PSI Scientific Report 2008

Research focus and highlights – Synchrotron light  25

ever, the two layers do not have to be preexisting as currently postulated, but they may be formed rapidly and locally by angiogenesis when needed. Because microvascular maturation takes place during alveolarization, we defined the entire time when new septa/alveoli are formed during lung development and growth as “developmental alveolarization”. This term distinguishes the developmental processes from any kind of lung regeneration, which we called “regenerative alveolarization”. Synchrotron-radiation tomographic microscopy was essential for the structural understanding on how new alveoli are formed throughout lung development and growth. We could show that new alveoli are formed not only before, but also after, the maturation of the alveolar microvasculature. During the latter, the requirement of a double-layered capillary network at the site where a new septum will be formed is overcome by a local duplication found at the sides of septation. Most likely, many of these duplications were not preexisting. We defined the classically described alveolarization “phase one of developmental alveolarization” and the newly described form “phase two”. Until now, the understanding of phase two is based on structural evidence only. However, due to its clinical significance, we believe that these structural findings will be the starting point for investigations of the molecular mechanisms Figure 2: 3D visualizations of the capillary network of single

involved. The description of phase two will most likely force

alveoli. The entrances of the alveoli are labeled with a yellow

us to rethink our views of (i) lung regeneration and of (ii) side

dotted line. Mercox vascular casts of 21-day-old rat lungs

effects on the structure of the lungs during the treatment of

were imaged at 12.6 keV with a pixel size of 0.7 microns. Scale bars are 10 microns. See text for details.

children and adolescents with glucocorticoids and retinoids.

of a double-layered capillary network inside the existing

References

alveolar septa may be overcome. For this purpose, we studied

[1] J. H. Caduff, L. C. Fischer and P. H. Burri, Scanning

3D tomographic data sets of vascular casts of rat lungs ob-

electron microscopic study of the developing

tained at the TOMCAT beamline of the SLS.

microvasculature in the postnatal rat lung, Anat Rec 216

Figure 2 shows the lumen of the capillaries. Inside the cavity of an alveolus, the up-folding of the single-layered capillary network is observed (blue dashed lines in A, C, and E). The folding is indicative of the formation of a new septum. The 3-D

154–164 (1986). [2] E. R. Weibel, Morphometry of the Human Lung (Springer-Verlag, Heidelberg, 1963). [3] J. C. Schittny, S. I. Mund and M. Stampanoni, Evidence

visualization enabled us, for the first time, to look at the re-

and structural mechanism for late lung alveolarization,

verse side of the same septum (B, D, and F). At the basis of

Am J Physiol Lung Cell Mol Physiol 294 L246–L254

the folding, we detected a local duplication of the existing

(2008).

capillary network (covering of the blue dashed line in B, D, and F). Whereas most duplications are already formed in these examples (arrowhead), one is most likely just forming by sprouting angiogenesis (arrow in B). In addition, (forming) tissue posts inside the capillary network (holes in the vascular cast, green asterisk) are indicative for intussusceptive angiogenesis (the growth of the capillary network to allow the up-folding). We were able to show that the requirement of a double-layered capillary network at the site of septation is still valid; how-