Microvascular maturation of the septal capillary layers takes place in parallel to alveolarization in human lungs.

Schmid, Lukas; Hyde, Dallas M; Schittny, Johannes (2023). Microvascular maturation of the septal capillary layers takes place in parallel to alveolarization in human lungs. American journal of physiology - lung cellular and molecular physiology, 325(5), L537-L541. American Physiological Society 10.1152/ajplung.00425.2022

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Primary and secondary septa formed during lung development contain a double-layered capillary network. To improve gas-exchange the capillary network is remodeled into a single-layered one, a process that is called microvascular maturation (MVM). It takes place during classical and continued alveolarization. Classical alveolarization is defined as a formation of new septa from immature septa and continued alveolarization as a formation from mature septa. Until now, MVM was never quantitatively evaluated in human lungs. To correlate alveolarization and MVM, and to determine the transition point from classical to continued alveolarization, the degree of MVM was stereologically estimated. In 12 human lungs (0.1-15 years) the alveolar surface area of immature and mature septa was estimated stereologically by intersection counting. A MVM-quotient (RMVM) was defined as the mature alveolar surface area over total alveolar surface area. The MVM-quotient increased logarithmically over age and showed a bi-phasic increase similar to alveolarization. It did not reach 100% maturity in these samples. A linear correlation between the MVM-quotient and the logarithm of the number of alveoli was observed. We conclude that MVM increased logarithmically and biphasically in parallel to alveolarization until alveolarization ceased. However, at 2-3 years of age three quarters of the alveolar microvasculature are mature. This result may explain a previous postulate that MVM is finished at this age. We hypothesize that as long as alveolarization takes place, MVM will take place in parallel. We propose that the transition from classical to continued alveolarization takes place between the ages of 1-3 years in humans.

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