What is a cell?

Ever so often, we need to repeat the basic questions before we can go on to the more difficult ones. So, what's a cell?

The cell is the smallest complete unit of a living organism. Bacteria are one-cell organisms; humans have many millions of cells. For multicellular organisms, the cell (zygote) created by the fertilization of a egg by a sperm can multiply, differentiate into many different tissue types (e.g., muscle, nervous, cardiac), form different organ systems (e.g., cardiovascular, skin, digestive tract), and become an entire human being. In humans, as with all mammals, development into an independent living organism is only possible if the zygote implants itself in a female's uterus within a limited number of cell disivions after fertilization.

During differentiation, most cells follow a one-way path towards one of the many different tissue types needed by the body. Although nearly all cells of a human being contain the same genetic information (genotype), cells of one tissue type (e.g., muscle) are different in appearance and function (phenotype) from all cells of other tissue types (e.g., nerve and skin).

Because the body often needs different numbers of certain cells, the body's remarkably efficient construction system maintains a supply of cells that have not completed their differentiation. In particular, stem cells in bone marrow can produce lymphocytes, leukocytes, and other blood cells. Except for these "undifferentiated" cells, one tissue type cannot produce another tissue type; e.g., muscle cells cannot produce nerve cells.

Two cell types of the human body are "special". The sex cells -- oocytes and spermatocytes -- produce specialized reproductive cells -- eggs (ova) and sperm -- which have only half the genetic material of other body cells. Human females produce all their egg cells before birth; males produce their sperm cells over many years. An egg or sperm must combine with its matching type (forming a zygote) before it can multiply. The other special type of cell is red blood cells (erythrocytes), which have differentiated so far they no longer contain any genetic material and can no longer produce new cells.

Marie Godfrey, PhD

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