The connective or connective tissue is a class of tissue that functionally integrates the body by forming a continuum with epithelial, muscular and nervous tissues, as well as with the components of other connective tissues.
Connective tissues and their cells originate from the migration of mesenchymal cells during embryonic development. It is convenient to remember that the mesenchyme is the product of the maturation of the cells of the mesoderm, one of the three germ layers of embryonic tissue..
Among the tissues that form with the migration of mesenchymal cells are bones, cartilage, tendons, capsules, blood and hematopoietic cells, and lymphoid cells..
Connective tissues, as will be seen later, are classified into embryonic connective tissue, proper connective tissue, and specialized connective tissue, which includes cartilage, bone, and blood..
Its wide distribution in the body and its functional importance means that any defect in these essential tissues ends in serious clinical pathologies, related either to their formation and establishment processes from the embryonic state, or to diseases acquired in the course of life..
Among the various pathologies associated with this type of tissue are numerous types of cancer, capable of affecting bones (carcinomas), blood (leukemias), adipose tissue (liposarcomas) and others.
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Absolutely all connective tissues are composed of cells, extracellular fibers and an amorphous substance or ground substance.
The extracellular fibers and the amorphous substance constitute the extracellular matrix, essential for intercellular contact and communication and determining the physical properties of the tissue..
Connective tissue cells can be grouped according to their mobile capacity into resident or fixed cells and mobile, free or transient cells..
Fixed cells are enormously diverse and constitute a population of cells that developed and remained in place within connective tissue from their formation; they are stable and long-lived cells.
Cells mobile or free originate from the bone marrow and are mainly in circulation. They are short-lived cells, constantly replaced and whose function they exert by migrating towards the tissues after receiving specific stimuli and signals..
The extracellular fibers of the connective tissues are the fundamental components of the extracellular matrix. They are represented mainly by collagen fibers, elastic fibers and reticular fibers..
Collagen fibers are not elastic, but they provide tensile strength to the tissue and more than 15 different types of collagen fibers are known, classified into six different types, namely:
-Type I collagen: present in the connective tissue itself, in bone and dentin
-Type II collagen: in hyaline and elastic cartilage
-Type III collagen: found especially in reticular fibers
-Type IV collagen: in the dense region of the basement membrane
-Type V collagen: observed in the placenta
-Type VII collagen: present at the junctions between the basement membrane and the reticular membrane
Elastic fibers, on the other hand, are made up of the protein elastin and multiple microfibrils. Their elasticity is such that they can stretch more than 100% of their length at rest.
The ground substance is a gel-like hydrated substance and that found in connective tissues is essentially composed of proteoglycans, glycosaminoglycans, and glycoproteins..
Glycosaminoglycans are long, unbranched polymers of repeating disaccharide units. Usually these units are composed of an amino sugar, which can be N-acetyl glucosamine or N-acetylgalactosamine..
Two types of glycosaminoglycans have been described: sulfated and unsulfated. The sulfated ones present molecules of keratan sulfate, heparan, heparin, chondroitin sulfates and dermatan, while the non-sulfated ones have hyaluronic acid residues.
Proteoglycans are nothing more than protein nuclei to which glycosaminoglycans bind.
The gel characteristics of the ground substance and therefore of the extracellular matrix of the connective tissue are given by lattices between proteoglycans and hyaluronic acid molecules that form voluminous molecules known as aggrecan aggregates..
Glycoproteins are large adherent proteins that function in the fixation of both matrix components and some components of the plasma membrane..
There are different types of glycoproteins, including laminins (present in the basement membrane); chondronectin and osteonectin in cartilage and bone, respectively, and fibronectin, dispersed throughout the extracellular matrix.
The mineralization of the amorphous substance in the connective tissue contributes to the hardness of mineralized tissues such as bone, dentin and enamel..
Also known as supporting or supporting tissues, connective tissues fulfill various functions, which usually depend on where they are located..
The connective tissue that constitutes the capsules that enclose the organs and the stroma that makes up the structure of these have support functions and mechanical support.
Skeletal muscles are attached to each other thanks to the presence of connective tissues and in turn are attached to bones by ligaments and tendons, which are also a specialized class of connective tissue.
An essential function of these tissues is also to provide an adequate medium for the exchange between cells and tissues, that is, for cellular communication through different molecular mechanisms (metabolic waste, nutrients, oxygen, among others)..
They contribute to the defense and protection of the body thanks to the participation of phagocytic cells, antibody-producing cells that work for the immune system and other cells that participate in the secretion of “pharmacological” substances during the inflammatory response..
They also include tissues that represent one of the body's largest energy reserves: lipid deposits in adipose tissue..
The traditional classification of connective tissues includes embryonic connective tissue, the connective tissue itself, and specialized connective tissues..
In this group are the mesenchymal and mucous tissues. The first is present only in the embryo and consists of mesenchymal cells embedded in an amorphous substance composed of disordered reticular fibers.
The cells present in this tissue have an oval-shaped nucleus, with little cytoplasm. These cells give rise to many cells of other connective tissues and do not exist in the adult organism, except in the dental pulp..
Mucosal tissue, as could be inferred, is a loose, amorphous-looking connective tissue, the matrix of which consists mainly of hyaluronic acid with little collagen content. Found only in the umbilical cord and subdermal tissue of the embryo.
This type of connective tissue "fills" the spaces in the body deep within the skin. It is found lining the internal body cavities, surrounding the parenchyma of the glands and in the adventitious layer of the blood vessels.
In mucous membranes like those of the digestive tract is a special type of loose tissue known as "own sheet ".
Loose tissue is characterized by its abundant ground substance and extracellular fluid. It normally contains a wide variety of cells, both fixed and mobile. Of the former, it can have fibroblasts, fat cells, macrophages, and mast cells, as well as undifferentiated cells..
In addition, this tissue has few loosely cross-linked reticular, elastic, and collagenic fibers. The cells of loose connective tissue are maintained thanks to the contributions of oxygen and nutrients that come from small blood vessels and small nerve fibers.
Since it is located immediately below the thin epithelia of the digestive and respiratory tracts, it is the first site in the body to attack by antigens and invading microorganisms, therefore it has many mobile cells that participate in immune, inflammatory and allergic responses.
As its name implies, this type of tissue is seen histologically as a more compact tissue. It essentially has the same components as loose connective tissue, with a greater number of extracellular fibers per unit volume and fewer cells..
According to the orientation and arrangement of the extracellular fibers that compose it, dense connective tissue can be further classified into regular and irregular dense connective tissue.
This type of dense tissue has a large number of extracellular fibers arranged in regular patterns. Examples of this type of tissue are the ligaments, tendons and the stroma of the cornea..
It is divided into two types of tissues: collagenous and elastic tissue, which differ in the proportions and arrangement of collagenic and elastic fibers..
Irregular dense connective tissues also have a large number of extracellular fibers, especially collagenic, but these are arranged in random and disorderly patterns. They are rich fibroblast-like cells.
This form of connective tissue is especially present in the skin, capsules or lining of organs such as the liver and spleen and in the periosteal tissue that surrounds the bones..
Composed primarily of reticular fibers (type III collagen fibers secreted by fibroblasts), reticular connective tissue is a special connective tissue that exists only in some thin vascular channels of the liver, spleen, lymph nodes, and bone matrix..
Of this class of tissues, two types are known: white and brown adipose tissue. The former is characterized by the presence of unilocular adipocytes (with a large fat vacuole), while the latter contains multilocular adipocytes (with many small fat vacuoles).
A large proportion of adipocytes are found in adipose tissue. They have a low number of collagen fibers, fibroblasts, leukocytes, and macrophages. It is found in the subcutaneous compartments and is particularly abundant in the abdomen area and around the hips and buttocks..
Brown or brown adipose tissue is highly vascular tissue. It is particularly abundant in hibernating mammals and infants, but its presence in adult humans has not been fully elucidated..
In this group of connective tissues are blood, bones and cartilage.
Cartilage is rich in cells called chondrocytes. The substance of this tissue is not vascularized nor does it have nerve endings or lymphatic vessels, so the nutrition of its cells is given through blood vessels of surrounding tissues by diffusion..
The cartilage is subdivided into hyaline cartilage, rich in type II collagen; elastic cartilage, with abundant elastic fibers and type II collagen and fibrocartilage, with thick type I collagen fibers.
Bone is a specialized connective tissue whose extracellular matrix is calcified. Provides structural support for the body, protection of vital organs, and attachment sites for skeletal muscles.
Stores 99% of body calcium. Its central cavity houses the bone marrow, a hematopoietic tissue (which gives rise to blood cells). Its main cellular components are osteoprogenitor cells and osteoblasts.
Blood is a fluid specialized connective tissue that is distributed throughout the body. Like all connective tissues it contains cells, fibers and fundamental substance.
Its cellular components include erythrocytes, leukocytes, and platelets. It has “potential” fibers called fibrinogen and the fundamental substance, together with its proteins, makes up the fluid region or blood plasma.
Its main function is to transport oxygen and nutrients, as well as waste products to the kidneys and lungs, and to maintain body homeostasis..
Fixed connective tissue cells are:
-Fibroblasts: the most abundant type, in charge of the synthesis of the extracellular matrix
-Pericytes: surround the endothelial cells of capillaries and small veins
-Fat cells: present in adipose tissue, they function in the synthesis, storage and metabolism of fats
-Mast cells: the largest type; work on inflammatory processes and hypersensitive reactions
-Macrophages: they can be fixed or mobile. They work in the elimination of cellular debris and in the protection against foreign agents (antigen-presenting agents)
Mobile connective tissue cells are:
-Plasma cells: derived from B lymphocytes, produce and secrete antibodies
-Leukocytes: circulating white blood cells that participate in inflammatory processes and the immune response
-Macrophages: mobile macrophages participate in the presentation of antigens to antibody-producing cells
Specialized connective tissues also have their specific cells, chondrocytes (cartilage tissue), osteocytes (bone tissue), and blood cells (which are classified within motile cells)..
Good examples of connective tissues have been named above:
-Bones and blood, both with primary functions in the human body
-Cartilage, which is present in the nose, larynx, bronchi, ears, ear canals, intervertebral discs, joints between bones, etc..
-The mucous membranes of the digestive tract
-Capsules that coat and give internal organs their characteristic shape, as well as adipose tissues that store energy in the form of fats, are also excellent examples.
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