Special Histology and Embryology: Practical Course - V. K. Napkhanyuk 2001

Digestive System
Midgut and Hindgut
Liver

The Liver (hepar) is the largest exocrine gland of the digestive tract, responsible for Bile secretion.

The external surface of the liver is covered by a Connective Tissue capsule that tightly fuses with the visceral layer of the Peritoneum.

The Structural and functional unit of the liver is the hepatic lobule, which has the shape of a hexagonal prism with a flat base and a convex apex.

Hepatic lobules are formed by hepatic cords and intralobular sinusoidal Blood capillaries.

These cords are composed of hepatocytes—liver epithelial Cells arranged radially. Blood capillaries run between them in the same direction, from the periphery to the center of the lobules.

Interlobular connective tissue forms the stroma of the organ. It contains Blood Vessels and bile ducts that are structurally and functionally connected to the hepatic lobules. In humans, interlobular connective tissue is poorly developed, which makes the boundaries between hepatic lobules indistinct.

The liver parenchyma consists of epithelial cells called hepatocytes (hepatocyti), which account for 60% of all cellular elements in the liver. They perform the vast majority of liver Functions. Hepatocytes have an irregular polygonal shape, with a diameter of 20–25 µm. A significant proportion of these cells (up to 20%) are binucleate or multinucleate.

The nuclei of hepatocytes are rounded, measuring approximately 7 to 16 µm in diameter.

The Cytoplasm of hepatocytes stains with both acidic and basic Dyes due to its high ribonucleoprotein content. It contains all common types of Organelles. The rough Endoplasmic reticulum is well developed, appearing as narrow tubules studded with Ribosomes. The smooth endoplasmic reticulum, taking the form of tubules and vesicles, is found either in restricted areas or scattered throughout the cytoplasm.

Hepatocytes contain various inclusions, such as Glycogen, Lipids, and pigments. Hepatocytes are arranged in hepatic plates (lamina hepatica). Between adjacent hepatocytes within a plate, bile canaliculi are formed as expansions of the intercellular space. Part of a hepatocyte's surface contacts other hepatocytes, while another part contacts sinusoidal capillaries. To understand The Structure of a hepatic lobule, one must examine the Blood supply to the liver.

The portal vein and hepatic artery enter the porta hepatis; branching repeatedly, they form lobular and subsequently segmental Arteries and Veins.

Segmental vessels divide into interlobular arteries and veins. Interlobular arteries and veins give rise to perilobular arteries and veins. Perilobular arteries and veins give origin to capillaries which, converging at the periphery of the lobule, form sinusoidal vessels (vasa sinusoideum). In the center of the lobule, the latter drain into the central vein (vena centralis). Central veins empty into sublobular veins (vena sublobularis).

Sublobular veins converge to form hepatic veins, which empty into the INFERIOR VENA CAVA.

An interlobular bile duct runs alongside the interlobular artery and vein, receiving drainage from the bile canaliculi.

The interlobular arteries, vein, and bile duct together form the portal triad (trias hepatica).

The Structural Features of sinusoidal capillaries are crucial for liver function. There are gaps between endothelial cells. A basement membrane is practically absent; instead, There is a perisinusoidal space (spatium perisinusoideum) containing individual bundles of Collagen fibrils. Thus, Blood Plasma bathes the hepatocytes freely, ensuring direct contact between the blood plasma and the organ parenchyma.

These vascular features facilitate the detoxification and metabolic Functions of the liver. Located among the endothelial Cells of the sinusoidal capillaries are hepatic macrophages, or Kupffer cells (macrophagocytus stellatus), which phagocytose microorganisms, damaged erythrocytes, and other particulate matter.

Perisinusoidal lipocytes (lipocytus perisinusoideus), which participate in Lipid METABOLISM, reside in the perisinusoidal space.

Slide 21. Section of the liver (Figs. 58, 59).

Low power. Examine and sketch the slide. Under low power, note the indistinct lobular structure due to the minimal amount of interlobular connective tissue; the BOUNDARIES OF THE lobules are determined by the arrangement of the hepatic plates and the interlobular vessels that form the triad.

High power. Study the STRUCTURE OF THE hepatic lobules, as well as the blood vessels and bile ducts running between them. The bulk of the lobule consists of liver cells—hepatocytes—grouped into double-rowed, radially arranged cords known as hepatic cords. These cords are surrounded by sinusoidal (venous) capillaries which, converging at the center of the lobule, form a non-muscular central vein running along the longitudinal axis of the lobule. The venous sinusoids of the hepatic lobules originate from perilobular veins, which in turn are Branches of the interlobular veins.

The liver has an Arterial System and two venous systems.

Arteries: hepatic, lobular, segmental, interlobular (possessing sphincters), perilobular, and capillaries; they are short, penetrate the peripheral Regions of the lobule, and empty into the venous sinusoids.

Veins: a) PORTAL VEIN SYSTEM: portal vein, lobular, segmental, interlobular, perilobular veins, and capillaries, which lead into intralobular venous sinusoids (containing mixed blood); b) hepatic vein system: central veins, collecting veins (also known as sublobular veins), and hepatic veins, numbering three to four.

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Fig. 58. Liver section. Hematoxylin and eosin stain. x 100:

1 — central vein; 2 — hepatic cords; 3 — venous capillaries (sinusoids); 4 — interlobular artery; 5 — interlobular vein; 6 — bile duct; 7 — interlobular connective tissue

Fig. 59. Liver section. Hematoxylin and eosin stain. x 200:

1 — interlobular connective tissue septum; 2 — interlobular duct; 3 — interlobular hepatic artery; 4 — interlobular vein; 5 — venous capillary; 6 — hepatic cord; 7 — central vein

Intralobular venous capillaries (sinusoids) are referred to as a remarkable network (rete mirabile) because they are situated between Blood vessels of the same name.

Label the following on the figure: 1) central vein; 2) hepatic cords; 3) venous capillaries; 4) interlobular artery; 5) interlobular vein; 6) bile duct; 7) interlobular connective tissue; 8) interlobular duct; 9) central vein.