Role of nonciliated cells in renewal of the bronchial epithelium of rats exposed to NO2.

The purpose of the present study was to identify the proliferative cell types in the nonciliated cell population of the upper airways and determine the capacity of each to act as progenitor cells. Sprague-Dawley rats (30 days old) were exposed to 20 ppm NO2 for 24 hours to stimulate cell division, given injected tritiated thymidine (3H-TdR), sacrificed 1 hour and 1, 3, 5, and 7 days later, and prepared for light- and electron-microscopic autoradiography. One hour after injection of 3H-TdR, the mean labeling index (LI) was 1.6% in control animals and 5.2% in exposed animals. Mean grain counts per cell decreased from 15.6 at 1 hour after 3H-TdR to 6.9 on the third day, indicating that the labeled cell population had divided. Labeled cells in the control and exposed cell populations were identified with electron microscopy. At 1 hour after injection of 3H-TdR, basal cells and nonciliated columnar cells were labeled. However, only nonciliated columnar cells were stimulated to divide by NO2. The labeled nonciliated columnar cell population was made up of serous, "intermediate" and goblet cells. Each of these cell types was stimulated to divide to the same degree. After cell division (1-7 days) labeled cells of all types were observed with labeled ciliated cells appearing on the third day. It was concluded that the basal cell is not a primary progenitor cell. The primary progenitor cell for epithelium in the upper airway is the total columnar secretory cell population (serous, "intermediate," and goblet cells).

Morphological basis of tolerance to ozone.

The purpose of this research was to study Type 1 epithelial cells in the ozone (O3)-tolerant lung epithelium. Rats were made tolerant by exposure to 0.5 ppm O3 for 2 days and allowed to recover in air. Reexposure to a lethal concentration of O3 (6 ppm) at 3, 7, and 15 days of recovery revealed that tolerance was present at 3 days but almost absent at 7 and 15 days of recovery. Using Type 2 cell proliferation as a means of quantitating Type 1 cell injury, it was observed that when the preexposed rats were reexposed to 0.5 ppm at 3, 7, and 15 days, very little Type 1 cell injury occurred at 3 days. However, at 7 and 15 days the amount of Type 1 cell injury was the same as that associated with the original exposure. To determine whether there was any change in the alveolar epithelial cell populations between the periods of tolerance (3 days) and its decline (7 and 15 days), the percentage of tritiated thymidine [( 3H]TdR-labeled Type 1 and 2 cells at these times were determined. There was a significant decrease in [3H]TdR-labeled Type 1 and 2 cells between the third and fifteenth days of recovery as excess cells were sloughed off and the tissue returned to normal. Using electron microscopic morphometry, Type 1 and 2 cells were then studied during the decline of tolerance. No change was found in the morphology of Type 2 cells; however, the morphology of Type 1 cells revealed a 58% decrease in surface area and a 25% increase in the arithmetic mean thickness when tolerance was present at 3 days. As tolerance declined (7 and 15 days), Type 1 cell morphology returned to normal. It was concluded that tolerance exists when the surface area of a cell exposed to a particular concentration of ozone is small enough so that the existing antioxidant mechanism contained within that cell volume can protect it from damage.

Quantitation of damage to the alveolar epithelium by means of type 2 cell proliferation.

The purpose of this study was to determine whether the amount of alveolar epithelial tissue damaged during exposure of NO2 could be quantified by measuring the proliferative response to Type 2 cells. To accomplishe this, we used tissues from previously published experiments in which rats had been exposed to NO2 and the proliferative response to Type 2 cells had been measured during a 5-day period. The proportion of alveolar epithelium damaged was determined by stereologic examination with electron microscopy of tissue sections from those rats exposed to NO2 for 24 hours. These values were then compared with the total proliferative response to Type 2 cells for the 5 days of exposure. The study demonstrated that increasing tissue damage is assocaited with a greater proliferative response to Type 2 cells. The high degree of correlation (r = 0.93) indicates that the proliferative response of Type 2 cells can be used as an indirect means to quantify acute damage to the alveolar epithelium.

Erythropoiesis in the yolk sac of the bat Tadarida brasiliensis cynocephala.

The process of erythropoiesis and vasculogenesis in the yolk sac of the bat (Tadarida brasiliensis cynocephala) has been studied through the use of both light and electron microscopy. Stem cells arise from the leading edge of the migrating splanchnic mesoderm and transform into primitive erythroblasts. Differentiation involves either contact or association with the endodermal cells, since all erythropoietic activity occurs on the endodermal side of the expanding vascular bed, and many of the cells are in close apposition to the lateral or basal plasma membranes of the endodermal cells. Endodermal cells also phagocytize developing primitive erythroblasts during the later stage of the process when erythropoiesis is subsiding in the yolk sac. Cells destined to become the endothelium of the expanding vascular bed also arise from the leading edge of the migrating splanchnic mesoderm. Their process of differentiation involves the development of cytoplasmic extensions that may surround a group of differentiating erythroblasts, enclosing them in the newly formed lumen of the blood vessel. The cytoplasmic extensions make contact and develop junctional complexes with similar processes from other cells to complete the lumen of the lengthening vascular bed. Cells of the granulocyte series or megakaryocytes are not observed in the yolk sac of the bat as has been described in certain other species.