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EMBRYONIC PERIOD
Third to Eighth Weeks / Organogenesis
Neuroectoderm or neural plate is induced by the notochord.
Neurulation is the formation of the neural plate and neural folds to make the neural tube.
Closure of the anterior neuropore is about day 25. Nonclosure results in anencephaly.
The posterior neuropore closes a bit later about day 27.
Anencephaly has a frequency of 0.3 per 1000 in the United States.
It has slight predominance for females.
There is little cerebral tissue: the diencephalon is present, hence the eyes.
FORMATION OF SOMITES IN
PARAXIAL MESODERM
Note the regression of the primitive streak
If it does not regress a sacrococcygeal teratoma will result which may become malignant.
A sacrococcygeal teratoma is the result
of failure of primitive streak regression.
In the United States the occurrence is
about 1 in 40,000 live births.
Teratomas from any origin are benign
tumors that arise from germ cells.
They may contain a variety of structures
such as skin, neurons, glands, and
cartilage. These may be seen grossly
or microscopically. The tissues are
not organized.
90% of sacrococcygeal teratomas up to 2
months are benign. However, half of them
diagnosed later in life are malignant.
CROSS SECTION OF NEURULATION
Neural crest cells form dorsal root ganglia, sympathetic and parasympathetic
ganglia, melanocytes, and many other structures.
SOMITES FORM IN THE PARAXIAL MESODERM
SCLEROTOMES AND DEMOMYOTOMES FORM FROM SOMITES
FORMATION OF VERTEBRAE
Somites in the paraxial mesoderm
will give rise to sclerotomes.
Anterior (A) designates toward the
head.
Posterior (B) is caudal.
The caudal part of the sclerotome
above will invade the cranial part
of the below.
Note that a vertebral body is made
of a P and an A.
P
A
A.
B.
C.
D.
Cross section at day 17
Cross section at day 19
Cross section at day 20
Cross section at day 21
Paraxial mesoderm makes somites.
Intermediate mesoderm makes urogenital.
Lateral plate mesoderm makes body walls
A. Beginning of folding about day 20.
B. Cross section through midgut.
C. Cross section below midgut.
Presomite
before day 19
about day 25
about day 22
end of 1st month
Sagittal section through embryo at various stages.
ENDODERMAL GERM LAYER
MAKES GLANDS AND EPITHELIAL LININGS.
NEURAL
CREST
CELL
DERIVATIVES