Download Early embryology

CLEAVAGE
The zygote is totipotent.
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It has the potential to produce all the cell types of the new individual.
Cleavage is a series of rapid mitotic divisions. These cells are called blastomeres.
At about 32-cell stage the embryo is a solid ball called morula.
BLASTULATION
Eventually a hollow ball of cells form the blastula with a fluid filled blastocoel.
Cells at these stages are capable of ameboid movement.
Surface proteins allow cells to recognize one another and determine which ones to adhere to form tissues.
Cytoplasm is not homogeneous and helps determine the course of development.
The amount of yolk determines the pattern of cleavage.
Yolk is a mixture of proteins, phospholipids and fats.
Yolk is evenly distributed in the eggs of most invertebrates and simple chordates: isolecithal eggs.
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Entire egg divides producing cell of even size (holoblastic cleavage).
Cleavage may be radial (deuterostomes) or spiral (protostomes).
Many vertebrate eggs have the concentrated at one end of the cell, known as the vegetal pole: telolecithal
eggs. The other pole is more metabolically active and is called the animal pole.
GASTRULATION
Gastrulation is the embryological process by which the blastula becomes a three-layered gastrula.
Zygote  morula  blastula  gastrula.
The three germ layers are the endoderm, mesoderm and ectoderm.
Each layer gives rise to specific structures.
There are different patterns of gastrulation.
In amphioxus and sea star, cells from the blastula wall invaginate and eventually meet the opposite wall.
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The cavity formed is called the archenteron.
This is the developing gut and the opening is called the blastopore.
In amphibians, the yolk-laden cells obstruct invagination.
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Cells from the animal pole move down over the yolk rich cells and invaginate forming the
dorsal lip of the blastopore.
In birds, epiblast cells migrate toward the midline to form a thickened cellular region, known as the
primitive streak.
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There is narrow furrow at its center, the primitive groove.
Cells migrate in from the epiblast, sink down and move out laterally and anteriorly in the
interior. These cells form the notochord.
ORGANOGENESIS
Organogenesis is the process of organ formation.
Brain, notochord and spinal cord are among the first organs to develop
First the notochord, in all chordate embryos, develops as a cylindrical rod of cells on the dorsal side
The developing notochord induces the overlying ectoderm to thicken and form the neural plate.
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Induction: certain cells stimulate or influence the differentiation of neighboring cells.
Cells from the neural plate move downward and form the neural groove flanked by the neural folds.
The ridges of the neural folds increase and eventually meet forming the neural tube.
The neural tube is formed beneath the surface. Its anterior portion will form the brain and the rest will
differentiate into the spinal cord.
The neural crest consists of cells that lie near the neural tube and will differentiate into sensory neurons
(dorsal root ganglia and postganglionic sympathetic neurons.
The digestive track is formed as a separate foregut and hindgut.
These two simple tubes are lined with endoderm.
Outpocketings give rise to the liver, pancreas and trachea.
The trachea as it grows downward gives rise to the lung buds.
A series of small outpocketings of the pharynx are the pharyngeal pouches. There are corresponding
inpocketings from the ectoderm, the branchial grooves.
EXTRAEMBRYONIC MEMBRANES
Terrestrial vertebrates have four extraembryonic membranes: chorion, allantois, yolk sac and amnion.
Chorion and amnion enclose the entire embryo.
The amniotic cavity is the space between the amnion and the embryo. It becomes filled with the amniotic
fluid.
In humans the allantois is nonfunctional except that its blood vessels form part of the umbilical cord.
In reptiles and birds, the allantois stores nitrogenous wastes.
The yolk sac in higher vertebrates contains little or no yolk.
In the early mammalian embryo, the yolk sac absorbs nutrients and oxygen from the mother and then
degenerates.
MAMMALS
Mammals fall into three groups that differ in the way they use the amniotic egg.
Monotremes
These primitive mammals produce a shelled egg like their reptilian ancestors. Only three species exist
today: two species of spiny anteater (echidna) and the duckbill platypus. [View (48K)]
Marsupials
Marsupials do not produce a shelled egg. The egg, which is poorly supplied with yolk, is retained for a
time within the reproductive tract of the mother. The embryo penetrates the wall of the uterus. The yolk
sac provides a rudimentary connection to the mother's blood supply from which it receives food, oxygen,
and other essentials. However, this interface between the tissues of the uterus and the extraembryonic
membranes never becomes elaborately developed, and the young are born in a very immature state.
Despite their tiny size, they are able to crawl into a pouch on the mother's abdomen, attach themselves to
nipples, and drink milk from her mammary glands.
Marsupials are still abundant in Australia, but only the opossum is found in North America.
Placental mammals
In placental mammals, the extraembryonic membranes form a placenta and umbilical cord, which
connect the embryo to the mother's uterus in a more elaborate and efficient way. The blood supply of the
developing fetus is continuous with that of the placenta. The placenta extracts food and oxygen from the
uterus. Carbon dioxide and other wastes (e.g., urea) are transferred to the mother for disposal by her
excretory organs.
Humans are placental mammals.
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