Folding of the Embryo

Folding of the Embryo

1. Folding of the EmbryoFolding of the Embryo
Muhammad Ramzan UL RehmanMuhammad Ramzan UL Rehman

2. Folding Of EmbryoFolding Of Embryo
Flat trilaminar disc folds into a somewhat Flat trilaminar disc folds into a somewhat
cylindrical embryocylindrical embryo
Folding occurs in both median & Folding occurs in both median &
horizontal planeshorizontal planes
Results from rapid growth of the embryoResults from rapid growth of the embryo
Long axis increases rapidly than the sidesLong axis increases rapidly than the sides
Occurs simultaneously on both axisOccurs simultaneously on both axis
Constriction at the junction of embryo & Constriction at the junction of embryo &
yolk sacyolk sac

4. Folding in Median PlaneFolding in Median Plane
Occurs in the cranial and caudal endsOccurs in the cranial and caudal ends
Causing head and tail foldsCausing head and tail folds
Moving ventrally as the embryo elongates Moving ventrally as the embryo elongates
cranially and caudallycranially and caudally

5. Head FoldHead Fold
At the beginning of the 4At the beginning of the 4
thth
week week
Neural folds in the cranial region thickened to Neural folds in the cranial region thickened to
form primordium of the brainform primordium of the brain
Initially the developing brain projects dorsally Initially the developing brain projects dorsally
into the amniotic cavityinto the amniotic cavity
Later grows cranially beyond the oropharyngeal Later grows cranially beyond the oropharyngeal
membranemembrane
Overhangs the developing heartOverhangs the developing heart

7. Head FoldHead Fold
Septum transversum, primordial heart, Septum transversum, primordial heart,
pericardial coelom & oropharyngeal membrane pericardial coelom & oropharyngeal membrane
move onto the ventral surfacemove onto the ventral surface
Endoderm of the yolk sac is incorporated into Endoderm of the yolk sac is incorporated into
the embryo as a foregutthe embryo as a foregut
The foregut lies between the brain & heartThe foregut lies between the brain & heart
Oropharyngeal membrane separates the foregut Oropharyngeal membrane separates the foregut
from the stomodeumfrom the stomodeum

8. Head FoldHead Fold
Septum transversum lies caudal to heart Septum transversum lies caudal to heart
after the folding and develops into central after the folding and develops into central
tendon of diaphragmtendon of diaphragm
Head fold also affects the arrangement of Head fold also affects the arrangement of
the primordium of body cavity which the primordium of body cavity which
consists of a flattened horseshoe shaped consists of a flattened horseshoe shaped
cavity before foldingcavity before folding

10. Tail FoldTail Fold
Results primarily from growth of the distal Results primarily from growth of the distal
part of the neural tubepart of the neural tube
This is primordium of the spinal cordThis is primordium of the spinal cord
As embryo grows, the caudal eminence As embryo grows, the caudal eminence
projects over the cloacal membraneprojects over the cloacal membrane
During folding, part of endoderm is During folding, part of endoderm is
incorporated into the embryo as a hindgut incorporated into the embryo as a hindgut

12. Tail FoldTail Fold
Terminal part of the hindgut soon dilates Terminal part of the hindgut soon dilates
to form the cloacato form the cloaca
Cloaca is the primordium of urinary Cloaca is the primordium of urinary
bladder and rectumbladder and rectum
Before folding primitive streak lies cranial Before folding primitive streak lies cranial
to the cloacal membraneto the cloacal membrane
After folding it lies caudal to itAfter folding it lies caudal to it

13. After Tail FoldAfter Tail Fold
The connecting stalk (primordium of The connecting stalk (primordium of
umbilical cord) is attached to the ventral umbilical cord) is attached to the ventral
surface of the embryosurface of the embryo
Allantois (a diverticulum of yolk sac) is Allantois (a diverticulum of yolk sac) is
partially incorporated into the embryopartially incorporated into the embryo

15. Folding in Horizontal PlaneFolding in Horizontal Plane
Folding on sides of the embryo produces Folding on sides of the embryo produces
right and left lateral foldsright and left lateral folds
Is produced by rapidly growing spinal cord Is produced by rapidly growing spinal cord
and somitesand somites
Ventrolateral rolling of the edges of Ventrolateral rolling of the edges of
embryonic disc form roughly cylindrical embryonic disc form roughly cylindrical
embryo embryo

16. Folding in Horizontal PlaneFolding in Horizontal Plane
As the abdominal walls form, part of As the abdominal walls form, part of
endoderm is incorporated into the embryo endoderm is incorporated into the embryo
as the midgutas the midgut
Initially there is a wide connection Initially there is a wide connection
between midgut & yolk sacbetween midgut & yolk sac
After folding the connection is reduced to After folding the connection is reduced to
yolk stalkyolk stalk

19. Folding in Horizontal PlaneFolding in Horizontal Plane
Umbilical cord forms from the connecting stalkUmbilical cord forms from the connecting stalk
As it forms, ventral fusion of the lateral folds As it forms, ventral fusion of the lateral folds
reduces the region of communication between reduces the region of communication between
intraembryonic and extraembryonic coelomic intraembryonic and extraembryonic coelomic
cavities to a narrow communicationcavities to a narrow communication
Amniotic cavity expands and obliterates Amniotic cavity expands and obliterates
extraembryonic coelomextraembryonic coelom

20. Derivatives of EndodermDerivatives of Endoderm
Endoderm gives rise to the epithelial lining Endoderm gives rise to the epithelial lining
of:of:
TracheaTrachea
BronchiBronchi
lungslungs

21. Derivatives of EndodermDerivatives of Endoderm
Endoderm gives rise to the epithelial lining of:Endoderm gives rise to the epithelial lining of:
Gastrointestinal tractGastrointestinal tract
LiverLiver
PancreasPancreas
Urinary bladderUrinary bladder
urachusurachus

22. Derivatives of EndodermDerivatives of Endoderm
Endoderm gives rise to the epithelial lining of:Endoderm gives rise to the epithelial lining of:
PharynxPharynx
ThyroidThyroid
Tympanic cavityTympanic cavity
Pharyngotympanic tubePharyngotympanic tube
TonsilsTonsils
Parathyroid glandsParathyroid glands

23. Formation of GutFormation of Gut
Primordial gut at the beginning of the 4Primordial gut at the beginning of the 4
thth

week is closed at its:week is closed at its:
Cranial end by oropharyngeal membraneCranial end by oropharyngeal membrane
Caudal end by the cloacal membraneCaudal end by the cloacal membrane

24. Formation of GutFormation of Gut
Primordial gut forms during the 4Primordial gut forms during the 4
thth
week as week as
the head, tail and lateral fold incorporate the head, tail and lateral fold incorporate
the dorsal part of the yolk sac into the the dorsal part of the yolk sac into the
embryoembryo
The endoderm of the primordial gut gives The endoderm of the primordial gut gives
rise to most of the epithelium and glands rise to most of the epithelium and glands
of the digestive tractof the digestive tract

25. Formation of GutFormation of Gut
The epithelium at the cranial and caudal The epithelium at the cranial and caudal
ends of the tract is derived from ectoderm ends of the tract is derived from ectoderm
of the stomodeum (mouth) proctodeum of the stomodeum (mouth) proctodeum
(anal pit)(anal pit)
The muscular, connective tissue, and The muscular, connective tissue, and
other layers of the wall of the digestive other layers of the wall of the digestive
tract are derived from the splanchnic tract are derived from the splanchnic
mesenchyme surrounding the primordial mesenchyme surrounding the primordial
gutgut

26. Formation of GutFormation of Gut
For descriptive purposes the primordial For descriptive purposes the primordial
gut is divided into 3 parts:gut is divided into 3 parts:
ForegutForegut
MidgutMidgut
HindgutHindgut