Folding of the EmbryoFormation of GutEndodermal DerivativesBy: Dr. Mujahid Khan

Folding Of EmbryoFlat trilaminar disc folds into a somewhat cylindrical embryoFolding occurs in both median & horizontal planesResults from rapid growth of the embryoLong axis increases rapidly than the sidesOccurs simultaneously on both axisConstriction at the junction of embryo & yolk sac

Folding in Median PlaneOccurs in the cranial and caudal ends

Causing head and tail folds

Moving ventrally as the embryo elongates cranially and caudally

Head FoldAt 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

Head FoldSeptum 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

Head FoldSeptum 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

Tail FoldResults 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

Tail FoldTerminal 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

After Tail FoldThe 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

Folding in Horizontal PlaneFolding 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

Folding in Horizontal PlaneAs 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

Folding in Horizontal PlaneUmbilical 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

Derivatives of Endoderm Endoderm gives rise to the epithelial lining of:

Trachea

Bronchi

lungs

Derivatives of EndodermEndoderm gives rise to the epithelial lining of:

Gastrointestinal tract

Liver

Pancreas

Urinary bladder

urachus

Derivatives of EndodermEndoderm gives rise to the epithelial lining of:

Pharynx

Thyroid

Tympanic cavity

Pharyngotympanic tube

Tonsils

Parathyroid glands

Formation of GutPrimordial gut at the beginning of the 4th week is closed at its:

Cranial end by oropharyngeal membrane

Caudal end by the cloacal membrane

Formation of GutPrimordial 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

Formation of GutThe 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

Formation of Gut For descriptive purposes the primordial gut is divided into 3 parts:

Foregut

Midgut

Hindgut