Download 29-1 PRENATAL (BEFORE BIRTH) DEVELOPMENT 1. The

PRENATAL (BEFORE BIRTH) DEVELOPMENT
1. The germinal period is approximately the first 2 weeks of development following
fertilization. The germ layers form during the germinal period.
2. The embryonic period is from approximately the second week to the eighth week.
Development of the organ systems. Embryo is about 1.25 inches long and weighs about 1
ounce.
3. The fetal period is the last seven months. Growth and maturation of the organ systems. The
fetus is about 20 inches in length and weighs about 7.8 pounds.
During which period do you think the developing organism is most susceptible to damage?
FERTILIZATION
1. The female produces oocytes (egg cells) and the male produces sperm cells.
2. The union of an oocyte and sperm cell is called fertilization and the resulting single cell that
is produced is called a zygote.
FIGURE 29.1 and 29.2
Oocyte
Sperm cell
Fertilization
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Zygote
(Single cell)
DEVELOPMENTAL EVENTS.
1. The zygote divides (at 18 - 39 hours) to form two cells. These divide to form four cells, and
so on.
FIGURE 29.2 and 29.3
2. The morula is a mass of 12 or more cells.
3. The blastocyst is 32 or more cells (formed at 3 - 4 days) that form a hollow ball of cells.
A. The trophoblast is the outer layer of the hollow ball. It combines with part of the uterine
lining to become the placenta, the site of exchange of gases, nutrients and waste products
between the mother and developing organism.
B. The inner cell mass is a group of cells at one end of the blastocyst. It becomes the
embryo.
C. The blastocele is the hollow space inside the trophoblast.
4. The developing blastocyst implants in the uterine lining about seven days after fertilization.
5. The amniotic cavity, yolk sac, and embryonic disk develop within the inner cell mass.
A. The amniotic cavity is a space lined by a layer of cells called the amnion. The amniotic
cavity eventually expands and holds the developing embryo or fetus.
B. The yolk sac is so name because in animals that lay eggs it contains yolk, a source of
nutrients necessary for development. In humans there is no yolk.
Why isn't yolk important in human development?
1) Some of the yolk sac cells migrate into the embryo and become reproductive cells
(oocytes or sperm cells to be) or blood cells.
2) Part of the yolk sac becomes a nonfunctional part of the umbilical cord, which
connects the placenta and the embryo or fetus.
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C. The embryonic disk is a circular plate consisting of two layers of cell.
FIGURE 29.6
1) Ectoderm is associated with the amniotic cavity and endoderm with the yolk sac.
2) Some of the ectoderm becomes mesoderm by migrating to a position in-between the
original ectoderm and mesoderm layers. The place where migration occurs is called
the primitive streak.
3) Ectoderm, mesoderm, and endoderm are the germ layers, which are the beginning of
the embryo.
Primitive streak
Ectoderm
Mesoderm
Embryonic disk
Transverse section of
the embryonic disk
Endoderm
6. The embryo folds over to form a tube.
Amniotic cavity
Ectoderm
Mesoderm
Yolk sac
Endoderm
Amniotic cavity
Yolk sac
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7. The germ layers give rise to the four types of tissues and hence to all the structures of the
body.
A. Ectoderm gives rise to "outer" structures and the nervous system.
B. Endoderm gives rise to the "innermost" structures.
C. Mesoderm gives rise to everything in between.
Ectoderm
Mesoderm
Endoderm
Epithelium
Epidermis
Hair
Nails
Lining of mouth,
nasal cavity, and anus
Lining of serous
membranes, blood
vessels, kidneys, and
reproductive organs
Lining of digestive
and respiratory
systems, urinary
bladder, and urethra
Connective tissue
Bone (facial)
Bone (most)
Cartilage
Tendons
Ligaments
Dermis of skin
Fat
Blood
Loose connective
tissue
Muscle
Skeletal muscle (some Skeletal muscle
in the head)
(most)
Cardiac muscle
Smooth muscle
Nervous tissue
Brain
Spinal cord
Nerves
8. Ectoderm gives rise to specialized cells called neural crest cells, which give rise to some
bone and muscles in the head.
9. Mesoderm and neural crest cells give rise to an embryonic tissue called mesenchyme (G.
middle infusion), from which connective tissues arise. Some mesenchymal cells remain in
the adult and give rise to new connective tissue cells during tissue repair.
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TWINNING
Types of Twins
1. Fraternal twins (“womb mates”) develop when two oocytes are fertilized (each with a
different sperm cell) at approximately the same time. The result is two zygotes that develop
into two individuals. These twins are no more nor less alike than any siblings.
2. Identical twins (“cell mates”) develop when one oocyte is fertilized by one sperm cell to
form one zygote. At some point later in development the cells from this one zygote separate
to form two or more individuals. Since they arose from a single cell, these individuals are
genetically identical.
Timing of Identical Twin Formation.
1. About 30% split before the inner cell mass forms.
2. About 70% split after the inner cell mass forms but before the amnion forms. Therefore each
embryo develops its own amniotic sac.
3. Rarely the split occurs after the amnion forms. Therefore two primitive streaks develop and
both embryos share the same amniotic sac.
What would happen if the two primitive streaks were not completely separate, i.e., were
touching each other?
Pluripotent
1. Twinning is possible because some of the early cells in development are pluripotent: what
they will develop into has not yet become determined. Therefore pluripotent cells can
potentially become anything, such as a separate organism.
A. The first 8 to 16 cells formed from the zygote are pluripotent.
B. Cells of the inner cell mass (but not the trophoblast) are pluripotent.
C. Cells of the embryonic disk are pluripotent.
2. The reverse of twinning is also possible. Since the cells are pluripotent it is possible to join
pluripotent cells together to form a single individual.
A. Experimental work with mice has resulted in the fusion of four developing mice into a
single individual. Therefore this mouse had eight parents!
B. The same phenomenon has also been found in humans (six known cases). Fraternal
twins, one male and one female have fused together to form a single individual.
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