Each adult human consist of about 1013 (10,000,000,000,000) cells. These cells live in a gigantic "commune".
The human body contains about 200 different types of cells. Each cell has a very specialized role. Some examples:
Unicellular blood cells:
Some "tissues" in humans are populations of single cells that circulate throughout the body via the circulatory system. Examples: Erythrocytes: red blood cells transport oxygen and carbon dioxide [Image link: light micrograph]. Phagocytic white blood cells: "eat" foreign materials, infectious agents, debris [Movie link: 509K 'mov.' file, from Cells Alive! Web site. Used with permission.]
Multicellular tissue cells:
Look at any human organ, such as the liver [Image link: light micrograph]. Then look at higher magnification -- it is made of individual liver cells [Image link: light micrograph]. View an electron micrograph of one liver cell [Image link: electron micrograph]. These are often used in textbooks as examples of "typical" cells, because they lack specialized structures that make other cells so distinctively recognizable. Actually, liver cells are biochemical specialists, converting thousands of chemicals into other chemicals so that your body can either digest or excrete them. Think "detoxification center" and you'll have a good image of a liver cell. Liver cells can be damaged by excess alcohol. Look at the liver cells [Image link: light micrograph] in this image -- can you spot any changes from the healthy cells seen above?
Each cell type has a characteristic lifetime, after which it commits suicide (apoptosis). Some examples:
Neutrophils live about 1 day. [Image: light micrographs, including animation]
Each cell is totally responsible for it's own "housekeeping" duties:
maintenance and repair
acquiring food from blood or lymph
disposal of wastes into blood or lymph
synthesis and regulation of all its large biomolecules
dividing (when allowed to do so) to create new cells by the process of mitosis, and not dividing unless properly signaled (cells that divide without responding to normal signals produce CANCER).
carrying out its own specialized tasks, such as: contracting, making antibodies, firing nerve impulses, secreting digestive enzymes, etc. Examples: (1) nerve cells (communications specialists); (2) heart muscle cells (contraction specialists) [electron micrographs from Dennis Kunkel Microscopy. Used with permission].
Each cell must be able to communicate with other cells and respond appropriately to signals that regulate its activity.
Each cell must differentiate from one common ancestral cell, the fertilized egg, by a complex process of differentiation and development. Example: Developing embryo, starting from single fertilized egg. [Movie: 3.1 meg 'mov.' file, from Univ. of Penn. Health System Web site]
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