PATHOLOGY

Detailed explanation-1: -Endocrine signaling: a cell targets a distant cell through the bloodstream. A signaling molecule is released by one cell, then travels through the bloodstream to bind to receptors on a distant target cell elsewhere in the body.

Detailed explanation-2: -Chemical signals between cells are called ligands. A ligand is a molecule that binds another specific molecule. In the case of cell signaling, the ligand binds a receptor, a protein in or on the target cell. Examples of ligands include hormones and neurotransmitters.

Detailed explanation-3: -Signals from distant cells are called endocrine signals, and they originate from endocrine cells. (In the body, many endocrine cells are located in endocrine glands, such as the thyroid gland, the hypothalamus, and the pituitary gland.)

Detailed explanation-4: -Direct-Contact Signaling: Gap junctions-tiny channels that connect neighboring cells-are found in plants and animals. These gap junctions are full of water and allow small signaling molecules to travel across the channel. This is cell signaling through direct contact.

Detailed explanation-5: -A second type of specialized signaling cell that controls the behavior of the organism as a whole is an endocrine cell. These cells secrete their signal molecules, called hormones, into the bloodstream, which carries the signal to target cells distributed widely throughout the body (Figure 15-4D).