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

The research article presents a comparative study of the superior cervical ganglion in rats, capybaras, and horses. It explores the correlation between the size and number of neurons in these different species and their respective body weight.

Methodology

• The researchers conducted the study using adult rats, capybaras, and horses.
• The ganglia of these animals were examined under light microscopy.
• To do this, they were vascularly perfused, embedded in resin, and then cut into semi-thin sections.
• An unbiased stereological procedure known as the ‘disector method’ was used to estimate the size of the ganglion neuron, the total number of neurons, and the neuronal density.

Findings

• The volume of the ganglion was found to be 0.5 mm3 in rats, 226 mm3 in capybaras, and 412 mm3 in horses.
• The total number of neurons per ganglion was found to be 18,800, 1,520,000, and 3,390,000 in rats, capybaras, and horses, respectively.
• The number of neurons per cubic millimetre was found to be 36,700, 7,000, and 8,250 in rats, capybaras, and horses, respectively.
• The average size of the largest sectional profile of a neuron was found to be 358, 982 and 800 microm2, in rats, capybaras, and horses, respectively.
• In rats, capybaras, and horses, respectively, the percentage of volume occupied by neurons was 33, 21, and 17%.

Comparative Analysis

• The researchers observed that, across the three species, most of the neuronal parameters change in line with the variations of body weight.
• An exception was found in the capybara, where the average size of the neurons was found to be larger than in the horse, which differs from the general pattern based on body weight.
• The rat showed great variability in all the neuronal parameters between individuals.

Conclusions

• The researchers concluded that there seemed to be a correlation between average neuron size and body size, and between the total number of neurons and body size.
• However, they caution that these correlations are only approximations based on averages for large populations of neurons, and might not hold true when considering single quantitative parameters.
• The study suggests that multiple quantitative features of nervous tissue need to be considered together, rather than individually, when exploring possible evolutionary trends related to size.