l General structure
l Elongated, lobulated, compound, acinar gland, with a very thin CT capsule and septa.
2 Long duct system and its CT provide support.
3 Exocrine part is major with very many serous acini and some ducts.
4 Endocrine part is minor: many small clusters of cells staining palely (with HE) - islets of Langerhans.
2 Acinar structure
l Pyramidal epithelial cells line the acini; are rich in basal granular ER (deeply basophil); have a prominent supranuclear Golgi complex and apical zymogen granules (precursors of several digestive enzymes).
2 Electron-radioautography with labelled leucine showed the secretory pathway through the cell and its time aspects; see Chapter l6.C.l.
3 A pale duct cell (or a pair) may be seen intruded into the centre of the acinus as a centroacinar cell.
1 Commence as narrow intercalated ducts within the acini, although vagaries of section plane result in one finding centroacinar cells in only some acini.
2 Beyond the intercalated ducts, ducts have pale cuboidal cells, with few organelles and some microvilli, changing to columnar epithelial cells in the larger ducts.
3 Ducts are less often seen than in the serous parotid gland, and probably actively change the secretions only in the smaller, early ducts.
4 Ducts are accompanied by less connective tissue than in the salivary glands, which are exposed to masticatory forces.
4 Exocrine function
l Formation of alkaline secretions, which counter the gastric fluid's acidity, thereby activating pancreatric pro-enzymes for digestion.
2 The release of alkaline and enzymatic secretions is under the hormonal control of secretin, and cholecystokinin/CCK, respectively.
5 Islet structure and functions
l No ducts, but rich in capillaries with a fenestrated endothelium.
2 Pale cells contain granules differing in alcohol-solubility and staining characteristics (distinguishable also in EM and immunocytochemically) for the differentiation of:
2 Liver lobule
l First impression is of a uniform mass of large glandular cells throughout the liver substance.
2 Closer examination shows that the cells are arranged in perforated plates, one cell wide. Between the plates are sinusoidal blood channels 9-l2 µm wide, lined by endothelial cells.
3 Scattered in the glandular mass are blood vessels, alone and accompanied by other vessels.
4 The distribution of these vessels defines or marks out the classic hepatic lobules.
5 Varieties of liver vessel
7 Hepatic lobular blood flow is:
9 Rappaport's liver acinus was a functional unit comprising parts of
three or so lobules. It sought to account for differences in exposure to the
blood supply among various parts of lobules. Such differences are reflected
in varied functional activities and degrees of susceptibility to toxic agents -
a metabolic zonation (Gebhardt R. Pharmacol Therapeut 1992;53:275-354;
Cell Biol Toxicol 1997;13:263-272).
The territory of an acinus has, as its axis, one final branch of the portal vein, and is subdivided into: 1 periportal, 2 intermediate, and 3 perivenous (close to the central vein) zones, with the initial periportal zone being roughly spheroid, and isolated from periportal zones of adjacent acini.
The concept is not easy for students to follow, nor, it seems, for hepatocytes, which, for many processes, heed different patterns. To best fit events to the architecture, hepatologists are now more likely to employ the simpler concept of separately continuous periportal and perivenous/pericentral zones, than that of discrete acini.
pp pp pp pv . pp _ _ _ _ _ _ _ _ _ _ . pv / pp pp pp pv . pp/ . pv portal veins / pp pv pv pv pv . - - - - - - - - . . . . . . . . . . . . . . . . . .central veins pp pp pp \ pv pv pv pv \ pp \_pp_ _ pp_ _ _pp_ _ _ pp pp pp pp
3 Liver sinusoids
l Are lined by fenestrated endothelial cells, loosely attached, and hold
2 phagocytic Kupffer cells (larger, stellate, with a pale oval nucleus), demonstrated by the vital intravascular injection of trypan blue or carbon particles, or latex particles for microscopy in vivo.
3 Fenestrated lining cells are not tightly attached and rest on microvilli of underlying hepatic cells, without a BL intervening.
4 Plasma can thus pass through the sieve plate, formed by the lining cells, out into the perisinusoidal space of Disse to interact with the hepatocytes. Some of this fluid may pass to the periphery of the lobule to be collected as lymph.
5 Disse's 'space' contains ECM materials, but not a visible basal lamina.
6 Scarce, fat-storing, stellate cells of Ito lie outside the endothelial cells. They store vitamin A. They respond to a variety of insults by making collagen and causing cirrhosis (fibrosis).
7 The sinusoidal wall provides for:
.. (a) blood cleansing, e.g., of gut bacterial toxins;
.. (b) haemopoiesis in the embryo;
.. (c) bringing plasma into intimate contact with the hepatic cell for its many metabolic functions of storage, transformations, syntheses, regulation of plasma concentrations, detoxifications, the production of bile, and assisting defence by producing acute-phase proteins.
4 Hepatocyte/hepatic cell
l Large, polyhedral, 30 µm x 20 µm cell with:
5 Bile pathways
l System of canaliculi (seen easily only with EM or special impregnation) between the hepatic cells leads to
2 canals of Hering/cholangioles, with both hepatocytes and pale duct cells in their walls. Next come, in the portal areas,
3 bile ductules with only small, pale cuboidal cells, firmly held by membrane interdigitations and junctional complexes, and having a few luminal microvilli.
4 Bile ducts' epithelium changes to columnar mucous cells and, extrahepatically, the ducts acquire smooth muscle as well as CT.
5 Cystic duct allows reflux into the gallbladder, when Boyden's sphincter choledochus at the duodenal outlet of the common bile duct is closed.
l Extensively folded mucosa of tall, simple, columnar epithelial cells with many microvilli, lying on a loose lamina propria.
2 Goblet cells are absent, but in the neck there may be small glands of uncertain function.
3 The middle layer has variously disposed (mainly circular) smooth muscle bundles.
4 Outermost is a serosa of mesothelium-covered areolar CT with vessels and nerves, except where the gallbladder attaches to the liver.
5 Function - stores and concentrates the bile by actively absorbing sodium, coupled with water and anions. The hormone - cholecystokinin - released from gut endocrine cells in response to fat or amino acids causes contraction of the muscle to expel the bile.