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The integument is the outer covering of the body, a protective layer that includes the skin and all structures associated with the skin such as; hair, setae, scales, feathers and horns. In the majority of animals, it is tough and pliable, providing mechanical protection from abrasions and the elemen...

The integument is the outer covering of the body, a protective layer that includes the skin and all structures associated with the skin such as; hair, setae, scales, feathers and horns. In the majority of animals, it is tough and pliable, providing mechanical protection from abrasions and the elements, as well as a barrier against infective agents such as fungi and bacteria. It can also prevent water loss, or gain, and protect the underlying layers from the sun's damaging ultraviolet rays. In addition to providing a protective cover, the skin can serve a number of regulatory functions. In endothermic animals, it is vitally concerned with temperature regulation, as most of the body's heat is lost through the skin. The skin contains mechanisms that cool the body when overheated, and it slows heat loss when cool. Skin sensory receptors are vital to provide information about the immediate environment, including the presence of prey or threat. It also has excretory functions, and for many invertebrates, (and some vertebrates) respiratory functions as well. It can also function as a method of camouflage, threat or advertisement through pigmentation. Skin secretions can make an animal attractive or repugnant to other members of the same species or others. Skin can, therefore, affect other individuals through vision, touch and chemical production. Invertebrate integument Many protozoa have delicate cell or plasma membranes as external coverings, although others such as paramecium (see unit 3) have a protective pellicle. Most multicellular invertebrates have more complex tissue coverings, usually the most primitive form being a single layer epidermis, while some have developed a secreted non-cellular cuticle over the epidermis providing additional protection. Molluscan epidermis is delicate and soft, and contains mucous glands, which in shell bearing species secrete the shell. Cephalopod molluscs have a more complex integument, consisting of cuticle, a simple epidermis, a layer of connective tissue, a layer of reflecting cells (iridocytes) and a final thicker layer of connective tissue. Arthropods possess the most complex invertebrate skin, which provides not only protection but also skeletal support. Development of a hard exoskeleton and jointed appendages with the added ability to have muscle attachments is a key feature of the success of this group. Arthropod integument consists of a single-layered epidermis, which secretes a two-layered cuticle: the upper layer is a thin epicuticle (composed of a complex system of proteins and lipids, providing a protective moisture proofing barrier); and the inner layer (procuticle), which is thicker and composed of layers of protein and chitin. In decopod crustaceans, the arthropod cuticle is further stiffened by calcification of the procuticle. In insects, a process called sclerotisation involves the fusing together of the layers in the procuticle, resulting in one of the toughest materials synthesised by animals. Arthropods do not lose this material when they moult as prior to shedding the outer layer, enzymes secreted by the epidermis digest and absorb the material so it can be reused in the new 'shell'. Vertebrate integument The basic plan of vertebrate integument includes a thin, outer stratified epithelial layer (epidermis of ectodermal origin) and a thicker inner layer (dermis), which is of mesodermal origin. Despite being thin and appearing simple in structure, the epidermis gives rise to most derivatives of epidermis, including, hair, feathers, claws and hooves. The dermis is a layer of connective tissue containing blood vessels, collagen fibres, nerves, pigment cells, fat cells, and connective tissues cells (fibroblasts). The dermis supports, cushions, and nourishes the epidermis, which has no blood vessels. The epidermis is made up of layers of undifferentiated cells, with basal cells undergoing frequent mitosis to renew the layers above. Keratin displaces most of the metabolic active cytoplasm in 'older' cells, causing them to die and be sloughed off. Keratinised cells, which are said to be cornified, are highly resistant to abrasion and water diffusion. The epidermal layer becomes especially thick in areas exposed to persistent pressure or wear, such as calluses, the foot pads of mammals, and the scales of reptiles and mammals. When present, true bony structures (such as the heavy bony plate of Palaeozoic ostracoderms, and in sturgeons) are dermal in origin. In fishes, dermal bone form their characteristic scales. In reptiles, dermal bone provides the armour for crocodilians, and contributes to the shells of chelonians. In mammals, dermal bone gives rise to antlers and the bony core of horns. Reptilian scales are not made of dermal bone but the much lighter and flexible epidermal keratin. Mammals continue to utilise keratin's advantages, using it as hair, hooves, claws, and nails. As a result of its keratin content, hair is by far the strongest material in the body; it has a tensile strength comparable to rolled aluminium, as is nearly twice as strong (weight for weight) as the strongest bone. Animal colouration is a fascinating subject. It can be vivid and dramatic when serving as a recognition feature, or as warnings, or subdued when used for camouflage. Integument colour is usually produced by pigments, but in many invertebrates and some birds, colours are produced by the physical structure of the surface tissue, reflecting or not reflecting certain wavelengths of light. More common than these structural colours, are pigments (biochromes), a varied group of large chemicals that reflect light rays. In crustaceans and ectothermic vertebrates, these pigments are contained in large cells with branching processes called chromatophores. Pigment molecules may be concentrated in the centre of the cell, producing no visible colour, or be spread throughout, producing a vivid display. Molluscan chromatophores are different in that each is a small sac-like cell filled with pigment granules and surrounded by muscle cells. When contracted, the muscles spread the pigment sac into a sheet maximising its effect, and the colour becomes invisible. Thus, squids and octopuses can alter their colouration more rapidly than any other animals. The most common animal pigments are the melanins (brown/black); the caroteniods (yellows/reds) are usually obtained from ingesting red/orange plant material. Skeletal systems Skeletons are supportive systems that provide rigidity to the body, surfaces for muscle attachment, and protection for vulnerable body organs. The familiar bone of the vertebrate skeleton is only one of several types of supportive and connective tissues serving various binding and weight bearing functions. The single-celled protozoan ancestors of animals had their weight supported by water and were able to move by cilia or other simple organelles. The evolution of large and more complex organisms (animals) necessitated the development of support and locomotion systems. Animals use their muscular and skeletal systems for support, locomotion, and maintaining their shape. Hydrostatic skeletons are common in invertebrate groups and consist of fluid-filled closed chambers. Internal pressures generated by muscle contractions, cause movement as well as maintain the shape of the animals, such as sea anemones and annelids. Alternating contractions of the circular and longitudinal muscles of the body wall enable a worm to thin and thicken, producing backward moving waves of motion that propel the animal forward. Earthworms and other annelids are helped by septa that separate their body into more or less independent compartments. Worms that lack internal compartments, e.g. the lugworm, arenicola, are helpless if body fluid is lost through a wound. There are many examples throughout the animal kingdom of muscles that produce not only locomotion but also provide support. The elephant's trunk is an example of a structure that lacks skeletal support, but is capable of intricate and strong movement. Along with tongues of mammals and reptiles, and tentacles of cephalopods, trunks are examples of muscular hydrostats, which work, because they consist of incompressible tissue that remains at a constant volume, in the same way as the worm's hydrostatic skeleton. Rigid skeletons are hard elements, which are moved by antagonistic pairs of muscles, and consist of two types, exoskeletons, typical of molluscs, arthropods and many invertebrates, and endoskeletons, characteristic of echinoderms, vertebrates and some cnidarians. Exoskeletons take the form of shells, spicules, or a calcareous, proteinaceous or chitinous plate. It may be rigid as in molluscs or jointed and moveable as in arthropods. Exoskeletons restrict the growth of the animal. Thus, it must shed its exoskeleton (or moult) to form a new one that has room for growth, although a few such as the shells of snails and bivalves grow with the animal. The bulk and weight of the exoskeleton and associated mechanical problems limits the size animals can attain. The arthropod exoskeleton is thought to be a much more efficient arrangement for small animals than a vertebrate endoskeleton, because a hollow cylindrical tube can support more weight without collapsing unlike a solid rod of the same material. The vertebrate endoskeleton is formed inside the body and is composed of bone and cartilage, which are specialised forms of connective tissue. The vertebrate skeleton is composed of two main divisions, axial skeleton, which comprises the skull, vertebral column, strum and ribs, and the appendicular skeleton, which includes the limbs (or fins/wings) and pectoral and pelvic girdles. The vertebrate skeleton has undergone dramatic changes in form. With increased cephalisation, the skull became the most intricate portion of the skeleton and it housed the specialised sense organs and increasingly large brain. Some early fishes had as many as 180 skull bones, but through the loss of some and the fusion of others, their number was reduced during the evolution of the tetrapods. Amphibians and lizards have between 50 and 95 cranial bones, mammals have 35 and fewer, and humans have 29. Bone is a relatively hard and lightweight composite material, formed mostly of calcium phosphate. It has a relatively high compressive strength (weight bearing), but poor tensile strength (sideway/twisting). While bone is essentially brittle, it does have a degree of significant elasticity contributed by its organic components (mainly collagen). Bone has an internal mesh-like structure, the density (and therefore strength) of which may vary at different points. All bones consist of living cells embedded in a mineralised organic matrix that makes up the main bone material. Bone can be either compact or cancellous (spongy). Cortical (outer layer) bone is compact and makes up a large portion of skeletal mass; but, because of its density, it has a low surface area. Cancellous bone has an open, meshwork structure with a relatively high surface area, and forms a smaller portion of the skeleton. Long bones are tubular in structure (e.g. tibia). The central shaft of a long bone (diaphysis) has a central medullar cavity filled with bone marrow. Surrounding the medullar cavity is a thin layer of cancellous bone that also contains marrow. The extremities of the bone are called the epiphyses, and are mostly cancellous bone covered by a relatively thin layer of compact bone. In the growing animal, there is a special area (growth plate or epiphysis line). Activity in this area results in the lengthening of the bone. Short bones (e.g. phalanges) have a similar structure to long bones, except that they have no medullar cavity. Flat bones (e.g. the skull and scapula) consist of two layers of compact bone with a zone of cancellous bone sandwiched between them. Irregular bones are bones, which do not conform to any of the previous forms (e.g. vertebrae). Joints can be classified according to structure, function and appearance. Structurally, joints are classified as the following: Fibrous -- bones, which are joined by tight and inflexible layers of dense connective tissue, consisting mainly of collagen fibres. Movement can occur in young animals, but in mature animals, these joints are fixed. Cartilaginous -- bones, which are connected entirely by cartilage and allow only slight movement. Examples of cartilaginous joints are the mandibular symphysis between the two sides of the lower jaw in felines, the joints between the ribs and the sternum, and the cartilage connecting the growth regions of immature long bones. Another example is in the spinal column, the cartilaginous region between adjacent vertebrae. the movement they allow. Synovial - there is a space (synovial cavity) between the articulating bones. These joints are covered by a fibrous joint capsule, and contain synovial fluid, which reduces friction between the bones. Functionally, they can be classified as the following: Synarthrosis - permit no movement. Amphiarthrosis - permit little movement. Diarthrosis - permit a variety of movements. Only synovial joints are diarthrosis. Synovial joints can be further grouped by their shape, which controls 1. Ball and socket, such as the hip joint. These allow a wide arrange of movement. 2. Condyloid, such as the knee. When the knee is extended, there is no rotation, when it is flexed some rotation is possible. A condyloid joint is where two bones fit together with an odd shape and one is concave, the other convex. 3. Saddle, such as that between the metacarpal and carpal, which permits the same movements as the condyloid joints. 4. Hinge joints, such as the elbow (between the humerus and the ulna), allowing flexion and extension in just one plane. 5. Pivot joints, such as the elbow (between the radius and the ulna). This is where one bone rotates about another. 6. Gliding joints, such as in the carpals of the wrist. These joints allow a wide variety of movement, but not much distance. Tendons attach the muscle to the bone, they are fibrous extensions of the muscle sheath, and ligaments attach the various bones with a joint. Animal movement Movement is a vital characteristic of animals, and occurs from the cellular level to the whole animal. Most animal movement depends on a single fundamental mechanism, contractile proteins, which can change their form to elongate or contract. The most important of these systems is the actomyosin system, which is composed of two proteins, actin and myosin. This is an almost universal biomechanical system found in a myriad of forms from protozoa to vertebrates. However, cilia and flagella are composed of different proteins. Amoeboid movement is characteristic of amoebas and other unicellular forms, but it is also found in the white blood cells of Metazoans. Amoeboid movement takes place by extending and retracting pseudopodia (see Module 3 for more details). Cilliary movements are produced by hair-like processes, which extend from the surfaces of many animal cells. They are a notable feature of ciliate protests, but except for nematodes, in which motile cilia are absent and arthropods (rare occurrence) they are present in all major animal groups. Cilia perform many functions, either in moving small organisms (unicellular ciliates, flagellates and ctenophores) through their aquatic environments, or in propelling fluids and materials across epithelial surfaces of larger organisms. Electron microscopy shows that each cilium has a basal body similar to a centriole, which gives rise to a spherical circle of nine double microtubules arranged around two single microtubules in the centre, which form the structural support and machinery for movement within each cilium. Each microtubule is composed of a spiral array of tubulin protein subunits. The microtubule doublets around the periphery are connected to each other and to the central pair by a complex system of connective elements. Also extending from each doublet is a pair of arms composed of the protein, dynein. These act as bridges between doublets and operate to produce a sliding force between microtubules, attaching, swivelling and releasing to produce movement, much as in muscle. (See below) Muscular movement Muscles are normally arranged in opposition so that as one group of muscles contract, another group relaxes or expands. Skeletal muscle cells are stimulated by acetylcholine, which is released at neuromuscular junctions by motor neurons. This excites the cells and induces muscular contraction via a process known as the sliding filament mechanism. All muscle cells are composed of a number of actin and myosin filaments in series. The basic unit is called the sarcomere. It consists of a central bidirectional thick filament flanked by two actin filaments, orientated in opposite directions. When each end of the myosin thick filament ratchets along the actin filament with which it overlaps, the two actin filaments are drawn closer together. Thus, the ends of the sarcomere are drawn in and the sarcomere shortens. Sarcomeres are connected together by so-called \'Z lines\', which anchor the ends of actin filaments so that the filaments on each side of the Z line point in opposite directions. By this means, sarcomeres are arranged in series. When a muscle fibre contracts, all sarcomeres contract simultaneously so that force is transmitted to the fibre ends. The actual mechanism is fairly complex, a summary is below. However, it will not be necessary to understand all the detail. Myosin is a molecular motor that acts like a ratchet. Chains of actin proteins form high tensile passive \'thin\' filaments that transmit the force generated by myosin to the ends of the muscle. Myosin also forms \'thick\' filaments. Each myosin \'paddles\' along an actin filament repeatedly binding, ratcheting and letting go, sliding the thick filament over the thin filament. 1. Myosin heads bind to the passive actin filaments at the myosin binding sites. 2. Upon strong binding, myosin and actin undergo an isomerisation (myosin molecule rotates at the myosin-actin interface) extending an extensible region in the neck of the myosin head. 3. Shortening occurs when the extensible region pulls the filaments across each other (like the shortening of a spring). Myosin remains attached to the actin. 4. The binding of ATP allows myosin to detach from actin. While detached, ATP hydrolysis occurs 'recharging' the myosin head. If the actin binding sites are still available, myosin can bind actin again. 5. The collective bending of numerous myosin heads (all in the same direction), combine to move the actin filament relative to the myosin filament. This results in muscle contraction. There are several different types of vertebrate muscle, based on the appearance of muscle cells when viewed under the light microscope. Skeletal muscle or 'voluntary muscle' appears striated with alternating light and dark bands and anchored by tendons to bone, and is used to affect skeletal movement such as locomotion and maintaining posture. Smooth muscle or 'involuntary muscle' is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, and blood vessels, and unlike skeletal muscle, smooth muscle is not under conscious control. Cardiac (heart) muscle is also an 'involuntary muscle' but is a specialised kind of muscle found only within the heart. Cardiac and skeletal muscles are 'striated', as they contain sarcomeres and are packed into highly-regular arrangements of bundles; smooth muscle has neither. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles (called intercalated discs). Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions. Smooth and striated muscle are also characteristic of invertebrates, but there are a great many variations on each type, some even have features of both. The thickest muscle fibres known occur in Giant Alaskan King Crabs and measure 3mm in diameter and 6cm long. The fibrillar muscle of insects contract faster than any known vertebrate muscle, allowing wings to beat up to 1000 times a second. Nervous system Irritability is a fundamental property of life; the ability to respond to environmental stimuli is the basis on which the nervous systems developed. The response may be very simple, such as a protozoan moving towards a food particle, or infinitely more complex, such as a bird responding to elaborate signals of courtship. A protist receives and responds to stimuli all within a single cell, the evolution of Metazoans requires an increasingly complex system of communication between cells, organs, organ systems and the animal as a whole. Relatively rapid communication is by neural mechanisms, and involves the propagated electrochemical changes in cell membranes; relatively long term or less rapid adjustments are controlled by hormonal mechanisms. Neurons (nerve cells) are the core components of the brain and spinal cord in vertebrates, and ventral nerve cord, and peripheral nerves in invertebrates. Neurons are typically composed of a nucleated soma, (cell body) and a single axon. The majority of vertebrate neurons receive input on the cell body and dendritic tree, and transmit output via the axon; input and output can be both excitatory and inhibitory. The single axon is relatively consistent in diameter and may be many metres in length. It typically carries information away from the cell body. In vertebrates and advanced invertebrates, the axon has a myelin sheath. Neurons can be classified by structure and function. Most neurons can be anatomically characterised into one of three categories: Unipolar - the dendrite and axon emerge from same process (most sensory neurons). Bipolar - § single axon and a single dendrite on opposite ends of the soma. Multipolar - more than two dendrites. Neurons can also be classified according to function: Afferent neurons (sensory) convey information from tissues and organs into the central nervous system. Efferent neurons (motor) transmit signals from the central nervous system to the effectors cells. Interneurons (connecting) connect neurons within specific regions of the central nervous system. In addition to neurons, there are several types of accessory cells (glial cells) within nervous tissue. 1. Astrocytes - these cells anchor neurons to blood vessels, regulate the microenvironment of neurons, and regulate the transport of nutrients and waste to and from neurons. 2. Microglia - these cells are phagocytic to defend against pathogens. They may also monitor the condition of neurons. 3. Ependymal cells - these cells line the fluid-filled cavities of the brain and spinal cord. They play a role in production, transport, and circulation of the cerebrospinal fluid. 4. Oligodendrocyte - these cells produce the myelin sheath in the CNS, which insulates and protects axons as well as providing for impulse conduction. 5. Schwann cells -- these cells produce the myelin sheath in the PNS. The myelin sheath protects and insulates axons, maintains their microenvironment, and enables them to regenerate and re-establish connection with receptors or effectors. 6. Satellite cells -- these cells surround cell bodies of neurons in ganglia. Their role is to maintain the microenvironment and provide insulation for the ganglion cells. Neurons communicate with one another and to other cells through synapses, where the axon terminal of one cell impinges upon a dendrite or soma of another, or, less commonly, to an axon. Synapses can be excitatory or inhibitory and will either respectively increase or decrease activity in the target neuron. Synaptic transmission occurs when an action potential (electrical impulse) reaches the axon terminal. This causes synaptic vesicles filled with neurotransmitter molecules to fuse with the membrane and then release their contents into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and activate receptors on the postsynaptic neuron.