As secretion ( latin secretio "segregation", verb secrete of lat. Secernere "secrete") the delivery of liquid substances which is (unlike the excreta in the excretion meet) a specific function, by individual cells or glands , respectively. The secretion is released involuntarily; The regulation takes place through the vegetative nervous system as well as through feedback mechanisms, in which hormones are partly involved.
According to the direction of the secretion
If the substances are released to internal or external body surfaces, one speaks of exocrine secretion . On the other hand, if they are released into the extracellular space , one speaks of internal secretion; If the secretion enters the bloodstream from there , it is called endocrine secretion .
According to the secretion mechanism
- Eccrine secretion: Transport of small ions or molecules dissolved in the cytosol across the cell membrane by means of transport proteins . When osmotically active particles are transported, water follows, which is often the real purpose of this secretion. Example: sweat glands in humans .
- Merocrine secretion: secretion by exocytosis : the secreted substances are located in the cell in vesicles, the contents of which are released to the outside by fusing with the cell membrane. This is the classic route for protein secretion . Examples: parotid gland , exocrine pancreas , mammary gland (proteins and lactose ). Exocrine merocrine glands also operate eccrine secretion to make the fluid in which the proteins are dissolved.
- Apocrine secretion: secretion vesicles with the surrounding apical cytoplasm are pinched off by the gland cell by part of the cell membrane . Examples: human scent glands , prostate , seminal vesicle , mammary gland (fat droplets).
- Holocrine secretion: The whole cell is released to produce secretion and perishes. The secretion fills the cell, the nucleus becomes pycnotic and finally the cell disintegrates. Examples: sebum glands , crop milk .
According to the function of the secretion
A secretion can fulfill different tasks:
- Keeping the mucous membranes moist and binding pathogens ( e.g. nasal secretions )
- Digestion within the body (for example, saliva , gastric juice , bile )
- Digestion outside the body (for example in spiders )
- Nutrition of offspring ( mammary gland of mammals )
- Thermoregulation ( e.g. sweat )
- Oiling of skin and hair ( sebum glands )
- Secretion of scent marks ( scent glands )
- Addition of nutrient media ( e.g. accessory sex glands )
- Keeping stab wounds or bite wounds open (for example in mosquitoes , leeches and vampire bats )
- Defense against predators with poisons or smelly secretions ( defense secretion , reflex bleeding )
- Poisoning of prey (e.g. snake venom )
A secretion can fulfill several of these functions at the same time: the bile, for example, serves both excretion and digestion.
At the cellular level, the release of a single protein is also known as secretion. These proteins (for example immunoglobulins ( antibodies ) or components of the extracellular matrix ) are then also called secretory proteins.
Furthermore, a distinction is made between constitutive secretion and regulated secretion (see also exocytosis ):
- The constitutive secretion affects proteins without special signals ( signal transduction ). They remain in the so-called default - vesicles , which fuse with the membrane and to secrete the protein.
- In regulated secretion , proteins are packaged in specialized vesicles after they have bound to a specific receptor. They can be modified or stored in these vesicles until a stimulus stimulates the cell to secrete. The proteins secretogranin II and chromogranin B probably have a certain function here, because they ensure that the proteins to be released are aggregated. However, this process has not yet been researched for certain.
Eukaryotic protein secretion
The secretion path is basically the same in all eukaryotic cells: vesicles move the proteins produced from the endoplasmic reticulum (ER) via the Golgi apparatus to the plasma membrane. Only the amount of secretory proteins produced varies. For example, products like collagen are secreted in plant cells, while some enzymes are produced in the human digestive system.
The steps of the secretory path
First, ribosomes synthesize proteins in the membrane of the endoplasmic reticulum. This is called translation . These proteins are either cotranslationally incorporated into the ER membrane (integral proteins) or enter the ER lumen, where their signal peptide is cleaved off. There the proteins are folded and N- glycosylation can take place.
Second, proteins are packaged in transport vesicles. These transport vesicles detach themselves from the donor organelle and fuse with the target membrane in order to release their cargo (= proteins).
The third step is the fusion of the transport vesicles either with the cis -side of the Golgi apparatus or with another vesicle to form a new cis -Golgis. From the cis -Golgi some vesicles also go back to the ER.
The proteins then move within the Golgi from the cis to the trans side, while the Golgi apparatus adds O- glycosylations or modifies the N- glycosylations. If the protein is to be transported to the lysosome , it is also given a mannose-6-phosphate label.
Finally, the modified proteins are either transported from the trans -Golgi to the plasma membrane or to the lysosome. The proteins that were initially synthesized as integral proteins are now still in the membrane.
Different transport vesicles for different routes
In general, all cell compartments have different molecular markers on their membrane. Similarly, transport vesicles have different coats that select different charges. Subunits in the sheaths lead them to different cell compartments. The three main types of coated vesicles are clathrin vesicles, COPI vesicles, and COPII vesicles . COPII vesicles bud from the endoplasmic reticulum. COPI vesicles transport proteins within the Golgi from cisternae to cisternae. Clathrin vesicles transport proteins from the trans -Golgi either to endosomes or directly to the plasma membrane.
- Excretory tissues of plants
- Georg Löffler, Petro E. Petrides (Ed.): Biochemistry and Pathobiochemistry. 7th edition. Springer, Berlin et al. 2002, ISBN 3-540-42295-1 , p. 192.
- Renate Lüllmann-Rauch: pocket textbook histology. 4th edition. Thieme, 2012, ISBN 978-3-13-129244-5 , p. 110 ff.