Introduction to Cells

Basic Structures of Cells

Cells are the basic units of human life. Billions of cells compose your skin, muscles, bones, and brain. A recent estimate shows that the human body is composed of 3.7 trillion cells! Moreover, all of these cells, when closely examined, can be divided into about 200 different cell types, each with their own size, shape, and job to do.

During early development, the cells of the embryo were much simpler looking. They all had a round shape, and they all had 2 features in common: 1) a cell Nucleus in the center of the cell, and 2) a mass of material surrounding the nucleus called the Cytoplasm. The cytoplasm is surrounded by a barrier called the Cell Membrane, and the nucleus is contained within the Nuclear Envelope. When scientists first began studying cells, they had difficulty viewing these features of cells using a microscope. This is because cells are 90% water and are almost transparent! To overcome this problem, scientists devised dyes or stains that stain the cytoplasm pink and the nucleus blue.

Varieties of Cells

All parts of the body contain cells with very different appearances. Tissue in the bone marrow, for example, contains 1) a cell with a huge, folded, pale-staining nucleus and an enormous cytoplasm, 2) a cell with tiny, dark-staining nucleus, 3) a cell with a horseshoe-shaped nucleus, and 4) even a cell with no nucleus at all! In this cell, the contents of the nucleus have spilled into the cytoplasm, because the nuclear envelope has disappeared. These rod-shaped nuclear components are called chromosomes. During cell division, the chromosomes have been duplicated into two identical sets. These two sets of chromosomes are pulled apart from each other and sent to opposite poles of a cell. The cell then divides into two new cells, each with its own nucleus.

This amazing variability in cell shape and function poses a problem. How are we to understand this bewildering variety of cells? Fortunately for us, Mother Nature has provided us with a solution to this problem. All of these types of cells can be sub-divided into only four main categories of cells, each with a major type of job to fulfill.

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Written by John Young

John K. Young is a retired professor of Cell Biology. He worked in the Department of Anatomy at Howard University College of Medicine in Washington, DC for 35 years, teaching his students about cells. During his career, Dr. Young published scientific articles about a part of the brain called the hypothalamus and also wrote a number of books about cells and about the brain.

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