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CHAPTER OUTLINE
22.1 Fertilization and Early Stages of Development
Animal development begins with a single cell that multiples and changes to form a complete
organism.
Fertilization
Fertilization is the union of a sperm and an oocyte, resulting in a zygote.
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. Cleavage is cell division without growth.
Tissue Stages of Development
The tissue stages of development are: the early gastrula and the late gastrula.
Gastrulation involves the formation of three layers of cells that will develop into
adult organs.
Organ Stages of Development
The organs of an animal’s body develop from the three embryonic germ layers:
ectoderm, mesoderm, and endoderm.
22.2 Processes of 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
Specialized cells produce only certain proteins and specialization is due to
differential gene expression. Certain genes and not others are turned on in
differentiated cells.
Cytoplasmic Segregation
Cytoplasmic segregation is the parceling out of maternal determinants as
mitosis occurs. These determinants consists of RNAs and proteins
synthesized from the maternal genome and store in the egg, they
influence the course of development.
Induction
Induction is the ability of one embryonic tissue to influence the
development of another tissue by the use of signals called inducers,
chemical signals that alter the metabolism of the receiving cell and
activate particular genes.
Induction and Frog Experiments
A frog embryo’s gray crescent becomes the dorsal lip of the
blastopore, where gastrulation begins. 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
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Pattern formation is the ultimate in morphogenesis.
Morphogen genes
In fruit flies, investigators have discovered certain genes, now called
morphogen genes, which determine the relationship of individual parts.
Homeotic Genes
Homeotic genes act by controlling the identity of each segment. They
encode master regulatory proteins that control the expression of other
genes, which in turn are responsible for the development of segment-specific
structures.
Apoptosis
Apoptosis (programmed cell death) is an important part of pattern formation in
all organisms.
22.3 Human Embryonic and Fetal Development
Human development before birth is often divided into embryonic development (months one and
two) and fetal development (months three to nine). Embryonic development consists of early
formation of the major organs, and fetal development is the refinement of these structures.
Extraembryonic Membranes
Humans have extraembryonic membranes, which lie outside of the embryo: 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 uterine tube closest to the ovary, and cleavage begins
even as the embryo passes down this duct to the uterus. By the time the embryo
reaches the uterus on the third day, it is a morula. 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 single layer of outer cells of the blastocyst begin to
secrete human chorionic gonadotropic (HCG),which is the basis for the
pregnancy test and serves to maintain the corpus luteum past the time it normally
disintegrates.
The Third Week
The nervous system and the heart begin to develop during the third week.
The Fourth and Fifth Weeks
The umbilical cord, which connects the developing embryo to the placenta is
fully formed. Little flippers called limb buds appear, the head enlarges, and the
sense organs become more prominent.
The Sixth through Eighth Weeks
During the sixth through eighth weeks of development, the embryo becomes
more recognizable as human.
Fetal Development
Fetal development includes the third through ninth months of development.
The Third and Fourth Months
Head growth now begins to slow down as the rest of the body increases in length.
Epidermal refinements appear. Cartilage begins to be replaced by bone.
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
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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.
22.4 Human Pregnancy, Birth, and Lactation
Pregnancy
Many changes that take place in the mother’s body during pregnancy are due to the
hormones progesterone and estrogen. Others are due to the increasing size of the uterus.
Morning Sickness and Energy Level
About six weeks into her pregnancy, the mother may experience nausea and
vomiting, loss of appetite and fatigue. These symptoms usually subside by
around the 12th week.
Effects of Smooth Muscle
Progesterone decreases uterine motility by relaxing smooth muscle, including the
smooth muscle in the walls of arteries. Arteries expand, leading to low blood
pressure and blood volume increases, causing cardiac output to increase.
Heartburn and constipation can also result from the relaxation of other smooth
muscles.
Other Effects
Compression of the ureters and urinary bladder by an enlarged uterus can result
in involuntary leakage of urine from the bladder. Compression of the inferior
vena cava results in fluid accumulation and varicose veins. Gestational diabetes,
“stretch marks” and darkening of certain areas of the skin all may occur during
pregnancy.
Birth
A positive feedback mechanism regulates the onset and continuation of labor. Uterine
contractions are induced by stretching of the cervix, which brings about the release of
oxytocin. Oxytocin stimulates uterine contractions, which push the fetus downward,
stretching the cervix.
Stage 1
Parturition, the process of giving birth to an offspring, can be divided into three
stages. The first stage of parturition ends once the cervix is dilated completely.
Stage 2
During the second stage, uterine contractions occur every 1 to 2 minutes and last
about one minute each; the baby is born during this stage.
Stage 3
The placenta, or afterbirth, is delivered during the third stage of parturition.
Female Breast and Lactation
Usually no milk is produced during pregnancy. Once the baby is delivered, the
pituitary begins secreting prolactin, which causes the formation of milk.
22.5 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. Gerontology is the study of
aging.
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The Effects of Aging on Organ Systems
Aging and death are as much a part of biology as are embryonic development and birth.
Integumentary System
As aging occurs, the skin becomes thinner and less elastic. There is less adipose
tissue in the subcutaneous layer. Together these changes typically result in
sagging and wrinkling of the skin.
Cardiovascular System
Common problems with cardiovascular function are usually related to diseases,
especially atherosclerosis. Even with normal aging the heart muscle weakens
somewhat and may increase slightly in size.
Immune System
As people age, many of their immune system functions become compromised,
which can play a major role in the aging process.
Digestive System
The digestive system is perhaps less affected by the aging process than other
systems.
Respiratory System
Cardiovascular problems are often accompanied by respiratory disorders, and
vice versa. Decreasing elasticity of lung tissues means that ventilation is reduced.
Excretory System
Blood supply to the kidneys is reduced, they become smaller and less efficient at
filtering waters. Salt and water balance are difficult to maintain.
Nervous System
Between the ages of 20 and 90, the brain loses about 20% of its weight and
volume. Recent studies show that decreased brain function may occur due to
alterations in complex chemical reactions, or increased inflammation in the brain.
Sensory Systems
In general, with again more stimulation is needed for taste, smell, and hearing
receptors to function as before.
Musculoskeletal System
Beginning in the twenties or thirties, muscle mass generally decreases with age,
due to decreases in both the size and number of muscle fibers. Regular exercise
can slow this decline. Bones tend to shrink in size and density with age.
Endocrine System
Some hormone levels tend to decrease with age, while others increase. The
activity of the thyroid gland generally declines, resulting in a lower basal
metabolic rate.
Reproductive System
After age 30, testosterone levels decrease in men. Females undergo menopause,
the period in their life during which the ovarian and uterine cycles cease.
Hypotheses About Why We Age
Aging is a complex process and multiple factors can affect it.
Preprogrammed Theories
Most scientist who study gerontology believe that aging is partly genetically
preprogrammed, which is supported by the observation that longevity runs in
families. Experts have estimated that in most cases, genes account only for about
25% of what determines the length of life.
Damage Accumulation Theories
A second group of hypotheses postulate that aging involves the accumulation of
damage over time. Cell damage may be avoidable or unavoidable.
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