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| Gastrulation |
| Pattern Formation and Induction |
Blastulation and gastrulation establish the main body axis. Organ formation occurs in the next stage of the development of the embryo. During organ formation, cell division is accomplished by migration and aggregation.
Pattern formation is the result of cells "sensing" their position in the embryo relative to other cells and to form structures appropriate to that position. Gradients of informational molecules within the embryo have been suggested to provide the positional information to cells. Homeobox genes are pattern genes; they coordinate with gradients of information molecules to establish the body plan and development of organs.
Induction is the process in which one cell or tissue type affects the developmental fate of another cell or tissue. As a cell begins to form certain structures, certain genes are turned on, others are turned off. Induction affects patterns of gene expression through physical contact or chemical signals. Formation of the vertebrate eye is a well known example.
Fertilization, the fusion of the sperm and egg, usually occurs in the upper third of the oviduct. Thirty minutes after ejaculation, sperm are present in the oviduct, having traveled from the vagina through the uterus and into the oviduct. Sperm traverse this distance by the beating of their flagellum. Of the several hundred million sperm released in the ejaculation, only a few thousand reach the egg.
Only one sperm will fertilize the egg.
One sperm fuses with receptors on the surface of the secondary oocyte, triggering a series of chemical changes in the outer oocyte membrane that prevent any other sperm from entering the oocyte. The entry of the sperm initiates Meiosis II in the oocyte. Fusion of the egg and sperm nuclei forms the diploid zygote. |
| Travels of a Young Zygote |
Cleavage of the zygote begins while it is still in the oviduct, producing a solid ball of cells (morula). The morula enters the uterus, continuing to divide and becomes a blastocyst.
Stages in the journey of a fertilized egg from fertilization in the Fallopian tube to implantation in the uterus.
The uterine lining becomes enlarged and prepared for implantation of the embryo in the trophoblast layer. Twelve days after fertilization, the trophoblast has formed a two-layered chorion. Human chorionic gonadotropin (hCG) is secreted by the chorion, and prolongs the life of the corpus luteum until the placenta begins to secrete estrogen and progesterone. Home pregnancy tests work by detecting elevated hCG levels in the woman's urine.
Maternal and embryonic structures interlock to form the placenta, the nourishing boundary between the mother's and embryo's systems. The umbilical cord extends from the placenta to the embryo, and transports food to and wastes from the embryo.
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| Changes in the zygote from fertilization to implantation. |
The period of time from fertilization to birth (usually 9 months) is divided into trimesters, each about three months long. During pregnancy the zygote undergoes 40 to 44 rounds of mitosis, producing an infant containing trillions of specialized cells organized into tissues and organs.
The three embryonic tissue layers form. Cellular differentiation begins to form organs during the third week. After one month the embryo is 5 mm long and composed mostly of paired somite segments. During the second month most of the major organ systems form, limb buds develop. The embryo becomes a fetus by the seventh week. Beginning the eighth week, the sexually neutral fetus activates gene pathways for sex determination, forming testes in XY fetuses and ovaries in XX fetuses. External genitalia develop.
The fetus increases in size during this trimester, and bony parts of the skeleton begin to form. Fetal movements can be felt by the mother.
During this trimester the fetus increases in size. Circulatory and respiratory systems mature in preparation for air breathing. Fetal growth during this time uses large parts of its mother's protein and calcium intake. Maternal antibodies pass to the fetus during the last month, conferring temporary immunity.
Birth is a positive feedback hormonal mechanism. During birth the cervix dilates to allow passage of the fetus. Uterine contractions propel the fetus through the birth canal, usually head first. Hormonal control of the birth process involves the release of oxytocin and prostaglandins, which are stimulated by uterine contractions, which stimulate more hormones that cause more contractions....etc.
The first stage of birth lasts from beginning of contractions to the full (10 cm) dilation of the cervix. Membranes of the amniotic fluid rupture, lubricating the vagina.
Strong uterine contractions of a minute in duration separated by two to three minute intervals propel the fetus down the birth canal. Abdominal muscles relax in synchrony with the uterine contractions.
After delivery of the baby, the umbilical cord is clipped and cut. The placenta (or afterbirth) in expelled through the vagina.
Nursing mothers have their hormone levels and uterine size return to normal much faster than non-nursing mothers. Breasts develop the capability for milk secretion about the mid point of pregnancy. Secretion of milk does not occur until delivery, and the action of prolactin. Suckling by the infant causes production of oxytocin to promote release of milk into the ducts emptying into the nipple.
| The Male Reproductive System |
Testes are suspended outside the abdominal cavity by the scrotum, a pouch of skin that keeps the testes close or far from the body at an optimal temperature for sperm development. Seminiferous tubules are inside each testis, and are where sperm are produced by meiosis. About 250 meters (850 feet) of tubules are packed into each testis. Spermatocytes inside the tubules divide by meiosis to produce spermatids that in turn develop into mature sperm.
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| The male reproductive system |
Sperm production begins at puberty at continues throughout life, with several hundred million sperm being produced each day. Once sperm form they move into the epididymis, where they mature and are stored.
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| Human Sperm |
The anterior pituitary produces follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Action of LH is controlled by the gonadotropin-releasing hormone (GnRH). LH stimulates cells in the seminiferous tubules to secrete testosterone, which has a role in sperm production and developing male secondary sex characteristics. FSH acts on cells to help in sperm maturation. Negative feedback by testosterone controls the actions of GnRH.
Sperm pass through the vas deferens and connect to a short ejaculatory duct that connects to the urethra. The urethra passes through the penis and opens to the outside. Secretions from the seminal vesicles add fructose and prostaglandins to sperm as they pass. The prostate gland secretes a milky alkaline fluid. The bulbourethral gland secretes a mucus-like fluid that provides lubrication for intercourse. Sperm and secretions make up semen.
Details of the testes.
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