Final answer:
The categories of chemical messengers for intercellular communication are autocrine, paracrine, endocrine, and direct signaling across gap junctions.
Step-by-step explanation:
The categories of chemical messengers for intercellular communication are autocrine signaling, paracrine signaling, endocrine signaling, and direct signaling across gap junctions.
In autocrine signaling, the signaling molecule acts on the same cell that released it. In paracrine signaling, the signaling molecule acts on nearby cells. In endocrine signaling, the signaling molecule is released into the bloodstream and acts on distant target cells. In direct signaling across gap junctions, the signaling molecule passes through gap junctions to reach neighboring cells.