Page 244 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

226 Veterinary Histology of Domestic Mammals and Birds

and lymphatic organs’). Glands (glandulae proctodaeales) ingrowth of mesodermal umbilical vessels (vv. ompha-
VetBooks.ir are present in the dorsolateral wall. lomesentericae). Endothelial cells lining the capillaries that
At the external opening of the cloaca, there is a dorsal enter the liver tissue retain their mesodermal character,
and ventral lip (labium venti dorsale and ventrale) each of becoming sinusoidal capillaries (liver sinusoids).
which contains glands (glandulae venti). In the chicken, Through its mesodermal components, the functions of
the lips contain numerous sensory Herbst corpuscles. the liver are expanded to include:

Glands associated with the intestine · phagocytosis and
· embryonic haematopoiesis.
Liver (hepar)
The liver of domestic mammals and birds is one of the Cells migrate from the bone marrow to the liver and,
few glands of the body that performs a diverse range of as components of the mononuclear phagocyte system
functions. Most of these relate to one cell type, the liver (MPS), perform non-specific functions such as uptake of
cell or hepatocyte (hepatocytus). In greatly simplified molecules, particles and cell fragments from circulating
terms, the liver performs three central functions that have blood (Kupffer cells). Haematopoiesis occurs in the liver
a significant influence on metabolic processes throughout from early in the middle phase to the end of embryonic
the body: development.
The distribution of the embryonic vascular network
• synthesis and immediate secretion (e.g. proteins), gives rise to the characteristic lobulation of the liver. This
• synthesis, storage and secretion (e.g. glycogen) and manifests to a varying extent in different species, depend-
• metabolism and detoxification (e.g. steroids, drugs). ing on the degree of connective tissue development. The
capacity of the liver to perform its many different functions
From nutrients absorbed in the intestine, the liver synthe- relies upon a close association between the intra-hepatic
sises various endogenous substances: glycogen is produced vascular network and the spatial arrangement of liver cells.
from the monosaccharide glucose and amino acids are
coupled to form complex proteins. Some of these pro- Structure of the liver
teins pass into the circulation as plasma proteins (albumin, The liver is enclosed in a capsule (tunica fibrosa) contain-
fibrinogen, prothrombin, enzymes and glycoproteins). ing collagen fibres and some elastic fibres. The outside of
Lipoproteins and ketone bodies are metabolised within the capsule is lined by a tunica subserosa and a simple
the hepatocytes. tunica serosa. Bundles of type 1 collagen fibres extend
An important function of liver cells is the synthesis and from the connective tissue capsule into the parenchyma
secretion of bile via a specialised system of capillaries and as interstitial tissue. Afferent and efferent vessels, bile pas-
collecting channels. Hepatocytes secrete bile acids, biliru- sages and nerve fibres pass through the connective tissue.
bin and water, as well as cholesterol and steroid hormones. The arrangement of connective tissue fibres and passages
After synthesising glycogen and triglycerides, liver cells divides the liver into distinct lobules (lobuli hepatici, see
may periodically – often in a diurnal pattern – store these below). The interlobular septa are particularly well devel-
substances in the cytoplasm before releasing them into the oped in the pig (Figure 10.70). In this species, lobulation
blood. Liver cells also act as reservoirs of fat-soluble vitamins can be appreciated macroscopically on the surface of the
(A, D and K). A further key function of liver cells is deamina- liver. In other mammalian species, lobulation is less distinct
tion of amino acids. Additional processes occurring within (Figure 10.71) but is nevertheless evident in terms of struc-
liver cells include hydroxylation, acetylation, methylation ture and function.
and oxidation-reduction, thus resulting in detoxification of
numerous metabolic products that are then eliminated from VESSELS OF THE LIVER
the body in the bile or through the kidneys. The liver receives venous and arterial blood through the
Embryologically, the liver is a compound organ that hepatic porta (hilus of the liver). These vessels divide and
develops from endodermal and mesodermal compo- extend widely throughout the parenchyma. Blood entering
nents. Hepatocytes originate from the endoderm from through both the venous and arterial systems drains into
buds of the hepatopancreatic ring of the embryonic fore- a common outflow.
gut. The developing liver cells and pancreatic cells separate Within the liver, the nutrient-rich functional blood sup-
from the gut but remain attached by ducts (ductus chole- ply from the intestine and associated unpaired abdominal
dochus, ductus pancreaticus). Both the liver and pancreas organs (portal system) combines with the oxygen-rich
exert a continuous influence on metabolism. nutritional supply originating from the hepatic artery.
Early in embryonic development, the endodermal Upon entering the liver at the porta, the portal vein
liver cell anlage is divided into sheets of liver cells by the divides within the interstitial tissue into interlobular veins.

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