Embryology Alimentarysystem2 Chap 13

8/12/2019 Embryology Alimentarysystem2 Chap 13

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ALIMENTARY SYSTEM II

GASTROINTESTINALTRACT


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Profile view of a human embryo estimated at twenty or twenty-one days old. (Dorsal aorta labeled at center left.)


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Dorsal aorta

Each primitive aortareceives anteriorly a veinthe vitelline veinfrom the yolk-sac, and isprolonged backward on the lateral aspect of the

notochordunder the name of the dorsal aorta. The dorsal aort give branches to the yolk-sac,

and are continued backward through the body-stalk as the umbilical arteriesto the villiof the

chorion. The two dorsal aortae combine to become the

descending aortain later development
http://en.wikipedia.org/w/index.php?title=Primitive_aorta&action=edit&redlink=1http://en.wikipedia.org/wiki/Vitelline_veinhttp://en.wikipedia.org/wiki/Yolk-sachttp://en.wikipedia.org/wiki/Notochordhttp://en.wikipedia.org/wiki/Umbilical_arterieshttp://en.wikipedia.org/wiki/Villihttp://en.wikipedia.org/wiki/Chorionhttp://en.wikipedia.org/wiki/Descending_aortahttp://en.wikipedia.org/wiki/Descending_aortahttp://en.wikipedia.org/wiki/Chorionhttp://en.wikipedia.org/wiki/Villihttp://en.wikipedia.org/wiki/Umbilical_arterieshttp://en.wikipedia.org/wiki/Notochordhttp://en.wikipedia.org/wiki/Yolk-sachttp://en.wikipedia.org/wiki/Yolk-sachttp://en.wikipedia.org/wiki/Yolk-sachttp://en.wikipedia.org/wiki/Vitelline_veinhttp://en.wikipedia.org/w/index.php?title=Primitive_aorta&action=edit&redlink=1


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TOPICS

Highlights.

Introduction.

Derivation of individual parts of alimentary tract./foregut,midgut,hindgut/

Rotation of the gut.

Fixation of the gut. Timetableof some events described in this lecture.


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HIGHLIGHTS

ENDODERM

At first it is in the form of a flat sheet,

Converted into a tubeby formation of head, tail and lateral folds

of embryonic disc.

This tube is the gut.

THE GUT consists of foregut, midgutandhindgut.

The midgut is at first in wide communication with the yolk sac.

Later it becomes tubular.

Part of midgut forms a loop that is divisible into prearterial and

postarterial segments.


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Cloaca

It is themost caudalpart of the hind gut.

It is partitionedto form the primitive rectum (dorsal) and

the primitive urogenital sinus. The oesophagusis derived from the foregut.

The stomach is derived from the foregut.

DUODENUM

The superior part and the upper part of the descending partis

derived from the foregut,

The restof the duodenum develops from the midgut loop.

HIGHLIGHTS (continue)


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Thejejunum and ileumare derived from the prearterialsegment of themidgut.

The postarterial segment of the midgut loop gives off a caecal bud.

The caecum and the appendix are formed by enlargement of the caecal

bud. The ascending colon develops from the postarterial

segment of the midgut loop. After ascending colon formation the gut undergoes rotation.

As a result of rotation; the caecum and ascending colon come tolie on the right side;

The jejunum and ileum lie mainly in the left-half of the abdominalcavity.

HIGHLIGHTS (continue)


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INTRODUCTION Epithelial lining of the various parts of the gastrointestinal tract is

endodermal origin. In the mouth and anal canal, some of the epithelium is derived from

ectoderm(stomatodaeum, proctodaeum).

Head and tail folds

Part of the Yolk sac is enclosed within the embryo to form the

primitive gut. Gut is in free communication with the yolk sac.

Foregut cranial to communication,

Hind gut ------?

Midgut --------??

Cranially Buccopharyngeal membrane separates the foregut fromthe stomatodaeum.

Caudally Cloacal membrane separate the hindgut from theproctodaeum.

Later both membrane disappear and gut opens to the exterior at 2 ends.


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While the gut is being formed, the circulatory system of the embryoundergoes considerable development.

A midline artery, the dorsal aorta, is established and comes to liejust dorsal to the gut.

It gives off a series of branches to the gut.

Vitelline arteries connect midgut with the yolk sac. Most of the ventral arteries disappear, only three of them remain;

Coeliac, superior and inferior mesenteric arteries. SMA, IMA

Wide communication between midgut and yolk sac is gradually

narrowed down, The midgut assumes the form of a loop.

The superior mesenteric artery runs in the mesentery of this loop to its apex.

The loop has Prearterial (proximal) andpostarterial (distal)segments.

INTRODUCTION (continue)


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After a number of weeks, the midgut loop comes to lie outside theabdominal cavity of the embryo.

It passes through the umbilical opening into a part of the extra-embryonic coelom (that persists in relation to the most proximal part of the umbilical cord).

The loop is subsequently withdrawn into the abdominal cavity. Allantoic diverticulum opens into the ventral aspect of the hindgut.

The part of the hindgut caudal to the attachment iscalled the cloaca.

The cloaca shows subdivision into a broad ventral part and narrowdorsal part.

Urogenital septum separate the two parts.



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The ventral subdivision is called the primitive urogenital sinusand gives origin to some parts of the urogenital system.

The dorsal subdivision is called the primitive rectum. It formsthe rectum and part of the anal canal.

The urogenital septum grows towards the cloacal membraneand fuses with it.

The cloacal membrane is divided into

ventral urogenital membrane (related to the urogenital sinus).

and dorsal anal membrane (related to the rectum). Mesoderm around the anal membrane becomes heaped up

with the result that the anal membrane comes to lie at thebottom of a pit called the anal pit, or proctodaeum.

The anal pit contributes to the formation of the anal canal.



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DERIVTIVES OF THE FOREGUT

1) Part of the floor of the mouth, including the tongue.

2) Pharynx.

3) Various derivatives of the pharyngeal pouches, andthe thyroid.

4) Oesophagus.

5) Stomach.

6) Duodenum: (1st part +1st of 2ndpart up to the major duodenal papilla)

7) Liver and extra-hepatic biliary system.8) Pancreas.

9) Respiratory system.


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1. Duodenum: (2nd of the 2ndpart distal to the majorduodenal papilla; horizontal and ascending part).

2. Jejunum.

3. Ileum.

4. Caecum and appendix.

5. Ascending colon.6. Right 2/3rdof the transverse colon.

DERIVTIVES OF THE MIDGUT


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Left 1/3rdof the transverse colon.

Descending and pelvic colon.

Rectum. Upper part of the anal canal.

Parts of the urogenital system derived from

the primitive urogenital sinus.

DERIVTIVES OF THE HINDGUT


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Note

At this stage,

The endodermof the foregut, midgut and

hindgut gives rise only to the epithelial lining

of the intestinal tract.

The smooth muscle, connective tissue and

peritoneum are derived from

splanchnopleuric mesoderm.


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DERIVATION OF INDIVIDUAL PARTS OF THE

ALIMENTARY TRACT

OESOPHGUS

The oesophagus is developed from the foregut,

Between the pharynx and the stomach.

At first, it is short,butelongates with the;

Formation of the neck,

Descent of the diaphragm,

Enlargement of the pleural cavities.

The musculature is derived from mesenchymesurrounding the foregut.

Around the upper 2/3rdthe mesenchyme forms striatedmuscle.

Around lower 1/3rdthe mesenchyme forms smooth muscle (asover the rest of the gut).


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At first, it is seen as a fusiform dilatation of the foregut, just distal to the

oesophagus.

Dorsal mesogastrium attaches the dorsal borderof the stomach to the

posterior abdominal wall.

Ventral mesogastriumattaches the ventral borderof the stomach tothe septum transversum.

The liver and the diaphragm are formed in the substance of the

septum transversum.

The ventral mesogastrium now passes from the stomach to the liverand from the liver to the diaphragm and anterior abdominal wall.

Lesser omentum = ventral mesogastrium between liver and stomach.

Coronary ligament= ventral mesogastrium between liver and diaphragm.

Falciform ligament = ventral mesogastrium between liver and anterior abdominal wall.


ALIMENTARY TRACT

STOMACH


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The dorsal mesogastrium is divided by the development ofthe spleen into;

Gastrosplenic ligament = between stomach and spleen.

Lienorenal ligament = between spleen and posterior

abdominal wall. The stomach undergoes differential growth resulting in

considerable alteration in its shape and orientation:

The original left surface becomes anterior surface.

The original right surface becomes the posterior surface.

The original ventral border comes to face upward and to theleft = lesser curvature.

The original dorsal border comes to face downwards and tothe left = greater curvature.


ALIMENTARY TRACT

STOMACH (continue)


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The superior (1st) part + the upper half of the descending (2nd)part of the duodenum are derived from the foregut.

The rest of the duodenum develops from the most proximal part ofthe midgut.

Mesoduodenum is a mesentery attaches the duodenum to theposterior abdominal wall.

Mesoduodenum then fuses with the peritoneum of the posteriorabdominal wall, with the result that most of the duodenum becomesretroperitoneal.

Mesoduodenum persists in relation to a small part of the duodenumadjacent to the pylorus. (duodenal cap/ radiograph).

Branches of Coeliac artery supply the proximal part of the duodenum.

Branches of Superior mesenteric artery supply the distal part of theduodenum.


ALIMENTARY TRACT

DUODENUM


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The jejunum and most of the ileum are derived from

the prearterial segment of the midgut loop.

The terminal portion of the ileum is derived from the

postarterial segmentproximal to the caecal bud.


ALIMENTARY TRACT

JEJUNUM AND ILEUM


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CAECUM AND APPENDIX

Caecal budis derived from the postarterial segment of themidgut.

The caecum and the appendix are formed by the enlargement of

this bud. The proximal part of the caecal bud grows rapidly to form the

caecum.

The distal part of the caecal bud remains narrow and forms theappendix.

The appendix arises from the apex of the caecum. The lateral (right) wall of the caecum grows much more rapidly

than the medial (left) wall,

The point of attachment of the appendix with caecum comes to lieon the medial side.


ALIMENTARY TRACT


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Ascending colondevelops from the postarterial segment of the midgutloop distal to the caecal bud.

Transverse colon; The right 2/3rddevelop from the postarterial segment of the midgut loop.

The left 1/3rdarises from the hindgut.

The right 2/3rdare supplied by the superior mesenteric artery.

The left 1/3rdis supplied by the inferior mesenteric artery.

Descending colondevelops from the hindgut.

The rectum is derived from the primitive rectum(dorsal subdivision of the cloaca).

Anal canalis formed partly from the endoderm of the primitive rectum, And partly from the ectoderm of the anal pit (proctodaeum).

Pectinate line= the line of junction of the endodermal and ectodermalparts of the anal canal is represented by the anal valves.

DERIVATION OF INDIVIDUAL PARTS OF THE ALIMENTARY TRACT


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ROTATION OF THE GUT

After its formation,the midgut loop lies outside the

abdominal cavity of the embryo (in a part of the extra-embryonic coelom that persists near the umbilicus).

The loop has a prearterial (proximal) segment and postarterial(distal) segment.

Initially,the loop lies in the sagittal plane,its proximal segmentbeing cranial and ventral to the distal

segment.

The midgut loopnow undergoes rotation.

This rotation plays a very important part in establishingthe definitive relationships of various part of the intestine.


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1. The loop undergoes Anticlockwise rotation by 90, (theprearterial segment lies on the right, and the postarterial segment lies on the left).

2. The prearterial segment undergoes great increase in lengthto form the coils of the jejunum and ileum.(loops still out side theabdominal cavity, to the right of the distal limb).

3. The coils of the jejunum and ileum (proximal segment)return to the abdominal cavity.As they do so, the midgutloop undergoes further anticlockwise rotation. Jejunumand ileum pass behind the SMA into the left half of theabdominal cavity. Duodenumcomes to lie behind the

artery. The jejunum and ileum occupy the posterior and left partof the abdominal cavity.

ROTATION OF THE GUT

STEPS OF ROTATION

(viewed from ventral side)


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4. Finally,the postarterial segmentof the midgut loop returns to

the abdominal cavity. It also rotates in an anticlockwise

direction. With the result the transverse colon lies anterior to the SMA,

and the caecum comes to lie on the right side.

5. At this stage the caecum lies below the liver, and an ascending

colon cannot be demarcated. Gradually, the caecum descendsto the right iliac fossa, and the ascending, transverse and

descending parts of the colon become distinct.

ROTATION OF THE GUT

STEPS OF ROTATION

(viewed from ventral side)


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FIXATION OF THE GUT

At first all partsof the small and large intestines have amesentery by which they are suspended from the posteriorabdominal wall.

After the completion of rotation of the gut, the duodenum,

theascending colon, the descending colonand the rectumbecome retroperitoneal(by fusion of their mesenteries with the

posterior abdominal wall).

The original mesentery persists as; The mesentery of small intestine,

The transversemesocolon,

Thepelvic mesocolon.


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ANOMALIES OF THE GUT

1. CONGENITAL OBSTRUCTION.

2. ABNORMALCOMMUNICATIONOR FISTULA.

3. DUPLICATION.4. DIVERTICULA.

5. ERRORS OF ROTATION.

6. ERRORS OF FIXATION.

7. SITUS INVERSUS.


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COGENITAL OBSTRUCTION (SKIP)

This maybe due to a variety of causes.

1) Atresia (continuity of the lumen is interfered by, a segment of the gutmay be missing, replaced by fibrous tissue, or by a septum block the

lumen).

2) Stenosis (abnormal narrowing).3) Non-development of nerve plexusesin the wall of a part of the

intestinal tract. (megacolon or Hirschsprungsdisease)

4) Abnormal thickening of muscular wall. (congenital pyloric stenosis)

5) External pressureby abnormal band or abnormal blood vessels.(bands seen in relation to the duodenum or compressed by annularpancreas)

6) Imperforate anus. (caused by stenosis or atresia of the lower part ofthe rectum or anal canal).


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ABNORMAL COMMUNICATION OR FISTULAE (SKIP)

Fistula is an abnormal communication with other cavities or with thesurface of the body.

Fistulae are most frequently seen in relation to the oesophagus and

the rectum and usually associated with atresia of the normal passage.

1. Tracheo-oesophageal fistula.

2. Incomplete septation of the cloaca. The rectum maycommunicate with the ;

1. Urinary bladder.

2. Urethra.

3. Vagina.

4. Or open onto the perineum at an abnormal site.

These conditions are associated with imperforate anus.


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DUPLICATION

Varying length of the intestinal tract may be

duplicated.

The duplication may form only a small cyst,

Or may be considerable length.

It may or may not communicatewith the rest

of the intestine.


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DIVERTICULA

Diverticula may arise from any part of the gut.

Diverticula are most common in and near theduodenum. (pylorus, fundus of stomach)

Meckels diverticulum (diverticulum ilei); Persistence of vitello-intestinal duct.

It is of surgical importance.

May contain pancreatic tissue.

May contain gastric mucosa.

May give rise to fecal fistula, umbilical sinus, cyst(enterocystoma or vitelline cyst), fibrous band or growths.


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ERRORS OF ROTATION

I. Non-rotation of the midgut loop. (small intestinelies towards the right side of the abdominal cavity, and the largeintestine towards the left).

II. Reversed rotation. (the transverse colon crosses behind theSMA and the duodenum crosses in front of it).

III. Non-return of umbilical hernia;

I. Omphalocoele or exomphalos (herniated parts arecovered only by omentum).

II. Congenital umbilical hernia(muscle layer and skinare absent in the region of the umbilicus, creating adefect).


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ERRORS OF FIXATION

a) Volvulus; where parts of intestine, that arenormally retroperitoneal, may have mesentery.

b) Adhesion; where parts of intestine, which

normally, have a mesentery, may be fixed byabnormal peritoneal attachment.

c) Sub-hepatic caecum, or may descend only to the

lumbar region. Alternatively, it may descend intothe pelvis.


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SITUS INVERSUS

All the abdominal and thoracic viscera are laterally transposed.

All parts normally on the right side are seen on the left side,

and vice versa.

For example, the appendix and duodenum lie on the left side

and the stomach on the right side.

TIMETABLE OF SOME EVENTS DESCRIBED ABOVE


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Age Developmental events

16 days Allontoic diverticulum starts appearing

3 weeks Gut begins to acquire tubular form because of head and tailfoldings.

At the end of 3rdweek the buccopharyngeal membrane ruptures.

4 weeks The fusiform shape of the stomach becomes visible.

5 weeks Stomach rotates and dilates. Intestinal loop begins to form.

Caecal bud can be identified.6 weeks Intestinal loop is well formed.

Urorectal septum starts dividing the cloaca.

Allantois and appendix become clearly visible.

Stomach complete its rotation.

7 weeks Septation of cloaca into rectum and urogenital sinus is completed.Intestinal loop herniates out of the abdominal cavity.

8 weeks Intestinal loop rotates counterclockwise.

9 weeks Anal membrane breaks down.

3 months Head and tail foldings are completed.

H i t d il f i t ti t t th bd i l it

TIMETABLE OF SOME EVENTS DESCRIBED ABOVE

Embryology Alimentarysystem2 Chap 13

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