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Doug Peterson
Content Narrative #3
Embryonic Development
Fertilization
Although it is crucial in the process of embryonic development, we are going to skip over
gametogenesis. It is essential, but it would add another twenty minutes to my presentation, so for
the sake of brevity we are going to skip over it and proceed directly to fertilization.
Fertilization is the combination of haploid sets of chromosomes from two individual
organisms (in our case animals) into a single diploid cell, called a zygote. Fertilization also,
functions in activating the egg, which initiates metabolic reactions that trigger the onset of
embryonic development.
In mammals, secretions in the female reproductive tract alter molecules on the surface of
sperm cells and increase its motility. The egg is surrounded by follicle cells, released during
ovulation, which the sperm must travel through before reaching the extracellular matrix of the
egg (zona pellucida). The zona pellucida acts as a sperm receptor, and binds to a complementary
molecule in the sperm head. This ensures that the sperm is off the same species. Binding of the
sperm to the egg induces the acrosomal reaction. In this process, the sperm releases hydrolytic
enzymes that penetrate the zona pellucida and reach the plasma membrane of the egg. The
release of these enzymes exposes a protein in the sperm membrane that binds with the egg’s
plasma membrane.
The binding of sperm and egg triggers changes in the egg which leads to a cortical
reaction, in which cortical granules are released to the outside of the cell via exocytosis. The
cortical granules catalyze the alteration of the zona pellucida that prevents the entry of additional
sperm.
After the sperm and egg membranes fuse, the entire sperm is taken into the egg. The egg
lacks a centrosome, but a centrosome forms around the centriole that was the basal body of the
sperm’s flagellum. The centrosome then duplicates to form two centrosomes, which will
generate the mitotic spindle in the first mitotic division of the zygote. The haploid nuclei from
the sperm and egg do not fuse, but their nuclear envelopes disperse. The two sets of
chromosomes will share a spindle apparatus during the first division of the zygote, so the
chromosomes will not come together in the same nucleus until after the first division.
Cleavage
Once fertilization is complete, a succession of rapid cell divisions occurs. This period is
called cleavage. The cells undergo S and M phase of the cell cycle, but virtually skip G1 and G2,
which results in the production of a very small amount of protein. The embryo does not grow
because cleavage simple partitions the cytoplasm of the large zygote cell into smaller cells called
blastomeres, each of which have their own nucleus.
The first five to seven divisions form a dense ball of cells known as the morula. As cell
division continues, liquid fills the morula and pushes the cells out to form a circular cavity
surrounded by a single layer of cells. The hollow sphere is known as the blastula and the cavity
is the blastocoel.
With the exception of mammals, zygotes of other animals have a polarity. This polarity
causes the divisions of the zygote to have a specific pattern. The polarity is defined by the
uneven distribution of substances in the cytoplasm (mRNA, proteins, and yolk). In many
animals, the distribution of yolk is the key factor that influences the pattern of cleavage. Yolk is
more concentrated towards one pole known as the vegetal pole, the opposite pole is known as the
animal pole.
In some organisms, early cleavages are polar, dividing the egg into segments that stretch
from pole to pole. Other cleavages are parallel with the equator. In deuterostomes, early
cleavages are radial, forming cells at the animal and vegetal poles that are aligned together, the
top cells directly above the bottom cells. In protostomes, cleavages are spiral, forming cells on
top that are shifted with respect to those below them. Radial cleavages are usually indeterminate,
producing blastomeres that can independently complete normal development. Spiral cleavages
are often determinate; producing blastomeres that cannot develop into a complete embryo if
separated from other blastomeres.
Gastrulation
Gastrulation is a dramatic rearrangement of the cells of the blastula to form a threelayered embryo with a primitive gut. In all species, gastrulation occurs when some cells at or
near the surface of the blastula invaginates, establishing three germ layers. The three-layered
embryo is called the gastrula. The ectoderm forms the outer layer; the endoderm lines the
embryonic digestive tract; and the mesoderm partly fills the space between the two. The
positioning of these layers allows cells to interact with each other in new ways. Eventually these
three cell layers develop into all the tissues and organs of the adult animal.
The center cavity formed by gastrulation is called the archenteron and is surrounded by
the endoderm cells. The opening into the archenteron is the blastopore. This will become the
mouth in protostomes or the anus in deuterostomes.
Extraembryoinic Membrane
In the amniotes (birds, reptiles, and humans) extraembryonic membrane develops which
allows the embryo to survive in its developmental environment. The chorion is the outer
membrane, which acts in gas exchange in birds and reptiles. The chorion implants into the
endometrium in mammals, and eventually forms the placenta (which allows for gas, nutrient, and
waste exchange). From the archenteron, the allontois buds off. This eventually encircles the
embryo forming a layer below the chorion. In animals and reptiles, it initially stores waste, but
later fuses with the chorion and acts as a membrane for gas exchange with blood vessels below.
In mammals, the allantois transports waste to the placenta, and eventually becomes the umbilical
cord and transports gasses, nutrients, and wastes between the embryo and placenta. Enclosing the
amniotic cavity is the amnion, a fluid filled cavity that cushions the embryo. In birds and reptiles,
the yolk sac membrane digests the enclosed yolk and blood vessels transfer nutrients to the
embryo. In mammals, the yolk sac is empty and nutrition is obtained through the placenta.
Organogenesis
During the process or organogenesis, the three germ layers develop into the rudiments of
organs. Cell differentiation is responsible for the cells taking on the characteristics of specific
tissues and organs. The first organs that begin to take form are the neural tube and the notochord.
The notochord is formed from the dorsal surface of the mesoderm that condenses above the
archenteron. The notochord is a still rod that provides support in lower chordates and the
vertebrae of higher chordates.
Signals sent from the notochord to the ectoderm above it cause that region to become the
neural plate. The neural plate curves inward, rolling itself into the neural tube, which runs along
the anterior-posterior axis of the embryo. The neural tube will later become the central nervous
system.
Along the border where the neural tube pinches off from the ectoderm, is a band of cells
called the neural crest. Cells from the neural crest migrate to various parts of the embryo,
forming nerves, teeth, skull bones, and many other different cell types.
As organogenesis progresses, morphogenesis and cell differentiation refines the organs
that arise from the germ layers.
Presentation
Similar to my mitosis/ meiosis presentation, I will post a set of PowerPoint slides with
mostly pictures and maybe a few notes. The majority of the notes will be given in a hand out that
my peers will fill out as I explain development. I will post pictures that depict the major
components of development (fertilization, cleavage, gastrulation, and organogenesis).
References
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Campbell, N. A., & Reece, J. B. (2005). Biology. (7th ed.). Benjamin-Cummings Pub Co.
Pack, P. E. (2008). Cliffsap biology. Cliffs Notes.