The liver, adrenal gland, lung and testis tissues were obtained from 30 groups of normal ddY strain mice, each consisting of 3 litter mates of both sexes, total 90, aged from embryonic day 19 to postnatal day 1, 3, 9 and 14 and month 1, 2, 6, 12 and 24. The embryonic age was based on observation of the vaginal plug of the female mice (vaginal plug = day 0). All the animals were housed under conventional conditions and bred with normal diet (mouse chow Clea EC2, Clea Co., Tokyo, Japan) with access to water ad libitum in our laboratory. They were administered with ³H-thymidine or ³H-uridine or ³H-leucine, DNA, RNA and protein precursors, respectively, and the liver, adrenal and testis tissues were processed for light and electron microscopic radioautography [34,35]. All the procedures used in this study concerning the animal experiments were in accordance with the guidelines of the animal research committee of Shinshu University School of Medicine as well as the principles of laboratory animal care in NIH publication No. 86-23 (revised 1985).

All the animals were injected intraperitoneally with ³H-thymidine (Amersham, England, specific activity 877 GBq/mM) or ³H-uridine (Amersham, England, specific activity 1.11 TBq/mM) or ³H-4,5-leucine (Amersham, England, specific acitivity 1002 GBq/ mM) in saline, at 9 a.m., one hour before sacrifices. The dosage of injections was 370 KBq/gm body weight. The animals were perfused at 10 a.m., one hour after the injection, via the left ventricles of the hearts with 0.1 m cacodylate-buffered 2.5% glutaraldehyde under Nembutal (Abbott Laboratories, Chicago, ILL, USA) anesthesia. The right medical lobe of the liver, the superior lobe of the right lung, the right adrenal gland and the right testis were taken out, excised and 3 small tissue pieces from the respective organs (1 mm × 1 mm ×1 mm) were immersed in the same fixative at 4˚C for 1 hr., followed by postfixation in 1% osmium tetroxide in the same buffer at 4˚C for 1 hr., dehydrated in graded series of ethanol and acetone, and embedded in epoxy resin Epok 812 (Oken, Tokyo, Japan).

For light and electron microscopic radioautography, thick (2.0 µm) or semithin (0.2 µm) sections were cut in sequence on a Porter-Blum MT-2B ultramicrotome (Dupont-Sorvall, Newtown, MA, USA) using glass knives. The sections were collected on either collodion coated copper glass slides or grid meshes (VECO, Eerbeek, Netherlands), coated with either Konica NR-M2 (LM) or NR-H2 (EM) radioautographic emulsion (Konica, Tokyo, Japan) by either dipping or wire-loop method [18,35-37] They were stored in dark boxes containing silica gel (desiccant) at 4˚C for exposure. After the exposure for several months, the specimens were processed for development in either SD-X1 developer at 20˚C (LM) or freshly prepared gold latensification solution for 30 sec at 16˚C and then in fresh phenidon developer for 1 min at 16˚C in a water bath, rinsed in distilled water and dried in an oven at 37˚C overnight, stained with either toluidine blue (LM) or lead citrate solution for 3 min and coated with carbon (EM). The electron microscopic radioautograms were examined in a JEOL JEM-4000EX electron microscope (JEOL, Tokyo, Japan) at accelerating voltages of 400 kV for observing thick specimens [33-35].

For quantitative analysis of electron micrographs, twenty EM radioautograms showing cross sections of respective cell types from each group, based on the electron microscopic photographs taken after observation on at least 100 each cell type obtained from each animal, and at least 10 cells were analyzed to calculate the total number of mitochondria in each cell and the number of labeled mitochondria covered with silver grains by visual grain counting.

On the other hand, the number of silver grains in the same area size as a mitochondrion outside cells was also calculated in respective specimens as background fog, which resulted in less than 1 silver grain (0.03/mitochondrial area) almost zero. Therefore, the grain count in each specimen was not corrected with background fog. From all the data thus obtained, the averages and standard deviations in respective aging groups were computed with a personal computer (Macintosh type 8100/ 100, Apple Computer, Tokyo, Japan). The data were stochastically analyzed using variance and Student’s t-test. The differences were considered to be significant at P value < 0.01.

We studied the liver tissues of ddY strain mice at various ages from embryonic day 19 to postnatal 2 years [15,16]. Observing light microscopic microscopic radioautograms labeled with ³H-thymidine, the silver grains were found over the nuclei of some hepatocytes, demonstrating DNA synthesis (Figure 1). By electron microscopic radioautography, some nuclei and some mitochondria in hepatocytes in perinatal stages at embryonic day 19 (Figure 2), postnatal day 1, 3, 9 and day 14 (Figure 3) as well as young adult (Figure 4) and senesecent animals (Figure 5) were observed. However, those labeled hepatocytes were almost mononucleate cells (Figures 2-5) and only a few binucleate cells were found among the mononucleate hepatocytes [36]. In the labeled hepatocytes (Figures 1-5) the silver grains were mainly localized over the euchromatin of the nuclei and only a few or several silver grains were found over the mitochondria of these mononucleate hepatocytes [37]. To the contrary, most hepatocytes were not labeled with any silver grains in their nuclei nor cytoplasm, showing no DNA synthesis even after labeling with ³H-thymidine in aged adult and senescent  animals at postnatal month 1 (Figure 4), 2, 6, 12 (Figure 5) and month 24. On the other hand, labeled binucleate hepatocytes over their nuclei were very rarely found only at the perinatal stages from postnatal day 1, 3 (Figure 6), 9 and 14 (Figure 7) but not after postnatal month 1 to senescent stages up to month 12 or 24. Among many unlabeled hepatocytes, most mononucleate and binucleate hepatocytes were observed to be labeled with several silver grains over their mitochondria due to the incorporations of ³H-thymidine especially at the perinatal stages from embryonic day 19 to postnatal day 1, 3, 9 and 14 (Figure 3). The localizations of silver grains over the mitochondria were mainly on the mitochondrial matrices but some grains over the mitochondrial membranes and cristae when observed by high power magnification (Figure 3).

For preliminary quantitative analysis on the number of mitochondria in 10 mononucleate hepatocytes whose

nuclei were labeled with silver grains and other 10 mononucleate hepatocytes whose nuclei were not labeled in each aging group injected with either ³H-thymidine revealed that there was no significant difference between the number of mitochondria and the labeling indices in both types of hepatocytes (P < 0.01). Thus, the number of mitochondria and the labeling indices were calculated in both types of hepatocytes with labeled or unlabeled nuclei at respective aging stages. The results obtained from the number of mitochondria in mononucleate hepatocytes showed an increase from the prenatal day to postnatal day 14 (26.2-34.5/cell), then to postnatal month 1-6 (89.2-97.1/cell), reaching the maximum, then decreased to year 1-2 (80.4-85.7/cell) as is shown in Figure 7 (top).

The results of visual counts on the number of mitochondria labeled with silver grains obtained from 10 mononucleate hepatocytes of each animal labeled with ³H-thymidine demonstrating DNA synthesis in 7 aging groups at perinatal stages, prenatal embryo day 19, postnatal day 3, 9 and 14, month 1, 6 and 12, are plotted in Figure 7 (middle). The labeling indices in respective aging stages were calculated from the number of labeled mitochondria (Figure 7 middle) and the number of total mitochondria per cell (Figure 7 top) which were plotted in Figure 7 (bottom), respectively. The results showed that the numbers of labeled mitochondria with ³H-thymidine showing DNA synthesis increased from prenatal embryo day 19 (3.8/cell) to postnatal day 14 (6.2/cell), reaching the maximum, and then decreased to month 6 (3.7/cell) and again increased to year 1 (6.0/cell), while the labeling indices increased from prenatal day 19 (11.8%) to postnatal day 14 (16.9%), reaching the maximum, then decreased to month 6 (4.1%) and year 1 (6.4%) and year 2 (2.3%). The increase of the total number of mitochondria in mononucleate hepatocytes was stochastically significant (P < 0.01), while the changes of number of labeled mitochondria and labeling index in mononucleate hepatocytes were not significant (P < 0.01).

As for the binucleate hepatocytes, on the other hand, because the appearances of binucleate hepatocytes showing silver grains in their nuclei demonstrating DNA synthesis were not so many in the adult and senescent stages from postnatal month 1 to 24, only binucleate cells at perinatal stages when reasonable numbers of labeled hepatocytes were found in respective groups were analyzed. The number of mitochondria in binucleate hepatocytes at postnatal day 1 to 14 kept around 80 (77-84/cell) which did not show such remarkable changes, neither increase nor decrease, as shown in mononucleate cells. Thus, the number of mitochondria per binucleate cell (Figure 8 top), the number of labeled mitochondria per binucleate cell (Figure 8 middle) and the labeling index of binucleat cell (Figure 8 bottom) in 4 groups from postnatal day 1 to 14 were shown. The number of mitochondria and the number of labeled mitochondria were more in binucleate cells than mononucleate cells.

On the other hand, observing light microscopic radioautograms labeled with ³H-uridine, the silver grains were found over both the karyoplasm and cytoplasm of almost all the cells not only at the perinatal stages from embryo day 19 to postnatal day 1, 3, 9, 14, but also at the adult and senescent stages from postnatal month 1 to 2, 6, 12 and 24. By electron microscopic observation, silver grains were observed in most mononucleate hepatocytes in respective aging groups localizing not only over euchromatin and nucleoli in the nuclei but also over many

cell organelles such as endoplasmic reticulum, ribosomes, and mitochondria as well as cytoplasmic matrices from perinatal stage at embryonic day 19 (Figure 9), postnatal day 1, 3 (Figure 10), 9, 14 (Figure 11), to adult and senescent stages at postnatal month 1, 2 (Figure 12), 6 (Figure 13), 12 and 24. The silver grains were also observed in binucleate hepatocytes at postnatal day 1, 3, 9, 14, month 1, 2, 6 (Figure 13), 12 and 24 (Figure 14). The localizations of silver grains over the mitochondria were mainly on the mitochondrial matrices but a few over the mitochondrial membranes and cristae when observed by high power magnification (Figure 11).

As the results, it was found that almost all the hepatocytes were labeled with silver grains showing RNA synthesis in their nuclei and mitochondria [38]. Preliminary quantitative analysis on the number of mitochondria in 10 mononucleate hepatocytes whose nuclei were intensely labeled with many silver grains (more than 10 per nucleus) and other 10 mononucleate hepatocytes

whose nuclei were not so intensely labeled (number of silver grains less than 9) in each aging group revealed that there was no significant difference between the number of mitochondria, number of labeled mitochondria and the labeling indices in both types of hepatocytes (P < 0.01). Thus, the number of mitochondria and the labeling indices were calculated in 10 hepatocytes selected at random in each animal in respective aging stages regardless whether their nuclei were very intensely labeled or not. The results obtained from the number of mitochondria in mononucleate hepatocytes per cellular profile area showed an increase from the prenatal day (mean ± standard deviation 26.2 ± /cell) to postnatal day 1 to day 14 (38.4-51.7/cell), then to postnatal month 1-2 (53.7-89.2/cell), reaching the maximum, then decreased to year 1-2 (83.7-80.4/cell) as is shown in

Figure 15 (top) and the increase was stochastically significant (P < 0.01). The results of visual grain counts on the number of mitochondria labeled with silver grains obtained from 10 mononucleate hepatocytes of each animal labeled with ³H-uridine demonstrating RNA synthesis in 10 aging groups at perinatal stages, prenatal embryo day 19, postnatal day 1, 3, 9 and 14, month 1, 6 and year 1 and 2, are plotted in Figure 15 (middle). The labeling indices in respective aging stages were calculated from the number of labeled mitochondria (Figure 15, middle) and the number of total mitochondria per cellular profile area (Figure 15, top) which were plotted in Figure 15 (bottom), respectively. The results showed that the numbers of labeled mitochondria with ³H-uridine showing RNA synthesis increased from prenatal embryo day 19 (3.3/cell) to postnatal month 1 (9.2/cell), reaching the maximum, and then decreased to month 6 (3.5/cell) and again increased to year 1 (4.0/cell) and year 2 (4.3/cell), while the labeling indices increased from prenatal day 19 (12.4%) to postnatal month 1 (16.7%), reaching the maximum, then decreased to year 1 (4.8%) and year 2 (5.3%). Stochastical analysis revealed that the increases and decreases of the number of labeled mitochondria from the perinatal stage to the adult and senescent stage were significant in contrast that the increases and decreases of the labeling indices were not significant (P < 0.01). As for the binucleate hepatocytes, on the other hand, because the appearances of binucleate hepatocytes were not so many in the embryonic stage, only several binucleate cells (5-8 at least) at respective stages when enough numbers of binucleate cells available from postnatal day 1 to year 2 were analyzed.  The results were shown in Figure 16. The results of visual counts on the number of mitochondria labeled with silver grains

obtained from several (5 to 8) binucleate hepatocytes labeled with ³H-uridine demonstrating RNA synthesis in 8 aging groups at perinatal stages, postnatal day 1, 9, 14, and month 1, 2, 6, and year 1 and 2, are plotted in Figure 16 (middle) and the labeling indices in respective aging stages were calculated from the number of labeled mitochondria (Figure 16, middle) and the number of total mitochondria per cellular profile area (Figure 16, top) which were plotted in Figure 16 (bottom), respectively. The results showed that the number of labeled mitochondria increased from postnatal day 1 (2.3/cell) to day 9 (5.2/cell) and remained almost constant around 4-5, but the labeling indices increased from postnatal day 1 (2.1%) to postnatal day 9 (13.6%), remained almost constant around 13% (12.5-13.6%) from postnatal day 9 to month 1, then decreased to month 2 (6.1%) to month 6 (3.9%), and slightly increased to year 1 (6.3%) and 2 (5.3%). The increases and decreases of the number of labeled mitochondria and the labeling indices in binucleate hepatocytes were stochastically not significant (P < 0.01).

When the animals were injected with ³H-leucine, it was found that almost all the hepatocytes, from embryonic day 19, postnatal day 1, 3 (Figure 18), 9, 14 (Figure 19), to adult and senescent stages at postnatal month 1, 2 (Figure 20), 12 and 24 (Figure 21). The silver grains were also observed in binucleate hepatocytes at postnatal day 1, 3 (Figure 22), 9, (Figure 23), 14 (Figure 24), month 1, 2, 6, 12 (Figure 25) and 24. The localizations of silver grains observed over the mitochondria were mainly on the mitochondrial matrices but a few over their nuclei, cytoplasmic matrix, endoplasmic reticulum, ribosomes, Golgi apparatus and mitochondria [39]. In the mitochondria the silver grains were localized over the mitochondrial membranes and cristae when observed by high power magnification (Figure 19). Preliminary quantitative analysis on the number of mitochondria in 20 mononucleate hepatocytes whose nuclei were intensely labeled with many silver grains (more than 10 per nucleus) and other 20 mononucleate hepatocytes whose nuclei were not so intensely labeled (number of silver grains less than 9) in each aging group revealed that there was no significant difference between the number of mitochondria, number of labeled mitochondria

and the labeling indices in both types of hepatocytes (P < 0.01).

On the other hand, the numbers of mitochondria, the numbers of labeled mitochondria and the labeling indices were calculated in 10 binucleate hepatocytes selected at random in each animal in respective aging stages, regardless whether their nuclei were very intensely labeled or not, except the prenatal stage at embryonic day 19, because no binucleate cell was found at this stage. Thus, the numbers of mitochondria, the numbers of labeled mitochondria and the labeling indices were calculated in 20 hepatocytes selected at random in each animal in respective aging stages regardless whether their nuclei were very intensely labeled or not. The results obtained from the total numbers of mitochondria in mononucleate

hepatocytes showed an increase from the prenatal day (34.5/cell) to postnatal days 1 (44.6/cell), 3 (45.8/cell), 9 (43.6/cell), 14 (48.5/cell), to postnatal months 1 (51.5/cell)2 (52.3/cell), reaching the maximum at month 6 (60.7/cell), then decreased to years 1 (54.2/cell) and 2 (51.2/cell) as shown in Figure 26 (upper left). The increase and decrease were stochastically significant (P < 0.01). The results obtained from visual counting on the numbers of mitochondria labeled with silver grains from 20 mononucleate hepatocytes of each animal labeled with 3H-leucine demonstrating protein synthesis in 10 aging groups at perinatal stages, prenatal embryo day 19, postnatal day 1, 3, 9 and 14, month 1, 2, 6 and year 1 and 2, are plotted in Figure 26 (lower left). The labeling indices in respective aging stages were calculated from the numbers of labeled mitochondria and the numbers of total mitochondria per cell which were plotted in Figure 26 (right). The results showed that the numbers of labeled mitochondria with 3H-leucine showing protein synthesis increased from prenatal embryo day 19 (8.3/cell) to postnatal days 1 (9.6/cell), 3 (8.1/cell), 9 (8.9/cell), 14 (9.5/cell), and month 1 (11.2/cell), reaching the maximum, and then decreased to months 2 (9.1/cell), 6 (8.8/cell) to years 1 (6.7/cell) and 2 (2.2/cell), while

the labeling indices increased from prenatal day 19 (20.1%) to postnatal days 1 (21.2%), 3 (21.6%), 9 (22.2%), 14 (23.1%), reaching the maximum, then decreased to month 1 (21.7%), 2 (17.4%), 6 (14.6%), and years 1 (12.4%) and 2 (4.4%). Stochastical analysis revealed that the increases and decreases of the numbers of labeled mitochondria as well as the labeling indices from the perinatal stage to the adult and senescent stages were significant (P < 0.01).

On the other hand, the results obtained from the numbers of mitochondria in binucleate hepatocytes showed an increase from the postnatal days 1 (66.2/cell), to 3 (66.4/cell), 14 (81.8/cell), to postnatal months 1 (89.9/cell), 2 (95.1/cell), and 6 (102.1), reaching the maximum at month 12 (128.0/cell), then decreased to years 2 (93.9/cell) as shown in Figure 27 (upper left). The increase and decrease were stochastically significant (P < 0.01). The results obtained from visual counting on the numbers of mitochondria labeled with silver grains from 10 binucleate hepatocytes of each animal labeled with ³H-leucine demonstrating protein synthesis in 10 aging groups at postnatal day 1, 3, and 14, month 1, 6 and year 1 and 2, are plotted in Figure 27 (lower left). The labeling indices in respective aging stages were calculated from the numbers of labeled mitochondria and the numbers of total mitochondria per cell which were plotted in Figure 27 (right). The results showed that the numbers of labeled mitochondria with ³H-leucine showing protein synthesis increased from postnatal day 1 (7.3/cell) to day 3 (6.8/cell), 14 (10.2/cell), and month 1 (15.0/cell), 2 (15.9/cell), reaching the maximum at month 6 (19.6/cell), then decreased to year 1 (8.3/cell) and 2 (5.1/cell), while the labeling indices increased from postnatal day 1 (11.8%) to 3 (10.2%), 14 (12.5%), month 1 (18.3%) and 2 (18.7%), reaching the maximum at month 6 (19.2%), then decreased to year 1 (6.4%) and 2 (5.5%). Stochastical analysis revealed that the increases and decreases of the numbers of labeled mitochondria as well as the labeling indices from the newborn stage to the adult and senescent stages were significant (P < 0.01).

We studied the adrenal tissues at various ages from embryo to postnatal 2 years [30,31]. The adrenal tissues obtained from ddY strain mice at various ages from embryo day 19 to postnatal day 30, consisted of the adrenal cortex and the adrenal medulla. The former consited of 3 layers, zona glomerulosa, zona fasciculata and zona reticularis, developing gradually with aging as observed by light microscopy. At embryonic day 19 and postnatal day 1, the adreno-cortical cells were composed mainly of polygonal cells, while the specific orientation of the 3 layers, zona glomerulosa, zona fasciculata and zona reticularis,was not yet well established. At postnatal day 3, orientation of 3 layers, especially the zona glomerulosa became evident. At postnatal day 9 and 14, the specific structure of 3 layers was completely formed and the arrangements of the cells in respective layer became typical especially at day 14 and month 1 (Figure 28). Observing the ultrastructure of the adreno-cortical cells by electron microscopy, cell organelles including mitochondria were not so well developed at perinatal and early postnatal stages from embryonic day 19 to postnatal day 9. However, these cell organelles, mitochondria, endoplasmic reticulum, Golgi apparatus, appeared well developed similarly to the adult stages at postnatal day 14. The zona glomerulosa (Figures 29, 32, 35, 38) is the thinnest layer found at the outer zone, covered by the capsule, consisted of closely packed groups of columnar or pyramidal cells forming arcades of cell columns. The cells contained many spherical mitochondria and well developed smooth surfaced endoplasmic reticulum but a compact Golgi apparatus in day 14 animals. The zona fasciculata (Figures 30, 33, 36, 39) was the thickest layer, consisted of polygonal cells which were larger than the glomerulosa cells, arranged in long cords disposed radially to the medulla containing many lipid droplets (Figures 33, 36, 39). The mitochondria were less numerous and were more variable in size and shape than those of the glomeruloza cells, while the smooth surfaced endoplasmic reticulum were more developed and the Golgi apparatus was larger than the glomerulosa. In the zona reticularis (Figures 31, 34, 37, 40), the parallel arrangement of cell cords were anastomosed showing networks continued to the medullar cells. The mitochondria were less numerous and were more variable in size and shape than those of the glomeruloza cells like the fasciculata cells, as well as the smooth surfaced endoplasmic reticulum were developed and the Golgi apparatus was large like the fasciculata cells. However, the structure of the adrenal cortex tissues showed changes due to development and aging at respective develop-

mental stages.

Observing EM radioautograms, the silver grains were found over the nuclei of some adreno-cortical cells labeled with ³H-thymidine, demonstrating DNA synthesis mainly in perinatal stages at embryonic day 19 (Figures 29-31), postnatal day 1 (Figures 32-34) and day 3 (Figures 35, 36), while less at day 9 (Figure 37), and day 14 (Figures 38, 39, 40).

However, those labeled cells were found in all the 3 layers, the zona glomerulosa (Figures 29, 32, 35, 38), the zona fasciculata (Figures 30, 33, 36, 39) and the zona reticularis (Figures 31, 34, 36, 37, 40), at respective aging stages. In the labeled adreno-cortical cells in 3

layers the silver grains were mainly localized over the euchromatin of the nuclei and only a few or several silver grains were found over the mitochondria of these cells. To the contrary, most adreno-cortical cells were not labeled with any silver grains in their nuclei nor cytoplasm, showing no DNA synthesis even after labeling with ³H-thymidine. Among many unlabeled adrenocortical cells, most cells in the 3 layers were observed to be labeled with several silver grains over their mitochondria due to the incorporations of ³H-thymidine especially at the perinatal stages from embryonic day 19 (Figure 29-31) to postnatal day 1 (Figure 32-34), day 3 (Figure 35, 36), day 9 (Figure 37) and 14 (Figure 38-40). The localizations of silver grains over the mitochondria were mainly on the mitochondrial matrices (Figure 39).

The pulmonary tissues obtained from ddY strain mice at

embryonic to early postnatal stages consisted of undifferentiated cells (Figure 44). However, they differentiated into several types of cells due to aging, the type I epithelial cell (Figure 45, 51) aging, the type I epithelial cell (Figure 45, 51) or the small alveolar epithelial cell, the type II epithelial cell (Figure 46,52" target="_self"> 52) or the large alveolar epithelial cell, the interstitial cell (Figure 47,53" target="_self"> 53), the endothelial cell and alveolar phagocyte or dust cell as we had formerly observed [19-22]. At embryonic day 16 and 18, the fetal lung tissues appeared as glandular organizations consisting of many alveoli bordering undifferentiated cuboidal cells and no squamous epithelial cells were seen (Figure 44). Mitotic figures were frequently observed in cuboidal epithelial cells. After birth, the structure of the alveoli was characterized by further development of the alveolar-capillary networks from postnatal day 1 to 3 and 7. During the development, the cellular composition of the alveolar epithelium resembled that of the adult lung, with a mixed population of the type I and type II epithelial cells. Up to 1 and 2 weeks after birth, the lung tissues showed complete alveolar structure and single capillary system almost the same as the adult after 1 month (Figure51-53) to 2 to 6 months, and further to senescent stage over 12 months to 22 months.

On electron microscopic radioautograms of the pulmonary tissues labeled with ³H-thymidine, silver grains were observed over the nuclei of some pulmonary cells corresponding to the DNA synthesis in S-phase as observed by light microscopic radioautograpy (Figure 44). On the other hand, some mitochondria in both S-phase cells and interphase cells which did not show any silver grains over their nuclei were labeled with silver grains showing intramitochondrial DNA synthesis. The intramitochondrial DNA synthesis was observed in all the cell types, the type I epithelial cell (Figure 45, 51), the type II epithelial cell (Figure 46, 52), the interstitial cell (Figure 47, 53) and the endothelial cell. Because enough numbers of electron photographs (more than 5) were not obtained from all the cell types in respective aging groups, only some cell types and some aging groups where enough numbers of electron photographs were available were used for quantitative analysis. The numbers of mitochondria per cell profile area, the numbers of labeled mitochondria per cell and the labeling indices of the type I epithelial cells in only a few aging groups was shown in Figure 48 (top). Likewise, the similar results from the type II epithelial cells (Figure 49 top), the interstitial cells (Figure 50 top) were shown. The labeling indices in respective aging stages were calculated from the number of labeled mitochondria and the number of total mitochondria per cellular profile area which were plotted in Figures 48-50 (middle and bottom), respectively. These results demonstrated that the labeling indices in these cell types decreased from perinatal stages to the adult and senescent stage due to aging.

On electron microscopic radioautograms of pulmonary tissues labeled with ³H-uridine, silver grains were observed over the nuclei of some pulmonary cells corresponding to the RNA synthesis in most cells in respective aging groups as observed by light microscopic radioautograpy. The silver grains were observed to localize not only over euchromatin and nucleoli in the nuclei but also over many cell organelles such as endoplasmic reticulum, ribosomes, and mitochondria as well as cytoplasmic matrices of all the cell types. The intramitochondrial RNA synthesis was observed in all the cell types, the interstitial cell (Figure 51) the type I epithelial cell (Figure 52), the type II epithelial cell (Figure 53), and the endothelial cell. Because enough numbers of electron photographs (more than 5) were not obtained from all the cell types in respective aging groups, only some cell types and some aging groups when enough numbers of electron photographs were available were used for quantitative analysis similarly to DNA synthesis. The numbers of mitochondria per cell profile area, the numbers of labeled mitochondria per cell and the labeling indices of the type I epithelial cells in only a few

aging groups was shown in Figure 54. Likewise, the similar results from the type II epithelial cells (Figure 55 bottom), the interstitial cells (Figure 56 bottom), and the endothelial cell (Figure 57 bottom) were shown. The labeling indices in respective aging stages were calculated from the number of labeled mitochondria and the number of total mitochondria per cellular profile area were also shown in Figures 54-57 (top and middle), respectively. These results demonstrated that the numbers of labeled mitochondria in these cell types increased from perinatal stages to the adult stage, reaching maxima at postnatal month 1, and decreased to the senescent stage due to aging.