The Elementary Tissues of the Body

There are four groups of tissue in the body which are known as the elementary tissues. They are:

1. Epithelial Tissue

2. Muscular Tissue

3. Nervous Tissue

4. Connective Tissue

1. Epithelial Tissue:

An epithelium consists of cells which cover surface of the body, e.g. skin, or which line hollow organs, tubes or cavities e.g. blood vessels, and the air cells. There are two main classes of epithelial tissue, each containing several varieties.
a) Simple Epithelium

b) Compound Epithelium

All epithelial cells lie on and are held together by a homogeneous substance called a basement membrane.

a) Simple Epithelium:

This class consists of a single layer of cells, and is subdivided into three varieties.

1. Pavement or Squamous Epithelium

2. Columnar Epithelium

3. Ciliated Epithelium

1. Pavement or Squamous Epithelium:

They are fine thin plates placed edge to edge like the particles in a mosaic pattern or the stones of a pavement. These cells form the alveoli of the lungs. They are found whenever a very smooth surface is essential as in the lining of the heart (serous membrane), lining of blood vessels and lymphatics. When lining these structures the epithelial covering or lining is called endothelium.

2. Columnar Epithelium:

They form a single layer or they are the cells which line the ducts of most glands, the gall bladder, nearly the whole of the digestive tract, in which goblet cells are interspersed, and parts of the genitor urinary tract. In intestine, these have a slightly striated border. In some situations, as when lining the alveoli of secreting glands, the cells of columnar epithelium are short and have a cubical appearance. They are then described as cubical cells.

3. Ciliated Epithelium:

It is found lining the air passages and their ramifications such as the frontal and maxillary sinuses. It also lines the uterine tubes or oviducts and part of the uterus and the ventricles of the brain.

Ciliated cells are like columnar cells in shape, but they have in addition fine hair like processes attached to their free edge. These processes are called cilia. The ciliary processes keep up a continual movement directed towards the external opening. This movement has been likened to the movement seen in a field of corn, blown in one direction by the wind. In the respiratory passages the constant movement prevents dust, mucus, entering the lungs, and in the uterine tubes the movement conveys the ovum into the uterus.

Goblet cells are mucus-secreting cells which lie in the walls of glands and ducts lined by columnar cells, either plain or ciliated. Goblet cells secrete mucus or mucin and express it on to the surface; they act as mucus-secreting glands and are most numerous where a considerable amount of mucus covers the surface as in the stomach, colon, and trachea.

b) Compound Epithelium: It consists of more then one layer of cells. Stratified Epithelium forms the epidermal layers of the skin. It also lines the mouth, pharynx, oesophagus, the lower part of the urethra, the anal canal and the vagina, and covers the surface of the cornea. In these areas it does not become cornified.

Transitional Epithelium is a compound stratified epithelium consisting of three layers of cells. It lines the urinary bladder, the pelvis of the kidney, the ureters and the upper part of the urethra. The deeper layers of cells in transitional epithelium are of the columnar type of cell with rounded ends which make them pyriform or pear shaped.

Functions of Epithelial Tissue:

The epithelial tissue which forms the covering of the body, the skin and the lining of the cavities which open on to the surface is mainly protective.

It prevents injury to the underlying tissues.

It prevents the loss of fluid from these tissues and also prevents the passage of fluid into the structures which are covered by skin.

The micro organisms cannot pass through healthy skin but they can and do pass through abraded skin.

(a) Glands:

A gland is a secretory organ which may exist as a separate organ such as the liver, pancreas and spleen or may be simply a layer of cells as the simple tubular glands of the alimentary canal, body cavities etc. All glands have a rich blood supply. Their special function is to select from the blood stream certain substances, which they then elaborate into their important juices or secretions.

There is a tremendous variety of glands, each with its different function, making a collective description and classification difficult. A simple classification is as follows:

a. Glands which pour their secretion directly on to the surface include the sweat glands, sebaceous glands, and the gastric and intestinal glands.

b. Glands which pour their secretion indirectly, by means of ducts, on to the surface include the salivary glands, pancreas, and liver.

c. Ductless glands: These form the group described as endocrine organs. These are the glands of internal secretion. A great deal of the well being of the body depends on these glands, which through their secretions exert an important chemical control on the functions of the body.

(b) Membranes:

Layers of specialized cells which line the cavities of the body are described as membrane. The three principal membranes are:

1. Mucous membrane

2. Synovial membrane

3. Serous membrane

All these membranes secrete a fluid to lubricate or moisten the cavity they line.

1. Mucous membrane is found lining the alimentary tract, the respiratory tract, and parts of the genitor urinary tract. It varies in character in the different areas. In the digestive tract it consists of columnar epithelial cells closely packed together. Some of them become distended with mucous secretion and are then called goblet cells. The cell becomes more and more distended and finally ruptures and discharges it secretion on to the surface.

Mucus is the secretion of the membrane and consists of water, salts, and a protein, mucin, viscid character to the secretion.

2. Synovial Membrane lines the cavities of joints. It consists of fine connective tissue, with a layer of squamous endothelial cells on the surface. The secretion of synovial membrane is thick and glairy in character.

3. Serous Membranes are found in the chest and abdomen, covering the organ contained therein and lining these cavities.

The pleura covers the lungs and lies the thorax.

The pericardium covers the heart as a double layer.

The peritoneum covers the abdominal organs and lines the abdomen.

The characteristics which are common to all three serous membranes are, that each consists of a double layer of membrane having an intervening potential cavity which receives the fluid secreted by the membrane. This serous fluid is very similar to blood serum or lymph. It acts as a lubricant, and in addition it contains protective substances and removes harmful products, passing these on to the lymphatic system to be dealt with.

2. Muscular Tissue: Muscle is a tissue which is specialized for contraction, and by means of this, movements are performed. It is composed of cylindrical fibres which correspond to the cells of other tissues. These are bound together into little bundles of fibres by a form of connective tissue which contains a highly specialized contractile element.

Types of Muscle:

1. Striped (striated skeletal or voluntary muscle):

The individual muscle fibres are transversely striated by alternate light and dark markings. Each fibre is made p of a number of myofibrils and enclosed in a fine membrane. The sarcolemma (meaning – muscles sheath).

2. Unstriped (unstriated, smooth or involuntary muscle):

This type will contract without nervous stimulation although in most parts of the body its activity is under the control of the autonomic (involuntary) nervous system. This variety is composed of elongated spindle shaped muscle cells which retain the appearance of a cell.

Involuntary muscle is found in the coats of blood and lymphatic vessels, in the walls of the digestive tract and the hollow viscera, trachea, and bronchi, in the iris and ciliary muscle of the eye, and in the involuntary muscles in the skin.

3. Cardiac Muscle is found only in the muscle of the heart. It is striated like voluntary muscle. But it differs in that its fibres branch and anastomose with each other; they are arranged longitudinally as in striated muscle, are characteristically red in colour and not controlled by the will.

Cardiac muscle possesses the special property of automatic rhythmical contraction independent of its nerve supply.

3. Nervous Tissue: 

The nervous tissue consists of three kinds of matter (a) grey matter, forming the nerve cells, (b) white matter, the nerve fibres and (c) neuroglia, a special kind of supporting cell, found only in the nervous system, which holds together and supports nerve cells and fibres. Each nerve cell with its processes is called a neurone.

Nerve cells are composed of highly specialized granular protoplasm, with large nuclei and cell walls on other cells. Various processes arise from the nerve cells; these processes carry the nerve impulses to and from the nerve cells.

4. Connective Tissue: Connective tissue provides the frame work of the body. There are several varieties of connective tissue.

Areolar Tissue: This consists of loosely woven tissue which is distributed widely throughout the body. It is placed immediately beneath the skin and mucous surfaces forming the subcutaneous and sub-mucous tissue, and it also forms the sheaths of fascia which support and bind and connect together muscles, nerves, blood vessels, and other organs.