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Microscopic and submicroscopic anatomy of the parabronchi air sacs and respiratory space of the budgerigar (Melopsittacus undulatus).

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Microscopic and Submicroscopic Anatomy of
the Parabronchi, Air Sacs, and Respiratory Space of the
Budgerigar (Melopsittacus undulatus)
Department of Pathology and Laboratory Medicine, College of Medicine,
Texas A and M University, College Station, Texas 77843-1114(J.H.S.,
I?J.G.N.); and Departments of Pathology (J.L M., C.L) and Microbiology
(E.D.B.), University of Texas Medical Branch, Galweston, Texas 77550
The normal microscopic and submicroscopic structure of the
lower respiratory tract of the budgerigar (Melopsittacus undulatus)is described
and compared with other birds and mammals. Granular (type 11)pneumocytes
are confined to linings of air sacs, parabronchi, and their atria; however, their
secretions (surfactant) cover the surfaces of the infundibula and respiratory
space. Infundibula extend from the atria and give rise to the air capillaries,
which branch and anastomose freely with those of adjacent infundibula and
other parabronchi (interparabronchial septa are not found). Infundibula and
the respiratory labyrinth are lined by a continuous epithelium of squamous
pneumocytes, whose perikarya are concentrated in the infundibula and whose
peripheral cytoplasm is markedly attenuated. The squamous pneumocytes of
the respiratory labyrinth share a basal lamina with the blood capillaries that
they envelop.
In 1978 the protozoan Sarcocystis falcatula
was discovered to complete its asexual schizogony in many families of birds (Duszynski
and Box, 1978; Box and Smith, 1982; Box et
al., 1984). This breadth of intermediate host
range was remarkable, considering that previously described Sarcocystis species had single genera as intermediate hosts. Thus its
development and comparative pathology in
different avian families needed study. Early
generations of the sporozoan develop within
specific pulmonary vessels and produce distinctive pulmonary lesions whose description
demanded precise anatomic definition of normal pulmonary structures. Review of the literature at that time revealed careful
descriptions and illustrations of air passages
down to parabronchi (Krause, 1922; Akester,
1960; King, 1966; Evans, 1969; King and
Atherton, 1970; Lasiewski, 1972) and brief
notes regarding the ultrastructure of the respiratory anatomy of some birds (Tyler et al.,
1961; Pattle and Hopkinson, 1963; Tyler and
Pangborn, 1964; Fugiwara et al., 1970), but
there was no integrated comprehensive
description of the microscopic and s u b
0 1986 ALAN R. LISS, INC.
microscopic structure of the budgerigar (Melopsittacus undulatus) respiratory system.
Light and electron microscopy
Six adult budgerigars were sacrificed by
COZ inhalation and the anterior neck was
immediately opened. An 18-gauge hypodermic needle was inserted and tied into the
upper trachea, and the lungs were inflated
with cold (0-4 "C) half-strength Karnovsky's
solution (Karnovsky, 1965) until fixative
flowed freely (without bubbles) out of the ruptured thoracic, abdominal, and cervical air
sacs. After 15 min the lungs were removed
and placed in cold (0-4°C) half-strength Karnovsky's solution for 30 min, then cut into
0.5- to 1.0-mm slices and 1.0-mm cubes, and
fixed for 1-2 h r more. Blocks were postfixed
in ethanol, and embedin 0 ~ 0 4 dehydrated
ded in Polybed 812. Thick (1pm) sections for
light microscopy were stained with 1% toluidine blue in 1% sodium borate. Thin sections
Received April 18,1983. Accepted May 25,1986.
smooth muscle fascicles defined the adventitia of the parabronchi (Figs. 4,5 ) , and basement membranes of blood capillaries abutted
on the abluminal surface of this adventitia.
The upper portion of the sides of the atria
was buttressed by the parabronchial muscle
fascicles (Figs. 4, 5). A fornix was formed
below these fascicles where the sides and
floor of the atria joined; this recess extended
for a short distance beneath the abluminal
margins of the muscle fascicles (Fig. 4). Atrial
epithelial cells covered the sides, filled the
While light microscopy of 4-to 5-pm-thick fornices, and lined the floor of the atrium.
paraffin sections afforded insufficient resolu- The atrial epithelium was supported by a
tion for detailed study, examination of tolui- basal lamina through which coursed colladine blue-stained semithin (1-pm) plastic gen fibers, peripheral processes of fibrosections allowed definition of parabronchi blasts, and rare smooth muscle cells (Figs. 4,
and their atria, air sacs, infundibula, and the 5, 12).
The atrial cell (Figs. 4-8) varied from squarespiratory space. Vessels consisted of arteries, arterioles, two orders of capillaries, pri- mous to cuboid. Distal processes of atrial cells
mary and secondary venules, and veins. extended over the luminal surface of the parTerminal secondary bronchi were lined by abronchial smooth muscle fascicles and
mucociliary columnar epithelia with some formed junctional complexes with the s q u a
nuclear pseudostratification (Smith et al., in mous cells lining infundibula (Fig. 12). The
press). Where secondary bronchi and para- apical surface of some cells had numerous
bronchi connected, cuboid cells covered the microvilli, but others were smooth. The latjunction (Fig. 1); these cells like bronchial eral margins were undulating and connected
basal cells contained tonofibrils and like to adjacent cells with junctional complexes
atrial cells contained myelinoid bodies. Tu- (Figs. 8, lo), but maculae to the basal lamina
bular parabronchi with shallow atria coursed were rarely seen. These junctional complexes
through the pulmonary parenchyme and were principally composed of long gap juncwere perforated by infundibular ostia in tions that were often reinforced by maculae
depths of the atria (Fig. 2). Both longitudinal adherentia subapically. Occasionally, apparand cross sections of parabrohchi showed a ent zonulae occludentes were observed within
scalloped profile with atria forming the evag- the gap junctions (Fig. 10). The ovoid nucleus
inations of the lumina (Fig. 3). The structure frequently had a corrugated margin. There
of the atrial and parabronchial walls differed was marked variation in the clumping of hetslightly, but both differed markedly from erochromatin at the nuclear membrane, and
more proximal and distal respiratory linings. the euchromatin had moderate density. NuThe wall of the parabronchus (Figs. 3-8) cleoli were not prominent. Mitochondria were
was composed of widely dispersed fibroblasts,
collagen fibers, and occasional elastic fibers
surmounted by fascicles of smooth muscle
(Figs. 3-5). It was lined by extensions of the
peripheral cytoplasm of atrial cells. A layer
of myelinoid material of varying depth overlay this epithelial surface. Junctional com- Fig. 1. Electron micrograph of the junction of a secplexes joined these cells at their lateral ondary bronchus (S)parabronchus (PI. The thick fibromargins. The basal plasma membrane rested muscular wall of the secondary bronchus is composed of
smooth muscle cells (s) and collagen (D and is surupon the basal lamina surrounding subja- mounted
by an attenuated process of a cell with tonocent smooth muscle cells; this basal lamina filaments (black-on-white arrow), resembling a bronchial
occasionally contained collagen fibrils. All basal cell; but a myelinoid body (as in atrial cells) is also
collagen fibrils observed in the budgerigar’s seen @lack arrow). The cytoplasm of an adjacent atrial
with an overlying myelinoid (surfactant) layer (white
lung measured 42 & 2.3 nm in diameter and cell
arrows) is seen. A thick basal lamina with collagen fiwere banded with a periodicity of 62.5 If- 5.9 bers separates the smooth muscle from the epithelium.
nm. The fibroelastic layer subjacent to the x 21,600.
were stained with uranyl acetate and lead
citrate (Reynolds, 1963) and viewed with a
Philips EM-200 transmission electron
All measurements are given as the mean
~ f -1.0 standard error of the mean unless otherwise specified. Anatomic terms used conform to standard terminology (King, 1979)
except where indicated.
ovoid with tubular cristae. Granular endoplasmic reticulum and polysomes were abundant. Microfilaments, both 5 and 8-10 nm in
diameter, were frequent but were not aggregated into tonofibrils. The most remarkable
feature of these cells was the prominent myelinoid bodies (Figs. 6-8). These bodies disgorged their contents onto the apical surface,
where the contents appeared to spread and
form the fibrillar or myelinoid coat of both
the parabronchi and the respiratory space
(Figs. 6-8). Another common cytoplasmic inclusion was an electron-lucent cleft (Figs. 7,
8) which was not membrane-bound; these
elongate “splinters” often intersected, rarely
contained loosely concentric membranous
profiles, and occasionally fused to myelinoid
Some atrial cells (less than 10%) were
larger, more globular, and more “watery”
than the “dense” atrial cells. Heterochromatin condensation at the nuclear membrane
was more uniform and narrower in these
atrial cells, and the euchromatin was more
electron lucent. Nucleoli were often prominent. Granular endoplasmic reticulum, polysomes, and microfilaments were widely
separated. Mitochondria were larger and less
numerous than in “dense” cells. Large myelinoid bodies and cleft inclusions were
Fig. 2. Photomicrograph of cut surface of budgerigar
lung. Longitudinal sections through parabronchi (p) show
shallow atria with infundibular ostia (black-on-white arrow). Infundibula (black arrow) radiate from parabronchi into the respiratory labyrinth (1). Half-strength
Karnovsky fixation, unstained, x 50.
Fig. 3. Photomicrograph of two longitudinally oriented parabronchi (P) and intervening respiratory space.
Muscular wall of parabronchus is noted above lower P,
and the atria (A) are well defined. Several infundibula
(arrows) extend into the respiratory labyrinth. No fibrous septum separates the respiratory space of the two
parabronchi. Veins (V) course through the respiratory
labyrinth perpendicular to the long axis of the parabronchi. Toluidine blue, x 320.
Fig. 4. Low-magnification view of wall of parabronchus with an atrium (A); atrial floor is noted (small
arrow) as well as atrial wall (large arrow). A prominent
fascicle of smooth muscle (s) defines the wall of the parabronchus and is overlain by processes of atrial cells.
Note that atrial cells (a) extend into the fornix formed
beneath the fascicle of smooth muscle. x 2,400.
A i r sacs
Some parabronchi emptied into air sacs
within the thorax. These thin-walled structures were lined by cells that were identical
to the atrial cells but were more flattened
(Fig. 9). All stages in formation of the myelinoid inclusions were seen, including multivesicular bodies and disgorgement of
myelinoid contents t o form the surface coating over the apical plasma membranes (Fig.
10).Polysomes, granular endoplasmic reticulum, and mitochondria were abundant. The
apical plasma membrane was smooth or had
small microvilli. Adjacent cells had undulant
or oblique lateral surfaces attached by long
junctional complexes (Fig. 10). The basal
plasmalemma had myriads of pinocytotic and
coated vesicles; and the epithelia rested on a
thin basement membrane, but hemidesmosomes were rare. The mesenchymal wall of
the air sacs was composed principally of two
to five layers of fibrocytes, abundant collagen
fibers, and elastic fibers oriented parallel to
the surface. Intermittent smooth muscle fascicles were noted (Fig. 9).
Respiratory space (Figs. 2, 3, 11-19)
As seen in the dissecting microscope (Fig.
21, the floor or base of the atrium was perforated by numerous small ostia that opened
into elongate wedges whose diameter decreased progressively from their ostia; these
were the infundibula of the respiratory space
(Figs. 3,ll-13). The structure of the infundibulum was slightly different from numerous
air passages that originated from it. When
parabronchi were expanded and atria were
distended, the ostia of the infundibula were
open (Fig. 11);but when atrial muscle was
contracted and the atria were shallow, the
ostia appeared closed (Fig. 11,inset).
The infundibulum was lined by distal processes of squamous pneumocytes whose perikarya were usually located near the ostium
(Figs. 11, 13).The basal lamina of the atria
extended to the margin of the ostia and
turned perpendicular to the atrial floor to
form the wall of the infundibulum (Fig. 12).
There often were redundant, globular deposits of basal lamina matrix at its reflection
down the infundibulum (Fig. 12). Near the
ostium, the infundibular basal lamina contained a few collagen fibers; but these progressively disappeared distally (Figs. 12, 13).
The infundibular basement membrane had a
thickness that was similar to that in the
Fig. 5. Wall and floor of distended atrium. Smooth
muscle (s) defining the wall of a parabronchus is noted
in the upper right. Abundant collagen fibers (f, right)
are noted along the atrial side wall finteratrial septum),
which is overlain by atrial cells (a). One atrial cell con-
tains numerous myelinoid inclusions (it. Collagen fibers
(f, below) are less frequent in the atrial floor. An artifactually extravasated nucleated erythrocyte lies in the
atrial lumen (A). x 8,600.
Fig. 6. Electron micrograph of atrial floor with atrial (black-on-white arrow) and fibroblasts (0. x 15,400.
cells (a) containing laminated myelinoid inclusions (9.
Fig, 7. Electron micrograph of atrial cell with myeliOne of these has spread along the surface of the cell noid inclusions (i) and cleft-like inclusions (c). The super(black arrows). Numerous mitochondria (m) are noted. ficial myelinoid layer (arrows) has a fibrillar appearance.
Subjacent to the atrial cells, there are collagen fibers x 70,000.
Fig. 8. Atrial surface showing portions of four atrial plexes (arrows). X 18,400.
cells. The surface is covered by a fibrillar myelinoid
Fig. 9. This portion of the wall of a thoracic air sac is
layer, and the superficial plasma membrane has numer- composed principally of leiomyocytes (s),elastin (el, and
ous microvilli. The cytoplasm contains cleft-like (c) and collagen fibers (0. A lining of flattened cuboid to squamyelinoid (i) inclusions as well as numerous mitochon- maus cells with numerous mitochondria (m) and myelidria (m). Adjacent cells are linked by junctional corn- noid inclusions (i) is seen. x 13,600.
Fig. 10. High-magnification view of cells lining air
sac. The prominent microvillous surface is partly COV.
ered by a myelinoid surfactant layer (large arrows). Multivesicular bodies (mb), laminated myelinoid inclusions
(i), and polysomes are numerous. Mitochondria (m) contain tubular cristae (black-on-white arrow). One inclu-
sion appears to be disgorging its contents to the surface
(small black arrows). Adjacent cells have apical junctional complexes that principally consist of long gap
junctions with an apparent zonula occludens (arrowhead). x 46,000.
Fig. 11. Ostia (asterisks) of infundibula (I)and atrium
(A). Atrial wall is present at small arrows. Infundibular
ostia and respiratory labyrinth are open and distended.
Squamous pneumocytes (arrowheads) are noted at the
lip of a n infundibulum and along the infundibula.
x 3,710. Inset: Low-power electron micrograph of a
nearly closed infundibular ostium with prominent cuboid atrial cells (a). x 3,100.
Fig. 12. Higher magnification of junction of atrium
(A) and infundibular ostium (I) seen in Figure 11. A
smooth muscle cell (s)underlies the attenuated process
of the atrial cell (a). Numerous collagen fibers (white
arrows) are noted in the basal lamina under this atrial
cell. This basal lamina has redundant folds (black ar-
rows). The squamous pneumocyte (p) lies on a basal
lamina with few collagen fibers (white arrowhead). The
pneumocyte’s processes extend down the infundibular
wall and along the lip of the ostium. Erythrocyte-filled
capillaries (B) lie along the abluminal surface of the
atrial and infundibular wall. X 22,400.
Fig. 13. Junction of infundibulum 0) and “air capillary” (1). The squamous pneumocyte (p) extends its cytoplasm over the infundibular wall to line the air capillary
(arrow). The basal lamina of the infundibulum is thick
but becomes attenuated as it continues as the basal
lamina of the air capillary. No collagen fibers are seen
in this basal lamina from the depths of the infundibulum. x 8,600.
Fig. 14. The squamous pneumocyte (p) within the respiratory labyrinth conforms to the angle of two adjacent
blood capillaries 03). Pneumocyte cytoplasm extends in
long attenuated processes (arrows) over the capillary
basal lamina. The blood capillaries are lined by a continuous endothelium. 1, “air capillary.” X 13,000.
Fig. 15. Two blood capillaries intersect with endothe- toplasm of' two pneumocytes is shown at broad arrow
lial perikarya (E) in approximation. Note marked atten- (and inset). X 12,000. Inset: The apical interface of the
uation (slender arrows) of capillary endothelium, basal two pneumocytes is closed by a gap junction with a
lamina, and squamous pneumocyte opposite the endo- subjacent macula adherens forming a typical epithelial
thelial cell nuclei. Junction of expanded peripheral cy- junctional complex. X 50,400.
atria but was nearly twice that of the respiratory passages (air capillaries) emanating
from it (Fig. 13). The abluminal surface of
the infundibular basement membrane abutted capillaries and was overlain by distal
extensions of squamous pneumocytes.
Multiple respiratory passages (air capillaries) branched off the infundibula at acute or
obtuse angles (Figs. 3,11,13).These passages
varied markedly in diameter and shape of
profile. They branched and anastomosed
freely to form the labyrinth that is the respiratory space. They were lined by the markedly attenuated processes of squamous
pneumocytes, which were applied over the
basement membranes of capillaries, the
abluminal aspects of atrial and infundibular
walls, and the adventitia of larger vessels.
The squamous pneumocytes (Figs. 11-18)
completely enveloped the air capillaries and
infundibula and formed junctional complexes
with atrial cells (Fig. 12) and with each other
(Fig. 15). These junctional complexes con-
Fig. 16. High-magnification view of air-blood barrier.
The erythrocyte (r) lies within the capillary lumen separated from the apical plasma membrane of the endothelial cell. Pinocytotic vesicles appear to fuse with both
apical and basal endothelial plasma membranes. The
basal endothelial plasma membrane lies on the basal
lamina, which is markedly attenuated. Squamous pneumocytic cytoplasm is extremely attenuated and barely
separates its apical and basal trilaminate plasma membranes; at two points it appears that the inner leaflets of
the apical and basal pneumocytic plasma membranes
fuse (slender arrows). A thin myelinoid surfactant layer
(broad arrows) rests upon the surface of the apical plasma
membrane of the pneumocyte. X 104,000.
Fig. 17. Air-blood barrier. The capillary lumen (bottom) is lined by a continuous endothelium that rests
upon the basal lamina. The apical and basal trilaminate
plasma membranes of the squamous pneumocyte are
barely separated. The trilaminate myelinoid surfactant
layer (slender arrow) forms a bleh (broad arrow) in the
lumen of the respiratory space. The pneumocytic membrane appears everted at the bleb. x 130,000.
Fig. 18. Air-blood barrier. The erythrocyte (below) is
barely separated from the endothelial cell plasma membrane. Numerous pinocytotic vesicles appear to have
joined the apical and basal plasma membranes of endothelial cell (arrows), suggesting formation of a channel.
x 121,000.
sisted of apical gap junctions with subjacent
zonulae or maculae adherentia (Fig. 14).
Their nuclei and perinuclear cytoplasm were
most frequently seen along the infundibula
(Figs. 11, 13) but also were found within the
respiratory labyrinth (Figs. 14, 15). In the
infundibula, these perikarya were usually
flattened; but in the air passages they were
globose or angular as they conformed to the
corners formed by branching or intersecting
blood vessels. Their nuclei resembled those
of endothelial cells and were round to ovoid
or compressed and crescentiform with scalloped margins and prominent irregular
clumping of heterochromatin at the nuclear
membrane. Nucleoli were not prominent. The
cytoplasm contained polysomes and small
ovoid mitochondria with lamellar cristae, but
granular and agranular reticulum were
sparse. Microfilaments were not prominent,
and pinocytosis from any surface was rare.
The basal plasmalemma rested on the basement membrane common to both the pneumocyte and endothelial cell, but no hemidesmosomes were seen (Fig. 14). Basal lamina were not observed subjacent to squamous
pneumocytes, where “air capillaries” abutted on atrial, arterial, or venous adventitia.
The apical plasma membrane was overlain
by a myelinoid layer (Figs. 16-18); sections
perpendicular to the surface usually showed
this myelin to be one or more trilaminar
membranes with occasional breaks or submembranous bullae (Fig. 17). Sections
oblique or parallel to the surface, however,
revealed a fibrillar or tubular myelin structure. Throughout the respiratory space the
cytoplasm between the apical and basal
plasma membrane of the pneumocyte was
markedly attenuated (Figs. 14-18), often less
than 10 nm; occasionally the inner leaflets of
the apical and basal plasmalemma appeared
to fuse (Fig. 16). Apical and basal plasma
membranes diverged to include more cytoplasm at the periphery where junctions between squamous cells occurred (Fig. 15).
The other components of the respiratory
space were blood capillaries, their basal lamina, their contents, and their afferent and
efferent vessels. Cells serving as “mesangial
cells” were not identified. The capillaries
possessed continuous endothelia differing in
no way from previous descriptions (Fawcett,
1981) of continuous endothelia (Figs. 11-21).
Their cytoplasm was thinned adjacent to the,
attenuation of pneumocytic processes but not
to the same extent as that of the pneumo-
Fig. 19. Junction of wall of arteriole (lower left, and
small muscular artery (upper right). The arterial and
arteriolar lumens are lined by continuous endothelia
(arrows). The wall of the arteriole is made up of a single
layer of smooth muscle cells (s), whereas that of the
small muscular artery contains several layers of smooth
muscle cells. N o internal or external elastic layer is
noted in either vessel. A markedly attenuated adventitia
is seen. Air and blood capillaries abut upon this adventitia. Outlined rectangle is enlarged in Figure 20.
x 12,300.
Fig. 20. Higher magnification of area indicated in
Fig. 21. Secondary venular wall showing continuous
Figure 19, showing continuous endothelia (E), smooth endothelium (E), fibroblast processes 0,and banded Colmuscle media (s),and absence of elastics. Fibroblasts (0 lagen fibers (arrows) with only occasional leiomyocyte
and collagen (arrow) of adventitia abut directly on air 0 ) processes noted. Air (1) and blood (B) capillaries abut
and blood (B) capillaries. x 14,700,
directly on the adventitia. X 14,800.
cytes (Figs. 16-18). Marked pinocytotic activity was evident at apical and basal surfaces,
often forming apparent channels from apex
to base (Fig. 18); and cells were joined by gap
junctions. As noted before, the endothelial
cells shared their basal lamina with the
pneumocytes. The basal lamina conformed to
the blood capillaries’ conformation; thus, the
air capillary shape became the complement
of the adjacent blood capillaries by default.
The basal lamina was thinner in areas of
endotheliaVpneumocytic cytoplasmic attenuation and thicker near endothelial perikarya. Rarely, a collagen fibril was seen
within the basal lamina where the blood capillaries approximated. Endothelial perikarya
of approximated blood capillaries most frequently backed upon each other, rather than
locating opposite the point of capillary conjunction (Fig. 15).
The blood-gas barrier was composed (from
air space to capillary lumen) of a myelinoid
layer (8.1 f 0.7 nm), apical pneumocytic
plasmalemma (7.2 f 0.2 nm), pneumocytic
cytoplasm (0-10.0 f 1.3 nm), basal pneumocytic plasma membrane (7.1 f 0.3 nm), basal
lamina (9.0 f 2.4 to 41.4 f 4.0; range =
0-69 nm), endothelial basal plasma membrane (7.8 f 0.3 nm), endothelial cytoplasm
(14.3 f 3.5 to 88.2 f 12.4 nm; range = 0147 nm, excepting channels), and endothelial
apical plasmalemma (7.8 f 0.3 nm). The resulting interface was 61-178 nm thick, a very
narrow separation between air and blood.
The major afferent and efferent blood uessels of the capillary ran midway between the
parabronchi. Arteries appeared to course
parallel to the axis of the parabronchi, and
veins ran perpendicular or at angles to the
major arteries. The right and left pulmonary
arteries had the structure of an elastic artery, whereas most of the “major” arteries at
the parabronchial level looked like arterioles
being characterized by an internal elastic
lamina, smooth muscular media, absence of
external elastic lamina, and scant adventitia. Branches of these small arteries emerged
at obtuse angles (with respect to blood flow)
and had two or three layers of smooth muscle
cells, no elastic laminae, continuous endothelia, and a narrow adventitia with collagen
fibers and occasional fibroblasts (Figs. 19,201.
These in turn branched at nearly perpendicular angles to give a vessel with a single
layer of circumferentially oriented smooth
muscle cells, continuous endothelia, and ru-
dimentary collagenous adventitia (Figs. 19,
20). These in turn gave rise to 20-pm-diameter capillaries (precapillaries) that branched
into 10-pmcapillaries. Several of these small
capillaries anastomosed to form two orders of
venules (20-30 pm and 40-50 pm in diameter) that had continuous endothelia, basal
lamina, no smooth muscle (or occasional,
widely dispersed, single smooth muscle cells),
occasional pericytes, and scant collagen fibers (Fig. 21). These emptied at nearly right
angles into secondary veins that consisted of
a continuous endothelium, one or two layers
of smooth muscle cells and/or pericytes, and
scant collagen. Primary veins were similar
but had several layers of pericytes and
smooth muscle cells and modest adventitial
collagen. Significantly, no connective tissue
septa enveloped the vessels coursing between
the parabronchi. Thus, the respiratory labyrinth emanating from one parabronchus was
continuous with that of the adjacent
The essential features of the budgerigar
lower respiratory tract (parabronchialatrial-infundibular and respiratory labyrinthine) structure are schematically presented
in Figure 22. The light microscope morphology of the parabronchi of various species of
birds including the budgerigar has been described (Krause, 1922; King and Molony,
1971; Duncker, 1971, 1974; Lasiewski, 1972;
Dubach, 1981; Evans, 1982; Drescher and
Welsch, 1983). The atria of the budgerigar
parabronchi are shallower and less distinctly
separable from the parabronchus proper than
in other birds. Interatrial septi are less substantial. Interparabronchial connective tissue septi are practically absent. It is
uncertain whether this difference represents
evolutionary divergence in families and orders of birds or is merely a matter of the size
of the bird. Nevertheless, the aseptate nature
of the budgerigar parabronchial system permits potential free exchange of gas (or liquid)
between adjacent parabronchi via the respiratory labyrinth.
The fine structure of the avian parabronchi
and the atrial cells, but not that of the budgerigar, have also been described (Pattle and
Hopkinson, 1963; Tyler and Pangborn, 1964;
Petrik and Riedel, 1968a,b; Lambson and
Cohn, 1968; Akester and Mann, 1969; King
and Molony, 1971; Powell and Mazzone, 1983;
Fig. 22. Schematic representation of parabronchial
atrium, infundibulum, and adjacent respiratory labyrinth.
Drescher and Welsch, 1983). The morphologic similarity of the atrial cell and type 2
(granular) pneumocytes of the mammalian
lung (Weibel, 1973; Kuhn, 1976; Sorokin,
1977) has been noted in chickens (Tyler and
Pangborn, 1964) and geese (Lambson and
Cohn, 1968); these authors postulated that
the atrial cell gave rise t o the continuous
osmiophilic laminated membrane which
coated not only parabronchi and atria but
also the surfaces of the respiratory labyrinth.
Other workers, however, have demonstrated
in essence the same findings (ie., the continuous osmiophilic layer and the atrial cells
with laminated myelinoid inclusions in fetal
and newborn chicks as well as adult chickens, sparrows, and pigeons) but have come to
the conclusion that the material was produced by squamous pneumocytes and phagacytosed by cells lining the parabronchi and
atria (Petrik and Riedel, 1968a,b). Additionally, this myelinoid material has been postulated to be the surfactant material in turkeys
(Fugiwara et al., 1970) and in chickens (Pat-
tle and Hopkinson, 1963; Hylka and Doneen,
1982). Carlson and Beggs (1973) described
similar cells lining the abdominal air sacs of
chickens. In the budgerigar, parabronchi,
atria, and thoracic air sacs are lined by these
granular cells; and there is convincing morphologic evidence of active secretion of these
myelinoid granules to form the osmiophilic
trilaminar membrane that overlies all airways distal to the secondary bronchi, including parabronchi and their atria, air sacs, and
resFiratory spaces.
It is notable in the budgerigar that these
cells are found nowhere else in the respiratory lining; thus, there is sequestration of
granular pneumocytes remote from the gasexchange areas in contrast to the mammalian lung where granular pneumocytes are
distributed throughout alveolar ducts and alveoli (Weibel, 1973; Kuhn, 1976; Sorokin,
Standard nomenclature (King, 1979) indicates that the appropriate term for the cell
lining the atria is a “granular cell”; the pres-
ent authors believe that the term “granular
pneumocyte,” “type 2 pneumocyte,” or
“atrial cell” would be more appropriate, since
it would avoid confusion with “granular
cells” of the trachea and bronchi, which are
APUD, neuroendocrine cells with dense-core
In the budgerigar, there are apparently two
populations of granular pneumocytes. A minority of granular pneumocytes are somewhat larger and have larger, more ovoid
nuclei, prominently “active” nucleoli, and
less dense cytoplasm; these are presumably
differentiating postmitotic cells, whereas the
more common granular pneumocytes with
more dense cytoplasm are mature differentiated cells. Another function of granular
pneumocytes in mammalian lungs appears
to be re-epithelialization of alveolar ducts and
alveoli after damage to the squamous pneumocytes (Kuhn, 1976); a similar function of
granular pneumocytes in the budgerigar
lung, with proliferation of lucent granular
pneumocytes and extension of granular
pneumocytes down infundibula and into the
respiratory labyrinth has been observed in
the reparative phase of squamous pneumocytic injury (Smith, Meier, Neill, and Box,
unpublished data).
While myelinoid inclusions have been
noted in avian and mammalian granular
pneumocytes, we have not seen descriptions
of the cleft-like electron-lucent inclusions.
Their spicular shape suggests a crystalline
composition, but the low electron density
suggests a lipid, soluble in the dehydrating
and clearing agents used in the plastic impregnation process. It is tempting to speculate that these bodies may be cholesterol or
some other sterol precursor of “surfactant,”
since some of these non-membrane-boundinclusions appear to be fusing with the myelinoid inclusions.
The fibromuscular wall, granular pneumocytic lining, and myelinoid coating of the thoracic air sacs suggest that these structures
are comparable to, if not embryologically derived from, the parabronchi and their atria.
The relatively avascular wall of these sacs
would seem to be a poor locus of gas exchange between blood and inspired air. The
rich investment of “surfactant” over the surfaces of these air sacs suggests that the surfactant may function more to decrease
friction and facilitate air flow through the
small airways rather than to facilitate gas
exchange. Alternatively, this surfactant
layer of the air sacs may act as a reserve for
the respiratory labyrinth, which produces no
myelin in the budgerigar.
Krause described infundibula at the light
microscopic level in 1922, but they had been
largely omitted from descriptions of avian
lung anatomy until the pioneering work of
Duncker (1971, 1974). The infundibula provide an intermediate passage between the
large airways and the respiratory labyrinth,
and their ostia probably control air entry to
and exit from the respiratory labyrinth (in
concert with dilatatioddistention of parabronchi and their atria). Their structure is
distinct from either the atria or the air capillaries in that the principal component of
their walls is a thickened basal lamina, yet
infundibula share an epithelium composed of
squamous pneumocytes with the remainder
of the respiratory labyrinth. In the budgerigar, squamous pneumocytic perikarya are
concentrated in the infundibula. These squamow pneumocytes appear comparable to
squamous pneumocytes of mammals (type 1
or membranous pneumocytes) (Weibel, 1973;
Kuhn, 1976; Sorokin, 1977). The squamous
pneumocytes of the infundibula and respiratory labyrinth in the budgerigar were always
overlain by a myelinoid “surfactant” layer,
yet we never saw myelinoid granules or complex elaborations of the surface membrane in
these cells such as described by others (Petrik and Riedel, 1968a,b). Thus, we conclude
that in the budgerigar this myelinoid layer
originates exclusively from the granular
pneumocytes of more proximal air ways.
The term air capillaries seems to us a misnomer for the passages that emanate from
the infundibula. “Air capillary” conjures the
image of a uniformly tubular structure that
is quite unlike the respiratory space of the
budgerigar. These respiratory passages are
markedly tortuous and variable in caliber;
and they branch and anastomose with fellow
passages from the same infundibulum, other
infundibula, other atria, and even other parabronchi. Thus the term respiratory laby
rinth seems to us a more apt, descriptive
term that may have salutary functional
It seems difficult to reconcile this architecture with its embryologic derivation from a
budding duct or the more familiar mammalian “blind-ended” alveolar structure. The
negative pressure in the pleural space in
mammals keeps the mammalian lungs distended into globular alveoli. Without this
negative pressure (as in birds, which have no
pleural cavity), mammalian alveoli collapse
(i.e., atelectasis) and lose their globular conformation, and their walls become contorted
in ribbon-like folds; thus, we can roughly reconcile the racemose rather than globular
conformation of avian respiratory units
whose ostia are the infundibula. Furthermore, the freedom of intercommunication between these units can be conceptualized as
a n exaggeration of alveolar pores noted in
mammalian lung (pores of Cohn). While this
simplistic conceptualization may be temporarily or emotionally satisfying to the microscopist, further work on the development
of the avian respiratory system at a n ultrastructural resolution is indicated. Such efforts may have real value in producing
understanding of the basic processes of repair in injured avian lung.
The fine structure of the respiratory labyrinth has been studied extensively in the
chicken (Tyler et al., 1961; Tyler and Pangborn, 1964; Petrik and Riedel, 1968a,b; King
and Malony, 1971; Abdullah et al., 1982; Kazachka, 19841, in geese (Lambson and Cohn,
1968; Powell and Mazzone, 1983), in the pigeon (Bargmann and Knoop, 1961; Policard
et al., 1962; Petrik and Riedel, 1968a), in the
sparrow (Petrik and Riedel, 1968a; Dubach,
19811, in penguins (Drescher and Welsch,
19831, and in many species of birds including
the budgerigar (Dubach, 1981; Maina and
King, 1982; Maina et al., 1982). Indeed, elaborate morphometric studies with correlation
to respiratory-function factors and type of
flight have now been beautifully documented
(Dubach, 1981; Abdullah et al., 1982; Maina
and King, 1982; Maina et al., 1982; Powell
and Mazzone, 1983; Drescher and Welsch,
1983) and compared with similar data from
mammals and reptiles (Weibel, 1973; Meban,
1980). Our findings confirm those structural
data of others and integrate the fine structure of the respiratory labyrinth with that of
the infundibula, parabronchi, and their atria
in a manner that permits understanding of
pathologic alterations encountered in the
budgerigar lung. Squamous pneumocytes of
the respiratory labyrinth appear applied to
the abluminal surface of the basal lamina of
the blood capillaries except in the infundibula. Moreover, where “air capillaries” abut
upon adventitia of atria and larger blood vessels, squamous pneumocytic basal laminae
are often not observed. While the ultrastructural ontogeny of the respiratory labyrinth
has not been examined, regeneration of its
lining after pathologic denudation shows
granular pneumocytes extending pseudopods
along “bare” basal lamina of intact blood
capillaries. These granular pneumocytes
then differentiate into squamous pneumocytes without ostensibly adding to the basal
laminae that they overlie (Smith, Meier,
Neill, and Box, unpublished data). This suggests that avian squamous pneumocytes possess their own basal lamina only in the
infundibula. The apical surfaces of the squamous pneumocytes are overlain by a n osmiophilic trilaminar (“surfactant”) membrane.
They are connected with adjacent squamous
pneumocytes by typical epithelial junctional
complexes, and in places their cytoplasmic
matrix is so attenuated that the squamous
pneumocyte consists solely of a n apical and
basal plasma membrane. Attenuation of
blood capillary endothelial cell cytoplasm is
most marked, is remote from the perinuclear
cytoplasm, and is usually associated with the
complementary attenuation of the squamous
pneumocytic cytoplasm and basal lamina,
creating a very narrow air-blood barrier. Our
estimation of the thickness of this barrier is
similar to that noted by previous authors
(Dubach, 1981; Maina and King, 1982; Maina
et al., 1982).
This work was supported in part by Public
Health Service grant DHHS 5ROl A1 15945
from the National Institute of Allergy and
Infectious Diseases. We greatly appreciate
the secretaria1 assistance of Edna Sue Davis,
M. Imogene Wiley, Julie Smylie, and Lori
Mohr and the advice and encouragement of
Drs. E.S. Reynolds, J.S. Davis, and H.W.
Sampson. This paper was presented a t the
lOlst Stated Meeting of the American Ornithologists Union, New York, NY, September,
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air, anatomy, melopsittacus, undulatus, space, respiratory, microscopy, parabronchial, sacs, submicroscopic, budgerigar
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