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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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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 • Development is all the changes that occur during the life cycle of an organism. • The embryo is the first stage in human development. • Following fertilization, the zygote undergoes cleavage, a period of cell division without growth. • Cleavage leads to a ball of cells called the morula.
The morula becomes a blastula when an internal cavity, the blastocoel, appears. 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.
The Effect of Yolk • Yolk is a dense nutrient material found in various amounts in the eggs of animals. • The amount of yolk affects the process of gastrulation, the formation of the three germ layers. • The presence of yolk causes cells to cleave more slowly. • The aquatic frog and lancelet need less yolk as they develop rapidly.
A chick egg has so much yolk that the embryo lies flat and endoderm formation does not occur by invagination. 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.
Neurulation and the Nervous System • The notochord forms from mesoderm. • During neurulation, the nervous system develops from midline ectoderm, just above the notochord; the notochord induces formation of the nervous system. • A neural plate is seen first, then a neural tube; the anterior neural tube becomes the brain. • At the neurula stage, cross sections of all chordate embryos appear similar.
Cytoplasmic Segregation • The cytoplasm of an egg is not uniform but contains maternal determinants that are parceled out during mitosis. • Cytoplasmic segregation helps determine how the various cells of the morula will develop. • The gray crescent in a frog’s egg is required for an embryo to develop.
Induction • Induction occurs when embryonic cells influence one another to develop in a particular way. • A molecular concentration gradient may act as a chemical signal to induce germ layer differentiation. • The presumptive (potential) notochord tissue induces the formation of the nervous system. • The vertebrate eye likewise forms by a series of inductions.
Model Organisms • The Roundworm Experiments • The roundworm Caenorhabditis elegans develops into an adult of 959 cells; researchers have traced every cell division from the first fertilized egg and developed a fate map. • Work with the roundworm shows that induction involves signals that activate new genes that provide new signals, and that induction requires the regulation of genes in a particular sequence.
Programmed cell death (apoptosis) plays a role during development. 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 Fruit Fly Experiments • Research with fruit flies has shown how morphogenesis comes about; that morphogen genes determine the pattern of an animals and its parts. • Each morphogen gene codes for a protein that is present in a gradient. • Homeotic genes control the organization of differentiated cells into specific three-dimensional structures.
Homeotic genes form protein gradients that determine if a segment will bear antennae or legs or wings. 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.
Embryonic Development • The First Week • Fertilization occurs in the upper third of the oviduct; a zygote is produced. • The embryo is ball of cells called a morula when it reaches the uterus on the third day. • By the fifth day, the morula is transformed into a blastocyst which consists of an outer trophoblast and an inner cell mass.
The Second Week 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.
The Third Week 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.
The Sixth Through Eighth Weeks 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.
Fetal Development and Birth • The Third and Fourth Months • During the third and fourth months, the body increases in size, and epidermal refinements (eyelashes, nipples) become apparent. • Bone is replacing cartilage. • It is now possible to distinguish males from females, and the heartbeat is audible with a stethoscope.
The Fifth Through Seventh Months 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).
Fetal Circulation 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.
One umbilical vein takes nutrients to the systemic system when the umbilical vein joins the vena cava by a venous duct. If the oval opening fails to close, it causes a “blue baby” that receives a mixture of oxygenated and unoxygenated blood.
The Structure and Function of the Placenta 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.
Fetal and maternal blood cells do not mix within the placenta. 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.
Birth • Stage 1 • Prior to parturition (giving birth), contractions of labor move the baby’s head downward, causing effacement and dilation of the cervix. • The amnion (bag of waters) breaks, releasing amniotic fluid. • The cervix is dilated completely at the end of this stage.
Stage 2 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. Stage 3 The afterbirth (placenta) is delivered.
Female Breasts and Lactation 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.
Suckling of the baby at the breast stimulates the hypothalamus to direct the pituitary to secrete oxytocin that, in turn, causes milk to flow. Breast milk contains antibodies that supplements the baby’s immature immune system.
