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