Gland

A gland is a specialized cell or organ within an animal's body. Its primary purpose is to produce and secrete substances required by the organism. These substances can be released into the bloodstream or into a body cavity. Glands may also function to remove unwanted substances, such as urine, from the body. Most glands consist mainly of epithelial tissue. They typically possess a supporting framework made of connective tissue and a protective capsule.

Glands develop through a specific growth process starting from an epithelial surface. This surface grows inward to form the gland. Initially, this ingrowth might possess a simple tubular structure. In other cases, a gland may begin as a solid column of cells. This column then becomes tubulated as it develops. As growth continues, the cell column may split or produce offshoots. This branching process creates what is known as a compound gland.

The complexity of a gland is often defined by its branching and shape. In compound glands, the secretory epithelium forms the terminal portion of each branch. The parts of the gland that unite to form ducts are lined with less modified epithelial cells. Most gland branches do not unite with one another. The liver is a notable exception because it produces a reticulated compound gland. Glands are also classified by their physical shape. Tubular glands maintain a tube shape throughout. Alveolar or saccular glands have an enlarged secretory portion with increased lumen size.

Functionally, glands are divided into two major groups: endocrine and exocrine. The primary difference lies in how they secrete their products. Endocrine glands are ductless systems. They secrete products called hormones through a basal lamina. This basal lamina is a layer surrounding the glands. More than a million tiny blood vessels attach to this layer. These hormones enter the interstitial spaces and are taken up by the bloodstream. This process is vital for maintaining homeostasis, which is the body's steady internal balance.

Specific examples of endocrine glands include the pineal, thymus, pituitary, and thyroid glands. The two adrenal glands are also part of this endocrine group. In contrast, exocrine glands use a structure called a duct. These ducts carry secretions onto an outer surface, like the skin, or an inner surface, like the gastrointestinal tract. In exocrine glands, secretion occurs directly onto the apical surface. This method allows for the targeted delivery of substances to specific areas of the body.

Exocrine glands are further categorized by their method of secretion. Merocrine glands use a process called exocytosis to release substances. Examples include salivary glands, tear glands, and major human sweat glands. Apocrine glands involve the loss of a portion of the cell's body during secretion. While often used to describe certain sweat glands, scientists believe some may not use this exact method. Examples include mammary glands and sweat glands in the armpit or pubic region. Holocrine glands are different because the entire cell disintegrates to release its contents. Sebaceous glands, such as the meibomian glands, are examples of holocrine glands.

Exocrine glands can also be grouped by the type of product they create. Serous glands produce a watery, protein-rich fluid. Sweat glands are a common example of this type. Mucous glands secrete a viscous product that is rich in carbohydrates, such as glycoproteins. Goblet cells function this way. Sebaceous glands, also known as oil glands, secrete a lipid product. Examples include Fordyce spots. If a gland undergoes abnormal formation or development, the condition is called adenosis. This abnormal tissue can sometimes become tumorous.