In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream ( endocrine gland) or into cavities inside the body or its outer surface ( exocrine gland).

Structure

Development

Every gland is formed by an ingrowth from an epithelial surface. This ingrowth may in the beginning possess a tubular structure, but in other instances glands may start as a solid column of cells which subsequently becomes tubulated. As growth proceeds, the column of cells may split or give off offshoots, in which case a compound gland is formed. In many glands, the number of branches is limited, in others (salivary, pancreas) a very large structure is finally formed by repeated growth and sub-division. As a rule, the branches do not unite with one another, but in one instance, the liver, this does occur when a reticulated compound gland is produced. In compound glands the more typical or secretory epithelium is found forming the terminal portion of each branch, and the uniting portions form ducts and are lined with a less modified type of epithelial cell. Glands are classified according to their shape. * If the gland retains its shape as a tube throughout it is termed a tubular gland. * In the second main variety of gland the secretory portion is enlarged and the

lumens The lumen (symbol: lm) is the unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time, in the International System of Units (SI). Luminous flux differs from power ( radiant flux) in that radia ...

variously increased in size. These are termed

alveolar Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * ...

or saccular glands.

Types of glands

Glands are divided based on their function into two groups: Endocrine vs

Endocrine glands

Endocrine glands secrete substances that circulate through the blood stream. The glands secrete their products through

basal lamina The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often incorrectly referred to as the basement membrane, though it does constitute a portion of the basement membrane. The ba ...

into the blood stream. Basal lamina typically can be seen as a layer around the glands to which a million, maybe more, tiny blood vessels are attached. These glands often secrete hormones which play an important role in maintaining homeostasis. The

pineal gland The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep, sleep patterns in both circadian rhythm, circ ...

, thymus gland, pituitary gland,

thyroid gland The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobe (anatomy), lobes. The lower two thirds of the lobes are connected by a thin band of Connective tissue, tissue cal ...

, and the two adrenal glands are all endocrine glands.

Exocrine glands

Exocrine glands secrete their products through a duct onto an outer or inner surface of the body, such as the skin or the

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...

. Secretion is directly onto the apical surface. The glands in this group can be divided into three groups: * Apocrine glands – a portion of the secreting cell's body is lost during secretion. 'Apocrine glands' is often used to refer to the apocrine sweat glands, however it is thought that apocrine

sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin that produce sweat. Sweat glands are a type of exocrine gland, which are glands that produce and secrete substances onto an epithelial sur ...

s may not be true apocrine glands as they may not use the apocrine method of secretion, e.g. mammary gland, sweat gland of arm pit, pubic region, skin around anus, lips and nipples. * Holocrine glands – the entire cell disintegrates to secrete its substances, e.g. sebaceous glands: meibomian and zeis glands. * Merocrine glands – cells secrete their substances by

exocytosis Exocytosis () is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell ('' exo-'' + ''cytosis''). As an active transport mechanism, exocytosis requires the use o ...

, e.g. mucous and serous glands; also called "

eccrine Merocrine (or eccrine) is a term used to classify exocrine glands and their secretions in the study of histology. A cell is classified as merocrine if the secretions of that cell are excreted via exocytosis from secretory cells into an epithelia ...

", e.g. major sweat glands of humans, goblet cells, salivary gland, tear gland and intestinal glands. The type of secretory product of exocrine glands may also be one of three categories: * Serous glands secrete a watery, often protein-rich, fluid-like product, e.g. sweat glands. *

Mucous Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...

glands secrete a viscous product, rich in

carbohydrates In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or may ...

(such as glycoproteins), e.g.

goblet cell Goblet cells are simple columnar epithelial cells that secrete gel-forming mucins, like mucin 5AC. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their secre ...

s. * Sebaceous glands secrete a lipid product. These glands are also known as oil glands, e.g.

Fordyce spots Fordyce spots (also termed Fordyce granules)James, William; Berger, Timothy; Elston, Dirk (2005). ''Andrews' Diseases of the Skin: Clinical Dermatology''. (10th ed.). Saunders. . are visible sebaceous glands that are present in most individuals ...

and

meibomian gland Meibomian glands (also called tarsal glands, palpebral glands, and tarsoconjunctival glands) are sebaceous glands along the rims of the eyelid inside the tarsal plate. They produce meibum, an oily substance that prevents evaporation of the eye' ...

Clinical significance

Histopathology of sclerosing adenosis of the breast

Adenosis is any disease of a gland. The diseased gland has abnormal formation or development of

glandular tissue In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...

Structure

Development

Types of glands

Endocrine glands

Exocrine glands

Clinical significance

Additional images

References

External links