Liver Anatomy and Functions

Liver

• Approximately 1500g
• Largest gland
• Endocrine and exocrine (bile) functions in the same cell (hepatocyte)
• Converts noxious substances into nontoxic materials

General Hepatic Structure

• Covered by Glisson's capsule (dense irregular connective tissue)
• Loosely attached except at porta hepatis (blood & lymph vessels & bile ducts)
• Sparse connective tissue within Glisson's capsule

General Hepatic Structure & Vascular Supply

• Porta hepatis
• Left & right hepatic artery
• Portal vein (nutrients, iron-rich blood from spleen)
• Right and left hepatic ducts
• Hepatic veins

Blood Supply

• Hepatic artery → distributing arterioles → inlet arterioles
• Peribiliary capillary plexus around interlobular bile ducts
• Portal vein → larger distributing veins → smaller inlet venules
• Central vein: initial branch of hepatic vein
• Hepatocytes radiate from central vein & separated by hepatic sinusoids

Types of Liver Lobules

• Classical lobule
• Portal lobule
• Hepatic acinus (acinus of Rappaport)

Classical Lobule

• Closely packed classical lobules (hexagon-shaped)
• Boundaries can only be approximated
• 3 lobules contact portal areas (triads)
• Hepatic artery, portal vein, and bile duct

Portal Lobule

• All hepatocytes that deliver their bile to a particular bile duct constitute a portal lobule
• Triangular region
• Portal area at the center
• Imaginary straight lines connecting 3 central veins

Hepatic Acinus (Acinus of Rappaport)

• Based on blood flow from distributing arteriole
• Hepatocytes degenerate subsequent to toxic or hypoxic insults
• Ovoid to diamond-shaped lobule
• Zone 1, Zone 2, Zone 3

Hepatic Sinusoids & Hepatocyte Plates

• Plates of hepatocytes radiate from central vein toward periphery of classical lobule
• Spaces between plates: sinusoids
• Sinusoidal lining cells
• Gaps between lining cells
• Fenestrae in clusters: sieve plates

Kupffer cells in Sinusoids

• Endocytose particulate matter and cellular debris, defunct erythrocytes

Perisinusoidal Space of Disse

• Narrow space between a plate of hepatocytes and sinusoidal lining cells
• Microvilli of hepatocytes, exchange of materials between blood & hepatocytes

Perisinusoidal Space of Disse

• Type 3 collagen (reticular) fibers, supporting sinusoids
• Type 1 & 4 collagen fibers
• No basal lamina
• Nonmyelinated nerve fibers

Space of Disse

• Hepatic stellate cells (Ito cells): fat storing cells
• Store vitamin A
• Manufacture & release type 3 collagen
• Secrete growth factors
• Form fibrous connective tissue to replace damaged hepatocytes
• Pit cells
• Natural killer cells

Hepatic Ducts

• Bile canaliculi
• Canals of Hering (Cholangioles)
• Interlobular bile ducts
• Right & left hepatic ducts

Hepatocytes

• 5-12 sided polygonal cells
• 20-30 µm diameter
• Closely packed to form one or two cell-thick plates
• Show variations in their structural, histochemical & biochemical properties depending on their location within lobules

Domains of Hepatocyte Plasmalemma

• Lateral domains
• Formation of bile canaliculi
• Bile leakage prevented by tight junctions
• High levels of Na, K-ATPase activity & adenylate cyclase
• Gap junctions

Domains of Hepatocyte Plasmalemma

• Sinusoidal domains
• Microvilli projecting into space of Disse
• Increase surface area, facilitating exchange between hepatocyte and plasma in perisinusoidal space
• Mannose-6-phosphate receptors
• Na, K-ATPASE
• Adenylate cyclase
• Endocrine secretions of hepatocytes released & enter sinusoidal blood
• Material from blood transported into hepatocyte cytoplasm

Hepatocyte Organelles and Inclusions

• Manufacture primary bile
• Abundant free ribosomes, RER, Golgi
• 2000 mitochondria
• Rich in endosome, lysosome & peroxisome
• SER
• Certain drugs & toxins increase SER

Hepatocyte

• Lipid droplets (mostly VLDLs)
• Glycogen
• Electron-dense granules (20-30 nm in size)
• Abundant after eating, fewer after fasting

Acute Pancreatitis

• Pancreatic digestive enzymes become active within cytoplasm of acinar cells
• Often fatal
• Inflammatory reaction, necrosis of blood vessels, proteolysis of parenchyma, enzymatic destruction of adipose cells

Islets of Langerhans

• Scattered among acini
• Richly vascularized
• Spherical conglomeration of 3000 cells
• 1 million islets
• Greater number in tail
• Surrounded by reticular fibers

Cells of Islets

• 5 types:
• Beta
• Alpha
• Delta (D&D1)
• PP (pancreatic polypeptide-producing)
• G (gastrin-producing)

Differences from Parotid Gland

• Exocrine part similar in structure to parotid gland
• Absence of striated ducts
• Presence of islets of Langerhans
• Initial parts of ICDs penetrate lumens of acini
• Centroacinar cells (nuclei surrounded by pale cytoplasm)

Diabetes Mellitus

• Hyperglycemic metabolic disorder
• Lack of insulin production from B cells or defective insulin receptors
• If uncontrolled: circulatory disorders, renal failure, blindness, gangrene, stroke, myocardial infarct

Type 1 Diabetes

• Usually affects persons <20 years of age
• Polydipsia (constant thirst)
• Polyphagia (undiminished hunger)
• Polyuria (excessive urination)

Type 2 Diabetes

• Non-insulin-dependent diabetes
• The most common type
• Affects persons >40 years of age

Verner-Morrison Syndrome

• Explosive, watery diarrhea
• Hypokalemia
• Caused by excessive manufacture and release of VIP due to adenoma of D1 cells
• Frequently, tumors of D1 cells are malignant

Gallbladder

• Simple columnar epithelium
• Mucinogen
• Short microvilli
• Lamina propria
• Smooth muscle
• Serosa/adventitia

Gallbladder Epithelial Cells

• Basally positioned oval nuclei
• Short microvilli coated by glycocalyx (carb rich coat → acid barrier)
• Basal region rich in mitochondria, providing abundant energy for Na-K ATPase pump in basolateral membrane
• Deep diverticula called Rokitansky-Aschoff sinuses

Gallbladder Intercellular Spaces

• Feature associated with transport of fluid across epithelium
• Commonly seen in intestinal absorptive cells

Gallbladder Lamina Propria

• Loose connective tissue
• Elastic & collagen fibers
• Neck: simple tubuloalveolar glands (mucus)

Gallbladder Smooth Muscle Layer

• Mostly obliquely oriented fibers
• Others longitudinally oriented
• Adventitia attached to Glisson's capsule
• Nonattached surface invested by peritoneum

Extrahepatic Ducts

• Right hepatic duct
• Left hepatic duct
• Common hepatic duct
• Cystic duct
• Common bile duct
• Pancreatic duct
• Ampulla of Vater

Pancreas

• Posterior body wall deep to the peritoneum
•  Four regions: Uncinate process, Head, Body, Tail
• 25 cm long, 5 cm wide, 1-2 cm thick, 150 g

Exocrine Pancreas

• Compound tubuloacinar gland
• Produces bicarbonate-rich fluid & digestive proenzymes (1200 ml/day)
• 40-50 acinar cells form a round to oval acinus
• Lumen occupied by 3-4 centroacinar cells (distinguishing characteristic, beginning of duct system)
• Surrounded by BL supported by reticular fibers

Acinar Cell

• Like a truncated pyramid
• Base positioned on BL, separating cell from CT
• Basally located round nucleus, surrounded by basophilic cytoplasm
• Apex faces the lumen
• Apex filled with zymogen granules, eosinophilic
• Number diminishes after meal
• Golgi region between nucleus and zymogen granules varies in size in inverse relation to zymogen granule concentration
• Base contains many parallel cisternae of GER (basophilic staining in LM)
• Golgi apparatus fluctuates in size (smaller when zymogen granules numerous, larger after granules release their content)
• Microvilli from apical surface
• Z. occludentes (tight junction) at an apicolparcian

Islets of Langerhans

• Lightly stained polygonal or rounded cells arranged in cords separated by a network of fenestrated capillaries.
• Both parenchymal cells & blood vessels are innervated by ANS.

Pancreatic Duct

• ICD (Intralobular ducts)
• Interlobular duct
• Main pancreatic duct
•  Common bile duct
• Papilla of Vater

Intercalated Ducts

• Receive secretions from acini
• Flattened to cuboidal epithelium that extends up into the lumen of the acinus to form centroacinar cells

Intralobular Ducts

• Cuboidal epithelium
• Within lobules
• Receive secretions from intercalated ducts

Interlobular Ducts

• Between lobules, within the connective tissue septae
• Vary considerably in size
• Smaller forms have cuboidal epithelium
• Larger ducts have columnar epithelium
• Transmit secretions from intralobular ducts to major pancreatic duct

The Main Pancreatic Duct

• Receives secretion from interlobular ducts
• Penetrates through the wall of duodenum
• Joins the bile duct prior to entering the intestine

Pancreas H&E

• Acinar cells
• Centroacinar cells
• Connective tissue
• Exocrine secretory tissue

Secretory Portion

• Basal cell membranes of acinar cells have receptors for CCK & ACh
• Abundance of basally located RER
• Rich supply of polysomes
• Numerous mitochondria
• Golgi well developed

Duct System

• Begins within center of acinus with terminus of intercalated ducts (ICD)
• Pale, low cuboidal centroacinar (CA) cells
• CA cells & ICDs have receptors on basal plasmalemma for secretin & possibly for ACh

Explore the intricate structure and vital functions of the liver in this quiz. From its role as the largest gland in the body to its unique vascular supply system, you'll test your knowledge on various liver lobules and their significance. Understand how hepatocytes work to detoxify substances within the liver.