Morphometric examination of equine adult and foal lung

Abstract

To fully understand the mechanisms of lower airway inflammation associated with many equine diseases such as heaves or Rhodococcus equi infection, which are age specific, we must first identify baseline “normal” structural characteristics of the horse lung. To develop a detailed understanding of the morphology of the horse lung, stereological methods were adapted and applied to the lungs from healthy adult horses (n=4) and one day (n=5) and 30 day (n=5) old foals. The left lung from each animal was fixed in situ and was then removed from the body cavity and remained in fixative overnight before beginning an unbiased sampling procedure. The tissue samples were fixed in plastic and paraffin blocks for stereological evaluation and immunohistochemistry, respectively. The lung was characterised into parenchyma and non-parenchyma, where median parenchymal density (alveolar airspace, ductal airspace and tissue) was 81.0% in one 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 one day old, 55.5% in 30 day and 66.9% in adult horse lungs. Ductal airspace and alveolar tissue volume density was unchanged between the age groups. The median alveolar surface area (m^2) seemed to increase with age, from about 205.3m^2, 258.2m^2 and 629.9m^2 in one day old foals, 30 day old foals, and adults, respectively. While the median alveolar surface density decreased with age, the mean linear intercept increased with age. Alveolar surface area was consistently greater than endothelial surface area (m^2) within each lung, however the ratio between alveolar and endothelial surface density remains unchanged with age. The median endothelium surface area was 106.2m^2 in one day, 147.5m^2 in 30 day and 430m^2 in adult lungs. The data show that the foal is born with a functionally developed lung and its basic architecture changes with age. Foal lung development and remodelling postnatally is a result of alveolar expansion paralleled with angiogenesis.