ह्यूमन REPRODUCTION

Sexual Reproduction in Humans
Index to this page
Sex Organs of the Human Male
Spermatogenesis
Sperm
LH
FSH
Sex Organs of the Human Female
Oogenesis
Ovulation
Copulation and Fertilization
Pregnancy
The placenta is an allograft
Assisted Reproductive Technology ("ART")
In Vitro Fertilization (IVF)
Intracytoplasmic Sperm Injection (ICSI)
Ooplasmic Transfer
Birth and Lactation
Birth Control
The Problems to be SolvedSexual reproduction is the formation of a new individual following the union of two gametes. In humans and the majority of other eukaryotes — plants as well as animals — the two gametes
differ in structure ("anisogamy") and
are contributed by different parents.Gametes need
motility to be able to meet and unite
food to nourish the developing embryo.In animals (and some plants), these two rather contrasting needs are met by anisogametes:
sperm that are motile (and small)
eggs that contain food.
Sex Organs of the Human MaleThe reproductive system of the male has two major functions:
production of sperm
delivery of these to the reproductive tract of the female. Sperm production — spermatogenesis — takes place in the testes.
Each testis is packed with seminiferous tubules (laid end to end, they would extend more than 20 meters) where spermatogenesis occurs.
Spermatogenesis
The walls of the seminiferous tubules consist of diploid spermatogonia, stem cells that are the precursors of sperm. Spermatogonia
divide by mitosis to produce more spermatogonia or
differentiate into spermatocytes.
Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete.
Then the spermatids differentiate into sperm, losing most of their cytoplasm in the process. For simplicity, the figure shows the behavior of just a single pair of homologous chromosomes with a single crossover. With 22 pairs of autosomes and an average of two crossovers between each pair, the variety of gene combinations in sperm is very great.
SpermSperm cells are little more than flagellated nuclei. Each consists of
a head, which has
an acrosome at its tip and
contains a haploid set of chromosomes in a compact, inactive, state.
a midpiece containing mitochondria and a single centriole
a tail
This electron micrograph (courtesy of Dr. Don W. Fawcett and Susumu Ito) shows the sperm cell of a bat. Note the orderly arrangement of the mitochondria. They supply the ATP to power the whiplike motion of the tail.
An adult male manufactures over 100 million sperm cells each day. These gradually move into the epididymis where they undergo further maturation. The acidic environment in the epididymis keeps the mature sperm inactive. In addition to making sperm, the testis is an endocrine gland. Its principal hormone, testosterone, is responsible for the development of the secondary sex characteristics of men such as the beard, deep voice, and masculine body shape. Testosterone is also essential for making sperm.
Link to more on testosterone.Testosterone is made in the interstitial cells that lie between the seminiferous tubules.
LHInterstitial cells are, in turn, the targets for a hormone often called interstitial cell stimulating hormone (ICSH). It is a product of the anterior lobe of the pituitary gland. However, ICSH is identical to the luteinizing hormone (LH) found in females, and I prefer to call it LH.
FSHFollicle-stimulating hormone (also named for its role in females) acts directly on spermatogonia to stimulate sperm production (aided by the LH needed for testosterone synthesis). [Discussion]
Sex Organs of the Human Female
The responsibility of the female mammal for successful reproduction is considerably greater than that of the male. She must
manufacture eggs
be equipped to receive sperm from the male
provide an environment conducive to fertilization and implantation
nourish the developing baby not only before birth but after.
Oogenesis
Egg formation takes place in the ovaries. In contrast to males, the initial steps in egg production occur prior to birth. Diploid stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes. By the time the fetus is 20 weeks old, the process reaches its peak and all the oocytes that she will ever possess (~4 million of them) have been formed. By the time she is born, 1–2 million of these remain. Each has
begun the first steps of the first meiotic division (meiosis I) and then
stopped.No further development occurs until years later when the girl becomes sexually mature. Then the primary oocytes recommence their development, usually one at a time and once a month.
The primary oocyte grows much larger and completes the meiosis I, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance.
In humans (and most vertebrates), the first polar body does not go on to meiosis II, but the secondary oocyte does proceed as far as metaphase of meiosis II and then stops.
Only if fertilization occurs will meiosis II ever be completed. Entry of the sperm restarts the cell cycle
breaking down MPF (M-phase promoting factor) and
turning on the anaphase promoting complex (APC).Completion of meiosis II converts the secondary oocyte into a fertilized egg or zygote (and also a second polar body).
As in the diagram for spermatogenesis, the behavior o