonia divide by mitosis to form either more type A spermatogonia to maintain the supply or type B spermatogonia. Under the influence of estrogen released during the first half of the menstrual cycle , three changes take place in the uterine tubes to facilitate its capture of the egg:.
The uterine tubes move closer to the ovaries physical approximation. The fimbriae on the ends of the tubes beat more rapidly increased fluid current. The number of ciliated cells in the epithelium of the fimbriae increase increase in ciliation.
Sperm are deposited in the upper vagina and must overcome several obstacles to reach an egg in the ampulla of one of the uterine tubes. The egg itself is viable for only about 24 hours.
Table 1 - Obstacles to Sperm Transport. The alkaline seminal fluid temporarily neutralizes the normal acidity pH 4. The composition of cervical mucus changes during menstrual cycle. Sperm can most easily penetrate the thinner E-mucus that predominates during the last few days before ovulation, as opposed to the thicker G-mucus. Two modes of transport:. Slow — the rest of the sperm swim their way up the last part of the cervical tube, are stored in cervical crypts folds of the cervix , and are slowly released into the uterus over days.
Table 2 - Karyotypes of Germ Cells and Gametes. About once a month, during ovulation, an ovary sends a tiny egg into one of the fallopian tubes. Unless the egg is fertilized by a sperm while in the fallopian tube, the egg leaves the body about 2 weeks later through the uterus — this is menstruation. Blood and tissues from the inner lining of the uterus combine to form the menstrual flow, which in most girls lasts from 3 to 5 days. A girl's first period is called menarche MEH-nar-kee.
It's common for women and girls to have some discomfort in the days leading to their periods. Premenstrual syndrome PMS includes both physical and emotional symptoms that many girls and women get right before their periods, such as:. PMS is usually at its worst during the 7 days before a girl's period starts and disappears after it begins.
Many girls also have belly cramps during the first few days of their periods caused by prostaglandins, chemicals in the body that make the smooth muscle in the uterus contract.
These involuntary contractions can be dull or sharp and intense. It can take up to 2 years from menarche for a girl's body to develop a regular menstrual cycle. During that time, her body is adjusting to the hormones puberty brings. On average, the monthly cycle for an adult woman is 28 days, but the range is from 23 to 35 days.
If a female and male have sex within several days of the female's ovulation, fertilization can happen. When the male ejaculates when semen leaves the penis , a small amount of semen is deposited into the vagina. Millions of sperm are in this small amount of semen, and they "swim" up from the vagina through the cervix and uterus to meet the egg in the fallopian tube.
It takes only one sperm to fertilize the egg. About 5 to 6 days after the sperm fertilizes the egg, the fertilized egg zygote has become a multicelled blastocyst. A blastocyst BLAS-tuh-sist is about the size of a pinhead, and it's a hollow ball of cells with fluid inside. The blastocyst burrows itself into the lining of the uterus, called the endometrium. The hormone estrogen causes the endometrium en-doh-MEE-tree-um to become thick and rich with blood.
Progesterone, another hormone released by the ovaries, keeps the endometrium thick with blood so that the blastocyst can attach to the uterus and absorb nutrients from it. This process is called implantation. As cells from the blastocyst take in nourishment, another stage of development begins. In the embryonic stage, the inner cells form a flattened circular shape called the embryonic disk, which will develop into a baby. The outer cells become thin membranes that form around the baby.
The cells multiply thousands of times and move to new positions to eventually become the embryo EM-bree-oh. After about 8 weeks, the embryo is about the size of a raspberry, but almost all of its parts — the brain and nerves, the heart and blood, the stomach and intestines, and the muscles and skin — have formed.
During the fetal stage, which lasts from 9 weeks after fertilization to birth, development continues as cells multiply, move, and change. It gets oxygen and nourishment from the mother's blood via the placenta pluh-SEN-tuh. This disk-like structure sticks to the inner lining of the uterus and connects to the fetus via the umbilical um-BIL-ih-kul cord. The amniotic fluid and membrane cushion the fetus against bumps and jolts to the mother's body.
Pregnancy lasts an average of days — about 9 months. When the baby is ready for birth, its head presses on the cervix, which begins to relax and widen to get ready for the baby to pass into and through the vagina. Mucus has formed a plug in the cervix, which now loosesn.
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