[1] Photocytes typically occur in select layers of epithelial tissue, functioning singly or in a group, or as part of a larger apparatus (a photophore).
[1] Light production may first be triggered by nerve impulses which stimulate the photocyte to release the enzyme luciferase into a "reaction chamber" of luciferin substrate.
The exact combinations of luciferase and luciferin types found among photocytes are specific to the species to which they belong.
Additionally, a series of action potentials have been shown to lead to the sporadic, discontinuous emission to light.
[3] It has been found that the neurons that control the light mechanism terminate at the tracheal cells rather than the photocytes themselves.
Exposure to neurotransmitters including epinephrine, norepinephrine, and synephrine, results in the emission of light but without any corresponding depolarization of the photocyte membrane.
Gastric cells form a barrier that keep the photocytes away from the opening of the radially canal which they are found to exist along.
As a response to being triggered by an norepinephrine, epinephrine, or phenylephrine, the photocyte exhibits a quick flash and then emits light that slowly fades in intensity.
The amplitude of the quick flash, referred to as the "fast response", was higher when the concentration of neurotransmitter stimulating it increased.
Phentolamine was shown to inhibit the effect of stimulation by phenylephrine completely and of epinephrine and norepinephrine to a lesser degree.
[8] In Amphipholis squamata, bioluminescence has been observed to come from the spines emanating from the arms from photocytes within the spinal ganglia.
Phuphania have even been shown to be able to preserve their ability to produce light even after long periods of hibernation.
It is believed that sympathetic nervous impulses provide the stimulus that causes photocytes to emit light.
The subsequent maturation of the photocytes and intensification of light produced develop rapidly, occurring within ten hours of the first observed instance of bioluminescence.
[6] Luciferins have been shown to be largely conserved among different species while luciferases show a greater degree of diversity.
[12] Overall, the evolution of light producing cells (photocytes) is believed to have happened twice in sharks through convergence.
[12] All five families of luminescent beetle, Phengodidae, Rhagophthalidae, Elateridae, Sinopyrophoridae, and Lampyridae are categorized into the Lampyroid clade.
In fact, every bioluminescent beetle species studied has been shown to use very similar mechanisms for light production in the photocyte.
Biolumiescence in beetles has been shown to serve multiple purposes including the deterrence of predators and the attraction of mates.
[2] Bioluminescence in Amphiura filiformis and other species of sea star is widely believed to function in protection against predators.
[10] The mitochondria is believed to be important in controlling the supply of oxygen available for making light in fireflies.
An increased rate of respiration decreases the intracellular oxygen concentration which reduces the amount available for light production.
[5] It is worth noting that not all bioluminescence in the firefly light organ occurs in the granules of the photocyte.
[16] The photocyte of Arachnocampa luminosa was found to contain a circular nucleus, and large amounts of ribosomes, smooth endoplasmic reticulum, mitochondria, and microtubules.
The mitochondria of the photocytes were found to be very large with abnormally organized cristae surrounding the nucleus of the cell.
Several small vesicles, on the order of 0.25 micrometers, were found in the cell, and differently shaped granules containing diverse contents were also observed.
[18] The photocytes present in Amphipholis squamata have been found to contain a Golgi apparatus and rough endoplasmic reticulum.
Additionally, the tracheolar end organ was found to contain a high concentration of the enzyme nitric oxide synthase.
A reaction among D-luciferin, luciferase, and ATP has been implicated in the mechanism of light production in firefly photocytes.
In addition, the matrix of the type I granule lacks a uniform shape or structure with ferritin distributed throughout.