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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 participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate physiology and / or behavior. Hormones are required for t ...

hormone
s) for release into the
blood Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to anima ...
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

This image shows some of the various possible glandular arrangements. These are the compound tubular, compound acinar, and compound tubulo-acinar glands. 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 Cell (biology), cells. Epithelial tissues line the ou ...
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 glands are categorized by shape, tubular glands contrast with alveolar glands. Tubular glands retain their shape as a tube throughout their length whereas alveolar glands have a saclike secretory portion. Tubular glands are further classified as ...
. * 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 ...
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 participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate physiology and / or behavior. Hormones are required for t ...

hormone
s which play an important role in maintaining
homeostasis In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
. 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 ...

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 upright=1.2, The Hypothalamus-Pituitary Complex. In vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingd ...
,
thyroid gland The thyroid, or thyroid gland, is an endocrine gland in the neck consisting of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thyroid is located at the front of th ...

thyroid gland
, and the two
adrenal gland The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer adrenal cortex ...
s are all endocrine glands.


Exocrine glands

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, ...
s 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, GIT, digestive tract, digestion tract, alimentary canal) is the tract from the mouth to the anus which includes all the organs of the digestive system in humans and other animal Animals (also c ...
. Secretion is directly onto the
apical Apical means "pertaining to an Apex (disambiguation), apex". It may refer to: *Apical ancestor, refers to the last common ancestor of an entire group, such as a species (biology) or a clan (anthropology) *Apical (anatomy), an anatomical term of loc ...
surface. The glands in this group can be divided into three groups: *
Apocrine Apocrine () is a term used to classify exocrine glands Exocrine glands are gland 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 an ...

Apocrine
glands – a portion of the secreting
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
's body is lost during secretion. 'Apocrine glands' is often used to refer to the
apocrine sweat glands An apocrine sweat gland (; from Greek ''apo–'' "away" and ''krinein'' "to separate") is composed of a coiled secretory portion located at the junction of the dermis and subcutaneous fat, from which a straight portion inserts and secretes into t ...

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 Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection ...
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 Holocrine is a term used to classify the mode of secretion in 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 gl ...
glands – the entire cell disintegrates to secrete its substances, e.g.
sebaceous gland A sebaceous gland is a microscopic exocrine 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, cer ...
s: meibomian and zeis glands. *
Merocrine Merocrine (or eccrine) is a term used to classify 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, s ...
glands – cells secrete their substances by
exocytosis 300px, Exocytosis of neurotransmitters into a synapse from neuron A to neuron B.1. Mitochondrion 2. Synaptic vesicle with neurotransmitters 3. Autoreceptor4. Synapse with neurotransmitter released (serotonin) 5. Postsynaptic receptors activated by ...
, 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 In physiology, the term serous fluid or serosal fluid (originating from the Medieval Latin word ''serosus'', from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. La ...
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 membrane A mucous membrane or mucosa is a membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers ...
glands secrete a viscous product, rich in
carbohydrates is a disaccharide found in animal milk. It consists of a molecule of galactose, D-galactose and a molecule of glucose, D-glucose bonded by beta-1-4 glycosidic linkage. A carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) an ...
(such as glycoproteins), e.g.
goblet cell Goblet cells are simple columnar epithelial cell Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. ...
s. *
Sebaceous A sebaceous gland is a microscopic exocrine gland in the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, ...
glands secrete a
lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
product. These glands are also known as oil glands, e.g.
Fordyce spots Fordyce may refer to: People Given name * Fordyce L. Laflin (1824–1887), New York businessman and politician * Fordyce R. Melvin (1832–1915), Wisconsin businessman and politician Surname * Alexander Fordyce (died 1789), Scottish banker * Br ...
and
meibomian gland Meibomian glands (also called tarsal glands) are holocrine Holocrine is a term used to classify the mode of secretion in exocrine gland Exocrine glands are glands that secrete substances onto an Epithelium, epithelial surface by way of a Duct ( ...
s.


Clinical significance

Adenosis is any
disease A disease is a particular abnormal condition that negatively affects the structure A structure is an arrangement and organization of interrelated elements in a material object or system A system is a group of Interaction, interacting ...
of a gland. The diseased gland has abnormal formation or development of glandular tissue which is sometimes tumorous.


Additional images


References


External links

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