Corpus: Connective tissue

Connective tissue is a fundamental type of tissue composed of connective tissue cells and an intercellular substance. It originates from the mesoderm during embryonic development.

Connective tissue is broadly categorized into two types: connective tissue in the narrower sense, which primarily differs based on the quality and density of its fibers, and supporting tissue, which includes bone and cartilage.

Histologically, connective tissue is composed of mobile and stationary cells (e.g., fibrocytes) embedded in an extracellular matrix. This matrix consists of collagen, other fibrillar proteins, and an amorphous basic substance. Together with the cells, the extracellular matrix forms a three-dimensional network filled with proteoglycans. The cell-fiber framework provides tensile strength to the connective tissue, while the proteoglycan-rich ground substance absorbs and distributes compression forces. The interaction of these properties maintains the structural integrity and shape of organs.

The stem cells of different types of connective tissue are round to spindle-shaped and lack polarity. These cells can differentiate into mobile blast cells, which are involved in forming the extracellular matrix of connective tissue, or into differentiated cells responsible for maintaining tissue homeostasis.

Connective tissue has diverse functions and therefore exhibits a wide variety of histological appearances. Depending on its location, connective tissue fills spaces between structures, protects and encases organs, serves as a framework for pathways, or acts as a sliding and displacement layer. Specialized connective tissues are involved in storage, substance production, and the structural support and stabilization of the body.

Connective tissue can be classified based on the ratio of cells to extracellular matrix into two main types: dense (molded) and loose (unformed) connective tissue. Further classification is based on morphological characteristics:

• Fibrous connective tissue
  • Dense connective tissue (example: dura mater)
    • Dense parallel-fibre connective tissue
    • Dense braided-fiber connective tissue
    • Dense elastic connective tissue
  • Loose connective tissue
    • Areolar connective tissue (example: greater omentum)
    • Interstitial connective tissue
• Cell-rich connective tissue
  • Embryonic connective tissue (mesenchyme)
  • Gelatinous connective tissue (example: Wharton's sulcus)
  • Reticular connective tissue
    • Lymphoreticular connective tissue (example: spleen)
    • Haemoreticular connective tissue (example: bone marrow)
  • Spinocellular connective tissue (example: ovary)
• Specialised connective tissue
  • Cartilage
  • Bone
  • Adipose tissue
  • Blood