In animals and plants, cells are grouped together in tissues and organs. Each organ is formed of a mixture of tissues which themselves contain several different cell types that work together to perform the tasks needed for the survival and reproduction of the organism. Connective tissue is characterized by large amounts of extracellular matrix secreted by well-separated cells, providing skeletal tissues such as bone, cartilage, tendons, and ligaments that make up the structural framework of the body. The more sophisticated the organism, the more complex and numerous are its cell types. In evolutionary terms, this allows for the creation of specialist cells that can respond to and survive a wide range of challenges. In this chapter we examine varied examples of cell specialisms that allow an organism to protect itself and respond to its environment.

Cells at surfaces

In both plants and animals there are common features as to how cells group and function together. Starting from the outside, there is a protective layer of cells. In plants this is called the epidermal layer, which secretes a waxy coating or cuticle that helps the plant retain water. In woody species that undergo secondary growth, the periderm, commonly called bark, replaces the epidermis and consists of cork cells giving the plant further protection from pathogens and thermal insulation. Insects are different, producing an exoskeleton made up of layers of chitin, a dense horny waterproof substance providing a protective cover which doubles up as their skeleton. Resembling a suit of armour, the exoskeleton is composed of jointed plates and the membranes connecting these give flexibility to the insect body. The organs and muscles are attached to the inner surfaces of the exoskeleton. In vertebrates (animals having a backbone), body structure is formed by an internal skeleton that is carefully laid down by a group of bone-producing cells (osteoblasts) starting early in development.

Skin in animals follows similar principles to the protective layer of cells in plants but has much more functional complexity and flexibility. All surfaces of our bodies are covered by epithelia, and we can contrast the day-to-day existence of the foot soldiers of the epithelial cells at these surfaces, on the outside and inside. Both are replaced on a daily basis, but in different ways, determined by the job that they fulfll.

Our external epithelium is called the epidermis, which when viewed from the surface is comprised of fiattened plates of keratin called squames ( Figure　13 ). Squames begin life as normal cells in the lower layers of the epidermis but, as they travel towards the surface, they progressively lose all recognizable contents, becoming plates of mainly keratin protein, based on a progressive deposition of protein on the intermediate filaments of the cytoskeleton. Because our skin is warm and moist, it provides an attractive surface that is constantly colonized by bacteria and fungi. Our response to this is to shed the outer layer of squames on a daily basis, together with the attached hitch-hikers. Shedding takes place one cell at a time, not as an entire sheet in the manner of a snake. As cells at the skin surface become detached, they are replaced by cell division in the basal layer of the epidermis. Between the basal layer and the surface (around 14 layers in humans), the differentiating cells are arranged in vertical columns. Basal epidermal cells have in their cytoplasm many bundles of keratin intermediate filaments, as well as actin and microtubule networks, which mediate changes in shape from cuboidal to fiattened as the cells travel upwards, until the cell reaches the upper layers. By this time the whole of the cytoplasm has become a concentrated network of keratin filaments, interspersed with granules packed full of lipids. About halfway up, the nucleus is broken down and reabsorbed. Although the epidermal surface appears quite disorganized, if the ‘loose’ cells are removed, then a striking geometric arrangement is revealed, where the squames have a regular hexagonal shape (Fig ure　13 b, c, d). The squames are not completely flat, as they have facets around their edges where they overlap with their neighbours. Two large flat surfaces and twelve edge facets make each squame a fourteensided solid or tetrakaidecahedron. This is exactly the same minimum area surface confi guration adopted by bubbles in a stable foam. This demonstrates that the shape of cells follows the laws of physics, ensuring the maximum surface coverage for the minimum use of material for each squame. This arrangement also guarantees that the cells at the top fall off as individual cells, because each cell is only free to detach when all its six neighbours have gone, and its own six edges are free ( Figure　13d ). Loss of surface cells one at a time maintains a constant overall thickness, with no tearing which might allow bacteria to penetrate more deeply. Sharp edges and solid geometry are not the first thing one associates with cells, but are there as the most efficient solution for maintaining our external surfaces and as a result of the meeting of natural selection with the laws of physics.

While skin provides a remarkably efficient watertight and mechanical barrier to the external environment, these parameters are exactly the opposite of the requirements of gut epithelium, where we need to optimize our uptake of nutrients, while at the same time inhibiting the uptake of anything potentially harmful. Anything we swallow is subject to a journey of around 30 metres over the course of 35 hours. Ingested food is subjected first to the strongly acidic environment in the stomach, followed by powerful enzymes in the small intestine, then fluid absorption in the large intestine, and the final evacuation of waste material. We will concentrate on the small intestine, where most nutrient uptake occurs. Here, in total contrast to the multiple layers of cells making the barrier of skin, the cells form a layer that is just one cell thick, directly above the network of tiny blood vessels (capillaries) that absorb nutrient material directly into the bloodstream. Patrolling this single cell boundary and guarding against potential entry into the bloodstream by gut dwelling bacteria, is the gut-associated lymphoid tissue. This part of the immune system produces more immune cells (see later in this chapter) than anywhere else in the body at specialized areas called Peyers patches, reacting to any potential threat in the gut contents ( Figure　14a ). This is a crucial part of our digestive system, because around 1000 different species of bacteria reside in our gut, which together outnumber the total number of cells in our body by a 1000 fold. Most gut bacteria are harmless or even beneficial, and are tolerated by the powerful immune system in the intestine. When we inadvertently ingest harmful organisms, these are dealt with by the large numbers of mononuclear cells stimulated by antigens already in the gut and keeping the gut in a state of defensive readiness, which is called ‘physiological infi ammation’. In most cases, after a few days of unfortunate symptoms, things return to normal. Thus, there is a delicate balance between tolerance and immunity. Over-reaction by our immunological defences leads to allergies and food intolerances, or more serious conditions such as irritable bowel or coeliac disease.

13. Surface views of skin cells at increasing magnifications (a–c), showing the stacked, hexagonal organization, and release of a single squame (d), (e) section through the stack of cells, with the last stage of a cell with a nucleus (nuc) before differentiation into squames (squ) above

The vast majority of the cells lining the small intestine are called enterocytes, although there are other cell types with important functions. Goblet cells secrete the mucus that covers the surface of the entire small intestine. Paneth cells reside in the epithelial crypts (of which more later) and secrete a variety of anti-microbial enzymes. Without the uptake function of the small intestine we would very quickly starve, and without the anti-microbial barrier, we would rapidly die of infection.

14. Gut epithelium. (a) Surface view of villi surrounding three dome-like Peyers patches, (b) a fractured villus, showing its single cell thickness, (c) section through a microvillus, showing the two layers of the membrane and internal actin filaments, (d) surface and edge view of microvilli

In order to maximize the surface area available for absorption, the gut lining is neatly organized into finger-like projections called villi, each made of around 2000 cells ( Figure　14 ). Each villus cell has its luminal surface organized into microvilli ( Figure　14c , d) supported by a central core of actin filaments, forming a ‘brush border’. These specializations increase the area of uptake in the small intestine to something like an entire football field. Gut contents are absorbed across the brush border of the enterocytes, and then diffuse across the capillary walls and into the bloodstream. All this frenzied metabolic activity, as well as exposure to the continual threat of bacterial invasion, has resulted in the life of an enterocyte being a short one, with each cell lasting no more than two or three days before replacement. The supply of new enterocytes comes from invaginations rather like small pockets found at the base of the villi, called crypts. Each villus has five to ten crypts where precursor cells divide at a rate that produces around 1400 cells each day. These new enterocytes migrate towards the tip of the villus, where, having usefully absorbed nutrient for a couple of days, they are then shed at a rate of one every minute. This continual (lifetime) replacement of cells adds up to a staggering production that is equivalent on a yearly basis to about three times the body weight (as worked out in mice). The exact mechanisms of this mass migration are still unknown, although there are several theories, including the idea of pressure generated by cell division in the crypts literally forcing cells upwards. Alternatively, the cells may migrate up the basement membrane in much the same way as migration elsewhere. Having spent their short (but useful) life at the top of the villus, the enterocytes lose attachment to the basement membrane, at which point the detached cells get ejected by crowding of surrounding cells rather like grasping and squeezing a bar of soap until it shoots out of your hand.

Blood cells

Blood is classified as a connective tissue (like bone), but with a liquid rather than solid matrix. This fluid transports nutrients and removes waste around the animal. Insects and crustaceans often have yellow or green ‘blood’ as they absorb oxygen directly into their tissues through small tubes throughout their body and their blood lacks our own red oxygen-carrying protein called haemoglobin.

Blood is formed from a collection of various cell types, which are continuously produced from a small number of stem cells (cells capable of developing into many cell types). Our understanding of how blood cells develop has acted as a model for many other tissue systems. In adult mammals, blood stem cells reside in the bone marrow ( Figure　15 ) and are characterized by a ‘self renewing’ division, in which half of their daughter cells remain as stem cells, with the other half becoming progenitor cells. These progenitor cells divide many thousands of times, progressively undergoing a process of differentiation that changes biochemistry, shape, and size until, finally, a mature blood cell is produced. The site of human blood cell formation changes throughout life. In embryos, it occurs in aggregates of blood cells in the yolk sac (called blood islands). As development continues, blood is produced in the spleen, liver, and lymph nodes. When the bones develop, blood production begins in the marrow of juvenile long bones (thigh and shin) but in adults it moves to the marrow of the pelvis and sternum. Humans make around 150 billion new blood cells per hour. Most are the red cells that carry oxygen around the body. Immature red cells develop into their final stage (erythrocytes) under the infi uence of a protein growth factor (erythropoietin or Epo) switching their protein production over to the two globin genes needed to form haemoglobin ( Figure　15b , c). Red cells live for a little over 100 days in the body. If you live or train for sports at high altitude where oxygen levels are lower, the production of Epo is stimulated and your blood contains more red cells. One of the biggest challenges to keeping sport ‘clean’ has been the scientists’ ability to discover whether sportsmen have gained unfair advantage from the artificial administration of Epo to boost their performance.

15. Blood cells. (a) Cells newly formed in the bone marrow, entering the bloodstream at the central venous sinus, (b) a developing red cell losing its spherical nucleus, (c) a group of developing red cells on the surface of a macrophage, (d) platelets make long extensions to form a clot, and (e) white cells in cultured bone marrow

Battling and defending cells

Specialist cells have evolved to protect multicellular organisms from attack by bacteria, viruses, and parasites. White blood cells perform these defensive and cleaning-up roles within the body. Originally produced in the bone marrow, some of these white cells travel to the spleen, thymus, and lymph glands for their final differentiation. Response times for white cell production are staggering. For example, if we catch flu, our white cell production can be tripled within hours of the infection. Understanding how these blood immune cells protect the body from infection has led to a vibrant area of biomedical sciences called immunology.

The cornerstone cells of the immune system are lymphocytes or T cells (so named because these cells mature in the thymus) and B cells (B from bursa, the organ in which they mature in birds; in other animals the B cell develops in the bone marrow) ( Figure　15a ). T cells are further divided into various classes—memory, helper, cytotoxic, and regulatory or suppressor. T-memory cells carry information about harmful substances and biological infections (known as antigens), long after the body has resolved the infection. They can also be triggered by inoculations. These cells often survive for a lifetime and continuously monitor for the presence of antigens specific to types of infection. Upon re-exposure to this antigen, the T-memory cells rapidly divide and signal to the immune system that there is a problem via proteins on their cell surface. Given the massive numbers of potential antigenic sequences, which can be as small as two or three consecutive amino acids in a protein, a chemically modified sugar molecule, or even the shape of the protein, there are millions of different T-memory cells.

Regulatory T cells are important in the process known as immunological tolerance, which damps down or suppresses reactive T cells if they mistake your own proteins as foreign following an immune reaction. Helper T cells stimulate the growth of other immune cells. If the other T-cell types are the diplomats and generals of the immune system, then cytotoxic and natural killer T cells are the infantry. They recognize virally infected and tumour cell targets and kill them by injecting the target cell with proteins that induce cell death.

Mature B cells produce complex proteins called immunoglobins which combine to form antibodies, each capable of binding to a specific molecular structure. The necessarily massive repertoire of antibodies is created by splicing separate pieces of the large immunoglobin genes together to make immunoglobulin proteins with different active sites, each capable of binding to just one specific antigen (one small part of a protein or sugar structure).

The other major group of white cells are the myeloid cells, more varied in structure than T and B cells and comprised of granulocytes, megakaryocytes, and macrophages. All are involved in immunity and the removal of foreign biological material by different mechanisms to that of the T and B cells. Granulocytes, which contain granules, are further subdivided into neutrophils, eosinophils, and basophils. One litre of human blood contains about five billion neutrophils (around half of all white blood cells). If neutrophils receive a signal from a site of injury, it takes around 30 minutes for them to leave the blood and reach the site of potential infection. Neutrophils are serious killing cells, ferociously engulfing invading bacteria that have been targeted by an antibody and then digesting their target. Once their job is finished they turn into pus cells. The less common eosinophils (from the Latin for their ‘acid loving’ reaction to chemical dyes) are responsible for destroying parasites by injecting them with hydrogen peroxide (commonly used as a hair bleach and disinfectant). Fortunately, eosinophils in the circulation can only live for a few hours when activated. Eosinophils are mediators of allergic responses and are active in the development of asthma. They are also involved in many other biological processes, including graft rejection and cancer. Basophils (they react with alkaline chemical dyes) are found in great numbers at the site of parasite infection, for example ticks. Mast cells were first observed by Paul Ehrlich in the late 19th century. The large granules they contained led to the idea that they existed to feed the surrounding cells, and thus Erlich named them M astzellen (from the German Mast, meaning food). Their granules contain histamine which, when released, triggers allergic reactions such as hay fever.

Megakaryocytes are ten times larger than red blood cells and each produces large numbers of the small platelets that are responsible for blood clotting. They normally account for 0.001% of bone marrow cells. As they mature, the DNA is replicated several times but the cell does not divide, a condition known as polyploidy which allows cells to increase in size. Some cells have as many as 64 copies of their DNA (a normal cell contains just two copies). At this stage, the cell matures and, in response to the protein hormone thrombopoietin, produces ‘pro-platelet’ bodies. The whole megakaryocyte undergoes a controlled ‘explosion’, fragmenting into a few thousand platelets, which often persist as ribbons in the blood vessels. Humans produce a billion platelets per day having a life span of four to five hours. Platelets (the only cells besides red blood cells that lack a nucleus) aggregate as clumps at the endothelial cell lining to prevent blood loss from damaged blood vessels. As they are activated to aggregate they change shape, extending long finger-like processes that interlock with each other ( Figure　15d ). The remaining myeloid cells are large lumbering amoeba-like cells called macrophages. Their role is rubbish collection. They engulf cellular debris and pathogens and swallow them whole (a process known as phagocytosis) before breaking their components down for further use. The average macrophage can digest a hundred bacteria before bursting as it succumbs to its own excesses. Foreign material is phagocytosed whole and its membranes are biochemically degraded, producing small ‘foreign’ protein fragments (antigens). These are then exported to the outer surface of the macrophage where they are ‘presented’ to T cells. Once the T cell has memorized this protein sequence, it rapidly divides, stimulated by a growth factor produced by the macrophage.

What has become clear by examining the types, the functions and molecular mechanisms of immune cells, especially in mammals, is that all are complex, interlinked, and sometimes redundant in function. This allows the body to cleanse itself of infection and to cope with most errant cells that present a threat to the organism.

Responding to the physical world

How have cells evolved to react and respond to external physical, chemical, and biological effects? Not surprisingly, cells both respond to, and make use of the environmental factors of light and gravity. Photosynthesis was established some billion years ago by ancient bacterial precursors of modern cyanobacteria. These photosynthesizing organisms were engulfed by early cells to produce chloroplasts, leading to the evolution of plants. Photosynthesis is a two-stage process involving the chemical trapping of light energy and its conversion to complex carbon–carbon bonded substances such as sugars. These sugar molecules can be used by the plant for growth but also, either directly or indirectly, act as a food source for the growth of all the non-photosynthesizing organisms on the planet. Bacteria of the deep sea and also deep terrestrial cave systems are the only cells that survive in the total absence of sunlight, as they use volcanic or inner earth heat as their energy source and have a biochemistry based on sulphur rather than oxygen. As we discover more about the processes that occurred in the first billion years of Earth’s history, it seems that all bacteria must have sustained themselves in the absence of oxygen. As things cooled down between three and four billion years ago, aquatic bacteria were able to use carbon dioxide in the sunlit oceans to produce oxygen and complex carbohydrates. The rise in atmospheric oxygen content increased in several stages, beginning with a significant rise about 2.5 billion years ago known as the ‘Great Oxygenation Event’, although it was only after further rises following the melting of the so-called ‘Snowball Earth’ glaciers that oxygen levels were sufficient to sustain multicelluar animals. The chemical conditions for the majority of complex life forms were established by about 600 million years ago, perhaps the most truly earth-changing event.

All living organisms respond to light (phototropism) and gravity (gravitropism). The upper parts of plants usually grow away from gravity and towards light. The effect of earth’s gravitational pull on plants was elegantly shown when mosses grown on the International Space Station grew in a spiral fashion instead of their normal upright structure. The growth control mechanism in higher plants relies on the presence of tiny dense starch-filled particles (amyloplasts) found free within the cytoplasm of specialized cells called statocytes. These cells are found in the column of the root and the joining layer of the shoot. Gravity normally pulls these particles downwards in the cell but in microgravity conditions of space they ‘float’, and fail to produce the usual growth pattern. The normal sedimentation of amyloplasts, within a cytoskeletal network of fine actin filaments, is thought to activate a molecular signalling pathway that leads to the redistribution of the plant hormone auxin. Changing the levels of auxin within cells is complex, as it can promote cell elongation in the shoot but inhibit it in the root.

Cells working together

We can learn much about how cells work with each other from studying simple animals such as　Caenorhabditis elegans , a roundworm the size of a comma on this page, less than one millimetre long. The developmental biologist Lewis Wolpert may have called it ‘the most boring animal imaginable’ but the phylum of nematoda to which　C. elegans belongs probably makes up 80% (numerically) of all animals in the world. Although it possesses no brain it does have a simple nervous system, a wiggling mechanism, a digestive tract, and egg-laying capacity. As mentioned in the previous chapter, it begins life with 1090 cells, but 131 are eliminated by apoptosis during development. These nematodes enjoy a soil-borne life of eating bacteria and reproducing usually with themselves as hermaphrodites by sequentially producing sperm and then eggs (only 1 in 2000 worms are truly male). For its size,　C. elegans has a surprisingly large number of genes, around 20,000 (humans have around 24,000). Many of its genes are involved in cell division and 50% of them are shared with the banana. Over a third of its genes have a direct human counterpart. How do we explain this high number of genes in such a simple organism? One suggestion is that there are an infi ated number of chemical receptor genes, allowing the worm to efficiently detect many different smells when hunting its bacterial food. Nematode worms are found in every imaginable soil type and climate. To survive in these widely different ecological niches, nematodes have evolved by collecting more and more protective or adaptive genes to allow them to survive the challenges made by all the varied species of soil bacteria, fungi, and other microbes on which they dine. The development of C. elegans is characterized better than any other multicellular organism, thanks to the Nobel Prize winning work of Sydney Brenner and his team, and we now understand precisely which cell develops into every other cell, including how all the 302 nerve cells connect to each other.

Moving up through the evolutionary scale, the cell development of the fruit fly, D rosophila melanogaster , has also been carefully mapped. Within three hours of fertilization, the fly embryo cells already start to show the first visible signs of differentiation, and the particular organ or tissue that develops depends on the exact position of each cell. Just how various organs and tissues, including brain, blood, fat tissue, thorax, and retina, develop from the embryo has been carefully established. With its extremely well defined genetics, this model organism has helped us to understand exactly how the entire cell lineage of the insect is laid down. In the next chapter, we examine how these embryo-derived cells give rise to all the adult cells found in the body of an organism.

The nervous system

Most organisms have a mechanism of movement to find food or escape from danger. The simple flagellum on a bacterium allows it to swim, whereas insects, fish, and mammals have sophisticated and coordinated sets of muscles, tendons, and nerves that can move the entire body with amazing speed and agility. Feeling and reacting to external stimuli begins with the network of nerve cells. The nematode worm has no brain but does have an interconnected collection of responsive nerve cells that performs a basically similar function to the massively complex organization of the human brain and nervous system.

The human brain contains around ten billion neurons. Each cell forms connections with thousands of others, allowing the brain to store and transmit information by passing electrical signals within the cell and chemical signals (neurotransmitters) between cells in a complex network extending throughout the body. Millions of sensory neurons have receptors that convert stimuli from the environment (light, touch, sound, smell) into electrical signals that feed back to the brain. Other motor neurons send information from the brain to the muscles and hormonesecreting glands. Interneurons mediate the information between the sensory and motor neurons. Neurons have specialized extensions called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body. Extended axons run large distances and are surrounded by a specialized structure called a myelin sheath, made up of cells that wrap the axon in multiple layers of membrane. The myelin sheath acts as insulation to promote the passage of nerve impulses and isolate the nerve axon from external interference. Synapses are the junctions between the cells where chemical or electrical signals are transferred. Neurons are our longest and most long-lived cells. Corticospinal neurons (which connect the motor cortex to the spinal cord) and primary afferent neurons (which extend from the organs such as skin and gut into the spinal cord and up to the brain stem) can be several feet long. Neurons last a lifetime but numbers do decrease with age. Glia cells are closely associated with all neurons, having a protective and nurturing role. We cannot understand the complex functions of vision, consciousness, and memory at the single cell level but only in terms of interconnections between many billions of cells, and perhaps the most extreme emergent property in biology. Important insights have been provided by the continuing study of the nervous system in nematodes, fruit flies, and other simple organisms which will, in time, shed light on how our own brain functions.

Cellular change

Over time, all cells accumulate multiple genetic changes to their DNA sequence. These changes are often the result of radiation damage (ultraviolet light, cosmic rays, radioactivity) or exposure to low levels of toxic chemicals. As they occur at random, the majority will miss the 2% of coding (genetically active) DNA, and have no major effect on the organism. Other rarer mutations that result in useful adaptations may arise by the alteration of single amino acids of a particular protein. This can modify the three-dimensional structure of the protein which can raise, lower, or negate its normal activity. Other changes can result in either complete or partial deletion of genes (including gene control sequences) or a subtraction or duplication of genes resulting in potentially lower or higher amounts of a particular protein. In rare cases, a major rearrangement of the DNA in the nucleus may result in a completely novel hybrid gene. This is the mechanism of evolution at its simplest and if such genetic changes adversely affect a single-cellular organism it dies. Should an advantage be conferred, it will spread throughout a population and eventually become established.

Multicellular organisms are subject to the same evolutionary pressure through DNA changes but each tissue or organ has billions of cells and at any moment of time only a proportion will be subject to genetic insult or change resulting in a disease. For example, if such a change results in a higher rate of cell division then, in time, these cells will eventually overgrow their normal counterparts (as seen in tumour growth). Adaptation relies on more subtle changes in particular cells to respond to environmental changes. Within the complex genome of most organisms there are alternative multiple pathways of proteins which can help the individual cell survive a variety of insults, for example radiation, toxic chemicals, heat, excessive or reduced oxygen. Often this is performed by repairing the damage or slowing the activity of the cell-growth machinery, waiting for the insult to go away. Many of these responsive biochemical pathways have evolved from those found in single cell genomes whose genetic make-up comes from a distant past when the Earth’s climate was far more extreme. In some circumstances, the cells cannot repair damage and die through apoptosis (as outlined in the last chapter). With time, disabling mutations accumulate and coping with their consequences proves too much for the organism. If the result of these faults is tissue or organ failure, this accelerates the demise of the organism. Only if the majority of a population fails to adapt does a species die out. Changes in ‘the cell’ as a driving force in evolution can really only be conjecture—dinosaurs were built of the same building blocks as ourselves, yet they disappeared dramatically, having spent infi nitely longer dominating the earth than our own miserable 10,000 years. It would seem that the increasingly unsteady edifice of emergent properties from combining cells into tissues, tissues into organs, and organs into creatures itself makes the end product infi nitely more sensitive to sudden environmental change than its individual basic building blocks. After all, we know that single cells (sperm and eggs) can be successfully frozen for decades, whereas those individuals who had their corpses (or just heads) frozen in the hope of future revival are merely wasting their money.