Stereological estimation of surface area and barrier thickness of fish gills in vertical sections

Abstract

SummaryPrevious morphometric methods for estimation of the volume of components, surface area and thickness of the diffusion barrier in fish gills have taken advantage of the highly ordered structure of these organs for sampling and surface area estimations, whereas the thickness of the diffusion barrier has been measured orthogonally on perpendicularly sectioned material at subjectively selected sites. Although intuitively logical, these procedures do not have a demonstrated mathematical basis, do not involve random sampling and measurement techniques, and are not applicable to the gills of all fish. The present stereological methods apply the principles of surface area estimation in vertical uniform random sections to the gills of the Brazilian teleost Arapaima gigas. The tissue was taken from the entire gill apparatus of the right-hand or left-hand side (selected at random) of the fish by systematic random sampling and embedded in glycol methacrylate for light microscopy. Arches from the other side were embedded in Epoxy resin. Reference volume was estimated by the Cavalieri method in the same vertical sections that were used for surface density and volume density measurements. The harmonic mean barrier thickness of the water-blood diffusion barrier was calculated from measurements taken along randomly selected orientation lines that were sine-weighted relative to the vertical axis. The values thus obtained for the anatomical diffusion factor (surface area divided by barrier thickness) compare favourably with those obtained for other sluggish fish using existing methods.

Implications for Osmorespiratory Compromise by Anatomical Remodeling in the Gills of Arapaima gigas

“…At low magnification, scanning electron microscopy (SEM) revealed welldeveloped lamellae in the gill filaments of 10-100 g fish (22-25 cm; 2-3 months old) but not in those of 724 g and 1,000 g fish (Brauner et al, 2004;Gonzalez et al, 2010). The lamellar organization of a 100 g A. gigas is similar to that of other teleosts, and its respiratory surface area is similar to those of facultative air-breathing fish of a similar size (Costa et al, 2007); the average respiratory surface area of the swim bladder exceeded that of the gills by a factor of 2.8 indicating the importance of the swim bladder for respiration, even in juvenile fish (Fernandes et al, 2012). Data concerning ion regulation has demonstrated that the rate of diffusive Na 1 loss is higher in 724 g fish (45 cm; 5 months old) than in 67 g fish (22 cm, 2 months old) (Gonzalez et al, 2010) and Brauner et al (2004) identified high density of mitochondria-rich cells (MRCs) along the outer cell layer of the filament epithelium of 1,000 g fish.…”

Histomorphometric comparison after fixation with formaldehyde or glyoxal

“…The measured thickness of random sections tends to overestimate the true thickness and varies with variations in the sectioning plane. It is generally accepted, however, that this effect is random and the true thickness can be obtained by applying a correction factor of π/4 to the measured thicknesses (Jensen et al 1979, Da Costa et al 2007). The harmonic mean estimation has been suggested to be statistically more efficient than the arithmetic mean thickness, because it minimizes the influence of large values on the mean.…”

Gill dimensions in near-term embryos of Amazonian freshwater stingrays (Elasmobranchii: Potamotrygonidae) and their relationship to the lifestyle and habitat of neonatal pups

“…The harmonic mean is a better measure of the water-blood distance than the arithmetic mean, because calculation give special weight to the shorter distance (minimum thickness). The anatomical diffusion factor of the water-blood barrier was estimated as the quotient of the mass-specific respiratory surface area (S gill /M b ) and 2/3 of the harmonic mean of diffusion barrier thickness (Costa et al, 2007).…”