thumb|right| Ommatidium: A – cornea, B – crystalline cone, C & D – pigment cells, E – rhabdom, F – photoreceptor cells, G – membrana fenestrata, H – optic nerve
thumbnail|Ommatidia of a [[krill]]
The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlaid with a transparent cornea. Each ommatidium is innervated by one axon bundle (usually consisting of 6–9 axons, depending on the number of rhabdomeres) and provides the brain with one picture element. The brain forms an image from these independent picture elements. The number of ommatidia in the eye depends upon the type of arthropod and range from as low as five as in the Antarctic isopod Glyptonotus antarcticus, or a handful in the primitive Zygentoma, to around 30,000 in larger Anisoptera dragonflies and some Sphingidae moths.
Description
Ommatidia are typically hexagonal in cross-section and approximately ten times longer than wide. The diameter is largest at the surface, tapering toward the inner end. At the outer surface, there is a cornea, below which is a pseudocone that further focuses the light. The cornea and pseudocone form the outer ten percent of the length of the ommatidium.
The specific organ of ommatidia, or eye units, vary between different organisms. The butterfly compound eye consists of multiple ommatidia, each of which consist of nine photoreceptor cells (numbered from R1–R9), and primary and secondary pigment cells. Nymphalid butterflies have the simplest eye ommatidium structure, consisting of eight photoreceptor cells (R1–R8) and a tiny R9 cell organized into a different tier. the Drosophila compound eye is a simple repetitive pattern of 700 to 750 ommatidia, initiated in the larval eye imaginal disc. Each ommatidium consists of 14 neighboring cells: eight photoreceptor neurons in the core, four non-neuronal cone cells and two primary pigment cells. A microlens array can be seen as an elementary, biomimetic analogy of ommatidia.
Mechanism of eye determination
Retinal cell fate determination relies on positional cell–cell signaling that activates signal transduction pathways, rather than cell lineage. Cell–cell signals released from R8 photoreceptors (already differentiated retinal cells) of each ommatidium are received by neighboring progenitor retinal cells, stimulating their incorporation into developing ommatidia. The undifferentiated retinal cells select their appropriate cell fates based on their position with their differentiated neighbors. The local signal, Growth Factor Spitz, activates the epidermal growth factor receptor (EGFR) signal transduction pathway, and initiates a cascade of events that will result in transcription of genes involved in cell fate determination. This process leads to the induction of cell fates, starting from the R8 photoreceptor neurons and progresses to the sequential recruitment of neighboring undifferentiated cells. The first seven neighboring cells receive R8 signaling to differentiate as photoreceptor neurons, followed by the recruitment of the four non-neuronal cone cells.
See also
- Pseudopupil
- Arthropod eye
- Apposition eye
- Superposition eye