sexual reproduction

reproduction

[re″pro-duk´shun] 1. the creation of a similar object or situation; replication or duplication.2. the process by which a living entity or organism produces a new individual of the same kind. The sex glands, or gonads (the ovaries in the female and the testes in the male) produce the germ cells (ova and sperm) that unite and grow into a new individual. Reproduction begins when the germ cells unite, a process called fertilization.Production of Germ Cells. The germ cells are the male spermatozoon and the female ovum (secondary oocyte). The secondary oocyte (mature ovum) is a large round cell that is just visible to the naked eye. Spermatozoa, on the other hand, can be seen only under a microscope, where each appears as a small, flattened head with a long whiplike tail used for locomotion.

In the female, maturation of an ovum is a remarkable process controlled by hormones secreted by the endocrine glands. The menstrual cycle is ordinarily 28 days long, measured from the beginning of one menstrual period to the beginning of the next. During the first 2 weeks of the usual cycle, one of the ova becomes mature enough to be released from the ovary. At the time of ovulation this mature ovum (secondary oocyte) is released and at this point can be fertilized. If fertilization occurs, the fertilized ovum (zygote) is then discharged into the abdominal cavity. Somehow, by mechanisms that are not clear, it moves into a fallopian tube and begins its descent toward the uterus. If the ovum remains unfertilized, menstrual bleeding occurs about 2 weeks later.
In the male there is no sexual cycle comparable to the cyclical activity of ovulation in the female. Mature sperm are constantly being made in the testes of the adult male and stored there in the duct system.Fertilization, or Conception. During coitus, semen is ejaculated from the penis into the back of the vagina near the cervix uteri. About a teaspoonful of semen is discharged with each ejaculation, containing several hundred millions of spermatozoa. Of this enormous number of sperm, only one is needed to fertilize the ovum. Yet the obstacles to be overcome are considerable. Many of the sperm are deformed and cannot move. Others are killed by the acid secretions of the vagina (the semen itself is alkaline). The sperm must then swim against the current of secretions flowing out of the uterus.

The sperm swim an average of 0.4 to 2.5 cm (0.1 to 1.0 inch) per minute. When one or more vigorous sperm are able to reach the ovum, which is normally in the outer half of the fallopian tube, fertilization occurs. The head end of the sperm plunges through the thick wall of the ovum, leaving its tail outside. The genetic materials, the chromosomes, are injected into the ovum, where they unite with the chromosomes inherited from the mother (see heredity). The sex of the child is determined at this instant; it depends on the sex chromosome carried by the sperm.
If by chance two ova have been released and are fertilized by two sperm, fraternal (dizygotic) twins are formed. Identical (monozygotic) twins are produced by a single fertilized ovum that divides into two early in its development.Ovulation and Fertilization. Fertilization typically can occur only (on the average) on 4 days of every menstrual cycle. The mature ovum lives only 1 or 2 days after ovulation, and the sperm have only about the same amount of time before they perish in the female reproductive tract. To fertilize the ovum, coitus must take place within the time that begins 1 or 2 days before ovulation and lasts until 1 or 2 days after ovulation. There is much variation, however, in the time when ovulation occurs. Most women ovulate between 12 and 16 days after the beginning of the last period, but others ovulate as early as 8 or as late as 20 days after the last period began.Pregnancy. The ovum, now known as a zygote, begins to change immediately after fertilization. The membrane surrounding it becomes impenetrable to other sperm. Soon the zygote is dividing into a cluster of two, then four, then more cells, as it makes its way down the fallopian tube toward the uterus. At first it looks like a bunch of grapes. By the time it reaches the uterus, in 3 to 5 days, the cells are formed in the shape of a minute ball, the blastocyst, which is hollow on the inside with an internal bump at one side where the embryo will form. The blastocyst quickly buries itself in the lining of the uterus (implantation). Occasionally implantation takes place not in the uterine lining, but elsewhere, producing an ectopic pregnancy.

As soon as the blastocyst is implanted, its wall begins to change into a structure that eventually develops into the placenta. Through the placenta the fetus secures nourishment from the mother and rids itself of waste products. Essentially the placenta is a filtering mechanism by which the mother's blood is brought close to the fetal blood without the actual mixing of blood cells.
During the early stages of pregnancy, the fetus grows at an extremely rapid rate. The mother's body must undergo profound changes to support this organism. The muscles of the uterus grow, vaginal secretions change, the blood volume expands, the work of the heart increases, the mother gains weight, the breasts prepare for nursing, and other adjustments are made throughout the mother's body.

Reproduction.asexual reproduction reproduction without the fusion of germ cells.assisted reproduction assisted fertility.cytogenic reproduction production of a new individual from a single germ cell or zygote.sexual reproduction reproduction by the fusion of female and male germ cells or by the development of an unfertilized ovum.somatic reproduction production of a new individual from a multicellular fragment by fission or budding.