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is an organ
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
in an animal's body that synthesizes a substance for release of substances such as hormones or breast milk
Breast milk, more specifically human milk, is the milk produced by the breasts of a human female for her infant offspring...
, often into the blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
stream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland
Exocrine glands are a type of ductal glands that secrete their products into ducts that lead directly into the external environment...
Glands can be divided into 2 groups:
- Endocrine glands — are glands that secrete their products through the basal lamina
The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below....
and lack a duct system.
- Exocrine glands — secrete their products through a duct or directly onto the apical surface, the glands in this group can be divided into three groups:
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. This method is also called decapitation secretion...
glands — a portion of the secreting cell
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
's body is lost during secretion. Apocrine gland is often used to refer to the apocrine sweat glands
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 glands
Holocrine glands are a type of secretory gland where the secretory product is formed inside the cytoplasm and the secretion is released by the disruption of the plasma membrane and destruction of the cell. An example is the sebaceous glands of the skin....
— the entire cell disintegrates to secrete its substances (e.g., sebaceous glands)
Merocrine 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 epithelial-walled duct or ducts and thence onto a bodily surface or into the...
glands — cells secrete their substances by exocytosis
Exocytosis , also known as 'The peni-cytosis', is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane...
(e.g., mucous and serous glands). Also called "eccrine".
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.
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
glands — secrete a viscous product, rich in carbohydrates (e.g., glycoproteins).
- Sebaceous glands — secrete a lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
product.These glands are also known as oil glands.
Every gland is formed by an ingrowth from an epithelial
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
surface. This ingrowth may from the beginning possess a tubular structure, but in other instances glands may start as a solid column of cells which subsequently becomes tubulated.
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
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
A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...
variously increased in size. These are termed alveolar
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.
- List of human endocrine glands.
- List of human exocrine glands.The synapse (from Greek, Sin = together = haptein touch, grasp, that is to say connection), means an area of active contact. There is from 1 to 1'00'000 synapses per neurone. The relay that ensures the transmission of nerve impulses is the synapse. There are two types of synapse:
1. The electrical synapses which are found primarily in invertebrates and lower vertebrates, but rarely in mammals.
2. Chemical synapses which are very common in mammals and