Concepts: Animal tissues and the organ system

1.1 Tissues and the division of labour

A single-celled organism does everything with one cell. A multicellular animal divides the same work among groups of cells, and a group of similar cells together with the material between them is a tissue. Four basic kinds of tissue appear in every complex animal, and the proportions in which they are mixed differ from organ to organ. That mixing is what makes a tissue set build a stomach, a lung, a heart or a kidney. Two or more organs that work together through physical or chemical interaction make an organ system, and the systems together keep the body alive. The heart is the standard example of an organ that holds all four tissues: epithelium lines it, connective tissue gives it frame, muscle pumps it and nerve tissue sets its rhythm.

1.1.1 Epithelial tissue

Epithelium is a sheet of cells that covers a surface or lines a cavity, a duct or a tube. One face of the sheet is free, facing the outside world or a body fluid, and the cells are joined to one another at junctions. Epithelium rests on a basement membrane, and it has no blood vessels of its own, so it draws its food by diffusion from the tissue below. Two broad classes are named by the number of cell layers. A simple epithelium is one cell thick. Its cells may be flat, as in the squamous type that lines the mouth and the alveoli, cube-shaped in the cuboidal type found in kidney tubules and glands, or tall in the columnar type that lines the intestine. Simple epithelia are where exchange and secretion happen. A compound epithelium has more than one layer and stands up to wear; the skin is the example, and its outer layers are shed and replaced. Cells in an epithelium are held together by three kinds of junction: tight junctions seal the space between cells, adhering junctions stick cells together with material such as calcium, and gap junctions leave a channel through which small molecules and ions pass so that cells can communicate.

1.1.2 Connective tissue

Connective tissue links other tissues and gives the body its frame. Its cells are spread out in a matrix, the material between them, and the matrix decides the property of the tissue. Loose connective tissue, called areolar tissue, binds the skin to the muscles below and holds cells in place. Adipose tissue stores fat in large cells and insulates the body. Dense connective tissue packs collagen fibres into cords and sheets, and it forms tendons, which join muscle to bone, and ligaments, which join bone to bone. Cartilage is a firm but flexible tissue; bone is a hard tissue with a mineralised matrix. Blood is the fluid connective tissue: its matrix is plasma, and its cells are red cells, white cells and platelets. Blood carries food, gases and wastes, which is connective work done in a fluid.

1.1.3 Muscular and neural tissue

Muscular tissue is built for movement. Its elongated cells, called muscle fibres, carry proteins that slide past each other and shorten the cell. Skeletal muscle is attached to bone, shows cross-stripes and works under conscious control. Smooth muscle lines the gut and blood vessels and works without conscious control. Cardiac muscle forms the heart, is striped, and beats without rest. Neural tissue is built for conduction. A neuron has a cell body with dendrites that receive signals and an axon that carries the impulse away; neural tissue carries messages between organs and coordinates their work.

1.1.4 Epithelium in one table

The table links shape to place. A flat cell keeps the barrier thin for exchange, a cube-shaped cell has room for secretion, a tall cell has space for absorption, and a stack of layers survives friction. The same four rows also answer the question of why epithelium has so many forms: the form follows the surface it has to protect or serve.

1.1.5 Why the four tissues are enough

Four tissue types look too few for an animal as complex as a frog, but each type can vary its form and its proportion. Epithelium changes shape to line a mouth, a tubule or a skin, connective tissue changes its matrix from soft fat to hard bone, muscle changes its control from voluntary to involuntary, and nerve tissue grows long processes to reach distant organs. The frog's body shows the result: its skin is epithelium over connective tissue, its limbs are skeletal muscle on bone, its gut is smooth muscle lined by epithelium, and its behaviour is run by neurons. The four types are a small alphabet that writes a large number of organs.