بسم الله الرحمن الرحيم

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).

Yolk Sac

Secondary yolk sac:- Primary yolk sac is reduced in size to be transformed to the Secondary yolk sac

After folding: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).

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 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.

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 stalkFunction:- The mesoderm surrounds the allantios give umbilical blood vessels

Allantois

Fate:1. The extra-embryonic part lies inside the umbilical cord.2. The intra-embryonic part obliterates forming the urachus in the fetus. Later on the urachus transforms into ligament called the median umbilical ligament.

Allantois and yolk sac3 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 closed3. 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 cordEarly: 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 arteriesOne umbilical veinMucoid connective

tissue (Wharton’s jelly)AllantoisHerniating 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.

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3. Abnormality in the number: double cord.4. Single umbilical artery .5. True node . (in1 % of pregnancies).6.Exompholos(omphalocoele) Intestine may remainherniated 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