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Fine structure of mammalian renal cilia.

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Fine Structure of Mammalian Renal Cilia
Department of Anatomy, The University of British Columbia,
Vancouver, British Columbia, Canada
Studies were carried out, using transmission electron microscopy, of the cilia of the nephrons of rat and human kidneys. Cilia were observed
in the parietal layer of Bowman's capsule, in the proximal tubule, the distal
tubule, including the macula densa, and the collecting duct. They had a number
of characteristic features, including the presence of a centriole adjacent to the
basal body, long, slender cross-striated rootlets, and a typically organized basal
body. The shaft of the cilia differed from the typical 9 2 pattern of organization. Near the base of the cilia the pattern was 9 0. In the middle portion,
one or more of the peripheral doublets had been displaced centrally to give an
1 or a 7 2 pattern, while towards the tip the pattern became more irregular and the doublets were reduced to single microtubules. We have hypothesized
that these cilia may be vestigial. They may, if motile, have some minor stirring
function, or they may play a sensory role, as has been postulated for similarly
structured cilia in other sites.
Cilia have long been known to be present in large numbers in the renal tubules
of many animal species (Latta et al., '61;
Zimmerman, 1898) and have been observed to have a propulsive function where
they are numerous (Fawcett and Porter,
'54). In the mammalian kidney they have
frequently been observed as single, isolated
structures to which little significance has
been ascribed. Recently, with the extensive use of scanning electron microscopy,
a number of authors have reported the general pattern of ciliation in the kidney of
both man and rat (Andrews and Porter,
'74; Bulger et al., '74; Webber, '74). On the
parietal layer of Bowman's capsule, the
proximal tubule, the distal tubule, and in
the light but not the dark cells of the collecting duct, each cell usually bears a single cilium, although occasionally two are
present. This uniform pattern has raised
the question as to whether these cilia are
vestigial or whether they may have some
specific function.
Cilia are usually motile and serve either
to move the organism or to move material
relative to a surface. These cilia typically
have an internal structure consisting of
nine peripherally placed doublets of microtubules and a pair of single centrally
placed microtubules, the 9 2 pattern. A
ANAT.REC., 182: 339-344.
variety of other patterns has also been described, for example in insect sperm (Baccetti et al., '74). In many sites, however,
cilia occur singly in locations where motility would not appear to be an important
function (Scherft and Daems, '67). A number of these sites are in sense organs or
part of the nervous system, some are
in contractile cells, notably myoepithelial
cells where they may be always present
(Tandler et al., '70), while others are in
a variety of cells with neither contractile
nor sensory functions, such as chondrocytes (Scherft and Daems, '67). In the
cilia associated with the nervous system,
it has been reported that the fine structure
differs from the typical 9 2 pattern in
that the central pair of single microtubules
are absent, giving a 9 0 pattern. In addition, these cilia have a centriole adjacent
to the basal body which is not usually seen
in typical motile cilia except during development (Dahl, '63). In certain respects,
the cells of the nephron resemble contractile cells. For example, they frequently contain basally located fibrils which have been
shown to be actin-like (Harper et al., '70;
Newstead, '71; Pease, '68; Ross and Reith,
'70). Contractility has not, however, been
Received Dec. 10, '74. Accepted Feb. 24, '75.
conclusively demonstrated. Furthermore,
in the macula densa of the nephron it has
been postulated that some form of sensing
device exists which allows monitoring and
response to the composition of the tubular
fluid (Thurau, '66). Because of these associations, and the fact that it seemed unlikely that the renal cilia perform a major
propulsive role, we were interested in determining whether or not these cilia had
the typical 9 2 pattern or some other
arrangement of microtubules, and whether
they showed any other special ultrastructural characteristics. We were also interested in knowing whether their structure
was uniform throughout the nephron and
whether it was similar in both man and
rat. We therefore undertook a study of the
ultrastructure of the cilia in a number of
parts of the nephron in these two species.
Male and female rats of the Wistar
strain were used. The animals were anaesthetized with a combination of phenobarbital and pentobarbital, and the left carotid
artery cannulated. The kidney was fixed
by retrograde aortic perfusion with Karnovsky's fixative (Karnovsky, '65) diluted
50% with cacodylate buffer at 100 mm
Hg. Following perfusion, the tissue was
prepared for either scanning or transmission electron microscopy using standard
techniques (Webber and Blackbourn, '71).
The sections were examined in either a
Philips 200 or 300 electron microscope.
Cross sections of cilia were examined in
the parietal layer of Bowman's capsule, in
the proximal tubule, in the distal tubule
generally, and in the macula densa and in
the collecting duct.
Human tissue obtained at autopsy after
being fixed by immersion was processed
and examined in similar fashion to the rat
kidney. Comparison was then possible of
the structural features of the cilia from the
various segments of the nephron in both
Since the cilia are relatively sparsely
distributed, ideal cross sections are rarely
encountered. The results reported here are
the result of observations of at least 50
cross sections of cilia in each site on the
tubule in both species studied.
The fine structure of the cilia was found
to be similar in all segments of the nephron
in both species. The following structural
Eeatures were seen:
( a ) There were fine cross striated rootIets (R) extending into the cytoplasm from
the basal body (fig. 1).
( b ) In appropriate sections, there was
a centriole (C) present, oriented at right
angles to the basal body (fig. 1). Th'is association has been observed previously in
the tubule by Latta ('61) and in the parietal layer of Bowman's capsule (Tyson and
Bulger, '72).
( c ) The basal body (B) was structurally
typical, having nine peripherally arranged
triads of microtubules (fig. 2).
( d ) The base of the shaft, which ranged
from 0.20-0.25 in diameter, was atypical
in that it contained nine peripherally located doublets of microtubules but lacked
single central microtubules (fig. 3).
(e) The pattern rapidly changed to 8
1 or 7 2 pairs of doublets (figs. 4,
5, 6 ) .
( f ) More distally, there was a reduction in the number of doublets, and some
doublets changed to single microtubules
(fig. 7).
The cilia of the mammalian nephron
which are characteristically present as a
single structure in each cell are not normal
cilia with a 9 2 pattern of microtubules.
Rather they resemble the cilia which have
been reported singly in a considerable
number of sites (Scherft and Daems, '67).
Thus our results differ somewhat from
those of Latta ('61) who, on the basis of
longitudinal sections, reported the cilia of
the rat nephron to be typical kinocilia. In
at least some of these other sites a relationship with sensory function exists although
i t is not known what role, if any, the cilia
play in the sensory process. At the present
time we know little about the function of
the cilia in the nephron. It is unlikely that
they play a major propulsive role as they
do in lower animals in which they are numerous (Fawcett and Porter, '54). If they
are motile, and this has not been unequivocally demonstrated, they could at most
have some stirring action on the tubular
34 1
Fig. 1 Longitudinal section of a cilium in the
distal tubule of the rat showing the basal body, a n
adjacent centriole ( C ) and a striated rootlet (R).
X 45,000.
Fig. 2 Cross section of a cilium in the distal
tubule of the rat showing the appearance of a
the 9
+ 0 pattern of microtubules. x 49,500.
an 8
+ 1 arrangement of microtubules. x 49,500.
contents. It is of course possible that they
are simply vestigial and constitute the remnants of the much more numerous cilia
seen in other species. In support of this
concept it is worth noting that during development the central microtubules have
been reported to develop after the outer
ring (Roth, '60) and these cilia may simply be incompletely developed. On the
other hand, however, the widespread occurrence of similar cilia in sensory structures, in nervous tissue and in contractile
cells, raises the interesting possibility that
they may have some sensory role and that
they may provide a means by which the
tubular contents can be monitored. Such
an arrangement would allow for a local
response to altered composition of the tubular fluid and provide for one element
of a system maintaining glomerulo-tubular
balance. In this regard it is fairly well
established that individual nephrons tend
to maintain glomerulo-tubular balance and
i t has been postulated that through the
macula densa the juxtaglomerular apparatus is responsive to some parameter of
tubular fluid (Thurau, '66). It would appear, therefore, that these cilia which show
interesting structural variations warrant
detailed functional study.
The support of the British Columbia
Heart Foundation and the technical assistance of Mrs. Patricia Hollingdale are gratefully acknowledged.
Andrews, P. M., and K. R. Porter 1974 A scanning electron microscope study of the nephron.
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Fig. 6 Cross section of a more distal portion
of a cilium in the distal tubule of the rat showing
a somewhat irregular 7 2 pattern of microtubules. x 49,500.
Fig. 7 Cross section of a cilium in the distal
tubule of the rat showing reduction of the microtubular doublets to single microtubules in a 7 2
pattern. X 49,500.
Fig. 5 Cross section of a more distal portion
of a cilium from human proximal tubule showing
a similar arrangement of microtubules to figure 4.
X 49,500.
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structure, mammalia, renar, fine, ciliv
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