Clusters of rough endoplasmic reticulum and free ribosomes in the perikaryon are called

Clusters of rough endoplasmic reticulum (RER) and free ribosomes in certain regions of the perikaryon are termed Nissl bodies, named after the German microscopist Franz Nissl, who initially characterized them. These structures contribute to the gray coloration observed in areas containing neuron cell bodies, commonly referred to as gray matter during gross dissection of the brain and spinal cord.