Table 1.
Body mass and lung volume during postnatal development.
Fig 1.
a) 3 days old rats, before alveolarization: saccules surrounded by thick primary septa containing a double layered capillary bed are (arrows) seen (scale bar; 200μm). b) 7 days old rats, during alveolarization: Protrusions of secondary septa are seen sporadically (arrows). The septa appear still thick (scale bar; 200μm).
Fig 2.
a) 14 days old rats. alveolarization is well advanced. Some alveoli and a lot of alveolar ducts (DA) are visible. Protrusions of secondary septa are often seen. Double (red arrows) and single layered (black arrow heads) capillary beds can be seen. (scale bar; 200μm). b) 21 days old rats: at the end of bulk alveolarization and microvascular maturation numerous alveoli with more or less small alveolar septa and alveolar ducts (DA) are seen. The capillary bed is single layered. The size of airspaces is smaller than in the younger developmental stages (scale bar; 200μm).
Fig 3.
Adult rats: alveolar ducts (numerous small alveoli with thin alveolar septa and (scale bar; 200μm).
Fig 4.
Various stereological parameters characterizing lung parenchyma and AEII before, during and after alveolarization as well as in adults.
a) Compared to 3 days old rats a significant increase of the total alveolar surface (S(alveoli, lung)) was determined after the end of bulk alveolarization and in adults. This modified figure is reused from PLOS ONE, published first as Fig 3b by Roeder F, Knudsen L, Schmiedl A. The expression of the surfactant proteins SP-A and SP-B during postnatal alveolarization of the rat lung. PLoS One. 2024 Mar 14;19(3):e029. b) The total number of alveolar epithelial cells type II (N(AEII, lung)) shows a significant increase after the end of bulk alveolarization and in adults compared to 3 days old rats. c) During alveolarization the number of AEII per mm2 alveolar surface (N(AEII, lung)/S(alveoli, lung)) decreases continuously reaching the significant lowest values in adults. d) The number of AEII related to lung volume decreases continuously during alveolarization. The significant lowest values are found in adults. Thus, the lung volume increases faster than the number of AEII in the postnatal development.
Table 2.
Regression analysis between different parameters during alveolarization.
Table 3.
Regression analysis between different parameters over all age groups.
Fig 5.
a) Number weighted mean volume of AEII during postnatal development and in adults. There are no volume alterations before and at the end of bulk alveolarization. A significant increase was determined in adults compared to 3 and 7 days old pups. b) Size of AEII presented as VS-ratioAEII during postnatal lung development into adulthood. Before and during bulk alveolarization there are no size changes. Adults show a significant increase compared to values before and at onset of alveolarization. c) Number weighted mean volume of AEII per lung volume during postnatal development and in adults. The highest values are seen before alveolarization. During alveolarization the quotient decreases and reaches significance compared to 3 days old pups at postnatal day 21. Thus, the lung volume has increased more rapidly than the AEII volume at the end of bulk alveolarization. d) Number weighted mean volume of AEII per alveolar surface area during postnatal development and in adults. Compared to adults, the significantly highest values are seen before alveolarization. e) VS-ratio AEII related to lung volume before and during postnatal development up to adulthood. Lungs in the saccular stage show the highest values compared to 21 days old and adult rats. Therefore, lung volume increases much more than the does AEII increase. f) VS-ratio AEII related to alveolar surface area before and during postnatal development up to adulthood. The significant highest values are seen before alveolarization, the lowest in adults.
Fig 6.
Representative ultrastructure of AEII from different postnatal development stages.
a) 3 days old: AEII in a corner of the alveolar epithelium between capillaries. The nucleus contains small clumps of heterochromatin on the inner side of the nuclear envelope. In the cytoplasm, organelles such as mitochondria with densely packed cristae and dark matrix, ER, Golgi complex and vesicles of different origin are normally distributed. Normally size distributed Lb are visible. b) 7 days old: AEII below the alveolar surface partly adjacent to lipofibroblasts. The nucleus contains a lot of heterochromatin. Organelles are regularly distributed. Large looking Lb show densely packed lamellae. c) 14 days old: AEII with a visible nucleolus in the nucleus and numerous Lb of different size. d) 21 days old: AEII with nucleus and numerous normally distributed Lb of different size. e) Adult: AEII with normally distributed and preserved organelles.
Fig 7.
Ultrastructure of AEII of different stages of development showing giant Lb.
a, b) 3 days old pups, c) 7 days old pups, d) 21 days old rat.
Fig 8.
Various parameters characterizing Lb before, during and after bulk alveolarization (adulthood).
a) Volume density of Lb. In lungs of 21 days old rats significant higher values compared to 3 days and 7 days old rats were determined. b) Before and during alveolarization the surface density of Lb doesn´t change. A significant increase was found in adults compared to 3 and 7 days old rats. c) The total volume of Lb in AEII obtained by multiplying VV(Lb/AEII) with νN(AEII, lung) is comparable before and during bulk alveolarization. A significant increase was determined in AEII of adults. d) The total surface of Lb in AEII obtained by multiplying SV(Lb/AEII) with νN(AEII) doesn´t change until postnatal day 21. A significant increase compared to 3 and 7 days old lungs was found in adults. e) The total volume of Lb in the lung calculated by multiplying VV(Lb/AEII) with νN(AEII) and N(AEII, lung) over all age groups. Between 3 and 21 days old lungs a significant increase was found. Compared to 3 and 7 days old pups the significant highest values are seen in adults. f) The total surface of Lb in the lung calculated by multiplying SV(Lb/AEII) with νN(AEII) and N(AEII, lung) over all age groups. Between 3 and 21 days old lungs a significant increase was found.
Fig 9.
Various parameters characterizing Lb related to lung volume or alveolar surface before, during and after bulk alveolarization (adulthood).
a) Total volume of Lb related to lung volume. Independent of the age similar values are visible. b) Total surface of Lb related to lung volume. All age groups show similar values. c) Total volume of Lb related to alveolar surface area. During alveolarization values remain constant. At pnd 21 a significant decrease compared to 3 days old pups is visible. Thus, at pnd 21 the increase of alveolar surface is much more pronounced then the increase of Lb volume. d) Total surface of Lb related to alveolar surface area. During alveolarization no differences were found. At pnd 21 values are significantly lower compared to 3 days old pups. Thus, on pnd 21 the increase of alveolar surface is much more pronounced then the increase of Lb surface. e) The volume weighted mean volume of Lb (νV(Lb, AEII)) exhibits comparable values before, during and after alveolarization. Values determined in adults are significant lower.f) νVLb, AEII) related to lung volume shows the highest values before onset of alveolarization. The significantly lowest values are found in adults. Thus, lung volume increases more rapidly than the volume weighted mean volume of Lb.
Table 4.
Volume densities of subcellular compartments in AEII.
Table 5.
Volume fraction, total volume per AEII and total volume per lung of mitochondria.