Impaired adaptation of pulmonary circulation to extrauterine life in newborn pigs exposed to hypoxia: An ultrastructural study

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Abstract
Twelve Large White pigs aged 0.05) because endothelial and smooth muscle cells (SMCs) retained their fetal shape, position, overlap, interdigitation and the low surface/volume ratio characteristic of fetal life. In all older animals, the cells had a normal postnatal shape and surface/volume ratio. In the elastic vessels hypoxia did not prevent the normal postnatal reduction in mean SMC diameter of animals exposed from birth. SMC hypertrophy did not occur in any age group, but all animals save those first exposed at 14 days, showed an increase in SMC myofilament volume density (p < 0.01). Connective tissue volume density also increased (p < 0.01), mainly due to an increase in elastin and ground substance. Thus a short period of neonatal hypoxia impaired adaptation and appeared to potentiate contractile capacity in stiff-walled arteries but elicited a less marked response from animals first exposed at 14 days.