In paracrine and autocrine signaling systems, chemical messengers differ from classical endocrine systems in that they are:

• A. Transported through blood vessels
• B. Not transported by blood vessels
• C. Only received by neighboring cells
• D. Secreted by the same type of cells only

Answer: (B)

In paracrine and autocrine signaling, the key characteristic that distinguishes these signaling mechanisms from classical endocrine systems is that the chemical messengers are not transported by blood vessels.

In autocrine signaling, cells produce signaling molecules that bind to receptors on their own surface, thus affecting the same cell that released them. This means the signal does not enter the bloodstream but acts locally on the secreting cell itself.

In paracrine signaling, the signaling molecules are released by one cell and act on nearby target cells. These signals also do not travel through the circulatory system; rather, they diffuse across a small distance to affect neighboring cells.

This local action contrasts with endocrine signaling, where hormones are released into the bloodstream and can affect distant target cells throughout the body. Thus, the defining feature of paracrine and autocrine signaling is their local action and the lack of transport via the blood vessels.