type 2 epithelial cells, these decline rapidly after day 10 and are undetectable on day 21.1° Therefore, we assume the bulk of alveoli to be formed within a week. The ultrastructure of these newly formed secondary septa is very similar to the primary ones, that is, capillaries can be found on either side of a connective tissue sheet. On higher crests, however, the capillary lumina are often narrowed or closed towards the tip, suggesting temporary closing of capillary segments. This would enable the capillary loops to be lengthened, in order to allow the short tissue ridges to increase rapidly in height, thus deepening the at first shallow alveoli. As emphasized by several authors,lzJs our observations also suggest that the elastic tissue is likely to assume an important role in alveolar formation. Elastic fibers were exclusively found to run along the edge of the secondary septa, increasing in caliber with age and with the height of the crests. The elastic network seems to form the abutment against which alveolar formation can occur. In lungs at three weeks, a great deal of interalveolar septa are mature, eg, their main feature is a single capillary, whose endothelium forms part of a thin air-blood barrier on both sides of the septum. This means that both the primary and the secondary septa, which in the second postnatal week still possess double capillaries, have to undergo a complicated structural modification within a few days. Morphometry reveals that between days 13 and 21 the interstitial tissue mass is significantly decreased (Table 1).This is confumed by our observations that "redundant" tissue, especially space f l i n g connective tissue in between capillaries, has virtually disappeared on day 21. Simultaneously the central interstitial layer of the septa has also been thinned out. Furthermore, there is some evidence for fusions between capillaries of both sides of the septum. Although the latter remains yet to be proved, the above "excavation" of the primitive septa may represent the key principle accounting for the configurational changes transforming the primitive septa to the mature ones.
the lung. Phil Trans Roy Soc ( B ) , London, 235:s-86, 1950 9 Burri PH, Dbaly J, Weibel ER: The postnatal growth of the rat lung. Part I: Morphometry. Anat Rec 178:711-730, 1974 10 Kauffman SL, Bum PH, Weibel ER: The postnatal growth of the rat lung. Part II: Autoradiography. Anat Rec ( in press ) 11 Burri PH: The postnatal growth of the rat lung. Part 111: Morphology Anat Rec ( in press ) 12 Dubreuil G, Lacoste A, Raymond R: Observations sur le developpement du poumon humain. Bull Histol Appliq Physiol Pathol 13:23&245, 1936 13 Loosli C,Potter EL: Pre- and postnatal development of the respiratory portion of the human lung. Am Rev Resp Dis 80:5-23, 1959 Discussion
Dr. Euanc: How long after labeling were your animals sacrificed, and what significance do you attach to the failure of type 1 alveolocytes to take up much thymidine? Dr. Burri: One hour. Karen O'Hare showed that type 1 cells became labeled 14 days after other cell types in the developing rat lung in utero. Several investigators have shown that in short term labeling studies, the type 2 cells take label, but type 1 cells do not. These data support the currently accepted notion that type 1 cells are derived from type 2 cells, and that the latter constitute the only "reserve" epithelial cells of the alveolus. Dr. Reeves: How does hypoxia affect the early postnatal development of the lung? Dr. Burri: The effect of hypoxia is variable; in general it results in a retardation of alveolar development which is reversible. Hypoxic animals have increased alveolar surface area, increased capillary surface area, increased capillary volume.
Growth and Aging of the Normal'
8
1 Engel S: The structure of the respiratory tissue in the newly born. Acta Anat 19:353-365, 1953 2 Dingler ECh: Wachtum der Lunge nach der Geburt. Acta Anat Suppl30, 32: 1-86, 1958 3 Dunnill MS : Postnatal growth of the lung. Thorax 17:329333,1962 4 Boyden EA, Tompsett TH: The changing patterns in the developing lungs of infants. Acta Anat 61:164-192, 1965 5 Weibel ER: Postnatal growth of the lung and pulmonary gas-exchange capacity. In Ciba Foundation Symposium on Development of the Lung. (De Reuck AVS, Porter R, eds) London, J and A Churchill Ltd, 1967, pp 131-148 6 Emery JL, Mithal A: The number of alveoli in the terminal respiratory unit of man during the late intrauterine life and childhood. Arch Dis Child 35:544-547, 1Qw 7 Davies G, Reid L: Growth of the alveoli and pulmonary arteries in childhood. Thorax 25:669-681, 1970 8 Short RDH: Alveolar epithelium in relation to growth of
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
Human ~ u n ~ * William M. Thurlbeck, M.B. Ch.B.,and C.Elspeth Angus, M.Sc.
D
unnill's classic study indicated that the respiratory portion of the human lung mainly grows by alveolar multiplication up to age eight years and thereafter by change in dimension of alveoli.' Dunnill found that there were 20 x lo8 alveoli at birth, 257 x lo8 alveoli in a child of four years and 280 x 108 in a child of eight. Available data at the time suggested that all human lungs had 300 x lo0 alveoli and it seemed reasonable to assume that the child of four was still adding alveoli, whereas alveolar multiplication had ceased in the child *From the Midhurst Medical Research Institute, Midhurst, Sussex, England. This study was supported in part by the Medical Research Council of Canada.
17TH ASPEN LUN6 CONFERENCE 3S
of eight. Recently, we showed that there was a range of 200 to 600 x 106 alveoli in normal human adult lungs and that the total number of alveoli was directly related to body length, ie that taller subjects had more alveoli than shorter ones.2 This normal wide range suggested to us that the conclusions that had been drawn from Dunnill's data were not necessarily correct. For example, the child of four might already have reached his adult complement of alveoli or, alternatively, the child of eight may have been destined to have 600 x 106 alveoli and thus half of the alveoli might be added after this age. We, therefore, examined the lungs of children of various ages to investigate the phenomonon of alveolar multiplication and lung growth more closely. In addition, we reanalyzed our previously reported data in adults2 to assess the effect of increasing age on lung structure.
Dunnill o
Weibel Angus
We examined the lungs from 14 subjects under the age of 19 years and 32 subjects older than this. The lungs were free
from disease and came from patients who died acutely. The techniques of alveolar counting have been described com-
0'6 i
4
b i I;li ;4 k i b i o AGE yrs
FIGURE2. The total number of alveoli (NAT)as found by different investigators is plotted against age. The regression line is that calculated by Dunnill from his data1 and once again the range in normal adults is shown at the right. pletely elsewhers as has the method of measuring interalveolar wall distance of Lm.3 All reported measurements are those at the volume of the lung distended at a transpulrnonary pressure of 25 cm of formalin. The possible effect of overidlation in adult lungs was examined by correcting the measurements to predicted lung volume at age 20 for a given stature* and the data re-analyzed. This did not alter any of the conclusions reached from the uncorrected data and these are the data illustrated in the figures. Where we have been able to include the results of others from published sourcesl+5?6we have done so. Correlation coefficients and regression lines were calculated and drawn by computer and we have shown the best fit and the linear regression if the latter was not also the best fit.
The Gowing Lung
AGE
yn
FIGURE1. When the volume of the fully distended lung (VL) is plotted against age, a close relationship becomes apparent. The circle and the bars represent the mean f 2 standard deviations found in a group of adults reported previously.2 4S 17TH ASPEN LUNG CONFERENCE
The best relationship that can be demonstrated in the growing lung is the simplest one (Fig l ) , that of age and volume of the distended lung (V,). Lung growth is not linear. The greatest rate of growth is in the first two years of life, where, on the graph, the points fall below the regression line. The growth spurt of puberty does not show because of the paucity of cases. Individual con-
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
,350.
Dunnill We~bel Dugu~d Angus
a 0
x
,315-
,280- 181
---
r2 = 0.3838
-r2 = 0.3953
AGE yrs
FIGUFIE 3. The average interalveolar distance (Lm) is plotted
against age and the adult range is shown to the right. An inflection point, which might indicate when alvedar multiplication ceased, is not apparent. 6
Dunn~ll Wribel Angus ' 0,6754 Ch~ldrrnto byrr - -re 0,I807 Adults 8
o
r
--
'0
10
20
30
40 50 60 AGE yrs
70
80
90
100
FIGURE 4. The number of alveoli per unit area ( n ) is plotted against age. Note that the regression lines calculated separately from children and from adults meet.
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
ducting airways increase in dimension as is shown by the direct relationship between anatomic deadspace and lung volumes and age7vs and because there is no increase in airway number as airway development is complete in ~ t e r oThe . ~ alveolated portion of the lung grows by alveolar multiplication (Fig 2) but our data (solid circles) show far more scatter than do Dunnill's original results. The scatter is in keeping with the observation that there is a wide normal range of alveolar number in adult lungs. It is not clear from our data when alveolar multiplication stops. It is apparent that, because of the wide range of the normal total number of alveoli, it would be necessary to examine a very large number of children's lungs to determine the age when alveolar multiplication ceases. Since this would be a very tedious way of solving the problem, we thought that another approach might be easier. Let us consider what would happen if lung growth was due entirely to alveolar multiplication until a certain age, at which time alveolar multiplication ceased and the lung now grew entirely by increase in alveolar size. In the first phase, alveolar dimensions would remain constant and thus the interalveolar distance, Lm, should remain about the same. In the second phase of growth, alveolar dimensions would increase and this would be reflected by an increase in Lm. Thus, one would anticipate a sharp inflection point at the age at which alveolar multiplication ceased. It seems unlikely that the two phases of lung growth would be entirely separate and more likely that both alveolar multiplication and enlargement of alveoli would occur. One could likely dominate over the other at different ages and thus one might also anticipate that an inflection point would exist under these circumstances, but it would not be as sharp as if the two phases were completely separate. No such inflection point is seen in our own data (Fig 3) or when all available data are plotted. Since Lm is not an entirely satisfactory number-it is a function of both the "signal" of alveolar size and the "noise" of alveolar duct dimensions (see below)-we have also plotted the number of alveoli per unit area against age. We would anticipate that the number of alveoli per unit volume (and hence per unit area) would remain more or less constant during the phase of alveolar multiplication and then diminish rapidly as lung volume increased without new alveoli being added. No inflection point is apparent when the number of alveoli per unit area is plotted against age (Fig 4 ) . The main conclusions that can be reached from these data are (1) that this approach does not solve-the problem of finding out when alveoli stop multiplying, and ( 2 ) alveolar multiplication contributes more to the increase in lung volume than does enlargement of alveoli. V, increases about 2 6 fold from birth to adult life (from 0.2 liters to 5.25 liters in the cases studied). Alveoli increase in number 18-fold (from 20 x 106 to 375 x 106) and Lm increases by about one-third. The volume change that the average alveolus undergoes is uncertain since this can be determined in a number of ways using different assumptions. The simplest data (the volume proportion of alveolar air divided by the number of alveoli) indicate that alveolar
17TH ASPEN WN6 CONFERENCE 99
"I
% Alveolar Air
r Z = 0 1857
-
15-
% Alveolor Duct and Resp~rotoryBronch~oleAlr r 2 : 0.2161 -
--- -
14
Dunnill Angus
-
-
r2 = 0.3607
13-
Yo 1 2 Alveolar Furmhyma II 10
-
9
-
8
-
7
-
6
-
=A 2 5l 0
I0
20
30 40
50 AGE
60
70
80
90
K)O
yrs
FIGURE 5. The proportion of alveolar air decreases with age and the proportion of air in the lumen of alveolar ducts and respiratory bronchioles increases.
volume doubles from shortly after birth to age 18 years. The conclusion must not be reached from our data that it is definite that alveoli continue multiplying at about the same rate as the increase in lung volume, a tempting assumption from Figures 3 and 4. However, as pointed out, Lm is a function of both alveolar size and alveolated airway dimension. The relative contribution of these two varies with age and thus their average value is difficult to interpret. Further, absolute measurements such as Lrn depend on the degree of distension of the lung and it is not certain that the lungs of children of different ages will be inflated to the same degree using a standard transpulmonary pressure. It might be thought that the average alveolar diameter would provide better information, but we have found this a very difficult measurement to make accurately and reproducibly (R. Fraser, W.M.Thurlbeck and J. C. Hogg, unpublished data). The number of alveoli per unit area will also be affected by the degree of inflation. In addition, the number of alveoli seen per unit area depends not only on the number per unit volume (in which we are really interested) but also on the alveolar shape constant, the p r e portion alveoli form of the lung, and the distribution coefficient of alveolar size.= These variables will likely vary continuously as alveoli multiply. An additional feature of the growing lung is that there is progressively more air per gram of lung tissue until about age eight years.1° Put another way, there is more tissue per unit lung volume in children than in adults.
a
; O A , ; O , A & ~ A AGE yrs
F I G ~6. R The ~ proportion that alveolar parenchyma (alveolar
walls ) forms of the lung decreases with age.
The Aging Lung One of us has previously shown that the alveolar surface area, normalized for variations in stature, diminishes with age.12 This is due to the fact that the average interalveolar distance increases more with age than does lung volume. (Alveolar surface area varies directly with lung volume and reciprocally with the interalveolar distance.) Figure 5 shows how this comes about-with increasing age the volume proportion of alveolar air decreases and the volume proportion of air in the lumen of alveolar ducts and respiratory bronchioles increases. The latter increases more than the former decreases and this rearrangement of the geometry of the lung-alveolar kttening and duct enlargement6.12-results in the increase in interalveolar distance. Figure 6 shows that the proportion of alveolar parenchyma (alveolar walls) forms of the lung decreases with age. This is loss of tissue per alveolus since alveoli are not lost with age. ( A signi6cant correlation, r2 = 0.18, exists between age and the number of alveoli per cm2. Since this relationship is parabolic we have chosen to ignore it because this would imply first a loss, then a new growth of alveoli. The linear correlation was not significant.) The diminution of proportion of alveolar parenchyma is also noted when correction is made for possible overidation and this change is interpreted as being a loss of capillary bed. The latter could not be measured directly by the techniques we used.
The lung grows primarily by alveolar multiplication, but
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
alveoli also double in size from infancy to adult life. The time at which alveolar multiplication ceases is obscure. As the lung ages it loses alveolar surface area due to alteration in the internal geometry of the lung. Alveolar wall tissue is also lost, thought to reflect loss of capillary bed.
1 Dunnill MS: Postnatal growth of the lung. Thorax 17:329,
1962 2 Angus GE, Thurlbeck WM: Number of alveoli in the human lung. J Appl Physiol32:483, 1972 3 Thurlbeck WM: The internal surface area of nonemphysematous lungs. Am Rev Resp Dis 95:765,1967 4 Matsuba K, Thurlbeck WM: The number and dimensions of small airways in nonemphysematous lungs. Am Rev Resp Dis 104:516,1971 5 Weibel ER: Morphometry of the Human Lung. Springer, Berlin 1963 6 Duguid JB, et al: The internal surface area of the lung in emphysema. J Path Bad 88:405,1964 7 Hart MC, et al: Relation between anatomic respiratory deadspace and body size and lung volume. J Appl Physiol 18:519, 1963 8 Wood LDH, et al: Relationship between anatomic deadspace and body size in health, asthma and cystic fibrosis. Am Rev Resp Dis 104:215,1971 9 Reid L: Embryology of the Lung in Development of the Lung. AVS de Reuck, K Porter, eds. J & A Churchill, London, 1967, 109 10 Stigol L, et al: Studies on elastic recoil of the lung in a pediatric population. Am Rev Resp Dis 105:552,1972 11 Thurlbeck WM:Internal surface area and other measurements in emphysema. Thorax 22:483,1967 12 Ryan SF, et al: Ductectasia: An asymptomatic pulmonary change related to age. Med Thorac 22: 181, 1965 Discussion
Dr. Lyons: What are the physiologic consequences of the aging changes you have discussed? Dr. Thurlbeck: There is progressive loss of diffusing capacity and elastic recoil with advancing age. Dr. Murray: Physiologically, the membrane component of diffusion is reduced to a much greater extent in older individuals than is the capillary component, suggesting that the membrane must become thickened or at least less permeable; I doubt a sijpificant reduction in capillaries with age.
Dr. Thurlbeck: Our data showing net loss of alveolar tissue volume in old age but constancy of alveolar number make it extremely unlikely that alveolar-capillary membranes become thicker.
Dr. Kluss: Are alveoli "permanent" structures, or do they come and go?
Dr. Thurlbeck: They are indeed permanent structures.
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
The Destruction of Type 2 Pneumocytes by Airborne Influenza PR8-A Virus; Its Effect on Surfactant and Lecithin Content of the Pneumonic Lesions of ice* Clayton G. LoosN, M.D.; Sherman F. Stinson, M.S.; Dennis P. Ryan, B.S., M.S. Hertweck, B.S.; John D. Hardy, M.S.; and R. Serebrfn, B.S.
S
tudies of the pathogenesis and pathology of experimental airborne influenza PR8-A virus infections in mice show that all types of cells lining the trachea, bronchi and alveoli may become parasitized by the virus. In doing so it destroys them. Animals subjected to s u b lethal inhalation of nebulized virus develop progressive pneurnonitis, involving one or more lobes. The postinfluenza1 lesions are characterized by pulmonary atelectasis, epithelial nodule formation and As the bronchial membranes as well as the alveolar type 2 pneumocytes are destroyed by the influenza virus, it seemed important to study the effect of this cell destruction on the surfactant and phospholipid (lecithin) content of noninfected and infected lobes in relation to onset of infection. MATERIAIS AND MF~TIoDS
In this study, specific pathogen free (SPF) young adult white male mice (CD-1 strain, Charles River Breeding Laboratories, Inc., Wilmington, Mass.) were used. At the time of exposure, the mice were approximately six weeks old. Two hundred thirty mice were subjected to the virus aerosol. A 16 liter sterile bell jar with a capacity for 50 mice served as the exposure chamber. Five sequential exposures were carried out employing a DeVilbiss 40 atomizer at one atmosphere pressure using filtered compressed air. At each exposure, the mice wer subjected to a sublethal cloud of 10-6 dilution of supernate rom a mouse lung suspension of PR8-A mouse adapted inlluenza virus for 15 minutes. After exposure smaller groups of mice, were sacrificed, either by intraperitoneal injection of pentobarbital sodium or etherization at zero hour and at daily intervals for 12, then 14, 18, 24, 30 and 80 days. Lung virus titers and serum antibodies were measured respectively by the standard chicken red cell agglutination (CCA) and agglutination inhibition (CCAI) procedures. For light and electron microscopy, the lungs of three mice were 6 x 4 intratrachedy with 2 percent gluteraldehyde in phosphate b&r (pH 7.2). Specimens of lungs for EM study were further 6xed in 1 percent osmium tetroxide for one hour. Proaxlures for virus and antibody determinations and for light and EM studies are outlined in previous publications.1-8 For lactate dehydrogenase (LDH) and g l u d phosphatedehydrogenase ( GL-6-PDH ) enzyme activity the
\
3
*From the Departments of Patholo and Medicine, UniverLaa Angela. sity of Southern California School ~edidne, Supported in part by the Council for Tobacco ResearchUSA, The Environmental Protection A ency, The Howard Hughes Em loyees Give Once Club a n t h e Hastings Foundation Funzof the University of Southern California.
l7TH ASPEN LUN6 CONFERENCE 7s
the lung. Phil Trans Roy Soc ( B ) , London, 235:s-86, 1950 9 Burri PH, Dbaly J, Weibel ER: The postnatal growth of the rat lung. Part I: Morphometry. Anat Rec 178:711-730, 1974 10 Kauffman SL, Bum PH, Weibel ER: The postnatal growth of the rat lung. Part II: Autoradiography. Anat Rec ( in press ) 11 Burri PH: The postnatal growth of the rat lung. Part 111: Morphology Anat Rec ( in press ) 12 Dubreuil G, Lacoste A, Raymond R: Observations sur le developpement du poumon humain. Bull Histol Appliq Physiol Pathol 13:23&245, 1936 13 Loosli C,Potter EL: Pre- and postnatal development of the respiratory portion of the human lung. Am Rev Resp Dis 80:5-23, 1959 Discussion
Dr. Euanc: How long after labeling were your animals sacrificed, and what significance do you attach to the failure of type 1 alveolocytes to take up much thymidine? Dr. Burri: One hour. Karen O'Hare showed that type 1 cells became labeled 14 days after other cell types in the developing rat lung in utero. Several investigators have shown that in short term labeling studies, the type 2 cells take label, but type 1 cells do not. These data support the currently accepted notion that type 1 cells are derived from type 2 cells, and that the latter constitute the only "reserve" epithelial cells of the alveolus. Dr. Reeves: How does hypoxia affect the early postnatal development of the lung? Dr. Burri: The effect of hypoxia is variable; in general it results in a retardation of alveolar development which is reversible. Hypoxic animals have increased alveolar surface area, increased capillary surface area, increased capillary volume.
Growth and Aging of the Normal'
8
1 Engel S: The structure of the respiratory tissue in the newly born. Acta Anat 19:353-365, 1953 2 Dingler ECh: Wachtum der Lunge nach der Geburt. Acta Anat Suppl30, 32: 1-86, 1958 3 Dunnill MS : Postnatal growth of the lung. Thorax 17:329333,1962 4 Boyden EA, Tompsett TH: The changing patterns in the developing lungs of infants. Acta Anat 61:164-192, 1965 5 Weibel ER: Postnatal growth of the lung and pulmonary gas-exchange capacity. In Ciba Foundation Symposium on Development of the Lung. (De Reuck AVS, Porter R, eds) London, J and A Churchill Ltd, 1967, pp 131-148 6 Emery JL, Mithal A: The number of alveoli in the terminal respiratory unit of man during the late intrauterine life and childhood. Arch Dis Child 35:544-547, 1Qw 7 Davies G, Reid L: Growth of the alveoli and pulmonary arteries in childhood. Thorax 25:669-681, 1970 8 Short RDH: Alveolar epithelium in relation to growth of
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
Human ~ u n ~ * William M. Thurlbeck, M.B. Ch.B.,and C.Elspeth Angus, M.Sc.
D
unnill's classic study indicated that the respiratory portion of the human lung mainly grows by alveolar multiplication up to age eight years and thereafter by change in dimension of alveoli.' Dunnill found that there were 20 x lo8 alveoli at birth, 257 x lo8 alveoli in a child of four years and 280 x 108 in a child of eight. Available data at the time suggested that all human lungs had 300 x lo0 alveoli and it seemed reasonable to assume that the child of four was still adding alveoli, whereas alveolar multiplication had ceased in the child *From the Midhurst Medical Research Institute, Midhurst, Sussex, England. This study was supported in part by the Medical Research Council of Canada.
17TH ASPEN LUN6 CONFERENCE 3S
of eight. Recently, we showed that there was a range of 200 to 600 x 106 alveoli in normal human adult lungs and that the total number of alveoli was directly related to body length, ie that taller subjects had more alveoli than shorter ones.2 This normal wide range suggested to us that the conclusions that had been drawn from Dunnill's data were not necessarily correct. For example, the child of four might already have reached his adult complement of alveoli or, alternatively, the child of eight may have been destined to have 600 x 106 alveoli and thus half of the alveoli might be added after this age. We, therefore, examined the lungs of children of various ages to investigate the phenomonon of alveolar multiplication and lung growth more closely. In addition, we reanalyzed our previously reported data in adults2 to assess the effect of increasing age on lung structure.
Dunnill o
Weibel Angus
We examined the lungs from 14 subjects under the age of 19 years and 32 subjects older than this. The lungs were free
from disease and came from patients who died acutely. The techniques of alveolar counting have been described com-
0'6 i
4
b i I;li ;4 k i b i o AGE yrs
FIGURE2. The total number of alveoli (NAT)as found by different investigators is plotted against age. The regression line is that calculated by Dunnill from his data1 and once again the range in normal adults is shown at the right. pletely elsewhers as has the method of measuring interalveolar wall distance of Lm.3 All reported measurements are those at the volume of the lung distended at a transpulrnonary pressure of 25 cm of formalin. The possible effect of overidlation in adult lungs was examined by correcting the measurements to predicted lung volume at age 20 for a given stature* and the data re-analyzed. This did not alter any of the conclusions reached from the uncorrected data and these are the data illustrated in the figures. Where we have been able to include the results of others from published sourcesl+5?6we have done so. Correlation coefficients and regression lines were calculated and drawn by computer and we have shown the best fit and the linear regression if the latter was not also the best fit.
The Gowing Lung
AGE
yn
FIGURE1. When the volume of the fully distended lung (VL) is plotted against age, a close relationship becomes apparent. The circle and the bars represent the mean f 2 standard deviations found in a group of adults reported previously.2 4S 17TH ASPEN LUNG CONFERENCE
The best relationship that can be demonstrated in the growing lung is the simplest one (Fig l ) , that of age and volume of the distended lung (V,). Lung growth is not linear. The greatest rate of growth is in the first two years of life, where, on the graph, the points fall below the regression line. The growth spurt of puberty does not show because of the paucity of cases. Individual con-
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
,350.
Dunnill We~bel Dugu~d Angus
a 0
x
,315-
,280- 181
---
r2 = 0.3838
-r2 = 0.3953
AGE yrs
FIGUFIE 3. The average interalveolar distance (Lm) is plotted
against age and the adult range is shown to the right. An inflection point, which might indicate when alvedar multiplication ceased, is not apparent. 6
Dunn~ll Wribel Angus ' 0,6754 Ch~ldrrnto byrr - -re 0,I807 Adults 8
o
r
--
'0
10
20
30
40 50 60 AGE yrs
70
80
90
100
FIGURE 4. The number of alveoli per unit area ( n ) is plotted against age. Note that the regression lines calculated separately from children and from adults meet.
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
ducting airways increase in dimension as is shown by the direct relationship between anatomic deadspace and lung volumes and age7vs and because there is no increase in airway number as airway development is complete in ~ t e r oThe . ~ alveolated portion of the lung grows by alveolar multiplication (Fig 2) but our data (solid circles) show far more scatter than do Dunnill's original results. The scatter is in keeping with the observation that there is a wide normal range of alveolar number in adult lungs. It is not clear from our data when alveolar multiplication stops. It is apparent that, because of the wide range of the normal total number of alveoli, it would be necessary to examine a very large number of children's lungs to determine the age when alveolar multiplication ceases. Since this would be a very tedious way of solving the problem, we thought that another approach might be easier. Let us consider what would happen if lung growth was due entirely to alveolar multiplication until a certain age, at which time alveolar multiplication ceased and the lung now grew entirely by increase in alveolar size. In the first phase, alveolar dimensions would remain constant and thus the interalveolar distance, Lm, should remain about the same. In the second phase of growth, alveolar dimensions would increase and this would be reflected by an increase in Lm. Thus, one would anticipate a sharp inflection point at the age at which alveolar multiplication ceased. It seems unlikely that the two phases of lung growth would be entirely separate and more likely that both alveolar multiplication and enlargement of alveoli would occur. One could likely dominate over the other at different ages and thus one might also anticipate that an inflection point would exist under these circumstances, but it would not be as sharp as if the two phases were completely separate. No such inflection point is seen in our own data (Fig 3) or when all available data are plotted. Since Lm is not an entirely satisfactory number-it is a function of both the "signal" of alveolar size and the "noise" of alveolar duct dimensions (see below)-we have also plotted the number of alveoli per unit area against age. We would anticipate that the number of alveoli per unit volume (and hence per unit area) would remain more or less constant during the phase of alveolar multiplication and then diminish rapidly as lung volume increased without new alveoli being added. No inflection point is apparent when the number of alveoli per unit area is plotted against age (Fig 4 ) . The main conclusions that can be reached from these data are (1) that this approach does not solve-the problem of finding out when alveoli stop multiplying, and ( 2 ) alveolar multiplication contributes more to the increase in lung volume than does enlargement of alveoli. V, increases about 2 6 fold from birth to adult life (from 0.2 liters to 5.25 liters in the cases studied). Alveoli increase in number 18-fold (from 20 x 106 to 375 x 106) and Lm increases by about one-third. The volume change that the average alveolus undergoes is uncertain since this can be determined in a number of ways using different assumptions. The simplest data (the volume proportion of alveolar air divided by the number of alveoli) indicate that alveolar
17TH ASPEN WN6 CONFERENCE 99
"I
% Alveolar Air
r Z = 0 1857
-
15-
% Alveolor Duct and Resp~rotoryBronch~oleAlr r 2 : 0.2161 -
--- -
14
Dunnill Angus
-
-
r2 = 0.3607
13-
Yo 1 2 Alveolar Furmhyma II 10
-
9
-
8
-
7
-
6
-
=A 2 5l 0
I0
20
30 40
50 AGE
60
70
80
90
K)O
yrs
FIGURE 5. The proportion of alveolar air decreases with age and the proportion of air in the lumen of alveolar ducts and respiratory bronchioles increases.
volume doubles from shortly after birth to age 18 years. The conclusion must not be reached from our data that it is definite that alveoli continue multiplying at about the same rate as the increase in lung volume, a tempting assumption from Figures 3 and 4. However, as pointed out, Lm is a function of both alveolar size and alveolated airway dimension. The relative contribution of these two varies with age and thus their average value is difficult to interpret. Further, absolute measurements such as Lrn depend on the degree of distension of the lung and it is not certain that the lungs of children of different ages will be inflated to the same degree using a standard transpulmonary pressure. It might be thought that the average alveolar diameter would provide better information, but we have found this a very difficult measurement to make accurately and reproducibly (R. Fraser, W.M.Thurlbeck and J. C. Hogg, unpublished data). The number of alveoli per unit area will also be affected by the degree of inflation. In addition, the number of alveoli seen per unit area depends not only on the number per unit volume (in which we are really interested) but also on the alveolar shape constant, the p r e portion alveoli form of the lung, and the distribution coefficient of alveolar size.= These variables will likely vary continuously as alveoli multiply. An additional feature of the growing lung is that there is progressively more air per gram of lung tissue until about age eight years.1° Put another way, there is more tissue per unit lung volume in children than in adults.
a
; O A , ; O , A & ~ A AGE yrs
F I G ~6. R The ~ proportion that alveolar parenchyma (alveolar
walls ) forms of the lung decreases with age.
The Aging Lung One of us has previously shown that the alveolar surface area, normalized for variations in stature, diminishes with age.12 This is due to the fact that the average interalveolar distance increases more with age than does lung volume. (Alveolar surface area varies directly with lung volume and reciprocally with the interalveolar distance.) Figure 5 shows how this comes about-with increasing age the volume proportion of alveolar air decreases and the volume proportion of air in the lumen of alveolar ducts and respiratory bronchioles increases. The latter increases more than the former decreases and this rearrangement of the geometry of the lung-alveolar kttening and duct enlargement6.12-results in the increase in interalveolar distance. Figure 6 shows that the proportion of alveolar parenchyma (alveolar walls) forms of the lung decreases with age. This is loss of tissue per alveolus since alveoli are not lost with age. ( A signi6cant correlation, r2 = 0.18, exists between age and the number of alveoli per cm2. Since this relationship is parabolic we have chosen to ignore it because this would imply first a loss, then a new growth of alveoli. The linear correlation was not significant.) The diminution of proportion of alveolar parenchyma is also noted when correction is made for possible overidation and this change is interpreted as being a loss of capillary bed. The latter could not be measured directly by the techniques we used.
The lung grows primarily by alveolar multiplication, but
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
alveoli also double in size from infancy to adult life. The time at which alveolar multiplication ceases is obscure. As the lung ages it loses alveolar surface area due to alteration in the internal geometry of the lung. Alveolar wall tissue is also lost, thought to reflect loss of capillary bed.
1 Dunnill MS: Postnatal growth of the lung. Thorax 17:329,
1962 2 Angus GE, Thurlbeck WM: Number of alveoli in the human lung. J Appl Physiol32:483, 1972 3 Thurlbeck WM: The internal surface area of nonemphysematous lungs. Am Rev Resp Dis 95:765,1967 4 Matsuba K, Thurlbeck WM: The number and dimensions of small airways in nonemphysematous lungs. Am Rev Resp Dis 104:516,1971 5 Weibel ER: Morphometry of the Human Lung. Springer, Berlin 1963 6 Duguid JB, et al: The internal surface area of the lung in emphysema. J Path Bad 88:405,1964 7 Hart MC, et al: Relation between anatomic respiratory deadspace and body size and lung volume. J Appl Physiol 18:519, 1963 8 Wood LDH, et al: Relationship between anatomic deadspace and body size in health, asthma and cystic fibrosis. Am Rev Resp Dis 104:215,1971 9 Reid L: Embryology of the Lung in Development of the Lung. AVS de Reuck, K Porter, eds. J & A Churchill, London, 1967, 109 10 Stigol L, et al: Studies on elastic recoil of the lung in a pediatric population. Am Rev Resp Dis 105:552,1972 11 Thurlbeck WM:Internal surface area and other measurements in emphysema. Thorax 22:483,1967 12 Ryan SF, et al: Ductectasia: An asymptomatic pulmonary change related to age. Med Thorac 22: 181, 1965 Discussion
Dr. Lyons: What are the physiologic consequences of the aging changes you have discussed? Dr. Thurlbeck: There is progressive loss of diffusing capacity and elastic recoil with advancing age. Dr. Murray: Physiologically, the membrane component of diffusion is reduced to a much greater extent in older individuals than is the capillary component, suggesting that the membrane must become thickened or at least less permeable; I doubt a sijpificant reduction in capillaries with age.
Dr. Thurlbeck: Our data showing net loss of alveolar tissue volume in old age but constancy of alveolar number make it extremely unlikely that alveolar-capillary membranes become thicker.
Dr. Kluss: Are alveoli "permanent" structures, or do they come and go?
Dr. Thurlbeck: They are indeed permanent structures.
CHEST 67: 2, FEBRUARY, 1975 SUPPLEMENT
The Destruction of Type 2 Pneumocytes by Airborne Influenza PR8-A Virus; Its Effect on Surfactant and Lecithin Content of the Pneumonic Lesions of ice* Clayton G. LoosN, M.D.; Sherman F. Stinson, M.S.; Dennis P. Ryan, B.S., M.S. Hertweck, B.S.; John D. Hardy, M.S.; and R. Serebrfn, B.S.
S
tudies of the pathogenesis and pathology of experimental airborne influenza PR8-A virus infections in mice show that all types of cells lining the trachea, bronchi and alveoli may become parasitized by the virus. In doing so it destroys them. Animals subjected to s u b lethal inhalation of nebulized virus develop progressive pneurnonitis, involving one or more lobes. The postinfluenza1 lesions are characterized by pulmonary atelectasis, epithelial nodule formation and As the bronchial membranes as well as the alveolar type 2 pneumocytes are destroyed by the influenza virus, it seemed important to study the effect of this cell destruction on the surfactant and phospholipid (lecithin) content of noninfected and infected lobes in relation to onset of infection. MATERIAIS AND MF~TIoDS
In this study, specific pathogen free (SPF) young adult white male mice (CD-1 strain, Charles River Breeding Laboratories, Inc., Wilmington, Mass.) were used. At the time of exposure, the mice were approximately six weeks old. Two hundred thirty mice were subjected to the virus aerosol. A 16 liter sterile bell jar with a capacity for 50 mice served as the exposure chamber. Five sequential exposures were carried out employing a DeVilbiss 40 atomizer at one atmosphere pressure using filtered compressed air. At each exposure, the mice wer subjected to a sublethal cloud of 10-6 dilution of supernate rom a mouse lung suspension of PR8-A mouse adapted inlluenza virus for 15 minutes. After exposure smaller groups of mice, were sacrificed, either by intraperitoneal injection of pentobarbital sodium or etherization at zero hour and at daily intervals for 12, then 14, 18, 24, 30 and 80 days. Lung virus titers and serum antibodies were measured respectively by the standard chicken red cell agglutination (CCA) and agglutination inhibition (CCAI) procedures. For light and electron microscopy, the lungs of three mice were 6 x 4 intratrachedy with 2 percent gluteraldehyde in phosphate b&r (pH 7.2). Specimens of lungs for EM study were further 6xed in 1 percent osmium tetroxide for one hour. Proaxlures for virus and antibody determinations and for light and EM studies are outlined in previous publications.1-8 For lactate dehydrogenase (LDH) and g l u d phosphatedehydrogenase ( GL-6-PDH ) enzyme activity the
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*From the Departments of Patholo and Medicine, UniverLaa Angela. sity of Southern California School ~edidne, Supported in part by the Council for Tobacco ResearchUSA, The Environmental Protection A ency, The Howard Hughes Em loyees Give Once Club a n t h e Hastings Foundation Funzof the University of Southern California.
l7TH ASPEN LUN6 CONFERENCE 7s
