Chapter 3 — Animal Tissues

Just as in plants, the cells of an animal are organised into tissues, groups of cells similar in origin and structure that perform a common function. But animal tissues differ from plant tissues in several ways, and the reasons lie in the way animals live.

Animals move about in search of food, mates and shelter and must respond quickly to danger. They therefore need tissues that can contract (muscle) and tissues that can carry messages rapidly (nerve), neither of which plants have. Because they move, animals cannot afford heavy dead supporting tissue; nearly all animal tissues are living and consume energy continuously, whereas much of a plant is dead wood and cork. Animal growth is not confined to a few growing points as in plants; it is more uniform over the body, and tissue repair goes on everywhere throughout life. Animals also have no cell walls, so their tissues are held together by cell junctions and by intercellular material secreted by the cells rather than by cemented walls.

Animal tissues are classified into four basic types on the basis of structure and function:

• Epithelial tissue covers the body and lines its cavities and organs; it protects, absorbs, secretes and senses.
• Connective tissue binds other tissues together, supports the body, fills spaces and transports materials; it includes bone, cartilage, fat, blood and the packing tissues.
• Muscular tissue is specialised to contract and so produces movement of the body and of materials within it.
• Nervous tissue receives stimuli and conducts impulses, coordinating and controlling the body.

Every organ is made of more than one of these. The stomach, for instance, has an epithelial lining that secretes digestive juice, layers of smooth muscle that churn the food, connective tissue that holds the layers together and carries blood vessels, and a network of nerves that regulates the whole process. A tissue is therefore not an organ; an organ is an assembly of tissues.

The study of tissues, histology, is done on very thin slices of tissue that are stained and mounted on slides. In your laboratory you will look at prepared slides of the main tissues and at a smear of blood, and the sections of this chapter will help you to identify what you see by the shape of the cells, the amount of material between them, and the presence of fibres, striations or long processes.

Epithelial tissue (epithelium) is the covering and lining tissue of the body. It forms the outer layer of the skin, the lining of the mouth, food canal, stomach and intestines, the lining of the air passages and lungs, the lining of blood vessels and the heart, the covering of internal organs, and the tubes of the kidney. It also forms the glands, which are derived from epithelium. Because it lies at every surface, epithelium is the tissue through which everything that enters or leaves the body must pass.

Epithelial tissue has a set of common features whatever its type. Its cells are packed tightly together with very little intercellular material and almost no space between them; the cells are joined to each other by special junctions, so that the layer forms a continuous sheet and a barrier that keeps substances from leaking between cells. The sheet rests on a thin non-cellular layer called the basement membrane, made of fibres and proteins secreted partly by the epithelial cells and partly by the connective tissue below, which anchors the epithelium and separates it from the underlying tissue. Epithelium has no blood vessels of its own; its cells receive oxygen and food by diffusion from the capillaries in the connective tissue beneath the basement membrane. Epithelial cells divide readily and the tissue has a high power of regeneration, which is necessary because surface cells are constantly being worn away; the lining of the intestine is replaced every few days and the skin surface every few weeks.

The functions of epithelium follow from its position at the surface. It protects the underlying tissues from mechanical injury, from drying, from chemicals and from the entry of microbes. It absorbs digested food in the intestine and water in the kidney tubules. It secretes mucus, enzymes, sweat, hormones and other products through its glands. It permits exchange of gases in the lungs and of nutrients across capillary walls. It carries out filtration in the kidney and sensation in the taste buds, nose and inner ear.

Epithelial tissues are classified on two bases: the shape of the cells (flat or squamous, cube-shaped or cuboidal, tall or columnar) and the number of layers (a single layer is simple epithelium; several layers form stratified epithelium). Special types have cilia on their surface (ciliated epithelium) or are modified for secretion (glandular epithelium). The next sections describe each type and where it is found.

A simple epithelium is a single layer of cells resting on the basement membrane. Three types are named after the shape of their cells.

Simple squamous epithelium is made of extremely thin, flat cells with irregular outlines, fitted together like the tiles of a floor; for this reason it is also called pavement epithelium. Each cell has a flattened nucleus that bulges slightly at the centre. Squamous epithelium lines surfaces where materials must pass across quickly by diffusion or where a smooth lining is needed: the alveoli of the lungs, where oxygen and carbon dioxide are exchanged; the inner lining of blood vessels and the heart (called endothelium); the lining of the mouth and oesophagus (in a stratified form); the Bowman's capsule of the kidney; and the membranes lining the body cavities. Its thinness is what makes gas exchange in the lungs possible.

Simple cuboidal epithelium consists of cube-shaped cells, about as tall as they are wide, each with a central round nucleus. It is found in the tubules of the kidney, in the ducts of glands such as the salivary glands and the pancreas, in the lining of the ovary and the tubules of the testis, and in the thyroid gland. Its jobs are secretion and absorption; the kidney tubule cells reabsorb water, glucose and salts from the filtrate, and in some places the free surface of the cells carries tiny finger-like projections called microvilli that increase the absorbing area. Cuboidal cells also give mechanical support to the ducts they line.

Simple columnar epithelium consists of tall, pillar-like cells, much taller than wide, with an oval nucleus usually near the base of the cell. It lines the stomach and the intestines, where its cells absorb digested food and secrete mucus and enzymes; the lining of the gall bladder; and the ducts of many glands. In the small intestine the free surface of the columnar cells bears a dense brush of microvilli, which along with the villi of the intestinal wall multiply the surface for absorption many times. Among the columnar cells lie flask-shaped goblet cells that secrete the slippery mucus that protects the lining and lubricates the passage of food.

The shape of the cell matches the job: flat cells for rapid exchange and smooth lining, cubical cells for moderate secretion and absorption in tubes, tall cells for heavy secretion and absorption on the surfaces of the gut. A good rule for identifying a slide: if the nucleus is flattened and the layer is thin, it is squamous; if the cells look square with a central nucleus, cuboidal; if they are tall with basal nuclei, columnar.

Beyond the three basic simple epithelia, three special types are important.

Ciliated epithelium is a columnar (sometimes cuboidal) epithelium whose cells bear numerous tiny hair-like projections called cilia on their free surface. The cilia beat rhythmically in one direction, like a field of wheat in the wind, and so move fluid, mucus or particles along the surface. Ciliated epithelium lines the respiratory tract, the nose, trachea and bronchi, where the cilia sweep dust particles and germs trapped in mucus upward to the throat so that they are swallowed or coughed out; this is the body's dust filter, and it is damaged by smoking. It also lines the oviducts (Fallopian tubes), where the cilia move the egg towards the uterus, and the ventricles of the brain and the central canal of the spinal cord, where they circulate the cerebrospinal fluid. Sperm move by a single long cilium-like structure, the flagellum.

Glandular epithelium consists of epithelial cells modified to secrete substances. Some glandular cells lie singly among other epithelial cells, such as the goblet cells of the intestine and respiratory tract that secrete mucus. Others fold inward from the surface to form multicellular glands. Glands that release their products through a duct on to a surface are exocrine glands: the salivary glands, sweat glands, oil (sebaceous) glands of the skin, gastric glands, the pancreas (digestive part) and the liver. Glands that have no duct and release their products, the hormones, directly into the blood are endocrine glands: the thyroid, pituitary, adrenal and the islet cells of the pancreas. Glandular epithelium therefore produces all the enzymes, mucus, sweat, milk and hormones of the body.

Stratified epithelium consists of several layers of cells piled one above another. Only the lowest layer rests on the basement membrane and divides; the new cells are pushed upward, flatten, and are finally shed from the surface. Because there are many layers the tissue withstands wear and tear. Stratified squamous epithelium forms the outer layer of the skin (epidermis), where the surface cells are dead and filled with the tough protein keratin, making the skin waterproof and resistant to abrasion, and it lines the mouth, tongue, oesophagus and vagina, where the surface cells are living and moist. Wherever the body surface is rubbed, scraped or stretched, stratified epithelium is present.

A special type, transitional epithelium, lines the urinary bladder and ureters; its cells can slide over one another so that the lining stretches as the bladder fills without tearing.

Connective tissue is the most abundant and widely distributed tissue of the animal body. As its name says, it connects: it binds organs and tissues together, supports the body, fills the spaces between organs, protects delicate organs, stores fat, and, in the form of blood, transports materials. Bone, cartilage, tendons, ligaments, fat, the packing tissue under the skin and the blood are all connective tissues, different as they look.

What they have in common is their structure, which is the reverse of epithelium. In epithelium the cells are packed tightly with almost no material between them; in connective tissue the cells are few and loosely scattered in a large amount of non-living intercellular substance or matrix that the cells secrete. The nature of this matrix decides the properties of the tissue. It may be a soft jelly (areolar tissue), a firm rubbery gel (cartilage), a hard mineralised solid (bone) or a liquid (blood). Embedded in the matrix, except in blood, are fibres of two main kinds: collagen fibres (white fibres), which are strong, tough and inelastic, made of the protein collagen, and elastic fibres (yellow fibres), made of the protein elastin, which stretch and recoil. Reticular fibres are fine branching collagen fibres that form supporting networks in soft organs. Most connective tissues are richly supplied with blood vessels, unlike epithelium; cartilage is the exception.

The cells of connective tissue are varied: fibroblasts secrete the fibres and matrix; macrophages wander through the tissue engulfing bacteria and debris; mast cells release histamine during inflammation and allergy; fat cells (adipocytes) store fat; plasma cells make antibodies; and in the skeletal tissues chondrocytes (cartilage cells) and osteocytes (bone cells) sit in small cavities of the matrix.

Connective tissues are classified by the nature of the matrix:

• Connective tissue proper, with a soft matrix: loose areolar tissue, adipose (fat) tissue, and dense regular tissue (tendons and ligaments).
• Skeletal or supporting connective tissue, with a firm or hard matrix: cartilage and bone.
• Fluid or vascular connective tissue, with a liquid matrix: blood and lymph.

All connective tissues arise from the same embryonic tissue, the mesoderm, which is one reason they are grouped together in spite of their differences.

Areolar tissue (loose connective tissue) is the most widespread connective tissue and the simplest. It is found beneath the skin, joining the skin to the muscles, between muscles, around blood vessels and nerves, around and inside organs as packing, and beneath every epithelium. Its matrix is a soft, transparent, jelly-like ground substance in which run loosely interwoven bundles of white collagen fibres and single branching yellow elastic fibres, leaving many small spaces (areolae) that give the tissue its name. Scattered in it are fibroblasts, which make the fibres, macrophages, which engulf germs and debris, mast cells, plasma cells and a few fat cells. Areolar tissue fills spaces inside organs, binds the skin to the underlying muscle and one tissue to another, supports the blood vessels and nerves that run through it, and helps repair tissues after injury; its macrophages and mast cells make it the battleground of inflammation. The swelling and redness around a cut or an insect bite is areolar tissue at work.

Adipose tissue (fat tissue) is areolar tissue in which the fat cells have become so numerous that they crowd out everything else. Each fat cell (adipocyte) is a large round cell almost entirely filled by a single drop of fat, with the cytoplasm and nucleus pushed to one side into a thin rim, so that the cell looks like a signet ring. Adipose tissue is found under the skin (the subcutaneous fat that gives the body its contours), around the kidneys, heart and eyeballs, in the bone marrow, in the mesentery of the intestine, and in the hump of a camel and the blubber of whales. Its functions are to store energy as fat, the most concentrated food reserve, to act as a heat insulator that reduces loss of body heat through the skin, and to cushion delicate organs such as the kidneys and eyes against shock. A well-fed person may carry 10 to 20 kg of adipose tissue.

Dense regular connective tissue has a matrix packed with collagen fibres running parallel in the same direction, with rows of fibroblasts squeezed between the bundles. It forms two important structures. Tendons connect muscle to bone; they are cords of great tensile strength but limited flexibility, made almost entirely of white collagen fibres, and they transmit the pull of the muscle to the bone. Ligaments connect bone to bone at joints; they contain collagen fibres along with many yellow elastic fibres, so that they are strong but also somewhat elastic, which allows a joint to move while holding the bones in place. A sprain is a stretched or torn ligament. Dense irregular tissue, with fibres running in all directions, forms the dermis of the skin and the capsules around organs.

Cartilage is a firm, smooth, flexible supporting tissue with a rubbery matrix. It is the tissue you feel in the tip of your nose and the flap of your ear, both of which can be bent and spring back. The skeleton of the embryo is made almost entirely of cartilage, which is later replaced by bone; in sharks and rays the whole adult skeleton remains cartilage.

Structure. The matrix of cartilage is a solid gel of proteins and carbohydrates (chondroitin sulphate) strengthened by fine collagen fibres. The cells, called chondrocytes, lie singly or in groups of two to four in small cavities in the matrix called lacunae; each group arose from one cell by division. Cartilage is covered by a fibrous membrane, the perichondrium, from which it grows. Unlike most connective tissues cartilage has no blood vessels and no nerves; the chondrocytes are fed by diffusion through the matrix. This is why an injury to cartilage, such as a damaged knee, heals very slowly.

There are three kinds of cartilage. Hyaline cartilage is glassy, bluish-white and translucent, with a fine, almost invisible network of collagen; it covers the ends of long bones at joints, forms the rings of the trachea and bronchi, the ends of the ribs and the tip of the nose, and the embryonic skeleton. Elastic cartilage contains many yellow elastic fibres in the matrix and is springy; it forms the pinna of the ear, the epiglottis and the tip of the nose. Fibrocartilage contains dense bundles of collagen fibres and is very tough; it forms the discs between the vertebrae and the pad in the knee joint, and joins the two halves of the pelvis.

Functions. Cartilage gives support with flexibility to parts that must keep their shape but also bend, such as the ear, nose and trachea; the trachea's cartilage rings keep the airway open but let the neck bend. It provides smooth, slippery surfaces at joints so that bones glide over one another with little friction. It acts as a shock absorber in the intervertebral discs and in the knee. And it is the template for bone: most of the bones of the body are first laid down as cartilage models which are gradually converted to bone (ossification), and the cartilage plates at the ends of long bones are where the bone grows in length during childhood.

Compared with bone, cartilage is softer, flexible, non-vascular, has cells in lacunae usually in groups, and its matrix is not calcified.

Bone is the hard, rigid connective tissue that forms the skeleton of vertebrates. It is the hardest tissue in the body after tooth enamel, yet it is living, has a rich blood supply, grows, repairs itself after fracture and is constantly being remodelled.

Structure. The matrix of bone is made of about one-third organic material, mainly collagen fibres, and two-thirds inorganic mineral, chiefly calcium phosphate with some calcium carbonate, deposited on the collagen framework. The collagen gives bone toughness and slight flexibility; the mineral gives hardness and rigidity. Together they make a material as strong as reinforced concrete at a fraction of the weight; if the mineral is dissolved away with acid a bone becomes rubbery, and if the collagen is burnt off it becomes brittle and crumbles. The bone cells, osteocytes, lie in small cavities called lacunae in the matrix. The matrix is laid down in concentric rings (lamellae) around a central canal, the Haversian canal, which carries blood vessels and nerves; each set of rings with its canal is a Haversian system or osteon, the unit of compact bone. Tiny channels called canaliculi radiate from each lacuna, and through them the osteocytes send out fine processes that contact one another and the canal, so that the cells deep in the hard matrix receive food and oxygen. Bone is covered by a tough membrane, the periosteum, from which it thickens and repairs.

There are two kinds of bone tissue. Compact bone is dense and solid and forms the outer shell of all bones and the shaft of long bones. Spongy bone has a honeycomb of thin plates with spaces between, found at the ends of long bones and inside flat bones; the spaces are filled with bone marrow, in which red blood cells, white blood cells and platelets are manufactured. The central cavity of a long bone also contains marrow.

Functions. Bone gives the body its shape and framework and holds it upright. It protects delicate organs: the skull encloses the brain, the ribs protect the heart and lungs, the vertebrae shield the spinal cord. It provides attachment for muscles, and bones act as levers so that muscle contraction moves the body. It is the main store of calcium and phosphorus, which are released into the blood when needed. And its marrow is the site of blood cell formation.

Bone forms from cartilage in the embryo and from membranes in the skull, and it grows in length at the cartilage plates near the ends of long bones until about the age of eighteen to twenty. A broken bone heals because bone-forming cells (osteoblasts) from the periosteum lay down new matrix across the break.

Blood is a connective tissue with a liquid matrix. It consists of a fluid, the plasma, in which three kinds of cells or cell fragments are suspended: red blood cells, white blood cells and platelets. An adult human has about 5 litres of blood, about 7 percent of body weight. Blood is red because of the pigment haemoglobin in the red cells; its pH is about 7.4 and it is slightly heavier and much thicker than water.

Plasma is a pale yellow fluid making up about 55 percent of the blood by volume. It is about 90 percent water and 10 percent dissolved substances: proteins (albumin, which holds water in the vessels; globulins, which include antibodies; fibrinogen, needed for clotting), glucose, amino acids, fats, salts such as sodium chloride and bicarbonate, hormones, dissolved gases and wastes such as urea. Plasma transports all these, distributes heat, and maintains the water balance and pH of the body. Plasma from which fibrinogen has been removed by clotting is called serum.

Red blood cells (erythrocytes, RBCs) are the most numerous cells, about 5 million per cubic millimetre of blood, 4.5 to 5.5 million in men and slightly fewer in women. They are tiny biconcave discs about 7 micrometres across, and in mammals they have no nucleus when mature, which leaves the whole cell free to be filled with haemoglobin, the red iron-containing protein that combines loosely with oxygen. Their job is to carry oxygen from the lungs to the tissues and to help carry carbon dioxide back. The biconcave shape gives a large surface for gas exchange and lets the cell bend to squeeze through capillaries. RBCs are made in the red bone marrow, live about 120 days and are destroyed in the spleen and liver; a shortage of them or of haemoglobin is anaemia.

White blood cells (leucocytes, WBCs) are larger, colourless, nucleated cells, far fewer in number, about 5,000 to 10,000 per cubic millimetre, but they increase sharply during infection. They are the defence force of the body. Some (neutrophils and monocytes) are amoeboid and engulf bacteria by phagocytosis, squeezing out of the capillaries into infected tissue to do so; pus is largely dead white cells. Others (lymphocytes) produce antibodies that neutralise germs and toxins and remember them for next time, which is how vaccination works. Eosinophils fight parasites and take part in allergy; basophils release histamine.

Platelets (thrombocytes) are small cell fragments without nuclei, about 2 to 3 lakh per cubic millimetre, budded off from giant cells in the bone marrow. When a blood vessel is cut they gather at the wound, stick together and release substances that convert the soluble plasma protein fibrinogen into a mesh of insoluble fibrin threads, in which cells are trapped to form a clot. Clotting stops bleeding and keeps germs out.

Blood as a whole thus transports oxygen, carbon dioxide, food, wastes and hormones, defends against disease, regulates body temperature and pH, and seals wounds. The related fluid connective tissue lymph is a colourless fluid that drains from the tissues into lymph vessels, contains lymphocytes, carries absorbed fats and returns fluid to the blood.

Muscular tissue is the tissue of movement. Its cells, called muscle fibres because they are long and thin, contain special contractile proteins, actin and myosin, arranged as fine threads (myofibrils) along the length of the cell. When the muscle is stimulated the actin and myosin threads slide over one another and the fibre becomes shorter and thicker; when the stimulus stops the fibre relaxes and can be stretched back by another muscle. Muscles can only pull, never push, so they work in pairs, one bending a joint and the other straightening it. Muscle contraction uses a great deal of ATP, which is why muscle cells are packed with mitochondria and why muscles produce most of the body's heat. Muscle makes up about 40 percent of the weight of a human body. There are three kinds of muscular tissue: striated, smooth and cardiac.

Striated muscle is also called skeletal muscle because it is attached to the bones by tendons and moves the skeleton, and voluntary muscle because it contracts when we will it to. It forms the muscles of the limbs, trunk, face, tongue and diaphragm and the muscles that move the eyes. Its fibres are very long, cylindrical, unbranched cells, from a few millimetres to 30 cm long, formed in the embryo by the fusion of many cells, so that each fibre has many nuclei lying just under the cell membrane at the edge of the fibre. Under the microscope the fibres show regular alternating light and dark bands or stripes across their width; these are the striations that give the tissue its name, and they come from the orderly arrangement of the actin and myosin threads in the myofibrils. Each fibre is enclosed in a delicate membrane called the sarcolemma, and the fibres are bound into bundles by connective tissue that runs on to form the tendon.

Striated muscle contracts quickly and powerfully but tires easily; you cannot hold a heavy bag at arm's length for long. It is controlled by the somatic nervous system through nerves from the brain and spinal cord, and each fibre is stimulated by a nerve ending; if the nerve is cut the muscle is paralysed and wastes away. Striated muscle is responsible for locomotion, posture, facial expression, speech, breathing movements and the maintenance of body temperature by shivering.

You can see striated muscle fibres by teasing a small piece of meat (which is skeletal muscle) apart with needles on a slide, staining with methylene blue and examining under the microscope: long fibres with many nuclei near the edges and faint cross-stripes.

Smooth muscle is also called non-striated, because its fibres show no cross bands, involuntary, because it works without our conscious control, and visceral, because it is found in the walls of the internal organs (viscera). Its cells are spindle-shaped, that is long and thick in the middle and tapering at both ends, much shorter than striated fibres (usually 20 to 500 micrometres), unbranched, with a single oval nucleus in the centre of each cell. The actin and myosin threads are present but are not arranged in regular bands, so the cell looks smooth. The cells are arranged in sheets or layers, often one layer running lengthwise and another circularly, as in the wall of the intestine.

Smooth muscle is found in the walls of the alimentary canal from the oesophagus to the rectum, where its waves of contraction (peristalsis) push food along; in the walls of blood vessels, where it controls their diameter and hence blood pressure; in the bronchi, urinary bladder, uterus and ureters; in the iris of the eye, where it changes the size of the pupil; and attached to the hairs of the skin, where it makes the hair stand up in cold or fear (goose flesh). Smooth muscle contracts slowly, with less force than striated muscle, but it can remain contracted for a long time without tiring; the bladder wall and the walls of arteries stay in tone all day. It is controlled by the autonomic nervous system and by hormones, not by the will.

Cardiac muscle is found only in the wall of the heart. It combines features of the other two. Its fibres are striated like skeletal muscle but involuntary like smooth muscle. The cells are cylindrical, relatively short, with a single central nucleus (occasionally two), and, uniquely, they are branched; the branches of neighbouring cells join end to end at special thickened junctions called intercalated discs, which appear as dark lines across the fibres. The intercalated discs bind the cells firmly and let the electrical impulse for contraction pass rapidly from cell to cell, so that the whole heart wall contracts as one unit.

Cardiac muscle contracts and relaxes rhythmically, about 72 times a minute at rest, throughout life without ever tiring; it is generously supplied with blood and with mitochondria to sustain this. It has its own built-in pacemaker and will keep beating even when all nerves to the heart are cut, though the nervous system and hormones adjust its rate. If cardiac muscle cells die because their blood supply is blocked, as in a heart attack, they are not replaced by new muscle but by scar tissue.

Nervous tissue is the tissue that receives stimuli, converts them into electrical messages called impulses, conducts the impulses at high speed, and so coordinates and controls all the activities of the body. It makes up the brain, the spinal cord and the nerves. Its cells are the most specialised in the body and among the longest.

The unit of nervous tissue is the neuron or nerve cell. A neuron has three parts. The cell body (cyton) contains the nucleus and most of the cytoplasm, with granular material (Nissl granules) that makes the proteins of the cell. From the cell body extend short, branched processes called dendrites, which receive impulses from sense organs or from other neurons and carry them towards the cell body. A single long process, the axon, carries impulses away from the cell body to another neuron, a muscle or a gland. The axon may be more than a metre long, as in the neurons that run from the spinal cord to the toes, and it is often wrapped in a fatty insulating sheath, the myelin sheath, laid down by supporting cells; gaps in the sheath called nodes let the impulse jump from node to node, greatly increasing its speed. The axon ends in fine branches with swollen tips, the nerve endings or synaptic knobs.

Neurons do not touch one another directly. Between the ending of one neuron's axon and the dendrite of the next is a microscopic gap called a synapse. When an impulse reaches the ending it releases a chemical (a neurotransmitter such as acetylcholine) that crosses the gap and starts a new impulse in the next neuron. Synapses ensure that impulses travel in one direction only, from dendrite to cell body to axon.

Neurons are classified by function. Sensory neurons carry impulses from receptors in the sense organs to the brain and spinal cord. Motor neurons carry impulses from the brain and spinal cord to effectors, the muscles and glands. Association or relay neurons, found inside the brain and spinal cord, connect sensory and motor neurons. A nerve is a bundle of many axons (nerve fibres) wrapped in connective tissue, like wires in a cable; a nerve that carries only sensory fibres is a sensory nerve, one with only motor fibres is a motor nerve, and most are mixed.

Besides neurons, nervous tissue contains supporting cells called neuroglia, which are far more numerous than neurons; they hold the neurons in place, feed them, insulate the axons and remove waste. Nervous tissue has very little intercellular material and, once developed, its neurons do not divide, which is why damage to the brain or spinal cord is permanent. The speed of an impulse in a myelinated human nerve fibre may reach 100 metres per second, so that you withdraw your hand from a hot plate in a fraction of a second, before you feel the pain.

The four tissues rarely work alone. Every organ is a partnership of several tissues, each contributing its special ability. Consider the skin, the largest organ of the body. Its outer layer, the epidermis, is a stratified squamous epithelium whose surface cells are dead and keratinised for protection. Beneath it the dermis is dense irregular connective tissue with collagen and elastic fibres giving strength and stretch, richly supplied with blood vessels, sweat and oil glands of glandular epithelium, hair follicles with tiny smooth muscles that raise the hairs, and nerve endings of nervous tissue for touch, pressure, heat, cold and pain. Under the dermis lies adipose tissue that insulates and cushions. Four tissues, one organ.

The stomach tells the same story. Its inner lining is a simple columnar epithelium with gastric glands secreting acid and enzymes; beneath is areolar connective tissue carrying blood vessels; then three layers of smooth muscle that churn and mix the food; and a network of nerves that controls secretion and movement. The heart is cardiac muscle lined inside by squamous epithelium (endothelium), covered outside by a serous epithelium, held together by connective tissue including fibrous rings and valves, and regulated by nervous tissue. The tongue is striated muscle covered by stratified epithelium containing taste buds.

This is why we say that the levels of organisation are cell, tissue, organ, organ system and organism: the stomach, intestines, liver and pancreas together form the digestive system, and all the systems together make the animal.

To identify a tissue on a slide or in a diagram, ask a few questions in order. Are the cells tightly packed with little between them? Then it is epithelium; decide squamous, cuboidal or columnar by cell shape, simple or stratified by layers, and look for cilia or goblet cells. Are there few cells in a lot of matrix or fibres? Then it is connective tissue; a jelly with loose fibres is areolar, ring-shaped cells with a big drop are adipose, parallel dense fibres are tendon, cells in lacunae in a smooth matrix are cartilage, cells in lacunae arranged in rings around a canal are bone, and cells floating in fluid are blood. Are the cells long fibres? Then it is muscle; stripes with many nuclei at the edge are striated, spindle cells with one central nucleus and no stripes are smooth, branched striped fibres with intercalated discs are cardiac. Are there cells with long branching processes? Then it is nervous tissue.

Finally, remember why the animal has these tissues and the plant does not: the animal moves, hunts, escapes and regulates its body, so it needs muscle to move, nerve to coordinate, blood to deliver fuel fast, and living, self-repairing supporting tissue rather than dead wood. The tissues of an organism are its way of life written in cells.