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Gland
 
 
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A gland is an organ
Organ (anatomy)

In biology, an organ is a biological tissue that performs a specific function or group of functions. Usually there is a main tissue and sporadic tissues....
 in an animal's body that synthesizes a substance for release such as hormones or breast milk
Breast milk

Breast milk refers to the milk produced by a mother to feed her baby. It provides the primary source of nutrition for newborns before they are able to eat and digest other foods; older infants and toddlers may continue to be breastfeeding....
, often into the blood
Blood

Blood is a specialized bodily fluid that delivers necessary substances to the body's Cell s ? such as nutrients and oxygen ? and transports waste products away from those same cells....
stream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland
Exocrine gland

Exocrine glands are glands that secrete their products into duct s . They are the counterparts to endocrine glands, which secrete their products directly into the bloodstream or release hormones that affect only target cells nearby the release site....
).

ds can be divided into 2 groups: The type of secretory product of an Exocrine gland may also be one of three categories:

y gland is formed by an ingrowth from an epithelial
Epithelium

In biology and medicine, epithelium is a Biological tissue composed of cell s that line the cavities and surfaces of structures throughout the body....
 surface.






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Gray1026
A gland is an organ
Organ (anatomy)

In biology, an organ is a biological tissue that performs a specific function or group of functions. Usually there is a main tissue and sporadic tissues....
 in an animal's body that synthesizes a substance for release such as hormones or breast milk
Breast milk

Breast milk refers to the milk produced by a mother to feed her baby. It provides the primary source of nutrition for newborns before they are able to eat and digest other foods; older infants and toddlers may continue to be breastfeeding....
, often into the blood
Blood

Blood is a specialized bodily fluid that delivers necessary substances to the body's Cell s ? such as nutrients and oxygen ? and transports waste products away from those same cells....
stream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland
Exocrine gland

Exocrine glands are glands that secrete their products into duct s . They are the counterparts to endocrine glands, which secrete their products directly into the bloodstream or release hormones that affect only target cells nearby the release site....
).

Types

Glands can be divided into 2 groups:
  • Endocrine glands — are glands that secrete their product directly onto a surface rather than through a duct.
  • Exocrine glands — secrete their products via a duct, the glands in this group can be divided into three groups:
    • Apocrine
      Apocrine

      Apocrine is a term used to classify exocrine glands in the study of histology. Cells which are classified as apocrine bud their secretions off through the plasma membrane producing membrane bound vesicles in the Lumen ....
       glands       — a portion of the secreting cell
      Cell (biology)

      The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
      's body is lost during secretion. Apocrine gland is often used to refer to the apocrine sweat glands
      Sweating

      Perspiration is the production of a fluid, consisting primarily of water as well as various dissolved solids , that is excreted by the sweat glands in the skin of mammals....
      , however it is thought that apocrine sweat glands may not be true apocrine glands as they may not use the apocrine method of secretion.
    • Holocrine
      Holocrine

      Holocrine is a classification of exocrine glands in the study of Histology.Holocrine secretions are produced within the cell followed by the rupture of the plasma membrane, thus releasing the cellular contents into the Lumen ....
       glands       — the entire cell disintegrates to secrete its substances (e.g., sebaceous glands)
    • Merocrine
      Merocrine

      Merocrine is a classification of exocrine glands in the study of Histology.A cell is classified as merocrine if the secretions of that cell are excreted via exocytosis from secretory vesicles opening into a gland's acinus and flowing through an Epithelium duct or ducts and thence onto a bodily surface or into the Lumen ....
       glands       — cells secrete their substances by exocytosis
      Exocytosis

      Exocytosis is the durable process by which a cell directs the contents of secretory Vesicle_ out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane....
       (e.g., mucous and serous glands). Also called "eccrine
      Eccrine

      Eccrine may refer to:* Merocrine* Eccrine sweat glands...
      ."
The type of secretory product of an Exocrine gland may also be one of three categories:
  • Serous glands — secrete a watery, often protein-rich product.
  • Mucous
    Mucous

    Mucous may refer to:* The adjectival form of mucus, a slippery secretion of the lining of various membranes in the body* Mucous membrane, a membrane which secretes mucus...
     glands     — secrete a viscous product, rich in carbohydrates (e.g., glycoproteins).
  • Sebaceous glands — secrete a lipid product.


Formation

Every gland is formed by an ingrowth from an epithelial
Epithelium

In biology and medicine, epithelium is a Biological tissue composed of cell s that line the cavities and surfaces of structures throughout the body....
 surface. This ingrowth may from the beginning possess be 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 divide 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
    Tubular gland

    If a gland retains its shape as a tube throughout it is termed a tubular gland.Tubular glands are further classified as one of the following:...
    .
  • In the second main variety of gland the secretory portion is enlarged and the lumen
    Lumen (anatomy)

    A lumen in biology is the inside space of a tubular structure, such as an artery or intestine. By extension, a lumen can also be the inside space of a cellular component or structure, such as the endoplasmic reticulum....
     variously increased in size. These are termed alveolar
    Alveolar gland

    In contrast to tubular glands, in the second main variety of gland, the secretory portion is enlarged and the lumen variously increased in size. These are termed alveolar glands ....
     or saccular glands.


Specific glands

  • List of human endocrine glands
    Endocrine system

    The endocrine system is a system of small organs that involve the release of extracellular signaling molecules known as hormones. The endocrine system is instrumental in regulating metabolism, human development , and tissue and also plays a part in determining Mood ....
    .
  • List of human exocrine glands
    Exocrine gland

    Exocrine glands are glands that secrete their products into duct s . They are the counterparts to endocrine glands, which secrete their products directly into the bloodstream or release hormones that affect only target cells nearby the release site....
    .


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