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GI > Development of GI system > Flashcards

Development of GI system Flashcards

1
Q

What are the germ layers?

A

Ectoderm
- nervous system, face, skin, teeth, eyes and inner ear

Mesoderm
- skeletal, muscular, vascular, connective tissue, epithelia of genital and urinary system

Endoderm - gut, epithelium and digestive organs (liver, pancreas), respiratory tract epithelium

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2
Q

How does the gut tube form?

A

Trilaminar disc undergoes folding in a longitudinal (cephalocaudal) and lateral direction

By the end of WEEK 8 (embryonic period) the main organ systems have been established (organogenesis)

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3
Q

How does stomach develop from foregut?

A

Start with gut tube

Expansion 4th-5th week, 7th week, characteristic shape of stomach visible

Combined rotation –> correct positioning

Rotates 90 degrees on longitudinal axis so dorsal mesogastrium forms greater omentum (dorsal swings left, ventral swings right)

Also shunts down a bit (pylorus moves up, fundus moves down)

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4
Q

Peritoneum during stomach development?

A

Gut tube suspended in abdominal cavity by double layers of peritoneum called ‘mesenteries’
- mesenteries is term that remain used for adult

In the region of the stomach these are called mesogastrium

In the region of the stomach only (foregut only), there is an additional ventral mesogastrium to accompany the dorsal mesogastrium

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5
Q

Nerve supply of peritoneum?

A

Nerve supply to visceral peritoneum same as one supplying organ (autonomic)

Nerve supply to parietal peritoneum same as the one supplying the body wall (usually somatic)

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6
Q

Secondary retroperitoneal?

A

Retroperitoneal = organ covered with peritoneum but doesn’t leave posterior abdominal wall - no mesentery

Secondary peritoneal = used to have a mesentery but lost during development

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7
Q

Duodenum development?

A

Loses mesentery quite early in development - comes to lie against posterior abdominal wall and mesentery is absorbed

Quite easy to get to stomach, quite difficult to get to duodenum (front ok but not the back of it)

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8
Q

What are peritoneal pathways?

A

As the peritoneum cannot be pierced, vessels and nerves etc. must find suitable pathways to reach viscera

Aorta → branch into mesenteries → peritoneum

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9
Q

Liver development?

A

The HEPATIC DIVERTICULUM or LIVER BUD, buds from the gut tube and develops BETWEEN the layers of the VENTRAL MESOGASTRIUM (WEEK 3)

Liver cells proliferate as CORDS

Diverticulum NARROWS to form BILE DUCT, outgrowth giving rise to gall bladder and cystic duct

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10
Q

Spleen development?

A

Aggregation of mesenchymal cells BETWEEN the layers of the DORSAL mesogastrium in WEEK 5

Several SPLENULES which usually merge to form one organ (clinical)

Spleen:
Lymphatic tissue
mops up blood, gets rid of dodgy red blood cells
- sickle cell anaemia RBCs - spleen removes them so can remove their spleen (and accessory spleens otherwise they can grow)

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11
Q

Pancreas development?

A

WEEKS 5-8

The pancreas develops from the VENTRAL and DORSAL pancreatic BUDS which lie in the dorsal and ventral mesogastria

  • The VENTRAL bud is closely associated with the developing gall bladder and bile duct

Differential GROWTH of the wall of the DUODENUM and ROTATION of the duodenum brings the BUDS INTO CONTACT and moves the BILE DUCT POSTERIOR to the 1st part of the duodenum

This is clinically significant - relationship between gall bladder and pancreatic disease

  • tumour in head of pancreas (where C of duodenum is) - may present with jaundice due to obstruction of bile duct
  • gall stones may also cause pancreatitis
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12
Q

Ligaments formed by peritoneum?

A

Ventral mesogastrium forms:

- FALCIFORM ligament
- LESSER omentum

Dorsal mesogastrium forms:

- LIENORENAL ligament
- GASTROSPLENIC ligament
- GREATER omomentum
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13
Q

Summary - foregut?

A 
Up to 2nd part duodenum
Celiac artery
Veins follow but drain to HPV
Lymph ollow arteries to nodes around celiac
Para = vagus
Symp = T8 (celiac plexus)
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14
Q

Summary - midgut?

A 
Up to 2/3rds along transverse colon
Superior mesenteric artery
Veins follow but drain to HPV
\:lymph follow arteries to nodes around SM
Para = vagus
Symp = T10
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15
Q

Summary - hindgut?

A 
From 2/3rds transverse colon
Inferior mesenteric artery
Veins follow but drain to HPV
Lymph follow arteries to nodes around IM
Para = vagus
Symp = L1
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16
Q

Primary intestinal loop?

A

Gut grows very fast and with pressure from liver and kidneys is pushed out of cavity through umbilicus: physiological umbilical hernia (from about 6-8 weeks, returns to cavity 12-13 weeks)

Elongated midgut is the primary intestinal loop
Axis of the loop is SMA
Described as having proximal and distal limbs

Proximal limb - jejunum, ileum
Distal limb - caecum, appendix, colon

17
Q

Midgut development - 5th week?

A

Gut tube elongates and bulges ventrally

18
Q

Midgut development - 6th week?

A

Loop grows so long it enters umbilical coelom

19
Q

How does midgut rotate?

A
  • Anticlockwise rotation along axis of SMA
  • Proximal limb continues to elongate and becomes pleated
  • Caecum forms as a diverticulum of caudal limb
  • Vitelline duct regresses
  • Abdominal cavity becomes larger
  • Further rotation of 180 degrees
  • Week 10
  • Proximal limb re-enters first
  • Distal limb re-enters later and caecum lies in subhepatic position
  • Caecum descends to right iliac fossa
  • Appendix develops as outgrowth on caecum
20
Q

Peritoneum of large intestine?

A

Transverse and sigmoid colon retain their mesentery (mesocolon) which attaches them to the posterior abdominal wall

The ascending/descending colon lose their mesentery and become (secondary) retroperitoneal

  • Transverse mesocolon derived from dorsal mesentery
  • Greater omentum derived from dorsal mesogastrium
  • These become adherent so that the transverse colon appears stuck to the posterior aspect of the greater momentum
21
Q

Peritoneum of rectum?

A

Progressively loses peritoneum as it descends through the pelvis

Upper third of rectum: anterior and lateral peritoneal covering

Middle third: anterior covering only

Lower third: no peritoneum

Lower third of rectum is ‘sub peritoneal’

22
Q

Development of cloaca?

A

Blind ending - terminal portion of the hindgut (ENDODERM) - primitive cloaca (sewer) with ECTODERM forming CLOACAL MEMBRANE

URORECTAL SEPTUM (mesenchyme) separates cloaca into UROGENITAL SINUS anteriorly and RECTUM posteriorly

CLOACAL MEMBRANE divided into urogenital and anal membranes

Allantois degenerates into fibrous cord - the URACHUS (median umbilical ligament)

23
Q

Development of anal canal?

A

Upper part of anal canal derived from hindgut endoderm
Lower part of anal canal derived from ectoderm (proctadaeum)

Proliferation of endoderm OCCLUDES the anal canal (week 7)
Week 8 - ECTODERM proliferates so that ANAL PIT forms
Weel 9 - Recanalises, breakdown of membrane

Division between endoderm and ectoderm (PECTINATE line at base of anal columns)

Clinically significant because of nerve supply, blood supply and lymphatic drainage

24
Q

Vascular supply of rectum?

A

Arterial supply from superior, middle & inferior rectal arteries

Plexuses around the rectum/anal canal drain to superior, middle & inferior rectal veins

Superior rectal vein drains to inferior mesenteric vein and then to the liver (hepatic portal system)

Middle and inferior rectal veins drain into caval system

25
Q

Abnormalities of GI development?

A

Persistent urachus, cysts or fistulae - closure of allantois

Fistulae - hindgut and urethra or vagina - misconnection between parts of the cloaca

Malroatation, herniation - omphalocele (hernia where abdominal organs protrude into umbilical cord), remnants of vitello-intestinal duct - Meckel’s diverticulum, (slight bulge in small intestine with remnants of vitelline duct) ligaments and fistulae

Imperforate anus - anal membrane thickens/fails to rupture

Anal agenesis - anal pit fails to form or incomplete separation of the cloaca by the urorectal septum

Rectal atresia - abnormal recanalisation/damaged blood supply - thick layer of CT between the anal canal and rectum

Epispadias (urethra ends up on dorsum of penus) exstrophy of the bladder/rectum - associated abdominal wall defects

Hirschprung’s disease - neural crest may not develop - aganglionic segment

26
Q

Types of fistulae?

A

Rectourethral bulbar fistula (low)
Rectobladderneck fistula (high)
High rectovaginal fistula
Vestibular fistula

GI (12 decks)

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