Morphometric examination of the equine adult and foal lung.

To understand the mechanisms of airway inflammation associated with equine diseases such as Rhodococcus equi infection, we must identify baseline "normal" structural characteristics of the horse lung. To develop a detailed understanding of the morphology of the horse lung, we adapted and applied stereological methods to the lungs from healthy adult horses (N = 4) and 1-day (N = 5) and 30-day (N = 5) old foals. The left lung was fixed in situ by intrabronchial instillation of glutaraldehyde/paraformaldehyde fixative at 25 cm H2 O column and sampled using a fractionator design followed by embedding in glycol methacrylate. The lung was characterized into parenchyma and non-parenchyma, where median parenchymal density was 81.0% in 1-day-old foals, 84.4% in 30-day-old foals and 93.7% in adult lungs. The median volume density of alveolar airspace per lung was 45.9% in 1-day-old, 55.5% in 30-day and 66.9% in adult horse lungs. The median alveolar surface area increased with age, from 205.3 m(2) , 258.2 m(2) , and 629.9 m(2) in 1-day-old foals, 30-day-old foals, and adults, respectively. While the median alveolar surface density decreased with age, the mean linear intercept (mean free distance within acinar airspaces) increased with age. Alveolar surface area was greater than endothelial surface area within each lung. The ratio between alveolar and endothelial surface density remains unchanged with age. The median endothelium surface area was 106.2 m(2) in 1-day, 147.5 m(2) in 30-day, and 430 m(2) in adult lungs. The data suggest the foal lung is functionally developed and postnatal lung development and remodelling is driven by alveolar expansion paralleled with angiogenesis.