Dictionary > Exogen

Exogen

Definition
noun
(physiology) The phase of the hair cycle in which the hair is shed
(botany) Any of the plants characterised by having c wood bark, pith, commonly netted-veined leaves, and two (or, very rarely, several in a whorl) cotyledons
Supplement
In hair anatomy and physiology, the exogen pertains to a phase in the hair cycle characterized by the shedding of the hair. The hair cycle is the cyclical phases of hair growth. It is comprised of the following stages: anagen, catagen, telogen, and exogen. Exogen (or teloptosis) is the stage of the hair cycle when the hair is shed. It occurs as a stage following telogen. Telogen is the when the hair follicle is dormant and the hair shaft completely detaches from the dermal papilla. Exogen, though, may occur while a new hair shaft enters the next anagen phase and start a new hair cycle.
In botany, exogen pertains to a group of plants characterised by having c wood bark, and pith, the wood forming a layer between the other two, and increasing, if at all, by the animal addition of a new layer to the outside next to the bark. The leaves are commonly netted-veined, and the number of cotyledons is two, or, very rarely, several in a whorl. The exogens are distinguished from endogens. The endogens are plants that increase in size by internal growth and elongation at the summit, having the wood in the form of bundles or threads, irregularly distributed throughout the whole diameter, not forming annual layers, and with no distinct pith. The terms exogen and endogen though are not as widely used as before and at present refer to the dicots and monocots, respectively.


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