Transcript 8.Folding of Embryo

Folding of the Embryo
Formation of Gut
Endodermal Derivatives
By: Dr. Mujahid Khan
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
Folding in Median Plane
 Occurs
in the cranial and caudal ends
 Causing
 Moving
head and tail folds
ventrally as the embryo elongates
cranially and caudally
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
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
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
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
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
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
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
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
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
Derivatives of Endoderm
Endoderm gives rise to the epithelial lining
of:
 Trachea
 Bronchi
 lungs
Derivatives of Endoderm
Endoderm gives rise to the epithelial lining of:

Gastrointestinal tract

Liver

Pancreas

Urinary bladder

urachus
Derivatives of Endoderm
Endoderm gives rise to the epithelial lining of:

Pharynx

Thyroid

Tympanic cavity

Pharyngotympanic tube

Tonsils

Parathyroid glands
Formation of Gut
gut at the beginning of the 4th
week is closed at its:
 Primordial
 Cranial
end by oropharyngeal membrane
 Caudal
end by the cloacal membrane
Formation of Gut
gut forms during the 4th week
as the head, tail and lateral fold
incorporate the dorsal part of the yolk sac
into the embryo
 Primordial
 The
endoderm of the primordial gut gives
rise to most of the epithelium and glands
of the digestive tract
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
Formation of Gut
For descriptive purposes the primordial gut
is divided into 3 parts:
 Foregut
 Midgut
 Hindgut