Atoms form molecules that form cells that form tissues that form organs that form systems. Systems, in turn, form individuals that form populations that form (biological) communities that form the biotic components of ecosystems. All ecosystems on earth form the biosphere.
The formation of complex and distinct pluricellular organisms would not be possible if mitosis in embryos produced only daughter cells identical to their parent cell, since there would be no differentiation or structural or functional specialization among cells.
Evolution solved this problem by creating the cellular differentiation process by which, provoked by stimuli not yet well-known by science, different and specialized cell lineages produce different tissues, organs and systems, which, as a whole, form pluricellular organisms.
Cellular differentiation likely is a very intricate process that activates and inactivates some genes within the cell in response to some stimulus.
The main animal cell tissues are epithelial tissue, nervous tissue, muscle tissue and connective tissue.
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Epithelial tissues, also called epithelia, are tissues specialized in covering the external and internal surfaces of the body.
The general function of the epithelium is to provide protection and impermeability (or selective permeability) to structure it covers. This is the reason for the epithelium's typical feature: the juxtaposition of layers of very close cells with little or no intercellular space between each two neighboring cells.
Nervous tissue is made of neurons and glial cells.
The function of nervous tissue is to receive and to transmit neural impulses (the reception and transmission of information). This function is derived from the characteristic morphology of neurons, which contain membrane projections (dendrites) to get information and an elongated membrane projection (axon, or nerve fiber) to transmit information at distance. Glial cells support the neurons and facilitate this process (sometimes acting as insulators).
Muscle tissues are tissues made of cells able to perform contractions and therefore generate movement.
The function of muscle tissue is to pull bones (skeletal striated muscle), to contract and move viscera and vessels (smooth muscle) and to make the heart to beat (cardiac striated muscle). Muscle cells have internal structures called sarcomeres where myosin and actin molecules are deposited to create contractions and distensions (movement).
The typical function of connective tissues is to fill empty spaces along other body tissues.
This function is related to the ability of the cells of connective tissues to secrete substances that produce extracellular materials, such as collagen and elastic fibers, creating the distribution of space between these cells.
(There are other important biological features of connective tissues, such as substance transportation, defending the body, etc.)
Bones and cartilage, both are tissues with a large amount of intercellular material, are formed of connective tissue.
The relatively large space between cells is a typical feature of connective tissue. A large amount of intercellular material is generally secreted by these cells.
The main functions of connective tissues are: supporting and filling spaces; cellular nutrition; energy storage (fats); hematopoiesis (formation of blood, blood cells and blood components); and immune defense (specialized cells).
The name connective tissue proper is used to designate the connective tissue that fills interstitial spaces as opposed to specialized connective tissues (blood, bones, cartilage, adipose tissue, etc.). Connective tissue proper secretes collagen, elastin and reticular fibers.
The main cells of connective tissue proper are fibroblasts, which are cells that secrete intercellular material. These cells make up the majority of cells of the tissue. Fibroblasts are later transformed into fibrocytes, mature cells with limited role in secretion.
The intercellular substance that fills interstitial spaces is called interstitial matrix, or just matrix.
The matrix of connective tissue proper is made of collagen fibers, elastic fibers and reticular fibers.
There are different types of collagen. The main function of these proteins is to maintain the shape and the structural rigidity of tissue. (Collagen is the most abundant protein in the human body.)
Elastic fibers are made of a protein called elastin.
Elastic fibers are abundant in artery walls, helping to maintain arterial blood pressure in these vessels. They are also present in the lungs, providing them with elasticity (some respiratory diseases are caused by the destruction of these fibers). In many other organs and tissues, elastic fibers are found in the interstitial matrix.
Reticular fibers are very delicate interstitial fibers made of a special type of collagen known as collagen type III. They can be found in many organs and tissues such as in lymph nodes, the spleen, the liver, blood vessels and also covering muscle fibers.
Diseases of the connective tissue are hereditary or acquired diseases (many of them are autoimmune) characterized by a deficiency in the structure or function of components of connective tissue, including deficiencies of collagen, elastin, etc. Some of these diseases are lupus, dermatomyositis, keloid, scleroderma, mixed connective tissue disease, mucinosis and Marfan's syndrome.
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