Neuronal density and expression of calcium‐binding proteins across the layers of the superior colliculus in the common marmoset (Callithrix jacchus)

The superior colliculus (SC) is a layered midbrain structure with functions that include polysensory and sensorimotor integration. Here, we describe the distribution of different immunohistochemically identified classes of neurons in the SC of adult marmoset monkeys (Callithrix jacchus). Neuronal nuclei (NeuN) staining was used to determine the overall neuronal density in the different SC layers. In addition, we studied the distribution of neurons expressing different calcium‐binding proteins (calbindin [CB], parvalbumin [PV] and calretinin [CR]). Our results indicate that neuronal density in the SC decreases from superficial to deep layers. Although the neuronal density within the same layer varies little across the mediolateral axis, it tends to be lower at rostral levels, compared to caudal levels. Cells expressing different calcium‐binding proteins display differential gradients of density according to depth. Both CB‐ and CR‐expressing neurons show markedly higher densities in the stratum griseum superficiale (SGS), compared to the stratum opticum and intermediate and deep layers. However, CR‐expressing neurons are twice as common as CB‐expressing neurons outside the SGS. The distribution of PV‐expressing cells follows a shallow density gradient from superficial to deep layers. When normalized relative to total neuronal density, the proportion of CR‐expressing neurons increases between the superficial and intermediate layers, whereas that of CB‐expressing neurons declines toward the deep layers. The proportion of PV‐expressing neurons remains constant across layers. Our data provide layer‐specific and accurate estimates of neuronal density, which may be important for the generation of biophysical models of how the primate SC transforms sensory inputs into motor signals.