The study of tissues

Although Antonie van Leeuwenhoek (1632-1723) did not invent the microscope, he was the most influential of all the seventeenth-century microscopists. A draper by trade, the Dutchman was familiar with using a small lens to examine fabrics. It was a small step from studying the quality of cloth to studying the natural world in what was to become a life-long passion. By polishing and combining a number of small lenses, he built simple little microscopes. Some of them could reach a magnification of 500 times. He was thus the first to see single-celled organisms in rainwater, the bacteria in tartar deposits, and the activity of sperm. Histology deals with the study of the construction and function of tissues. Here, too, the microscope is of inestimable value. Very thin layers of tissue are laid on a glass slide and treated in such a manner that particular structures become more visible: a dye, for instance, can be added to increase the contrast between the nucleus and surrounding cytoplasm. The common light microscope cannot magnify more than around 1,200 times. With an electron microscope a magnification factor of 2 million can be achieved.

Epithelial tissue

This is essentially the covering tissue of the body. Every organ that is in contact with the outside world is lined by one or more layers of epithelial tissue. The most obvious example of epithelium is the epidermis of the skin, but there are many other places where epithelium can be found: for example, the mucous membranes in the digestive system, in the air passages or in the urinary tract. Epithelia can perform many different functions. On the outer surface of the body they prevent execessive water loss and protect against injury. Those lining internal surfaces are specialized in absorbing or secreting substances (food and urine, for example). Each specialized function is reflected in a characteristic structure, so that under a microscope it is possible to determine from which organ of the body a specimen of tissue is derived. Simple epithelium is only one cell layer thick. In one form, pavement epithelium, the cells are very flat, which means that substances can move relatively freely from one side of the cell membrane to the other. This type can be found in the tiny air sacs in the lungs (the alveoli) where oxygen from the air needs to reach the blood with the least amount of obstruction. In simple columnar epithelium the cells are more elongated. This type of epithelium is found in the stomach and in the intestines where it is concerned with absorption of food particles. To facilitate this process the cells have up to 500 very tiny projections at their top (microvilli). The purpose of these microvilli is to increase the surface area of absorption. Pseudostratified epithelium is found in large portions of the air passages. At the top of the cells small hairs (cilia) are present which constantly sway to and fro like a whip. Their function is to keep the air passages clean. Mucus embedded with dust particles and bacteria is carried to the throat from the nasal passages at a speed of sometimes more than 1cm a minute.

Multilayered epithelia are especially effective in providing protection against wear and tear. Most of the time, physical injury harms only the superficial layers which can be quickly replaced by deeper-lying cells. The mucous membranes of the mouth consist of stratified squamous epithelium. As all the layers consist of living cells, it is essential that the mucous membranes are kept wet. The epidermis of the skin is stratified epithelium too. There is a difference however: the superficial cell layers of the skin consist of dry dead scales. This is the result of the production of keratin. Because this substance is not permeable to water, the body is protected against drying out. Every gland in our body is derived from epithelial tissue. In its most simple form a gland consists of only one cell. Between the ciliated cells in the air passages, for example, there are goblet cells that secrete slimy mucus. This prevents the drying out of the mucous membranes and traps dust particles. More complex glands are derived from sac-like invaginations of epithelium and have a branched structure. These glands are responsible for the secretion of specific substances at a specific location. For example, the parotid gland in the mouth produces saliva containing enzymes for the digestion of starch. The way in which gland cells produce their secretions can differ to a large extent. Most of the time, the cells form tiny droplets that are eventually expelled from the cell. In breast glands, however, the tops of the cells are shed, to produce milk. In cells of the sebaceous glands the whole cell is eventually excreted. Before this occurs, the cell is filled with fat. This provides the hair and the skin with an oily layer which is helpful in destroying bacteria and preventing drying out.

Connective tissues

Unlike epithelial tissue, connective tissue is composed of cells that are only loosely attached to each other. Between the cells there are large intercellular spaces, filled with fluid. According to the demands made on them, connective tissue cells secrete various substances into this fluid. One of these is collagen, a thick and strong fibrous protein which can resist traction. If the connective tissue needs to be elastic, another fibre – elastin – is formed. Yet another type, areolar connective tissue, is found under the skin and consists of collagen and elastin fibres lying in different directions. In this tissue large amounts of fat cells can also be found, which gives the body a rounded appearance and acts as an insulator. When injury occurs, it is by the action of connective tissue cells that wound repair takes place: new fibres are produced very quickly to replace torn tissue, and after a few weeks the tissue is almost as strong again as before. Tendons do not have to be elastic, so they contain only collagen. These fibres lie parallel, providing a strong connection between muscle and bone. Cartilage is another type of connective tissue. It is an important component of joints, where it covers both ends of the bones. It provides a smooth polished surface, to prevent friction. It is also very resilient, which helps to absorb the shocks caused by walking. As cartilage has a poor blood supply, damage is serious, because repair is very slow. Bone is also a connective tissue, designed to support the whole body, make movement possible and protect some vital organs. The substance that is secreted by the bone cells gets mineralized by calcium salts. This makes bone one of the hardest substances in the body, but contrary to what most people believe, it is also flexible. The spring in bones comes from collagen fibres. Because in elderly people the amount of collagen decreases, their bones become brittle and more liable to fractures.