Tkanki

Multicellular organisms, like humans, are comprised of specialized cell communities called tissues, groups of cells that have similar structure to perform related functions.

They can be classified into four major types, nervous, controlling information, muscle, including skeletal, cardiac, and smooth for movement, epithelial, to cover and protect boundaries, and connective tissues, for further support.

Most organs contain all four types and their structural arrangements vary to provide distinguishable characteristics.

For instance, epithelial tissue, such as skin, displays an apical surface to the outside and are arranged either in single and simple layers or stratified, two or more layers stacked upon each other to protect against abrasion.

In addition, cell shape can vary, from squamous, flattened, to cuboidal and columnar, with the nucleus conforming to each form. These deeper sheets of epithelial cells are supported by connective tissue just below, where nutrients can diffuse up from blood vessels and provide the nourishment that cells need.