how to Introduction to Human Cytology

Human cells come in a wide range of sizes. A human egg cell (ovum) is comparatively large. At 300 μm in diameter, it is just about visible to the naked eye. Most other cells in the body are a fraction of this size.

The cells that line the stomach are relatively large. Like skin cells, these are gradually shed. Muscle fibers, or myofibrils, would at first glance appear to be the largest body cells since they can be anything from 1 mm to almost 3 cm long. However, although these fibers are bound by a single cell membrane, they are not really a single cell since they are multinucleated- they may contain many thousands of nuclei each. Red blood cells are at the smaller end of human cells. They lack many of the internal features of other cells. Single-celled organisms, such as yeast (a fungus) and bacteria tend to be small. It would take about 500 bacteria, each 1.5 μm long, to cover the period at the end of this sentence.

Levels of organization

The body is a single organism (living creature) made up of many different substances and structures. These can be arranged into levels of increasing complexity called levels of organization, beginning with the most basic:

1) Biochemistry: atoms and molecules make the substances that are the building blocks (for example, water and protein) of all the other levels.

2) Cells are the smallest living units of the body.

3) Tissues are formed by groups of cells with similar structures and functions (together with other substances).

4) Organs are formed by tissues.

5) Body systems result from various organs working together and include, for example, the digestive system.

6) The body (organism) is thesum of all the levels existing and operating together.

Cells, tissues, and organs

It is estimated that the human body contains about 100 trillion cells. These are specialized to perform certain functions and have a wide variety of shapes and sizes. They are organized into separate structures within the body.

Tissues

Cells of the same type are organized into layers or other types of groupings known as tissues

The cells in tissues are devoted to a narrow range of functions. Examples of tissues include:

• Cells arranged to form a thin layer of tissue (epithelium) as found in the lining of the mouth;

• Cells forming a tubule (small tube) as in a kidney tubule;

• Muscle cells arranged in a sheet of muscle tissue, as occurs in the wall of arterial blood vessels

Organs

Tissues of different kinds are themselves grouped together to form the organs of the body. These include the skin, heart, liver, and kidneys. For example, human skin contains several distinct layers of tissue:

• A protective layer of dead cells to prevent infection, and drying out;

• Epidermal cells, some of which form the hair follicles;

• A basement membrane separating the epidermis from the dermis;

• The dermis layer, which includes cells responsible for maintaining the strength and integrity of the skin, as well as sensory nerve endings, blood vessels, and muscle fibers.