In multicellular beings there may be different types of cells, each type being specialized in an specific function, and having the specific shape that allows them to ful fill that function the best. Each of those types is called acellular tissue; examples are the vascular tissue (plants) or the blood tissue (animals). One tissue may have several subtypes of cells (e.g. white blood cells and red blood cells). The human body contains over 200 different types of cells.
The four main types of human tissues are the following:
1. Epithelial tissue. Composed of layers of cells that line organ surfaces such as the surface of the skin or the inner lining of the digestive tract. Serves for protection of organs (as in the skin), secretion of substances (when it forms glands – in the skin, in the digestive tract, etc.), and absorption of substances (as in the intestine).
2. Muscle tissue. Composed of very long cells (up to several cm) called muscle fibres. They have more than one nucleus, are able to expand and contract (thanks to a dense protein network that takes up most of the cellular space), and so, are specialized in movements. There are three kinds: cardiac muscle (found in the heart), skeletal muscle (attached to bones and under voluntary control) and smooth muscle (not in the heart or attached to bones and under involuntary control, as in the wall of the stomach).
3. Nerve tissue. Composed of cells with many projections that are specialized in contacting other cells and transmitting messages via electrical signals.
4. Connective tissues. Usually specialized in holding together different organs or tissues. It is composed of cells usually very separated by an abundant extracellular matrix. The main types are the bone tissue (in bones, with matrix rich in apatite, a mineral rich in P and Ca), the cartilage tissue (in cartilages), the adipose tissue (as in the fatty layer under the skin – the hypodermis), the fibrous connective tissue (in ligaments and tendons), the loose connective tissue (as in the skin’s dermis) and the blood.