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Third to Eighth Week:
The Embryonic Period
The embryonic period is also known as the period of organonenesis. The three germ layers,
ectoderm, mesoderm, and endoderm develop into terminal tissue types.
Derivatives of the Ectodermal Germ Layer
Soon, after the formation of the prechordal plate and notochord, the ectoderm overlying those
structures is induced to thicken and form the neural plate. Tissue forming the neural plate
(derived from ectoderm) is now call neuroectoderm. Neuroectoderm is involved with the
occurrence called, neurulation. Neurulation is the lateral folding of the neural plate, resulting in
the elevation of each side of the neural plate along a midline neural groove.
The ventral midline of the neural plate, or median hinge point, acts like an anchoring point
about which the two sides of the neural plate become elevated at a sharp angle from the
horizontal. Elevation of the lateral edges of the neural plate, neural folds, is accomplished
largely by factors extrinsic to the neural epithelium lateral to the neural plate pushing up the
sides. Gradually, the neural folds approach each other and fuse, beginning in the cervical region
(days 21 to 22). Fusion proceeds in both a cranial and caudal direction, hence, forming the
neural tube. Until fusion is complete, the ends of the neural tube have open communication
with the amniotic cavity through cranial and caudal neuropores. At the same time, cells of the
neural crest begin to separate from the neural tube. The cranial neuropore closes about day 25
and the caudle neuropore closes a couple days later, about day 27. Closure of the neural tube
results in formation of a narrow spinal cord and a broader area cranially, brain vesicles.
Neurulation
Neural crest cells are the result of an inductive action by non-neural ectoderm on the lateral
cells of the neural plate. Neural crest cells break free from the neural plate or neural tube by
changing their shape and properties from those of typical neuroepithelial cells to those of
mesenchymal cells. Mesenchymal cells are able to migrate as individual cells if provided with
the proper extracellular environment, and then, differentiate into diverse tissue types. Most
mesenchymal cells are from the mesodermal layer, forming primitive connective tissue, muscle,
bone, and blood vessels. Also, some ectoderm in the head can form mesenchymal cells.
Crest cells that migrate first have the potential to differentiate into several different types
cells. Crest cells that migrate later are capable of forming only derivatives of more dorsal
locations (spinal ganglia) but not sympathetic neurons or adrenal medulla. Those that leave the
neural tube last can only form pigment cells. Many crest cells from the trunk differentiate into
sympathetic neurons that produce norepinephrine as the transmitter. In the cranial region, crest
cells give rise to parasympathetic neurons which produce acetylcholine. Crest cells from
different areas cannot develop into all possible tissue types. Although it is normal for cranial
neural crest to form skeletal or cartilage tissue in the head, trunk crest cells transferred into the
head will not form these tissues.
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Neural crest derivatives – Derivatives of crest cells are from either trunk (T) or cranial /
circumpharyngeal (C) cells or both (TC)
Connective tissue and bones of face and skull (C)
Cranial ganglia (C)
C-cells of thyroid gland (C)
Conotruncal septum in the heart (C)
Odontoblasts (C)
Dermis in face and neck (C)
Spinal (dorsal) ganglia (T)
Sympathetic chain and preaortic ganglia (T)
Parasympathetic ganglia of gastrointestinal tract (T)
Adrenal medulla (T)
Schwann cells (T, C)
Glial cells (T)
Arachnoid and pia mater (leptomeninges) (C)
Melanocytes (T, C)
By the time the neural tube is closed, two bilateral ectodermal thickenings will form: otic
placodes and the lens placodes. The otic placodes will invaginate and form otic vesicles that
will differentiate into hearing and equilibrium structures. The lens placodes will invaginate and
form the lenses of the eyes.
Derivatives of the Mesodermal Germ Layers
By day 17, the mesodermal layer of cells, close to the midline, proliferate and form the paraxial
mesoderm. More lateral to that is a thinner layer of mesoderm, the lateral plate. Part of the
mesoderm of the lateral plate will merge with mesoderm covering the amnion (somatic or
parietal mesoderm) and another will blend with mesoderm covering the yolk sac (splanchnic
or visceral mesoderm). With folding of the embryo, these layers will eventually fuse and form
a new cavity, the intraembryonic cavity. Of course, before fusion, there is a short period of
time where the intraembryonic cavity is continuous with the extraembryonic cavity. Intervening
between the paraxial mesoderm and lateral mesoderm is the intermediate mesoderm. It will be
involved with formation of the genitourinary tract.
Paraxial mesoderm – By the beginning of the third week, paraxial mesoderm is arranged into
segments, somitomeres. Somitomeres form from cephalad to caudad. In the head region they
are associated with subdivisions of developing brain segments called neuromeres. Not until
almost twenty pairs of somitomeres have formed and the primitive node has regressed quite far
caudally does the first pair of somites form behind the seventh pair of somitomeres. The first
seven pairs of somitomeres in the cranial region do not undergo further separation or
segmentation. The first pair of somites forms at the expense of the eighth pair of somitomeres.
There is a constant relationship between the most caudal pair of definitive somites and the
number of somitomeres (usually 10 – 11) that can be demonstrated behind them. Every few
hours the pair of somitomeres located caudal to the last-formed somites becomes transformed
into a new pair of somites. Concomitantly, a new pair of somitomeres is laid down at the caudal
end of the paraxial mesoderm near the primitive node, keeping the number of somitomeres
constant.
From about day 20 to the end of the fifth week, pairs of somites appear sequentially until 42
to 44 pairs are made. There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 –
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10 coccygeal pairs. The first occipital and last five to seven somites later disappear while the
remaining forms the axial skeleton.
While in the earliest stages of its formation, a somite also undergoes an internal subdivision
into anterior and posterior halves. Blocks of cells in the somite undergo transformation into
epithelial cells (epithelialization). Shortly after the formation of the somite, its ventromedial
wall is subjected to an inductive stimulus in the form of the signaling molecule originating from
the notochord and ventral wall of the neural tube. The result is the formation of the sclerotome
which will form the vertebral column and ribs.
Under the influence of products secreted by the neural tube, the dorsal half of the somite
becomes transformed into the dermomyotome, expressing its own characteristic genes.
Mesenchymal cells arising from the dorsomedial end of the dermomyotome forms a medial lip.
The medial lip spreads out under the dermomyotome to produce a separate muscle-forming
layer, the myotome. The lateral portion of the dermomyotome is now called the dermatome.
As their names imply, cells of the myotome produce muscle and cells of the dermatome
contribute to dermis.
Intermediate mesoderm – Intermediate mesoderm temporarily connects paraxial to lateral
mesoderm. It will form the transitory early kidney (pronephros) in the cranial region. Also, it
will make the pronephric ducts, which are important in the development of the adult urogenital
system.
Lateral plate mesoderm – The lateral mesoderm soon divides into two layers as the result of
the formation and coalescence of coelomic (body cavity) spaces within it. The dorsal layer,
which is closely associated with the ectoderm, is called somatic mesoderm. The combination of
somatic mesoderm and ectoderm is called somatopleure. Mesoderm in the somatopleure is the
parietal layer (outside lateral and ventral body wall). The ventral layer of lateral mesoderm,
called splanchnic mesoderm, is closely associated with endoderm. The combination of
splanchnic mesoderm and endoderm is called splanchnopleure. This splanchnic mesoderm is
the visceral mesoderm that will form the wall of the gut. Lateral folding of the embryo results in
formation of an intraembryonic coelom. Initially the intraembryonic coelom is continuous with
the extraembryonic (chorionic cavity) coelom, but as folding continues, the two coelomic
cavities are separated.
Mesoderm of the parietal layer of the intraembryonic cavity will form mesothelial
membranes, or serous membranes. This will line the peritoneal, pleural, and pericardial
cavities. Mesodermal cells of the visceral layer will form serous membranes adjacent to organs.
Blood vessels – Stimulated by an inductive interaction with the endoderm of the yolk sac, many
small blood islands, consisting of hemangioblasts, appear in the extraembryonic splanchnic
mesoderm of the yolk sac. Once blood islands form, the central cells become blood-forming
cells (hemocytoblasts or hematopoietic stem cells), whereas those on the outside acquire the
characteristics of endothelial lining cells or angioblasts.
Derivatives of the Endodermal Germ Layer
The gastrointestinal tract is the main organ system derived from the endodermal germ layer. As
soon as it forms during gastrulation, the endoderm receives information that determines anterior
or posterior characteristics of the appropriate regions. The intraembryonic endoderm constitutes
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the roof of the roughly spherical yolk sac. Expansion of the neural plate results in the formation
of the head fold and tail fold. This process, along with lateral folding, results in the formation
of the tubular foregut and hindgut. The sequence is likened to a purse string constricting the
ventral region of the embryo, although the actual mechanism is related to overall growth of the
embryo. The region of the "purse string" becomes the yolk stalk (omphalomesenteric or
vitelline duct). The portion of the gut that still opens into the yolk sac is called the midgut. The
portions of the tubular gut that are anterior and posterior to the midgut are the foregut and
hindgut respectively.
The anterior end of the foregut remains temporarily sealed off by an ectodermal-endodermal
bilayer called the oropharyngeal membrane. This membrane separates the future mouth
(stomodeum), which is lined by ectoderm, from the endodermally lined anterior portion of the
foregut, the pharynx. Without intervening mesoderm, the oropharyngeal membrane eventually
breaks down.
As the earliest signs of the tail are taking shape, a tubular evagination of the hindgut extends
into the mesoderm of the body stalk. This evagination is called the allantois. In most mammals
and birds the allantois represents a major structural adaptation for the exchange of gasses and
removal of urinary wastes. Because of the efficiency of the placenta the allantois never becomes
a prominent structure in the human embryo. By the end of the fifth week, the distal portion of
the allantois, yolk sac duct (vitelline duct), and the umbilical vessels are restricted to the region
of the umbilical ring (the oval line of reflection between the amnion and embryonic ectoderm
[amnio-ectodermal junction]).
Caudal to the allantois is another ectodermal-endodermal bilayer structure called the cloacal
plate, or proctodeal membrane. This membrane, which ultimately breaks down, covers the
cloaca. The cloaca represents a common outlet for both the digestive and urogenital systems in
the early embryo. The shallow depression outside the proctodeal membrane is called the
proctodeum.
The endodermal germ layer initially forms the epithelial lining of the primitive gut and
portions of the allantois and vitelline duct. Later the endoderm gives rise to the (1) epithelial
lining of the respiratory tract, (2) parenchyma of the thyroid, parathyroid, parathyroids, liver,
and pancreas, (3) reticular stroma of the tonsils and thymus, (4) epithelial lining of the urinary
bladder and urethra, and (5) epithelial lining of the tympanic cavity and auditory tube.
External Appearance During the Second Month
There is an increase in head size and formation of limbs, face, ears, nose, and eyes. By the
beginning of the fifth week, forelimbs and hindlimbs appear as paddle-shaped buds. The
hindlimbs appear slightly later than the forelimbs. With further development, grooves (rays)
form at the distal ends of the buds. The rays will form future digits and those of the forelimb are
slightly more advanced. Apoptosis (programmed cell death) is employed for digit development.
Later, it will be understood that joint formation occurs by the splitting of precartilagenous rods
rather than by forming new cartilagenous skeletal models in sequence.
Birth Defects
Most major organs and their systems are formed during the third to eighth week. This period is
called the period of organogenesis. Thus, this period is when most gross structural birth defects
are induced. There may be inescapable genetic defects, but the mother should avoid harmful
influences such as cigarette smoking and alcohol, particularly at this time.
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Questions
1.
The neural plate develops from the ____________.
a. the hypoblast
b. epiblast
c. cytotrophoblast
d. yolk sac
2.
Elevation of the lateral edges of the neural plate will immediately form ____________.
a. the neural tube
b. the posterior neuropore
c. neural crest cells
d. neural folds
3.
Which of the following is not a derivative of ectoderm?
a. anterior neuropore
b. neural tube
c. gut tube
d. neural crest cells
4.
Neural crest cells may give rise to many tissue types. Which of the following is not one
of them
a. enamel of teeth
b. dorsal root (sensory) ganglia
c. melanocytes
d. autonomic postganglionic nerve cells
5.
DiGeorge's syndrome and ___________ are the result of defective neural crest cell
migration.
a. persistent truncus arteriosus
b. sacrococcygeal teratoma
c. sirenomelia
d. situs inversus
6.
Lateral plate mesoderm associated with the yolk sac is called, _____________.
a. somatic mesoderm
b. extraembryonic mesoderm
c. visceral mesoderm
d. amniotic mesoderm
7.
Somites are formed at the expense of __________ which are segmentations of
the _____________ mesoderm.
a. neuromeres / paraxial
b. neuromeres / lateral plate
c. somitomeres / lateral plate
d. somitomeres / paraxial
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8.
A somite will undergo further development with its ventromedial part partitioning off
into a ___________.
a. dermomyotome
b. sclerotome
c. myotome
d. dermatome
9.
In the yolk sac, angioblasts are derived from ______________.
a. splanchnic mesoderm
b. endoderm
c. somatic mesoderm
d. ectoderm
10.
The midgut communicates with the yolk sac through the ___________.
a. allantois
b. vitelline duct
c. oropharyngeal membrane
d. cloacal plate
11. Parenchyma of the digestive-associated glands, gut lining, and respiratory system
epithelium are derived from __________.
a. the notochord
b. mesoderm
c. endoderm
d. ectoderm
12.
The time of gestation from 3 to 8 weeks is called __________ or period of organogenesis.
a. the embryonic period
b. the fetal period
c. bilaminar period
d. trilaminar period