ABSTRACT
Connective tissue (CT) is a structural functional complex of specialized cells, which are derivatives of the mesenchyma, fibrous structures and the integrating buffer metabolic medium (ground substance) carrying out the integrating, regulatory, trophic, biomechanical, morphogenetic, plastic and protective functions in an organism. The presence of an extracellular (intercellular) matrix (EM) in its structure is a distinct feature of the connective tissue. The EM considerably exceeds the mass of cell elements that produce it, carries out the basic functions of connective tissue, and serves as a medium for the existence of other cells, mesenchyma derivatives, as well as those involved in CT vital activities and in various metabolic processes. The extracellular matrix, in turn, is a significantly complicated component of the connective tissue, comprising collagen and elastic type fibrous structures, which are surrounded by the integrating buffer metabolic medium (ground substance). The specialized cellular elements of the connective tissue (the CT cells properly) are located between the fibrous structures and are surrounded by the integrating buffer metabolic medium. They produce and renew an intercellular matrix, thereby maintaining a quantitative proportion of its composition. The specificity of connective tissues’ organ structure is determined by a combination of qualitative and quantitative differences in the cell structure composition (the CT proper cells and the associated ones) and non-cellular components: the composition and structure of a fibrous framework, the proteoglycans’ composition, the physical and chemical properties of the intercellular matrix, and the interstitial space volume. The combination of these parameters is taken as a principle for the allocation of proper (fibrous) connective tissues, connective tissues with special properties, and skeletal tissues. It is necessary to emphasize that the convention of the proposed classification for fibrous structures is present in all types of connective tissue, but in the first of them, fibrous СТ, there is much more of them than in the second group – СТ with special properties. The group of skeletal СТ is distinguished by the mineralization of all bone tissue and partially cartilaginous tissue. The latter is subdivided into three types of cartilaginous tissue (hyaline, elastic and fibrous) and three types of bone tissue (lamellar, rough-fibrous and reticular-fibrous). In turn, fibrous structures’ orientation and their volume ratio to interfibrous and intercellular spaces form the basis for the allocation of loose, dense regular (oriented) and dense irregular (non-oriented) connective tissues (the dermis, tendons, ligaments, aponeuroses and others). An analysis of the group of connective tissues with
special properties has shown that reticular tissue, which belongs to this group, can undoubtedly be considered as loose irregular connective tissue as the reticular fibers are a type of the collagen fibers whose basis is composed of a type III collagenous protein. The jelly or mucous connective tissue normally encountered in the umbilical cord (Wharton’s jelly) of the fetus should be referred to as embryonic tissues, along with the connective tissue of the provisory organs. Therefore, the group of CT with special properties is comprised of adipose connective tissue consisting of two types: white and brown. Thus, three type of connective tissue will be subdivided as such: 1) fibrous connective tissue (loose, dense regular and dense irregular);
2) adipose connective tissue (white and brown); and 3) skeletal tissue (bone and cartilaginous). If the existence of the organ-specific peculiarities of connective tissues is taken into consideration, it is pertinent to allocate three groups of connective tissues: 1) intraorgan (organ) connective tissue; 2) extraorgan (interorgan) connective tissue with a prominent trophic function; and
3) connective tissue of organs with a biomechanical function. The aforementioned information is combined and presented in Fig.1.1.
