Download A zygote undergoes rapid cell divisions (cleavage)

A zygote undergoes rapid cell divisions (cleavage) to form a spherical
ball of cells: the blastula; this will further develop into a blastocyst.
LEARNING OBJECTIVE [ edit ]
Describe the events that occur from the formation of a zygote to gastrulation
KEY POINTS [ edit ]
A single­celled zygote will undergo multiple rounds of cleavage, or cell division, in order to
produced a ball of cells, called a blastula, with a fluid­filled cavity in its center, called a blastocoel.
In animals with little yolk in the egg, the zygote undergoesholoblastic cleavage, in which the entire
zygote is cleaved repeatedly; in animals with a lot of yolk in the egg, the zygote
undergoes meroblastic cleavage, in which only part of the zygote is cleaved.
The blastula eventually organizes itself into two layers: theinner cell mass (which will become the
embryo) and the outer layer or trophoblast (which will become the placenta); the structure is now
called a blastocyst.
During gastrulation, the blastula folds in on itself to form three germ layers, the ectoderm,
the mesoderm, and theendoderm, that will give rise to the internal structures of the organism.
TERMS [ edit ]
gastrulation
the stage of embryo development at which a gastrula is formed from the blastula by the inward
migration of cells
trophoblast
the membrane of cells that forms the wall of a blastocyst during early pregnancy, providing
nutrients to the embryo and later developing into part of the placenta
inner cell mass
a mass of cells within a primordial embryo that will eventually develop into the distinct form of a
fetus in most eutherian mammals
blastomere
any cell that results from division of a fertilized egg
blastula
a 6­32­celled hollow structure that is formed after a zygote undergoes cell division
meroblastic
undergoing only partial cleavage
holoblastic
cleaving, and separating into separate blastomeres
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Cleavage and Blastula Stage
The development of multi­cellular
organisms begins from a single­celled
zygote, which undergoes rapid cell
division to form the blastula. The rapid,
multiple rounds of cell division are
termed cleavage. After the cleavage has
produced over 100 cells, the embryo is
called a blastula. The blastula is usually a
spherical layer of cells (the blastoderm)
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surrounding a fluid­filled or yolk­filled cavity (the blastocoel). Mammals at this stage form a
structure called the blastocyst, characterized by an inner cell mass that is distinct from the
surrounding blastula. During cleavage, the cells divide without an increase in mass; that is,
one large single­celled zygote divides into multiple smaller cells. Each cell within the blastula
is called a blastomere.
Cleavage can take place in two ways: holoblastic (total) cleavage or meroblastic (partial)
cleavage. The type of cleavage depends on the amount of yolk in the eggs.
Inplacental mammals (including humans) where nourishment is provided by the mother's
body, the eggs have a very small amount of yolk and undergo holoblastic cleavage.
Otherspecies, such as birds, with a lot of yolk in the egg to nourish the embryo during
development, undergo meroblastic cleavage.
In mammals, the blastula forms the blastocyst in the next stage of development. Here the
cells in the blastula arrange themselves in two layers: the inner cell mass and an outer layer
called the trophoblast . The inner cell mass is also known as the embryoblast; this mass of
cells will go on to form the embryo. At this stage of development, the inner cell mass consists
of embryonic stem cells that will differentiateinto the different cell types needed by the
organism. The trophoblast will contribute to the placenta and nourish the embryo.
Formation of the blastocyst
The rearrangement of the cells in the mammalian blastula to two layers, the inner cell mass and the
trophoblast, results in the formation of the blastocyst.
Gastrulation
The typical blastula is a ball of cells. The next stage in embryonic development is the
formation of the body plan. The cells in the blastula rearrange themselves spatially to form
three layers of cells in a process known as gastrulation. During gastrulation, the blastula
folds upon itself to form the three layers of cells. Each of these layers is called a germ layer,
which differentiate into different organ systems .
Differentiation of germ layers
The three germ layers give rise to different cell types in the animal body: the ectoderm forms the nervous
system and the outer layer of skin, the mesoderm gives rise to muscles and connective tissues, and the
endoderm gives rise to the lining of the digestive system and other internal organs.
The three germs layers are the endoderm, the ectoderm, and the mesoderm. The ectoderm
gives rise to the nervous systemand the epidermis; the mesoderm gives rise to the muscle
cells and connective tissue in the body; and the endoderm gives rise to columnar cells found
in the digestive system and many internal organs.