EXAMPLES OF CELL DIFFERENTIATION
The unity of a cell of a single-celled organism will take forms and structures, the most diverse, depending on the environment, the type of metabolism, etc.
The increasing complexity of multicellular organisms and the individual cells that compose them come to assume increasingly specialized structures and functions, differentiating themselves in a varied (and more or less extreme) way from the cell type.
Just as in the human community the specialist loses the competence necessary to perform tasks other than his own, so the most differentiated cell gradually loses from some to many of the structures (or functions) of the type cell, to the point of becoming incapable of autonomous metabolism and reproduction.
Most of the billions of cells that make up man are differentiated, as much as less, to perform individual functions for the benefit of the "community".
LARGE CATEGORIES OF DIFFERENTIATION
First of all we find cells charged with constituting the "boundary" between the inside of the organism and the external environment. These are the cells of the so-called integumentary tissue or coating epithelium. We specify immediately that the boundary between inside and outside must be understood in a biological and not topographical sense. For example, the mouth and the entire digestive tract, while appearing to our eyes "internal" to the organism, are biologically external, in continuity with the environment that surrounds us. In general, the epithelium that covers our body is called skin, while what constitutes the wall of the cavities communicating with the outside is called mucosa.
The more it is subject to mechanical wear, the more the epithelium is stratified, as happens in the case of the skin, in which the germinative layer consists of cells in continuous division, generating the cells of the outer layers, which gradually proceed towards the surface, differentiating, hardening, to die and fall apart.
In the mucous membranes the hardening does not occur, and the cellular layers are much less numerous the more intense are the metabolic exchanges that must be performed there.
Since the epithelia are intended for contact with the outside, some epithelial cells further differentiate to take care of specific communication functions. The photoreceptors (retina of the eye), the chemoreceptors (taste buds), the organs of touch, hearing, etc., are made up of highly specialized epithelial cells.
Moreover, the entire nervous system derives similarly from a section of what was the superficial cellular layer in the early embryonic stages.
The epithelia never include veins or other vessels in their thickness. They are supported, with a more or less rigid or elastic anchorage, on a lower layer of connective tissue.
The connective, as the term itself says, ensures continuity between tissues and organs. It can be loose, elastic, fibrous or rigid. In its thickness we find the blood vessels, the more or less differentiated cells, the nerves, the fibers, etc. We distinguish fibers and cells of various types, the intercellular substance in which they are immersed (produced by the cells themselves) and the blood and lymphatic vessels (which in their connective find their natural seat). The connective, in establishing connections between all the tissues and organs of the body, fills the interior spaces and ensures the transport of various metabolites. The connectives are also called trophomechanical tissues. "Trofo" is a term of Greek origin that expresses the task of ensuring the metabolism, while "mechanical" expresses the task of supporting the organs and the organism itself.
Particular differences in this sense occur on the one hand in the blood, and on the other in the cartilaginous and bone tissue. The blood, continuously pumped by the heart through arteries, capillaries and veins, is the trophic component par excellence of the organism that collects oxygen through the wall of the pulmonary alveoli and nourishment through that of the intestinal villi, and then transport them to all the cells, of which it collects the catabolites, transferring them to the elimination sites (especially the kidneys).
Cartilages and bones are the main mechanical components of the body. The former are more elastic, with a high content of water and lubricating substances, engaged in sliding seats (joints) and flexibility. The bone tissue, rigid due to the abundant deposition of mineral salts in the intercellular substance, ensures above all the support function and the system of levers for the mechanics of motion.
Muscle tissue is divided into two broad classes: smooth and striated. The smooth one consists of single cells, with a relatively slow and lasting contraction, which ensure the functioning of the internal organs with non-voluntary innervation, such as the intestine. The striated muscle tissue, so called because under the microscope appears crossed by striations perpendicular to the direction of its contraction, constitutes the skeletal musculature, under the control of the central nervous system, for voluntary movements, and consists of parallel fibers, even very long, multinucleated, with rapid contraction but not lasting. Skeletal muscles, as a motor component of biomechanical phenomena, take on the role of protagonists in physical education and sports.
Next to the cartilages, bones and muscles, it is necessary to mention the nervous system, consisting of cells with specialization and differentiation pushed to the extreme, with characteristics of perennial tissue (as well as the muscular one) and that is with the loss of cellular reproduction capacity .
While a part of the nervous system (orthosympathetic and parasympathetic) presides over the functions of vegetative life and the control of the various internal organs, the somatic nervous system controls the striated muscles (voluntary movements) and is basically constituted by a system of receptors (sense organs ) peripheral, connected by means of fibers afferent to the brain (CNS), which processes and stores the impulses received, transmitting them, through other nerve fibers (those efferent), to the musculature.
The topic of cell differentiation is so complex that those mentioned here are only generic examples.
Edited by: Lorenzo Boscariol
