In some cases, a cell communicates with another cell through direct contact, without diffusion or circulation of the signaling molecule. This form of signaling requires that the two communicating cells be in physical contact with each other. A transmembrane protein on the surface of one cell acts as the signaling molecule, and a transmembrane protein on the surface of an adjacent cell acts as the receptor (Fig. 9.4d). In this case, the signaling molecule is not released from the cell, but instead remains associated with the plasma membrane of the signaling cell.

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This form of signaling is important during embryonic development. As an example, let’s look at the development of the central nervous system of vertebrate animals. In the brain and spinal cord, neurons transmit information in the form of electrical signals that travel from one part of the body to another. The neurons in the central nervous system are greatly outnumbered by supporting cells, called glial cells, which nourish and insulate the neurons. Both the neurons and the glial cells start out as similar cells in the embryo, but some of these undifferentiated cells become neurons and many more become glial cells.

During brain development, the amount of a transmembrane protein called Delta dramatically increases on the surface of some of these undifferentiated cells. These cells will become neurons. Delta proteins on each new neuron bind to transmembrane proteins called Notch on the surface of adjacent, undifferentiated cells. In this case, the signaling cell is the cell with elevated levels of Delta protein. The Delta protein in turn is the signaling molecule, and Notch is its receptor. Cells with activated Notch receptors become glial cells and not neurons. Because one signaling cell sends this same message to all the cells it contacts, it is easy to understand how there can be so many more glial cells than neurons in the central nervous system.

As you can see from these examples, the same fundamental principles are at work when signaling guides a developing embryo, allows neurons to communicate with other neurons or muscles, triggers DNA uptake by pneumococcal cells, or allows your body to respond to stress. All these forms of communication are based on signals that are sent from a signaling cell to a responding cell. These signaling molecules are the language of cellular communication.