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Derivatives of Endoderm (Embryonic Period)

The endoderm germ layer covers the ventral surface of the embryo forming the roof of the yolk sac. The GIT is the main organ system derived from the endodermal germ layer.

Grey’s Anatomy 20th edition

Folding of the embryo consists of two parts:
1. Cephalocaudal folding, occuring in the median plane caused by rapid longitudinal growth of central nervous system.
2. Lateral folding, occuring in the horizontal plane caused by rapidly growing somites.

Cephalocaudal Folding:

When the brain vesicles are developed, the embryonic disc begins to bulge into the amniotic cavity and begins to fold cephalocaudally. As a result, head fold and tail fold appears.

Head fold:

The cranial region of the embryonic disc, from caudal to cranial contains:

a.     The buccopharyngeal membrane

b.     The cardiogenic area

c.      Septum transversum

The cranial rim of the disc folds under the cephalic neural plate, wedging the septum transversum between the heart region and the yolk sac. Thus placing the cardiogenic area and the buccopharyngeal membrane in the future thoracic and mouth regions, respectively.

As a result of folding:
1. A large portion of the endoderm-lined cavity is incorporated into the body of the embryo proper.
2. Anterior part forms the foregut and tail region forms the hindgut.
3. The midgut temporarily communicates with the yolk sac by way of broad stalk, the vitelline duct.
Tail fold:
The caudal rim of the embryonic disc contains:
a. The cloacal membrane
b. The connecting stalk, with the slender allantois.
Caudal folding brings the cloacal membrane onto the ventral surface of the embryo, with the embryo rotating ventrally until the connective stalk lies against the neck of the yolk sac.

In the 4th week buccopharyngeal membrane ruptures and an open communication is established between amniotic cavity and the primitive gut. Cloacal membrane breaks down in the 7th week to create the opening for the anus. The mid gut remains in communication with the yolk sac though vitelline duct.

Lateral Folding:

Initial flat embryonic disc also folds laterally and embryo obtains round appearance As a result of this folding:

1. The ventral body wall is established except where yolk sac duct and connecting stalk are attached

2. When vitelline duct is obliterated, only then does the midgut looses its connection with original endoderm lined cavity and obtain its free position in abdominal cavity.

3. Partial incorporation of allantois into body of embryo takes place where it forms cloaca while its distal portion remains  in connecting stalk.

4. By 5th week yolk sac duct, allantois, and umblical vessels are restricted to the region of umblical ring.

In humans yolk sac is vestigial and in all probability has a nutritive role only in early stages of development. In second month it lies in chorionic cavity.

Derivatives of Endodermal Germ Layer:

The derivatives of the endodermal germ layer include:

1.  Epithelial lining of:

a. Primitive gut

b. Allantois & vitelline duct

c. Respiratory tract

d. Urinary bladder & urethra

e. Tympanic cavity & auditory tube

2. Parenchyma of thyroid, parathyroids, liver & pancreas

3. Reticular stroma of tonsils & thymus

External Appearance During The 2nd Month
During the second month:
1. Increase in head size and formation of the limbs, face, ears, nose and eyes occurs.
2. By the beginning of the 5th week, forelimbs and hind limbs appear as paddle shaped buds. The forelimb buds are located dorsal to the pericardial swelling at the level of the C4 – T1 somites. Hind limb buds appear at the level of the lumbar and upper sacral somites.
Terminal portions of buds flatten and circular constriction separates them from proximal more cylindrical segments. Soon 4 radial grooves separates 5 slightly thicker areas appear on distal portion of buds. These grooves are called rays which appear in hand region first and shortly afterwards in foot as upper limb is slightly more advanced in development than lower limb. While fingers and toes are formed, 2nd constriction divides the proximal portion of buds into two segments and 3 characteristic features of adult extremity are recognizable.

Birth Defects:

Most major organs and organ systems are formed during the 3rd week to 8th week. This period is called the period of organogenesis. Stem cell populations are establishing each of the organ primordia, any insult from genetic and environmental influences can result in birth defects. The 3rd and 4th weeks are particularly vulnerable.
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