CHAPTER TWENTY-EIGHT
Development and Inheritance
From Egg to Embryo A. Development is the gradual modification of anatomical structures and physiological characteristics during the period from fertilization to maturity. 1. Embryo = embryonic development is the first two months, or eight weeks after fertilization. The study of the events that characterize this period is described as embryology. 2. Fetus = from ninth week through birth. The study of the events occurring during this timeframe is referred to as fetal development. 3. Prenatal development = embryological and fetal development are sometimes referred to collectively as prenatal development. 4. Gestation = extends from the last menstrual period until birth (280 days). a. First trimester = period of embryological development and early fetal development; during this period, the rudiments of all the major organ systems appear. b. Second trimester = dominated by the development of organs and organ systems; a process that nears completion by the end of the sixth month. During this period, body shape and proportions change by the end of this trimester, the fetus looks distinctively human. C. Third trimester = characterized by the largest gain in fetal weight. Early in the third trimester, most of the fetus's major organ systems become fully functional. An infant
survival if appropriate medical care is available.
5. Postnatal development = begins at birth and continues through maturity, the state of full development or completed growth. 28.1 Accomplishing Fertilization A. Fertilization = fusion of the haploid sperm with the haploid egg to produce a zygote that contains 46 chromosomes. a. Ovulation releases a secondary oocyte, or egg, into the fallopian tube. It is surrounded by a corona radiata and zona pellucida that must be penetrated. b. The egg is only viable for 12 to 24 hours after ovulation. Sperm are viable for 24 to 72 hours after ejaculations. C. Consequently, coitus must occur no more than three days before ovulation and no later than 24 hours after ovulation. d. Only about 10,000 sperm in a males ejaculate actually make it to the uterine tubes and fewer than 100 reach the egg. Remember, in a healthy male, the ejaculate typically contains 200+ million sperm. a. Thousands are phagocytized in the uterus. b. Millions are destroyed by acidity of the vagina. c. Millions leak immediately from the vagina. e. Fertilization requires capacitation a. Capacitation = the ability of the sperm to swim to the egg and the sperm's outer membranes must become fragile so that the hydrolytic enzymes within their acrosomes can be released. b. Sperm penetration through the corona radiata and zona pellucida surrounding the oocyte is accomplished by release of hyaluronidase, acrosin, etc. by many sperm. C. Fusion of the plasma membrane of a single sperm with the oocyte membrane is followed by engulfment of the sperm by the oocyte cytoplasm and the cortical reaction.
d. Cortical reaction (or oocyte activation) involves the release of the contents of the oocytes cortical granules into the extracellular space preventing entry of more than one sperm = polysp