Mod.A Embryology Lec5 Flashcards
Gastrulation
It is differentiation of three germ layers by which the bilaminar
embryonic disc is converted into a trilaminar embryonic disc.
When gastrulation occurs
third week
gastrulation is the beginning of
morphogenesis (development of
various organs and parts of the body)
The process of gastrulation begins with the formation of the
primitive streak
Primitive streak is
a thickened linear band of epiblast.
•
where does the primitive streak appear
It appears on the dorsal aspect of the embryonic disc.
how gastrulation happens ?
A narrow primitive groove develops in the primitive streak that ends in a small depression in the primitive node.
This is place where gastrulation happens.
The cells around gap is actively folding and invade under epiblast layer to form
mesoderm
Once this process (gastrulation) is complete
mesoderm and endoderm are formed and epiblast becomes ectoderm.
what happens to the primitive streak after gastrulation is completed ?
The streak diminishes in relative size and becomes an insignificant structure in the sacrococcygeal region of the embryo.
Sacrococcygeal Teratoma (4)
- Remnants of the primitive streak may persist and give rise to a large tumor known as a sacrococcygeal teratoma.
- The tumor contains tissues derived from all three germ layers in incomplete stages of differentiation.
- Sacrococcygeal teratomas are the most common tumors in newborn infants.
- These tumors are usually surgically excised promptly, and the prognosis is good.
Notochord is
cord of cells extends between the ectoderm and
endoderm.
The notochordal process soon acquires a lumen, which is called
the notochordal canal
The notochord functions
the primary inductor in the early embryo.
• It induces the overlying embryonic ectoderm to thicken and form the neural tube.
• It also send signal to the surrounding mesoderm which causes 3 subdivisions;
paraxial, intermediate and lateral mesoderm.
when does the paraxial mesoderm differentiates and
divides into paired cuboidal bodies, somites, on each side of the developing neural tube ?
toward the end of the 3rd week
Each somite is differentiated into three parts
Sclerotome, Myotome, and Dermatome.
Lateral mesoderm split in two layers by formation of intraembryonic coelom
1-Theportion which is in contact with endoderm is called visceral/splanchnic lateralplate mesoderm
2-the portion in contact with ectoderm is called
somatic/parietal lateral plate mesoderm.
Neurulation: formation of the neural tube
It is the formation of the neural folds and closure of these folds to form the neural tube.
when is the neural tube formation completed ?
toward the end of the 4th week
The neural tube is formed from
ectoderm
Neural crest cells
are cells lying along the crest/tops of each neural fold.
The neural tube is formed, then is fully dissociate from
the ectoderm
The cranial part of the tube dilates to form
the brain vesicle
The brain vesicle
shows two constrictions which subdivide it into 3 primary brain
vesicles: Forebrain, Midbrain and Hindbrain.
The caudal part of the tube forms
the spinal cord
Abnormal Neurulation
Disturbance of neurulation may result in severe abnormalities of the brain and spinal cord.
• Neural tube defects are the most common congenital anomalies
Meroencephaly (anencephaly)
is the most severe defect.
• Failure of the neural folds to fuse and form the neural tube in the brain region results in anencephaly, and in the lumbar region, spina bifida cystica.
