Cytology, General Histology and Embryology - V. K. Napkhanyuk 2002

Tissues
Tissues of the Internal Environment

Internal Environment Tissues form a large group of tissues that, along with epithelia, belong to general-purpose tissues.

This group of tissues comprises Blood, Lymph, and all varieties of Connective Tissue. Although individual types of internal environment tissues differ significantly in appearance (for example, blood and Bone tissue), there are ample grounds for grouping them into a single tissue system, namely due to their common origin, Structure, and Functions.

Common origin is the most prominent feature, as all internal environment tissues are derivatives of the mesenchyme. Mesenchyme is the most primitive connective tissue, existing only during Cytology/cytology/16.html">Early stages of embryonic development. In its structure, mesenchyme resembles a network, since its Cells are stellate or spindle-shaped and contact one another via their processes. The meshes of the reticular framework contain a gelatinous mass—the Extracellular matrix, the density of which depends on METABOLISM. Through differentiation, blood, lymph, and all types of connective tissue develop from the mesenchyme.

Commonality of structure lies in the presence of an extracellular matrix, the quantity of which exceeds that of the cells.

Unity of function implies that all internal environment tissues perform trophic, protective, and supportive (mechanical) functions. The functional features of various types of internal environment tissues are largely determined by and dependent on the PHYSICOCHEMICAL PROPERTIES OF the extracellular matrix. All internal environment tissues are divided into two major groups that encompass all varieties of these tissues (Fig. 43).