Tissues are sets of the same cells carrying out particular functions these include; epithelial, connective, muscle and nervous.
Epithelial tissues are the inside layer of internal and external surfaces and body cavities, including ducts carrying secretions from glands. They may be composed of some layers of cells, called compound epithelia or just a single layer which is known as simple epithelia. The lowest or bottom layer of cells is attached to the basement membrane for support and connection. Part of the basement membrane is concealed by the epithelia but they are supplied with oxygen and nutrients from deeper tissues by diffusion which is the passage of the molecules from a high concentration to a low concentration. As they are surface tissues and exposed to friction, their ability for growth and repair is greater than other tissues and generally occurs during sleep.
Simple epithelial cells may be squamous, cuboidal, columnar or ciliated. Squamous epithelial cells are very flat, with each nucleus forming a lump in the centre. The word ‘squamous’ means ‘scaly’, referring to the flatness of the cells. They fit jointly, rather like crazy paving. Clearly, such delicate thin cells cannot offer much protection and their chief function is to allow materials to pass through via diffusion and osmosis. Simple squamous epithelium is found in the walls of; lung alveoli, blood capillaries and Bowman’s capsule of nephrons. As their names propose, cubiodal cells are cube-shaped, with spherical nuclei. They often line ducts and tubes and can allow materials to pass through in a similar way to squamous epithelia. They often occur in glandular tissues making secretions, these can be found in; Kidney tubules sweat ducts and glands like the thyroid gland and breast tissues.
Columnar epithelial cells are much taller, with slightly oval nuclei. They can often be linked with microscopic filaments known as cilia and are then called ciliated epithelia. Cilia move in