Formation of germ layers
FERTILIZATION AND STEM
CELLS
• The oocyte (female gamete) is released
from the ovary and then "pulled" into the
ampulla of the uterine tubes by its fingerlike projections, called fimbriae.
• Sperm (male gametes) travel through the
vagina, into the uterus, and continue into
the uterine tubes where fertilization of the
oocyte occurs.
• Fertilization most often takes place in
the ampulla of the uterine tube, which
is the widest area of the tube.
• Both gametes are haploid, carrying
only half the number of chromosomes
as a normal diploid cell.
• Fertilization restores the cell to a
diploid state and denotes the
formation of the zygote.
• The zygote moves through the
uterine tube toward the uterus.
• At this point, the zygote is a stem
cell that will give rise to daughter
cells with the maximum capacity for
differentiation.
• It is considered "totipotent" meaning
that this cell has the capacity to give
rise to cells that can become any type
of embryological tissue.
• This totipotent cell will
differentiate into every type of
fetal tissue as well as the support
structures for the fetus (such as
the placenta.)
• As development progresses, cells
become more and more
specialized, and simultaneously
lose their potentiality.
• III. BLASTOCYST AND IMPLANTATION
• It takes about 4 days for the zygote to
reach the uterus.
• As the zygote travels through the uterine
tubes, the original singe cell divides into a
two, four, eight, and sixteen cell unit
without increasing in overall size or mass.
• The sixteen cell unit is approximately the
stage at which the embryo enters the
uterus.
• The cells reach the first branch
point in differentiation here.
• The outer cells form a ring that
surrounds the inner cells, which
are known as the inner cell mass.
• The outer cell mass becomes the
trophoblast (green cells in
Figures), which will contribute to
the placenta.
• The inner cells form the embryoblast,
which will give rise to the embryo
(blue cells in Figures.)
• A cavity (or "cyst") begins to form
between the embryoblast and the
trophoblast and the embryo is now
considered a blastocyst.
• The blastocyst will penetrate the wall
of the uterus and implant into the
uterine mucosa.
• Cells of the embryoblast now
differentiate into two distinct layers.
• The hypoblast (yellow cells in
Figures) layer is most inward and the
• epiblast (blue cells in Figure) layer
lies adjacent to the trophoblast.
• The epiblast (blue) begins to form a
second space called the amniotic
cavity.
• Simultaneously, the hypoblast forms the
primitive yolk sac followed by the
secondary yolk sac which will
degenerate when fetal circulation is
established.
• The trophoblast continues to invade the
uterine wall and establish the foundation
for a placenta.
• The uterus responds to this invasion by
altering the vasculature in the uterine
tissue adjacent to trophoblast in order to
support the growing embryo.
IV. FORMATION OF THE THREE GERM
LAYERS
• The next major stage in the development
of the part of the blastocyst that will
become the fetus is the formation of three
distinctive layers that will go on to form
identifiable organ systems.
• In one half of the epiblast, a primitive
streak appears; this event determines the
orientation of the developing tissue.
• The end with the primitive streak will
become the head (cephalic) and the other
will become the rump (caudal).
• Along this streak, the cells begin to
proliferate and invaginate, folding inward
between the two layers.
• The resulting mass of tissue forms two
new cell layers between the epiblast and
hypoblast.
• The first inward moving cells that displace
the hypoblast make the endoderm.
• The next cells to move inward form the
mesoderm.
• The remaining epiblast cells are now
termed the ectoderm layer.
• These three germ layers will contribute to
all of the cell types in the body.
• The endoderm gives rise to the lining of
the gut, lungs and bladder.
• The mesoderm becomes muscles, bones,
blood cells, spleen, lymphatic tissues,
heart, lungs, reproductive and excretory
systems.
• Derivatives of the ectoderm include
the skin, nails, hair, mouth, anus, and
nervous system.
• All three layers give rise to different
types of connective tissue.
• Undifferentiated connective tissue is
called mesenchyme regardless of
the germ layer it is derived from.
• The cells in these three germ layers are
still stem cells because, without losing
their own generative capacity, they can
produce daughter cells that can
differentiate into multiple cell types.
• However, because they have made an
irreversible step towards a particular
lineage of cell types, they are "pluripotent"
(capable of giving rise to multiple cell
types) instead of being "totipotent"
(capable of giving rise to all cell types.)
NOTOCHORD AND NEURAL
TUBE FORMATION
• At the most medial part of the primitive
streak cells begin to proliferate in a linear
fashion and migrate toward the cephalic
end.
• They begin to form a chord of cells know
as the notochord. The notochord serves
as a guide for the axial skeleton.
• The ectoderm cells that lie directly
over the top of the notochord push
up from either side of the notochord
and form a layer that curl toward itself
until the cells meet in the middle,
where they fuse.
• The resulting cavity lined by these
cells is called the neural tube. The
neural tube eventually seals
completely to encase the brain and
spinal cord.
• As the edges of the ectoderm layer curl
upwards to form the neural tube, another
cell type forms at the tip of the curl. These
cells differentiate into neural crest cells.
• Neural crest cells eventually give rise to
the dorsal root ganglia, Schwann cells, the
autonomic nervous system, meninges,
sensory ganglia, bones of the face, teeth,
lens of the eyes, melanocytes, adrenal
medulla, and many glands. (The neural
crest cells are sometimes referred to as
the fourth germ layer.)
• Along the notochord, mesoderm
cells start to form segmented
columns that will become
somites.
• The each somite forms a specific
segment of muscle, bone, and
connective tissue.
• One of the most notable
structures formed from somites is
the spinal vertebrae.
• During this period, as the
notochord, neural tube, and
somites are forming, the
embryo develops a left and
right side.
• Some tissues and organs are
paired and form symmetrically
while others form unilaterally.
• They act as chemoattractants, growth
factors, and growth inhibitors.
• Signaling molecules guide the
migration of tissues to certain places,
such as the abdomen, in the embryo.
• They also guide the orientation
indicating the top, bottom, left and
right side of the organ.