Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 11

In the mid-eighteenth century, the theory of preformation gave way to the theory of epigenesis as biologists observed embryos specialize over time. Karl Ernst von Baer noted that early vertebrate embryos look alike but gradually become more distinctive. Ernst Haeckel proposed that embryonic stages repeat the adult structures of more primitive species. Modern scientists believe that vertebrate embryos simply share similar early stages.
Cells of the early embryo are pluripotent. Gradually, morphogen gradients establish biochemically distinct regions of the embryo, directing them to differentiate down specific developmental pathways.
A cell commits to a fate at a certain point in development. Before this time, a transplanted cell develops according to its new surroundings; after this point, it retains the specialization other cells exhibit in its original location. Differential gene expression underlies cell specialization.
Developmental biology has evolved from observing to disrupting development, often with a molecular focus. Model organisms are critical in studying development.
Human prenatal development begins at fertilization. A single, capacitated sperm cell at a secondary oocyte burrows through the zona pellucida and corona radiata. The two pronuclei join. Cleavage ensues, and a 16-celled morula forms. Between days 3 and 6, the morula arrives at the uterus and hollows to form a blastocyst, made up of individual blastomeres. The trophoblast layer and inner cell mass form. Implantation occurs between days 6 and 14. Trophoblast cells secrete hCG, which prevents menstruation.
During the second week, the amniotic cavity forms as the inner cell mass flattens, forming the embryonic disc. The primitive streak appears. Ectoderm and endoderm form, and then mesoderm appears, establishing the germ layers of the gastrula. Cells in a particular germ layer develop into parts of specific organ systems.
During the third week the chorion starts to develop into the placenta, and the yolk sac, allantois, and umbilical cord form as the amniotic sac swells with fluid. Organogenesis occurs throughout this embryonic period. Gradually structures appear, including the notochord, neural tube, arm and leg buds, heart, facial structures, skin specializations, and skeleton.
The fetal period begins in the third month. Organs specialize and body proportions come to more closely resemble those of a newborn. The fetus becomes active. During the fifth month, vernix caseosa and lanugo cover the fetus. In the last trimester, the brain develops rapidly and fat fills out the skin. The digestive and respiratory systems mature. At about 38 weeks, strong uterine contractions dilate the cervix and push the baby out.
Postnatal stages include the newborn, infant, child, adolescent, and adult. Drastic changes occur at birth, as the newborn takes on functions that the pregnant woman provided. Infancy and childhood are periods of rapid growth and maturation of organ systems. Hormonal changes dominate adolescence. Organ systems function during adulthood and begin to show signs of aging.
Aging is a part of life, and it begins even before birth. Various structures and functions reach their peaks at different ages.
In passive aging, structures break down, and DNA repair becomes less efficient. Free radicals build up, threatening cell stability, the thymus shrinks, and blood vessels narrow with cholesterol deposits. In active aging, lipofuscin accumulates in cells and cells die.
The theoretical maximum human life span is 120 years. The most common causes of death shift as technologies conquer various diseases.