REFLECTION 4: IMPORTANCE OF COMMUNICATION FOR DEVELOPMENT

Andrea Alonzo 812000494

Ria Ramcharitar 812000177

Kamine Saroop 812002265

Short Distance Communication Between Cells

The Drosophila, also commonly known as the fruit fly, has very complex eyes called Compound Eyes. These compound eyes contain 750 ommatida arranged in a spherical formation. Each ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The pigment cells separate each ommatidium from its neighbors. When visual information enters the eye via the retina, which contains photoreceptors R1-8 in individual ommatidium (six out of the eight photoreceptors are white, R7 is purple and R8 is green).

There are many cell communications happening in the compound eye in order for vision to occur, however we will be focusing specifically on the communication between the photoreceptor cells and its neighboring pigment cells. This will be local signaling since the cells are close to each other.

Autocrine signaling- cell secretes chemical messengers (autocrine agent) which binds to autocrine receptors of the same type of cell. A Photoreceptor cell will use autocrine signaling to communicate with another photoreceptor cell. There are 3 stages of signaling => signal transduction pathway:

·         Reception of the signal by a cell

Communication between R8 and R7: the autocrine agent [transmembrane ligand Bride of Sevenless (Boss)] binds to the receptor known as Tyrosin Kinase Sevenless (SEV).

·         Transduction (from the outside of the cell to the inside)

This mechanism is trasduced by Ras activation within the R7 cell.

·         Response

Recall: R7 is purple photoreceptor cell

Less light will be absorbed so that the fly is not blinded by ambient light.

Paracrine signalingthis is a type of cell-cell communication where cells produces signals to induce a change in neighboring cells (altering the behavior or differentiation of those cells).

·         Reception

·         Transduction

·         Response

Responsible for color vision

Below is a video discussing the structure and function of the ommatidium.

Importance of communication between photoreceptor cells and pigment cells

The red screening pigment of the pigment cells optically isolates the ommatida. The photoreceptor cells contain yellow screening pigment which is responsible for papillary responses. The light sensitive region of the photoreceptor cell is known as the rhabdomere. The Rhabdomere is composed of 30,000 microvilli which are responsible for capturing photons of light to a quantum bump, which is then responsible for the depolarization of the photoreceptor cell membrane. The membrane is packed with approximately 1000 pigment cells that make the Rhabdomere optically dense and provide a guide for light. The pigment cells ensures that only light entering the ommatidium parallel or nearly parallel to its long axis reaches the visual cells and triggers nerve impulses. Expression of pigments in photoreceptor cells determines color vision in the fruit fly.

Below shows the structure of the ommatidia.

intro2 

References:

http://images.nationalgeographic.com/wpf/media-live/photos/000/605/cache/nikon-small-world-2012-fruit-fly-eyeball_60559_600x450.jpg

http://dev.biologists.org/content/126/4/607.full.pdf

http://eol.org/pages/733739/details

http://starklab.slu.edu/eyecolor1.htm

http://neurophilosophy.wordpress.com/2006/10/02/loss-of-spam-de-evolves-the-fruit-flys-compound-eye/

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One response to “REFLECTION 4: IMPORTANCE OF COMMUNICATION FOR DEVELOPMENT

  1. Signalling from the Glia cells!!

    Photoreceptor neurons (also referred to as R cells) in the Drosophila compound eye essentially establish connections in various optic ganglia. If there is an absence of glial cells in the eye disc, photoreceptor axons can express posterior growth however, they are incapable of entering the optic stalk, thus implying that glia play a role in axon guidance. Additionally, several glia adjacent to the entry of the optic stalk are sufficient enough to direct the axons into the stalk, thereby indicating that the glia instruct axons via local communication. In the developing Drosophila visual system, glia migrate to specific sites within the photoreceptor axon fields in order to provide information for photoreceptor axon guidance. Glial migration as well as their development relies on the photoreceptor axons. Glial formation also depends on the activity of Focal adhesion kinase. Dpp is a member of the TGF-β superfamily of ligands. In Drosophila, Dpp manipulates the growth and patterning of imaginal discs. With respect to the eye disc, Dpp plays a role in the initiation of the morphogenetic furrow, which instigates photoreceptor differentiation. Dpp first binds to Type I serine threonine kinase receptors, Saxophone and Thick veins (Tkv), and the Type II receptor Punt after which the Type I receptors become phosphorylated by the Type II receptor, causing the activation of the receptor I kinase. The latter effectively phosphorylates the Receptor-Smad Mad, which subsequently binds the Co-Smad, Medea. The Smad complex moves to the nucleus, where target gene expression is commenced. After photoreceptor differentiation, the photoreceptor axons subsequently become wrapped by particular glial cells, which come from the developing optic stalk. Once in the eye disc, glia move anteriorly on the basal portion of the eye epithelium. Therefore, decreased Dpp signaling can adversely affect the accumulation of glia in the eye disc. The protein Hedgehog also has an important role in the propagation of the furrow. The Hh receptor, Patched (Ptc) is not expressed in the photoreceptors however, it is located in all glial cells. Dpp and hh are not expressed in the subretinal glial cells, thus suggesting that Dpp and Hh act via paracrine means. It can be implied that Dpp and Hh signaling both work in harmony in order to control the transcription of genes which are necessary for glial cell motility. It can be therefore be said that Dpp and Hh cooperate to guarantee that the development is synchronized.

    Glial cell migration is induced by the R cells of the photoreceptors. Laminal glia cells are intermediate targets for the R1-R6 cells and, in the absence of the laminal glia, the R1-R6 project onto the medulla.. The COP9 sigalosome (CSN) is required for the development of the laminal glia and the R1-R6 targeting. JAB1/CSN5 is a localized signal thatcomprises of the R cell growth cones and lie adjacent to the laminal glia cells to support the JAB1/CSN5 role to mediate the interactions between the growth cones and the glia. JAB1/CSN5 also plays a role in the early functioning of R cells development.
    The R cell growth cones ptoduce the Hedgehog and Spitz signals which initiate the final cell divisions in the laminal precursor and the differentiation of the laminal neurons. The R cells are aloso thought to produce a third signal that induces the glia development as the laminal glia are essential for R1-R6 targeting. If the glia cells are not formed, the R1-R6 cells target the medulla which leads to abnormalities. The JAB1/CSN5 protein (ligand) is required for signalling between the R cell growth cones and the glia. The JAB1/CSN5 protein regulates the lamina development indirectly by controlling the expression of signals necessary for the laminal glia development

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