Page 19 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

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The cell (cellula)

Eukaryotic cells contain a soluble matrix (cytosol) in which specific functions of a particular cell. As these are clearly
numerous organelles and inclusions are suspended. These distinguishable under the light microscope, the cytoplasm
cellular contents are referred to collectively as the cyto- and the nucleus are conventionally regarded as separate
plasm. Cellular processes associated with metabolism, compartments. The nucleus is basophilic (= binds with
respiration, energy transformation, contractility and cell basic stains, e.g. stains blue with haematoxylin), while the
motility take place within the cytoplasm. These processes cytoplasm is predominantly acidophilic (binds with acidic
involve functional interactions between the cytosol and stains, e.g. stains red with eosin). The cytoplasm is sur-
the organelles. The matrix is an aqueous gel incorporat- rounded by a highly differentiated biological membrane,
ing variably sized molecules and structural elements in the the plasma membrane (plasmalemma), that serves both
form of a cytoskeleton. to separate the cell from its surroundings and to establish a
Most organelles are bounded by a biological mem- connection between the cell and its environment.
brane that, by virtue of its specialised layered structure,
separates the individual organelles from the surrounding Cell membrane (cytolemma, membrana
matrix (compartmentation). This permits diverse meta- cellularis)
bolic processes to occur independently within the confines Cell membranes are biological membranes that constitute
of the same cell. The cellular organelles can be subdivided the structural foundation of most organelles and form the
as follows: outer boundary of the cell. The outer (surface) membrane
is highly specialised and is termed the plasmalemma.
· membranous organelles: All biological membranes have a similar basic structure
− nucleus, (unit membrane) comprising layers of phospholipid and
− endocytotic vesicles, protein molecules held together by non-covalent bonds.
− exocytotic vesicles, Chains of lipid molecules form a lipid bilayer that encloses,
− endosomes, or is traversed by, protein molecules and glycoproteins in a
− lysosomes, ratio of 2:1 (depending on cell type, this relationship varies
− peroxisomes, from 4:1 to 1:4).
− rough endoplasmic reticulum (rER), With a total thickness of 7.5 to 10 nm, the cell mem-
− smooth endoplasmic reticulum (sER), brane can only be seen using electron microscopy. When
− Golgi apparatus, prepared with osmic acid, the cell membrane appears
− mitochondria, as three layers: an electron-dense outer layer (lamina
· non-membranous organelles: externa), an inner layer (lamina interna) and an electron-
− ribosomes, lucent intermediate layer (lamina intermedia).
− microtubules, On the outer surface of the plasmalemma, carbo-
− filaments (actin filaments, intermediate fila- hydrates (oligosaccharide chains) attach to membrane
ments) and proteins, forming glycoproteins, and to lipids of the bilayer,
− centrioles. forming glycolipids. This gives rise to a layer known as the
glycocalyx (cell coat).
The nucleus directs all of the functions of the cell and
houses its genetic material. Strands of deoxyribonucleic Cell membrane structure
acid (DNA) within the nucleus store genetic information The cell membrane is a biological membrane (Figures 1.1
and serve as a regulatory and information-processing sys- and 1.2) comprising lipid-protein layers covered on their
tem for cellular metabolism. The intracellular organelles external surface by oligosaccharide chains. The individual
act synergistically, their combined actions giving rise to the components of the cell membrane comprise:

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