fundamentals
Cells
Introduction
Cells are the basic building blocks of the body. All Tissues and Organs are made of billions of different cells. Human cells vary in size, but all are quite small. Even the largest, a fertilized egg, is too small to be seen with the naked eye. All cells in a person's body are descendants of two cells, the mother's egg and the father's sperm. After the egg and sperm join together (fertilization), the fertilized egg is just a single cell. This cell, the zygote, divides many times, and as it divides, the descendant cells develop different characteristics and functions. These different cells eventually form the different organs (see also Stages of Development of the Fetus ). The body is composed of many different types of cells, each with its own structure and function. Some types of cells include Blood cells Muscle cells Skin cells Nerve cells Glandular cells Some cells, such as blood cells, move freely in the blood and are not attached to each other. Other cells, such as muscle cells, are firmly attached to one another. Some cells, such as skin cells, divide and reproduce quickly. Other cells, such as certain nerve cells, do not divide or reproduce except under unusual circumstances. In addition to human cells, the human body has foreign cells. The foreign cells are microorganisms, such as Overview of Bacteria and are not cells. Viruses contain 