23. Fibrous connective tissue. Types. Loose, collagenous fibrous, elastic and reticular connective tissues – structural, ultrastructural and functional characteristics.

Mesenchyme: The Precursor to All Connective Tissues

Mesenchyme is the primary embryonic connective tissue and the origin of all adult connective tissues. Mesenchyme is derived mainly from the mesoderm, the middle layer of the embryo. It first appears in the embryo during the early stages of development and serves as the precursor tissue for a wide variety of specialised connective tissues in the adult body.

Mesenchyme is characterised by:

Undifferentiated cells: Mesenchymal cells are undifferentiated, meaning they have not yet specialised into specific cell types. They have large nuclei with prominent nucleoli, indicating high levels of synthetic activity, and are often described as spindle-shaped with extended cytoplasmic processes.

Viscous ground substance: The extracellular matrix (ECM) of mesenchyme is primarily composed of a viscous ground substance rich in hyaluronan (hyaluronic acid), a type of glycosaminoglycan (GAG) that attracts water and contributes to the tissue's gel-like consistency.

Few collagen fibres: Mesenchyme contains relatively few collagen fibres compared to more mature connective tissue types.