‫بسم هللا الرمحن الرحيم‬
Yolk Sac
Yolk Sac
Origin:
Primary yolk sac
- Starts as vacuoles in the
hypoblastic cells of the
inner cell mass.
- These vacuoles coalease
together forming a single
cavity called primary yolk
sac. Its roof and side wall is
formed by hypoblasts
(endoderm).
Secondary yolk sac:
- Primary yolk sac is reduced in size to be transformed to the
Secondary yolk sac
After folding:
The secondary yolk sac is
divided into 3 parts:
- Intraembryonic yolk sac.
Part of the yolk sac will be
taken inside the embryo
(Foregut is made by head
fold, hind gut is made by
tail fold and midgut is
made mainly by lateral
fold).
- Extra-embryonic yolk sac
(part of the yolk sac
outside the embryo).
- Vitellointestinal duct or
yolk stalk connecting the
intra & extra- embryonic
parts of the yolk sac.
Function & fate:
1. It gives the primordial germ cells (future ova in female & future sperm
in male)
2. The endoderm will give the mucous membrane which lines the gut &
respiratory tract.
3. The splanchnopleuric primary mesoderm that surrounds the yolk sac
will give the vitelline arteries (future superior mesenteric artery) & vitelline
vein (future liver sinusoids- portal & hepatic veins).
Abnormalities
1. Faecal umbilical fistula: due to persistence of the yolk stalk, so the
umbilicus will charge faces.
2. Vitelline sinus: one end of the yolk stalk is opened & the other end is
closed.
3. Vitelline cyst: the 2 ends of the yolk stalk are closed but still a part of
the yolk stalk is opened between the 2 ends forming a cyst.
4. Fibrous band: obliteration of the yolk stalk occurs but it remains as a
Fibrous band. Intestinal obstruction may occur as a complication.
5. Meckel' diverticulum.
- It occurs due to patent intestinal end of the vitello-intestinal
duct (yolk stalk).
- The rest of the duct is obliterated forming a fibrous band.
- Meckel's diverticulum is a fingerlike pouch about 3-6 cm (2
inches) long that arises from the antimesentric border of the
ileum,
- It is 2 feet from the iliocecal junction .
- It occurs in 2-4% of people and is 3-5 times more prevalent in
males than females.
- Sometimes it becomes inflamed and causes symptoms that
mimic appendicitis.
- It may contain gastric mucosa leads to ulcer in this
diverticulum.
Allantois
Development& structure:
- It develops as a blind
diverticulum from the
caudal part of the yolk sac
(endodermal origin).
- It is surrounded by
splanchnopleuric primary
mesoderm. Later it is
embedded in the body
stalk
Function:
- The mesoderm
surrounds the allantios
give umbilical blood
vessels
Fate:
1. The extraembryonic part lies
inside the umbilical
cord.
2. The intraembryonic part
obliterates forming
the urachus in the
fetus. Later on the
urachus transforms
into ligament called
the median
umbilical ligament.
Allantois and yolk sac
3 wks
9 wks
3 month
Adult
Abnormalities:
1. Urachal fistula: (persistence of the Allantois). In
this defect there is a connection between the
urinary bladder & the umbilicus, so the urine will
charge from the umbilicus.
2. Urachal sinus: one end of the allantois is
opened & the other end is closed
3. Urachal cyst: the 2 ends of the allantois are
closed but still a part of the allantois is opened
between the 2 ends forming a cyst.
Umbilical cord
Development of Umbilical cord
Early: By folding the primitive
cord is formed. It consists of:
1. Yolk stalk (vitello-intestinal
duct) connecting the mid gut
and the extra embryonic yolk
sac with the umbilical cord.
2. Vitelline blood vessels which
are surrounded by
splanchnopleuric primary
mesoderm.
3. Extra-embryonic coelom .
4. Allantois (small diverticulum
attached to the hind gut).
5. Two umbilical arteries
& single umbilical
vein. These vessels
originated from
mesoderm around the
allantios
(connecting stalk).
- All of these contents
are surrounded by
amniotic membrane.
Structures forming the primitive
umbilical cord (Summary)
1. Yolk stalk and the
vitelline blood
vessels.
2. Ectoderm of the
amnion.
3. Body stalk.
4. Allantois.
5. Part of extraembryonic coelom. The small intestine (midgut loop) normally
herniates in extraembryonic coelom till its disappearance at the 10th week where
the intestine is reduced back into the peritoneal cavity.
6. Umbilical blood vessels.
Structures forming the definitive umbilical cord:
 Two umbilical arteries
 One umbilical vein
 Mucoid connective tissue
(Wharton’s jelly)
 Allantois
 Herniating loop of intestine
Characters of the
normal umbilical cord
at birth:
I. Shape:
-Macroscopically: it is 50-60-cm
long, 2 cm in diameter, tortuous,
has false node (due to the unequal
growth of the 2 umbilical arteries), it is
attached to the center of the
placenta.
-Microscopically: the definitive
cord is surrounded by amniotic
membrane. It has 2 umbilical
arteries & single umbilical vein. It
has also the allantois. All these
structures are embedded in
Wharton jelly (mesoderm of body
stalk).
III. Function:
The two umbilical arteries & single umbilical
vein are responsible for the nutrition of the
embryo. The vein carries oxygen to the embryo,
while the arteries carry CO2 & waste product of
the embryo to the mother.
Fate of the cord
(Postnatal changes in the
umbilical cord):
1- Allantois is transformed into
the median umbilical ligament.
2- Umbilical arteries forms the
medial umbilical ligaments.
3- Umbilical vein forms
ligamentum teres of the liver.
4- Extra-embryonic ceolom
disappears.
5- Vitalline arteries forms
superior mesenteric artery.
6- Yolk stalk disappears.
Abnormalities of the umbilical cord:
1. In length (long cord leads to strangulation of the
fetus or short cord leads to premature separation of
the placenta).
2. Abnormality in the attachment of the placenta:
a. Marginal attachment: the cord is attached
to the margin of the placenta.
b. Eccentric attachment: the cord is attached
away from the center of the placenta.
c. Velamentous attachment: the cord ends
before reaching the placenta & the umbilical
vessels reach the placenta via the amniotic
membrane.
 Long umbilical cord (Cord around neck leading to
fetal strangulation.
）
3. Abnormality in the
number: double cord.
4. Single umbilical artery .
5. True node . (in1 % of
pregnancies).
6.Exompholos(omphalocoele)
Intestine may remain
herniated in the cord, fails to
return to the fetal abdominal
cavity. So,the cord should be
ligated away from the
umbilicus .
7. Congenital umbilical
hernia (due to weakness of
abd.wall).
False knot
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