1. Folding of the Embryo Formation of Gut Endodermal Derivatives
By: Dr. Mujahid Khan

2. Folding Of Embryo
Flat trilaminar disc folds into a somewhat cylindrical embryo
Folding occurs in both median & horizontal planes
Results from rapid growth of the embryo
Long axis increases rapidly than the sides
Occurs simultaneously on both axis
Constriction at the junction of embryo & yolk sac

4. Folding in Median Plane
Occurs in the cranial and caudal ends
Causing head and tail folds
Moving ventrally as the embryo elongates cranially and caudally

5. Head Fold At the beginning of the 4th week
Neural folds in the cranial region thickened to form primordium of the brain
Initially the developing brain projects dorsally into the amniotic cavity
Later grows cranially beyond the oropharyngeal membrane
Overhangs the developing heart

7. Head Fold
Septum transversum, primordial heart, pericardial coelom & oropharyngeal membrane move onto the ventral surface
Endoderm of the yolk sac is incorporated into the embryo as a foregut
The foregut lies between the brain & heart
Oropharyngeal membrane separates the foregut from the stomodeum

8. Head Fold
Septum transversum lies caudal to heart after the folding and develops into central tendon of diaphragm
Head fold also affects the arrangement of the primordium of body cavity which consists of a flattened horseshoe shaped cavity before folding

10. Tail Fold
Results primarily from growth of the distal part of the neural tube
This is primordium of the spinal cord
As embryo grows, the caudal eminence projects over the cloacal membrane
During folding, part of endoderm is incorporated into the embryo as a hindgut

12. Tail Fold Terminal part of the hindgut soon dilates to form the cloaca
Cloaca is the primordium of urinary bladder and rectum
Before folding primitive streak lies cranial to the cloacal membrane
After folding it lies caudal to it

13. After Tail Fold
The connecting stalk (primordium of umbilical cord) is attached to the ventral surface of the embryo
Allantois (a diverticulum of yolk sac) is partially incorporated into the embryo

15. Folding in Horizontal Plane
Folding on sides of the embryo produces right and left lateral folds
Is produced by rapidly growing spinal cord and somites
Ventrolateral rolling of the edges of embryonic disc form roughly cylindrical embryo

16. Folding in Horizontal Plane
As the abdominal walls form, part of endoderm is incorporated into the embryo as the midgut
Initially there is a wide connection between midgut & yolk sac
After folding the connection is reduced to yolk stalk

19. Folding in Horizontal Plane
Umbilical cord forms from the connecting stalk
As it forms, ventral fusion of the lateral folds reduces the region of communication between intraembryonic and extraembryonic coelomic cavities to a narrow communication
Amniotic cavity expands and obliterates extraembryonic coelom

20. Derivatives of Endoderm
Endoderm gives rise to the epithelial lining of:
Trachea
Bronchi
lungs

21. Derivatives of Endoderm
Endoderm gives rise to the epithelial lining of:
Gastrointestinal tract
Liver
Pancreas
Urinary bladder
urachus

22. Derivatives of Endoderm
Endoderm gives rise to the epithelial lining of:
Pharynx
Thyroid
Tympanic cavity
Pharyngotympanic tube
Tonsils
Parathyroid glands

23. Formation of Gut
Primordial gut at the beginning of the 4th week is closed at its:
Cranial end by oropharyngeal membrane
Caudal end by the cloacal membrane

24. Formation of Gut
Primordial gut forms during the 4th week as the head, tail and lateral fold incorporate the dorsal part of the yolk sac into the embryo
The endoderm of the primordial gut gives rise to most of the epithelium and glands of the digestive tract

25. Formation of Gut
The epithelium at the cranial and caudal ends of the tract is derived from ectoderm of the stomodeum (mouth) proctodeum (anal pit)
The muscular, connective tissue, and other layers of the wall of the digestive tract are derived from the splanchnic mesenchyme surrounding the primordial gut

26. Formation of Gut
For descriptive purposes the primordial gut is divided into 3 parts:
Foregut
Midgut
Hindgut