Structural Organisation in Animals

An animal body is built up in steps. Cells that look alike and do the same job group into a tissue; different tissues working together form an organ; organs sharing one large task form an organ system. However complicated an animal looks from the outside, all of its tissues belong to just four families: epithelial, connective, muscular and neural. Learn those four well and the rest of this chapter becomes easy.

Epithelial tissue, or epithelium, always has a free surface that faces either the outside world or a body fluid such as the contents of the gut. Its cells are packed tightly with almost no material between them, and the whole sheet rests on a thin supporting layer, the basement membrane. Because the sheet is so compact, anything entering or leaving an organ has to pass through the epithelial cells themselves. That is precisely why the body uses epithelium wherever it needs to control what gets in and what stays out.

Simple epithelium is one cell thick and lines surfaces across which material has to move. Simple squamous epithelium is made of flat, tile-like cells with irregular boundaries; it forms the walls of blood vessels and the air sacs of the lungs, where its thinness makes it an easy diffusion boundary for gases. Simple cuboidal epithelium is made of cube-shaped cells and lines the ducts of glands and the tubular parts of the nephrons in the kidney, where it absorbs and secretes; in the proximal part of the nephron its free surface carries tiny projections called microvilli. Simple columnar epithelium is built of tall, pillar-like cells and lines the stomach and the intestine, again for secretion and absorption. When cuboidal or columnar cells carry cilia on the free surface, the tissue is called ciliated epithelium; it is present in the bronchioles and in the fallopian tubes, where the beating cilia sweep mucus or the egg steadily in one fixed direction.

Some cuboidal or columnar cells become glandular. A single secreting cell, such as a goblet cell in the lining of the gut, is a unicellular gland, while a salivary gland is multicellular. Glands that release their product through a duct are exocrine glands, and their products include mucus, saliva, earwax, oil, milk and digestive enzymes. Ductless glands that pour their hormones straight into the blood are endocrine glands.

Compound epithelium is made of more than one layer of cells, so it plays only a small part in secretion and absorption. Its real job is protection against mechanical rubbing and chemical attack. It covers the dry surface of the skin, the moist lining of the buccal cavity and pharynx, and the inner lining of the ducts of the salivary glands and of the pancreatic duct.

Cells in a tissue are not simply stacked next to one another; specialised cell junctions hold them in place, and three types are worth separating carefully. Tight junctions seal the space between neighbouring cells so that substances cannot leak across the sheet through the gaps. Adhering junctions work like cement, keeping neighbouring cells stuck to each other when the tissue is pulled or stretched. Gap junctions leave an open channel that connects the cytoplasm of two adjoining cells, letting ions, small molecules and sometimes larger molecules pass directly from one cell into the next; this is how cells of a tissue communicate quickly without sending anything through the blood.

Connective tissues are the most abundant and the most widely distributed tissues in the body of a complex animal. Their job is to link and support all the other tissues, and their design is the exact opposite of epithelium. Here the cells are few and scattered, while the matrix that the cells secrete around themselves does most of the work. The matrix contains protein fibres, mainly collagen and elastin, held in a ground substance that may be semi-fluid, tough or even solid. Change the amount, the arrangement and the hardness of that matrix and the same basic plan gives you something as soft as fat and something as hard as bone.

Loose connective tissue has cells and fibres arranged loosely in a semi-fluid ground substance. Areolar tissue is found beneath the skin and serves as a support framework for epithelium. It contains fibroblasts, which make and secrete the fibres, along with macrophages that engulf foreign particles, and mast cells. Adipose tissue is the other loose connective tissue and also lies mainly beneath the skin; its cells are specialised to store fat, so nutrients that are not used immediately are converted into fats and stored here.

In dense connective tissue the fibres and the fibroblasts are packed compactly. When the collagen bundles all run parallel in one direction, the tissue is dense regular connective tissue, and it can take an enormous pull along that single line. Tendons, which attach skeletal muscles to bones, and ligaments, which attach one bone to another, are both of this type. Students mix these two up constantly, so fix it now: tendon ties muscle to bone, ligament links bone to bone. When the fibres point in many different directions instead, the tissue is dense irregular connective tissue and it resists pulling from any side; the skin contains this kind.

Three connective tissues are called specialised because their matrix is unusual. In cartilage the intercellular material is solid but pliable, and it resists compression. Its cells, the chondrocytes, sit enclosed in small cavities inside the matrix that they themselves secreted. Most of the cartilage in a vertebrate embryo is replaced by bone in the adult, but cartilage remains in the tip of the nose, the outer ear, the joints, and between the adjacent bones of the vertebral column, the limbs and the hands.

Bone has a hard, non-pliable ground substance that is rich in calcium salts and collagen fibres, and this pairing is what gives bone its strength: the mineral resists crushing while the collagen keeps the bone from snapping like chalk. The bone cells, or osteocytes, lie in spaces called lacunae. Bone is the main tissue that provides a structural frame to the body. It supports and protects softer tissues and organs, and it works together with the skeletal muscles attached to it to bring about movement; the long bones of the legs, for instance, are weight-bearing. The bone marrow present in some bones is the site where blood cells are produced.

Blood is a fluid connective tissue. Its matrix is the liquid plasma, and suspended in that plasma are red blood cells, white blood cells and platelets. Blood is the main circulating fluid of the body and carries nutrients, gases, wastes and hormones from one place to another. It is grouped with the connective tissues for one clear reason: like every connective tissue, its cells are separated from one another by a matrix that the cells do not fill.

A muscle cell is so long and thread-like that it is usually called a muscle fibre. Every fibre is packed with contractile proteins, so it can shorten when it is stimulated and then relax again. Remember that muscles only pull, never push; the body gets around this by arranging muscles in sets that pull in opposite directions. There are three kinds of muscle tissue, and they differ in exactly three things: whether they show cross bands under the microscope, where they are found, and whether you can control them by will.

Skeletal muscle is closely attached to the bones of the skeleton. Its fibres lie in bundles wrapped in a sheath of tough connective tissue, and along the length of every fibre there are alternate light and dark bands running across it, which is why this muscle is called striated. It is under the control of your will, so it is voluntary, and it contracts quickly and powerfully but tires after a while.

Smooth muscle, also called visceral muscle, is built from spindle-shaped fibres that taper at both ends and show no striations at all. Cell junctions hold the fibres to one another and connective tissue bundles them together. Smooth muscle forms the walls of internal organs such as the blood vessels, the stomach and the intestine. You cannot start or stop it by wishing, so it is involuntary. It contracts slowly and can stay contracted for long periods without tiring, which is what the gut and the blood vessels need.

Cardiac muscle is found in only one place in the whole body: the wall of the heart. Its fibres are striated like skeletal muscle but involuntary like smooth muscle, so it borrows one feature from each. Cell junctions fuse the plasma membranes of neighbouring cardiac cells so that they stick firmly together, and at some of these fusion points there are communication junctions that let the excitation pass straight from one cell into the next. Because of this, the whole sheet of cardiac cells contracts as a single unit instead of fibre by fibre, which is exactly the behaviour a pump requires.

Neural tissue is what gives an animal fast control over everything else. It contains two kinds of cell. Neurons are the excitable ones. When a neuron is suitably stimulated, an electrical disturbance is set up and travels swiftly along its plasma membrane. When that disturbance reaches the endings of the neuron, it sets off events that either stimulate or inhibit the next neuron, or an effector such as a muscle fibre or a gland cell. This is why a signal can run from your fingertip to your brain in a fraction of a second.

The second kind of cell is the neuroglia. Neuroglial cells do not carry impulses of this sort; they support, insulate and protect the neurons and keep the chemical surroundings suitable for them. A point worth remembering is that neuroglial cells make up more than one-half of the volume of neural tissue in the body, so most of the bulk of the nervous system is not made of neurons at all.

To see how tissues combine into organs and organ systems, the frog is studied as a representative animal. The common Indian frog used for this study is Rana tigrina. It belongs to the class Amphibia of the phylum Chordata, and the word amphibian tells you its whole story: it can live both on land and in fresh water.

Frogs are poikilotherms, or cold-blooded animals, which means their body temperature changes along with the temperature of the surroundings instead of being held at a fixed value. They survive extreme weather by hiding rather than by heating or cooling themselves. They take shelter in deep burrows and pass the hot dry summer in a resting state called aestivation, and the cold winter in a resting state called hibernation. Frogs also change the colour of their skin to match the background so that predators do not spot them; this protective change of colour is camouflage, and it is also referred to as mimicry.

The body of a frog is divided into a head and a trunk, and that is all - an adult frog has neither a neck nor a tail. The skin is smooth and slippery because it is coated with mucus, and it is kept moist at all times. That moisture is not a small detail: the frog also uses its skin as a breathing surface, and a dry skin cannot exchange gases. Above the mouth lies a pair of nostrils. The eyes bulge outward and each is covered by a transparent nictitating membrane, which protects the eye while the animal is under water without blocking its view. A little behind each eye, lying flat on the surface of the head, is a membrane called the tympanum, which receives sound signals; a frog has no projecting external ear, so the tympanum is what you see from outside.

A frog has two pairs of limbs. The forelimbs are shorter and end in four digits, while the hind limbs are longer, thicker and far more muscular and end in five digits. The digits of the hind feet are joined to one another by webs of skin. Put those facts together and you can explain the frog's movement in words alone: the long muscular hind limbs deliver the thrust for swimming and for leaping, the webbed hind feet give a wide surface to push against the water, and the short forelimbs take the shock of landing and prop the body up while the animal sits, walks or burrows.

Male and female frogs can be told apart by looking for two features that only the male has. The first is a pair of vocal sacs, which amplify the croaking sound the male makes to attract a mate. The second is a rough, swollen copulatory pad on the first digit of each forelimb, which gives the male a firm grip on the female during breeding. A visible difference in body form between males and females of the same species, like this one, is called sexual dimorphism.

A frog is a carnivore, and flesh is far easier to break down than plant material, so its alimentary canal is short. That is a rule worth carrying with you: a long gut usually means a plant eater, a short gut usually means a flesh eater. The canal begins at the mouth, which opens into the buccal cavity and then continues as the oesophagus, a short tube that leads into the stomach. The stomach narrows into the intestine, which coils and then widens into the rectum, and the rectum opens into the cloaca. The cloaca is a small median chamber and it deserves careful attention, because three different systems empty into it: faecal matter from the gut, urine from the urinary system, and the sperms or ova from the reproductive system all leave the body through the single cloacal aperture.

The frog captures its prey with a bilobed tongue that is attached at the front of the mouth and free at the back, so it can be flicked far forward and pulled back with the insect stuck to it. Two large glands lie outside the canal but pour their secretions into it. The liver secretes bile, which is stored in the gall bladder; bile carries no enzyme of its own but breaks fat into fine droplets so that enzymes can act on a much larger surface. The pancreas produces pancreatic juice, which does contain digestive enzymes. Inside the stomach, hydrochloric acid and gastric juice act on the food, and the partly digested material, now called chyme, is passed on to the duodenum, the first part of the intestine. The duodenum receives bile from the gall bladder and pancreatic juice from the pancreas through the common bile duct, and here the fats are emulsified while carbohydrates and proteins are attacked by enzymes. Digestion is completed in the intestine, whose inner wall is thrown into finger-like folds called villi carrying still smaller microvilli on their cells; these folds multiply the surface available for taking the digested food into the blood. Whatever remains undigested moves into the rectum and leaves through the cloaca.

Breathing in a frog changes depending on where the animal is, and this is the most interesting part of its biology. In water the frog does not use its lungs at all. Oxygen dissolved in the water simply diffuses in through the moist, thin skin, which is richly supplied with blood vessels, and carbon dioxide diffuses out the same way. This is called cutaneous respiration, and it is also how the frog survives the long weeks of aestivation and hibernation, when it lies buried and barely moving.

On land the frog uses three surfaces: the skin, the lining of the buccal cavity, and the lungs. The lungs are a pair of elongated, pink, sac-like structures lying in the upper part of the trunk. Air enters through the nostrils into the buccal cavity and is then pushed on into the lungs. A frog has no diaphragm, so it cannot expand a chest to suck air in the way a mammal does; instead it lowers and raises the floor of its mouth to force air down into the lungs. Breathing with the lungs is called pulmonary respiration, and a frog uses it only when it needs more oxygen than the skin alone can supply.

The frog has a closed circulatory system, which means the blood always remains inside the heart and the blood vessels and never flows loose through open body spaces. Alongside the blood vascular system, the frog also possesses a well-developed lymphatic system made of lymph, lymph channels and lymph nodes.

The heart lies in the upper part of the body cavity, is muscular, and is enclosed in a membrane called the pericardium. It has three chambers: two atria above and a single ventricle below. Two further chambers are attached to it. A triangular sinus venosus joins the right atrium, and it receives blood arriving through the large veins called the vena cava. On the ventral side, the ventricle opens into a sac-like conus arteriosus, from which blood is despatched to the body. Arteries carry blood away from the heart and together form the arterial system, while veins collect blood from the different parts of the body and return it to the heart, forming the venous system. Because there is only one ventricle, the oxygenated blood returning from the lungs and the deoxygenated blood returning from the body are mixed to some degree before being pumped out again. That partial mixing is the cost of having a single ventricle: the blood sent out to the tissues is not fully oxygenated, unlike the blood leaving a four-chambered heart in which the two streams are kept completely apart.

Two special venous routes are present in the frog. In the hepatic portal system, a vein carries blood from the intestine to the liver before that blood is allowed to return to the heart. In the renal portal system, a vein carries blood from the hinder parts of the body to the kidney. Both are called portal systems for the same reason: the blood passes through a second bed of capillaries inside an organ on its way back to the heart, instead of going straight home.

Frog blood is composed of plasma and cells. The cells are the red blood cells or erythrocytes, the white blood cells or leucocytes, and the platelets. One point catches students out again and again: the red blood cells of a frog are nucleated and contain the red pigment haemoglobin, whereas the mature red blood cells of a human have lost their nucleus. Lymph is not the same thing as blood - it lacks some of the proteins present in blood and it contains no red blood cells.

The excretory system consists of the kidneys, ureters, urinary bladder and cloaca. The kidneys are a pair of dark red, bean-shaped organs lying a little towards the posterior end of the body cavity, one on either side of the vertebral column. Each kidney is built of a great many structural and functional units called uriniferous tubules or nephrons, and these separate the wastes out of the blood that is brought to the kidney. A ureter emerges from each kidney. In the male frog the ureter also conducts sperm, and for that reason it is called a urinogenital duct, opening into the cloaca; in the female the ureter and the oviduct open into the cloaca separately, because the female has no such shared duct. A thin-walled urinary bladder lies ventral to the rectum and also opens into the cloaca. The frog gets rid of its nitrogenous waste mainly in the form of urea, which makes it a ureotelic animal.

Control in a frog is shared between two systems that work at very different speeds. The nervous system sends fast electrical messages along nerves, while the endocrine glands send slower chemical messages as hormones carried in the blood. The endocrine glands of the frog include the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenal and the gonads.

The nervous system is organised into three parts. The central nervous system is made of the brain and the spinal cord. The peripheral nervous system is made of the cranial and the spinal nerves, and there are ten pairs of cranial nerves arising from the brain. The autonomic nervous system has sympathetic and parasympathetic parts and looks after the organs that are not under conscious control.

The brain lies inside a bony case, the brain box or cranium, and is divided into fore-brain, mid-brain and hind-brain. The fore-brain carries the olfactory lobes for the sense of smell, a pair of cerebral hemispheres, and an unpaired diencephalon. The mid-brain is marked by a pair of optic lobes, which deal with sight. The hind-brain contains the cerebellum, which coordinates movement and balance, and the medulla oblongata, which leaves the skull through a large opening called the foramen magnum and continues as the spinal cord, itself enclosed and protected inside the vertebral column.

The frog has organs for touch, taste, smell, sight and hearing. Touch is served by sensory papillae in the skin, taste by taste buds, and smell by the nasal epithelium; sight by the eyes and hearing by the tympanum together with the internal ear. Out of all of these, only the eyes and the internal ears are well-organised structures - the rest are simply groups of cells gathered around nerve endings. Each eye is a spherical structure lodged in a socket called the orbit in the skull, and it is a simple eye, meaning it is one single optical unit, unlike the compound eye of an insect which is built from many units. The frog has no external ear at all; the ear serves for balancing as well as for hearing.

In the male frog the reproductive organs are a pair of yellowish, oval testes, held against the upper part of the kidneys by a double fold of peritoneum called the mesorchium. Ten to twelve fine tubes, the vasa efferentia, arise from the testes, enter the kidney of the same side and open into Bidder's canal. From there the sperms travel into the urinogenital duct, which emerges from the kidney and opens into the cloaca, so that they can finally reach the exterior.

In the female there is a pair of ovaries lying near the kidneys, but unlike the testes they have no functional connection with the kidneys at all. A pair of oviducts arises from the ovaries and each opens separately into the cloaca. A mature female can lay 2500 to 3000 ova at one time. Fertilisation is external and takes place in water: the male grips the female with his copulatory pads, the female releases her eggs into the water and the male sheds sperms over them at the same moment. The fertilised egg develops into a swimming larva called a tadpole, which lives in water and then changes into the adult frog through a process called metamorphosis. Frogs are useful animals for people, because they eat large numbers of insects and so protect crops, and because they form an important link in the food chain of ponds and fields.