Tissue

Definition
Tissue: a functional 3-D organization of aggregations of morphologically and functionally similar cells, together with their products and derivatives.

In turn, different types of tissues come together to form organs and organ systems. The parenchyma is the functional tissue of an organ, while the stroma is the tissue that surrounds and supports the organ.

Basic Types
Epithelial


 * Continuous, avascular aggregations of polyhedral cells
 * Minimal ECM and intercellular space
 * Line surface or body cavities
 * Derived from all three germ layers

Muscle


 * Elongated contractile cells
 * Moderate ECM
 * Strong contractions
 * Mostly derived from mesoderm (but mm. sphincter and dilator pupillae from ectoderm)

Nerve


 * Elongated cells with fine processes
 * Minimal ECM
 * Receive and transmit nerve impulses
 * Mostly derived from ectoderm, but microglia from mesoderm

Connective


 * Fixed and wandering cells
 * Abundant ECM
 * Structural support and protection of organs
 * Derived from mesenchyme
 * Vascularization depends on type

Histogenesis
Though all cells start from the zygote, they differentiate into over 200 different types. Histogenesis happens via


 * 1) Proliferation: cells increase in number
 * 2) Differentiation: stem cells acquire new properties and functions (gene activation/inactivation)
 * 3) Migration: differentiated cells colonize other parts of the organism
 * 4) Apoptosis: programmed cell death used to shape structures
 * 5) Tissue patterning: cells receive regulator signals influencing their organization to determine final localization, orientation and morphology. Environment defines stem cell niche.