BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 3. CELL AND EXTRACELLULAR MATRIX

3.2. Cell Classification

Traditional Cell Classification is based on shape, Structure, and The chemical properties of Cells that allow them to bind stains. More refined Methods reveal new cell groups. For instance, modern immunology indicates the existence of more than ten types of lymphocytes. There are many varieties of smooth Muscle cells (in Blood Vessels and the intestine, for example). Connective Tissue cells from different Regions of the dermis are distinct, as the overlying epidermal cells behave differently depending on their influence. Therefore, any cell classification is imperfect. For biochemistry, it is best to classify cells by their Functions, though with certain limitations. For example, a keratinocyte (a cornified epidermal cell) successively acquires different names depending on its stage of maturation.

Biochemists need only two designations: for a differentiated cell and for a stem cell. With this in mind, we present the basic list of various genome expression patterns in the form of adult human phenotypes: cornified epithelial cells (Skin, Nails, Hair); cells of moist stratified barrier epithelia (Tongue, Esophagus, Urinary Tract); epithelial cells with exocrine function (salivary, mammary, lacrimal, sebaceous, Bowman's, Brunner's, Bulbourethral Glands, epithelial Cells of the prostate, endometrium, gastric mucous cells, Paneth cells, etc.); hormone-secreting cells (Endocrine glands and endocrine cells of the gastrointestinal tract), epithelial absorptive cells of the gastrointestinal tract, exocrine glands, and urogenital tract (microvillous cells, Gallbladder epithelial cells, etc.); cells responsible for METABOLISM and reserve storage (hepatocytes, lipocytes, etc.), epithelial cells with a primary barrier function (lining the Lungs, intestine, exocrine glands: pneumocytes, renal glomerular podocytes, etc.); epithelial cells lining closed internal cavities (endothelium, synovial, and serous cells, etc.); ciliated cells with a propulsion function (cells of the respiratory tract, oviduct, ependymal cells); cells that secrete Extracellular matrix components (ameloblasts - tooth enamel, fibroblasts, odontoblasts, chondrocytes, etc.); contractile cells (skeletal, cardiac, and smooth Muscles, myoepithelial cells of the iris); blood and immune system cells (erythrocytes,

megakaryocytes, macrophages, lymphocytes, etc.); sensory transducer cells (photoreceptors, auditory, taste receptor cells, thermoregulatory cells, and blood pH receptors, etc.); autonomic Neurons (cholinergic, adrenergic, peptidergic); supporting cells of Sensory Organs and peripheral neurons (supporting cells of The Organ of Corti and vestibular apparatus, Schwann cells, etc.); neurons and glial cells of the Central Nervous System (neurons, glial cells - astrocytes, oligodendrocytes); lens cells (epithelial cells and lens fibers - crystallin-containing cells); pigment cells (melanocytes, retinal epithelial cells); Germ Cells (oogonia, spermatogonia, spermatocytes); nutritive cells (egg follicle cells, Sertoli epithelial cells in the Testis).

Thus, more than 200 cell types in vertebrates possess over 200 Different types of specialization. As an example, we can characterize the neuromuscular complex, The formation of which involves three cell types: muscle, nerve, and Schwann cells. The cells in this complex perform strictly specialized functions inherent to each:

1) myocytes provide the contraction-relaxation process. This is facilitated by myofibrils of contractile and regulatory Proteins, for which ATP is supplied by Mitochondria located between the fibrils;

2) neurons deliver excitatory signals from the Brain AND SPINAL cord to myocytes. This specific function is maintained by Ion Channels and ion pumps that transport charge-carrying ions, which is equivalent to an electric current. In a neuron, the electrical impulse propagates from one end of The Cell to the opposite end (which is -100 µm) within milliseconds;

3) such Propagation of Excitation is possible only with electrical insulation of the neuron axon. This is provided by the Schwann cell: it wraps its Plasma Membrane layer by layer around the axon (like electrical tape), forming the Schwann sheath;

Thus, analyzing all cell types from an evolutionary perspective, one can conclude that one of the earliest steps toward a multicellular Organism was The Emergence of epithelium, in which cells are joined into layers that demarcate the internal environment of the organism. Alongside epithelial cells, the first primitive types of differentiated cells were nerve and muscle cells, as well as connective tissue cells. All of these cell types are found in the most primitive modern animals.