Page 22 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

4 Veterinary Histology of Domestic Mammals and Birds

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1.2 Freeze-fracture electron microscope image of the cell membrane (plasmalemma) of a lymphocyte showing
the lipid bilayer and membrane proteins (x40,000).

At the outer surface of the plasmalemma, membrane of the system – receptor, G-protein and enzyme – are able
proteins serve as receptors for signalling molecules. to move freely within the plane of the membrane. Binding
These include receptors involved in endocytosis and in of this group of receptors with ligands occurs as a result of
recognition of other cells or cellular parasites (protozoa, random interactions (mobile receptors).
viruses). Also present are receptors for substances that While the plasmalemma, with its lipid bilayer, serves as
act as primary messengers in signal transduction path- a barrier to the simple diffusion of most water-soluble mol-
ways, including hormones, growth factors, cytokines and ecules, passage of small molecules across the membrane is
neurotransmitters. enabled by membrane transport proteins including chan-
These receptors are particularly significant in deter- nel proteins and carrier proteins (Fig. 1.3) (see below).
mining the function and structure of each cell, and in Channel proteins are components of ionotropic neu-
establishing its specificity. The structure of numerous rotransmitter receptors, in which receptor subunits form
receptors has been characterised, enabling detailed exami- a centrally located ion conduit (ion channel). Nicotinic
nation of cell structure and function by means of cloning, acetylcholine receptors are included in this group.
mutagenesis and transfection. Protein tyrosine kinases Activation of the receptor results in opening of the ion
+
and G-protein coupled receptors represent two promi- channel and influx of Na , leading to membrane depolari-
nent groups of cell surface receptors that convey signals sation. This mechanism is integral to the transmission of
to regulatory systems within the cytoplasm. signals between presynaptic cells and postsynaptic cell
Protein tyrosine kinases include receptors (receptor membranes.
tyrosine kinases, RTKs) for growth factors such as epider- Transport facilitated by carrier proteins may be pas-
mal growth factor (EGF) and insulin-like growth factor sive or active (see below). Other membrane proteins have
I (IGF-I). This receptor type is activated by binding of a enzymatic or catalytic functions.
ligand with endogenous kinase on the external surface of
the plasmalemma. Subsequent autophosphorylation or Cellular metabolism
phosphorylation of other proteins results in the stimula- Cells are characterised by the capacity to engage in con-
tion of cell-specific metabolic reactions. tinuous synthesis, transformation, breakdown and, under
G-protein coupled receptors are activated by a range certain circumstances, release of materials. To sustain
of ligands including hormones and neurotransmitters. such metabolic activity, the cell takes up organic and inor-
This receptor type acts as a membrane modulator by reg- ganic substances (substrates) from its surroundings and
ulating the activity of the membrane-associated enzyme modifies these through a series of chemical reactions.
adenylyl cyclase. G-protein coupled receptors thus influ- These reactions take place within the cellular matrix or in
ence production of the second messenger cyclic AMP association with cellular organelles.
(cAMP) within the cytoplasm (stimulation through Through this process, a diverse range of dietary nutri-
G -proteins, inhibition through G -proteins). This diverse ents enters the cell and undergoes enzymatic cleavage
s i
family of receptors includes adrenergic, cholinergic, dopa- (cellular digestion). Substances such as monosaccharides,
minergic and peptidergic receptors. The three components fatty acids and amino acids, as well as ions and vitamins,

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