Epithelium
Epithelium is a type of tissue that serves various critical roles throughout the human body. It is primarily responsible for covering the body as skin, lining body cavities and organs, and forming glands. Depending on its location, epithelial tissue can provide protection, absorption, excretion, secretion, diffusion, and sensation. The structure of epithelial tissue comprises closely packed cells arranged in sheets or tubes, with one side exposed to air or fluid and the other attached to a basement membrane that connects it to underlying connective tissue. Notably, epithelial tissue is avascular, meaning it lacks blood vessels and derives nourishment from the connective tissue below.
Epithelial tissue is categorized into two main types: simple epithelium, which consists of a single layer of cells, and stratified epithelium, made up of multiple layers. Within these categories, the cells can be classified based on their shape into squamous, cuboidal, or columnar forms. For instance, simple squamous epithelium facilitates diffusion in areas like the lungs, while stratified squamous epithelium forms the outer layer of the skin for protection. The unique characteristics of epithelial tissue, including limited intercellular space and a high regenerative capacity, allow it to maintain its functions effectively despite constant wear and tear.
Epithelium
Epithelium is a type of tissue that is widespread throughout the human body. Epithelial tissue covers the body as skin, lines body cavities and hollow organs, and surrounds glands. It has a variety of functions depending on its location. These functions include protection, absorption, excretion, secretion, diffusion, and reception.
The cells in a layer of epithelial tissue are closely bound to one another and are generally arranged in sheets or tubes. A layer of epithelial tissue has a "free" side that is exposed to either air or fluid. On the other side is a basement membrane that connects the epithelial tissue to the connective tissue underneath it. Epithelial layers do not have blood vessels. They receive nourishment from the connective tissue beneath them.
Epithelium can have one or more layers. The free surface of a layer of epithelial tissue can be covered by additional layers. A single layer of epithelial tissue is called simple epithelium. Epithelial tissue with more than one layer is called stratified epithelium.
Background
The human body consists of four types of tissue: epithelial, connective, nervous, and muscle. Epithelial tissue covers the body, lines organs and body cavities, and forms some glands, such as sweat glands and sebaceous glands. Connective tissue joins together other tissues and supports cells. Connective tissue is under all epithelial tissue. Nervous tissue produces nerve impulses, which are sent from one body part to another.
Three types of muscle tissue are in the human body: skeletal, cardiac, and smooth. Skeletal muscle tissue is attached to bones. It produces movement when it contracts. It has striations, alternating light and dark bands. The movement of skeletal muscle is voluntary, meaning a person controls it. Cardiac muscle tissue is found in the walls of the heart. Its movement is involuntary, meaning the person cannot control it. Smooth muscle tissue does not have striations. It is found in the small intestines and the walls of hollow organs such as the kidneys. Its movement is involuntary.
The four types of tissue comprise cells. The cells that make up epithelial tissue can be squamous, columnar, or cuboidal. Squamous cells are thin and scalelike. Columnar cells are tall. In cuboidal cells, the length and width are equal, like a cube.
Epithelium has many functions, most of which depend on location. Some epithelium, such as the skin, protects the body from toxins and physical trauma. Absorption is a function of epithelium in the digestive tract and some glands. Other functions of epithelium are to secrete hormones into the blood and excrete chemicals between tissues in body cavities. Some epithelium allows materials to pass through it via diffusion or detects sensations.
Epithelial tissue has characteristics that differentiate it from other types of tissue: (1) It has very limited intercellular space, which is the space between cells; (2) epithelial tissue is polarized, which means each cell is attached to an underlying basement membrane; (3) epithelial tissue is made up of an uninterrupted layer of cells that form a sheet; (4) it can have a single layer of cells or multiple layers; (5) it is avascular, which means it lacks blood vessels; and (6) it has mitotic capability, which means it regenerates quickly. This is important because some epithelial tissue, such as the skin, is subjected to nearly constant wear and tear.
Overview
Epithelium is first classified into two broad types: simple and stratified. Simple epithelium consists of one layer of cells, while stratified epithelium consists of multiple layers. Within these two classifications, epithelium is categorized by the type of cells composing it. Simple epithelium classifications include simple squamous, simple cuboidal, simple columnar, and pseudostratified. Stratified epithelium classifications include stratified squamous, stratified cuboidal, stratified columnar, and stratified transitional epithelium.
Simple Epithelium
The following are types of simple epithelium:
Simple squamous epithelium is a flat layer of scalelike cells. It is found in the lining of organs such as the heart and lungs. It allows materials to pass through it by diffusion.
Simple cuboidal epithelium is a layer of cube-shaped cells found in the ducts of the kidneys, salivary glands, and pancreas. Its functions are to secrete and absorb.
Simple columnar epithelium contains cells that are taller than they are wide. It is found in the lining of the digestive tract and in the small intestines. Its functions are to secrete and absorb.
Pseudostratified columnar epithelium is actually simple epithelium; as it is used here, the prefix pseudo- means "false." This tissue looks as if it is multilayered, but actually consists of just one layer. This type of epithelium sometimes has cilia, which are hairlike extensions. When it does, it is called ciliated pseudostratified epithelium.
Pseudostratified columnar epithelium is found in the salivary glands, the upper respiratory tract, and in some parts of the male reproductive system. Its function is to secrete mucous. Ciliated epithelium is in the nose, bronchi, uterus, and fallopian tubes.
Stratified Epithelium
The following are types of stratified epithelium:
Stratified squamous epithelium is the epidermis, or outer layer of the skin. Its function is to protect. The cells in this tissue regenerate very quickly.
Stratified cuboidal epithelium contains cells with an equal length and width. They are found in sweat glands and the male urethra, as well as in other parts of the body. Their main function is to protect.
Stratified columnar epithelium contains tall cells. It is found in the lining of parts of the male urethra and the excretory ducts of glands. Its functions are to protect and secrete.
Stratified transitional epithelium is composed of cells that can change from being cube-like to flat and back again. These cells are connected in such a way that they can stretch and expand. The lining of the bladder is stratified transitional epithelium. When the bladder is full, the cells within the tissue stretch. After urine is released and the bladder is empty, the cells relax and return to their original shape.
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