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Classical and Continued Alveolarizatoin

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Fig. 8 Schematic of classical and continued alveolarization. The interairspace walls present in the saccular stage represent the primary septa. They contain a double-layered immature capillary network (a). Each layer appears as a perforated sheet of capillaries (see also Fig. 9). Smooth muscle cell precursors, elastic fibers and collagen fibrils (green spots) accumulate at sites where new septa (or secondary septa) will be formed (blue arrows, a). The secondary septa form by an upfolding (blue arrows) of one of the two capillary layers (red, b). The resulting newly formed secondary septa (gray arrows) subdivide preexisting airspaces and the alveoli are born (c). At this stage, most of the septa are immature because they possess two capillary layers. During microvascular maturation, the double-layered capillary network fuses to a single-layered one. As a first approximation, microvascular maturation takes place in parallel to alveolarization. Therefore, a significant fraction of new septa are formed starting from a preexisting mature septum containing only a single-layered capillary network (e). Now following the mechanism of continued alveolarization, new alveolar septa are still formed by an upfolding of the capillary layer (red, d–f), even if the alveolar surface opposing the upfolding is now missing its capillaries (d). This gap is immediately closed by angiogenesis (orange arrows in e , f). In both modes of alveolarization, a sheetlike capillary layer folds up (b , e) in order to form a double-layered capillary network inside the newly formed septum (c, f). Regardless of how and when a new septum is formed, it will mature shortly after by a fusion of the double-layered capillary network. (Altered and extended from Burri 1997; Burri 1999; Schittny and Mund 2008; Woods and Schittny 2016, by courtesy of Cambridge University Press, New York) ^[1]

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