Micro-Anatomy of the Liver Notes

• By the end of the session, students will be able to: - Describe the structure of the liver and explain its multifaceted roles in digestive processes, with an emphasis on microstructure and general functions that support metabolic activities. - Explain the microscopic structure and function of the gallbladder and biliary duct system in healthy animals, highlighting the importance of these structures in bile production and fat digestion. - Describe the structure and function of the hepatic portal system in healthy animals, outlining its role in nutrient and toxin transport from the gastrointestinal tract to the liver for processing and detoxification.

Liver - General Structure

• The liver is the largest gland in the body, weighing approximately 1.5 kg in an adult human, and it performs over 500 different functions critical to maintaining homeostasis.

• Understanding the liver's complex structure is essential for comprehending its diverse functions: - Lobes: The liver is divided into lobes, each covered by serosa and a connective tissue capsule that provides support and structure. - Connective tissue extends into liver lobes in forms of interlobular connective tissue, which surrounds liver lobules and supports the vascular system and bile ducts essential for liver function. - The organization of this connective tissue varies significantly among species; for instance, pigs exhibit distinct septa within the liver, while other species may have minimal connective tissue.

Liver Cells - Hepatocytes

• Hepatocytes are the primary cell type constituting liver parenchyma and are crucial for liver functions: - Structure: These polygonal epithelial cells possess eosinophilic granular cytoplasm and a centrally located large round nucleus that plays a key role in cell regulation. - Hepatocytes are abundant in organelles that facilitate their metabolic functions, including numerous mitochondria for energy production, and both smooth and rough endoplasmic reticulum, and Golgi apparatus, responsible for protein synthesis and lipid metabolism. - The cytoplasmic composition of hepatocytes can vary based on the nutritional state of the animal, often containing lipid droplets and lipofuscin pigment, which accumulates with aging and metabolic stress. - The appearance and metabolic activity of hepatocytes also change in response to dietary intake and physiological status of the animal.

Hepatocyte Arrangement

• Hepatocytes are organized in radiating cords known as hepatic cords, with sinusoids interspersed between them, allowing for efficient exchange of materials: - Sinusoids are specialized blood vessels that are difficult to discern in histological sections due to flattened nuclei; they facilitate nutrient and waste exchange, being surrounded by reticular fibers that provide structural support.

Cell Types in the Liver

Ito Cells

• Also referred to as stellate cells or lipocytes, Ito cells are located in the perisinusoidal region (space of Disse) and play several key roles: - Identified by their large lipid vacuoles, they are involved in the uptake and storage of fat-soluble vitamins, particularly vitamin A. - Their functions extend to the production of extracellular matrix, regulation of sinusoidal blood flow, and participation in hepatic tissue repair during injury, indicating their significance in maintaining liver health.

Kupffer Cells

• Kupffer cells originate from monocytes and reside within the vascular spaces of hepatic sinusoids: - Classified as phagocytes, they play a crucial role in the immune function of the liver by removing aged red blood cells, pathogens, and debris from the circulation. - They can be identified as individualized cells within sinusoids, particularly when they contain engulfed materials, aiding in the liver's detoxifying and immune responses.

Oval And Pit Cells

• Oval Cells: These are pluripotent stem cells that have the capability of repopulating hepatocytes and other liver cells following injury; they are not easily identifiable in healthy tissue, making their presence more meaningful during regeneration processes. - Pit Cells: A type of granular lymphocyte found in hepatic sinusoids that contributes to the immune defense of the liver.

Bile Canaliculi & Ducts

• Hepatocytes are responsible for the absorption and conjugation of bilirubin, which is then secreted into bile canaliculi, tiny channels located between adjacent hepatocytes: - The canalicular structure is secured by tight junctions that prevent bile from leaking into the bloodstream, thus ensuring proper bile formation and storage. - Bile is processed through canaliculi into larger ducts known as the Canals of Hering and eventually into bile ducts, which are lined with cuboidal epithelium to facilitate bile movement. - All ducts within the biliary tree are lined by simple columnar epithelium, aiding in the efficient transport and secretion of bile into the gastrointestinal tract.

Hepatic Blood Supply

• The liver receives blood from two primary sources: - Hepatic artery: This artery provides oxygenated blood crucial for the metabolic processes occurring within the liver. - Portal vein: It drains blood from the gastrointestinal tract, gallbladder, and spleen, carrying nutrients, toxins, and microbes that need processing; the portal circulation is essential for nutrient metabolism and toxin elimination. - The organization of the hepatic blood supply is highly efficient, with branches of the hepatic artery running parallel to the bile ducts to ensure that oxygenated blood reaches hepatocytes promptly.

Hepatic Sinusoids

• Sinusoids are specialized blood capillaries located between the hepatic laminae, lined with fenestrated endothelial cells and Kupffer cells, allowing for effective filtration: - Microvilli from hepatocytes extend into the perisinusoidal space, which facilitates direct exchange of substances between blood and liver cells, enhancing the liver’s functional capacity.

Structural Organization of the Liver

Hepatic Lobule

• The hepatic lobule is the functional unit of the liver, featuring a central vein at its center and radiating cords of hepatocytes extending outward like spokes on a wheel, vital for facilitating efficient blood and bile flow: - Portal triads, consisting of the hepatic artery, portal vein, and bile duct, are located at the periphery of each lobule, providing necessary blood supply and drainage for metabolic functions.

Portal Lobule

• The portal lobule emphasizes the liver's exocrine function, specifically bile secretion, structured as a triangular area encompassing three adjacent lobules with central veins forming the peripheral angles, ensuring synergy in bile production.

Hepatic Acinus

• This is the functional unit of the liver emphasizing vascular supply, characterized as diamond-shaped and incorporating parts of two classical lobules: - Zones: - Zone 1: Exhibits the best oxygen and nutrient supply, and is most metabolically active, prioritizing gluconeogenesis and bile production. - Zone 2: Represents an intermediate level of activity, buffering between the more active and less active zones. - Zone 3: Has the least favorable conditions for oxygen and nutrient supply, often being the first affected during ischemic damage.

Gallbladder Structure

• The gallbladder serves to store and concentrate bile produced by the liver, playing a key role in the digestion of fats: - The mucosa of the gallbladder forms folds (plicae) when contracted, allowing for flexibility and increased surface area when necessary. - It is lined with tall simple columnar epithelium rich in microvilli and tight junctions to maximize the absorption of water and electrolytes, which is essential for concentrating bile. - Goblet cells are present, particularly prevalent in ruminant species such as cattle, and are found in the propria submucosa adjoining smooth muscle (tunica muscularis) that aids in gallbladder contraction and bile release.

Summary

• The histological structure of the liver and its diverse cell types have been detailed, demonstrating their organization and significant roles related to liver health and function. - Additionally, the anatomy and function of the gallbladder and bile ducts have been thoroughly described, outlining their importance in digestive health.

Next Steps

• Further questions can be addressed via email at Rstloui2@sgu.edu. - The next lesson will explore liver functions in detail, including its role in metabolism, detoxification, and synthesis of important plasma proteins.