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Prelab Reading
Early Development in Animals
INTRODUCTION
e specifics of development differ for different species, but there are general patterns or
processes that all vertebrates undergo. e processes involve mitosis or cellular division,
differentiation or specialization of cells or tissues for a particular function, and morphogenesis,
the development of the animal’s shape, or body form, and organization. An animal’s embryonic
development can be divided into four major stages: (1) fertilization, when the sperm penetrates
the egg and syngamy or fusion of the egg and sperm nuclei occurs; (2) cleavage, the repeated
process of mitosis that divides the fertilized egg or zygote into many smaller cells; (3)
gastrulation, a process of inward growth and cellular movement resulting in three layers of
embryonic tissues and a tube-within-a-tube body plan; (4) organogenesis, the process by which
organs develop from the three embryonic layers of tissue.
FERTILIZATION
Development begins as sperm and egg prepare for fertilization. Sperm develop a flagellum, which
is used to propel the sperm towards the egg. e egg builds up energy and nutrient reserves called
yolk, which is primarily composed of protein and fat. e yolk can provide support to the early
embryo.
ere are essentially three stages in fertilization. First the sperm finds the egg through
chemotaxis. Egg cells release a specific peptide unique to that species’ eggs, essentially directing
the sperm to swim towards the egg. Next sperm activation occurs.
When the sperm contacts the outer layers of the egg,
swimming becomes much faster, but also more forceful
and erratic. In addition, the acrosome, or the outer
structure covering the tip of the head of the
sperm, releases enzymes that help to
breakdown the outer coating of the egg.
is allows for the final step, fusion
of the sperm with the egg. Finally
plasmogamy and syngamy
occur. e plasma membranes
of the sperm and egg join as
plasmogamy begins. is
activates the egg cell so
that a second sperm
cannot fertilize the egg.
As the cytoplasm of the
sperm and egg join
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during plasmogamy, the nucleus of the sperm cell enters the cytoplasm of the egg cell. Eventually
the haploid nucleus of the sperm cell fuses with the haploid nucleus of the egg cell during
syngamy, resulting in a single diploid cell or zygote.
CLEAVAGE
e next stage for development begins as this single, original cell divides to become two cells,
after which the two become four, the four eight, and so on. Once the zygote starts dividing, we
call it an embryo. As division occurs, the cells of the embryo are repeatedly divided into smaller
cells. e reason the cells get progressively smaller is that none of the usual cell growth occurs
between these cell divisions. e product of this process of cleavage is a morula, a tightly packed
ball or disc of early embryonic cells. is configuration soon changes, however, as the cells arrange
themselves into a blastula: an early embryonic structure composed of one or more layers of cells
surrounding a liquid- filled cavity (the blastocoel). e progression from zygote to blastula is
illustrated in Figure 2.
Figure 2. Stages of cleavage leading to the formation of the blastula.
e early cleavage events are affected by the amount of yolk present in the egg. erefore it is
important to be aware of the type of egg produced by the animal being studied. Classification of
egg type is based on the amount and distribution of yolk. Eggs with small amounts of yolk evenly
distributed through the egg are referred to as isolecithal. Animals with isolecithal eggs include
mammals and certain fish, namely lamprey, sturgeons, bowfins, and gars. Eggs containing a large
amount of yolk concentrated on one end are called telolecithal. e eggs of birds, reptiles,
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amphibians, and most fish are telolecithal. Eggs can also be centrolecithal, meaning yolk found
in the center of the egg, which is common on arthropods.
In telolecithal eggs, the presence of a large amount of yolk at one end of the egg cell with
cytoplasm and nucleus at the other end of the cell gives the zygote polarity. e region containing
the nucleus and cytoplasm is often referred to as the blastodisc, and the end of the zygote
containing the blastodisc is referred to as the animal pole. e end containing the yolk the
referred to as the vegetal pole. As you will see, this polarity helps to define the way the embryo
develops.
Although the end result of cleavage, the formation of the blastula, is essentially the same in all
animals, the patters of cleavage are dictated by the amount of yolk present. If the egg is
isolecithal, the impact of the yolk is minimal. As the zygote first divides, the entire fertilized egg
divides. As cleavage continues, each resulting cells completely divides in two during the next
round of mitosis. is type of cleavage in isolecithal eggs is called holoblastic. In these embryos,
the blastocoel forms in the center of the blastula.
In eggs that are moderately telolecithal, the yolk slows cytokinesis, but cleavage is still usually
holoblastic. Cells at the animal pole divide faster than the cells towards the vegetal pole. is
results in smaller, more numerous cells at the animal pole and the blastocoel develops closer
towards the animal pole in the blastula.
In eggs that are strongly telolecithal, cytokinesis only occurs within the blastodisc and so is
incomplete. is process is called meroblastic, and results in a cap of cells forming at the animal
pole. is cap or blastoderm consists of two layers of cells, between which forms the blastocoel.
Figure 3. Meroblastic cleavage in a telolecithal egg.
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GASTRULATION
Gastrulation is a process that transforms the blastula from a hollow ball or cap of cells into a
gastrula. A gastrula is a ball of cells with a tube passing through and consisting of three layers of
cells. ese embryonic or germ layers are called the ectoderm, mesoderm, and endoderm.
Gastrulation is a process of directed movement of groups of cells to particular places in the
developing embryo. Movements include ingression of cells, or an inward movement of individual
cells, an involution or infolding of cells as surface cells migrate towards the interior of the
embryo, delamination, in which a single sheet of cells splits in two, and epiboly in which the
entire embryo becomes surrounded by ectodermal cells.
e result of this directed movement is a new internal cavity, the archenteron lined with the
innermost layer of embryonic tissue, the endoderm. e endoderm and archenteron will
eventually give rise to the digestive tract and glands such as the liver and pancreas. In some
vertebrates it also gives rise to respiratory structures and some urinary structures. e cells that
remain on the outer surface of the embryo become the ectoderm, which gives rise to skin and
structures of the nervous system. Finally, the mesoderm forms between the ectoderm and
endoderm. In vertebrates, the mesoderm gives rise to the somites, the notochord, and the
mesenchyme, which give rise to the muscles, skeletal, and circulatory systems of the body as well
as the kidneys.
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Figure 5. Tissues derived from the three germ layers.
ORGANOGENESIS
In vertebrates, an interaction of two of the three germ layers marks the first steps on the path to
the development of organs. Organogenesis begins shortly after gastrulation with the process of
neurulation, the formation of a hollow, dorsal nerve cord. First, within the mesoderm there is the
development of a rod-shaped support organ found in all embryonic vertebrates, the notochord.
In addition to support, one of the notochords functions is to induce, or bring about, development
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in the ectodermal tissue that lies above it, toward the dorsal surface, or what will become the
animal’s back.
e ectodermal cells dorsal to the notochord elongate to
form a flattened surface called the neural plate. e neural
plate extends from the blastopore, or the initial opening to
the archenteron – what will become the anus, to the
anterior or “head” end of the embryo. e edges of the
neural plate begin to extend upward as the ectoderm
grows, forming neural folds; at the same time, the center
of the neural plate invaginates or sinks inward, forming a
neural groove. Eventually the edges of the neural folds
meet and fuse, forming a hollow neural tube. Developing
from anterior to posterior, this neural tube is a dorsal,
ectodermal structure that gives rise in vertebrates to both
the brain and spinal cord. Equally important are neural
crest cells: cells that break away from the top of the
vertebrate neural tube as it folds together and then
migrate to varying parts of the embryo, giving rise to
various tissues and organs. For example, the medulla or
inner portion of the adrenal glands (which in humans sit
atop the kidneys) is derived from neural crest cells.
As this ectodermal movement is taking place, mesodermal
tissue on both sides of the notochord is developing into
approximately 40 repeating blocks of tissue, called somites.
ese blocks then go on to give rise to several structures
that retain the somite’s repeating nature but that are much
more specialized: the vertebra that make up our spinal
column, the muscles that attach to each of these vertebrae,
and the ribs that extend from the vertebrae. All these
structures develop at least in part from the somites.
Organogenesis continues as cells of the ectoderm and
neural tube develop into the eyes, brain and spinal cord.
Cells of the mesoderm continue to differentiate to form a
heart, and the somites continue to differentiate to form
skeletal muscles.
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