2. CONTENTS UNDER
CONSIDERATION
• INTRODUCTION
• GROSS ANATOMY OF DIGESTIVE SYSTEM
• DIVISIONS OF THE GUT TUBE
• FOREGUT
• MESENTERIES OF STOMACH
• DEVELOPMENT OF PERITONEAL CAVITY
• DEVELOPMENT OF LIVER & PANCREAS
• DEVELOPMENT OF SPLEEN
• MIDGUT
• MESENTERIES OF INTESTINAL LOOPS
• HINDGUT
• IMPORTANT CLINICAL ASSOCIATED WITH EACH VISCERA
3. Formation of primitive gut
• Digestive system is mainly derived from
primitive or primordial gut.
• The primordial gut is endodermal in origin
and develops from the dorsal part of the
yolk sac.
• It (primordial gut) forms during the fourth
week of intrauterine life.
• It develops as a result of longitudinal and
transverse folding of the embryo.
4. Longitudinal folding
• It is mainly due to rapid growth of central
nervous system.
• Ends of the trilaminar germ disc folds
ventrally and produce head and tail folds.
6. • Later (day 22) the developing forebrain grows
cranially beyond the buccopharyngeal
membrane and overhangs the developing
heart.
• Concomitantly, the septum transversum,
primordial heart, pericardial cavity, and
buccopharyngeal or oropharyngeal membrane
move onto the ventral surface of the embryo.
• A small foregut and hindgut has developed.
• Buccopharyngeal membrane is vertical
(ectoderm is anterior, endoderm is posterior).
7. • Pericardial cavity is now inferior to heart
tube.
• Septum transversum is posterior to heart
tube and pericardial cavity.
• Cloacal membrane is vertical.
• Connecting stalk and allantois has gone
inferior to hindgut but they are directed
posteriorly.
8. • Small portions of yolk sac have been
engulfed and incorporated anteriorly and
posteriorly. This is the beginning of foregut
and hind gut formation.
9. • Day 25. Now the
buccopharyngeal/oropharyngeal membrane
and cloacal membrane have completed 180
degree turns (ectoderm is inferior and
endoderm is superior). Note that the rotation is
more in tail fold than head fold.
• Foregut and hindgut have elongated.
• The connecting stalk and allantois are now
directed (project) inferiorly/ventrally.
• The constriction of yolk sac is now obvious.
10. • Relatively advanced stage of cephalo-caudal
folding. Now foregut and hindgut are clearly
visible. Yolk sac is reduced in size. Midgut is
still widely communicating with yolk sac.
11. • Day 28. Oropharyngeal and cloacal
membranes have rotated more and now facing
each other.
• The yolk sac has further constricted. Midgut is
now clear but still connected to yolk sac
through narrow canal called vitelline duct.
• Connecting stalk along with allantois has gone
nearer to vitelline duct and associated
splanchnic mesoderm.
12. • Advanced stage of cephalo-caudal folding.
foregut, midgut and hindgut are clearly visible.
Yolk sac is reduced in size. Midgut is still
communicating with yolk sac through a narrow
duct called vitelline duct.
13. • During longitudinal folding, part of the
endoderm of the yolk sac is incorporated into
the embryo as the foregut and hindgut.
• The foregut lies between the brain and heart.
Oropharyngeal membrane separates the
foregut from the stomodeum.
• The terminal part of the hindgut soon dilates
slightly to form the cloaca (primordium of the
urinary bladder and rectum).
14. • Lateral view of developing 26 day old embryo
showing different parts of gut.
15. • After folding, the septum transversum lies
caudal to the heart where it subsequently
develops into the central tendon of the
diaphragm.
• Before folding, the primitive streak lies cranial
to the cloacal membrane; after folding, it lies
caudal to it.
• After folding the connecting stalk and the
vitelline duct join each other and finally form
umbilical cord.
16. • The folding also affects the
arrangement of the embryonic
coelom (primordium of body
cavities).
• Before folding, the coelom is a
flattened, horseshoe-shaped
single continuous cavity.
18. • After folding, the coelom has become divided
into thoracic and abdominal cavity. Thoracic
cavity or pericardial coelom lies ventral to the
heart and cranial to the septum transversum.
Abdominal (peritoneal) cavity lies caudal to
septum transversum.
• Thoracic (pericardial) and abdominal
(peritoneal) cavities communicate each other
through pleuropericardial canals. The canals
are posterior to septum transversum and lateral
to primitive developing vertebral column.
19. • Diagram of a five week old embryo showing:
• Septum transversum
• Pericardioperitoneal canals
20. Transverse folding
• The flat germinal disc not only folds
cephalo-caudally but also folds in
horizontal or transverse direction.
• The right and left lateral edges of
trilaminar germ disc not only grow laterally
but also move ventrally and finally
medially to meet in the midline.
21. • As the lateral edges of embryonic disc
grow they form right and left lateral folds.
• Transverse folding is produced by the
rapidly growing somites and neural tube.
• The primordium of the ventrolateral wall
grows laterally, ventrally and medially
towards the median plane, rolling the
edges of the embryonic disc and forming a
roughly cylindrical embryo.
22. • By the middle of 3rd week (day 17) a flat
trilaminar germinal disc has formed.
23. • The development of paraxial mesoderm and
formation of somites is mainly responsible for
transverse folding of embryo.
• As the paraxial mesoderm starts developing the
ectoderm is raised bilaterally and neural groove
forms between these bilateral ridges.
• Paraxial mesoderm later forms somites. The
formation of somites and neural tube further
raises surface ectoderm dorsally.
24. • By the day-19 paraxial mesoderm has formed.
• In the lateral plate of mesoderm small spaces
have started developing.
26. • Day-21. intra-embryonic coelom is well
developed now. The yolk sac is decreasing in
size. The embryo is assuming a globular form.
27. • Day-22. Somites have further enlarged. Neural
tube has formed. Intra-embryonic coelom is
wide. Embryo is more globular in shape.
28. • Day 23.
• Yolk sac has
further reduced in
size.
• Dorsal aortae have
joined to form one
aorta.
• Intra-embryonic
coelom is wider.
• Embryo is more
globular in shape.
29. • Day-25.
• amnion is almost
surrounding the
developing embryo.
• yolk sac is constricted.
after lateral folding the
connection is reduced
to a yolk stalk.
• Intra-embryonic coelom
and extra-embryonic
coeloms still
communicating.
30. • Day-28.
• right & left edges of amnion, surface ectoderm
and parietal mesoderm have joined each other.
• Intra-embryonic coelom is now within body
31. • As a result of cephalo-caudal (longitudinal) and
lateral (transverse) folding the embryo assumes
globular form.
• The area of attachment of amnion to the ventral
surface of the embryo is reduced to a relatively
narrow umbilical region.
• Connecting stalk and vitelline duct join and
form umbilical cord.
• The amnion surrounds the umbilical cord from
all sides.
32. • Lateral view of developing 26 day old embryo
showing different parts of gut. The allantois and
vitelline duct have come closer.
33. • As a result of longitudinal and
transverse folding, a portion of
endoderm-lined yolk sac is
incorporated into the
developing embryo to form
primitive or primordial gut.
34. • The endoderm of the primordial gut gives rise
to the epithelium and glands of digestive
system.
• Only a small portion of epithelium at the cranial
and caudal extremities of the tract is derived
from ectoderm of stomodeum (primordial
mouth) and proctodeum (anal pit), respectively.
• All other layers i.e., lamina propria, muscularis
mucosa, submucosa, mauscularis externa and
fibrosa/serosa of the wall of the digestive tract
are derived from the splanchnic mesenchyme
surrounding the primordial gut.
35. • For descriptive purposes the
primordial gut is divided into
four parts:
1.Pharyngeal gut
2.Foregut
3.Midgut
4.Hindgut.
36. • Pharyngeal gut extends from
oropharyngeal membrane to the
respiratory diverticulum.
• Primordial pharynx and its derivatives
(oral cavity, pharynx, tongue, tonsils,
salivary glands, and respiratory
system) develop from pharyngeal gut.
37. • I believe that Pakistan has come into being to
stay forever and is not like a dune of sand
which appears and vanishes with strong winds.
• Although the economy is confronted with
difficult times yet it is a temporary phase and
the country has the strength to get through it
sooner or later.
• Hence we should not lose hope to
make country a greater Pakistan in the days to
come.
