Epithelial and connective tissues

What makes a tissue

A tissue is not simply a collection of identical cells. It includes the cells and the material they secrete around themselves, and the two together perform a function the cells could not perform alone. That surrounding material is the extracellular matrix, often shortened to matrix. The amount and composition of the matrix vary enormously. In epithelial tissue there is almost none, so the cells touch one another and form a continuous sheet. In connective tissue the matrix dominates, and the cells are scattered within it. The four tissue families are epithelial, connective, muscle and nervous tissue, and most organs contain several of them. Recognising which family dominates a region tells you what that region is likely to do: cover, support, contract or signal.

For example: The lining of your mouth is mostly epithelium with almost no matrix, so it forms a smooth barrier. The tissue beneath it is mostly connective tissue, with abundant matrix that gives the cheek its flexibility.

Epithelial tissue: covering, lining and secreting

Epithelial tissue forms sheets of closely packed cells that cover the body surface, line internal cavities and tubes, and form glands. Because the cells are tightly joined, epithelium is an effective barrier: it protects underlying tissue, limits water loss, and resists injury and infection. It also specialises in exchange: a thin epithelium allows substances to filter or diffuse across it, and a thicker one protects. Epithelial cells have two distinct sides. The apical surface faces the outside or the lumen, the space inside a tube; the basal surface rests on a thin supporting layer called the basement membrane, which anchors the epithelium to the tissue beneath. Epithelium has no blood vessels of its own, a property described as avascular, so it receives nutrients by diffusion from the connective tissue below.

For example: The epidermis of the skin is a stratified epithelium that resists abrasion, while the single layer lining the air sacs of the lungs is thin enough for gases to cross.

Common misconception: Avascular does not mean inactive. Epithelium is nourished by diffusion from the connective tissue beneath it and is often very metabolically busy.

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Simple and stratified epithelia

Epithelia are classified in two ways: by the number of cell layers and by the shape of the cells. A simple epithelium has one layer, which makes it thin and suited to exchange, absorption or secretion. A stratified epithelium has several layers, which makes it durable and suited to protection in places exposed to friction, such as the skin, mouth and oesophagus. Cell shape adds the second part of the name. Squamous cells are flat and thin, cuboidal cells are about as tall as they are wide, and columnar cells are taller than they are wide. Combining the two descriptions gives names such as simple squamous, simple columnar and stratified squamous. The name predicts both the appearance and the job: thin and flat for exchange, tall and layered for protection or secretion.

For example: Simple squamous epithelium lines the air sacs because it must be thin; stratified squamous epithelium lines the oesophagus because swallowed food scrapes the surface.

Connective tissue: cells in a matrix

Connective tissue is the most varied family in the body, and it is defined less by its cells than by its matrix. The matrix has two parts: ground substance, a gel that fills the space and holds water, and protein fibres. Collagen fibres are strong and resist stretching, elastic fibres stretch and recoil, and reticular fibres form fine supporting networks. The main cell type, the fibroblast, produces the matrix. Because the matrix can be soft, firm or hard, connective tissue ranges from loose packing material to firm cartilage to rigid bone, and it also includes blood, whose matrix is the fluid plasma. This variety explains its many roles: binding organs together, supporting and protecting them, insulating the body, storing energy and transporting substances.

For example: Loose connective tissue beneath the skin allows it to move over the tissues underneath, while dense connective tissue in a tendon transmits the pull of a muscle to bone.

Comparing epithelial and connective tissue

Four contrasts summarise the difference. Cell arrangement: epithelial cells are tightly packed and form sheets; connective cells are scattered in abundant matrix. Blood supply: epithelium is avascular and nourished by diffusion; most connective tissue is well supplied with blood, except cartilage. Position: epithelium covers surfaces and lines cavities and ducts; connective tissue fills spaces, binds structures and forms the body’s supporting framework. Main functions: epithelium protects, absorbs, filters and secretes; connective tissue supports, binds, protects, insulates, stores and transports. These contrasts are not rules to memorise blindly but consequences of the matrix. A tissue with almost no matrix must be a sheet; a tissue with a large matrix must be a supporting or transporting material.

For example: The gut wall has epithelium facing the food, connective tissue holding the layers together, muscle moving the contents and nerve tissue controlling it — four families in one organ.