Conjunctive tissue
In histology, connective tissue (TC), also called connective tissue, is a heterogeneous set of organic tissues that share a common origin from the embryonic mesenchyme originated from the mesoderm.
Understood in this way, the connective tissues concur in the primary function of support and systemic integration of the organism. In this way, the TC participates in the cohesion or separation of the different tissue elements that make up the organs and systems and also becomes a logistical means through which the vascular-nervous structures are distributed.
With morphofunctional criteria, connective tissues are divided into two groups:
- Non-specialized connective tissues.
- The specialized conjunctive tissues.
Concept and nomenclature
The term connective tissue groups various subtypes of tissues; Understood in this way (without any clarification) reference is then made to "connective tissues" in general, specialized and non-specialized.
To refer exclusively to non-specialized connective tissue, without falling into ambiguities, the name "connective tissue proper" is used. The connective tissue proper is a type of ubiquitous connective tissue, more general in function, less differentiated from the histophysiological point of view.[citation needed]
Classification of connective tissues
The following primary classification differentiates them into specialized and non-specialized.
Non-specialized connective tissues:
- Laxo conjunctive fabric (always irregular):
- Conjunctive mucoso or gelatinoso fabric
- Conjunctive reticular fabric
- Mesenchymal fabric
- Conjunctive dense fabric:
- Conjunctive fabric dense regular
- Conjunctive fabric dense irregular
Specialized connective tissues:
- Tejido adiposo
- Cartilage fabric
- Bone fabric
- Hematopoietic tissue
- Blood tissue (blood)
- Lymph tissue
- Blood, a particular case
According to the histological criteria used for the classification of tissues, blood is considered by some to be a specialized type of connective tissue, whose matrix is liquid (blood plasma); Others understand blood as one more basic tissue, bringing the number of primordial tissues to five: epithelial, connective, blood, muscular, and nervous tissues.
- Mesénquima, origin
Embryonic mesenchyme is understood to be the set of mesenchymal tissues of the embryo. Mesenchymal tissue is the connective tissue of the embryonic organism, regardless of its origin. In general, embryonic connective tissues are considered to be of mesodermal origin.
With embryonic and then fetal development, mesenchymal tissue "maturing" and differentiating, not only towards the different types of connective tissue (loose, dense, adipose, cartilaginous, bone, hematopoietic and blood), but also towards muscle tissue. In this way, multiple structures start from the differentiation of the mesenchyme.
Dense patterned connective tissue
Dense patterned or regular connective tissue is formed by the parallel arrangement of collagenous fibers (stained blue), among which fibroblasts (ovoid nuclei of loose chromatin) and fibrocytes (elongated nuclei of dense chromatin) are observed. They are also parallel to the collagen fibers. Collagen fibers are the most abundant and thickest in connective tissue. There are 15 types, of which type 1 collagen is the most abundant. Viewed naturally, the fibers are whitish in color and are synthesized by: fibroblasts, osteoblasts, odontoblasts, chondroblasts and smooth muscle cells.
Components of connective tissue
Like all tissue, it is made up of cells and extracellular components associated with cells. The ground substance and the fibers are the extracellular components —generically known as extracellular matrix—, on which the morphophysiological characteristics of connective tissues in general depend mainly. The following is a description of the elements that make up unspecialized connective tissue (both loose and dense).
Ground Substance
The ground substance (SF) is a translucent, extensively hydrated material with a gelatinous consistency, in which cells and tissue fibers are immersed, along with other components in solution. The aqueous phase of the SF works as a solvent that allows the exchange of metabolites (nutrients and wastes) from one cell to another through the interstitial space.
The physicochemical characteristics of SF are given by its biological composition: proteins and associated glycosaminoglycans (GAGs) (proteoglycans). Initially known as acid mucopolysaccharides, nowadays identified as GAGs, they are mainly found: chondroitin sulfate, heparan sulfate, keratan sulfate and hyaluronic acid. GAGs are complex macromolecules of polysaccharides (hydrophilic polymers) associated with proteins, with an acid reaction and numerous anionic groups that attract soluble cations (mainly Na+) with a large osmolar effect (by "water drag") that contributes to to the turgidity of the intercellular matrix.
In conventional preparations "they wash off" polymers; for this reason, special histological techniques are applied to preserve the SF in the preparations:
- fastening with ether-formaldhyde vapours for optical microscopy; otherwise,
- pressurized freezing + cryosustitution + low temperature inclusion for ultrastructural microscopy.
Toluidine blue dye presents the phenomenon of metachromasia (turns purple) on contact with SF. Special stains are generally used: periodic acid Schiff (PAS +), Alcian blue, colloidal iron, etc.
- Other partner components
- glucoproteins of accession:
- fibronectin, laminine, thrombospondine
- integration
- cell excretion products (hormones, growth factors, chemotactics, etc.) and more.
Protein or protein fibers
The fibers that make up the extracellular matrix can be of several types: collagen fibers, elastic fibers, and microfibrils. Qualitatively and quantitatively, collagen is the most important and abundant fiber in our body. Fibroblasts are the main cells that produce collagen and elastic fibers; other types of cells of mesenchymal origin also synthesize fibers (smooth muscle, mesothelial cells, etc.), as well as epithelial cells.
- Collagen fibres
Collagenous fibers serve to resist stretching and are present in all types of connective tissue, particularly tendons, ligaments and fascia.
Reticular fibers
The reticular fiber is part of a support network, they are inelastic present enveloping organs. Formerly considered different fibers, they are fibers composed of type III collagen.
- Elastic fibres
Elastic fibers are composed of two types of proteins: elastin and fibrillin. They are thinner fibers than collagen fibers and are abundant in loose connective tissues. The elastic fibers have a branched, network-like appearance on the loose CT; or else a parallel fibrous and perforated band appearance on dense CT. In order to visualize these fibers, special staining techniques must be used, such as: the Weigert method (resorcin-fuscin), the Veroheff method and the Halmi method (aldehyde-fuscin), since they are difficult to distinguish with the common hematoxylin-fuscin stain. eosin.
They are extremely elastic and are adapted to stretching, since they can increase up to 1.5 times their length under traction and return to their normal position. Thus, elastic fibers are present in tissues and organs where this physical property is needed: the trachea, the vocal cords and the walls of the blood vessels (aorta).
Pancreatic elastase is the enzyme specialized in the digestion of this fibrillar protein.
lathyrism is a toxicological disease that affects the synthesis of elastic fibers; It is produced by ingesting the plant Lathyrus odoratus.
- Microfibs
Fibrillin is a 350 kD fibrillar glycoprotein especially associated with elastic fibers and abundant in the basal lamina of epithelia. Marfan Syndrome is an inherited (genetic) disorder of CT that affects the normal synthesis of fibrillin.
Cells of connective tissue
Although some of them are slightly mobile (free cells), connective tissue cells are essentially fixed and immobile (sessile cells).
- Mesenchymal cells. They are characteristics in embryonic and fetal states as a cellular element in the mesenchymal tissue. They are the ones that differentiate in the remaining types of connective cells. They can be located in the capillaries after birth.
- Fibroblasts. Highly basophilic cells due to their high endoplasmic Reticulous content. Calls fibrocytes in his inactive state.
- Adipocytes or adipous cells. They are cells that store fat, making this the maximum lump of your cytoplasm. They have vital functions in living beings. The adipocytes have the peculiar characteristic of not being able to execute the mitosis.
- Macrophages: Popular cells in the immune system, which enjoy the characteristic of being frontline phagocytes. Also called histiocytes.
- Mastocytes: They are found in most of the connective tissue, their function is basically secretive, in particular histamine (cause of allergic symptoms), and heparin anticoagulant.
- Plasmatic cells: Present in the digestive tract, its function is to secrete antibodies (especially IgG) to the bloodstream in response to a bacterial infection.
- Reticular cells. They have star shape and participate along with reticular fibers in glands and the lymphoid system.
- White blood cells. Cellular components of the immune system, of various types and functions. Also called leucocytes.
Loose connective tissue
Loose CT is characterized by the presence of cells and extracellular components of the matrix in more abundant proportions than the fibrillar components. There are several subtypes of lax CT.
Mucous connective tissue
It is a loose connective tissue in which the amorphous ground substance predominates, composed of hyaluronic acid. Cellularity is medium, mainly fibroblasts and macrophages, irregularly dispersed in the jellylike matrix.
It is not common to find this type of tissue in adults, but it is in the umbilical cord of the newborn, a material known as Wharton's jelly; also in the pulp of the teeth in small quantity.
Reticular connective tissue
The reticular type of loose connective tissue is characterized by an abundance of argyrophilic reticular fibers, composed of type III collagen. They give an appearance of a mesh-like network, in which the fibroblasts are distributed throughout the matrix. Reticular connective tissue composes the stroma of the bone marrow, spleen, lymph nodes, and thymus, giving sustenance and microclimatic framework to the parenchyma.
Mesenchymal tissue
Mesenchymal tissue makes up the embryonic mesenchyme, or all of the differentiated and differentiating connective tissues in the embryo. These tissues primarily have a loose consistency and are rich in mesenchymal cells that, by differentiation, provide specific cells for each type of mature tissue.
Dense or fibrous connective tissue
Dense connective tissue can take two basic types of configurations:
Dense regular connective tissue
It is the type of connective tissue that forms tendons, fascia, ligaments and, in general, structures that receive traction in the direction in which their collagen fibers are oriented. These fibers are arranged in an orderly fashion, parallel to each other, providing maximum strength.
In tendons, the conformation of the fibers are parallel to each other and with fibroblasts (called tendinocytes in this structure) between fiber and fiber. The tendon also presents a dense TC on its periphery, which presents not so parallel fibers, called the epitendon. Finally, around each fascicle of the tendon is a tissue called endotendon.
In the aponeuroses there are collagen fibers parallel to each other, but arranged in layers and in an orthogonal arrangement, that is, a layer placed at 90º on the lower layer.
In the ligaments, the shape of the tendons does not change, except for ligaments in certain parts of the body where more elasticity is needed, such as the yellow ligament in the spine. In these places, the ligaments have a greater quantity of elastic fibers than collagen, and in a not so regular way. They are called elastic ligaments.
Dense irregular connective tissue
It is present in the capsules of the liver, lymph nodes, kidney, small intestine, and dermis. It is basically part of the capsule of all organs, with the exception of the pancreas, which is a loose areolar connective tissue. Randomly arranged collagen fibers and very little ground substance are seen in this irregularly dense connective tissue. This provides protection against excessive stretching of the organs.
Histophysiology
The histophysiology of connective tissue includes:
Normal functions
- structural bra
- metabolic and nutritional support
- storage of energy reserves
- immunity
- protection
- inflammation
Inflammation and repair
- protection against infection
- injury repair
Diseases of connective tissue
- rheumatological diseases
- mixed diseases of connective and conjunctive tissue
- mucocele
- Ehlers-Danlos syndrome
- Marfan syndrome
Additional bibliography
- Fawcett D. W (1997) Bloom and Fawcett: a textbook of histology. 12th Sub edition. London: A Hodder Arnold Publication. (in English)
- J. Myllyharju, K. I. Kivirikko: Collagens, modifying enzymes and their mutations in humans, flies and worms. In: Trends Genet.. 20, Nr. 1, 2004, S. 33–43. PMID 14698617. (in English)