"Eicosanoids are not stored within cells, but are synthesized as required. They derive from the fatty acids that make up the cell membrane and nuclear membrane.
Eicosanoid biosynthesis begins when cell is activated by mechanical trauma, cytokines, growth factors or other stimuli. (The stimulus may even be an eicosanoid from a neighboring cell; the pathways are complex.) This triggers the release of a phospholipase at the cell membrane. The phospholipase travels to the nuclear membrane. There, the phospholipase catalyzes ester hydrolysis of phospholipid (by A2) or diacylglycerol (by phospholipase C). This frees a 20-carbon essential fatty acid. This hydrolysis appears to be the rate-determining step for eicosanoid formation...."
source: http://en.wikipedia.org/wiki/Eicosanoid

"The eicosanoids consist of the prostaglandins (PG), thromboxanes (TX), leukotrienes (LT) and lipoxins (LX). The PGs and TXs are collectively identified as prostanoids. The nomenclature of the prostanoids includes a subscript number which refers to the number of carbon-carbon double bonds that exist in the molecule. The majority of the biologically active prostaglandins and thromboxanes are referred to as series 2 molecules due to the presence of two double bonds. The predominant leukotrienes are series 4 molecules due to the presence of four double bonds. There are, however, important series 1 prostaglandins and thromboxanes as described below...

The eicosanoids produce a wide range of biological effects on inflammatory responses (predominantly those of the joints, skin and eyes), on the intensity and duration of pain and fever, and on reproductive function (including the induction of labor). They also play important roles in inhibiting gastric acid secretion, regulating blood pressure through vasodilation or constriction, and inhibiting or activating platelet aggregation and thrombosis.
The principal eicosanoids of biological significance to humans are a group of molecules derived from the C20 fatty acid, arachidonic acid. Additional biologically significant eicosanoids are derived from dihomo-γ-linolenic acid (DGLA) which is produced in the reaction pathway leading to arachidonic acid from linoleic acid (see Figure below). Minor eicosanoids are derived from eicosapentaenoic acid which is itself derived from α-linolenic acid or obtained in the diet. The major source of arachidonic acid is through its release from cellular stores. Within the cell, it resides predominantly at the C–2 position of membrane phospholipids and is released from there upon the activation of PLA2 (see Phospholipid Metabolism page)..."

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