Anatomy of the GI Tract

ANATOMY REVIEW COURSE 2009

GASTROINTESTINAL TRACT

ALFONSO C. DANAC, MD, FPCS, FPSGS, FPALESChairman & Associate Professor, Department of AnatomyAngeles University Foundation School of Medicine

Organ Systems of the Body

• Digestive system– Composed of the oral cavity, esophagus, stomach,

small intestine, large intestine, rectum, anus, and liver

– Breaks down food into absorbable units that enter the blood

– Eliminates indigestible foodstuffs as feces

Body Cavities

• The abdominopelvic cavity is separated from the superior thoracic cavity by the dome-shaped diaphragm

• It is composed of two subdivisions– Abdominal cavity – contains the stomach,

intestines, spleen, liver, and other organs– Pelvic cavity – lies within the pelvis and contains

the bladder, reproductive organs, and rectum

Abdominopelvic Regions• Umbilical• Epigastric• Hypogastric• Right and left iliac or

inguinal• Right and left lumbar• Right and left

hypochondriac

Figure 1.11a

Abdominopelvic Regions

Figure 1.11b

Abdominopelvic Quadrants

• Right upper• Left upper• Right lower• Left lower

Figure 1.12

Digestive System: Overview

• The alimentary canal or gastrointestinal (GI) tract digests and absorbs food

• Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine

• Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas

Digestive System: Overview

Figure 24.1

Digestive Process

• The GI tract is a “disassembly” line – Nutrients become more available to the body

in each step

Digestive Process• There are six

essential activities: – ingestion,

propulsion, and mechanical digestion

– chemical digestion, absorption, and defecation

Figure 24.2

Essential Activities of Digestion

• Ingestion – taking food into the digestive tract • Propulsion – swallowing and peristalsis

Peristalsis – waves of contraction and relaxation of muscles in the organ walls

• Mechanical digestion – chewing, mixing and churning food

Essential Activities of Digestion• Chemical digestion -

catabolic breakdown of food

• Absorption - movement of nutrients

from the GI tract to the blood or lymph

• Defecation - elimination of indigestible solid

wastes

Figure 24.2Figure 24.2

Digestive System Organs and Peritoneum

• Peritoneum – serous membrane of the abdominal cavity– Visceral – covers

external surface of most digestive organs

– Parietal – lines the body wall

Figure 24.5a

Digestive System Organs and Peritoneum • Peritoneal cavity

– Lubricates digestive organs

– Allows them to slide across one another

Figure 24.5a

Digestive System Organs and Peritoneum

• Mesentery - double layer of peritoneum that provides:

Vascular and nerve supplies to the

visceraA means to hold digestive

organs in place and store fat

• Retroperitoneal organs - organs outside the peritoneum

• Peritoneal organs (intraperitoneal) - organs surrounded by peritoneum

Figure 24.5b

Blood Supply: Splanchnic Circulation

• Arteries and the organs they serve include– The celiac trunk (hepatic, splenic, and left gastric):

spleen, liver, and stomach– Inferior and superior mesenteric: small and large

intestines • Hepatic portal circulation:

– Collects nutrient-rich venous blood from the digestive viscera

– Delivers it to the liver for metabolic processing and storage

Mouth • Oral or buccal cavity:

– Is bounded by lips, cheeks, palate, and tongue – Has the oral orifice as its anterior opening– Is continuous with the oropharynx posteriorly

• To withstand abrasions: – The mouth is lined with stratified squamous epithelium – The gums, hard palate, and dorsum of the tongue are

slightly keratinized

Mouth

Figure 24.7a

Lips and Cheeks • Have a core of skeletal muscles

– Lips: orbicularis oris – Cheeks: buccinators

• Vestibule – bounded by the lips and cheeks externally and teeth and gums internally

• Oral cavity proper– area that lies within the teeth and gums

• Labial frenulum – median fold that joins the internal aspect of each lip to the gum

Figure 24.7b

Lips and Cheeks

Figure 24.7b

Palate• Hard palate – underlain by palatine bones and palatine

processes of the maxillae– Assists the tongue in chewing– Slightly corrugated on either side of the raphe (midline

ridge)• Soft palate – mobile fold formed mostly of skeletal muscle

– Closes off the nasopharynx during swallowing– Uvula projects downward from its free edge

• Palatoglossal and palatopharyngeal arches form the borders of the fauces

Tongue• Occupies the floor of the mouth and fills the oral cavity

when mouth is closed• Functions include:

– Gripping and repositioning food during chewing– Mixing food with saliva and forming the bolus– Initiation of swallowing, and speech

• Intrinsic muscles change the shape of the tongue• Extrinsic muscles alter the tongue’s position• Lingual frenulum secures the tongue to the floor of the

mouth

Tongue• Superior surface bears three types of papillae

– Filiform – give the tongue roughness and provide friction

– Fungiform – scattered widely over the tongue and give it a reddish hue

– Circumvallate – V-shaped row in back of tongue• Sulcus terminalis – groove that separates the tongue into

two areas:– Anterior 2/3 residing in the oral cavity– Posterior third residing in the oropharynx

Tongue

Figure 24.8a

Salivary Glands• Produce and secrete saliva that:

– Cleanses the mouth– Moistens and dissolves food chemicals – Aids in bolus formation– Contains enzymes that breakdown starch

• Three pairs of extrinsic glands – parotid, submandibular, and sublingual

• Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa

Salivary Glands• Parotid – lies anterior to the ear between the

masseter muscle and skin– Parotid duct – opens into the vestibule next to the

second upper molar• Submandibular – lies along the medial aspect of

the mandibular body– Its ducts open at the base of the lingual frenulum

• Sublingual – lies anterior to the submandibular gland under the tongue– It opens via 10-12 ducts into the floor of the mouth

Salivary Glands II

Figure 24.9a

Teeth • Primary and permanent dentitions have formed

by age 21• Primary – 20 deciduous teeth that erupt at

intervals between 6 and 24 months• Permanent – enlarge and develop causing the

root of deciduous teeth to be resorbed and fall out between the ages of 6 and 12 years– All but the third molars have erupted by the end of

adolescence– There are usually 32 permanent teeth

Teeth

Figure 24.10.1

Teeth

Figure 24.10.2

Classification of Teeth• Teeth are classified according to their shape

and function– Incisors – chisel-shaped teeth adapted for cutting or

nipping– Canines – conical or fanglike teeth that tear or pierce– Premolars (bicuspids) and molars – have broad

crowns with rounded tips and are best suited for grinding or crushing

• During chewing, upper and lower molars lock together generating crushing force

Dental Formula• A shorthand way of indicating the number and

relative position of teeth– Written as ratio of upper to lower teeth for the mouth– Primary: 2I (incisors), 1C (canine), 2M (molars)– Permanent: 2I, 1C, 2PM (premolars), 3M

2I 1C 2PM 3M

X 2 (32 teeth)2I 1C 2PM 3M

Tooth Structure• Two main regions – crown and the root• Crown – exposed part of the tooth above the gingiva

(gum)• Enamel – acelluar, brittle material composed of

calcium salts and hydroxyapatite crystals is the hardest substance in the body– Encapsules the crown of the tooth

• Root – portion of the tooth embedded in the jawbone

Tooth Structure• Neck – constriction where the crown and root

come together• Cementum – calcified connective tissue

– Covers the root– Attaches it to the periodontal ligament

• Periodontal ligament– Anchors the tooth in the alveolus of the jaw – Forms the fibrous joint called a gomaphosis

• Gingival sulcus – depression where the gingival borders the tooth

Tooth Structure• Dentin – bonelike material deep to the enamel cap

that forms the bulk of the tooth• Pulp cavity – cavity surrounded by dentin that

contains pulp • Pulp – connective tissue, blood vessels, and nerves• Root canal – portion of the pulp cavity that extends

into the root• Apical foramen – proximal opening to the root canal• Odontoblasts – secrete and maintain dentin

throughout life

Tooth Structure

Figure 24.11

Pharynx• From the mouth, the oro- and laryngopharynx

allow passage of:– Food and fluids to the esophagus– Air to the trachea

• Lined with stratified squamous epithelium and mucus glands

• Has two skeletal muscle layers– Inner longitudinal – Outer pharyngeal constrictors

Esophagus

• Muscular tube going from the laryngopharynx to the stomach

• Travels through the mediastinum and pierces the diaphragm

• Joins the stomach at the cardiac orifice

Deglutition (Swallowing)• Involves the coordinated activity of the tongue,

soft palate, pharynx, esophagus and 22 separate muscle groups

• Buccal phase – bolus is forced into the oropharynx

• Pharyngeal-esophageal phase – controlled by the medulla and lower pons– All routes except into the digestive tract are sealed off

• Peristalsis moves food through the pharynx to the esophagus

Deglutition (Swallowing)

Figure 24.13a-c

Deglutition (Swallowing)

Figure 24.13d, e

Stomach• Chemical breakdown of proteins begins and food is

converted to chyme• Cardiac region – surrounds the cardiac orifice• Fundus – dome-shaped region beneath the diaphragm• Body – midportion of the stomach• Pyloric region – made up of the antrum and canal

which terminates at the pylorus• The pylorus is continuous with the duodenum through

the pyloric sphincter

Stomach

Figure 24.14a

Stomach• Greater curvature – entire extent of the convex

lateral surface• Lesser curvature – concave medial surface• Lesser omentum – runs from the liver to the lesser

curvature• Greater omentum – drapes inferiorly from the

greater curvature to the small intestine• Nerve supply – sympathetic and parasympathetic

fibers of the autonomic nervous system• Blood supply – celiac trunk, and corresponding

veins (part of the hepatic portal system)

Small Intestine: Gross Anatomy• Runs from pyloric sphincter to the ileocecal valve • Has three subdivisions: duodenum, jejunum, and ileum• The bile duct and main pancreatic duct:

– Join the duodenum at the hepatopancreatic ampulla – Are controlled by the sphincter of Oddi

• The jejunum extends from the duodenum to the ileum• The ileum joins the large intestine at the ileocecal

valve

Liver• The largest gland in the body• Superficially has four lobes – right, left,

caudate, and quadrate• The falciform ligament:

– Separates the right and left lobes anteriorly– Suspends the liver from the diaphragm and

anterior abdominal wall• The ligamentum teres:

– Is a remnant of the fetal umbilical vein– Runs along the free edge of the falciform ligament

Liver: Associated Structures• The lesser omentum anchors the liver to the stomach• The hepatic blood vessels enter the liver at the porta

hepatis• The gallbladder rests in a recess on the inferior surface

of the right lobe• Bile leaves the liver via

– Bile ducts which fuse into the common hepatic duct – The common hepatic duct fuses with the cystic duct– These two ducts form the bile duct

Liver: Associated Structures

Figure 24.20

The Gallbladder

• Thin-walled, green muscular sac on the ventral surface of the liver

• Stores and concentrates bile by absorbing its water and ions

• Releases bile via the cystic duct which flows into the bile duct

Pancreas• Location

– Lies deep to the greater curvature of the stomach– The head is encircled by the duodenum and the tail

abuts the spleen• Exocrine function

– Secretes pancreatic juice which breaks down all categories of foodstuff

– Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes

• The pancreas also has an endocrine function – release of insulin and glucagon

Large Intestine• Has three unique features:

– Teniae coli – three bands of longitudinal smooth muscle in its muscularis

– Haustra – pocketlike sacs caused by the tone of the teniae coli

– Epiploic appendages – fat-filled pouches of visceral peritoneum

• Is subdivided into the cecum, appendix, colon, rectum, and anal canal

• The saclike cecum:– Lies below the ileocecal valve in the right iliac

fossa– Contains a wormlike vermiform appendix

Large Intestine

Figure 24.29a

Colon• Has distinct regions: ascending colon, hepatic

flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon

• The transverse and sigmoid portions are anchored via mesenteries called mesocolons

• The sigmoid colon joins the rectum • The anal canal, the last segment of the large

intestine, opens to the exterior at the anus

Valves and Sphincters of the Rectum and Anus

• Three valves of the rectum stop feces from being passed with gas

• The anus has two sphincters:– Internal anal sphincter composed of smooth muscle– External anal sphincter composed of skeletal muscle

• These sphincters are closed except during defecation

Histology of the Alimentary Canal

• From esophagus to the anal canal the walls of the GI tract have the same four tunics– From the lumen outward they are the mucosa, submucosa,

muscularis externa, and serosa

• Each tunic has a predominant tissue type and specific digestive function

Histology of the Alimentary Canal

Figure 24.6

Mucosa• Moist epithelial layer that lines the lumen of the alimentary

canal

• Its three major functions are:– Secretion of mucus– Absorption of the end products of digestion– Protection against infectious disease

• Consists of three layers: a lining epithelium, lamina propria, and muscularis mucosae

Mucosa: Epithelial Lining• Consists of simple columnar epithelium and

mucus-secreting goblet cells• The mucus secretions:

– Protect digestive organs from digesting themselves– Ease food along the tract

• Stomach and small intestine mucosa contain:– Enzyme-secreting cells and – Hormone-secreting cells (making them endocrine

and digestive organs)

• Lamina Propria– Loose areolar and reticular connective tissue– Nourish the epithelium and absorb nutrients– Contains lymph nodes (part of MALT) important in

defense against bacteria

• Muscularis mucosae – smooth muscle cells that produce local movements of mucosa

Mucosa: Lamina Propria and Muscularis Mucosae

Mucosa: Other Sublayers• Submucosa – dense connective tissue containing

elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves

• Muscularis externa – responsible for segmentation and peristalsis

• Serosa - the protective visceral peritoneum

- replaced by the fibrous adventitia in the esophagus - retroperitoneal organs have both an adventitia and serosa

Enteric Nervous System• Composed of two major intrinsic nerve plexuses

– Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa

– Myenteric nerve plexus:• Major nerve supply that controls GI tract mobility

• Segmentation and peristalsis are largely automatic involving local reflex arcs

• Linked to the CNS via long autonomic reflex arc

Esophageal Characteristics• Esophageal mucosa – nonkeratinized stratified

squamous epithelium • The empty esophagus is folded longitudinally and

flattens when food is present• Glands secrete mucus as a bolus moves through

the esophagus• Muscularis changes from skeletal (superiorly) to

smooth muscle (inferiorly)

Microscopic Anatomy of the Stomach• Muscularis – has an additional oblique layer that

– Allows the stomach to churn, mix and pummel food physically

– Breaks down food into smaller fragments• Epithelial lining is composed of:

– Goblet cells that produce a coat of alkaline mucus• Gastric pits containing gastric glands that

secrete: – Gastric juice – Mucus – Gastrin

Glands of the Stomach: Fundus and Body• Gastric glands of the fundus and body have

a variety of secretory cells– Mucous neck cells – secrete acid mucus– Parietal (oxyntic) cells – secrete HCl and intrinsic

factor – Chief (zymogenic) cells – produce pepsinogen

• Pepsinogen is activated to pepsin by:– HCl in the stomach– Pepsin itself by a positive feedback mechanism

– Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria

Stomach Lining• The stomach is exposed to the harshest

conditions in the digestive tract• To keep from digesting itself, the stomach has

a mucosal barrier with:– A thick coat of bicarbonate-rich mucus on the

stomach wall– Epithelial cells that are joined by tight junctions– Gastric glands that have cells impermeable to HCl

• Damaged epithelial cells are quickly replaced

Microscopic Anatomy of the Small Intestine

• Structural modifications of the small intestine wall increase surface area– Plicae circulares: deep circular folds of the mucosa

and submucosa– Villi: fingerlike extensions of the mucosa– Microvilli: tiny projections of absorptive mucosal

cells’ plasma membranes

Microscopic Anatomy of the Small Intestine

Figure 24.21a-c

Small Intestine: Histology of the Wall• The epithelium of the mucosa is made up of:

– Absorptive cells and goblet cells – Interspersed T cells (intraepithelial lymphocytes),

and – Enteroendocrine cells

• Intestinal crypts cells secrete intestinal juice• Peyer’s patches are found in the submucosa • Brunner’s glands in the duodenum secrete

alkaline mucus

Large Intestine: Microscopic Anatomy

• Colon mucosa is simple columnar epithelium except in the anal canal

• Has numerous deep crypts lined with goblet cells• Anal canal mucosa is stratified squamous

epithelium • Anal sinuses exude mucus and compress feces• Superficial venous plexuses are associated with

the anal canal• Inflammation of these veins results in itchy

varicosities called hemorrhoids

Large Intestine: Microscopic Anatomy

Figure 24.29b

Large Intestine

• receive fluid waste products and store until released from body.

• excess water absorbed

• feces, stools• defecation, or bowel

movement

Functions of the Large Intestine

• Other than digestion of enteric bacteria, no further digestion takes place

• Vitamins, water, and electrolytes are reclaimed• Its major function is propulsion of fecal material

toward the anus• Though essential for comfort, the colon is not

essential for life

Mesenteries of Digestive Organs

Figure 24.30b

Mesenteries of Digestive Organs

Figure 24.30c

Mesenteries of Digestive Organs

Figure 24.30d

Embryonic Development of the Digestive System

• 3rd week – endoderm has folded and foregut and hindgut have formed

• The midgut is open and continuous with the yolk sac• Mouth and anal openings are nearly formed• 8th week – accessory organs are budding from

endoderm

Embryonic Development of the Digestive System

Figure 24.37

EMBRYOLOGY OF THE GASTROINTESTINAL TRACT

GIT – represented as a straight tube of uniform caliber suspended in the midline of the abdominal cavity by a ventral and dorsal mesentery - lies between 2 layers of mesentery thus has right and left surfaces

Dorsal mesentery – extends along entire length of the tube subdivided as mesogastrium, mesoduodenum, mesojejunum, mesoileum, and mesocolon

Ventral mesentery – extends as far as first inch of duodenum (or umbilicus) referred to as ventral mesogastrium

DIVISIONS OF THE GIT- based on the 3 branches of the aorta supplying the GIT

1. FOREGUT - stomach to second portion of duodenum (entrance of the bile ducts); supplied by

celiac axis

2. MIDGUT - entrance of the bile ducts up to junction of middle and distal third of the transverse colon;

supplied by superior mesenteric artery

3. HINDGUT - from distal third of transverse colon to rectum; supplied by inferior mesenteric artery

DEVELOPMENT, ROTATION AND FIXATION initial short segment becomes the abdominal esophagus- distal to this segment, foregut bulges to form the stomach- lower foregut and entire midgut lengthen to form the redundant umbilical loop which has a descending proximal and ascending distal limb > small intestines develop from descending limb and half of ascending limb > remaining ascending limb bulges to form the cecal bud- elongation of initial colic segment (midgut) forms the ascending colon and proximal 2/3 of transverse colon- terminal colic segment (hindgut) forms the remaining 1/3 of transverse colon, descending, sigmoid and pelvic portions of colon- rectum develops from the primitive cloaca

DEVELOPMENT, ROTATION AND FIXATION

- upper end of hindgut is fixed to posterior abdominal wall by the mesentery of teminal colic segment- colic angle is formed where the initial colic segment and terminal colic segment meets- extremities of the midgut as anchored by the fixed upper duodenum above and and the fixed “colic angle” *fixed points are close together and the mesentery filled space is known as “duodenocolic isthmus”- part of umbilical loop (descending limb and first half of ascending limb) grows rapidly making the embryonic peritoneal cavity unable to contain it *part of the loops grows into the umbilical cord (extra- embryonic celom) resulting in a temporary and physiologic umbilical hernia

DEVELOPMENT, ROTATION AND FIXATION

- SMA supplies the umbilical loop (midgut) which starts as a branch of the abdominal aorta that passes through the duodenocolic isthmus, sends branches to the umbilical loop and ends as the the vitelline artery which supplies the vitello-intestinal duct (omphalomesenteric duct)* *communication between the midgut and yolk sac which normally disappears before birth > may persist as a Meckel’s diverticulum

- SMA runs in the mesentery of the umbilical loop and sends branches upward to the descending limb (prearterial limb and prearterial mesentery) and downward to the ascending limb of the loop (postarterial limb and postarterial mesentery)

DEVELOPMENT, ROTATION AND FIXATION

5th week: rotation takes place - starts while midgut loop is still in the umbilical cord *growth of right lobe of liver starts rotation by pressing on prearterial segment pushing it to the right and downward; postarterial segment is pushed to the left and upward *liver growth rotates umbilical loop 90° counterclockwise >primitive left surface of mesentery now faces upward, primitive right surface faces downward >arrest of rotation: appendix and cecum is found on

on the left side of the body

DEVELOPMENT, ROTATION AND FIXATION10th week: umbilical loop returns to abdominal cavity in definite and orderly manner - starts with proximal part of prearterial segment *SMA is fixed firmly from its origin at the aorta to its termination at the umbilicus forming a taut cord - return of the small gut starts to the right of the SMA but since the intra-abdominal space to the right is small, coils that were reduced first are pushed to the left and behind the artery *as coils pass to the left, they encounter the dorsal mesentery of the hindgut which occupies the midline which is also pushed to the left ahead of the small intestines > descending colon now occupies left flank and “colic angle” is pushed upward and to the left to form the splenic flexure; cecum and right half of colon follows crossing in front of origin of the SMA

DEVELOPMENT, ROTATION AND FIXATION

- outcome: 1. Duodenum lies behind the SMA2. Transverse colon is in front of SMA3. Small gut travels from the left upper to the right lower

quadrant of the abdomen4. Descending colon is pushed to the left5. Cecum and appendix are under the liver6. No ascending colon

- as rotation continues, descent of cecum occurs until it reaches the right iliac fossa and fixation of the gut to the posterior abdominal wall takes place *ascending colon is now formed

DEVELOPMENT, FIXATION AND ROTATION- ascending, transverse and descending colon has now reached its final position forming an inverted U which embraces the jejunum and ileum- cecum, ascending colon and ascending mesocolon (postarterial) fuses with the right parietal peritoneum- upper limit of ascending mesocolon overlies a part of a loop of duodenum and pancreatic head, which aids in fixing these structures to the posterior abdominal wall- transverse colon and mesocolon hangs free- two transverse “colic angles” (hepatic and splenic flexure) are fixed- jejunal and ileal loops hang free by a common jejuno-ileal mesentery which passes obliquely from the duodeno- jejunal flexure to the ileocecal angle

DEVELOPMENT, ROTATION AND FIXATION- descending colon and mesocolon fuses with left parietal peritoneum- sigmoid colon does not fuse and forms an intersigmoid recess or fossa of variable depth- layers are not firmly fixed; with no vessels, nerves or other vital structures passing between them *may be restored to embryologic state during dissection through these cleavage planes-lateral fixation occurs in the git *to restore to embryologic state, incise lateral to the segment and dissect in medial direction thus preserving the primitive dorsal mesentery- fusion and fixations are variable

FETAL CIRCULATION- fetus receives oxygenated blood from the placenta through

the umbilical vein > vein passes through the liver and enters the IVC

through the ductus venosus > some blood circulate through the liver and enter

IVC through the hepatic veins- some of the blood from the pulmonary artery that is

pumped into the aorta flows through the hypogastricarteries to the umbilical arteries and out to the placenta for removal of waste products and oxygenation

Umbilical vein – becomes ligamentum teresHypogastric arteries – becomes the lateral umbilical ligaments

surface ectoderm•buccopharyngeal membrane membrane breaks down, creating opening. •anal pit

anal canal (caudal part) anal membrane perforates

primitive endoderm•foregut diverticulum (pocket)

opharyngeal region of foregut olaryngo-tracheal groove oesophageal region of foregut oesophagus

ostomach oglandular/proventricular/pyloric stenosis ofundus/pyloric antrum opyloric sphincter odorsal mesogastrium olieno-renal ligament osplenic primordium

ospleen ogastro-splenic ligament

oduodenum (rostral half)

•foregut-midgut junction oseptum transversum ohepatic diverticulum ocystic primordium

ogall bladder ocommon bile duct ohepatic ducts oliver/gall bladder

ohepatic primordium ohepatic parenchyma ohepatic sinusoids olobes of liver oliver/gall bladder

opancreatic primordium oleft & right lobes obody/tail uncinate process opancreas

•midgut region oduodenum (candal half) ojejunum omidgut ophysiological umbilical hernia omidgut loop returns to peritoneal cavity

•hindgut diverticulum (pocket) ohindgut

odistal hindgut (+urorectal septum) orectum (+perineal body) oanal canal (rostral part)

(+anal sphincter (external & internal)) oano-rectal junction

ocloaca (surface ectoderm?) ocloacal membrane

ourogenital sinus obladder (+perineal body)

bladder

SUMMARIESThe gastrointestinal tract (GIT) extending from the pharyngeal membrane to the cloacal membrane arises from the endoderm of the trilaminar embryo (week 2, 3).

During the 4th week, the 3 distinct portions (fore-, mid- and hind-gut) extend the length of the embryo and will contribute different components of the GIT.

The large mid-gut is generated by lateral embryonic folding which "pinches off" a pocket of the yolk sac, the 2 compartments continue to communicate through the vitelline duct.

The oral cavity (mouth) is formed following breakdown of the buccopharyngeal membrane (=oropharyngeal) and contributed to mainly by the pharynx lying within the pharyngeal arches.

The opening of the GIT means that it contains amniotic fluid, which is also swallowed later in development.

The hepatic diverticulum (liver bud) lies under the septum transversum and is the earliest associated GIT organ that has differentiated, and will occupy a substantial region of the abdomen during development.

Much of the midgut is herniated at the umbilicus external to the abdomen through development. A key step in development is the rotation of this midgut that must occur to place the GIT in the correct abdominal position with its associated mesentery.

The GIT itself differentiates to form significantly different structures along its length: esophagus, stomach, duodenum, jejunum, ileum (small intestine), colon (large intestine).

The mesenteries of the GIT are generated from the common dorsal mesentery, with the ventral mesentery contributing to the lesser omentum and falciform ligament.

The pancreas arises from 2 sources: the hepatic diverticulum (ventral) and the duodenum (dorsal). The pancreas must also differentiate to establish specific cells for endocrine and exocrine function.

The spleen arises in week 5 within the dorsal mesogastrium as proliferating mesenchyme. Cells required for its hemopoietic function arise from the yolk sac wall. The spleen generates both red and white cells in the 2nd trimester. Note that embryonic RBCs remain nucleated.

ESOPHAGUS

- stomach is separated from the primitive pharynx by a constriction which is the future esophagus > arises from the common tubular structure of the trachea and esophagus which becomes divided by the growth of two lateral septa which fuse and form the trachea anteriorly and esophagus posteriorly *lack of fusion of septa leads to development of tracheo-esophageal fistula > due to development of the lungs, constriction is lengthened > esophagus is converted into a solid cord of cells, losing its tubular structure

>> becomes canalized later to form the tube again *imperfect canalization leads to congenital stricture

STOMACH- developed from that part of the foregut which is situated between the esophagus and the pharynx in front and the liver bud and yolk sac behind *4th week: stomach is located at the neck > heart, lungs and stomach lie near the exit of the vagal fibers from the CNS *6th-7th week: growth of lung bud causes elongation of esophagus and backward migration of the stomach (from cervical structure goes down to lower thorax) >embryologic position: long axis is in the median plane and greater curvature is facing dorsally *changes its position during development becoming almost transversely placed with the greater curvature facing downward and to the left and the lesser curvature facing upward and to the right

STOMACH- change is due to the combined axial rotation along the long axis of the stomach and anteroposterior axis > first rotation (longitudinal axis) swings through an arc of 90° *primitive left gastric surface is directed forward, primitive right gastric surface, backward; greater curvature and dorsal mesogastrium to the left and lesser curvature and ventral mesogastrium to the right- lesser peritoneal cavity (omental bursa) forms retrogastric pouch; spleen and splenic artery to the left; and meso-esophagus becomes shortened making esophagus lie against the posterior wall- second rotation (anteroposterior axis) draws the pyloric end to the right and cardiac end to the left-opening of lesser sac is formed (epiploic foramen or Foramen of Winslow)

DUODENUM- rotation of intestines to their ultimate abdominal positions is produced by drawing the initial colic segment to the right so that the duodeno-jejunal junction and the small intestines lie toward the left *duodenojejunal angle is placed against mesentery of terminal small intestines, with which it fuses *degree of fusion accounts for fossae and peritoneal folds >deficient fixation increases depth and capacity of fossae (leading to internal herniation) >hyperfixation of duodenojejunal angle may cause obstruction *descending duodenum and part of tranverse duodenum fuses with primitive right parietal peritoneum *remaining transverse and ascending duodenum fuses with descending mesocolon *duodenojejunal angle fuses with transverse mesocolon

CONGENITAL ANOMALIES

Malrotation

Midgut Volvulus

Duodenal Webs

Meckel’s Diverticulum

Intestinal Atresia

Duplications

COLON- 10 weeks AOG, abdominal cavity increases sufficiently to permit return of intestines >because of cecal swelling, large intestine is the last to leave the umbilical cord and re-enter the abdominal cavity *cecum, ascending and proximal 2/3 of transverse colon is derived from the midgut *rest is derived from the hindgut >cecum becomes fixed on the right side close to the iliac crest >> colon passes obliquely upward to the left of the stomach, where it curves sharply to form the splenic flexure and continues as the future descending colon >> as liver decreases in size, hepatic flexure appears in the

originally oblique proximal colon *flexure demarcates ascending from transverse colon

COLON- posterior peritoneal fixations of the colon take place so that ascending mesocolon and colon fuse with the right parietal peritoneum and anterior surface of the descending duodenum and its mesentery;- descending colon fuses with left parietal peritoneum- mesentery to the rest of small bowel remains free and unfused- transverse colon and mesocolon remain free and suspended from anterior abdominal wall and remain fixed at the two colic angles-redundant sigmoid loop does not fuse with the left pelvic peritoneum-upper boundary of mesosigmoid is rectosigmoid or intersigmoid recess (fossa)-rectum is the only part of the GIT which maintains its primitive sagittal position and has no mesentery

RECTUM

- developing hindgut is terminated by the cloaca *6th week - cloaca is partitioned by the anorectal septum

into ventral urogenital sinus and dorsal rectum

8th week - anal canal is completely formed when the thin anal membrane ruptures >dentate line in the lower anal canal marks the transition from endodermal hindgut to ectodermal tissue

CONGENITAL ANOMALIESHirschsprung’s DiseaseImperforate Anus

ANTERIOR ABDOMINAL WALL

Boundaries: Superior - costal margins (7th-10th rib) and xiphoid processInferior - iliac crest, inguinal ligament, pubic crest and upper end of symphysis pubis

Surface AnatomySkin – loosely attached except in the umbilicusLinea alba – xiphoid to symphysis pubis; divided into supraumbilical and infraumbilical part - fibrous raphe formed by the decussation of 3 lateral abdominal musclesUmbilicus – navel; puckered scar marking site of umbilical cord

DIVISIONS OF THE ANTERIOR ABDOMINAL WALL

- divided into 9 regions by two horizontal and vertical lines *horizontal line drawn at the level of 9th costal cartilage and superior border of iliac crest *vertical line drawn from middle of subcostal margin

(midclavicular) and middle of inguinal ligament

- can also be divided into four quadrants by 2 perpendicular imaginary lines that will pass through the umbilicus

DIVISIONS OF THE ABDOMEN

RIGHT EPIGASTRIC LEFTHYPOCHONDRIAC HYPOCHONDRIACLiver Liver, Stomach, Fundus of Stomach, Pancreas Spleen

RIGHT LUMBAR UMBILICAL LEFT LUMBARAscending Colon Small intestines, Descending Colon Transverse Colon, Greater Omentum

RIGHT INGUINAL HYPOGASTRIC LEFT INGUINALIleocecal Junction, Small Intestines Sigmoid ColonAppendix Pregnant Uterus Full Urinary Bladder

ANTERIOR ABDOMINALWALLSkinCutaneous Nerve Supply: anterior rami of lower six thoracic and first lumbar nerves *thoracic (lower five intercostals and subcostal nerves) *lumbar (iliohypogastric and ilioinguinal nerves)Dermatome: T7 – epigastrium over xiphoid process T10 – umbilicus L1 just above inguinal ligament and xiphoid processBlood Supply: superior and inferior epigastric arteries branches from intercostal, lumbar and deep circumflex iliac arteriesVenous Drainage: axillary vein via lateral thoracic vein femoral vein via superior epigastric vein and great saphenous veinsLymphatics: above umbilicus – anterior axillary nodes below umbilicus – superficial inguinal nodes

BLOOD SUPPLY OF THE ANTERIOR ABDOMINAL WALL

1. Superior Epigastric Artery- arises from the internal thoracic artery an enters the rectus

sheath- descends behind the rectus muscle and supplies upper central part of anterior abdominal wall- anastomoses with inferior epigastric artery

2. Inferior Epigastric Artery- arises from external iliac artery above the inguinal ligament- runs medial to deep inguinal ring and enters rectus sheath- ascends behind the rectus muscle and supplies lower central part of anterior abdominal wall- anastomoses with superior epigastric artery

BLOOD SUPPLY OF THE ANTERIOR ABDOMINAL WALL

3. Deep Circumflex Iliac Artery- branch of external iliac artery; runs upward and laterally toward the ASIS- supplies the lower lateral part of abdominal wall

4. Lower two posterior intercostal arteries (from descending thoracic aorta) and four lumbar arteries (from abdominal aorta)- supplies lateral part of anterior abdominal wall- superficial epigastric artery, superficial circumflex iliac artery and superficial external pudendal artery branch of femoral artery also supply the lower anterior abdominal wall

VENOUS DRAINAGE1. Superior and Inferior Epigastric Veins and Deep Circumflex Iliac Veins - follow the arteries and drain into internal thoracic and external iliac veins2. Posterior intercostal veins – drain into azygos veins3. Lumbar Veins – drain into IVC4. Superficial epigastric, superficial circumflex iliac, and superficial external pudendal veins - drain into the great saphenous vein then to femoral vein5. Thoracoepigastric vein - anastomoses between the lateral thoracic vein and superficial epigastric vein, a tributary of the great saphenous vein - alternative path for venous blood in case of obstruction in the SVC or IVC

NERVE SUPPLY OF THE ANTERIOR ABDOMINAL WALL

Anterior Rami of Lower Six Thoracic and First Lumbar Nerves - run downward and forward between the internal oblique and transversus abdominis muscles. - supply the skin, muscles, and parietal peritoneum of anterior abdominal wall - lower six thoracic nerves pierce the posterior wall of the rectus sheath - first lumbar is represented by the iliohypogastric and ilioinguinal nerves, which do not enter the rectus sheath - iliohypogastric nerve pierces the external oblique aponuerosis above the superficial inguinal ring - ilioinguinal nerve passes through the inguinal canal to emerge through the ring

LAYERS OF THE COVERINGS OF ABDOMINAL WALL SPERMATIC CORD/

TESTES

1. Skin 1. Skin2. Superficial (Camper’s) Fascia 2. Colles’ Fascia and

Dartos Muscle3. External Oblique Muscle 3. External Spermatic

Fascia4. Internal Oblique Muscle 4. Cremasteric

Fascia and Muscle

5. Transversus Abdominis Muscle 5. Absent6. Transversalis Fascia 6. Internal Spermatic

Fascia7. Extraperitoneal Fat 7. Extraperitoneal Fat8. Parietal Peritoneum 8. Tunica Vaginalis

SUPERFICIAL FASCIA1. Camper’s Fascia (Superficial Fatty Layer) - continuous with the superficial fascia over rest of the body and extremely thick in obese individuals - represented as a thin layer of smooth muscle in the scrotum (Dartos muscle) 2. Scarpa’s Fascia (Deep Membranous Layer) - fades out laterally and above; passes over inguinal inguinal ligament to fuse with deep fascia of the thigh (fascia lata) - not attached to pubis in the midline but forms a tubular sheath for the penis (clitoris) - attached to one side to margins of pubic arch (Colles’ Fascia) - fuses posteriorly with perineal body and posterior margin of perineal membrane

DEEP FASCIA- thin layer of areolar tissue covering the muscle

Muscles of Anterior Abdominal Wall- three broad thin sheets that are aponeurotic in front External Oblique, Internal Oblique, Transversus Abdominis- Rectus Abdominis: found on either side of midline *aponeurosis of three sheets pass forward and enclose Rectus Abdominis to form the Rectus Sheath- Pyramidalis Muscle: may be present in the lower part of the Rectus Sheath

Rectus Sheath - long fibrous sheath that encloses rectus abdominis and pyramidalis (if present)- contains anterior rami of lower six thoracic nerves and superior and inferior epigastric vessels and lymph vessels- internal oblique aponeurosis splits to form an anterior and posterior laminae- external oblique aponeurosis fuses with anterior lamina- transversus abdominis aponeurosis fuses with posterior lamina- at level of ASIS, all 3 aponeurosis past anteriorly to the rectus muscle (sheath becomes deficient posteriorly)

Rectus Sheath- arcuate line: lower crescent-shaped edge of posterior wall

of rectus sheath- all aponeurosis fuse with each other and with the other side to form the linea alba- anterior wall of rectus sheath attached to tendinous intersections of rectus abdominis *xiphoid, umbilicus and in between Linea Semilunaris: lateral edge of rectus muscle

Conjoint Tendon: union of internal oblique and transversus abdominis muscle strenghtens medial half of floor of inguinal canal

Muscles of the Abdominal Wall

• The abdominal wall is composed of four paired muscles (internal and external obliques, transversus abdominis, and rectus abdominis), their fasciae, and their aponeuroses

• Fascicles of these muscles run at right and oblique angles to one another, giving the abdominal wall added strength

Muscles of the Abdominal Wall

• In addition to forming the abdominal wall, these muscles:– Are involved with lateral flexion and rotation of the

trunk– Help promote urination, defecation, childbirth,

vomiting, coughing, and screaming

Muscles of the Abdominal Wall

Figure 10.11a

Muscles of the Abdominal Wall

Figure 10.11b

Muscles of the Abdominal Wall

Figure 10.11c

Inguinal (Poupart’s) Ligament- connects ASIS to pubic tubercle- formed by lower border of external oblique aponeurosisLacunar (Gimbernat’s) Ligament- extends from medial end; goes backward and upward to the Pectineal Line on superior ramus of pubis; where it becomes continuous with pectineal ligament (periosteal thickening)- lower border attached to the fascia lataCooper’s Ligament- lateral continuation of lacunar ligament- extends from base of lacunar ligament laterally along the pectineal line to which it is attachedFascia Transversalis- thin layer that lines trasnversus abdominis- continuous with a similar layer lining the diaphragm and iliacus muscle- forms femoral sheath together with fascia iliaca

SPERMATIC CORD- collection of the following structures that pass through

the inguinal canal to and from the testis1. Vas Deferens2. Testicular Artery3. Testicular Vein (pampiniform plexus)4. Testicular Lymph nodes5. Autonomic Nerves6. Remains of the Processus vaginalis7. Cresmeasteric Artery8. Artery of the Vas Deferens

9. Gential branch of genitofemoral nerve, which supplies cremasteric muscleCoverings:1. External Spermatic Fascia – external oblique muscle2. Cremasteric Fascia – internal oblique muscle 3. Internal Spermatic Fascia - fascia trasnversalis

PROCESSUS VAGINALIS- peritoneal diverticulum formed in the fetus that passes throught the layers of lower abdominal wall to form the inguinal canal normally, cavity of tunica vaginalis is shut off from the upper part of processus and periotneal cavity just before birth; becomes a closed sac invaginated from behind by the testis

GUBERNACULUM TESTES- muscular ligamentous cord that connects the fetal testis to the floor of developing scrotum- plays an important role in the descent of the testis- homologous to female round ligament of the ovary and round ligament of the uterus

SCROTUM- outpouching of the lower part of the anterior abdominal wall and contains the testis, epididymis, and lower ends of the spermatic cord- layers

1. Skin 2. Superficial Fascia: Dartos muscle (smooth muscle)

replaces fatty layer3. External Spermatic Fascia from external oblique4. Cremasteric Fascia from internal oblique

*cremasteric muscle supplied by genital branch of genitofemoral nerve

5. Internal Spermatic Fascia – from fascia transversalis6. Tunica Vaginalis: closed sac that covers the

anterior, medial and lateral surface of each testis

TESTIS – paired ovoid organ responsible for the production of spermatozoa and testosterone. *descent to abdominal cavity allows normal spermatogenesis to take place - outer fibrous capsule is called tunica albugineaEPIDIDYMIS - lies on each side, posterior to the testis - has head, body and tail - coiled tube about 20 feet (6 m) long - vas deferens emerges from the tailBLOOD SUPPLY/DRAINAGE/LYMPHATICS - supplied by testicular artery which is a branch of the abdominal aorta- testicular veins emerges from testis and epididymis as a venous network called pampiniform plexus-right testicular vein drains to IVC and left joins left renal vein- lymphatics: para-aortic nodes at L1 level

INGUINAL CANAL- oblique passage through the lower part of anterior abdominal wall present in both sexes- about 4 cm long in adults; extends from deep inguinal ring, (hole in the fascia transversalis) downward and medially to superficial inguinal ring (hole in external oblique aponeurosis)- deep inguinal ring: oval opening in the fascia transversalis, about 1.3 cm above the inguinal ligament; margins of the ring give attachment to internal spermatic fascia- superficial inguinal ring: triangular-shaped defect in the external oblique aponeurosis; lies immediately above and medial to the pubic tubercle; margins give attachment to external spermatic fascia

WALLS OF THE INGUINAL CANALAnterior: external oblique aponeurosis, reinforced laterally by origin of internal oblique from inguinal ligamentPosterior: conjoint tendon medially, fascia transversalis laterallyRoof or Superior: arching fibers of internal oblique and transversus abdominisFloor or Inferior: inguinal ligament and lacunar ligament

FUNCTIONS OF INGUINAL CANAL- allows passage of structures to and from the testis to the abdomen in males or round ligament of uterus from uterus to labium major in females-transmits ilioinguinal nerve in both sexes

MECHANICS OF INGUINAL CANAL- site of potential weakness in both sexes- arching lowest fibers of internal oblique and transversus abdominis muscles contract during coughing and straining, which flattens out the arch so that the roof of the canal is lowered toward the floor and the canal is virtually closed

HERNIA - protrusion of a viscus through an opening in the wall of the cavity in which it is contained - important features: hernial orifice and sac- hernial orifice: defect in the innermost aponeurotic layer of the abdomen- hernial sac: outpouch of the peritoneum- can be internal or external

INGUINAL HERNIA- occurs above the inguinal ligament- two types: direct and indirect

Indirect Inguinal Hernia- hernial sac is the remains of processus vaginalis- most common form of hernia; more common in males- more common on the right side- usually seen in children and young adults - hernial sac enters the inguinal canal through deep inguinal ring and lateral to inferior epigastric vessels; neck is narrow- hernial sac may extend through the superficial inguinal ring above and medial to pubic tubercle- hernial sac may extend down to scrotum or labia majora

Direct Inguinal Hernia- common in elderly men with weak abdominal muscles - sac bulges forward through the posterior wall of the inguinal canal medial to the inferior epigastric vessels- neck is wide

PERITONEUM- serous membrane lining the abdominal and pelvic cavities and clothing the viscera- may be regarded as a balloon into which organs are pressed into from the outside- parietal layer lines the walls of the abdominal and pelvic cavities- visceral layer covers the organs- potential space between the parietal and visceral layer is called peritoneal cavity *closed cavity in males *communicates in the open in females through the fallopian tubes, uterus and vagina

Peritoneal Cavity- divided into two parts: greater and lesser sac- greater sac: main component of the peritoneal cavity and extends from the diaphragm down into the pelvis- lesser sac: smaller and lies behind the stomach- greater and lesser sac communicates through the epiploic foramen or foramen of Winslow- peritoneum secretes a small amount of serous fluid, which lubricates the surfaces of the peritoneum and facilitates free movement between the viscera- boundaries of epiploic foramen anteriorly: free border of lesser omentum, bile duct, hepatic artery and portal vein posteriorly: inferior vena cava superiorly: caudate process of the caudate lobe of the liver inferiorly: first part of the duodenum

PERITONEAL LIGAMENTS, OMENTA AND MESENTERIES- permits blood, lymphatic vessels and nerves to reach the viscusPeritoneal Ligaments- two-layered folds of peritoneum that connect solid viscera to the abdominal walls eg. Liver is connected to diaphragm by the falciform ligament, coronary ligament, and right and left triangular ligamentsOmenta- two-layered folds of the peritoneum that connect the stomach to another viscus- greater omentum connects the greater curvature of the stomach to the transverse colon; hangs down like an apron in front of the cells of the small intestines and is folded back in itself- lesser omentum suspends the lesser curvature of the stomach to the fissure of the ligamentum venosum and porta hepatis of the liver- gastrosplenic omentum (ligament) connects the stomach to the hilus of the spleen

Mesenteries- two-layered folds of peritoneum connect parts of the intestines to the posterior abdominal wall eg. Mesentery of the small intestines, transverse mesocolon, sigmoid mesocolon

Lesser Sac- lies behind the stomach and lesser omentum - extends upward as far as the diaphragm and downward between the layers of the greater omentum- left margin is formed by the spleen, the gastrosplenic omentum, lienorenal ligament- right margin of the sac opens into the greater sac through the epiploic foramen

PERITONEAL FOSSAE, SPACES AND GUTTERS

Duodenal Fossae – close to the duodenojejunal junction, there may be four small pouches of peritoneum called superior duodenal fossa, inferior duodenal fossa, paraduodenal fossa and retroduodenal fossaCecal Fossae - folds of peritoneum close to the cecum produce three peritoneal fossa called the superior ileocecal, inferior ileocecal and retrocecal fossaeSubphrenic Space - lies between the diaphragm and the liver and are called the right and left anterior and posterior subphrenic spaces are important since they may provide sites for the accumulation of pusParacolic Gutter - lies on the lateral and medial sides of the ascending and descending colons respectively; provides channels for the movement of infected fluid in the peritoneal cavity

NERVE SUPPLY OF THE PERITONEUM

Parietal Peritoneum (for pain, temperature, touch and pressure)- supplied by the lower six thoracic and first lumbar nerves - parietal peritoneum in the pelvis is mainly supplied by the obturator nerve

Visceral Peritoneum (for stretch only)- supplied by autonomic nerves that supply the viscera or are traveling in the mesenteries

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