Comparison of the size of the vascular compartment of the carotid body of the fetal, neonatal and adult cat.

The carotid bodies from full-term fetal cats, 3- to 4-day-old neonates and adult cats were perfusion-fixed at normal arterial blood pressure with 3% phosphate-buffered glutaraldehyde. Serial 5-microns sections were cut and stained by the MSB method. Using an interactive image analysis system, determinations were made of the volumes of the carotid body and of its vascular and extravascular compartments. Compared to the fetus, the carotid body of the neonate increased in volume by 51% and by 286% in the adult cat. There was a proportional increase in the volumes of the vascular compartment and of the small vessels (5-12 microns diameter) in that compartment. The volume of the small vessels, expressed as a ratio of the total volume of the organ, remained constant in the three animal groups at 5-7%. The small vessel endothelial surface area, expressed as a ratio of the extravascular volume (which was assumed to consist largely of type 1 and type 2 cells), was the same in the neonate as in the full-term fetus. Thus, there were no apparent quantifiable morphological features of the carotid body and its vasculature which would account for the resetting of the hypoxic sensitivity of the organ from the fetal to the adult range within a few days of birth.