Organogenesis,week4-8

Question 1

What is embryonic folding?

Answer 1

Folding of the flat trilaminar layer into a cylindrical embryo

Question 2

Where does embryonic folding occur?

Answer 2

Median and horizontal (transverse) planes

Question 3

Median plane folding divisions

Answer 3

Head and tail foldings

Question 4

Head folding

Answer 4

Cranial neural folds form the primordium of the brain
- At the beginning of the 4th week
Developing brain initially projects dorsally into the amniotic cavity
Later, the developing forebrain grows cranially
- Beyond the oropharyngeal membrane
- Overhangs the developing heart
Concurrent movement of structures to the ventral surface
- Septum transversum
- Primordial heart
- Pericardial coelom
- Oropharyngeal membrane

Part of the umbilical vesicle endoderm incorporates into embryo
- Will develop into the foregut
Foregut lies between
- Forebrain and primordial heart
Oropharyngeal membrane separates
- Foregut from the stomodeum (Primordial mouth)
After folding
- Septum transversum lies caudal to the heart
- Develops into central tendon of the diaphragm
Folding affects embryonic coelom arrangement
- Primordium of the body cavity
After folding, the pericardial coelom
- Lies ventral to the heart
- Lies cranial to the septum transversum
At this stage, intraembryonic coelom
communicates with extraembryonic coelom

Question 5

What is the embryonic coelom primordium to

Answer 5

Body cavity

Question 6

Tail folding

Answer 6

Folding of the caudal end of the embryo
- From growth of the distal part of the neural tube
Caudal eminence projects over cloacal membrane
Endodermal layer incorporates into embryo as hindgut
- Future transverse, descending and sigmoid colon and the rectum
- Its terminal part slightly dilates to form the cloaca

Question 7

What happens after folding?

Answer 7

After folding, primitive streak lies caudal to the cloacal membrane
Connecting stalk is now connected to the ventral surface
- The future umbilical cord
Allantois is partially incorporated into the embryo
- Projects into connecting stalk from the umbilical vesicle

Question 8

Horizontal folding

Answer 8

Produced by rapidly growing spinal cord and somites
Primordia of the ventrolateral abdominal wall
- Fold towards the median plane
- Rolls embryonic disc edges ventrally
Abdominal cavity formation incorporates endodermal layer
- To form the midgut (Primordial small intestines)

Midgut initially has a wide connection to the umbilical vesicle
- Reduction of the connection will form the omphaloenteric
(vitelline) duct
Attachment of the ventral region to the amnion also reduces
Communication area reduces as umbilical cord forms
- Between intra- and extraembryonic coelomic cavities
- Caused by ventral fusion of the lateral folds

Question 9

What forms the foregut

Answer 9

Endoderm and umbilical vesicle

Question 10

Which structures do the foregut lie between

Answer 10

Forebrain and primordial heart

Question 11

Which structure is the foregut separated from and what structure separates it

Answer 11

Separated from the stomodeum by the oropharyngeal membrane

Question 12

What forms the hind gut

Answer 12

Endoderm incorporating into embryo

Question 13

Primordium of hindgut

Answer 13

1.Transverse, sigmoid and descending colon
2.Rectum

Question 14

What forms cloaca?

Answer 14

Dilation of rectum

Question 15

What happens to the primitive streak after tail folding?

Answer 15

Lies caudal to cloacal membrane

Question 16

End of tail folding

Answer 16

Connecting stalk is now connected to the ventral surface
- The future umbilical cord
Allantois is partially incorporated into the embryo
- Projects into connecting stalk from the umbilical vesicle

Question 17

What forms midgut

Answer 17

Abdominal cavity and endoderm

Question 18

Which structure does the midgut have connection with

Answer 18

Umbilical vesicle

Question 19

Attachment of which structure reduces the connection btw umbilical vesicle and midgut

Answer 19

Ventral region of amnion

Question 20

What forms after reduction of connection btw umbilical vesicle and midgut

Answer 20

Vitelline duct

Question 21

How many somites are dorsally found on 4th week?

Answer 21

4-12 with neural tube opposite them

Question 22

When are first pharyngeal arches visible?

Answer 22

day 24

Question 23

Contributions of the first pharyngeal arches

Answer 23

1. Major part to mandible
   2.Cranial extension to maxilla

Question 24

What are first pharyngeal arches?

Answer 24

Mandibular arches

Question 25

When is large ventral heart prominence produced

Answer 25

Week 4

Question 26

What is visible at day 26?

Answer 26

Three pharyngeal arches
Cranial neuropore closed
Forebrain producing elevation on the head

Question 27

What is visible at day 26/7?

Answer 27

Upper limb buds
Lower limb buds
Caudal eminence
Lens placodes
4th pair of pharyngeal arch
Otic pits
Rudiments of CVS
Caudal neuropore close

Question 28

What forms the internal ears

Answer 28

Otic pits

Question 29

5th week

Answer 29

Head grows, exceeding other regions
- From rapid development of the brain
and facial prominences
Face soon contacts heart prominence

2nd pharyngeal arch outgrows the 3rd and 4th arches
- Forms lateral depressions (Cervical sinus)

Mesonephric ridges are visible
- Site of developing mesonephric kidneys

Question 30

When the 2nd pharyngeal arch outgrows the 3rd and 4th, what does it form?

Answer 30

Cervical sinus{lateral depression}

Question 31

When does an embryo show movements?

Answer 31

6th week

Question 32

6th week?

Answer 32

Upper limbs show
Digital rays in hand plates
Lower limbs
Auricular hillocks(small swelling)
Pharyngeal groove
Eyes become obvious and retinal pigmentation
Heart prominence

Question 33

What does auricular hillock form?

Answer 33

Auricle (pinna)

Question 34

What does pharyngeal groove form?

Answer 34

External acoustic meatus

Question 35

What are auricular hillocks?

Answer 35

Small swellings around the pharyngeal groove

Question 36

What happens 6th week later?

Answer 36

Later, the trunk and neck begin to straighten
Intestines enter extraembryonic coelom
- In the proximal part of the umbilical cord
- An event called umbilical herniation

Question 37

What occurs when part of the intestine protrudes through the umbilical opening in the abdominal muscles?

Answer 37

Umbilical herniation
(ISBHONO FORMATION)

Question 38

7th week

Answer 38

-Notches appear digital rays indicating hand and foot digits

-Communication btw gut and umbilical vesicle is reduced

-Upper limb ossification

Question 39

When do lower limbs show?

Answer 39

4-5 days after upper limbs

Question 40

8th week main events

Answer 40

1. Hand digits (clearly separated but webbed)
2. Digital feet rays (notches are clear)
3. Caudal eminence (visible but appears as stub)
4. Scalp vascular plexus (appears and form a band)
   PURPOSEFUL LIMB MOVEMENTS OCCUR

Question 41

Continuation of 8th week events

Answer 41

-primary ossification of femur begins
-caudal eminence disappears completely
-hands and feet move ventrally
-Distinct human characteristics present by end of week
-Eyelids established and fused by end of the week, United by epithelial fusion
-Intestines remain in the umbilical cord proximally
-External genitalia is visible, but not distinctive enough

Question 42

What forms the future umbilical cord? When?

Answer 42

Connecting stalk + ventral surface during tail folding

Question 43

Umbilical cord

Answer 43

Umbilical cord development begins in the embryologic period around week 3 with the formation of the connecting stalk. By week 7, the umbilical cord has fully formed, composed of the connecting stalk, vitelline duct, and umbilical vessels surrounding the amniotic membrane.

Question 44

Difference between day 24 and day26

Answer 44

1.24- straight embryo
26-curved embryo from
median folding

2. 24- first (3) pharyngeal arches
   26- 4th pair of pharyngeal arches
3. 24- open cranial neuropore
   26- closed cranial neuropore