Tissues

Tissues are defined as a group of similar cells that act together to perform a specific function. The study of tissues is known as histology.

Due to the complexity of the human body it is not possible for every cell to carry out all the functions required by the body. Some cells, therefore, become specialised to form of a group of cells or tissues. There are four major types of tissues in the human body:

• epithelial tissue

• connective tissue

• muscle tissue

All four types of tissue have special purposes and therefore correspondingly have varying different rates of cellular regeneration. Epithelial tissue is renewed constantly by the process of cell division or mitosis. Bone tissue and adipose connective tissue are highly vascular and therefore heal quickly. Muscle tissue takes longer to regenerate. Nervous tissue regenerates very slowly. The less vascular forms of connective tissue such as ligaments and tendons are even slower to heal than muscle tissue, and cartilage is amongst the slowest to heal.

The following table gives an overview of the four major types of tissue in the body.

Type of tissue

Main function

Epithelial tissue

Provides a protective covering for surfaces inside and outside of the body

Connective tissue

Protects, binds and supports the body and its organs

Muscle tissue

Provides movement

Nervous tissue

Initiates and transmits nerve impulses

Epithelial tissue

Epithelial tissue consists of sheets of cells which cover and protect the external and internal surfaces of the body and line the inside of hollow structures. They specialise in moving substances in and out of the blood during secretion, absorption and excretion. As they are subject to a considerable amount of wear and tear, epithelial cells reproduce very actively.

Usually there is little matrix (a ground material or base) present in epithelial tissues. The matrix present tends to form continuous sheets of cells, with the cells held very close together. A thin permeable basement membrane attaches epithelial tissues to the underlying connective tissue. Epithelial tissue, which consists of cells closely packed together, comes in various shapes. There are two categories of epithelial tissue:

• simple (single layered)

Simple epithelium

Simple epithelium tissue has only one layer of cells over a basement membrane. Being thin, it is fragile and is found only in areas inside the body which are relatively protected, such as the lining of the heart and blood vessels and the lining of body cavities. It is also found lining the digestive tract and in the exchange surfaces of the lungs where its thinness is an advantage for speedy absorption across it. There are four different types of simple epithelium, named according to their shape and the functions they perform.

The table on page 21 summarises the structure, location and function of the different types of simple epithelial tissue.

Compound epithelium

The main function of compound epithelium is to protect underlying structures. Compound epithelium contains two or more layers of cells. There are two main types:

• stratified

• transitional

Cuboidal epithelium

Found in the kidneys

Columnar epithelium

Found in the intestines

Cuboidal epithelium

Found in the kidneys

Columnar epithelium

Found in the intestines

Transitional epithelium

Found in the bladder

Stratified keratinised epithelium

Found in the skin

Transitional epithelium

Found in the bladder

Stratified keratinised epithelium

Found in the skin

Fig 1.6 Types of epithelial tissue

Stratified epithelium is composed of a number of layers of cells of different shapes. In the deeper layers the cells are mainly columnar in shape and as they grow towards the surface they become flattened.

Type

Structure

Location

Function

Simple squamous

A single layer of flat, scale-like cells with a central nucleus. The cells fit closely together, rather like a pavement, producing a very smooth surface

Lines the alveoli of the lungs Lines blood and lymphatic vessels and the heart

Allows for exchange of nutrients, wastes and gases

Simple cuboidal

Single layer of cube-like cells

Ovaries, kidney tubules, thyroid gland, pancreas and salivary glands

Secretion and absorption

Simple columnar

Single layer of tall, cylindrical column cells with nucleus situated towards base of cell

Lines the small and large intestine, stomach and gall bladder

Secretion and absorption

Simple ciliated (columnar)

A form of columnar epithelium, single layer of rectangular cells that has hair-like projections (cilia) from its surface

Lines the upper part of respiratory system

Also lines the uterine tubes

The beating of the cilia carries unwanted particles along with mucus out of the system

Helps propel the ova towards the uterus

There are two types of stratified epithelium:

• Non-keratinised stratified epithelium - this is found on wet surfaces that may be subject to wear and tear such as the conjunctiva of the eyes, the lining of the mouth, the pharynx and the oesophagus.

• Keratinised stratified epithelium - this is found on dry surfaces such as the lining of the skin, hair and nails. The surface layers of keratinised cells are dead cells. They give protection and prevent drying out of the cells in the deeper layers from which they develop. The surface layer of cells is continually being rubbed off and is replaced from below.

Transitional epithelium is composed of several layers of pear-shaped cells which change shape when they are stretched. This type of tissue is found lining the uterus, bladder and pelvis of the kidney.

Connective tissue

Connective tissue is the most abundant type of tissue in the body. It connects tissues and organs by binding together the various parts of the body and helps to give protection and support. Connective tissue consists of a matrix or ground substance which contains cells called fibroblasts and fibres made of protein.

Connective tissue cells are often more widely separated from each other than those forming epithelial tissue, and the space between cells is larger and is filled with a large amount of non-living matrix. There may or may not be fibres in the matrix, which may be either a semi-solid jelly-like consistency or dense and rigid, depending on the position and function of the tissue.

There are several different types of connective tissue, which are summarised in the table on page 23.

Fig 1.7 Types of connective tissue

Cartilage

For descriptive purposes, cartilage is divided into three types:

• hyaline cartilage

• white fibrous cartilage

• yellow elastic fibrocartilage.

Muscle tissue

Muscle tissue is very elastic and therefore has the unique ability to provide movement by shortening as a result of contraction. This tissue is made

Type

Structure

Location

Function

Areolar

Most widely distributed type of connective tissue in the body

A loose, soft and pliable tissue containing collagen, elastin and reticular fibres

Under the skin, between muscles, supporting blood vessels and nerves and in the alimentary canal

Provides strength, elasticity, connects and supports organs

Adipose

A type of areolar tissue containing fat cells (adipocytes)

Surrounds organs such as kidneys and the heart Under the skin (subcutaneous layer) between bundles of muscle fibres

In the yellow bone marrow of long bones and as a padding around joints

Provides insulation, support and protection; emergency energy reserve

White fibrous

Strong, connecting tissue made up of mainly closely packed bundles of white, collagenous fibres, with very little matrix Contains cells called fibrocytes between the bundles

Forms tendons which attach muscle to bone, ligaments which tie bones together and act as an outer protective covering for some organs such as the kidney and bladder

Provides strong attachment between different structures

Yellow elastic

Consists of branching yellow elastic fibres with fibrocytes in the spaces between the fibres

Arteries, trachea, bronchi and lungs

To allow the stretching of various organs, followed by a return to their original shape and size

Lymphoid

This tissue has a semi-solid matrix with fine branching fibres. The cells contained within this tissue are specialised and are called lymphocytes

In the lymph nodes, spleen, tonsils, adenoids, walls of the large intestine and glands in the small intestine

Forms part of the lymphatic system whose function is to protect the body from infection

Blood

Also known as liquid connective tissue. It contains the blood cells erythrocytes, leucocytes and thrombocytes which float within a fluid called plasma

Contained within blood vessels

Helps maintain homeostasis of the body by transporting substances throughout the body, by resisting infection and by maintaining heat

Bone

Hardest and most solid of all connective tissues Consists of tough, dense compact bones and slightly less dense cancellous bone

Bones

Protects and supports other organs and soft tissues

Cartilage

Much firmer tissue than any of the other connective tissues; matrix is quite solid

See table on page 24

See table on page 24

Type of cartilage

Description

Location

Function

Hyaline cartilage

Most abundant type of cartilage found in the body A smooth, bluish-white, glossy tissue

Contains numerous cells called chondrocytes from which cartilage is produced

Found on the surfaces of the parts of bones which form joints

Forms the costal cartilage which attaches the ribs to the sternum

Forms part of the larynx, trachea and bronchi.

Provides a hard-wearing, low-friction surface within joints

Provides flexibility in the nose and trachea

White fibrous cartilage

This type of cartilage is tough but slightly flexible

It is composed of bundles of collagenous white fibres in a solid matrix with cells scattered among them

Found as pads between the bodies of the vertebrae called the intervertebral discs and in the symphysis pubis which joins the pubis bones together

Its function is one of support and to join together or fuse certain bones

Yellow elastic fibrocartilage

Consists of yellow elastic fibres running through a solid matrix, between which chondrocytes are situated

Found forming the pinna (lobe of the ear) and forming the epiglottis

To provide support and to maintain shape

up of contractile fibres, usually arranged in bundles and surrounded by connective tissue. There are three types of muscle tissue:

• voluntary (skeletal) tissue

• involuntary or smooth tissue

• cardiac muscle tissue.

The different types of muscle tissue are discussed in more detail in Chapter 4, 'The muscular system'.

Nervous tissue

Nervous tissue consists of cells called neurones which can pick up and transmit electrical signals by converting stimuli into nerve impulses. Nervous tissue has the characteristics of excitability and conductivity. Its functions are to coordinate and regulate body activity. Nervous tissue and neurones are discussed in more detail in Chapter 8, 'The nervous system'.

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