Glands
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In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as
hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...

hormone
s) for release into the
blood Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to the Cell (biology), cells and transports metabolic waste products away from those same cells. In vertebrates, it is composed ...

blood
stream (
endocrine gland Endocrine glands are ductless glands of the endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating dista ...

endocrine gland
) or into cavities inside the body or its outer surface (
exocrine gland Exocrine glands are glands that secrete substances onto an Epithelium, epithelial surface by way of a Duct (anatomy), duct. Examples of exocrine glands include sweat gland, sweat, salivary, mammary, ceruminous, lacrimal gland, lacrimal, sebaceous, ...
).


Structure


Development

Every gland is formed by an ingrowth from an
epithelial Epithelium is one of the four basic types of animal Tissue (biology), tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed Cell (biology), cells with little Extr ...
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 If gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular or ...
. * In the second main variety of gland the secretory portion is enlarged and the
lumens The lumen (symbol: lm) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...
variously increased in size. These are termed
alveolar Alveolus (pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Alveolus may refer to: In anatomy and zoology in general * Pulmonary alveolus A pulmonary alveolus (plural: alveoli, from Latin ''alveolus'', "littl ...
or saccular glands.


Types of Glands

Glands are divided based on their function into two groups:


Endocrine glands

Endocrine gland Endocrine glands are ductless glands of the endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating dista ...
s 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular ...

basal lamina
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
hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...

hormone
s which play an important role in maintaining
homeostasis In , homeostasis is the state of steady internal, , and conditions maintained by . This is the condition of optimal functioning for the organism and includes many variables, such as and , being kept within certain pre-set limits (homeostatic r ...
. The
pineal gland The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland Endocrine glands are ductless glands of the endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by inte ...

pineal gland
,
thymus gland The thymus is a specialized primary lymphoid organ of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''or ...
, pituitary gland, thyroid gland, and the two adrenal glands are all endocrine glands.


Exocrine glands

Exocrine glands secrete their products through a Duct (anatomy), duct onto an outer or inner surface of the body, such as the skin or the human gastrointestinal tract, gastrointestinal tract. Secretion is directly onto the apical membrane, apical surface. The glands in this group can be divided into three groups: * Apocrine glands – a portion of the secreting cell (biology), cell's body is lost during secretion. 'Apocrine glands' is often used to refer to the sweating, apocrine sweat glands, however it is thought that apocrine sweat glands 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 gland, meibomian and Gland of Zeis, zeis glands. * Merocrine glands – cells secrete their substances by exocytosis, e.g. mucous and serous glands; also called "eccrine", e.g. max sweat gland 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. * mucus, Mucous glands secrete a viscous product, rich in carbohydrates (such as glycoproteins), e.g. goblet cells. * Sebaceous glands secrete a lipid product. These glands are also known as oil glands, e.g. Fordyce spots and meibomian glands.


Clinical significance

Adenosis is any disease of a gland. The diseased gland has abnormal formation or development of epithelium, glandular tissue which is sometimes neoplasm, tumorous.


Additional images


References


External links

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