Gland
A cell or organ that produces and secretes needed substances.
A gland is a cell or organ in an animal that produces and releases substances the body needs, either into the bloodstream, a body cavity, or onto an outer surface. Glands can also eliminate waste, like urine. They are mainly made of epithelial tissue, with a supporting framework of connective tissue and a capsule.
There are two main types of gland, differing in how they secrete. Endocrine glands have no ducts; they release hormones directly into the spaces between cells, where the hormones enter the bloodstream. Exocrine glands secrete their products through a duct to a body cavity or outer surface.
Every gland develops from an inward growth of an epithelial surface. This growth may start as a tube or as a solid column of cells that later becomes hollow. As it grows, the column may split or branch. If branching is extensive, a compound gland forms (as in the salivary glands or pancreas). Branches usually do not join, except in the liver, where a reticulated compound gland results. In compound glands, the secretory epithelium is at the ends of the branches, while the ducts are lined with less specialized cells.
Glands are classified by shape. If the gland stays tubular throughout, it is a tubular gland. If the secretory portion is enlarged and the lumen expanded, it is called an alveolar or saccular gland.
Functionally, glands are divided into endocrine and exocrine groups. Endocrine glands release substances (hormones) that circulate in the blood, passing through the basal lamina into tiny blood vessels. Examples include the pineal, thymus, pituitary, thyroid, and adrenal glands. Exocrine glands secrete through a duct to an outer or inner body surface, such as the skin or gastrointestinal tract. They are grouped by secretion method: merocrine glands release substances by exocytosis (e.g., sweat glands, goblet cells, salivary glands); apocrine glands lose a portion of the cell during secretion (e.g., mammary glands, some sweat glands); holocrine glands disintegrate entirely to release their product (e.g., sebaceous glands). Exocrine glands are also categorized by product: serous glands produce a watery, protein-rich fluid (e.g., sweat glands); mucous glands produce a viscous, carbohydrate-rich product (e.g., goblet cells); sebaceous glands produce a lipid product (e.g., Fordyce spots, meibomian glands).
Adenosis is any disease of a gland, involving abnormal formation or development of glandular tissue, sometimes tumorous.
- types
- Endocrine and exocrine
- endocrine_examples
- Pineal gland, thymus, pituitary gland, thyroid gland, adrenal glands
- exocrine_methods
- Merocrine, apocrine, holocrine
- exocrine_products
- Serous, mucous, sebaceous
- shape_classifications
- Tubular, alveolar (saccular)
- development
- Formed by ingrowth from an epithelial surface
Lore & Background
Every gland is formed by an ingrowth from an epithelial surface. This ingrowth may initially possess a tubular structure, or in other instances glands may start as a solid column of cells that subsequently becomes tubulated. As growth proceeds, the column of cells may split or give off offshoots, forming a compound gland. In many glands the number of branches is limited, while in others such as the salivary glands and pancreas, a very large structure is formed by repeated growth and subdivision. As a rule, the branches do not unite with one another, except in the liver, where a reticulated compound gland is produced.
Reader's Guide
Glands are fundamental to animal physiology, serving as the body's secretory organs. They are divided into two main types based on function: endocrine glands, which are ductless and secrete hormones directly into the bloodstream via interstitial spaces, and exocrine glands, which secrete products through a duct onto an outer or inner surface. Endocrine glands include the pineal gland, thymus, pituitary gland, thyroid gland, and adrenal glands, and their hormones play an important role in maintaining homeostasis. Exocrine glands are further categorized by their method of secretion—merocrine (exocytosis), apocrine (loss of part of the cell), and holocrine (disintegration of the entire cell)—and by their product, such as serous (watery), mucous (viscous), or sebaceous (lipid). Glands are mostly composed of epithelial tissue with a supporting framework of connective tissue and a capsule. Their development begins as an ingrowth from an epithelial surface, and they are classified by shape as tubular or alveolar (saccular). Clinically, adenosis refers to any disease of a gland involving abnormal formation or development of glandular tissue, sometimes tumorous.
Did You Know?
- Endocrine glands secrete hormones directly into interstitial spaces to be taken up into the bloodstream.
- Exocrine glands secrete through a duct onto an outer or inner surface, such as the skin or gastrointestinal tract.
- The liver is an exception to the rule that gland branches do not unite, forming a reticulated compound gland.
- Holocrine glands, such as sebaceous glands, secrete by the entire cell disintegrating.
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