Module
8
Icon module 8

Introduction

After the end of the third week, the three germinal layers begin to differentiate and transform so that the initially flat embryonic disk develops into a cylindrical structure like a "C".

The folding and genesis of the abdominal wall permits a delimitation of the embryo, that can now be clearly distinguished from the appending organs. Up to this stage 9 , the extraembryonic tissue went over into the intraembryonic tissue with no boundaries. The folding and the resulting formation of the abdominal wall lead to an enclosure of the mesoderm and the endoderm. They become surrounded by the ectoderm, which later forms the skin.

Two mechanisms lead simultaneously to the formation of the abdominal wall:

  • The cephalo-caudal flexion (in the longitudinal direction)
  • The lateral folding (in the transversal direction, rolling up)
Animation

- The cephalo-caudal folding (in the longitudinal direction)

- The lateral folding in the transversal direction (rolling up)

- Comparison of the two foldings

The cephalo-caudal folding (in the longitudinal direction)

What? How? Where?
  • Cephalo-caudal flexion takes place in the A plane
  • Lateral folding takes place in the C plane
media/multuse/i4_vueac3D.gif

In order to understand how this turning takes place, the structures must first be described that are found in the cephalic end before the folding:
In the cephalic region, rostral to the prechordal plate and the pharyngeal membrane, the mesenchymal cells form the cardiac plate (pericardium) and the septum transversum (which later becomes a part of the diaphragm and separates the coelom into thoracic and abdominal cavities).

With the 180° degree turn that results from the folding, the following occurs:

  • the pharyngeal membrane extends towards the lower front (mouth area)
  • the cardiogenic plate (which initially lay most cranially) into the thorax area
  • between the cardiac anlage and the umbilical vesicle a mesenchymal bridge forms, the septum transversum

After this movement is completed, the brain (encephalon) lies the most cranially, followed by the mouth, heart, and diaphragm (septum transversum).

During this folding the endoderm below the pharyngeal membrane becomes surrounded ventrally by the cardiac anlage. From this region, the throat (pharynx) arises and, subsequently, the thyroid gland, the lungs, and the esophagus. The pharyngeal membrane that for now separates the mouth (ectoderm) from the throat (endoderm) later atrophies.

 

In order to understand the process of folding better (after stage 9), the forgoing stages are also depicted.

1
Amniotic cavity
2
Ectoderm
3
Umbilical vesicle
4a
Endoderm
4b
Endoderm that joins together
again under the notochorde
4c
Completely joined up endoblast
5
Body stalk
6a
Allantois
6b
Lengthened allantois
7
Extraembryonic mesoblast
8
Cloacal membrane
9
Notochordal process
10a
Primitive streak
10b
Shrinking primitive streak
11a
Neural plate
11b
Neural folds
12
Central axial canal
13
Primitive node
14a
Ventrally merged
notochord
14b
Ventrally resorbed
notochord
14c
Free notochord
15
Prechordal plate
16
Canalis neurentericus
17
Pharyngeal membrane
18
Cardiac plate
19
Pericardial cavity
20
Fused neural tube
21
Septum transversum
22
Closure of the neuroporus cranialis
23
Recessus caudalis
24
Thyroid gland anlage
25
Lung anlage
26
Liver anlage
27
Closure of the neuroporus caudalis
28
Dorsal aorta
Fig. longplic01N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad6_17N.gif
Fig. longplic02N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad7_19N.gif
Fig. longplic03N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad7_21N.gif
Fig. longplic04N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad8_23N.gif
Fig. longplic05N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad8_24N.gif
Fig. longplic06N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad9_25N.gif
Fig. longplic07N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad9_27N.gif
Fig. longplic08N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad10_28aN.gif
Fig. longplic09N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad10_28bN.gif
Fig. longplic10N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad10_28cN.gif
Fig. longplic11N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad11_29N.gif
Fig. longplic12N - Longitudinal folding
Carnegie stages 6 - 12
media/module8/i4_astadNumeros/i4_astad12_30N.gif
 
Fig. 5 - Folding of the cephalic end
S9 (ca. 27th day)
media/module8/i4e_plicephal27.gif

  1. Future prosencephalon
  2. Notochorde
  3. Neural tube
  4. Pericardial cavity
  5. Cardiac tube
  6. Pharyngeal membrane
  7. Extraembryonic mesoderm

Fig. 6 - Folding of the cephalic end
S12 (ca. 30th day)
media/module8/i4f_plicephal30.gif

8
Foregut
9
Septum transversum

Legend
Fig. 5, 6

Schematic detail of the cephalic extremity, sagittal section.

Fig. 6

The folding of the caudal end occurs after the cephalic folding and has the result that the body stalk comes closer to the umbilical vesicle (yolk sac).
Due to the large axial growth the caudal end of the embryonic disk (with the cloacal membrane) comes to lie under the original embryonic disk and thus shoves the allantois and the body stalk in the ventral direction, up to the umbilical vesicle (yolk sac) and merges with its stalk.
To be noted is that now the end of the primitive streak, which initially lies dorsally after the flexion of the embryo, also comes to lie ventrally.

Fig. 7 - Folding of the caudal end
S9 (ca. 27th day)
media/module8/i4g_plicaudal27.gif

  1. Notochord
  2. Neural fold
  3. Amniotic cavity
  4. Primitive streak
  5. Primary endoderm
  6. Cloacal membrane
  7. Allantois
  8. Body stalk

Fig. 8 - Folding of the caudal end
S12 (ca. 30th day)
media/module8/i4h_plicaudal30.gif

1
Notochord
2
Neural tube
3
Amniotic cavity
5
Endoderm
6
Cloacal membrane
7
Allantois
8
Body stalk
9
Hind gut

Legend
Fig. 7, 8

Schematic detail of the folding of the caudal end.

Fig. 8