In the same way a house is built of simple components, we’ve seen that animals are made up of tissues — cells of similar structure and function.

But how are these tissues put together?

Look at these pictures of the human body.

The basic framework of an animal is the skeleton and other supportive tissues. These tissues are called connective tissues.

On top of this framework we find other types of tissue, such as muscle tissue and nerve tissue.

Covering the delicate internal tissues of the body, and covering the whole outside of all animals, are tissues called epithelia.

In this section we’ll see that the structure of the different tissue types is closely related to the function that the tissues perform.

Epithelial tissue, or epithelium, covers the outside of the body and its organs, and also lines the intestine.

The simplest type of epithelium – cuboidal epithelium - is found in tubes. The kidney, for example, has tubes that carry the body’s waste fluids.

Squamous epithelial tissue is flatter in appearance, and is found in the material lining the body cavities.

Columnar epithelia line the intestines; these cells secrete digestive juices and absorb nutrients.

More complex epithelia include glandular epithelia. A glandular epithelium contains secretory cells. The most common type of glandular epithelium contains cells called goblet cells.

Let’s look at this cross section through a gland on the tongue. This gland is made up of goblet cells that secrete saliva. The saliva goes into a central duct that opens on the surface of the tongue.

Another type of complex epithelium is multilayered, or stratified epithelium.

The outer layers of our skin are comprised of stratified squamous epithelium.

Skin cells divide at the bottom of the epithelium, and the older cells are gradually pushed upward by the division beneath them. Eventually they die and become the outer layer of the skin. These cells are periodically sloughed off and are replaced by the underlying cells.

Connective tissue is the packing material of the body. It suspends organs in place and is also found under the skin.

In loose connective tissue, cells are sparsely distributed through an extracellular matrix.

This tissue doesn’t have the dense, packed structure of epithelia, where the cells are lined up side by side.

There are fibers of collagen and an elastic protein in the extracellular matrix of connective tissue.

The star-shaped cells in connective tissue are called fibroblasts. These cells manufacture fibers of collagen and the elastic protein.

Macrophages are cells that move in amoeba-like fashion through the connective tissue digesting bacteria and the remains of dead cells.

Types of connective tissue differ in the density of the extracellular matrix. Let’s look at some of these, from the least to the most dense.

Although you might not think of blood as a connective tissue, it consists of a very fluid matrix in which the different types of blood cells are suspended.

Adipose tissue is loose connective tissue containing adipose cells, commonly called fat cells. Adipose tissue is distributed to pad and insulate the body, as well as to provide fat as a reserve supply of energy when food is scarce.

Cartilage is much firmer than loose connective tissue. It has more collagen fibers, but it also has some flexibility, making it an ideal material for body parts like joints, ears, and noses!

Bone tissue is a dense network made up of collagen and mineral deposits. This matrix gives bone its rigidity.

Three types of muscle tissue are found in vertebrates.

Muscle cells are highly specialized, elongated cells: they contain bundles of protein fibers involved in contraction.

Skeletal muscle, the best known type of muscle, powers our body’s movements. Cardiac muscle is found in the heart – its fibers form an interconnecting network that coordinates our hearts’ electrical impulses.

Smooth muscle surrounds our intestines and blood vessels. Smooth muscle is capable of slow, prolonged contractions.

The role of the nerve tissue in the nervous system is to sense stimuli and transmit signals to other parts of the body. Nerves perform the same function as the wiring of a house.

Nerve cells are called neurons. At one end of the neuron is a cell body with two or more branches. The main part of the cell body elongates into a long extension called an axon, and it may be surrounded by an insulating sheath of cells.

Now that we’ve looked at the body’s cell and tissue types, let’s consider how the different tissues cooperate to form organs and organ systems.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education