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Third Week of Development:
Trilaminar Germ Disc
Gastrulation – The formation of all three germ layers: ectoderm, mesoderm, and
endoderm. Gastrulation begins with the primitive streak, which is not clearly seen till
the 15 or 16-day embryo. The primitive streak is a midline condensation of cells derived
from the epiblast. With the appearance of the primitive streak, the anteroposterior
(craniocaudal) and left-right axes can be readily identified. As cells of the epiblast reach
the primitive streak, they change shape and pass through it on their way to forming a new
cell layer beneath (ventral to) the epiblast. The movement of the cells through the
primitive streak results in the formation of a primitive groove. At the anterior end of the
primitive streak is a small, but well-defined accumulation of cells, called the primitive
node or Henson's node. The node is an elevated area of cells that surrounds a primitive
pit. The node is of developmental significance because it is the area through which
migrating cells are channeled into a rod-like mass of mesenchymal cells called the
notochord and a group of cells anterior to that, the prechordal plate.
Cells in the epiblast are associated with the basal lamina underlying the epiblast, but,
as they enter the primitive streak they lose their basal lamina and take on morphological
characteristics giving them the name, bottle cells. Bottle cells have characteristics
mesenchymal cells. After the primitive streak is well established, the majority of cells
spread out between the epiblast and hypoblast to form embryonic (intraembryonic)
mesoderm.
By the time the mesoderm has formed a discrete layer in the embryo, the upper layer
(remains of the former epiblast) is called the ectoderm, and the lower germ layer, which
has displaced the original hypoblast, is called the endoderm. Mesodermal cells spread
out beyond the margin of the germ disc and make contact with extraembryonic mesoderm
covering the yolk sac and amnion. Also, mesodermal cells migrating from the primitive
streak pass around the prechordal plate.
After its initial appearance at the extreme caudal end of the embryo, the primitive
streak expands cranially until about 18 days post fertilization. Thereafter, it regresses
caudally, stringing out the notochord in its wake. In other words, the primitive node
seems to ratchet caudally through the primitive streak, hence the streak gets smaller until
it disappears. If the primitive streak remains, a teratoma will form in the sacrococcygeal
region.
Cranial to the notochord is a small region where embryonic ectoderm and endoderm
abut without any intervening mesoderm. This is the oropharyngeal (buccopharyngeal)
membrane: the future oral cavity.
A prechordal plate forms between the tip of the notochord and the buccopharyngeal
membrane. This is a small aggregation of mesodermal cells closely apposed to the
endoderm and was some of the first cells to pass through the primitive node. The
prechordal plate is instrumental in formation of the forebrain.
Formation of the Notochord
As the primitive streak regresses, the cellular precursors (prenotochordal cells) of first
the prechordal plate and then of the notochord migrate rostrally from the node, but then
are then left behind as a rod-like aggregation of cells, the notochordal process or plate.
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Shortly, the cells of the notochordal plate temporarily spread out and fuse with the
embryonic endoderm (from streak cells that replaced the hypoblast), resulting in the
formation of a transitory neurenteric canal that connects the emerging amniotic cavity
with the yolk sac. This canal is at where the primitive pit forms an indentation in the
epiblast. The neurenteric canal's function is obscure. Later, cells of the notochord
separate from the endodermal roof of the yolk sac and form the definitive notochord.
The cloacal membrane is formed at the caudal end of the embryonic disc and like the
buccopharyngeal membrane consists of adjoining ectoderm and endoderm. When the
cloacal membrane appears, the posterior wall of the yolk sac forms a small diverticulum,
the allantois. In some lower animals it is used for excretory products: in humans, it is
not.
PRECHORDAL PLATE AND NOTOCHORD
The first cells passing through the primitive node form a discrete midline mass of cells,
the prechordal plate, which is closely associated with endoderm in the region just caudal
to the oropharyngeal membrane.
The notochord is a major axial signaling center of the trunk of the early embryo and it
is important in the formation of many axial structures including the spinal cord.
The prechordal plate, sometimes known as the head organizer, consists of early
mesodermal cells passing through the primitive node. These cells are, structurally and
functionally, closely related with cells of the underlying anterior endoderm.
Molecular Basis for left-Right Asymmetry
Up to the time of gastrulation the embryo is bilaterally symmetrical, but at that time
mechanisms are set in place that ultimately result in the right-sided looping of the heart,
followed by asymmetrical looping of the gut and asymmetrical positioning of the liver,
spleen, and location of the lungs. In mammalian embryos, the earliest known signs of
asymmetry involve the movement of cilia around the primitive node.
In roughly one in 10,000 persons the left-right symmetry of the body is totally
reversed, a condition called situs inversus. This condition is often not recognized until
the individual is examined by a physician. Kartagener's syndrome, the situs inversus is
associated with respiratory symptoms due to abnormalities of the dynein arms in cilia
(immotile cilia). Partial situs inversus, such as right-sided heart (dextrocardia) can also
occur.
Growth of the Embryonic Disc
The embryonic disc is initially flat and round, but becomes elongated and broad,
especially in the cephalic region. Invagination and migration of surface cells in the
primitive streak continues until the end of the fourth week. At that stage, the primitive
streak shows regressive changes, rapidly shrinks, and soon disappears.
Further Development of the Trophoblast
By the beginning of third week, the trophoblast is characterized by primary villi that
consist of a cytotrophoblastic core covered by a layer of syncytial cells. Later,
mesodermal cells penetrate the core of the primary villi and grow toward the deciduas.
These are known as secondary villi.
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By the end of the third week, mesodermal cells in the core of the villus begin to
differentiate into blood cells and small blood vessels. The villus is now a tertiary villus
or definitive villus. Capillaries in tertiary villi make contact with capillaries in the
connecting stalk and chorionic plate, both formed by extraembryonic (somatopleuric)
mesoderm. See Lecture 3. The heart starts to beat and the intraembryonic circulation is
becoming established.
Cytotrophoblastic cells penetrate the syncytium until they reach the maternal
endometrium. When they meet the endometrium, columns of cytotrophoblast fan out and
establish contact with each other and form an outer cytotrophoblastic shell. The inner
cytotrophoblastic shell is the original cytotrophoblast with extraembryonic mesoderm
lining its inside. Anchoring or stem villi extend from the "inner" cytotrophoblastic shell
(chorionic plate) to the outer cytotrophoblastic shell abutting the endometrium. The
placenta consists of two components: (1) a fetal portion, derived from the chorion
frondosum or villous chorion, and (2) a maternal portion, derived from the decidua
basalis (functional layer of endometrium shed at parturition). Villi that hang free from
the stem villi are called free or terminal villi.
Teratogenesis Associated with Gastrulation
Holoprosencephaly - Holoprosencephaly includes a broad spectrum of defects, all based
on defective formation of the forebrain (prosencephalon) and structures whose normal
formation depends on influences of the forebrain. Defects can include altered facial
structures with, mental retardation, heart defects, and postnatal growth. Fetal alcohol
syndrome, from ingesting as little as 3 ounces of alcohol per day during the first 4 weeks
of pregnancy, can result in holoprosencephaly.
Sirenomelia (caudal dysgenesis) is caused by too little mesoderm formed in the caudalmost region of the embryo.
Tumors Associated with Gastrulation
Sacrococcygeal teratomas - If remnants of the primitive streak persist in the coccygeal
region, pleuripotent cells may proliferate and form these tumors. These contain a bizarre
mixture of fat, cartilage, muscle, hair, and glandular tissue.
Questions
1. Gastrulation begins with formation of ___________.
a. Henson's node
b. the notochord
c. primitive streak
d. placenta
2. From which of the following are all of the embryonic germ layers derived?
a. epiblast
b. hypoblast
c. amnion cells
d. trophoblast
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3. The notochord and prechordal plate develop from cells that passed through
the________.
a. primitive streak
b. allantois
c. primitive node
d. buccopharyngeal membrane
4. Which of the following structures have ectoderm and endoderm directly apposed to
each
other?
a. prechordal plate
b. notochord
c. yolk sac
d. cloacal membrane
5. The neurenteric canal connects the amniotic cavity with the _________.
a. chorionic cavity
b. lacunae
c. yolk sac
d. exocoelomic cyst
6. The _____________ is instrumental in development of the forebrain.
a. prechordal plate
b. notochord
c. buccopharyngeal membrane
d. allantois
7. Situs inversus, reversed left-right asymmetry, is due to abnormalities in _________.
a. blood flow
b. lefty-1
c. dynein
d. fate map
8. When __________ cells begin to penetrate primary villi, secondary villi are formed.
a. cytotrophoblastic
b. syncytiotrophoblastic
c. endometrial
d. mesodermal
9. Villi that extend from the inner (original) cytotrophoblast to the outer cytotrophoblast
are called, __________ villi.
a. primary villi
b. anchoring villi
c. tertiary villi
d. free villi
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10. If there is too little mesoderm formed in the caudal part of the embryo,
___________
will be the result.
a. holoprosencephaly
b. sirenomelia
c. sacrococcygeal teratoma
d. immotile cilia
11. A teratoma will form if the ___________does not disappear.
a. primitive node
b. prechordal plate
c. buccopharyngeal membrane
d. primitive streak