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Circadian rhythmicity plays a critical role in the behavior and physiology of nearly all organisms.  There are 4 well-established characteristics of circadian rhythmicity including a free running period (FRP) of about 24 hours, the ability to entrain to environmental cues, temperature compensation, and endogeny.  Although circadian rhythms are coordinated in the brain, almost every cell has been found to contain its own circadian clock, including individual brain cells.

Our group wanted to know if circadian clocks could be found in astrocytes. Astrocytes are star-shaped glial cells located in the brain and spinal cord near nutrient-filled vesicles, allowing them to produce ATP and maintain extracellular ion balance.  Previous data from our lab suggested that astrocytes contain photoreceptors, so this project aimed to determine if astrocytes located close to light-accessible regions of the brain, are capable of transducing light input directly.  To determine this, we collected and cultured astrocytes from 15 day old chick embryos, and placed them in a 12 hour light/12 hour dark cycle for 3 day entrainment.  Afterwards, photographs were taken of the astrocytes every 4 hours for 36 hours. The cell’s total area and process lengths were measured and recorded. It was found that both measurements increased during photoperiod and troughed during scotoperiod, showing that the astrocytes do contain photoreceptors and possibly express circadian rhythmicity. This conclusion could be reinforced by running the experiment again but with a dark/dark light cycle to determine if the astrocytes expressed circadian or diurnal rhythmicity.