Cell Signaling

Quorum Sensing Quorum sensing is a system of stimuli and response correlated to population density. Quorum sensing(QS) enables bacteria to restrict the expression of specific genes to conditions when cell density is high.

Vibrio fischeri The bioluminescence emitted by the marine bacterium Vibrio fischeri is a particularly striking result of individual microbial cells coordinating a group behavior. The genes responsible for light production are principally regulated by the LuxR-LuxI quorum-sensing system. In addition to LuxR-LuxI, numerous other genetic elements and environmental conditions control bioluminescence production.

Autocrine Signaling Autocrine signaling is a type of cell communication, that occurs when the cell secretes a substance that is capable of provoking a reaction in itself . The secreted molecules are known as local mediators, because they only affect cells in their immediate vicinity. An example of autocrine signaling can be found in the human immune response. T lymphocytes can stimulate their own proliferation when they detect a foreign antigen, by producing a growth hormone that they respond to. The result of this is a larger population of active T lymphocytes and thus, the body is capable of mounting a more effective immune response against the invader.

T-Lymphocytes T lymphocytes (T cells) are a type of white blood cell that is vital for the function of cell-mediated immunity. Once it is produced in the Thymus, naive T cells enter the bloodstream and migrate towards the lymphoid tissue until they are in contact with their specific antigen. Once it binds to the target cells, T lymphocytes begin to proliferate and differentiate into cells.

Cancer We can also see the effects of autocrine signaling in cancer. Overproduction of growth factors can lead to the formation of tumors, when cell division becomes uncontrollable due to a dysfunctional autocrine signaling pathway.

Juxtacrine Signaling In juxtacrine interactions, proteins from the inducing cell interact with receptor proteins of adjacent responding cells. The inducer does not diffuse from the cell producing it. There are three types of juxtacrine interactions. A protein on one cell binds to its receptor on the adjacent cell. 2. A receptor on one cell binds to its ligand on the extracellular matrix secreted by another cell. 3. The signal is transmitted directly from the cytoplasm of one cell through small conduits into the cytoplasm of an adjacent cell. An example is plasmodesmata in plant cells.

Gap Junctions Gap junctions are a specialized intercellular connection between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulated gate between cells.

Plasmodesmata Plasmodesmata are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them.

Paracrine Signaling  A form of cell-to-cell communication in which a cell produces a signal to induce changes in nearby cells, altering the behavior of those cells. Signaling molecules known as paracrine factors diffuse over a relatively short distance . Cells that produce paracrine factors secrete them into the immediate extracellular environment. Factors then travel to nearby cells in which the gradient of factor received determines the outcome. However, the exact distance that paracrine factors can travel is not certain.

Neurotransmittes Neurotransmitters, also known as chemical messengers that move from one nerve cell to another nerve cell in very close proximity but not touching. They can also transmit signals across a chemical synapse known as a neuromuscular junction. That is from one neuron (nerve cell) to a muscle cell. Neurotransmitters are released from synaptic vesicles in the nerve cell into the synaptic cleft, where they are received by receptors on the target cells.

Endocrine Signaling The term endocrine means “secreting internally,” and specifically refers to secretions that are distributed in the body by way of the bloodstream. Endocrine cells are made up of ductless glands that produce chemical messages called hormones, which are released into the internal environment of the body. Thus, endocrine signaling occurs when endocrine cells release hormones that act on distant target cells in the body and the hormones and receptors are very specific. This is why the body can release a hormone into the blood that can pass cells without the specific receptor and only target the ones that do.

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Let’s Review