Folding of Embryo

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