Folding of the Embryo Formation of Gut Endodermal Derivatives

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Folding of the EmbryoFormation of GutEndodermal
Derivatives

• Dr. Mujahid Khan

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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

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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

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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 the developing forebrain grows cranially
  beyond the oropharyngeal membrane
• Overhangs the developing heart

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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

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Head Fold

• Septum transversum lies caudal to the 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

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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

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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

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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

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Folding in Horizontal Plane

• Folding on sides of the embryo produces right and
  left lateral folds
• It is produced by rapidly growing spinal cord and
  somites
• Ventrolaterally rolling of the edges of embryonic
  disc form roughly cylindrical embryo

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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 lateral folding the connection is reduced
  to yolk stalk

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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

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Derivatives of Endoderm

• Endoderm gives rise to the epithelial lining
  of
• Trachea
• Bronchi
• Lungs
• Pharyngotympanic tube
• Tympanic cavity

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Derivatives of Endoderm

• Endoderm gives rise to the epithelial lining of
• Gastrointestinal tract
• Urinary bladder
• Urethra
• urachus

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Derivatives of Endoderm

• Endoderm gives rise to the parenchyma of
• Pharynx
• Thyroid
• Tonsils
• Parathyroid glands
• Liver
• Pancreas

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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

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Formation of Gut

• Primordial gut forms during the 4th week as the
  head, tail and lateral folds 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

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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

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Formation of Gut

• For descriptive purposes the primordial gut is
  divided into 3 parts
• Foregut
• Midgut
• Hindgut