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Microanatomy of the lung of the bowhead whale balaena mysticetus.

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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. Ellis, S. Gibson, L. Philo, L. Dalton, J.
Everitt, J.C. George, and R. Tarpley.
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