Title: Cells, Tissues and Overall Anatomy

Key words: the cell, cytoplasm, nucleus, mitochondria, ribosomes, DNA, differentiation, membranes, diffusion, active transport, tissues, chromosomes, receptors

Date: Aug 2000

Category: 1. The Cell

Type: Article

Author: DJE Candlish

Cells, Tissues and Overall Anatomy

Cells

The cell is regarded as the smallest functional unit in the human body, although it contains many smaller structures, each with their own form and function. In simple, one-celled organisms, the cell can carry out all the processes that define life: feeding, breathing, reproducing, moving and communicating with others. In complex organisms such as humans, cells have more specialised functions and are grouped together into tissues or organs.

Every cell in the body originates from the single cell formed by fusion of a sperm with an ovum at conception. This original cell divides countless times during its development into an embryo and then an adult. As it does so, the new cells that it forms will differentiate into the many different cell types that form the tissues, organs and systems of the body. Despite differences in terms of function and structure, which will be described later, there are also many similarities between cell types, due to their common origin.

To show the similarities, it is possible to describe a 'standard' human cell. This consists of a jelly-like mass, or cytoplasm, enclosed in a very thin cell wall or membrane called the plasma membrane.

The cytoplasm is mainly water, in which proteins, carbohydrates, fats and salts are dissolved. It also contains a number of specialised structures called organelles. These include the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes and lysosomes.

The nucleus contains long chains of deoxyribonucleic acid (DNA) in the form of chromosomes. These contain all of the information needed to recreate the human body.

The endoplasmic reticulum, ribosomes and Golgi apparatus produce the protein the body needs.

The mitochondria are the power source of the cell. They produce and store energy, assisted by the lysosomes.

The cell membrane separates the cell contents from the external environment. However, special proteins in the membrane can form a link between the internal and external environments and can receive and act upon chemical 'messages' from other cells. These proteins are called receptors.

The cell membrane is semi-permeable, so a range of substances can pass through it, either by active or by passive transportation. As the names suggest, active transportation involves expending energy, while passive transportation involves no energy, as it simply involves diffusion of substances through the membrane. Generally, small molecules like oxygen and carbon dioxide can diffuse easily through cell membranes, while larger molecules must be transported actively.

Tissues

A tissue is generally a collection of cells with similar size, shape and function. However, some tissues can contain more than one cell type, so this is not a rigid definition. Some examples of the four main types of tissue will make this clear:

· epithelial (covering) tissues

· supporting tissues

· muscle tissues

· nerve tissue

Epithelial or covering tissues

In epithelial tissues, the cells form a closed layer. They usually form coverings, such as over the outside of the body (see Skin section) and the inner surfaces of hollow structures like the heart, lungs, blood vessels, digestive tract, urinary tract, vagina and uterus. Covering tissue is classified by the shape and functions of the cells it contains (see figure X).

· one layered pavement epithelium

· cuboidal epithelium

· cylindrical epithelium

· ciliated epithelium

· multi-layered epithelium

· transitional epithelium

Ciliated epithelium, for example, gets its name from the cilia or tiny hair-like projections at the top of the cells. In the lungs, this tissue also contains mucus glands, which, together with the cilia, help rid the lungs of inhaled dust and so on. The mucus traps dust particles and the cilia act like tiny brushes, sweeping the dust and mucous back up over the lining of the lungs to the mouth, where it can be swallowed or spat out.

When epithelial-type tissue forms the inner lining of hollow structures like the blood vessels, it is referred to as endothelial tissue. This terminology simply indicates its position (epi means outer, endo means inner).

Support tissues

These are more varied than covering tissues in terms of both structure and function. There are four basic types:

· connective tissue

· cartilage tissue

· bony tissue

· blood.

The structure and function of connective tissue varies according to its location in the body. In the skin, for example, it is dense and firm and contains many criss-crossed, interwoven fibres of collagen. Around the intestines, the connective tissue is called loose or adipose, as it contains fatty tissue and has large spaces in it. Very few collagen fibres are present. In the walls of arteries, the connective tissue is more elastic. It contains many fibres, composed of elastin rather than collagen. This allows it to stretch more easily to adjust to changes in blood pressure and contraction or relaxation of the smooth muscle in the arterial walls.

Fatty tissue is present as a matrix (substance between the cells) in many connective tissues, as it is a valuable energy store and can also act as a protective cushion. Around the kidneys, for example, a thick layer of fatty tissue acts as a shock absorber to protect these vital organs.

Cartilage tissue

Cartilage tissue has a much firmer and more elastic matrix than most connective tissue. It consists of chondrin, a translucent substance produced by cartilage cells. These form into groups surrounded by a thick layer of collagen fibres (see Figure X). There are three types of cartilage:

· hyaline

· elastic

· fibrous.

Hyaline cartilage forms a smooth, slippery surface over the ends of bones where they come into direct contact in joints, such as the knee. This surface reduces friction between the bones during movement and also absorbs some of the shock of movement.

Elastic cartilage is found in the external ear, the epiglottis, the nostrils and the septum or division between the two sides of the nasal cavity. It contains elastin, which gives it flexibility.

Fibrous cartilage is a tight network of collagen fibres closely packed together, which has high resistance to pressure. The discs between the vertebrae of the backbone are made of this type of cartilage. It acts as a protective buffer against compression.

Bony tissue has a solid matrix, composed of calcium phosphates. It is also flexible, as it contains many collagen fibres. Bone forms the skeleton, the framework upon which all the other organs, the muscles and so on depend for support. Bone is a living tissue, constantly being broken down and rebuilt and for this reason it has a good blood supply.

The blood is also considered as a 'support' tissue. The matrix here is a fluid (plasma), so it can be pumped around the circulatory system supplying nutrients and oxygen, removing waste products and carbon dioxide.

Muscle tissues

In muscle tissue, the cells have developed in a highly specialised way, which allows them to contract or shorten. In combination with the leverage provided by bones in the skeleton, this muscle contraction is what makes movement possible. Muscle tissue uses a lot of energy and needs a very good blood supply. There are three subtypes of muscle tissue:

· smooth muscle

· striated muscle

· cardiac (heart) muscle.

Smooth muscle contracts fairly slowly but can continue to contract and relax for long periods, without tiring. In the digestive tract, smooth muscle provides the movements that mix food and move it along. Smooth muscle also gives blood vessels the strength and flexibility to resist and adjust to changes in blood pressure. Smooth muscle is usually not under conscious control and so is known as involuntary muscle.

Striated muscle is what enables us to run, jump and lift objects. It can contract very quickly, but because it uses so much energy, it soon tires and needs a rest period before it can contract again. Striated muscle fibres are longer than smooth muscle fibres and have a distinctive striped appearance under a microscope. They actually consist of many cells fused together during the embryo stage of development.

Cardiac muscle tissue is similar to striated muscle, with just a few important differences. The cells are separate, not fused together and, because they have a regular, automatic cycle of contraction and relaxation (the heartbeat), they do not become fatigued. In fact, your heart will continue beating, 24 hours a day, for your entire lifetime.

Nerve tissue

Nerve tissue has also become very specialised. This type of tissue contains two cell types, the neurones and glial cells. Neurones conduct messages to and from the central nervous system, to control every aspect of the body's function. They do so by being excitable - responding to stimuli with a change in the ratio of potassium and sodium ions inside and outside the cell. The glial cells are support cells that provide the neurones with nutrients and oxygen. Nerve cells are described in more detail in the section on the nervous system.