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Yolk Sac
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Origin:Primary yolk sac
- Starts as vacuoles in thehypoblastic cells of theinner cell mass.
- These vacuoles coaleasetogether forming a singlecavity called primary yolksac. Its roof and side wall isformed by hypoblasts(endoderm).
Yolk Sac
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Secondary yolk sac:- Primary yolk sac is reduced in size to be transformed to theSecondary yolk sac
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After folding:The secondary yolk sac isdivided into 3 parts:
- Intraembryonic yolk sac.Part of the yolk sac will betaken inside the embryo(Foregut is made by headfold, hind gut is made by
tail fold and midgut ismade mainly by lateralfold).- Extra-embryonic yolk sac(part of the yolk sacoutside the embryo).- Vitellointestinal duct oryolk stalk connecting theintra & extra- embryonic
parts of the yolk sac.
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Function & fate:1. It gives the primordial germ cells (future ova in female & future spermin male)2. The endoderm will give the mucous membrane which lines the gut &respiratory tract.
3. The splanchnopleuric primary mesoderm that surrounds the yolk sacwill give the vitelline arteries (future superior mesenteric artery) & vitellinevein (future liver sinusoids- portal & hepatic veins).
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Abnormalities1. 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 isclosed.3. Vitelline cyst: the 2 ends of the yolk stalk are closed but still a part ofthe yolk stalk is opened between the 2 ends forming a cyst.
4. Fibrous band: obliteration of the yolk stalk occurs but it remains as aFibrous band. Intestinal obstruction may occur as a complication.
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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 (2inches) 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 inmales than females.
- Sometimes it becomes inflamed and causes symptoms thatmimic appendicitis.
- It may contain gastric mucosa leads to ulcer in thisdiverticulum.
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Development& structure:- It develops as a blinddiverticulum from thecaudal part of the yolk sac(endodermal origin).- It is surrounded by
splanchnopleuric primarymesoderm. Later it isembedded in the bodystalk
Function:- The mesodermsurrounds the allantiosgive umbilical bloodvessels
Allantois
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Fate:1. The extra-
embryonic part lies
inside the umbilical
cord.
2. The intra-
embryonic partobliterates forming
the urachus in the
fetus. Later on the
urachus transformsinto ligament called
the median
umbilical ligament.
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Allantois and yolk sac
3 wks 9 wks
3 month Adult
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Abnormalities:
1. Urachal fistula:(persistence of the Allantois). Inthis 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 isopened & the other end is closed
3. Urachal cyst: the 2 ends of the allantois are
closed but still a part of the allantois is openedbetween the 2 ends forming a cyst.
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Umbilical cord
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Development of Umbilical cord
Early:By folding the primitive
cord is formed. It consists of:
1. Yolk stalk (vitello-intestinalduct) connecting the mid gutand the extra embryonic yolk
sac with the umbilical cord.2. Vitelline blood vessels which
are surrounded bysplanchnopleuric primarymesoderm.
3. Extra-embryonic coelomcontaining loop of herniatedmid gut.
4.Allantois (small diverticulumattached to the hind gut).
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5. Two umbilical arteries& single umbilical
vein. These vesselsoriginated frommesoderm around the
allantios(connecting stalk).
- All of these contentsare surrounded byamniotic membrane.
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Structures forming the primitive
umbilical cord (Summary)
1. Yolk stalk and the
vitelline bloodvessels.
2. Ectoderm of the
amnion.3. Body stalk.
4. Allantois.
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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.
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Structures forming the definitive umbilical cord:
Two umbilical arteries
One umbilical vein
Mucoid connective tissue
(Whartons jelly) Allantois
Herniating loop of intestine
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Characters of thenormal umbilical cord
at birth:I. Shape:-Macroscopically:it is 50-60-cmlong, 2 cm in diameter, tortuous,has false node (due to the unequal
growth of the 2 umbilical arteries), it isattached to the center of theplacenta.-Microscopically:the definitivecord is surrounded by amniotic
membrane. It has 2 umbilicalarteries & single umbilical vein. Ithas also the allantois. All thesestructures are embedded inWharton jelly (mesoderm of body
stalk).
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III. Function:The two umbilical arteries & single umbilicalvein are responsible for the nutrition of the
embryo. The vein carries oxygen to the embryo,while the arteries carry CO2 & waste product ofthe embryo to the mother.
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Fate of the cord(Postnatal changes in theumbilical cord):1- Allantois is transformed into
the median umbilical ligament.
2- Umbilical arteries forms themedial umbilical ligaments.
3- Umbilical vein formsligamentum teres of the liver.
4- Extra-embryonic ceolomdisappears.
5- Vitalline arteries formssuperior mesenteric artery.
6- Yolk stalk disappears.
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Abnormalities of the umbilical cord:1. In length(long cordleads to strangulation of the
fetus or short cordleads to premature separation ofthe 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 endsbefore reaching the placenta & the umbilical
vessels reach the placenta via the amniotic
membrane.
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Long umbilical cord (Cord around neck leading tofetal strangulation.
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3. Abnormality in thenumber:double cord.4. Single umbilical artery .
5. True node . (in1 % ofpregnancies).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 theumbilicus .7. Congenital umbilicalhernia(due to weakness of
abd.wall).
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False knot
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