Endocrine signaling is an example of long distance communication between hormone producing cells, tissues and glands and cells that express hormone receptor molecules.

Positive Regulation of the Cell Cycle Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are responsible for the progress of the cell through the various checkpoints. The levels of the four cyclin proteins fluctuate throughout the cell cycle in a predictable pattern (Figure 2).

Cell-cycle proteins are the proteins involved in regulation and maintenance of the cell cycle of eukaryotic cells. These include kinases and cyclins that regulate movement between the three phases of the cell cycle that leads to replication and division of a cell – these phases are interphase, mitosis and cytokinesis.

The central components of the cell-cycle control system are cyclin-dependent protein kinases (Cdks), whose activity depends on association with regulatory subunits called cyclins. Oscillations in the activities of various cyclin-Cdk complexes leads to the initiation of various cell-cycle events.

In both local and long-distance signaling, only specific target cells recognize and respond to a given signaling molecule. Local signaling includes Paracrine signaling (A secreting cell acts on nearby target cells by discharging molecules of a local regulator into the extracellular fluid.)

An endocrine cell, which is a cell that secretes a hormone, is the signaling cell in long-distance signaling. For example, epinephrine is a hormone produced in the adrenal glands, located superior to the kidneys. When an animal is under stress, the adrenal glands release epinephrine into the bloodstream.

The equipment for long distance communication is very important. Communication takes place either through heliograph signaling using a flashing mirror, electrical lamps, spot lights, colored disks, etc., or it is accomplished electrically by radio or by wires within the confined areas of the space station.