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The postnatal growth of the rat lung. I. Morphometry

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The Postnatal Growth of the R a t Lung'
I. MORPHOMETRY
PETER H. BURRI, JAROSLAV DBALY AND EWALD R. WEIBEL
Department of Anatomy, University o f Berne, Berne, Switzerland
ABSTRACT
The postnatal growth of the lung was quantitatively investigated
in rats aged 1 , 4 , 7, 10, 13, 21,44 and 131 days by light and electron microscopic
morphometry .
Lung volume (V,) increased first directly with body weight (W). After day 10
V, followed the function WO.?O.Based on the quantitative findings the postnatal
lung growth could be divided into three phases.
1. Lung expansion (up to day 4 ) : Lung volume increase resulted almost
exclusively from an 87% enlargement of the existing air spaces.
2. Tissue proliferation (day 4 to 13): All tissue compartments showed a
pronounced mass increase, followed by a high gain in capillary volume. Alveolar
and capillary surface areas (Sa, Sc) developed rapidly due to subdivision of the
primitive air sacs.
3. Equilibrated growth (third week to adult age): An initial period of redistribution of tissue mass with septa1 lengthening and further rapid increase
in Sa and Sc was followed by proportionate alveolar growth. In the adult further
lengthening of the interalveolar septa or continued alveolar formation could not
be excluded.
During the period of fundamental internal remodelling of the lung, its function, as determined by the morphometric pulmonary diffusing capacity, was not
impaired.
Until recently the question of whether
the development of the mammalian lung
ceased a t birth was unsettled. Based on
adult to newborn alveolar size relationships, Koelliker (1879) stated that the
structure of the adult lung resulted from
simple enlargement of the structures existing at birth. His concept, supported by the
work of others (Willson, '22; Barnard and
Day, '37; Norris, Kochenderfer and Tyson
'41) was opposed to the view that new
respiratory units are formed during postnatal growth of the lung (Broman '23;
Heiss '23; Bremer, '35, '36/'37; Willson
'28; Cohn, '40; Engel, '47, '53; Clemens,
'55).
Today this problem seems to be solved.
Within the last fifteen years, more and
more evidence indicates that in most mammalian species new alveoli are formed postnatally and hence the newborn lung cannot be considered to be a miniature of the
adult one (Dingler, '58; Emery and Mithal,
'60; Boyden and Tompsett, '61, '65; DunANAT. REC., 178: 711-730.
nill, '62; Neuhauser, '62; Emery and
Wilcock, '66; Reid, '67; Weibel, '67a;
Davies and Reid, '70; Burri and Weibel,
'71a). Disagreement is still great, however,
on the extent and mode of this postnatal
structural rearrangement and on the age
at which this process is completed. Hoping
to elucidate some of the events involved
in postnatal lung growth we attempted a
systematic analysis of the rat lung during the first three weeks after birth.
Our approach was based on three
methodological facets : ( 1) a quantitative
investigation by means of stereologic techniques; ( 2 ) a n autoradiographic analysis;
( 3 ) a light and electron microscopic
purely morphological study.
This paper presents the quantitative
data obtained on the growing rat lung. It
should provide the basis for the understanding of the autoradiographic findings
Received July 31, '73. Accepted Oct. 12,'73.
1This study was supported by grants 3.5.68 and
3.682.71 from the Swiss National Science Foundation.
711
712
P. H. BURRI, J . DBALY AND E. R. WEIBEL
as well as of the morphological alterations
described in two further publications
(Kauffman, Burri and Weibel, in prep.;
Burri, in prep.).
a t 180 EL intervals in a plane parallel to the
inferior surface of the lobe. They were
sequentially arranged on a glass slide and
stained with hematoxylin-eosin.
MATERIALS AND METHODS
D. Specimen preparation for
electron microscopic
investigation
A. Animals
Young rats from about seven litters of
the CFN-COBS-strain were killed in groups
of three at the age of 1, 4, 7, 10, 13 and
21 days, day 1 corresponding to the age of
24 -t 6 hours. No distinction regarding the
sex of the animals was made, since there
are no differences in growth rate between
sexes in the early postnatal period (Grab
and Munker, '64).
To facilitate interpretations, data from
rats, 44 and 131 days old, have been included in some tables or in the text. They
were obtained in previous studies with
identical techniques on rats of the same
strain but i n another context (Burri and
Weibel, '71a,b). The group of day 131 exclusively comprised male animals.
B. Lung fixation
The animals were anesthetized with
Hypnorm" (FentanylO.2 m g and Fluanison
10 m g l m l ) in a dose of 0.2 mljlOO gm
body weight, diluted 1:lO with saline.
After tracheotomy and careful perforation
of the diaphragm to create a pneumothorax, their lungs were fixed by intratracheal instillation of glutaraldehyde at a
standardized pressure of 20 cm of water
in a supine position, according to techniques used previously (Weibel, '70/'71;
Burri and Weibel, '71a). When flow of
fixative stopped, the trachea was ligated,
the chest organs were removed and stored
in toto for two to four hours in the fixative
at a temperature of 4°C. Then, the lungs
were neatly dissected and their volume
measured by saline displacement (Scherle,
'70 ) .
C . Specimen preparation f o r light
microscopic investigation
A light microscopic morphometric investigation of the lung was performed,
using step sections through a whole lobe.
For this purpose the right middle lobe of
each lung was embedded in paraffin wax;
step sections of 4 thickness were obtained
For electron microscopic morphometry
the lower part of the left lung was taken
as a representative sample for the analysis
of lung parenchyma. The reasons for this
choice have been discussed previously
(Burri and Weibel, '71a). Slices of the
lowerehalf of the left lung were cut into
small blocks, washed in phosphate buffer
for one and one-half hours and postfked
in O,O, for two hours. They were given a
second wash in maleate buffer, then immersed in 0.5% uranylacetate (block contrast) for one hour at room temperature,
dehydrated in a graded ethanol series, and
embedded i n Epon 812 (Luft, '61). Large
sections of regular thickness, cut on an
LKB-Ultrotome 111 with diamond knives,
were picked up on Parlodion 200 mesh
grids and additionally stained with leadcitrate (Reynolds, '63). Blocks containing
throughout large vessels or bronchioles
were discarded according to the morpliometric standards established below.
E. Solutions
The osmotic pressure of the following
solutions was adjusted to 340 mosm except for the uranyl-acetate block-contrast
step.
1. Glutaraldehyde fixation
For lung fixation a 2.5% glutaraldehyde
solution in 0.03 M K-phosphate buffer at
pH 7.4 was used. Prior to instillation the
solution was cooled to 4°C.
Postosmification
The washing of the blocks between aldehyde and osmium fixation was performed
in a n ice-cooled 0.2 M K-phosphate buffer
of pH 7.4. The blocks were postfixed in a
chilled 1% OsOl solution in 0.2 M s-collidinejHC1 buffer a t pH of 7.4.
2.
3. Block staining
The washing between osmium fixation
713
MORPHOMETRY OF POSTNATAL LUNG GROWTH
and block-staining was done in 0.05 M
maleate-NaOH-buffer of pH 5.0. For blockstaining 0.5% uranylacetate was added to
the above maleate buffer.
cytes); ( 2 ) the alveolar and capillary surface areas; (3) the harmonic mean thickness of the air-blood barrier (Weibel and
Knight, '64).
F. Morphometric analysis
( a ) Light microscopic evaluation
The major purpose of these measurements was to establish, whether there was
a shift in the volume density of lung
parenchyma, i.e., the gas exchanging parts
of the lung, during growth. Step sections
were evaluated at a magnification of 200
times, using the Wild M501 automatic
sampling stage microscope (Weibel,$70).
All sections were systematically covered
with a square test point lattice with a
period of 550 p. This was performed by
marking just one test point in the centre
of the test-screen and by traversing the section automatically and stepwise by one
width of the test-screen about every 1.5
sec. The volume densities of the following
compartments were determined : lung
parenchyma (= gas exchange region) and
non-parenchyma. The latter comprises the
connective tissue septa, the airways down
to the respiratory bronchioles, and the
blood vessels down to a diameter of 25 I".
Furthermore the point hits on air and
tissue in the whole lung and on lumina of
the blood vessels and of the airways in the
nonparenchyma were recorded.
(c)
( b ) Electron microscopic evaluation
In most cases three blocks from each
lung were randomly chosen to be cut and
from every block one section was taken to
be photographed. From each section 12
pictures were taken in the mesh-corners of
the supporting grid on a Philips EM 200 at
a primary magnification of about 1200
times.
The photographs were analysed at a final
magnification of 11,000 times on a projection screen containing a multipurpose test
system with 84 lines and 168 test
points for point and intersection counting
(Weibel, '69).
The parameters estimated were : ( 1)
volume densities of the air spaces, of tissue
and its components (epithelium, interstitium and endothelium), and of capillaries
and their content (plasma and erythro-
Calculation of pulmonary
diffusing capacity
We calculated for each animal the pulmonary diffusing capacity (DL)
using the
following equations (Weibel, '70) :
(1)
1
1
1
1
DL
Dt
Dp
De
-=-+-+-
where the right hand components, the diffusing capacities for tissue (Dt) plasma
(Dp) and erythrocytes (De), were obtained
by :
1
( 2 ) Dt == Kt . 1/2(Sa + S c ) . Tht
(4) D , = O . V ,
The structural parameters (alveolar and
capillary surface areas S, and S,, the capillary volume V, and the harmonic mean
tissue thickness T h t ) were morphometrically determined for each animal. For the
harmonic mean plasma thickness T h p a constant value of 0.18 was assumed. This
value represents the mean obtained from
measurement on a limited number of rats.
The choice of the physical coefficients
K,, K, and 0 influences the absolute values
for the diffusing capacity; following our
practice for comparative studies, the
highest values found in the literature were
applied in this study (Weibel, '70/'71,
Burri and Weibel, '71a, Siegwart, Gehr, Gil
and Weibel, '71).
@
(d )
Data processing
The point hits and intersection counts
were registered by means of a n electrical
counter with automatic data transfer to
tables and punch cards (Weibel, '67b).
Calculations and statistical analysis were
performed on a Bull-gamma 30 computer
with the Pocoster-program of Gnagi, B u d
and Weibel, '70. As a rule a result was considered to differ significantly from another
one, if the significance level a of 0.05 was
reached or surpassed in Student's t-test.
714
P. H. BURRI, J. DBALY AND E. R. WEIBEL
TABLE 1
N u m b e r o f animals, body weights, lung volumes and specific lung volumes for the various age-groups.
Values are m e a n s 2 1 standard error. Significant differences between
groups are given f o r specific lung volumes
Group
Ane in davs
Number of
animals
1
3
4
3
7
3
10
3
13
3
21
Body weight
Lune volume
Specific lung
volume
gm
cm3
cm3/lOO gm
*
7.2
0.1
12.0
2 0.2
15.6
2 0.2
18.5
& 0.3
0.57
& 0.01
0.90
7.89
t 0.32
-
7.49
No
* 0.03
2 0.42
1.22
t- 0.05
2 0.30
1.44
t- 0.07
& 0.29
7.82
25.7
1.80
& 0.06
7.00
&0.10
3
38.5
2 0.1
2.25
-to.11
44
8
140.5
t 7.1
131
3
456.7
12.0
RESULTS
A. Body weight and lung volumes
The data for the body weights ( W ) and
lung volumes (V,) as well as the specific
lung volumes (i.e., lung volume per 100
gm body weight) are contained in table 1.
During the first ten days the increase
of lung volume was directly proportional
to the increase in body weight, a s is reflected by the constant values of about 7.8
cm3/100 gm for the specific lung volume
(V,,/W) and by a n exponent of about 1 in
the allometric function (fig. 1). After day
10 the lung volume increased at a less steep
rate (fig. l ) , the values for VL/W falling
gradually to 5.86 cm3/100 gm on day 21
and to 2.93 cm3/100 gm at the age of 131
days.
B. Morphometric results
1. Light microscopic morphoinetric
results
A synopsis of the compartmental distribution of volume densities in the various
age groups is presented in table 2. The
parenchymal volume density (V,.,) ranged
between 0.82 and 0.85 and showed no
definite trend.
No
7.80
2 0.7
2
Difference to
previous group
sienificant?
No
Yes
P
< 0.05
r 0.44
P
< 0.05
2 0.20
4.54
rO.ll
P
< 0.005
13.37
t 0.52
5 0.04
P
< 0.005
6.34
Yes
5.86
Yes
Yes
2.93
The volume density of total tissue in the
lung (V>7tl)ranged from 0.29 to 0.38; the
relative amount of air in parenchyma and
non-parenchyma (V,,,) fluctuated from
0.54 to 0.66 and inversely with VVtl.For
both parameters, the only significant
change could be located between days 4
and 7, where relative tissue mass increased
by 17% at the expenses of the air compartment. The volume density of blood
vessel lumina (V,.,,) showed quite large
variations during the first three weeks; this
could however be due to chance, as indicated by the large standard errors of
?.Three different types of results must be considered:
( a ) Relative results
These represent the primary morphometric data reflecting the relative volumes (= volume densities Vv)
of the compartments of the lung and the relatlve surface areas ( = surface densities S V ) , i.e., the surface
areas per unit lung volume. The relative results a r e
directly derived from the point counts (Glagoleff, ,'33;
Chalkley, '43) and the lntersectlon counts (Tomkeieff,
451, obtained by superimposing the test screen to the
image to be analysed.
(b) Absolute sesults
Except for the harmonic and arithmetic mean thickness, these results are calculated by multiplication of
the relative results with the absolute volume of lung
parenchyma (= total lung volume x percentage of
lung parenchyma).
( c ) Specific results
These express the absolute results with respect to
body weight. They are given in volumes (cm3) or in
surface areas (m2) per 100 gm body weight.
715
MORPHOMETRY OF POSTNATAL LUNG GROWTH
50
VIL
cm 3
20
10
W
z 5
3
014
1
,
2
5
10
20
50
100
200
500
.
DO
BODY WEIGHT
Fig. 1 Bilogarithmic plot of lung volume (V,) against body weight ( W ) . Two growth
phases can be distinguished (days 1 to 10 and 10 to 1 3 1 ) with two distinct allometric relationships. The points represent the means of the age groups listed in table 1: brackets
include ? 1 standard error. The allometric functions were calculated with the values of
the individual animals.
some groups. On day 131 this parameter was however significantly increased
( P < 0.005).
The volume densities of bronchial and
bronchiolar lumina (V,,,) decreased significantly from 0.059 at day 1 to 0.042 at
day 21, a reduction of more than 28%
( P < 0.05). On day 44 it was only 0.032,
and on day 131 i t was once more significantly reduced to 0.023 (P < 0.05).
2. Electron microscopic morphometric
results
( a ) Relative results
The compartmental volume distribution
within the parenchyma of the lungs of the
various age groups is given in table 3.
The volume density of air spaces (Vva)
increased between days 1 and 4 from 0.63
to 0.75 and was reduced again from day 7
on to values around 0.69. On day 44 this
parameter was significantly higher by
about 15%.
The volume density of tissue (Vvt)
varied in the opposite sense: between days
1 and 4 there was a significant fall from
0.28 to 0.19, followed by a n increase to
0.25 on day 7. After day 13 i t was gradually reduced and reached 0.087 on day 131.
The tissue comprises a n epithelial, an
716
P. H . BURRI, J . DBALY AND E. R. WEIBEL
TABLE 2
Light microscopic evaluation of relative quantitative composition of lung tissue. V o l u m e densities (Vs)
are given as m e a n s f 1 standard error. p , parenchyma; n, non-parenchyma; t l , tissue in whole lung;
la, lung air spaces ( i n c l . a i r w a y s ) ; vl, lwmina of blood vessels ( d o w n to 25 p); bl, l u m i n a of airways
( d o w n to respiratory bronchioles).VVp+ Vs,= 1 . VVti+ V,(, + V l U r= 1 . Significant differences between groups with asterisks: * p 0.05; * * p 0.005
<
<
~~~
~~
~
Parameters
Group
Age in days
1
4
V\ u
VV”
0.822
2 0.006
0.178
f0.006
0.318
-t 0.015
0.824
0.176
20.013
* 0.003
2 0.013
7
10
13
21
0.820
0.180
-t 0.010
f0.010
0.295
VV”1
VVbl
0.047
t 0.010
0.059 *
2 0.005
**
0.047
t 0.007
0.055
t0.007
**
0.061
t 0.009
% 0.002
VVI,
VVti
0.635
f0.009
**
0.658
f0.006
0,357 :‘*
% 0.009
0.582
i0.009
0.817
0.183
0.375
0.544
0.081
f 0.026
20.026
2 0.026
? 0.032
2 0.018
0.049
0.037
t 0.004
0.854
0.146
0.339
0.605
0.056
0.037
f0.011
2 0.011
f0.035
? 0.036
& 0.002
% 0.003
0.823
0.177
f0.015
0.289
t 0.021
0.647
0.064
0.042
-C 0.025
-t 0.007
f0.004
f 0.015
*
TABLE 3
Electron microscopic estimation of volume densities of compartments in lung parenchyma.
Volume densities of air spaces ( V l u ) , of capillary blood ( V s c ) , and of tissue ( V s t ) . Values
are m e a n s -t 1 standard error. Important changes occur in t h e first week and a f t e r day 21
( s e e t e x t ) . Asterisks indicate that difference to n e x t younger group i s significant: * P < 0.05;
** P < 0.01; * * * P 0.005
<
Group
Age in days
1
4
7
10
13
21
44
131
VVa
Vve
0.630f 0.020
0.75220.014 * * *
0.687 2 0.027 *
0.691 ? 0.024
0.668 -t 0.024
0.699 f0.044
0.799C0.008 * * *
0.772 f0.005
0.088 f0.002
0.055t0.018
0.064 2 0.012
0.065e 0.003
0.087 f0.008
0.108k 0.020
0.092%0.005
0.141*0.005 ***
endothelial and an interstitial compartment. These components underwent a
biphasic shift consisting in a relative augmentation of the interstitial volume from
day 1 to 7 with a concurrent reduction of
epithelial volume density (table 4 ) . The
inverse phenomenon took place between
days 13 and 21. Both these changes were
highly significant for either compartment
(P < 0.005). The endothelial volume density showed the same tendency as that observed for the epithelium, however only the
increase to day 21 and day 44 reached a
significant level.
The volume density of capillary blood
(VVc,table 3 ) showed at day 4 a fall by
vst
0.282 2 0.022
0.19420.005 **
0.249fO.018 **
0.244 2 0.025
0.245 t 0.022
0.194 -t 0.029
0.109tO.006 * *
0,0872 0.001 *
more than 35% which had recovered on
day 13. For a certain time this parameter
stabilized around 9-10%. In adult rats
however, it was again significantly increased to 14%.
and capillary
Both the alveolar (S,.)
(Sv,) surface densities showed similar relationships with age (fig. 2). The sudden
drop by about 30% for Sv. and 40% for
S,, between days 1 and 4 was followed by
a fast recovery until day 10. On day 21
S\. was enlarged by 47% and Sv. by 30%,
when compared to day 1. On day 44 Sv.
was significantly reduced to 750 cm5/cm5
(Burri and Weibel, ’71a) and remained at
this level until day 131.
717
MORPHOMETRY OF POSTNATAL LUNG GROWTH
TABLE 4
Composition of parenchymal tissue. Volume densities of epithelium (V,,,), interstitium
(VV,,,) and endothelium (VV,,). Values are means 2 1 standard error. Asterisks
indicate that difference to next younger group is significant:
* P 0.05; *:'P 0.01; **'<'P 0.005
<
<
<
Group
Age in days
Vvell
vvin
Vven
1
4
7
10
13
21
44
131
0.237 2 0.007
0.191 20.009 **
0.16320.003 *
0.159 rt 0.01 1
0.151 f0.005
0.26320.007 ***
0.274 f0.013
0.282f 0.013
0.578 t- 0.006
0.647 f0.022 *
0.678 f0.017
0.659 0.014
0.664? 0.012
0.514f0.012 ***
0.464 C 0.017
0.463 2 0.018
0.18520.001
0.1622 0.022
0.159& 0.015
0.182&0.004
0.185t0.008
0.223 2 0.012 *
0.261 I?I 0.007 * *
0.255 f0.015
SV
m2
4rfi
t
0.12
m
2
w
5
W
V
a
LL
(L
3
*
0.08
t
(L
42
-
a
Q
V
n
f 0.04
.----a
sv,
sv
C
LL
4
s
3
Q
8
I
1
4
I
7
1
1
10
13
I
21 DAYS
AGE
Fig. 2 Plot of alveolar (S,.,) and capillary surface density (Sv.) against age. T h e rapid
fall on day 4 is rapidly recovered. Brackets include 2 1 standard error.
(b)
,
I
Absolute results
Volumes and surfaces
The absolute volumes of the different
compartments of lung parenchyma and the
absolute alveolar and capillary surface
a
areas are listed on table 5a and 5b together
with the rates of increase.
Changes between days 1 and 4. Va increased by 87% from 0.30 cm3 to 0.57 cm'
(table 5a j. The other parameters exhibited
no significant changes. The increase in
v
TABLE 5a
<
<
<
Total percent
change from
day 21 to 131
131
Percent change
from day 44 to 131
44
Percent change
from day 21 to 44
Total percent change
from day 7 to 21
21
Percent change
from day 13 to 21
13
Percent change
from day 10 to 13
10
Percent change
from day 7 to 10
7
Percent change
from day 4 to 7
1
4
Percent change
from day 1 to 4
Group
Age in days
66.3 * * *
965.02 35.9
31.0 **
1277.02 74.2
32.3 *+
2159.0 C 268.5
69.1 *
0.31720.036
69.5 *
0.365 2 0.036
0.512 C 0.022
40.3 *
0.551 2 0.051
7.6
0.251 2 0.024
73.1 * *
0.289 t 0.034
0.066 2 0.014
_f
0.019
520.1
650.8
159.6
335.2
208.1 ***
96.2 ***
264.9
102.4 ***
7857.5 2 379.3
129.0 ***
***
79.9
3882.9 f 192.2
193.1
15.6
90.0 * * *
1.0472 0.060
2.398 k 0.056
**
62.7 * * *
59.6
0.565 t 0.040
73.8
0.9192 0.041
143.7 ***
0.480 f 0.022
41 .O
-6.1
15.1
1.479k 0.025
***
198.5
*
0.354 2 0.031
30.4
0.377 k 0.023
15.1
8.130f0.337
216.0
4.1432 0.108
90.0
45.9
0.197 t 0.030
1.31120.173
26.7
77.6 * * *
0.135 2 0.011
15.2
0.076 2 0.005
27.1 *
1.0352 0.068
18.0 * * *
0.814
22.1 * * *
0.690 2 0.013
46.7
736.72 36.8
5.6
7.4
0.0
*
285.2
100.6 ***
8158.72 343.5
92.0 ***
4067.2% 196.9
212.6
59.4
2118.02271.5
48.6 * * *
1329.02 56.2
32.0
894.12 50.2
75.2 *
677.52 96.9
- 9.4
426.4-e 28.1
386.72 105.3
383.22 15.4
442.92 38.9
87.1 * * *
_f
cm3
0.042 2 0.001
0.042 t 0.014
cm3
0.3022 0.008
0.565 0.013
sc
CnY
Sa
CrnZ
VS
cm3
0.17720.012
0.187k0.015
Vt
cm3
0.13520.012
0.145f0.001
vc
Va
Absolute volumes and surfaces o f the parenchymal lung compartments. V a , air space volume; V c , capillary blood volume; V t , tissue volume;
V s , septa1 volume (volume o f tissue + capillary b l o o d ) ; Sa, alveolar surface area; Sc, capillary surface area. Values are means 2 1
standard error. Significance levels of percent changes: * P
0.05; ** P
0.01; *** P
0.005
719
MORPHOMETRY OF POSTNATAL LUNG GROWTH
TABLE 5b
Absofute volumes of tissue components. V o l u m e of epithelium (V,,), of interstitium (V,"),
o f endothelium (Ve,,). Values are m e a n s e 1 standard error. Significance levels f o r
differences: * P
0.05; ** P
0.01; * * * P
0.005
<
<
Group
Age in days
1
4
Percent change
from day 1 to 4
7
Percent change
from day 4 to 7
10
13
Percent change
from day 7 to 13
V,,
V.,
Vi"
mm3
3 2 . 0 t 2.0
27.72 1.4
mm3
mm3
78.4& 7.7
94.02 2.6
25.12 2.3
2 3 . 6 2 3.2
- 13.2
+ 19.8
- 6.0
40.8% 3.6
$47.2
*
45.5-t 3.9
56.92 3.9
+39.4
'2
21
Percent change
from day 13 to 21
93.22 8.5
44
Percent change
from day 21 to 44
155.02 12.7
131
Percent change
from day 44 to 131
262.4'- 18.9
Total percent
change from day
21 to 131
<
+63.8
**
66.3 *
69.3
***
181.6
lung volume was brought about almost exclusively by the expansion of air spaces,
whilst the small augmentation of tissue
mass was due to the enlargement of the
interstitium (tables 4, 5b).
Changes between days 4 and 7. In this
period Va increased only by 22% whereas
all the other parameters showed augmentations between 47 and 75% (table 5a).
The alveolar and capillary surface areas
were enlarged by 66 and 75% respectively.
Changes between days 7 and 21. This
period was characterized by a more equilibrated growth of air and tissue compartments. Before day 13 both showed similar
rates of increase: 18% and 15% from day
7 to 10 and 27% and 30% respectively
from day 10 to 13. After day 13 the air compartment grew much faster; thus i n the
whole period from day 7 to 21 the air space
volume almost doubled compared with a n
increase of only 41% for the tissue, All
tissue components proliferated u p to day
13, the endothelium showed a n especially
170.42 18.2
$81.4
*+
190.7t25.1
250.5 t 18.1
$47.0 *'
182.52 19.9
- 27.2 :
264.4 224.3
44.9 *'
422.72 2.3
59.9
131.6
***
4 0 . 1 2 5.0
+ 70.2 *
52.3? 5.9
69.32 3.1
$72.9
***
78.12 3.6
+ 12.7
146.02 6.8
89.6
**:'
234.1 223.1
60.3
ak
*1
199.7
high volume gain (table 5b). After day 13
only the epithelium increased markedly,
whereas the interstitium exhibited a significant loss of absolute volume!
It is important to notice the extremely
high growth rate of the capillary blood
compartment; it was more than doubled
between days 7 and 13 and was three
times as high on day 21 as on day 7.
The alveolar and capillary surface areas
increased three-fold between days 7 and
21 (table 5 a ) . The changes which occurred
in the alveolar volume and alveolar surface
area in the first three postnatal weeks are
well reflected by the surface to volume
ratio of air spaces (S,/V), (fig. 3 ) ; this
ratio estimates the degree of complexity of
the outlines of the air compartment. The
capillary loading of the alveolar surface
area, measured by the ratio Vc/Sa, fell
until day 10 and then increased sharply
between days 10 and 13. The mean septum
thickness, estimated by the ratio of septa1
volume to half the alveolar surface area
720
P. H. BURRI, J . DBALY AND E. R. WEIBEL
0.2-- 2
( 2 . Vs/Sa), showed a tendency to decrease with a marked fall after day 13.
Changes after day 21. Since the increase in lung volume was almost sixfold
from day 21 to day 131, all measured
parameters were found to be greatly augmented (table 5a). The lowest rate of increase was in the tissue compartment
which enlarged by a factor of 2.6, whereas
the highest rate was shown by the capillary
volume which increased 7.5 times.
The alveolar air compartment changed
at about the same rate as lung volume, by
a factor of 6.2. The alveolar and capillary
surface areas were enlarged to a similar
degree by factors of 3.6 and 3.9 respectively.
Within the tissue compartment Vep and
Ven underwent an almost threefold increase whereas the interstitial volume was
only augmented by a factor of 2.3 (table
5b).
fl Air-blood barrier thickness
The arithmetic as well as the harmonic
mean barrier thicknesses of tissue de-
-0.4
creased progressively (table 6). The arithmetic mean thickness (;) was highest on
day 4 with 3.8 p, dropped to 1.7 on day
21 and to 1.15 on day 131. This reduction was reflected by decreases in all three
components of the barrier, but the interstitial layer was most markedly reduced as
is illustrated in figure 4. The harmonic
mean thickness of the tissue barrier TI,^),
was lowered from 0.64 on day 1 to 0.39 I.1
on day 21 (fig. 5 ) ; the latter corresponds
to values in adult animals (0.40 on day
131, table 6).
Y Pulmonary diffusing capacity
The absolute values of the morphometric
pulmonary diffusing capacity D, is plotted
double-logarithmically against body weight
in figure 6. The broken line represents the
calculated regression line for the whole
period investigated (day 1to 131) and follows the allometric equation :
D L , - ~ S=~0.0123 . W0.*'
where W is given in grams. The correlation
MORPHOMETRY OF POSTNATAL LUNG GROWTH
721
-
7q 5.0
z_
m
m
W
z 4.0
Y
uI
I-
- 3.0
Lz
Lz
Q.
m
5
2.0
W
I
0
L
I
1.0
I
c-
[L
4
AGE I N
DAYS
Fig. 4 Arithmetic mean barrier thickness of tissue ( 7 ) and its components epithelium,
interstitiurn and endothelium. After day 13 the barrier thickness is greatly and significantly
reduced at each age step. Columns are means: brackets include f 1 standard error.
coefficient r for this curve was 0.986. A
significance test of the exponent showed
that the slope of the curve dfiered significantly from 1.0 (P < 0,001).
However, analysis of the data from the
animals aged day 1 to 21 compared with
the data from animals after day 21 showed
that there were significant differences in
the rate of increase of diffusion capacity.
The slope of the curve relating days 1
to 21 was 1.04, whereas the slope after
day 21 was 0.73. The difference is significant at P < 0.005. In the first period
the slope did not differ significantly from
1.0, whereas i n the later period the difference was highly significant.
( c ) Specific results
The results expressed per 100 gm body
weight are presented in table 7. The
parenchymal portion of the lung increased
proportionally to body weight during the
first ten days, but later on it lagged behind
body growth, resulting i n a lowering of the
quotient Vp/W.
Up to day 10 the other parameters
varied according to the shifts which
occurred in their relative values, i.e., either
in their volume or surface density. The
specific results thus reflect the expansion
of the air spaces between days 1 and 4:
Va/W increased, but there was a marked
diminution in the specific volumes of capillary blood and tissue a s well a s i n the
specific alveolar and capillary surface
areas. These considerable diminutions
however were already recovered to a major
part on day 7.
If we compare the specific values of day
21 with those of day 44 and of day 131, a
drastic and significant fall in all parameters of the older animals can be observed.
For some of these parameters (Vp/W,
Va/W, Vt/W) this tendency to decrease
becomes noticeable before day 21.
722
P. H. BURRI, J. DBALY AND E. R. WEIBEL
Th'
08
A
z
T
v,
06
.-
Y
I!
I
c
c
w
c 04
4
m
Z
Q
W
z
0
z
0
02
I
[L
6
I
4
L
7
+/AGE IN DAYS
Fig. 5 Harmonic mean thickness of air-blood barrier ( T h t ) . A significant decrease occurs
after day 13. On day 21 adult values are already reached. Points represent values of single
animals; columns are means; brackets include k 1 standard error.
DISCUSSION
A. Discussion of previous
quantitative findings
There are relatively few publications
devoted to the quantitative investigation of
TABLE 6
Arithmetic (T) and harmonic ( T h t ) mean thicknesses of air-blood barrier. Values are means & 1
standard error. Asterisks indicate that difference
to next younger group i s significant: * P
0.05;
* * P 0.01; + * + P < 0.005
<
<
Group
Age in days
-
T
P
1
4
7
10
13
21
44
131
3.45'0.29
3.82 ?z 0.75
3.69k 0.21
3.1720.33
2.9620.04
1.6920.14 ***
1.42 2 0.07 *
1.1520.04
Tht
P
0.637 i0.050
0.509 f0.045
0.506 k 0.015
0.449 i0.028
0.465i0.013
0.393 2 0.007 * +
0.373 2 0.014
0.40020.012
postnatal lung growth. Dunnill ('62)
counted the number of alveoli in lungs of
human infants and found that these increased from 24.10" at birth up to the age
of eight years, when adult values of around
300 million alveoli were reached. I n parallel he found a ninefold increase in the
number of respiratory airways and a linear
increase of air-tissue interface with body
surface area. Emery and Wilcock ('66)
counted alveoli on standardized transsections through the right middle lobe and
concluded that in the human lung new
alveoli can be formed up to the age of 20.
With a somewhat different technique
Emery and Mithal ('60) had already described a thousandfold increase in the
number of alveoli from birth to puberty.
More recently Davies and Reid ('70)
confirmed the findings of Dunnill, alveolar
number increasing from 17.106 at birth to
336 millions a t the age of 11 years.
The first stereologic approaches to post-
723
MORPHOMETRY OF POSTNATAL LUNG GROWTH
TABLE 7
Specific volumes and surface areas o f lung parameters during postnatal growth. Specific volume of
tung parenchyma ( V p / W ) , of parenchymal air spaces ( V a / W ) , of capillary blood ( V c / W ) , and of
parenchymal tissue ( V t / W ) . Specific surface areas o f parenchymal air spaces ( S a / W ) and o f capillaries ( S c / W ) . V / W z cm3/100 gm body weight; S / W = m z / l O O g m body weight. Values are m e a n s
k 1 standard error. Asterisks indicate that difference to n e x t younger group is significant: * P
0.05;
<
** P < 0.01; *** P < 0.005
Group
Age in days
1
4
7
10
13
21
44
131
Vp/W
Va/W
6.49
20 . 3 0
6.16
k 0 .3 2
6.42
k0.18
6.35
C 0.04
5.98 * * *
kO.01
4.84
k 0.44
3.73
c 0.10
2.30 * * *
k 0.03
*
***
4.17
i- 0.1I
4.72
k 0.19
4.42
C 0.05
4.41
20.15
4.02
rt 0.1 5
3.44
C 0.53
2.98
C0.11
1.78
0.03
*
vc/w
Vt/W
0.58
*
C 0.02
0.35
k0.13
_f
0.42
k 0.09
0.41
f0.02
0.53
C 0.05
0.51
2 0.08
0.34
***
* 0.01
1.88
C0.19
1.21 *
0.03
1.61 *
C 0.16
1.56
t 0.1 6
1.47
& 0.13
0.92 **
k 0.06
0.40 ***
* *
k 0.02
0.32
0.20
k 0.01
C 0.01
natal lung growth were made by Short
('50, '52). Mainly interested in the fate of
alveolar epithelium during growth he concluded that additional alveolar surface
area was formed postnatally by a subdivision process of alveoli which he interpreted
to be due to a purely physical factor: the
distension of the lung (Short, ' 5 0 ) , a
theory, which he confirmed for other
species including m a n i n his subsequent
paper (Short, '52).
The same parameters were estimated by
identical methods in kittens and cats by
Dingler ('58) who described the formation
of new alveoli after birth.
Of particular interest for our study is
the work of Neuhauser and Dingler ('62)
and Neuhauser ('62) who investigated the
development of the inner surface of the
lungs of suckling rats. They found a n
irregular growth rate with a slightly
steeper increase of alveolar surface area
during the second week. For this period
they also described the formation of new
alveoli. Similar findings have been reported
by Weibel ('67a). Using more elaborate
morphometric techniques his study revealed that the important structural alterations of the rat lung took place within the
first ten days of life. It was hence this short
period on which we focussed our interest.
***
Sa/W
sc/w
0.531
C 0.029
0.371
C 0.040
0.471
+- 0.018
0.523
k0.018
0.499
C 0.041
0.559
C 0.061
0.277
+- 0.009
0.172
C 0.006
0.592
f0.050
0.326
C 0.093
0.432
2 0.058
0.484
f0.020
0.519
f0.034
0.550
C 0.068
0.291
& 0.008
0.179
& 0.005
*
*
***
***
*
***
***
B. Discussion of some methodological
aspects
Because of the amount of work involved
our investigation had to be based on the
analysis of three animals per age group.
Although morphometric techniques are
rather accurate, it appears on closer inspection of our results, that for some
groups and some parameters the standard
errors are high. As a consequence some
relatively large changes do not become
statistically significant. This applies in
particular to the capillary parameters on
day 4 and 7, despite the fact, that for
these ages six blocks per animal instead
of three were evaluated. The most probable
reason for this inhomogeneity is biological
variation. Since this period is marked by
rapid transformations, differences in the
capillary development between the animals
of one group could easily occur. We can,
however, safely assume that the trend observed is not due to chance: for all species
and all ages so far investigated with our
techniques i t is known, that the capillary
surface area (fig. 2) follows alveolar surface area rather closely (Burri and Weibel,
'71a; Geelhaar and Weibel, '71; Siegwart
et al., '71 ; Weibel, '72). Furthermore Weibe1 ('67a) also observed a fall of S, on
day 5.
724
P. H. BURRI, J. DBALY AND E. R. WEIBEL
E
E
2 0-
E
E
\
0”
-E
>
1008-
06-
I-
i? 0 4 Q
a
Q
0
z
G
cn
.
02-
’..
./’ D, = 0.0073 . W
3
LL
& .lo0 .08$ 06Q
6
/
04-
//
f
3
a
0
.02-
2
4
6 810
40 60 80 100
BODY WEIGHT g
600
200
20
w
Fig. 6 Morphometrically determined pulmonary diffusing capacity (DL) for oxygen
plotted double logarithmically against weight ( W ) . Two growth phases with distinct d o metric relationships c a n be m a d e out (solid line). In a first phase (days 1 to 21) DL
increases in direct proportion with body weight; in t h e second phase it increases a t a
significantly less steep rate. The broken line represents the overall allometric function
DL = 0.0123 . W0.*j. Dots are animal values; open circle represents an aberrant value of
one animal of day 4, which has not been considered for the regression calculations.
The question may be raised, why did we
not count alveoli? In fact we did; we
counted alveoli or what appeared to be
“alveoli” in one animal per group.3 The
absolute number and the numerical density
of “alveoli” and the mean “alveolar” diameter are presented in table 8 and in figure
7 respectively. The two curves (fig. 7) indicate that between days 4 and 13 a rapid
increase in
density of “alveoli”
occurred combined with a rapid fall in
“alveolar” size. The interpretation is that
the original air spaces had become sub-
3 At a magnification of 400 times on the Wild M501
automatic sampling stage microscope we estimated on
ten fields per animal the volume density of all “alveolar like” air spaces VV.* (which is not identical with
the previously used Vva) and the mean number of
“alveolar” profiles NA=*’per field. From these parameters the number of “alveoli” per unit volume of lung
parenchyma (numerical density of “alveoli” Nva*)
could be derived usine the formula:
1
NA.* 312
Nva* = -
B
.
~
V”.”
112
The value of 1.38 for the shape coefficient 4 was used
(Weibel and Gomez, 6 2 ) , assuming alveoli to be
roughly spherical; NA=*was derived from NA.*’and
represents the number of alveoli” per unit area of
section. From the absolute number of “alveoli” and
the absolute “alveolar” volume we calculated the
sp$;;;:u”
diameter D.* by assuming “alveoli”
npg
725
MORPHOMETRY OF POSTNATAL LUNG GROWTH
TABLE 8
Absolute number of “alveoli” Na* and derived
mean “alveolar” diameter Da+, assuming “alveoli”
to be of constant spherical shape (see t e x t ) . Measurements were made o n one randomly chosen
animal per group
Age in days
1
4
7
10
13
21
Da*
Na*
106
P
2.59
4.73
10.46
18.38
25.76
33.82
54.3
51.5
39.3
34.7
33.3
37.7
divided. This is in good agreement with
our other results; the absolute values given
here however are certainly incorrect, since
a n “alveolus” at birth is obviously different
from a n alveolus of the adult lung. Thus,
we would like to emphasize that it would
be erroneous to deduce from this curve,
that true alveoli are present at birth
(Burri, in prep. ).
N ~ , * NUMERICAL DENSITY
OF
Even if one is aware of these transformations alveolar counts are inaccurate in the
growing lung:
( a ) In some stages it is difficult or
even impossible to identify safely the
alveoli in statu nascendi.
( b ) During development the alveoli
change their shape from very shallow to
rather deep excavations. The problem of
correction for alveolar shape variations
has not been properly solved so far. As
shape enters the formula for calculation
of alveolar number either in form of a
shape coefficient (Weibel, ’63, see footnote) or in form of the mean tangent diameter (De Hoff and Rhines, ’61), any
calculation not properly considering the
shape changes will be misleading.
C . Discussion of the results
1. Lung volume and light microscopic
morphometry
The pattern of lung volume growth with
“ALVEOLI ”
“ALVEOLAR” 01A M ETE R
Oa‘.
Y
58
18,
54
16.
14
50
12.
46
10
42
8
6
38
4
.34
2,
1
1
I
1
4
7
10
AGE
I
13
21
IN DAYS
Fig. 7 Numerical density (Nv,+) and mean diameter ( D a b ) of “alveoli.” Fall in D,*
and rise in Nv.* reflect the rapid partitioning of primitive air spaces occurring after birth;
the values are however not representative (see text). Each data point was obtained from
one animal only.
726
P. H. BURRI, J. DBALY AND E. R. WEIBEL
respect to body weight, especially the fall
in the ratio VJW after day 10 is in agreement with the findings of others (Neuhauser and Dingler, '62; Neuhauser, '62;
Weibel, '67a). Our data indicate that after
day 10 lung volume increases as the function W".'O.
From the light microscopic morphometric analysis i t appears that the relative
amount of parenchyma does not change
during growth.
Since the value of Vv, for day 4 is the
same as that for days 1 and 7, we can
refute the possible objection that the lung
could merely have been overinflated during instillation.
The airways do not grow in proportion
to the lung volume : their absolute volume
increases 2.8 times, lung volume four
times from day 1 to 21; from day 21 to
131 the airways enlarge 3.3 times and
lung volume six times.
2. Electron microscopic morphometry
Based on the quantitative analysis three
growth phases can be postulated in postnatal lung growth: ( a ) a short phase of
lung expansion, ( b ) a phase of intense
tissue proliferation , ( c ) a long period of
equilibrated growth.
(a)
Phase of lung expansion
The results of all parameters investigated show that from day 1 to 4 the lung
undergoes a n expansion of its air spaces.
Almost the entire change in lung volume
is due to the 87% increase of air volume,
which causes the volume density of tissue
and the surface densities of alveoli and
capillaries to fall on day 4 (fig. 2 ) . Analysis of the data of Dingler in cats ('58) and
Neuhiiuser and Dingler in rats ('62) reveals a n analogous trend of Sva.I n rabbits,
Short ('50) found the percentage of air
spaces lumen in the lung to increase
rapidly from 68.6% on day 1 to 80% on
day 5.
All these findings indicate that initially
after birth, lung growth follows chest
growth by lung expansion. The fact that
tissue mass drags behind, favors the assumption that this could be mainly a passive process.
( b ) Phase of tissue proliferation
This phase is marked by high rates of
increase i n tissue mass (Vt, Vs) and surface areas (Sa, Sc) and by a reversed
shift in lung compartments compared to
those of phase ( a ) . The surface densities
(S,,, Sv,) show a period of increase, a fact,
which has also been described for Sv. by
Short ( ' 5 0 ) , Dingler ( ' 5 8 ) , Neuhauser and
Dingler ('62), and Neuhauser ('62). The
morphologic findings presented in a companion paper (Burri, in prep.) will show
that this period is marked by a n extensive
and rapid proliferation of secondary septa1
crests, leading to a subdivision of the primary air spaces.
It is rather difficult to determine the duration of this proliferative phase, since it
depends on the parameter taken as reference. Based on some preliminary findings
of this study and considering especially the
explosive tissue proliferation we have postulated that this phase was very short and
lasted until day 7 (Burri, Dbaly and
Weibel, '72). It is however apparent that
certain parameters, capillary volume (Vc)
and surface area (Sc) continue to increase
markedly between days 10 and 13; the
capillary loading (Vc/Sa) expresses this
clearly. Thus evidently capillary development postdates tissue development.
The alveolar and capillary surface areas
increase steadily from day 4 u p to day
21, when plotted double-logarithmically
against lung volume (figs. 8,9). The curve
is flatter before day 4 and after day 21.
Weibel ('67a) has described a similar
steep increase of alveolar surface area between days 5 and 10 with a markedly lower
increase rate before and after this five day
period. Thus, the present study confirms
his data with the exception that the high
rate of augmentation lasts up to day 21.
It is obvious that the growth of alveolar
surface area which equals the 1.6 power
of lung volume during this second phase
implies some structural transformations,
as is also apparent from the curve of the
alveolar surface to volume ratio (fig. 3 ) . I n
a lung growing by mere enlargement of
its alveoli the surface area would increase
in the proportion to the two-thirds power
of lung volume.
These structural alterations comprise
not only formation of new septa but also
727
MORPHOMETRY OF POSTNATAL LUNG GROWTH
I
02
I
04
06
10
40
60
LUNG VOLUME IN 0 3
20
I
10
20
y
Fig. 8 Bilogarithmic plot of alveolar surface area ( S a ) and lung volume (VL) for 1
to 131 day old rats. Between days 4 and 21 the slope of the curve is very steep. The curve
is flatter before and later on. The two solid lines represent the calculated regressions for
days 1 to 21 and 21 to 131 respectively.
their lengthening. Both of these processes
will be responsible for higher rates of increase than those provided by a proportional expansion. The lengthening seems
to be achieved partially by stretching and
by a redistribution of the tissue mass
which does not increase between days 13
and 21.
This stretching is also indicated by the
marked decrease of Vvt after day 13, the
fall of the arithmetic and harmonic mean
barrier thicknesses, and by the reduction
of septal thickness, I n this context the
absolute mass changes in the tissue components are interesting. Apart from epithelium the rapid mass increase stops between
days 13 and 21, which is in accordance
with autoradiographic h d i n g s (Kauffman
et al., in prep.). The significant loss of
interstitial tissue mass recorded in the
third week is surprising. A possible explanation for this finding is that the freshly
formed intercellular fiber mass and the
ground substance could be condensed.
Since i t appears from our results that the
initial part (days 7-13) of the steep rise in
Sa and Sc is due to tissue proliferation,
whereas the second part of this steep increase (days 13 to 21) is probably due to
a rearrangement of tissue mass, we conclude that the phase of intense tissue proliferation lasts until the end of the second
week.
( c ) Phase of equilibrated growth
From our data we can deduce that a
period of more equilibrated growth starts
somewhere between days 13 and 21. During this phase, the newly formed respira-
728
P. H. BURRI, J. DBALY AND E. R. WEIBEL
SC
8000
6000
LOO0
N
r
5
z
= 095
2000
Q
w
(r
a
1000LQL
n
3
600
cn
>
(L
Q,
2
400
a
Q
0
200
0
02
04
06
10
2
LUNG VOLUME
4
IN
6
I
10
20
VL
cm3
Fig. 9 Bilogarithmic plot of capillary surface area (Sc) and lung volume (V,) for
rats aged 1 to 131 days. The very steep slope between days 4 and 21 is preceded and followed by flatter segments. The two solid lines correspond to the calculated regressions for
days 4 to 21 and 21 to 131 respectively. The open circle represents an aberrant value of
one 4 day old animal; it was excluded from the calculations.
tory units are progressively enlarged, the
quantitative relationship of their components however can still undergo some
shifts
After day 21 tissue mass is further increased but to a lesser degree than air
space volume (tables 3, 5a). On the other
hand, the volume of the blood compartment (capillaries and larger blood vessels)
showed the greatest increase.
Alveolar surface area was enlarged 3.6
times between days 21 to 131, the alveolar
air compartment 6.2 times. If one assumed alveolar surface area to increase
by simple enlargement, i.e., by proportional growth of the existing air spaces,
the surface should increase by the twothirds power of air volume change, or by
a factor of 3.4, a figure very close to that
which we measured. The exponent of the
allometric relationship between Sa and VL
is 0.71, i.e., slightly higher, but not significantly different from 0.67, the 95%
confidence interval extending from 0.57 to
0.85. Correspondingly, the alveolar surface
density decreased after day 21 but reached
the adult value already on day 44 (750
cm2/cm3; Burri and Weibel, '71a). Moreover, the alveolar surface to volume ratio
(S/V). remained also unchanged between
days 44 and 131; these findings thus would
favour the assumption that new gas exchange surface could be formed as long as
the lung continues to grow. In rats, lung
growth seems to be possible up to the adult
age, since in this species the thoracic cage
does not ossify until late adulthood (Dawson, '27). Weibel ('67a) and Bartlett ('71)
MORPHOMETRY OF POSTNATAL LUNG GROWTH
postulated that new alveoli continued to be
formed in adult rats. These authors found
by light microscopic morphometry Sa to
increase a s a function of VLo.'* and VLo.8'
respectively. We must consider however
that a higher rate of increase of Sa (as
compared to the function VL0.67)
could also
be achieved by expansion combined with
a n overproportional lenghthening of the
interalveolar septa. Based on the available
data this problem cannot be settled and
certainly needs further clarification.
3 . Morphometric estimation of
pulmonary diffusingcapacity
The pulmonary diffusing capacity in
growing rats had been found to increase
proportionally to body weight ( Weibel,
'67a). This is partially confirmed by our
data since we found this to be true for the
first three weeks of postnatal lung growth.
One can, however, notice that between
days 4 and 10 six out of eight points lie
below the curve (fig. 6). Analysis of the
calculations for DL shows that this is due
mainly to small capillary volumes. This
fact could perhaps point to the critical
aspect of postnatal pulmonary growth: the
capillarisation and the full deployment of
capillaries i n the newly formed septa is
the latest process involved in the morphological transformations (Burri, in prep.).
The finding that after day 21 DLincreased
as a function of W0.73reflected the smaller
rate of increase i n lung volume which
occurred with body growth during this
phase. Siegwart et al. ('71) in a study on
adult dogs of different strains (and hence
body weights) described DL to grow as a
function of W'.". I n interspecies comparisons Stahl ('67) found physiological DL to
increase with W'.'8 whereas Weibel ('72)
found morphometric DL to increase with
W0.96. These findings are contrasting with
ours. We must however emphasize that our
data were obtained on growing animals of
the same species. Therefore they probably
cannot be directly compared to those obtained by the authors cited above.
However that may be, it is important to
note that in the rat, lung function, a s defined by the morphometric pulmonary
diffusing capacity, is not seriously affected
in the immediate postnatal period by the
massive transformations of lung structure.
729
It should be emphasized however, that the
transformations here described pertain for
the rat lung; further work will have to
show to what degree differences in maturity at birth may shift the period of transformation with respect to birth.
ACKNOWLEDGMENTS
The authors wish to thank Mrs. Marlise
Witschi, Mrs. Silvia Stoller and Mr. Karl
Babl for their skillful technical assistance
and Mrs. Esther Hassler for the typing of
the manuscript.
We are indebted to Dr. J. B. Forrest for
his reading of the manuscript and his
suggestions.
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