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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.