L3: Third week of development (Gastrulation)

Question 1

What are the changes during the 3rd week?

Answer 1

❖ Changes in the structure of bilaminar embryonic plate:
 Development of prochordal plate
 Development of primitive streak (Groove)
 Development of notochord

❖ Formation of 3 germ layers:
 Gastrulation

Question 2

prochordal plate

Answer 2

Site:

✓ near the cranial end in the midline

Cause:

✓ elongation (in height) of the primary endoderm (hypoblast): • It is the site of the oropharyngeal membrane

Question 3

What is the site of the primitive streak?

Answer 3

✓ Opacity formed caudally in the midline (from caudal end to middle of the embryo)

Question 4

Formation of the primitive streak

Answer 4

✓ proliferation and migration of the epiblastic cells:
• Primitive node: the proliferation of cranial end of the primitive streak
• Primitive pit: depression in the middle of the primitive node

Question 5

What is the fate of the primitive streak?

Answer 5

✓ Diminishes in size and becomes an insignificant structure in the sacrococcygeal region of the embryo

✓ Disappears by the end of the fourth week (if it persists, it will lead to the formation of sacrococcygeal teratoma)

Question 6

What is the definition of notochord?

Answer 6

✓ The core around which the vertebral column formed

Question 7

What is the source of notochord?

Answer 7

✓ epiblast of primitive streak & nodes

Question 8

What is the site of notochord?

Answer 8

✓ in the midline between ectoderm & endoderm

Question 9

What are the stages of formation of notochord?

Answer 9

✓ Notochordal process: 17th day
✓ Notochordal canal
✓ Notochordal plate
✓ Notochord: 20th day

Question 10

What is the function of notochord?

Answer 10

✓ Axis for the formation of the vertebral column
✓ Produce intraembryonic mesoderm
✓ Initiate (stimulate) neutralization (neural tube formation)

Question 11

What is the fate of notochord?

Answer 11

✓ The cranial part: some parts of bones forming skull base

✓ Parts in the center of the bodies of vertebrae: Degenerate and disappear

✓ Parts in the intervertebral discs: Undergo mucoid degeneration to form nucleus bulbosus.

Question 12

What is the definition of gastrulation?

Answer 12

✓ the process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc.

Question 13

What is the time of gastrulation?

Answer 13

During the 3rd week

Question 14

What are the steps of the process of gastrulation?

Answer 14

1) Cells from primitive streak migrate and are arranged between epiblast & hypoblast forming intraembryonic mesoderm.

2) The epiblast of primitive streak displaces the hypoblast, forming the embryonic endoderm

3) The cells remaining in the epiblast from the embryonic ectoderm

Question 15

What is the first morphological sign of gastrulation?

Answer 15

✓ The first morphologic sign of gastrulation is the formation of the primitive streak

Question 16

What happens by the middle of 3rd week?

Answer 16

✓ By the middle of the 3rd week, the intraembryonic mesoderm separates the ectoderm and endoderm everywhere except:

 At oropharyngeal membrane cranially.

 In median plane cranial to the
primitive node (site of notochord)
 At the cloacal membrane caudally.

Question 17

What is the origin of the intraembryonic mesoderm?

Answer 17

Primitive streak, Primitive node & Notochord

Question 18

What are the steps of the development of the intraembryonic mesoderm?

Answer 18

✓ A longitudinal groove appears on each side of the notochord divides the intraembryonic mesoderm into:
• paraxial mesoderm: medial to the longitudinal groove
• intermediate cell mass: the floor of the longitudinal groove
• lateral plate mesoderm: lateral to the longitudinal groove

Question 19

What is the definition of paraxial mesoderm?

Answer 19

❖ the medial part of the intraembryonic mesoderm on each side of the notochord

Question 20

What are the steps of the development of paraxial mesoderm?

Answer 20

❖ The intra-embryonic mesoderm proliferates to form a thick column of paraxial mesoderm

❖ At end of 3week: the paraxial mesoderm divides into segments (somites) by transverse grooves

Question 21

What is the number of Somites?

Answer 21

✓ 42 to 44 pairs of somites:
• 4 occipital
• 8 cervical
• 12 thoracic 5 lumbar
• 5 sacral
• 8-10 coccygeal

Question 22

What is the sequence of appearance of somites?

Answer 22

✓ Appear in the craniocaudal direction

Question 23

What is the fate of somites?

Answer 23

✓ Ventromedial part (sclerotome):
• Give axial skeleton (vertebral column& ribs

✓ Dorsolateral part (dermo-myotome):
• Dermatome: gives dermis & fascia
• Myotome: gives skeletal muscles

Question 24

What is the definition of intermediate cell mass?

Answer 24

✓ mesoderm corresponds to the floor of the longitudinal groove

Question 25

What is the fate of intermediate cell mass?

Answer 25

✓ Kidney & Cortex of suprarenal gland
✓ Testis or ovary
✓ The male and female genital duct system

Question 26

What is the definition of the lateral plate mesoderm?

Answer 26

❖ mesoderm lateral to the longitudinal groove

Question 27

What are the steps of the development of lateral plate mesoderm?

Answer 27

❖ Small cavities appear in it

❖ These cavities unite to form a single horse-shoe cavity (intraembryonic coelom) that divide the lateral plate mesoderm into somatic (partial), splanchnic (visceral), and intraembryonic coelom

Question 28

What is the function of the somatic layer?

Answer 28

✓ Give rise to:
• Muscles of the chest and abdominal wall
• Parietal layer of pericardium, pleura and peritoneum

Question 29

What is the function of the splanchnic layer?

Answer 29

✓ Give rise to:
• Muscles of heart & Smooth muscles of the gut and bronchial tree

• visceral layer of pericardium, pleura, and peritoneum

Question 30

What is the function of the intraembryonic coelom?

Answer 30

give rise to:
• pericardium, pleura and peritoneum

Question 31

What are the derivatives of endoderm?

Answer 31

“3 Epi + pptt + salah”

 The epithelial lining of:
 Gastrointestinal tract
 Respiratory tract
 Urinary tract and reproductive system
 Thymus& thyroid glands
 Parathyroid glands & pancreas
 Liver

Question 32

What are the derivatives of mesoderm?

Answer 32

 The dermis of skin and fascia
 Skeletal system
 Muscular system (all muscles)
 Circulatory and lymphatic system
 Excretory system
 Reproductive system except for germ cells
 Suprarenal cortex
 The lining of the body cavity

Question 33

What are the derivatives of ectoderm?

Answer 33

 Nervous system

 Cornea and lens of the eye

 Pituitary and pineal gland

 Epidermis & skin appendage (sweet glands& hair follicles)

 Epithelium of anterior part of the mouth and lower part of the anal canal

 Derivatives of neural crest