General Structural Features
Like blood, tissues of this group consist of cells and a well-developed extracellular space. The primary feature of textus connectivus is that the extracellular matrix significantly predominates over cellular elements in volume and mass. Furthermore, it is not in a liquid state, but forms a gel or a solid structure.
The extracellular matrix consists of two main components:
- Fibers: Collagen, elastic, and reticular fibers.
- Ground amorphous substance: A non-fibrous, gel-like component.
Thus, any connective tissue is described by the classic triad: cells, fibers, and ground substance.
Classification Principles
Depending on the ratio and nature of their components, at least 15 types of tissues are distinguished. In histology, they are traditionally divided into three major groups:
- Proper connective tissues (fibrous).
- Connective tissues with special properties.
- Skeletal connective tissues.
Characteristics of Main Groups
Proper Connective Tissues Also referred to as fibrous connective tissues. Their main hallmark is the presence of collagen fibers (either independently or alongside elastic fibers) and the absence of mineralization. This group includes 4 types of tissues. They are fundamentally different from mucous and adipose tissues (which lack fibers), reticular tissue (different fiber type), and skeletal tissues (absence of minerals).
Tissues with Special Properties Includes four types:
- Reticular tissue: Consists of branching cells and reticular fibers forming a network. It forms the stroma of hematopoietic and lymphoid organs (spleen, lymph nodes, red bone marrow), where blood cells develop.
- Adipose tissues (white and brown): Formed by adipocytes. Brown adipose tissue contains numerous mitochondria, while white adipose tissue forms large depots (subcutaneous fat).
- Mucous tissue: Contains a jelly-like substance rich in hyaluronic acid. It is found exclusively in the fetus (umbilical cord).
Skeletal Connective Tissues A group of 7 tissues united by the solid state of their extracellular matrix due to mineralization.
- Cartilage tissues (3 types): Hyaline, elastic, and fibrocartilage (mineralization 4–7%).
- Bone tissues (2 types): Woven-bone (immature) and lamellar (mature) bone.
- Hard dental tissues (2 types): Dentin and cementum. It is important to remember that tooth enamel is epithelial tissue, not connective tissue.
Functions in the Body
The abundance of tissue types dictates a wide variety of functions, which are divided into two main categories.
Mechanical (Supportive) Functions
- The stroma divides organs into lobules and binds them into a cohesive whole.
- Fasciae, capsules, and fat pads protect organs and maintain their shape.
- Ligaments, bones, and cartilage form the musculoskeletal system.
- The dermis provides tensile strength to the skin.
- Hard dental tissues mechanically process food.
Homeostasis Maintenance
- Trophic: The stroma serves as a pathway for blood vessels.
- Metabolism: Creates a microenvironment for exchange between blood and organ cells.
- Protection: The stroma is the primary site for immune responses.
- Deposition and thermoregulation: White adipose tissue stores energy and provides thermal insulation, while brown adipose tissue generates heat.