third week embryology Flashcards
what is gastrulation?
the formation of the primitive streak at the 16th day .
where does the primitive streak form?
It is a thickened band that appears caudally in the median plane of of the epiblast.
why does gastrulation occur?
This occurs as a result of the proliferation & migration of cells of the epiblast from lateral to the median plane of the embryonic disc.
the appearance of the primitive streak determines what?
The appearance of a primitive streak determines the cranio-caudal axis of the embryo.
It will give rise to notochord & intra-embryonic mesoderm
there three primary germ discs in which all body tissues develop from them
ectoderm
mesoderm
endoderm
a narrow depression is known as ?
primitive groove
what is the cranial end of the groove called ?
primitive pit
the groove is surrounded by how many folds ?
2 primitive folds
The cranial end of the folds proliferates to form a?
primitive node
what is Sacro - coccygeal teratoma
Remnants of the primitive streak may persist & give rise to a large tumor known as a sacro - coccygeal teratoma
what is an intraembryonic mesoderm?
It separates ectoderm & endoderm except in the prochordal plate.
how is the intraembryonic mesoderm formed?
Intra embryonic mesoderm is formed by proliferation of the cells in the primitive streak
is there a mesoderm in the prechordal plate?
No
how is the prechordal plate formed?
Localized area of thickening of the Hypoblast(endoderm)
the prechordal plate determines which future site?
future site of oral cavity (mouth)
what is the notochordal process?
Extension of mesenchymal cells from primitive node to oropharyngeal membrane forming a median cellular cord
how does the notochord grow?
It grows cranially between the ectoderm & the endoderm until it reaches the oro -pharyngeal membrane, where it cannot grow further.
Does the process require a lumen which this lumen is called?
A notochordal canal
what is the notochordal plate?
the remaining roof & sides of the notochordal canal form the notochordal plate.
what does the definitive notochord mean?
when the plate enfolds to form the definitive notochord
what are the 3 functions of the notochord?
- Define the axis of the embryo.
- Serves as the basis for the development of the axial skeleton.
- Induces the formation of the neural tube from the overlying ectoderm.
The notochord degenerates & disappears when the vertebral bodies form, but parts of it?
persist as the nucleus pulposus of each intervertebral disc.
formation of the endoderm
Mesenchymal cells separate from the deep surface of the primitive streak.
some of the mesenchymal cells migrate ventrally to form what?
to form the definitive endoderm
the intraembryonic mesoderm is divided into three parts, what are they?
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
intraembryonic mesoderm is absent in?
Region of the oropharyngeal membrane.
Region of the cloacal membrane.
how is intraembryonic coelom developed?
The intra-embryonic coelom (body cavity) appears as small spaces in the cardiogenic mesoderm.
These spaces coalesce to form a single, horse shoe-shaped cavity (the intra-embryonic coelom).
uring the 2nd month ,the intra- embryonic coelom is divided into 3 body cavities which are?
The pericardial cavity (Heart) ,the pleural cavities ( lungs ) &the peritoneal cavity ( abdomen) .
The intra-embryonic coelom that divides the lateral mesoderm into 2 layers which are?
Somatic (the parietal layer that is in contact with the ectoderm
Splanchnic (the visceral layer that is in contact with endoderm)
what is an allontois?
It is dorso - caudal invagination of the endoderm of the umbilical vesicle into the connecting stalk
when does the extra embryonic vasculogenesis ( blood vessels ) formation occur?
At the beginning of 3rd week (16th day).embryonic nutrition is obtained from
the maternal blood by diffusion of nutrients through the chorion, extra-
embryonic coelom & umbilical vesicle
- Extra embryonic vasculogenesis begins in the?
chorionic membrane
connecting stalk
umbilical vesicle.
when does Extra-embryonic Blood formation (HEMATOGENESIS) begin? and where?
begins during the 3rd – 5th week in the walls of the umbilical vesicle &the connecting stalk as mesodermal blood islands.
in the Extra-embryonic Blood formation (HEMATOGENESIS) the peripheral cells form what and the central cells form what?
the peripheral cells of each island form the wall of blood vessels and the central cells form the blood cells
formation of the intra-embryonic blood vessels does not begin within the embryo until the ?
5TH week
Extra-embryonic Blood formation (HEMATOGENESIS)
this process occurs first in?
This process occurs first in the liver (6th week) , then in the spleen &later on, in the bone marrow.
the primary chorionic villi appears at the end of? and is formed by what?
appears at the end of the 2nd week, and is formed by cytotrophoblast and syncytiotrophoblast
the secondary chorionic villi appears when and is made up of what?
appears at the beginning of the 3rd week and is made up of cytotrophoblast+ syncytiotrophoblast + extraembryonic mesoderm
the tertiary ( definitive ) chorionic villli is made up of ?
mesenchymal cells
the mesenchymal cells at the core of the tertiary villi can differentiate to ?
into capillaries and blood cells
the capillaries in the chorionic villi fuse together to form?
to form networks and become connected with the embryonic heart
development of somites
At the end of the 3rd week (22nd day), the paraxial mesoderm begins to divide in a cranio- caudal direction into paired cuboidal masses called somites.
The 1st pair of somites appear near ?
appear near the cranial end of the notochord & on each side of the developing neural tube.
during the development of somites by the end of the 5th week how many pairs are formed ?
42 – 44 pairs are formed.
by the end of the 5th week 42-44 pairs of somites are formed in which they are categorized into:
they are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral & 8–10 coccygeal somites.
each somite is divided into?
Ventro - medial part which is called the sclerotome
Dorso - lateral part which is called dermomyotome.
the somites are divided into 2 parts one of them is Ventro - medial part which is called the sclerotome what will the sclerotome surround?
Sclerotome will surround the neural tube and notochord to form the vertebral column & (ribs, sternum, skull).
