THE ANATOMICAL RECORD 226:187-197 (1990) Microanatomy of the Lung of the Bowhead Whale Balaena mysticetus WILLIAM G. HENK AND JERROLD T. HALDIMAN Department of Veterinary Anatomy and Fine Structure, School of Veterinary Medicine, Louisiana State University and Agricultural and Mechanical College, Baton Rouge, Louisiana 70803 ABSTRACT The lungs from six bowhead whales harvested by Alaskan Eskimos have been examined with light and electron microscopes. Airways ranging from 1 to 40 mm in lumina1 diameter are lined by a pseudostratified ciliated epithelium containing numerous mucus-secreting cells. The underlying lamina propria-tela submucosa of these airways contains tubuloalveolar glands, plasma cells, and lymphatic accumulations in addition to both elastic and collagenous fibrillar elements. Cartilage extends to the level of the respiratory airways, but smooth muscle is absent from airways larger than 3 mm, and tubuloalveolar glands are absent from airways smaller than 3 mm. Respiratory airways are lined by pseudostratified, simple cuboidal, and simple squamous epithelia. Alveolar ducts are lined by simple squamous epithelium exclusively. A connective tissue core composed mostly of elastic fibers supports the walls of the alveolar ducts. Neither smooth muscle nor cartilage has been observed in these structures. Alveoli contain the typical cetacean double capillary bed separated by a thick septum composed mainly of collagenous connective tissue. Alveoli are lined by a simple squamous epithelium similar to that encountered in alveolar ducts and respiratory airways. This epithelium is composed of type I and I1 pneumocytes closely appressed to a n underlying capillary network. The type I1 pneumocytes contain typical lamellar bodies and tubular myelin can be seen in the air spaces. The lung is surrounded by a thick (X = 2.5 mm) visceral pleura rich in blood vessels and elastic fibers. The migratory arctic mysticete Balaena mysticetus, bowhead whale, was hunted to virtual extinction by commercial whalers during the late 19th and early 20th century. Since the cessation of commercial whaling some recovery of the population is evident (Zeh and Raftery, 1988). This large (to approximately 20 m) baleen whale is now harvested in regulated small numbers only by the native Eskimo peoples of western and northern coastal Alaska as a n integral component of their nutrition, culture, and economics. Despite this whale’s status as a n endangered species and the continued potential for hydrocarbon contamination of its Beaufort, Chukchi, and Bering Sea environments, few investigations have focused on determining the microscopic anatomy of the respiratory system of this animal. Thus far, descriptions of the respiratory system of B. mysticetus have been confined to a gross anatomical description (Henry et al., 19831, a description of the microscopic anatomy of the extrapulmonary airways (Haldiman et al., 1984a), and preliminary reports of specific components of lung microstructure (Fetter and Everitt, 1980; Haldiman e t al., 1981, 1984b). Various aspects of lung microstructure have been investigated in dolphins and other toothed whales (odontocetes) (Lacoste and Baudrimont, 1926, 1933; Wislocki, 1929, 1942; Goudappel and Slijper, 1958; Ito et al., 1967; Olsen e t al., 1969; Fanning and Harrison, 0 1990 WILEY-LISS, INC. 1974; Fanning, 1977), but relatively little is known about the pulmonary microanatomy of large baleen whales (Engel, 1966; Murata, 1951). Even in studies of odontocete lungs, little attention has been directed toward pulmonary ultrastructure. This report presents both light and electron microscopic observations of several important tissues of the bowhead whale lung. MATERIALS AND METHODS Tissue Collection and Initial Fixation Six Eskimo subsistence-harvested bowhead whales taken near Barrow, Alaska, served as sources of lung tissue. After the whales were killed and butchered in the traditional manner, intact lungs were fixed by immersion in 10% neutral-buffered formalin. Various smaller pieces of lung tissue were also similarly fixed at the collection sites. The specimens were shipped to Louisiana in fixative, which was replaced with fresh 10% neutral-buffered formalin upon arrival. Additional small samples from the lungs of two of the six whales were fixed at the collection sites for electron microscopy. This fixation took place in 1.25% glutaraldehyde and 2% formaldehyde in a 0.1 M sodium caco- Received July 22, 1988; accepted February 24, 1989. W.G. HENK AND J.T. HALDIMAN 188 dylate-buffered solution a t pH 7.4 for 6 hours. Following fixation, the samples were placed in 0.1 M sodium cacodylate containing 5% (w/v) sucrose and shipped to Louisiana. Light Microscopy (LM) Tissue samples destined for LM were trimmed to a n appropriate size, dehydrated, oriented, embedded in either paraffin or methacrylate, and sectioned at 2-6 pm. Slides were stained according to one of the following methods (Humason, 1966): 1) hematoxylin and eosin (H&E) for general morphology, 2) Verhoeff‘s elastic stain (VER) for elastic fibers, 3) periodic acidSchiff reaction (PAS) for polysaccharides, and 4) Masson’s trichrome (MAT). In addition, some samples were prepared as for TEM but 1-2 pm sections for LM were cut. These sections were stained with 1% methylene blue plus 1% azure I1 (MBA). Transmission Electron Microscopy (TEM) Upon arrival in Louisiana, samples were washed in 0.1 M sodium-cacodylate-buffered solution containing 5% (w/v) sucrose, postfixed in 1% Os04 in the same buffer without sucrose, dehydrated through a graded series of ethanol, embedded in Epon-Araldite (Mollenhauer, 1964), and sectioned on a n ultramicrotome a t 60-90 nm. Sections were poststained with uranyl acetate (Watson, 1958) and lead citrate (Reynolds, 1963) and viewed in either a Zeiss EM-10 or EM-109 transmission electron microscope. Some samples received treatment with a mordant of 1% tannic acid prior to osmication to enhance membrane staining (Simionescu and Simionescu, 1976). Scanning Electron Microscopy (SEM) Samples for scanning electron microscopy were processed as were those for TEM through the dehydration step. Following dehydration, the tissues were criticalpoint dried from COZ, mounted on aluminum stubs with either colloidal graphite or silver adhesive, coated with gold-palladium by using a D.C. sputtering device, and viewed in a Cambridge S-150 SEM. RESULTS Although the terms bronchi and bronchioli are frequently used in descriptions of cetacean lungs we have chosen to use the simple term airway for the conducting structures in the lung of the bowhead. This choice Fig. 1. A light micrograph of a portion of a 1 cm airway stained with H&E. Cilia (C) are seen forming the luminal surface of the pseudostratified epithelium (E). Numerous blood vessels (Bv) are seen in the subepithelial connective tissue. Numerous glands (G) are present in the fibrous connective tissue that forms a thick lamina propria-tela submucosa between the epithelium and the underlying cartilage. Plasma cells (pc) and collagenous fibers (cf) are abundant in this connective tissue layer. Fig. 2. A scanning electron micrograph of the surface of a n airway similar to the one illustrated in Figure 1. Both ciliated (C) and nonciliated cells with numerous microvilli (mv) are evident. Fig.3. A transmission electron micrograph of a ciliated and anonciliated epithelia1 cell from the airway epithelium. Cilia (C)and basal bodies (b) characterize the ciliated cell, whereas secretory granules is a result of difficulties encountered in making real structural or functional distinctions between small bronchi and bronchioli in this cetacean’s lung. A typical pseudostratified columnar epithelium (PCE) formed a lamina epithelialis mucosae in airways measuring from 40.0 to 1.0 mm in luminal diameter (Fig. 1). The apical regions of ciliated cells formed a complete ciliary carpet over the luminal surface in most regions (Figs. 1, 2). Small nonciliated patches where secretory cells dominated the surface were, however, not uncommon, especially in the larger airways (Fig. 2). Both ciliated and nonciliated cells possessed numerous microvilli (Fig. 2). Cilia possessed typical 9 + 2 microtubular axonemes that terminated in basal bodies (Fig. 3). Secretory cells were not ciliated; they contained apical accumulations of PAS-positive secretory vesicles (Fig. 3) and basally positioned nuclei. An irregular fibrous connective tissue formed the lamina propria mucosae-tela submucosa beneath the epithelium (Fig. 1). This extensive connective tissue layer was rich in blood vessels, simple tubuloalveolar glands, and plasma cells (Figs. 1, 4, 5). Plasma cells were typically oval with a n eccentric nucleus and a n extensive dilated rough endoplasmic reticulum (Fig. 4). These cells were widely distributed in the connective tissue just beneath the epithelium and in the connective tissue separating secretory elements of the glands but were relatively uncommon in the deeper connective tissue (Fig. 1). Accumulations of lymphocytes were also sometimes seen in this layer. The glands contained secretory cells with basally positioned nuclei and apical PAS-positive accumulations of secretory vesicles (Figs. 1, 5). At the ultrastructural level, the surface of these secretory cells possesses numerous microvilli and secretory vesicles of variable density (Fig. 5). Cartilaginous support continued as the luminal diameter of the airways was reduced to between 1.0 and 0.4 mm. This supporting hyaline cartilage was generally more pronounced in airways with luminal diameters greater than 3 mm. The cartilage is diminished to incomplete rings and plaques in airways with diameters between 3.0 and 0.4 mm. At about 1.0 mm in diameter, the airway walls were interrupted by openings into alveolar ducts, alveolar sacs, and alveoli (Fig. 6). Airways below this level do not contain any cartilaginous support. As the cartilaginous support was reduced, the thickness of the lamina propria mucosae-tela submucosa (Sg) are confined to the secretory cell. A junctional region (Jr) between these two cells can also be seen. Fig. 4. A transmission electrpn micrograph of a plasma cell containing a typical distribution of heterochromatin in the nucleus (Nu) and extensive dilated endoplasmic reticulum (er).This cell resides in a connective tissue matrix characterized by numerous collagenous fibers (Cf). Fig. 5. A transmission electron micrograph of a portion of one of the airway glands. Secretory cells with numerous apical secretory granules (Sg) and surface microvilli (mv) form the glandular epithelium. Fig. 6. A scanning electron micrograph of a respiratory airway. The cartilage (Ct) that supports these airways is evident as are the openings (0)of alveolar ducts and alveoli. Figs. 1-6 190 W.G. HENK AND J.T. HALDIMAN also declined, and its composition changed. In the larger airways, collagenous fibers were the predominant fibers of the connective tissue. Elastic fibers were also present and, in the larger airways, were arranged into distinct elastic laminae. As the diameter of the airways declined elastic fibers became more prominent (Fig. 7). The number of mucosal glands declined until they completely disappeared from airways with luminal diameters of less than 3 mm (Fig. 7). In all the cartilage-containing airways, the cartilage consisted of single chondrocytes and chondrocytic nests distributed in a matrix of collagenous fibers (Fig. 8). Numerous fine cytoplasmic processes extended outwardly from the body of each condrocyte (Fig. 9) toward similar processes from adjacent chondrocytes. Most chondrocytes contain lipid inclusions (Fig. 9). A continuous smooth muscle lamina muscularis mucosae was never observed in any of the airways examined. Interrupted bundles of smooth muscle were, however, frequently seen in airways between l and 3 mm in diameter (Fig. 10). These smooth muscle bundles, as well as the cartilaginous support, disappeared at a lumina1 diameter of 1-0.4 mm. Airways 1.0-0.4 mm in luminal diameter were lined with pseudostratified ciliated columnar epithelium (PCE), simple cuboidal epithelium (SCE), and simple squamous epithelium (SSE) (Figs. 10, 11). The PCE is composed of both ciliated cells and nonciliated, PASpositive, secretory cells. The PCE covered only a small initial portion of the airway surface. Although this epithelium was somewhat thinner, it otherwise appeared similar to the PCE lining larger airways and it continued t o contain secretory as well as ciliated cells. A short band of simple cuboidal epithelium was sometimes seen separating the terminus of the PCE and the leading edge of the SSE (Figs. 10, 11). This SCE was composed of PAS-negative cells with centrally positioned nuclei and no evidence of secretory granules. The SCE was not always present between the end of the PCE and the start of the SSE. When present, the SCE generally consisted of only 15-30 cells (Figs. 10, 11). The SSE that lined the remainder of the small airways was often appressed to an underlying capillary network (Figs. 10,121. Much of the airway surface was covered with this type of epithelium. At about 0.4 mm in diameter, the mural elements of the airway were reduced to an SSE overlying a fibrous connective tissue core that formed the alveolar duct wall (Figs. 13, 14). These passages contained many openings that gave them the appearance of chicken wire (Fig. 13). Well-organized bundles of elastic fibers predominated in the dense connective tissue core of these alveolar ducts (Figs. 14, 15). At the ultrastructural level, the connective tissue consists of large elastic fibers containing microfibrils embedded in a matrix of collagen fibrils (Fig. 16). Smooth muscle cells were not observed in the alveolar duct wall except where they were clearly part of a blood vessel wall. Some regions of this connective tissue did contain numerous fibroblastlike cells (Fig. 17). The SSE lining these passages was frequently appressed to an underlying capillary bed (Figs. 13, 18). This SSE consisted of type I and I1 pneumocytes. Type I1 pneumocytes were easily identified by their numerous apical microvilli and the presence of lamellar bodies in their cytoplasm (Fig. 19). The cell bodies of type I pneumocytes were less often seen but their attenuated cytoplasmic extensions covered a large portion of the alveolar duct surface (Fig. 19). Generally, the walls that surrounded the openings into alveolar sacs and alveoli were thickened, and these walls contained a prominent core composed mainly of elastic fibers (Figs. 20, 25). The lining epithelium throughout the alveolar sacs and alveolar lining epithelium consisted of both type I and type I1 pneumocytes (Figs. 21, 22). This thin epithelium resided atop an extensive capillary network (Figs. 20-22). Type 11 pneumocytes were also distinguished with the SEM (Fig. 21). Epithelia of adjacent alveoli were separated from one another by two capillary networks and a loose collagenous connective tissue septum (Fig. 22). This septal connective tissue was continuous with the elastic tissue swellings that formed the apertures opening into alveoli and alveolar sacs (Figs. 14, 25). Elastic fibers were not usually a prominent feature within the septa (Fig. 22). Fibroblasts were common within the septa, particularly in regions adjacent to the capillary beds (Fig. 22). Capillaries did not frequently cross the septal walls to lie beneath the epithelia of adjacent alveoli Fig. 7. A light micrograph of a VER-stained 2 mm airway. Glands are absent from the connective tissue between the epithelium (E) and the underlying cartilage. The relatively thin lamina propria-tela submucosa contains numerous elastic fibers (ef). Typical chondrocytes (cc) are evident in the cartilage. Fig. 8. A scanning electron micrograph of the fractured surface of a airway similar that illustrated in Figure 7. The epithelial surface (Es), lamina propria-tela submucosa (1s) and cartilage containing chondrocytes (cc) are evident. Bar = 50 pm. Fig. 9. A transmission electron micrograph of a chondrocyte embedded in a matrix of collagenous fibrils (cf). A lipid inclusion (1) and numerous cytoplasmic extensions (p) characterize this cell. Fig. 10. A light micrograph of a PAS-stained respiratory airway. Ciliated pseudostratified epithelium forms part of the airway surface. The secretory cells of this epithelium are clearly evident here as a result of the PAS staining of their secretory product (sp). A short region of cuboidal epithelial cells (ce) is evident between the pseudostratified epithelium and the simple squamous epithelium that lies atop a capillary network (cn). The connective tissue between the cartilage, which contains chondrocytes (cc), and the epithelium contains bundles of smooth muscle fibers (sm). Fig. 11. A higher magnification of a VER-stained region similar to the one illustrated in Figure 10. Secretory product (sp) can be clearly seen in the pseudostratified epithelium that abruptly becomes continuous with a cuboidal epithelium (ce). Elastic fibers (ef) are abundant in the subepithelial connective tissue. Fig. 12. A portion of a respiratory airway with a luminal surface consisting of a squamous epithelium closely appressed to a capillary network (cn). In this PAS-stained section, the region adjacent to the chondrocytes (cc) produces a positive reaction. Fig. 13. A scanning electron micrograph of the surface of an alveolar duct. The walls of the duct (dw) which superficially resemble those of the respiratory airway do not contain cartilage. The capillary network (cn) which underlies the thin epithelium lining the duct walls is clearly evident. Numerous openings (0)into alveolar sacs and alveoli are also evident. BOWHEAD WHALE LUNG Figs. 7-13. 191 192 W.G. H E N K AND J.T. HALDIMAN (Fig. 22). Most of the alveolar surface area was covered by thin cytoplasmic extensions of type I pneumocytes appressed to underlying capillaries. At the narrowest point measured, the blood-air barrier was reduced to 350 nm and consisted of a thin cytoplasmic extension from a type I pneumocyte (125 nm), a shared basal lamina (125 nm), and the thin endothelial cell cytoplasm (100 nm) (Fig. 23). Neither epithelial nor endothelial fenestrations were observed. The endothelial and epithelial cells contained numerous caveoli but few other cytoplasmic organelles were observed (Fig. 23). A surfactant layer atop the epithelial cell surfaces was not observed directly, but expanded lamellar bodies and tubular myelin were observed within all air spaces smaller than 1 mm that were examined (Figs. 24, 19). Within these luminal lamellar bodies, tubular myelin was arranged in a regular rectangular pattern (Fig. 24). Pulmonary macrophages were observed routinely in alveolar lumina and on the luminal surfaces of alveolar ducts. Occasionally they were noted in respiratory airways. A vascularized visceral pleura averaging 2.5 mm in thickness enveloped the lungs. The mesothelium rested on a thin, dense, collagenous connective tissue layer. Beneath this surface layer (arrowhead, Fig. 251, the remaining connective tissue contains numerous large elastic fibers, collagenous fibers, and a large number of blood vessels (Fig. 25). The connective tissue of the pleura is continuous with that forming the septal walls. The elastic fibers of the connective tissue cores of alveolar aperatures were sometimes seen extending into the pleura (Fig. 25). In some areas, immediately adjacent to the pleura, the alveolar lining became a simple cuboidal rather than simple squamous epithelium. DISCUSSION Conducting airways in mammalian lungs are called bronchi and bronchioles (Krahl, 1964; Weiss, 1983; Banks, 1986). In B. mysticetus, distinctions between bronchi and bronchioles are not clear. In this species, the ciliated pseudostratified columnar epithelium characteristic of larger airways undergoes a n abrupt transition to a n exchange epithelium characterized by type I and 11pneumocytes in airways less than 1mm in luminal diameter. Nonciliated bronchiolar epithelial (Clara) cells are sometimes used in terrestrial mammals as a n epithelial indicator of bronchiolar identity. Similar cells are seen in the bowhead whale, but only for a short distance in airways that open directly into Fig. 14. Light micrograph of a n alveolar duct (AD), the alveolar duct wall (dw), and its supporting elastic fiber (ef) core. The alveolar septum (as)and the associated capillary network (cn) are also evident. Fig. 15. A scanning electron micrograph of a portion of an alveolar duct wall showing elastic fibers (ef) that form the prominent fiber bundle core (FBI. Fig. 16. Transmission electron micrograph of the fiber bundle core of an alveolar duct wall. A large number of elastic fibers (ef) are seen surrounded by collagen fibrils (cf). Fig. 17. Transmission electron micrograph illustrating cellular elements of the fiber bundle core of an alveolar duct wall. Three fibroblasts with numerous cytoplasmic extensions are evident. The fibril- alveoli and alveolar ducts. Since these cells were not observed at the ultrastructural level, it is not possible to know how similar they are to the cells described by Fanning (1977) as microvillus cells in dolphin terminal (respiratory) bronchi. In the bowhead whale, these cuboidal cells do not resemble type I1 pneumocytes seen elsewhere in the lung. Cartilaginous rings and plaques extend to airways lined by simple squamous epithelium. Smooth muscle bands are found only in walls of airways with alveolar outpocketings or those that immediately give rise to them. For these reasons, none of the usual markers used to differentiate small bronchi and bronchioles were useful in the case of the bowhead whale lung. Therefore we elected to simply use the term airway. In describing cetacean lungs, Fanning and Harrison (1974) suggested using the term bronchus to describe all the airways to the level of the alveolar ducts. Others have referred to bronchi and bronchioles, but failed to report the basis for these distinctions (Wislocki and Belanger, 1940; Simpson and Gardner, 1972). We would suggest that both these approaches be abandoned in descriptions of cetacean lungs where morphological or functional distinctions are not yet clearly evident. The epithelium of the conducting airways of B. mysticetus is similar to that of other mammals, including other cetaceans. The PCE lining cetacean airways is composed of both ciliated cells and nonciliated secretory cells; the latter are often described as mucoussecreting, or goblet, cells (Krahl, 1964; Weiss, 1983; Banks, 1986). These epithelial secretory cells are absent or reduced in number in some cetaceans (Simpson and Gardner, 1972). It is clear from LM, SEM, and TEM, however, that both cell types are present in the bowhead whale lung to the level of those airways containing alveolar outpocketings. The PAS-positive secretory product and the basal position of the nucleus suggest t h a t these are mucus-producing cells. The relative proportion of ciliated to nonciliated cells in different airways was not investigated. Fanning (1977) also reported the presence of goblet cells to the level of the terminal airways in dolphins. The presence and abundance of mucus-secreting epithelial cells and of submucosal mucous glands in cetaceans is variable (Wislocki, 1929; Simpson and Gardner, 1972; Fanning and Harrison, 1974). In their review of cetacean microanatomy, Simpson and Gardner (1972) reported that cetaceans, in general, have few mucous glands and few epithelial mucus-secreting cells. The bowhead whale lung is clearly a n exception to that generalization. lar elements surrounding the cells contain both elastic fibers (ef) and collagen fibrils (cf). Fig. 18. A higher-magnification scanning electron micrograph of a portion of the alveolar duct wall similar to Figure 13 more clearly demonstrates the capillary network (cn)just beneath the surface. Regions devoid of a capillary network near the surface are also seen (arrowheads). Fig. 19. A transmission electron micrograph of the simple squamous epithelium that lines the alveolar duct (AD).Both type I (I) and I1 (11) pneumocytes are evident. Both surface microvilli (mv) and lamellar bodies (lb) are visible in the type I1 pneumocytes. BOWHEAD WHALE LUNG Figs. 14-19 193 194 W.G. HENK AND J.T.HALDIMAN The significance of the numerous blood vessels lying in the connective tissue beneath the airway epithelium is not apparent, but an abundance of anastomosing vessels has been described in this position in other cetaceans (Lacoste and Baudrimont, 1926, 1933; Wislocki and Belanger, 1940; Goudappel and Slijper, 1958). Goudappel and Slijper suggested that these vessels may serve to heat inspired air or as hydrodynamic cushions. These vessels may serve similar functions in the bowhead whale lung. The prominent myoelastic sphincters characteristic of the terminal airways of most of the smaller toothed whales (Belanger, 1940; Wislocki and Belanger, 1940; Wislocki, 1942; Goudappel and Slijper, 1958; Fanning, 1974; Harrison and Fanning, 1974; Fanning and Harrison, 1974) have not been seen in the bowhead whale. Similarly, these sphincters are absent from the airways of most other large cetaceans (Wislocki and Belanger, 1940; Goudappel and Slijper, 1958) and at least one smaller odontocete, the bottlenosed whale (Hyperodon ampullatus) (Goudappel and Slijper, 1958). Because the behavior of the bottlenosed whale includes deeper dives and longer intervals between breaths than most small odontocetes, it has been suggested that the sphincters are more characteristic of animals that breath more often, dive less deeply, and have larger relative lung capacities than large cetaceans (Goudappel and Slijper, 1958). In other large whales, myoelastic connective tissue cores are found in the walls of alveolar ducts and alveoli; smooth muscle dominates in some species, whereas elastic tissue dominates in others (Belanger, 1940; Murata, 1951; Engel, 1966; Ito et al., 1967). The apparent absence of smooth muscle from the connective tissue cores of the walls of the alveolar ducts and apertures at the openings of alveoli and alveolar sacs in bowhead whales appears to be unusual in large cetaceans. In dolphins, however, Fanning and Whitting (1969) reported that smooth muscle is not found beyond the bronchiolar termination. Similarly, Ito et al. (1967) reported that muscle fibers are almost completely absent from the elastic bands of the alveolar ducts of the striped dolphin, Prodelphinus (Stenella) caeruleoalbus. Pseudostratified, ciliated, columnar epithelium changes to simple squamous epithelium in airways 1-0.4 mm in diameter with alveoli and alveolar sacs outpocketing from their walls. Similar changes have been reported in other cetaceans (Fanning, 1977; Simpson and Gardner, 1972).The presence of a simple cuboidal epithelium in these transition regions has been re- ported previously in the small airways of other cetaceans (Fanning and Harrison, 1974; Simpson and Gardner, 1972). In the bowhead whale, these simple cuboidal cells are confined to regions immediately adjacent to the simple squamous epithelium which mediates gas exchange between blood and air. The functional significance of the small numbers of nonciliated, nonmucous-secreting, cuboidal epithelial cells is not known. In the bowhead whale, the epithelium across which gases are exchanged, consisting of type I and type I1 pneumocytes, extends well past the borders of the alveoli to form a part of the airway lining. The morphology of type I and I1 pneumocytes is typically mammalian (Weibel, 1985). Attenuated cytoplasmic extensions with numerous caveoli characterize the type I cell, whereas lamellar bodies and numerous lumina1 microvilli distinguish the type I1 cell. Type I and I1 pneumocytes have been described in certain odontocetes (Simpson and Gardner, 1972; Fanning, 1977) where they appear to be similar to those of the bowhead whale. The presence of tubular myelin in the small airways confirms the presence of active type I1 pneumocytes and suggests that they function in the production of pulmonary surfactant phospholipids as they do in terrestrial mammals (Weibel, 1985). We made no attempt t o determine mean blood-air barrier thickness in the bowhead lung because only specimens from collapsed lungs were available. The minimal thickness (350 nm) reported here is slightly greater than minimal thicknesses reported in other mammals (Meban, 19801, including the porpoise Tursiops truncatus (Fanning, 1974,1977).It has been suggested that the minimum thickness of the blood-air barrier is an indicator of the degree to which respiratory tissues can become attenuated without compromising its mechanical stability (Meban, 1980). A thicker blood-air barrier along with thick connective tissue septa and heavily supported airways may suggest that bowhead whale lungs are subjected to greater mechanical stress than those of terrestrial mammals and T. truncatus. A quantitative investigation of bloodair barrier thickness as well as other aspects of pulmonary morphology in both inflated and collapsed bowhead lungs might provide additional insights. The thick septal wall and double capillary bed of the bowhead lung is typical of cetaceans (Wislocki, 1929; Haynes and Laurie, 1937; Wislocki and Belanger, 1940; Baudrimont, 1959; Ito et al., 1967). Some reports have indicated that each alveolus has its own capillary Fig. 20. A scanning electron micrograph illustrating the surfaces of several alveoli with their dense subsurface capillary networks (cn). The opening from an alveolar duct (AD) into a n alveolar sac (A) can also be seen. Fig. 21. A scanning electron micrograph of a portion of the surface of a n alveolus. Type I1 pneumocytes (11) and a portion of the capillary network beneath the surface are visible. Fig. 22. A transmission electron micrograph of a portion of an alveolar septum (S). The alveolar air space (A) is separated from the septal connective tissue by an epithelium and a capillary bed on each surface. The septal connective tissue contains collagen fibrils (cf) and fibroblasts (fb)that usually lie just beneath the capillary bed. Fig. 23. A higher-magnification transmission electron micrographof a thin area of the alveolar surface. The alveolar air space (A) is separated from the blood by cytoplasmic extensions of epithelial and endothelial cells and a shared basal lamina (bl) that lies between them. Both epithelial and endothelial cell extensions contain numerous caveoli (cv). Fig. 24. A transmission electron micrograph of a lamellar structure from the air space of a n alveolus. This structure contains tubular myelin (tm) arranged in typical system of packed square tubules. Fig. 25. A montage of three light photomicrographs illustrating the pleura. The numerous blood vessels (bv) and elastic fibers (ef) are visible. Arrowhead indicates the thin layer of connective tissue which lies immediately beneath the mesothelium. BOWHEAD WHALE LUNG Figs. 20-25. 195 196 W.G. HENK AND J.T. HALDIMAN bed (Haynes and Laurie, 1937; Wislocki and Belanger, 1940; Murata, 1951; Ito et al., 1967). We have presented no convincing evidence that these capillaries interconnect across the septal connective tissue. If they do, then they probably do so at relatively infrequent intervals. Such trans-septal anastomoses are reported in T. truncatus (Fanning and Harrison, 1974; Fanning, 1977). The absence of smooth muscle in the septal walls is similar to what has been observed in most of those other cetaceans that have been examined (Wislocki, 1929; Fanning and Whitting, 1969); however, some smooth muscle has been reported in the septa of a few cetaceans (Belanger, 1940). The importance of the observed absence of elastic fibers from the narrowest portions of the interalveolar partition in the bowhead lung is not clear. The distribution of elastic fibers in the alveolar septa of other cetacea varies considerably (Haynes and Laurie, 1937; Goudappel and Slijper, 1958; Simpson and Gardner, 1972). The presence of plasma cells and lymphocytic aggregations in the airway connective tissue and the common occurrence of pulmonary macrophages indicate that the respiratory system of the bowhead whale receives antigenic stimulation. A conspicuous absence of alveolar macrophages has been reported in some cetaceans (Simpson and Gardner, 1972; Slijper, 19791, whereas in others they are common (Fanning, 1977). Whether these differences are real or result from the problem associated with tissue sampling and processing is not known. The visceral pleura of cetaceans is generally described as thick, measuring from l to 5 mm depending on the species (Haynes and Laurie, 1937; Belanger, 1940; Simpson and Gardner, 1972). The 2.5 mm highly elastic pleura of the bowhead whale is, therefore, not exceptional. In the humpback whale (Megaptera nodosa, now M . novaeangliae), Belanger (1940) reported that the pulmonary epithelium immediately adjacent to the pleura is composed of cuboidal rather than squamous cells. We found no continuous or extensive layer of cuboidal cells adjacent to the pleura in the bowhead but did find some regions adjacent to the pleura lined by cuboidal cells. Belanger (1940) suggests that epithelia1 cells adjacent to the pleura may be exposed to greater mechanical stresses that result in the development of a more stress-tolerant cuboidal epithelium. Our observations on the bowhead lung clearly indicate that the pulmonary microanatomy of this large mysticete is quite different from that of terrestrial mammals and that of smaller odontocetes. Considering the dramatic difference in breathing patterns and in diving behaviors, it is not surprising that significant morphological differences are present. Unfortunately, the small number of investigations focused on the pulmonary microstructure of large mysticetes coupled with generally poor photographic documentation and sometimes unreported methodology make interpreting these differences difficult. Continued studies of the lungs of the bowhead whale as well as those of other mysticetes, especially studies that provide documentation from a variety of imaging systems, may provide a clearer understanding of the significance of these morphological variations. ACKNOWLEDGMENTS This study was funded by the Bureau of Land Management through the University of Maryland (DOII BLM, AA851-CTO-22) and by the North Slope Borough, Barrow, AK. The Department of Veterinary Anatomy and Fine Structure, Louisiana State University, Baton Rouge, LA, has also provided support throughout the study. We appreciate the cooperation of the Eskimo whaling captains, Dr. T.F. Albert, senior scientist, the North Slope Borough, and the technical assistance of P. 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