Connective tissues are defined as such because of their function to connect, functionally and structurally, other tissues within the body. They provide structural and metabolic support to other tissues, which is why the term “support tissue” has been proposed. They are also considered to be tissues with trophic function (in that through them nutrient exchanges occur) and tissues with mechanical function (in that they include tissues for support, protection of internal organs, or tissues, such as tendon, that allow musculoskeletal movement).
Connective tissue (and therefore all its different types) is derived from the mesenchyme. Mesenchymal cells, of mesodermal origin, are irregularly shaped, generally elongated, and have an amorphous, fiberless intercellular substance. Mesenchymal cells are pluripotent, that is, they have the ability to differentiate into each of the different types of connective cells (fibroblasts, chondroblasts, osteoblasts, mast cells, adipocytes, white blood cells and macrophages), as well as into muscle fibrocells. They are therefore a very heterogeneous group of tissues, which have some common features:
- the cells are separated from each other by a substance called matrix or intercellular substance, consisting of an amorphous (or anist or fundamental) component and a fibrillar component (collagen, elastic, reticular fibers). The amorphous substance is a semi-fluid gel, rich in water, which promotes the exchange of metabolites and catabolites through the connective tissue; it contains glucosaminoglycans, some of which are sulfurated and bound to proteins to form proteoglycans;
- cells belong to several categories, each performing a specific function:
- synthesis of components of intercellular substance (fibroblasts);
- reserve and metabolism of lipids (adipocytes);
- immune and non-immune defense (mast cells, macrophages, leukocytes);
- a distinction is made between fixed cells (fibroblasts and adipocytes, which carry out the entire life cycle in the connective tissue) and mobile or free or migrating cells, which during the life cycle can pass from the connective tissue to the blood and vice versa;
- in some connective tissue the intercellular substance is very abundant (connective tissue proper, cartilage, bone), in others it is very reduced (adipose tissue) or exceptionally absent (chordoid tissue). Some connective tissues have typical cells (bone, cartilage, tendon tissue), others have many different cell types (lax connective tissue);
- They all derive from a particular embryonic connective tissue (mesenchyme) that lacks its own form and fills the spaces of differentiating organs. The mesenchyme has a very fluid intercellular substance and multipotent, star-shaped stem cells that differentiate, giving rise to different cell types;
- they can replace each other in the organism, such as the cartilaginous tissue of the embryonic skeleton of vertebrates, which in most adult vertebrates is replaced by bone tissue; instead some cell types can transform into each other: e.g. monocytes can transform into macrophages, B lymphocytes into plasma cells.
Connective tissue membranes
The connective tissue membrane is formed solely from connective tissue. These membranes encapsulate organs, such as the kidneys, and line our movable joints. A synovial membrane is a type of connective tissue membrane that lines the cavity of a freely movable joint. For example, synovial membranes surround the joints of the shoulder, elbow, and knee. Fibroblasts in the inner layer of the synovial membrane release hyaluronan into the joint cavity. The hyaluronan effectively traps available water to form the synovial fluid, a natural lubricant that enables the bones of a joint to move freely against one another without much friction. This synovial fluid readily exchanges water and nutrients with blood, as do all body fluids.
Types of connective tissue
Connective tissue is divided into three main categories:
- mucous connective tissue;
- connective tissue proper;
- specialized connective tissue.
Mucous connective tissue
Mucous connective tissue (also referred to as embryonic tissue in some texts) is a connective tissue poor in fibers that forms during embryonic development and persists in the adult limited to the pulp of the tooth and the vitreous humor of the eye. It is predominantly composed of an extracellular matrix of a hydrophilic nature that gives it a gelatinous consistency. It looks very similar to the embryonic mesenchyme in that the basic substance is very fluid; it contains collagen fibers (not present in the mesenchyme) and fibroblasts. It is also present in the pulp of the tooth and in the umbilical cord where it takes the name of Wharton’s jelly.
Connective tissue proper
The cells of connective tissue proper are: fibroblasts and fibrocytes, mast cells, histiocytes or macrophages, plasma cells. In connective tissues are also found pigmented cells (which do not originate from the mesenchyme) and cells from circulating blood: the role of immune defense of leukocytes is realized in connective tissues, where leukocytes arrive after having crossed the wall of blood capillaries (diapedesis). The classification criteria of connective tissue proper are based on the quantity, quality and arrangement of fibers (collagen, reticular, elastic) present in the amorphous or fundamental substance.
The connective tissue proper is divided into two subclasses: loose connective tissue and dense or compact connective tissue. In loose connective tissue fibers are less abundant and loosely intertwined (with a prevalence of amorphous substance). In dense connective tissue the fibers are abundant and tightly packed (gathered in large parallel bundles or with irregular and disordered arrangement) and give considerable consistency to the tissue.
Connective tissue proper is divided into:
- fibrillar connective tissue: distinguished in two types, one that presents itself with abundant amorphous component (loose fibrillar tissue) and one that presents itself with collagen fibers thickened among them (dense fibrillar tissue):
- loose fibrillar connective tissue (or areolar);
- dense (or compact) fibrillar connective tissue;
- reticular connective tissue;
- elastic connective tissue.
Loose fibrillar connective tissue (or areolar)
The loose fibrillar connective tissue is characterized by the presence of numerous cells of variable type while the amorphous substance is poor in fibers and not particularly dense whose component is the tissue fluid (or interstitial fluid), a substance containing hyaluronic acid. It is flexible and elastic and makes possible the movement of anatomical formations connected to it with respect to each other. In this tissue the fibers intertwine loosely, it is also called areolar because of the way it appears to be made up of small spaces containing almost exclusively the amorphous component.
It forms the tonaca propria of the lining epithelia and the submucous tonaca of the hollow organs communicating with the outside (digestive, respiratory, etc.). Wraps all organs, constituting the stroma or interstitial connective tissue (the fat cells present in this tissue perform a function of mechanical protection by associating in voluminous masses: fat pads). It forms the tonaca intima (together with the endothelium) and the adventitious tonaca of the arteries, the middle and adventitious tonaca of the veins (together with smooth muscle fibers). It surrounds muscles and nerves, penetrating into them by surrounding fiber bundles and individual fibers. The fundamental substance is abundant, fairly fluid, among the fibers collagen prevails over elastic fibers and rare reticular fibers can be seen, interwoven to form large meshes; there are connective cells of all types and leukocytes from the bloodstream.
It is also the site of the body’s defense through phagocytic activity and antibody production. Most of the processes of tissue damage repair are attributable to the loose connective tissue, which fills the damaged solutions first with blood vessels and tissue with numerous fibroblasts that produce collagen fibers, then with the formation of a scar tissue in which, over time, cells and vessels decrease (sclerosis).
Dense (or compact) fibrillar connective tissue
The dense fibrillar connective tissue is characterized by numerous fibers formed by type I collagen, organized in bundles, even very thick, immersed in an amorphous substance and oriented in different directions in space: parallel fibers (tendons), cross-fiber (cornea), interwoven fibers (dermis). It is a tissue particularly suited to resist mechanical stress. The cells are less numerous than in the loose connective tissue. It contains almost exclusively fibrocytes.
The dense connective tissue does not have clear and precise boundaries with the loose connective tissue. Based on the arrangement of fibers, three types of fibrous connective tissue are distinguished:
- dense fibrillar connective tissue with intertwined bundles. It is found particularly in the dermis. It has few cells (mainly fibroblasts) and little fundamental substance. The fibers (predominantly collagen) are gathered in large, crammed bundles, which may have parallel or disordered courses. The orientation of the bundles generally relates to the different direction of mechanical stress. Sometimes they intertwine to form an outer capsule of various organs, and we speak of capsular fibrous tissue. In other cases, however, the fibers have an annular course to form concentric lamellae and we speak of lamellar fibrous tissue. They are also present in tissues where increases in internal pressure may occur;
- dense fibrillar connective tissue: with parallel bundles (in tendons and ligaments), with crossed bundles (in the sclerotic and in the cornea), lamellar (in Pacini’s corpuscles). Scarce is the presence of the amorphous component and the cellular component, consisting only of fibrocytes. This results in structures that are very resistant to traction, such as tendons and ligaments. The fibrocytes present, also called tendinous cells, are arranged in rows aligned parallel to the fibers, and from their elongated body detach thin expansions that in cross sections resemble wings; they are called, therefore, also winged cells;
- dense fibrillar connective tissue with crossed bundles: it consists of overlapping lamellae in each of which the fibers are all parallel to each other and cross at different angles the fibers of contiguous lamellae. An example of tissue of this type is the cornea, where the lamellae are overlapping and oriented almost orthogonally and the fibrocytes take the name of corneal cells or keratocytes.
Reticular connective tissue
It is found, as a support tissue, in various organs: hematopoietic and lymphoid, exocrine and endocrine glands, liver, spleen, nerve fibers, muscle fibers, kidney. It is a connective tissue rich in reticular fibers consisting of type III collagen. These fibers, despite being made of collagen, because of the diversity of organization, size, specific function and, above all, the strong affinity to metallic silver (hence the name of argirophilic fibers) have kept their original name, which is reticular fibers or fibers of the lattice. The greater affinity to Silver-based dyes, which is the basis of Bielschowsky staining for reticular fibers, seems to be attributable, more than to the biochemical characteristics of the constituents of the fibers, to the conformation of the fibers themselves and the matrix that surrounds them.
From a histological point of view, the only real difference with other collagen fibers is that while these are aggregated in bundles, reticular fibers are isolated and tend to form thin lattices. These fibers surround individual muscle fibers and peripheral nerve fibers in such a way as to isolate them from each other, associate with the basement membrane, just below the epithelia, surround the adipocytes, and form the thin reticular weave that constitutes the connective stroma of lymphoid organs and large glands, both exocrine and endocrine. In embryonic life, during the transformation from mesenchymal tissue to connective tissue, reticular fibers are the first to appear and only later are most of them replaced by collagen fibers.
Elastic connective tissue
Constitutes the elastic membranes (fenestrated membranes) of all arteries and the middle tonaca of the great arteries. It is also found in the yellow ligaments of the vertebrae, the vocal cords, and the nuchal ligament. Elastic fibers are prevalent, present to the naked eye a characteristic yellowish color (because they are thick and concentrated), and are united in parallel bundles.
The various structures that make up the body must often respond to mechanical stress or perform sliding or gliding functions. The connective tissue, which has the task of supporting these movements, must have an elastic structure capable of responding to any stress but, once finished, can bring the structure to its original shape and size, without deformation. This type of connective tissue is called elastic. Elastic connective tissue is composed of non-birefringent fibers, therefore different from those of collagenous connective tissue, formed by elastin, amorphous substance and fibrillin.
In its microscopic organization, elastic connective tissue can be organized mainly in parallel fibers (as in the nuchal ligament of ruminants, in tendons and ligaments of the spine in humans) or in fibers, more or less numerous, scattered among the collagen fibers (epiglottis, outer ear, urinary bladder). In the wall of the arteries there are no fibers, and elastin appears in the form of fenestrated laminae called elastic membranes.
Macroscopically, the elastic connective tissue is easily distinguishable by the slight yellowish coloration that elastin gives to this tissue, however, at the microscopic level, we can recognize it only thanks to specific elective staining based on resorcin-fuchsin (Weigert) or aldehyde-fuchsin (Halmi) that give to this tissue the classic intense purple-black coloration.
Highly specialized connective tissues
Highly specialized connective tissue is defined as a type of tissue that due to its unique functions takes on histological characteristics that clearly distinguish it from other connective tissues. Highly specialized connective tissues are:
- adipose tissue;
- cartilage tissue;
- bone tissue;
- lymphoid tissue;
Note: cartilaginous tissue and bone tissue are commonly referred to as supportive connective tissues.
Cellular connective tissue
These connectives are characterized by the presence of very little fundamental substance between the cells. The connectives with a cellular constitution are:
- chordoid tissue;
- adipose tissue (white and brown).
- Anatomy and Physiology. OpenStax. Authors: J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. https://openstax.org/books/anatomy-and-physiology/pages/1-introduction