Mod.A Embryology Lec5

Question 1

Gastrulation

Answer 1

It is differentiation of three germ layers by which the bilaminar
embryonic disc is converted into a trilaminar embryonic disc.

Question 2

When gastrulation occurs

Answer 2

third week

Question 3

gastrulation is the beginning of

Answer 3

morphogenesis (development of

various organs and parts of the body)

Question 4

The process of gastrulation begins with the formation of the

Answer 4

primitive streak

Question 5

Primitive streak is

Answer 5

a thickened linear band of epiblast.

•

Question 6

where does the primitive streak appear

Answer 6

It appears on the dorsal aspect of the embryonic disc.

Question 7

how gastrulation happens ?

Answer 7

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.

Question 8

The cells around gap is actively folding and invade under epiblast layer to form

Answer 8

mesoderm

Question 9

Once this process (gastrulation) is complete

Answer 9

mesoderm and endoderm are formed and epiblast becomes ectoderm.

Question 10

what happens to the primitive streak after gastrulation is completed ?

Answer 10

The streak diminishes in relative size and becomes an insignificant structure in the sacrococcygeal region of the embryo.

Question 11

Sacrococcygeal Teratoma (4)

Answer 11

• 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.

Question 12

Notochord is

Answer 12

cord of cells extends between the ectoderm and

endoderm.

Question 13

The notochordal process soon acquires a lumen, which is called

Answer 13

the notochordal canal

Question 14

The notochord functions

Answer 14

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.

Question 15

when does the paraxial mesoderm differentiates and

divides into paired cuboidal bodies, somites, on each side of the developing neural tube ?

Answer 15

toward the end of the 3rd week

Question 16

Each somite is differentiated into three parts

Answer 16

Sclerotome, Myotome, and Dermatome.

Question 17

Lateral mesoderm split in two layers by formation of intraembryonic coelom

Answer 17

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.

Question 18

Neurulation: formation of the neural tube

Answer 18

It is the formation of the neural folds and closure of these folds to form the neural tube.

Question 19

when is the neural tube formation completed ?

Answer 19

toward the end of the 4th week

Question 20

The neural tube is formed from

Answer 20

ectoderm

Question 21

Neural crest cells

Answer 21

are cells lying along the crest/tops of each neural fold.

Question 22

The neural tube is formed, then is fully dissociate from

Answer 22

the ectoderm

Question 23

The cranial part of the tube dilates to form

Answer 23

the brain vesicle

The brain vesicle
shows two constrictions which subdivide it into 3 primary brain
vesicles: Forebrain, Midbrain and Hindbrain.

Question 24

The caudal part of the tube forms

Answer 24

the spinal cord

Question 25

Abnormal Neurulation

Answer 25

Disturbance of neurulation may result in severe abnormalities of the brain and spinal cord.

• Neural tube defects are the most common congenital anomalies

Question 26

Meroencephaly (anencephaly)

Answer 26

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.