Module
8
Icon module 8
Embryonic phase

Condition of the folding at the end of the 4th week

For a better understanding of the folding at the end of the fourth week (stage 12) several regions have to be looked at:

  • The region above the umbilical cord (Fig. 9, section B)
  • The region in the area of the umbilical cord (Fig. 9, section C)
  • The region below the umbilical cord (Fig. 9, section D)
Fig. 9 - Schematic diagram of the finished folding,
the abdominal wall (ectoderm) has been formed (stage 12)
media/module8/i4i_plicature.gif

Legend
Fig. 9

Transversal section B above the umbilical cord.

 

Transversal section C at the level of the umbilical cord.

 

Transversal section D below the umbilical cord.

In the region above the umbilical cord (Figs. 10 - 11) the liver anlage (ventral) (stage 11) is formed from the endoderm and somewhat later the dorsal pancreas anlage (stage 12).

The liver diverticulum appears cranially to the umbilical vesicle (yolk sac). Liver cell bands penetrate into the mesoderm of the septum transversum. This is formed at the ventral location between the liver and cardiac anlage and contains the umbilical vein.

Fig. 10 - Region above the umbilical
cord (stage 11, ca. 29th day)
media/module8/i4j_coupB_st11.gif

  1. Nephrogenic cord
  2. Aortae
  3. Dermatomyotoma
  4. Ectoblast
  5. Notochord
  6. Pancreas anlage
  7. Intraembryonic coelom

Fig. 11 - Region below the umbilical
cord (stage 12, ca. 30th day)
media/module8/i4k_coupB_st12.gif

8
Amnion
9
Somatopleura mesoderm and ectoderm
10
Gut
11
Liver anlage
12
Septum transversum
13
Splanchnopleura mesoderm and endoderm
14
Amniotic cavity

Legend
Fig. 10

The ventral abdominal wall now closes itself. From the endoderm the liver anlage is formed ventrally. In stage 12 the pancreas anlage is visible.

Fig. 11

The ventral mesenterium has disappeared and, except for the umbilical cord, the embryo is completely surrounded by the amnion (later the peritoneal cavity will form at this level).

In the region below the umbilical cord (Fig. 13), the ventral abdominal wall closes up, whereby the primitive gut remains connected to it by the ventral mesenterium. After resorption of the ventral mesenterium, the gut is attached to the dorsal torso wall solely through the dorsal mesenterium.

Fig. 12 - Region below the umbilical
cord (stage 11, ca. 29th day)
media/module8/i4l_coupD_st11.gif

  1. Nephrogenic cord
  2. Aortae
    (their fusion occurs in stage12)
  3. Dermatomyotoma
  4. Ectoblast
  5. Notochorde
  6. Mesenterium dorsal
  7. Intraembryonic coelom

Fig. 13 - Region below the umbilical
cord (stage 12, ca. 30th day)
media/module8/i4m_coupD_st12.gif

8
Amnion
9
Somatopleura mesoderm and ectoderm
10
Gut
11
Allantois
12
Mesoderm
13
Splanchnopleura mesoderm and
endoderm
14
Amniotic cavity

Legend
Fig. 12

The embryo becomes totally enveloped by the amnion now. The intraembryonic coelom encloses the gut and temporarily forms a ventral mesenterium, that attaches the gut to the ventral abdominal wall.

Fig. 13

The ventral mesenterium has disappeared and, except for the umbilical cord, the embryo is completely surrounded by the amnion (later the peritoneal cavity will form at this level).