Histology

What is it?

According to NCI SEER, a tissue is "a group of cells with a similar structure that function together as a unit". Between the cells is a non-living material called the intercellular matrix, whose amount varies, and it may contain salts and fibres that give the tissue its properties.

The training material identifies four main types of tissue: epithelial, connective, muscle and nervous, "each with specific functions".

In the levels of body organisation, tissues come after cells and before organs: an organ according to SEER is an organisation of several types of tissue arranged so that together they perform a special function, such as the stomach.

MedlinePlus Genetics states that cells are the basic building blocks of all living things, and that the human body is made of trillions of cells that build the body's structure, take in nutrients and convert them to energy, and perform specialised functions. The source page did not address how cells come together into tissues and organs.

Why is it done?

Understanding the four tissues clarifies how organs and body systems are built from specialised units, as systems are made of organs, organs of tissues, and tissues of cells (according to SEER).

Key points

**Epithelial tissue:** according to SEER a tissue widespread in the body that forms the covering of all body surfaces, lines body cavities and hollow organs, and is the main tissue in glands. Its functions: protection, secretion, absorption, excretion, filtration, diffusion and sensory reception.

Epithelial cells are tightly packed with very little intercellular material, each cell has a free surface that does not touch other cells, and they are attached to the tissue beneath by a non-cellular basement membrane of carbohydrates and proteins. The cells may be squamous, cuboidal or columnar, in one layer or several layers.

Examples of epithelium mentioned: simple cuboidal (in glandular tissue and kidney tubules), simple columnar (lining the stomach and intestines), pseudostratified columnar (in parts of the respiratory tract and male reproductive ducts), transitional (able to stretch), and glandular (specialised in producing and secreting substances).

**Connective tissue:** according to SEER it binds structures together, forms a framework and support for organs and the body, stores fat, transports substances, protects against disease, and helps repair damage. It is characterised by abundant intercellular material and relatively few cells, its cells multiply but more slowly than epithelial ones, and most types have a good blood supply (and some have none).

Common connective tissue cells: fibroblasts, macrophages and mast cells. The page lists its types: loose connective, adipose, dense fibrous, elastic, cartilage, bone, blood, and areolar connective tissue.

**Muscle tissue:** its cells are able to contract to produce movement of body parts, and it is rich in cells and blood vessels. Its cells are elongated and called muscle fibres, and contain the contractile proteins actin and myosin, usually organised in bundles or layers surrounded by connective tissue.

The three muscle types according to SEER: skeletal (cylindrical multinucleated fibres, striated and voluntary), smooth (spindle-shaped cells with a single central nucleus, non-striated and involuntary), and cardiac (branched fibres with one nucleus per cell, striated with intercalated discs, and involuntary contraction).

**Nervous tissue:** found in the brain, spinal cord and nerves, and coordinates and controls many body activities through electrical communication between cells. Its cells include neurons (dendrites that carry signals to the cell body, a cell body, and an axon that carries them away) and supporting glial cells that bind, insulate and protect neurons and supply them with nutrients.

Limits of the test

The sources of this page are educational materials from NCI SEER, which are introductory summaries; this page does not cover diseases of tissues or histopathological examination.

The general tissues page does not contain details of each type; the details were taken from the sub-pages cited for each tissue. This page does not cover advanced histological details, whose place is histology textbooks.

Common questions