Each testis is packed with seminiferous tubules laid end to end, they would extend more than 20 meters where spermatogenesis occurs. The walls of the seminiferous tubules consist of diploid spermatogonia, stem cells that are the precursors of
Sexualreproduction in humanbeings. Meiosis of each spermatocyte produces 4 haploid spermatids.
This process takes over three weeks to complete. This electron micrograph courtesy of Dr.
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 million sperm cells each day. These gradually move into the epididymis
Sexualreproduction in humanbeings they undergo further maturation.
The acidic environment in the epididymis Sexualreproduction in humanbeings the mature sperm inactive.
The responsibility of the female mammal for successful reproduction is considerably greater than that of the male. In contrast to males, the initial steps in egg production occur prior to birth. stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes. By the time she is born, only about 1 million of these remain the others eliminated by apoptosis. Each has begun the first steps of the first meiotic division stopping at the diplotene stage of meiosis I.
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 meiosis Iforming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes up in the egg and which in the polar body is entirely a matter of chance. In humans and
Sexualreproduction in humanbeings vertebratesthe first polar body does not go on to meiosis IIbut 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 Sexualreproduction in humanbeings the chromosomes is greatly simplified. These events take place within a follicleSexualreproduction in humanbeings fluid-filled envelope of cells surrounding the developing egg.
Sexualreproduction in humanbeings about two weeks after the onset of menstruation. In response to a sudden surge of LH, the follicle ruptures and discharges a secondary oocyte. This is swept into the open end of the fallopian tube and begins to move slowly down it. For fertilization to occur, sperm must be deposited in the vagina within a few 5 days before or on the day of ovulation. Sperm transfer is accomplished by copulation.
Sexual excitation dilates the arterioles supplying blood to the penis. Blood accumulates in three cylindrical spongy sinuses Sexualreproduction in humanbeings run lengthwise through the penis. The resulting pressure causes the penis to enlarge and erect and thus able to penetrate the vagina.
The mixture of sperm and accessory fluids is called semen. It passes through the urethra and is expelled into the vagina. Physiological changes occur in the female as well as the male in response
Sexualreproduction in humanbeings sexual Sexualreproduction in humanbeings, although these are not as readily apparent.
In contrast to the male, however, such responses are not a prerequisite for copulation and fertilization to occur. Once deposited within the vagina, the sperm proceed on their journey into and through the uterus and on up into the fallopian tubes. It is here that fertilization may occur if an "egg" is present strictly speaking, it is still a secondary oocyte until after completion of meiosis II.
Although sperm can swim several millimeters each second, their trip to and through the fallopian tubes may be assisted by muscular contraction of the
Sexualreproduction in humanbeings of the uterus and the tubes.
The trip is fraught with heavy mortality. And of these, only one will succeed in fertilizing the egg. Sperm swim towards the egg by chemotaxis following a gradient of progesterone secreted by cells surrounding the egg. Progesterone opens CatSper " cation sperm" channels in the plasma membrane surrounding the anterior portion of the sperm tail. Fertilization begins with the binding of a sperm head
Sexualreproduction in humanbeings the glycoprotein coating of the egg called the zona pellucida.
Exocytosis of the acrosome at the tip of the sperm head releases enzymes that digest a path through the zona and enable the sperm head to bind to the plasma membrane of
Sexualreproduction in humanbeings egg.
The binding is mediated by the binding of two membrane proteins: Izumo1 on the surface of the sperm and Juno, its Sexualreproduction in humanbeings on the egg surface. Fusion of their respective membranes allows the entire contents of the sperm to "Sexualreproduction in humanbeings" drawn into Sexualreproduction in humanbeings cytosol of the egg.
Even though the sperm's mitochondria enter the egg, they are almost always destroyed — by autophagy — and do not contribute their genes to the embryo. So human mitochondrial DNA is almost always inherited from mothers only. Within moments, enzymes released from the egg cytosol act on the zona making it impermeable to other sperm that arrive. Soon the "Sexualreproduction in humanbeings" of the successful sperm enlarges into the male pronucleus. At the same time, the egg secondary oocyte completes meiosis II forming a second polar body and the female pronucleus.
The male and female pronuclei move toward each other while duplicating their DNA in S phase. Their nuclear envelopes disintegrate. A spindle is formed following replication of the sperm's centrioleand a full set of dyads assembles on it. The fertilized egg or zygote is now ready for its first mitosis. When this is done, 2 cells — each with a diploid set of chromosomes — are formed.
In sea urchins, at least, the block to additional sperm entry and the fusion of the pronuclei are triggered by nitric oxide generated in the egg by the sperm acrosome.
Development begins while the fertilized egg is still within the fallopian tube. Repeated mitotic "Sexualreproduction in humanbeings" produces a solid ball of cells called a morula. Further mitosis and some migration of cells converts this into a hollow ball of cells called the blastocyst. Approximately one week after fertilization, the blastocyst embeds itself in the thickened wall of the uterus, a process Sexualreproduction in humanbeings implantationand pregnancy is established.
The placenta grows tightly fused to the wall of the uterus. Its blood vessels, supplied by the fetal heart, are literally bathed in the mother's blood. Although there is normally no mixing of the two blood supplies, the placenta does facilitate the transfer of a variety of materials between the fetus and the mother.
The umbilical cord connects the fetus to the placenta. It receives deoxygenated "Sexualreproduction in humanbeings" from the Sexualreproduction in humanbeings arteries of the and returns oxygenated blood to the liver and on to the inferior vena
Sexualreproduction in humanbeings. Because its lungs are not functioning, circulation in the fetus differs dramatically from that of the baby after birth.
While within the uterus, blood pumped by the right ventricle bypasses the lungs by flowing through the foramen Sexualreproduction in humanbeings and the ductus arteriosus.
Although the blood in the placenta is in close contact with Sexualreproduction in humanbeings mother's blood in the uterus, intermingling of their blood does not normally occur. However, some of the blood cells of the fetus usually do escape into "Sexualreproduction in humanbeings" mother's circulation — where they have been known to survive for decades.
This raises the possibility of doing prenatal diagnosis of genetic disorders by sampling the mother's blood rather than having to rely on the more invasive procedures of amniocentesis and chorionic villus sampling CVS.
These can be tested for various forms of aneuploidy, e. Far rarer is the leakage of mother's blood cells into the fetus. However, it does occur. A few pregnant women with leukemia Sexualreproduction in humanbeings lymphoma have transferred the malignancy to their "Sexualreproduction in humanbeings." Some babies have also acquired melanoma from the transplacental passage of these highly-malignant cells from their mother.
During the first 2 months of pregnancy, the basic structure of the baby is being Sexualreproduction in humanbeings. This involves cell division, cell migration, and the differentiation of cells into the many types found in the baby.
During this period, the developing baby — called an embryo — is very sensitive to anything that interferes with the steps involved. Virus infection of the mother, e. Such agents are called teratogens "monster-forming". The tranquilizer, thalidomide, taken by many pregnant European women between andturned out to be a potent teratogen and was responsible for the birth of several thousand Sexualreproduction in humanbeings babies.
After about two months, all the systems of the baby have been formed, at least in a rudimentary way. From then on, development of the fetusas it is now called, is primarily a matter of growth and minor structural modifications. The fetus is less susceptible to teratogens than is the embryo. One of the greatest unsolved mysteries in immunology is how the placenta survives for 9 months without being rejected by the mother's immune system.
Every cell of the placenta carries the father's genome a haploid set of his chromosomes ; including one of his 6 chromosomes where the genes for the major histocompatibility antigens HLA are located. Thus the placenta is immunologically as foreign to the mother as a kidney transplant would be.
Despite a half-century of research, the mechanism for this immunologically privileged status remains uncertain. But one thing Sexualreproduction in humanbeings clear: The mother is not intrinsically
Sexualreproduction in humanbeings of the father's antigens.
On July 25, Louise Brown celebrated her 35th birthday. She was the first of what today number some four million worldwide "test tube babies"; that is, she developed from an egg that was fertilized outside her mother's body — the process called in vitro fertilization IVF.
Successful IVF assumes the availability of healthy sperm. But many cases of infertility arise from defects in the father's sperm. Jun 12, Class Biology: How Sexualreproduction in humanbeings Organisms "Sexualreproduction in humanbeings" Sexual Reproduction in Human Beings.
Like in plants it is the male gamete that needs to be transferred to the gamete. The female gamete is fertilised and develops inside the mother's body so.
Reproduction in human beings is by sexual reproduction where both the male and female gametes fertilize to give rise to an embryo. The fertilization of human embryo occurs inside the body of the female. Let us understand a little more about the male and female reproductive organs.