Advanced pulmonary development in newborn guinea pigs (Cavia porcellus).

Morphological and morphometric evidence is presented to support the hypothesis that lung growth is advanced in mammals born at a relatively mature stage. The lungs from fetal and postnatal guinea pigs ranging in age from gestational age 56 days (normal gestation is 68 days in this species) through 16 days postpartum were fixed in situ by intratracheal glutaraldehyde. Morphometry included measurements of lung volume (VL), tissue and air-space volumes, fraction of respiratory parenchyma, alveolar (SA) and capillary (SC) surface areas, and the arithmetic mean thickness of the tissue barrier (tau t). VL, SA, and SC all increased monoexponentially versus body weight (W) from birth to adulthood; the lungs appeared to be in the equilibrated growth phase, with no postnatal period of pronounced tissue proliferation as reported in the newborn rat and mouse. The prepartum value of tau t was 1.96 micrometers; this value decreased by parturition of 1.27 micrometers and did not change significantly with additional age. At the light-microscopical level, respiratory bronchioles could be visualized giving rise to alveolar ducts by a gestational age of 58 days (10 days preterm) with well-developed alveolar septal partitioning evident. Structures resembling the primitive pre-alveolar saccules of newborn rats were never seen in even the youngest fetal animals. Elastin fibers were also evident at this age, both in bronchiolar and duct walls, as well as in alveolar septa. Using electron microscopy, the air-blood barrier appeared mature by a gestational age of 61 days and thereafter, double capillary layers were only rarely seen in septal walls.