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CHAPTER 22: DEVELOPMENT AND AGING LECTURE OUTLINE 22.1 Principles of Animal Development Animal development begins with a single cell that multiples and changes to form a complete organism. Fertilization Fertilization results in a zygote. It requires that the sperm and oocyte interact. Early Stages of Animal Development The early stages of animal development occur at the cellular, tissue, and organ levels of organization. Cellular Stages of Development The cellular stages of development are cleavage resulting in a multicellular embryo, and formation of the blastula. Tissue Stages of Development The tissue stages of development are the early gastrula and the late gastrula. Organ Stages of Development The organs of an animal’s body develop from the three embryonic germ layers. Processes of Animal Development Aside from growth, the process of development requires cellular differentiation and morphogenesis. Cellular differentiation occurs when cells become specialized in structure and function. Morphogenesis produces the shape and form of the body. Cellular Differentiation Specialization is due to differential gene expression. Certain genes and not others are turned on in differentiated cells. Induction and Frog Experiments The gray crescent of a frog’s egg marks the dorsal side of the embryo where the mesoderm becomes notochord and ectoderm becomes nervous system. Experiments have shown that presumptive notochord tissue induces the formation of the nervous system. Induction and Roundworm Experiments Work with the roundworm Caenorhabditis elegans has also shown that induction is necessary to the process of differentiation. Fate maps, diagrams that trace the differentiation of developing cells, have been developed that show the destiny of each cell as it arises following successive cell divisions. Morphogenesis Pattern formation is the ultimate in morphogenesis. The Fruit Fly Experiments In fruit flies, investigators have discovered certain genes, now called morphogen genes, which determine the relationship of individual parts. Homeotic genes act by controlling the identity of each segment. Apoptosis Apoptosis is an important part of pattern formation in all organisms. 22.2 Human Embryonic and Fetal Development Human development before birth is often divided into embryonic development (months 1 and 2) and fetal development (months 3-9). Embryonic development consists of early formation of major organs, and fetal development is the refinement of these structures. Extraembryonic Membranes Humans have extraembryonic membranes, including the chorion, amnion, allantois, and yolk sac. Embryonic Development Embryonic development encompasses the first two months of development following fertilization. The First Week Fertilization occurs in the distal third of an oviduct, and cleavage begins even as the embryo passes down this duct to the uterus. By about the fifth day, the morula is transformed into the blastocyst. The Second Week At the end of the first week, the embryo begins the process of implanting in the wall of the uterus. The Third Week The nervous system and the heart develop during the third week. The Fourth and Fifth Weeks Little flippers called limb buds appear, the head enlarges, and the sense organs become more prominent. The Sixth through Eighth Weeks The embryo changes to a form that is easily recognized as human. Fetal Development and Birth Fetal development includes the third through ninth months of development. The Third and Fourth Months Cartilage begins to be replaced by bone. Epidermal refinements appear. Sometime during the third month it is possible to distinguish males from females, and during the fourth month, the fetal heartbeat is loud enough to be heard when a physician applies a stethoscope to the mother’s abdomen. The Fifth through Seventh Months During the fifth through seventh months the mother begins to feel movement. At the end of this period, the length has increased to about 300 mm, and the weight is about 1,380 grams. Fetal Circulation The fetus has circulatory features that are not present in the adult circulation because the fetus does not use its lungs for gas exchange. Structure and Function of the Placenta The placenta is a structure that functions only before birth. It functions in gas, nutrient, and waste exchange between the embryonic and maternal circulatory systems. Birth A positive feedback mechanism regulates the onset and continuation of labor. Stage 1 The first stage of parturition, the process of giving birth to an offspring, ends once the cervix is dilated completely. Stage 2 During the second stage, the baby is born. Stage 3 The placenta, or afterbirth, is delivered. Female Breast and Lactation Usually no milk is produced during pregnancy. Once the baby is delivered, however, the pituitary begins secreting prolactin which causes the formation of milk. 22.3 Human Development after Birth Development does not cease once birth has occurred but continues throughout the stages of life: infancy, childhood, adolescence, and adulthood. Aging encompasses these progressive changes, which contribute to an increased risk of infirmity, disease, and death. Hypotheses about Aging Genetic in Origin Several lines of evidence indicate that aging has a genetic basis. These are based on the number of times a cell divides and the production of free radicals. Whole-Body Process A decline in the hormonal system and/or immune system can affect many different organs of the body. It is possible that aging is not due to the failure of a particular system that can affect the body as a whole, but to a specific type of tissue changes that affects all of the organs and even the genes. Extrinsic Factors It is possible that much of what we attribute to aging is instead due to years of poor health habits. Effect of Age on Body Systems It is possible that in the future age will not have the same effects or at least not to the same degree as those described here. Skin As aging occurs, skin becomes thinner and less elastic. There is less adipose tissue in the subcutaneous layer. Processing and Transporting Cardiovascular disorders are the leading cause of death today. Cardiovascular problems are often accompanied by respiratory disorders. Integration and Coordination Contrary to previous opinion, recent studies show that few neural cells of the cerebral cortex are lost during the normal aging process. Loss of skeletal muscle mass is not uncommon, but it can be controlled by a regular exercise program. Aging is accompanied by a decline in bone density. The Reproductive System Females undergo menopause, and thereafter, the level of female sex hormones in the blood falls markedly. Sexual activity need not decline with age, however, due to hormone therapy and other treatments. Conclusion It is important to emphasize that many of the adverse effects of aging are not inevitable. We must discover any extrinsic factors that precipitate these adverse effects and guard against them.