Serotonin is stored in eccentric, electron-opaque granules that are referred to as dense core granules or dense core vesicles.
Can you spot the dense core vesicles in these rabbit platelets?
Some of the vesicles are empty, some are half-filled with an asymmetric density, and some are completely filled.
Electron microscopy is one of the best tools available for viewing the tiny parts of the cell, and it has been very useful for studying the diameter of dense core vesicles, which is between 750-1500 Angstroms.
Dense core vesicles are of interest because they are the storage sites of 5-HT in the cell. In addition to 5-HT, trace amounts of Mg2+, Ca2+, ATP and cyclic AMP have been found in dense core vesicles.
Dense core vesicles can be highly purified and obtained in large quantity from rabbit platelets, which contain 10X more 5-HT than human platelets. Figure 2 below is an electron micrograph of purified 5-HT dense core vesicles from rabbit platelets.
These dense osmiophilic organelles were first identified in the adrenal medulla of the gut. The cytoplasm of enterochromaffin cells stains readily with potassium bichromate, showing fine brown granules as chromium salts. The investigation of these "chromaffin vesicles" in the gut led to the discovery of amine-containing vesicles in the brain (Wolfe et al., 1962).
The electron micrograph below shows dense core vesicles forming a synapse in the cortex of 6-day-old rat. Vesicles near the cell membrane can fuse with it, emptying the contents into the extracellular space or the adjacent cell. The dense core vesicles can be discerned very clearly using electron microscopy.
Dense core vesicles are found in the axon terminals of noradrenaline-, dopamine-, and 5-HT-containing neurons, suggesting that 5-HT may not be the only neurotransmitter that is packaged into these vesicles.
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