Development and function of the liver
In stage 13 (ca. 32 days) the embryonic liver cells (pars hepatica) form themselves into acini and cords that grow into the capillary network that has arisen in the transverse septum between the two omphalomesenteric veins. Below it arises the gall bladder diverticulum (stage 11, ca. 29 days) (pars cystica), which also grows into the transverse septum.
- Capillary network of the omphalomesenteric vein
- Liver bud
- Intestinal tube (duodenum)
- Gall bladder
- Dorsal pancreas anlage
- Capillary network of the omphalomesenteric vein
- Liver bud
- Intestinal tube (duodenum)
- Gall bladder
- Omphalomesenteric vein
- Omphalaomesenteric duct
- Umbilical vein
Through sprouting and divisions of the intestinal bud, the first liver acini arise in the capillary network of the omphalomesenteric vein.
Fig. 33Through the relocation of the entire cardiac circulation system to the right the liver also is shifted more into that position.
The complex pattern of parenchyma and sinusoids arise in that sheets composed of liver cells are engendered from the cell cords and the capillares expand to become sinusoids. The hepatic laminas are ca. 5-7 cells thick. This organization is still retained until several years after birth. Until after birth no multi-nuclear hepatocytes are found.
All intrahepatic bile passages stem from the cells of the hepatic bud. The liver cells form canaliculi between them and also the extrahepatic hepatic duct that joins with the cystic duct to become the bile duct.
The central vein-lobule is considered the classic histological structural unit of the liver, whereby the lobule consists of radially organized sheets of hepatocytes separated by the sinusoids converging toward the central vein. Some physiologic and pathologic observations on the liver cannot be explained based on the classic liver lobule concept. For this reason further functional concepts of the histological organization have been suggested.
It is to be noted that the blood and bile flows in opposite directions.
The blood flows from the periphery over the sinusoids into the central veins while the bile flows from the centrally located hepatocytes into the periphery and is collected by the bile capilaries in the portal fields.
If one observes a small section of a classic liver lobule one sees that the branches of the portal vein as well as the hepatic artery discharges their blood into the sinusoids while the bile flows via tiny bile canaliculi that are each formed by two neighboring hepatocytes into the bile canals of the portal fields.
- Branch of the portal vein
- Branch of the hepatic artery
- Bile duct
- Boundary lamina of hepatocytes
- Central vein
- Liver acinus
- Sinusoid
The classic liver lobule is enveloped by a limiting plate of hepatocytes, surrounded by various vessels embedded in connective tissue. The blood flows from the periphery via the sinusoids into the central vein (violet arrow). The bile flows in the opposite direction (green arrow).
In a liver cell, the blood pole (facing the sinusoid) can be distinguished from a bile pole (between two hepatocytes). The bile canaliculi are formed by the hepatocytes themselves and are sealed by tight junctions.On the boundary between the sinus and hepatocytes there are a number of specialized cells.
- Ito cells with fat vacuoles
- Sinusoidal space
- Disse's space
- Bile canaliculi
- Hepatocyte
- Tight junction
- Kupffer's cells while phagocytizing
- Discontinuous endothelial layer
Section of a classic liver lobule with a sheet of hepataocytes.
Fig. 37The enlargement shows a sheet of hepatocytes, the sinusoids and a variety of specialized cells close to (Ito-cells) or inside (macrophages) the sinusoids.