23. Anatomy, Histology, & Embryology of the Pancreas

*Pancreas is secondary retroperitoneal, with the exception of the tail, the foregut.

Anatomy of Pancreas

• Location: Within the curve of the duodenum, located in the epigastric and left hypochondriac regions

• Surface Projection: Surface projection of is different depending on the part of it, and will be entailed later.

• Topography: touches the doudenum, the spleen, the left kidney, the lower border of the stomach.

• Function: releases secretions in an exocrine fashion, namely pancreatic juice, which it then secretes into the duodenum through the major duodenal papilla, or papilla of Vater. This major pancreatic duct is also called the duct of Wirsung. Endocrine secretions include glucagon and insulin into the blood.

• Parts:

  • Head – located between L1-2 in curve of the duodenum, to the descending and horizontal parts of it. develops mainly from the ventral pancreatic bud, except the upper part, which is from the dorsal pancreatic bud. Is located right to the sup mesenteric a/v.SP = head of pancreas is anterior loc to IVC, right renal a/v, left renal v Uncinate process– at lower head behind sup mesenteric a/v
  • Neck – short, 1-2cm, sup mesenteric a/v go behind the neck, @ notch below neck, called pancreatic notch. Ant surface of neck covered with peritoneum, and moves along with movements of pylorus of stomach. Sup mesenteric v and splenic v combine behind the neck of pancreas, to form portal v.
  • Body – loc @ L1-2, in front of vert column, posterior to lesser sac Has 3 surfaces:
  • • Ant – covered by peritoneum, and indirectly contacts post stomach, lies in floor of lesser sac, helps form gastric bed >
    • Post – contact with aorta, sup mesenteric a/v, L kidney & renal vessels, and L suprarenal gland, Splenic a runs in tortous path superior to it, and has groove for splenic v.
    • Inf – touches 3rd and 4th part of duod Omental tuberosity – in sup surface of body below celiac trunk, contact with visceral surface of liver. (T12-L1 midline)
  • Tail – loc. at left costal arch, 2-3 fingers lat to T12, runs to hilum of spleen.
    • Can be retroperitoneal or intraperitoneal.
    • If shorter, than usually retroperitoneal, like the rest of pancreas. Then a small peritoneal ligament = pancreaticolineal lig extends from tip of tail –> hilum of spleem. Splenic a/v enter spleen through this ligament.
    • If longer, intraperitoneal, and splenic a/v can go directly to spleen.
  • Ducts:
    • Main pancreatic duct = Duct of Wirsung, duct of ventral pancreatic bud, begins in tail and runs along entire pancreas, till head, where it turns inf and runs with common bile duct. Opens at major duodenal duct – Ampulla of Vater, which is gaurded by sphincter of Oddi, after joining bile duct.
    • Accessory pancreatic duct = Santorini’s duct can open into main or minor duodenal papilla, sometimes both. minor duodenal papilla located 2cm above major one, Begins in lower portion of the head, and drains small portion of head and body.
    • Clinical NOTE – Bile ducts is embedded into a groove on the post/sup side of the head. If tumors are located within the head, this can block bile flow into the duodenum, resulting in jaundice.
• Blood Supply: B/c develops from FOREGUT – supplied by branches of celiac trunk.
  • Splenic a – supplies derivatives of dorsal pancreatic bud –> neck, body, tail of pancreas located on shallow groove on sup/post side of pancreas. 10 branches, that can form arterial arcades (networks) with branches from gastroduodenal a and sup mesenteric a
  • Sup pancreaticoduodenal a (gastroduodenal a) & Inf pancreaticoduodenal a (sup mesenteric a ) splits into ant/post branches to supply the head of the pancreas
  • Most veins empty into splenic v, which is embedded into the back of the pancreas itself

• Lymph Drainage
  • lymph vessels follow the arteries and drain into pancreatiosplenic nodes that lie along splenic arteries. The ones by the head go to pyloric lymph nodes, as well.
  • These two sets of nodes drain into celiac, hepatic, and sup mesenteric lymph nodes

• Innervation
  • PNS – (+) digestion, secretion, vasodilation.
    • From CN X, carries pre ggl fibers to submucosal and myenteric plexus within walls of organs, with short post ggl PNS fibers.
    • Go to parenchyme (acinar cells and islets of Langerhans)
    • Inc Insulin secretion, Inc glucagon secretion
  • SNS – (-) digestion, secretion, vasoconstriction
    • SNS –> (T5-T9) from thoracic splanchnic n –> aortic hiatus –> celiac ggl plexus around the celiac trunk –> post ggl fibers run with branches of celiac trunk.
    • Go to pancreatic glands and parenchyme (pancreatic acinar cells and islets of Langerhans)
    • dec insulin secretion, inc glucagon secretion

Peritoneal ligaments:Peritoneal ligaments: none ,except for the tail sometimes, which can be located with in the pancreaticolienal ligament.

Histology Pancreas slide # 59 *H&E

Structures to Identify:

• intercalated duct

• intralobular duct

• a/v/capillaries

• Islets of Langerhans

• Secretory acinus

• A,B,D, cells

• CT septa lobules

General Information

• Digestive gland with head, neck, body , tail regions

• Mixed gland with endocrine and exocrine functions

Exocrine:

• Tubuloacinar glands manily composed of serous secretory units

• Secretions proteolytic enzymes

• Proteolytic Endo-peptidases: trypsinogen, chemotrypsinogen

• Exopeptidases: procarboxypeptidases, proaminopeptidases, alpha amylases, lipases nucleolytic enzymes

• Has hormonal/ neural control regulation: Secretin – inc. fluid secretion, CCK – pro enzyme secretion Chyme of stomach and duodenum stimulate enteroendocrine cells to secrete CCk and Secretin → causes pancreas to secrete

• Forms most of pancreas, has tightly packed serous acini – circular gland, small circular lumen, darkly stained. zymogen cells arranged in lobule fashion

• Lobules seperated by thin intralobular, and interlobular CT septa – have blood vessels, interlobular ducts, pacinian corpsucles

• Pacinian corpsucle – concentric circles, lighter than surrounding acini, is a sensory receptor

Endocrine:

• Islets of Langerhans (Pancreatic Islets) – highly vascularized epitheloid tissue

• alpha, beta, delta, cells in periphery, beta cells also in center.

• light staining, globular, with thin CT capsule surrounding it.

• Pyramid shaped cells facing central lumen,

• Secretions go to large vacular network of capillaries just outside the islets,

• Excretes into intercalated duct, lined by low cuboidal epithelium

Alpha cells – glucagon – inc amount of glucose in blood

Beta cells – insulin – dec amount of glucose in blood

Delta cells -Somatostatin – inhibits the other two

Duct flow:

Within the islets, flow into pale staining centro acinar cells within lumen → intercalated (same as intralobular) ducts → interlobular ducts (simple cuboidal epith) in interlobular CT septa → larger ducts ( also interlobular, but with stratified columnar epith)

Embryology – Development of Pancreas

• Endodermal lining of forgut forms two outgrowths, ventral pancreatic bud and dorsal pancreatic bud

• Within both buds, endodermal tubules surr. by mesoderm –> branch to from acinar cells and ducts (exocrine part).

• Clumps of cells within exocrine part form, and become the future islets (endocrine part)

• Remember how the duodenum rotates 90 degrees clockwise? Because of this, the ventral bud (ant) rotates also – dorsally (post) and fuses with the dorsal bud already there to form adult pancreas

• Ventral bud = uncinate process and lower part of head of pancreas

• Dorsal bud = rest of pancreas

• Main pancreatic duct (of Wirsung) = formed by distal dorsal bud and all of ventral bud

• Acc pancreatic duct (of Santorini) = prox part of dorsal bud

• Endoderm origin = acinar cells, islet cells, simple columnar and cuboidal lined parts of duct system

• Visceral mesoderm = CT surrounding them, and a/v/capillaries

Anim : http://www.mcb.harvard.edu/melton/animation.html

18. The peritoneum. The peritoneal cavity. The development of the peritoneum.. The histology of tonsils.

Anatomy of the peritoneum & the peritoneal cavity.

• Definition of Peritoneum = is  a continuous, glistening+ slippery transparent serous membrane,  lines the andominopelvic cavity+ invests the viscera.

• The peritoneum consists of two continuous layers, both layers of peritoneum consists of mesothelium, a layer of simple squamous epithelial cells:
  • Parietal peritoneum, which lines the internal surface of the abdomino-pelvic wall
    • has same a/v/n/lymphatics, as the region of wall that it covers
    • is sensitive to pressure, pain, heat+ cold+ laceration.
    • Remember = Parietal = Pain* same goes for parietal pleura in thoracic cavity
    • Pain from FOREGUT = expressed in EPIGASTRIC region, MIDGUT = UMBILICAL region, HINDGUT = PUBIC region.
    • nerve supply = phrenic n, lower IC n, subcostal n, Iliohypogastric n, Ilioinguinal n
  • Visceral peritoneum, which covers visceral organs like the stomach+ intestines.
    • has same a/v/n/lymphatics, as the organ it covers
    • Stimulated primarily by stretching + chemical irritation
    • nerve supply = visceral n, ANS pathways

RELATIONSHIP of the VISCERA TO THE PERITONEUM:

• Intraperitoneal organs:  are almost covered with visceral peritoneum (e.g. the stomach+ spleen)
• Extraperitoneal – only organ that is extra– peritoneal is the ovary
• Retroperitoneal – 2 types – more on this later
  • Primary  – always has been located behind the peritoneum
  • Secondary – was originally intraperitoneal, but now is located behind the peritoneal cavity
• Infraperitoneal – located below the peritoneal cavity, usually covered superiorly with peritoneum

PERITONEAL REFLECTIONS – support viscera and contain a/v/n

1 Sup point of peritoneum, 2 inner aspect of the abdominal wall , 3 superior surface of the urinary bladder, 4 over the uterus in the female, 5 into the pouch of Douglas, 6 anterior surface of the rectum onto the posterior abdominal wall, 7 root of the mesentery of the small intestine. 8 horizontal part of the duodenum, 9 gastrocolic ligament, GO= greater omentum (11), 12 anterior surface of the stomach, 13 lesser omentum, EF = epiploic foramen, LPC = lesser peritoneal cavity (lesser sac)

Omentum

• Lesser Omentum – double layer peritoneum, from porta hepatis –> lesser curve + sup hor part of duodenum
  • hepatogastric & hepatoduodenal ligaments
  • form ant wall of lesser sac
  • carry L & R gastric a/v b/w 2 layers of peritoneum
  • free lower margin for = proper hepatic a, bile duct, and portal v
• Greater Omentum – hangs down like apron from gr. curve of stomach –> covering transverse colon & other ab viscera
  • carry R & L gastroepiploic a/v along greater curve
  • adheres to areas of inflammation and wraps around inflammed areas
  • prevents serous diffuse peritonitis = accumulating peritoneal fluid w/ fibrin & leukocytes

Mesentaries

• Mesentary Proper – fan shaped double fold of peritoneum, suspends jejunum & ileum from post ab wall
  • forms a root (duod-jej flexure –> R iliac fossa)
  • free border encloses SI
  • contains sup mesenteric & SI a/v/n/lymph vessels
• Transverse Mesocolon – connect post surfac of transv. colon –> post ab wall
  • fuses w/ gr. omentum to form gastrocolic lig
  • contains middle colic a/v/n/lymphatics
• Sigmoid Mesocolon – inverted V shaped peritoneal fold
  • connects sigmoid colong to pelvic wall
  • contains sigmoid a/v
• Mesoappendix – connects appendix to mesentery of ileum
  • contains  appendicular a/v

Peritoneal Folds –  reflections w/ free edges

• Umbilical folds – 5 folds of peritoneum below umbilicus
  • Lat umbilical folds = contain inf epigastric a/v
  • Medial umbilical folds = contain umbilical a
  • Median umbilical folds = contain remnant of urachus = connects urinary bladder of the fetus with the allantois, a structure that contributes to the formation of the umbilical cord
• Retrouterine folds – extension from cervix of uterus, along side of rectum to pelvic wall (post) and form Rectouterine pouch of Douglas
• Ileocecal fold – terminal ileum –> cecum

Peritoneal Ligaments

• Gastrosplenic lig – from L greater curve –> hilus of spleen, has short gastric a/v, L gastroepiploic a/v
• Splenorenal lig – Hilus of spleen –> L Kidney, has splenic a/v, has tail of pancreas
• Gastrophrenic lig – Upper greater curve –> diaphragm
• Gastrocolic lig – Greater curve –> transverse colon, absorbed into greater omentum, usually
• Phrenicocolic lig – Colic flexure –> diaphragm
• Falciform lig – sickle shaped peritoneal fold, connects liver –> diaphragm & ant ab wall
  • border b/w R & L Lobe (ant)
  • contains ligamentum teres hepatis, and paraumbilical v, which cxts L portal v w/ subcut v in umbilical regions
• Ligamentum Teres Hepatis – aka round ligament of liver, lies in lower free marginof falciform ligament, is L border of quadrate lobe on visceral surface of liver, remnant of umbilical v
• Coronary Lig – peritoneal reflection from diaphragmatic surface of liver  onto diaphragm, encloses bare area of liver
  • has R & L extensions that form R & L triangular ligaments
• Ligamentum Venosum – fibrous remnant of ductus venosus, lies in fissure  on inf surface of liver, forms L border of caudate lobon visceral surface of liver 

 

Start @ 1 and follow around the peritoneal cavity. 2. Back of the abdomen, anterior surface of the right kidney, pass through the epiploic foramen, along the posterior wall of the lesser peritoneal cavity, 3 then up along the renal lienal ligament 4 onto the posterior surface of the stomach 5. Your finger will continue through the epiploic foramen again to turn around the free margin of the lesser omentum 6, then over the anterior surface of the stomach again 7. Continue to follow around the greater curvature of the stomach 8 until you reflect again along the gastrolienal ligament 9. Your finger will now pass around the spleen, onto the left kidney to the parietal peritoneum and back to the falciform ligament fl.

THE PERITONEAL CAVITY

• located within the abdominal cavity &  continous inf. to the pelvic cavity.
• =  a potenial space between the parietal+ visceral layers of peritoneum
• contain no organs
• contains a thin film of peritoneal fliud = which is composed of water, electrolytes+ other substances derived from interstitial fliud in adjacent tissues.
• peritoneal fluid lubricates the peritoneal surfaces, enabling the viscera to move over each other without friction and allowing the movements of digestion
• Contains leukocytes+ antibodies that resists infection.
• Lymphatic vessels, particularly on the inf.surface of the unceasingly active diaphragm, absorb the peritoneal fluid.

In  Males: the peritoneal cavity is completely closed

In Females: connected to extra-peritoneal cavity through the uterine tubes, uterine cavity, & vagina

• split into Lesser Sac & Greater Sac

Lesser Sac = Omental Bursa

• irregular space that lies behind liver, lesser omentum, stomach, upper ant part of greater omentum
• closed sac, except for cxn w/  greater sac via epiploic foramen
• 3 recesses:
  • Sup. recess – being liver, stomach, lesser omentum
  • Inf recess – behind stomach, extends into layers of greater omentum
  • Splenic recess – extends to the L to the hilus of spleen

Greater Sac

• extends across entire area of abdomen and from diaphragm –> pelvic floor
• 5 recesses:
  • Subphrenic recess – peritoneal pocket b/w diaphram & ant/sup part of liver
    • separates into R & L recesses by falciform lig
  • Subhepatic recess – peritoneal pocket b/w liver & transverse colon
  • Hepatorenal recess – deep peritoneal pocket b/w liver (ant) & kidney (post)
  • Morison’s pouch = formed by R subhepatic & hepatorenal recess
    • comminucates w/ subphrenic recess, lesser sac via epoploic foramen, and R paracolic gutter(to pelvic cavity)
  • Paracolic recess – (aka gutters) – lies lat to asc/desc colon

Epiploic foramen (of Winslow) – natural opening b/w lesser and greater sacs

• Sup = peritoneum of caudate lobe of liver
• Inf = peritoneum of 1st part of duodenum
• Ant = free edge of lesser omentum
• Post = peritoneum covering IVC

Retroperitoneal Space

The retro peritoneal space is seperated into the 3 compartments by the renal fasica. This fascial covering is like a tent that is closed susuperiorly and open inferiorly.

Ant Chamber = b/w peritoneum and renal fascia, has all secondary retroperitoneal organs

• asc colon
• desc colon
• duodenum (except sup hor part)
• pancreas (except tail, sometimes)
• Br. of sup mesenteric a, celiac trunk, sup/inf mesenteric v, portal v, common bile duct

Middle Chamber = w/in renal fasica, has primary retroperitoneal organs.

• kidneys
• suprarenal glands
• ureters
• Ab aorta + branches
• IVC + branches
• Thoracic duct
• Cisterna Chyli

Post Chamber = b/w renal fascia and transverse fascia(post ab wall)

• asc lumbar v (becomes azygos v, once crosses diaphragm into thoracic cavity)
• Greater/Lesser splanchnic n
• SNS trunk
• Subcostal n.
• Lumbar plexus + branches
• Ilioinguinal n.
• Inohypogastric n
• Obturator n
• Genitofemoral n
• Gonadal a/v

Histology – Tonsils

Slide #25 Palatine Tonsils *H&E

 Structures to Identify:

• tonsillar crypts
• str. sq. non keratinizing epith
• lymph nodules (primary and secondary)
• muscle bundles
• germinal centers
• CT capsule 

With naked eye: dark, blue, partially encapsulated specimen w/ deep crypts

General Info:

• The palatine tonsilles(faucial tonsils)are paired, ovoid structures that consits of dense accumulation of lymphatic tissue located in the mucous membrane of the fauces(the junction of the oropharynx + oral cavity).
• The epithelium that forms the surface of the tonsil dips into the underlying CT in numerous places, forming crypts known as tonsillar crypts.
• Numerous lymphatic nodules are evident in the walls of the crypts.
• Tonsils guard the opening of the pharynx, the common entry to the respiratory+ digestive tracts.
• CLINCAL NOTE: can become inflamed because of repeated infection in the oropharynx+ nasopharynx+ can even harbour,
  • bacteria can cause repeated infections if they are overwhelmed.
  • debris and abcteria that collects in tonsilar crypts are hard to clean, as not enough saliva to clean them
  • When this occurs, the inflamed palatine tonsils+ pharyngeal tonsils ( also called adenoids) are removed surgically.

Important Histological Features

• C.T. capsule on one side, oral mucosa on other side

• Stratified squamous nonkeratinizing epithelium, lymphocytes invade epithelium within the crypt

  • this epith is present in both palatine and lingual tonsils

• Mucous membrane, lamina propria enlarged contains lymphatic nodules

  • NOTE = W/in nodule s= B lymphocytes, b/w them = T lymphocytes 

• Stroma, each lobules has a cortex+ medulla, the cutting plane of the section determines whether you can see both or not

• surrounded by a  dense fibroelastic CT capsule (red)

• Extends trabeculae to the margin of the cortex and medulla, which can contain fat, a/v

• Below CT capsule = skeletal m fibers,  but not as much as in lingual tonsil

• CORTEX- darker stained(blue)

  • Blood vessels with epithelioreticular cell sheath, cytoreticulum

  • Different than CT has no reticular fibers

  • Contains epithelioreticular cells as the stroma. Ovoid nucleus, larger cell, lighter colour = lymphoreticular mesrk

• large # of High Endothelial Venules (HEVs)

Embryo

• Develops form endoderm instead of mesoderm, unlike regular ct, mostly small lymphocytes
• epithelial lining of 2nd pharyngeal pouch – forms buds that penetrate surrounding mesenchyme
• mesenchyme => becomes palatine tonsil primordium
• in 3rd – 5th, invaginated by lymph tissue, forming tonsil

Slide #26 Lingual Tonsils *H&E

Structures to Identify:

• tonsilar crypts
• CT
• salivary glands
• Str. Sq. non-keratinizing epith
• lymph nodules
• skeletal m

With Nake Eye: A solid specimen with a darker region on one side

General Info:

• aggregation of lymph tissue located at root of tongue, posterior to sulcus terminalis
• not usually inflammed, as very accessible to saliva, and tonsilar crypts are not that deep for debris to collec

Histological Characteristics:

• has a str. squamous non-keratinized epith – very characteristic of oral mucosa, lines surface, and dips down in very shallow tonsillar crypts
• tonsillar crypts form deep invaginations on surface of tongue, ext. deep into LP
• Many lymph nodules, some secondary.
  • nodules =  B lymphocytes, b/w nodules = T lymphocytes
• LP = adipose tissue,  mucus acini of lingual glands, ducts of glands, lymphoreticular tissue
• Below LP, is the skeletal musc. coming from the tongue – bright red color

To be sure it is lingual tonsil = look for the str sq non kerat epith, large amts of skeletal m, lingual mucus glands, NO CT capsule

Embryology – The development of the peritoneum

The peritoneum develops ultimately from the mesoderm of the trilaminar embryo. As the mesoderm differentiates, one region known as the lateral plate mesoderm splits to form two layers separated by an intraembryonic coelom. These two layers develop later into the visceral and parietal layers found in all serous cavities, including the peritoneum.

As an embryo develops, the various abdominal organs grow into the abdominal cavity from structures in the abdominal wall. In this process they become enveloped in a layer of peritoneum. The growing organs “take their blood vessels with them” from the abdominal wall, and these blood vessels become covered by peritoneum, forming a mesentery