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.