Chapter 22: Aging. Fertilization. Depolarization of the egg’s plasma membrane after the sperm touches the egg and separation of the zona pellucida prevent a second sperm from fertilizing the egg. The sperm enters the egg and the sperm nucleus fuses with the egg nucleus. Embryonic Development.
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The sperm enters the egg and the sperm nucleus fuses with the egg nucleus.
At the gastrula stage, invagination of cells into the blastocoel results in formation of the germ layers: ectoderm, mesoderm, and endoderm; mesoderm arises from pouches in endoderm.
Two layers of mesoderm form, and the space between them becomes the coelom.
The three germ layers will have different developmental fates.
Instead an upper layer of cells becomes ectoderm, and a lower layer becomes endoderm; mesoderm invaginates between the two layers, and the furrow that develops is called a primitive streak.
Because of a shared evolutionary history, gastrulation in humans is like that of the chick even though the human egg has little yolk.
A good example is the development of fingers and toes in humans due to death of cells between the digits.
The fate maps of C. elegans indicate that apoptosis occurs in 131 cells as development takes place.
The same sequence of genes is found in many organisms; the same sequence of nucleotides is a homeobox.
A homeobox codes for a sequence of 60 amino acids called a homeodomain.
Homeodomain proteins bind to DNA and determine which genes are turned on.
The embryo begins to implant in the uterine lining at end of first week.
The trophoblast secretes human chorionic gonadotropin (HCG), a hormone that maintains the corpus luteum.
The yolk sac and amnion form.
Gastrulation occurs and the inner cell mass becomes the embryonic disk while the trophoblast becomes the chorion.
Neurulation occurs and the nervous system is the first visible organ system.
The heart begins to form and pump blood when right and left heart tubes fuse.
The Fourth and Fifth Weeks
The allantois forms and is contained within the umbilical cord.
Limb buds appear and sense organs develop.
By end of eight weeks, the embryo is only 38 mm (1.5 inches) long but is easily recognized as human.
All organ systems are established, even though the embryo weighs no more than an aspirin tablet at this point.
The thin skin is covered with lanugo and coated with a vernix caseosa.
The eyelids open fully.
At the end of seven months, the fetus can possibly survive if born prematurely.
The fetus is now 300 mm (12 inches) in length and weighs 1,380 grams (3 lb).
Blood passes from the right to the left atrium through an oval opening, the foramen ovale, and an arterial duct, the ductus arteriosus, shunts blood between the pulmonary trunk and aorta.
These features enable blood to bypass the non-funtioning lungs.
Two umbilical arteries that branch off the iliac arteries lead to the placenta.
If the oval opening fails to close, it causes a “blue baby” that receives a mixture of oxygenated and unoxygenated blood.
Chorionic villi project into maternal tissue as the placenta develops.
By the tenth week, the placenta is fully formed and secretes estrogen and progesterone that maintains the lining and prevents further menstrual cycling and ovulation.
Carbon dioxide and wastes diffuse from the fetal to the maternal side, and oxygen and nutrients diffuse from the maternal to the fetal side.
Harmful chemicals can cross the placenta and some alter normal fetal development.
Uterine contractions occur each 1–2 minutes and the mother experiences a desire to push.
An episiotomy may be performed to prevent tearing.
The baby is pushed out during this stage, and the umbilical cord is cut and tied.
The afterbirth (placenta) is delivered.
The breast contains 15–25 lobules with milk ducts.
No milk is produced during pregnancy, but milk ducts and alveoli proliferate during that time, and breasts enlarge.
Once the baby is delivered, the pituitary secretes prolactin, and milk is produced.
Breast milk contains antibodies that supplements the baby’s immature immune system.
A second theory of aging suggests that a hormonal decline can affect many different organ systems.
Type II diabetes is due to cells lacking receptors to take up insulin; menopause is a similar failure by ovaries to take up the follicle-stimulating hormone.
The thymus gradually gets smaller with age.
Aging may also be due to a tissue change that affects all organs throughout the body.
Collagen fibers become cross-linked which leads to loss of elasticity throughout many body organs.
A third theory on aging suggests that years of poor health habits contribute most to aging.
Insufficient calcium intake and smoking increase osteoporosis, for example.
Exercise and adequate servings of fruits and vegetables help eliminate cardiovascular disease.
Cardiovascular disorders are the leading cause of death among the elderly; the heart shrinks with age, and fatty deposits clog arteries.
Lungs lose elasticity, so ventilation is reduced.
A reduced blood supply to the kidneys results in the kidneys becoming smaller and less efficient.
The digestive tract may lose muscle tone but still absorbs nutrients efficiently.
Normal aging results in the loss of few nerve cells; short-term memory may decline but overall cognitive skills remain.
After age 50, there is a decline in the ability to hear higher frequencies, and the lens of the eye does not accommodate as well.
Loss of skeletal muscle mass and bone density is common but can be controlled through exercise and adequate calcium intake.
Females undergo menopause and are no longer reproductive.
In males, sperm production declines after age 50 but continues until death.
Women as a group outlive men.
Good health habits, started when young, slow the aging process and contribute to a long, healthy life span.