The Cells

The cells – our body’s 100 million million building blocks

Did you know that the human body consists of approximately 100 million million cells, each of which contains 10,000 times as many molecules as there are stars in the Milky Way? Note that the Milky Way comprises at least 100,000 million stars. The smallest building blocks of the human body, as well as of plants and animals, are the cells. Body structures consist of cells, along with intercellular material. In the case of unicellular organisms (protozoa), the cell is the entire organism. A cell is the smallest possible morphological entity capable of independent life. In metazoa (manycelled creatures) the cells are the basic functional organic units, or building blocks, which make up the whole organism. Cells differ widely in regard to size, shape, and function  Cell membrane Section of a dendrite Cytoplasm with mitochondria and endoplasmic reticulum Nucleus with nucleolus Post-synaptic unit Synaptic vesicles Node of Ranvier Myelin sheath Pre-synaptic nerve terminal

Schematic diagram of a neural (nerve) cell.

Size:

The sizes of human cells vary considerably.

Very few of them are visible to the unaided eye or with a magnifying glass. The largest cells, the ova (egg cells), become just barely visible at a diameter of 0.15 mm (150 μm) or more. (1 μm = 1 micrometre = 1 thousandth of a millimetre).

Many of the larger neural (nerve) cells have a diameter of between 0.12 and 0.2 mm = 120 to 200 μm. Many neural cells have extensions (processes) up to one metre or more in length, but their diameter is only a few micrometres. The sizes of most other human cells vary between 5 μm and 20 μm, depending on the tissue type.

Even so, some neural cells and some giant cells in bone marrow can measure up to 80 μm or more, several times the average cell size. Measuring 4 to 5 μm, the glial cells (Latin glia = glue) are, along with sperm cells, among the smallest human cells. The diameter of the “heads” of sperm cells is between 3 and 5 μm. With their average diameter of 7.5 μm, the red blood corpuscles also count among the smallest cells. The median cell size is between 30 and 50 μm.

Shape:

Because of their special functions the shapes of most cells differ widely. They are eminently suited to their locality. Cells occurring in covering layers, like an epithelium, can be cubical, flat, or prismatic, and are as densely packed as their shapes allow. Many cells are spherical, or spindle shaped like those in the smooth muscle tissue. Nerve and connective tissue cells can have long, branching processes.

Multitasking:

Even though the basic plan of all cells is the same, they may differ significantly according to their function. Every kind of cell is specialised to perform certain tasks in the organism.

The red blood cells (erythrocytes) transport oxygen, the nerve cells carry information, glandular cells secrete specific substances, muscle cells are responsible for body movements, and sex cells serve reproduction. The various functions of a cell are all coded for by way of specific genetic information, stored in certain sections of the DNA (desoxyribonucleic acid) of the cell itself.

Pre-programmed here are processes like cell division and the synthesis of all the necessary proteins.

These two processes are essential prerequisites for the development of a multi-cellular organism from a fertilised ovum. It is remarkable that such diversified cells as are found in the brain, lungs, muscles, or liver, all develop from undifferentiated precursors. Cells with different functions also have different “life expectancies”.

Some cells last for only a few hours or days (eg intestinal epithelial cells), and others, like neural cells, live as long as the organism itself.

Number:

The estimated total number of cells constituting the human body is an astonishing 100 million million (1014)! It is practically impossible to visualise a number like 1014. To count up to this number, counting non-stop at the rate of one per second, day and night, would take three million years. The number of red corpuscles in the blood, the most numerous of all the cell types, is 25 x 1012 = 25 million million. But, unlike all other cells, they have neither a nucleus nor cellular organelles. The total number of brain cells is about hundred thousand million.

Taking the average size of a cell as 40 μm, all the cells of a human body, placed side by side, would form a chain reaching 100 times around the equator.