Estimate of size and total number of neurons in superior cervical ganglion of rat, capybara and horse

The superior (cranial) cervical ganglion was investigated by light microscopy in adult rats, capybaras (Hydrochaeris hydrochaeris) and horses. The ganglia were vascularly perfused, embedded in resin and cut into semi-thin sections. An unbiased stereological procedure (disector method) was used to estimate ganglion neuron size, total number of ganglion neurons, neuronal density. The volume of the ganglion was 0.5 mm³ in rats, 226 mm³ in capybaras and 412 mm³ in horses. The total number of neurons per ganglion was 18,800, 1,520,000 and 3,390,000 and the number of neurons per cubic millimetre was 36,700, 7,000 and 8,250 in rats, capybaras and horses, respectively. The average neuronal size (area of the largest sectional profile of a neuron) was 358, 982 and 800 µm², and the percentage of volume occupied by neurons was 33, 21 and 17% in rats, capybaras and horses, respectively. When comparing the three species (average body weight: 200 g, 40 kg and 200 kg), most of the neuronal quantitative parameters change in line with the variation of body weight. However, the average neuronal size in the capybara deviates from this pattern in being larger than that of in the horse. The rat presented great interindividual variability in all the neuronal parameters. From the data in the literature and our new findings in the capybara and horse, we conclude that some correlations exist between average size of neurons and body size and between total number of neurons and body size. However, these correlations are only approximate and are based on averaged parameters for large populations of neurons: they are less likely to be valid if one considers a single quantitative parameter. Several quantitative features of the nervous tissue have to be taken into account together, rather than individually, when evolutionary trends related to size are considered.