TISSUE

Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.The study of tissue is known as histology or, in connection with disease, histopathology.

The classical tools for studying tissues are the paraffin block in which tissue is embedded and then sectioned, the histological stain, and the optical microscope. In the last couple of decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis. Based on morphology, animal tissues can be grouped into four basic types.

Multiple tissue types comprise organs and body structures. While all animals can generally be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism. For example, the origin of the cells comprising a particular tissue type may differ developmentally for different classifications of animals. The epithelium in all animals is derived from the ectoderm and endoderm with a small contribution from the mesoderm which forms the endothelium. By contrast, a true connective tissue is present only in a single layer of cells held together via occluding junctions called tight junctions, to create a selectively permeable barrier. This tissue covers all organismal surfaces that come in contact with the external environment such as the skin, the airways, and the digestive tract. It serves functions of protection, secretion, and absorption, and is separated from other tissues below by a basal lamina. Endothelium, which comprises the vasculature, is a specialized type of epithelium. Connective tissues are fibrous tissues.They are comprised of cells separated by non-living material, which is called extracellular matrix. Connective tissue holds other tissues together such as in the formation of organs, and has the ability to stretch and contract passively. Bone, often referred to as osseous tissue, and blood are examples of specialized connective tissues. Muscle cells form the active contractile tissue of the body known as muscle tissue. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs.

Muscle tissue is separated into three distinct categories: visceral or smooth muscle, which is found in the inner linings of organs; skeletal muscle, in which is found attached to bone providing for gross movement; and cardiac muscle which is found in the heart, allowing it to contract and pump blood throughout an organism. Parenchyma is Greek word where "parn" means besides and "enchien" means to pour. Parenchyma is the most specialized primitive tissue. It mainly consist of thin-walled cells which have intermolecular spaces between them. The cell wall is made up of cellulose. Each parenchymatous cell is iso-diametric, spherical, or oval in shape.