third week embryology

Question 1

what is gastrulation?

Answer 1

the formation of the primitive streak at the 16th day .

Question 2

where does the primitive streak form?

Answer 2

It is a thickened band that appears caudally in the median plane of of the epiblast.

Question 3

why does gastrulation occur?

Answer 3

This occurs as a result of the proliferation & migration of cells of the epiblast from lateral to the median plane of the embryonic disc.

Question 4

the appearance of the primitive streak determines what?

Answer 4

The appearance of a primitive streak determines the cranio-caudal axis of the embryo.
It will give rise to notochord & intra-embryonic mesoderm

Question 5

there three primary germ discs in which all body tissues develop from them

Answer 5

ectoderm
mesoderm
endoderm

Question 6

a narrow depression is known as ?

Answer 6

primitive groove

Question 7

what is the cranial end of the groove called ?

Answer 7

primitive pit

Question 8

the groove is surrounded by how many folds ?

Answer 8

2 primitive folds

Question 9

The cranial end of the folds proliferates to form a?

Answer 9

primitive node

Question 10

what is Sacro - coccygeal teratoma

Answer 10

Remnants of the primitive streak may persist & give rise to a large tumor known as a sacro - coccygeal teratoma

Question 11

what is an intraembryonic mesoderm?

Answer 11

It separates ectoderm & endoderm except in the prochordal plate.

Question 12

how is the intraembryonic mesoderm formed?

Answer 12

Intra embryonic mesoderm is formed by proliferation of the cells in the primitive streak

Question 13

is there a mesoderm in the prechordal plate?

Answer 13

No

Question 14

how is the prechordal plate formed?

Answer 14

Localized area of thickening of the Hypoblast(endoderm)

Question 15

the prechordal plate determines which future site?

Answer 15

future site of oral cavity (mouth)

Question 16

what is the notochordal process?

Answer 16

Extension of mesenchymal cells from primitive node to oropharyngeal membrane forming a median cellular cord

Question 17

how does the notochord grow?

Answer 17

It grows cranially between the ectoderm & the endoderm until it reaches the oro -pharyngeal membrane, where it cannot grow further.

Question 18

Does the process require a lumen which this lumen is called?

Answer 18

A notochordal canal

Question 19

what is the notochordal plate?

Answer 19

the remaining roof & sides of the notochordal canal form the notochordal plate.

Question 20

what does the definitive notochord mean?

Answer 20

when the plate enfolds to form the definitive notochord

Question 21

what are the 3 functions of the notochord?

Answer 21

• 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.

Question 22

The notochord degenerates & disappears when the vertebral bodies form, but parts of it?

Answer 22

persist as the nucleus pulposus of each intervertebral disc.

Question 23

formation of the endoderm

Answer 23

Mesenchymal cells separate from the deep surface of the primitive streak.

Question 24

some of the mesenchymal cells migrate ventrally to form what?

Answer 24

to form the definitive endoderm

Question 25

the intraembryonic mesoderm is divided into three parts, what are they?

Answer 25

Paraxial mesoderm Intermediate mesoderm Lateral plate mesoderm

Question 26

intraembryonic mesoderm is absent in?

Answer 26

Region of the oropharyngeal membrane. Region of the cloacal membrane.

Question 27

how is intraembryonic coelom developed?

Answer 27

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).

Question 28

uring the 2nd month ,the intra- embryonic coelom is divided into 3 body cavities which are?

Answer 28

The pericardial cavity (Heart) ,the pleural cavities ( lungs ) &the peritoneal cavity ( abdomen) .

Question 29

The intra-embryonic coelom that divides the lateral mesoderm into 2 layers which are?

Answer 29

Somatic (the parietal layer that is in contact with the ectoderm Splanchnic (the visceral layer that is in contact with endoderm)

Question 30

what is an allontois?

Answer 30

It is dorso - caudal invagination of the endoderm of the umbilical vesicle into the connecting stalk

Question 31

when does the extra embryonic vasculogenesis ( blood vessels ) formation occur?

Answer 31

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

Question 32

* Extra embryonic vasculogenesis begins in the?

Answer 32

chorionic membrane connecting stalk umbilical vesicle.

Question 33

when does Extra-embryonic Blood formation (HEMATOGENESIS) begin? and where?

Answer 33

begins during the 3rd – 5th week in the walls of the umbilical vesicle &the connecting stalk as mesodermal blood islands.

Question 34

in the Extra-embryonic Blood formation (HEMATOGENESIS) the peripheral cells form what and the central cells form what?

Answer 34

the peripheral cells of each island form the wall of blood vessels and the central cells form the blood cells

Question 35

formation of the intra-embryonic blood vessels does not begin within the embryo until the ?

Answer 35

5TH week

Question 36

Extra-embryonic Blood formation (HEMATOGENESIS) this process occurs first in?

Answer 36

This process occurs first in the liver (6th week) , then in the spleen &later on, in the bone marrow.

Question 37

the primary chorionic villi appears at the end of? and is formed by what?

Answer 37

appears at the end of the 2nd week, and is formed by cytotrophoblast and syncytiotrophoblast

Question 38

the secondary chorionic villi appears when and is made up of what?

Answer 38

appears at the beginning of the 3rd week and is made up of cytotrophoblast+ syncytiotrophoblast + extraembryonic mesoderm

Question 39

the tertiary ( definitive ) chorionic villli is made up of ?

Answer 39

mesenchymal cells

Question 40

the mesenchymal cells at the core of the tertiary villi can differentiate to ?

Answer 40

into capillaries and blood cells

Question 41

the capillaries in the chorionic villi fuse together to form?

Answer 41

to form networks and become connected with the embryonic heart

Question 42

development of somites

Answer 42

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.

Question 43

The 1st pair of somites appear near ?

Answer 43

appear near the cranial end of the notochord & on each side of the developing neural tube.

Question 44

during the development of somites by the end of the 5th week how many pairs are formed ?

Answer 44

42 – 44 pairs are formed.

Question 45

by the end of the 5th week 42-44 pairs of somites are formed in which they are categorized into:

Answer 45

they are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral & 8–10 coccygeal somites.

Question 46

each somite is divided into?

Answer 46

Ventro - medial part which is called the sclerotome Dorso - lateral part which is called dermomyotome.

Question 47

the somites are divided into 2 parts one of them is Ventro - medial part which is called the sclerotome what will the sclerotome surround?

Answer 47

Sclerotome will surround the neural tube and notochord to form the vertebral column & (ribs, sternum, skull).