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Ultrastructural study of the coiled body and a new inclusion the mykaryon Э in the nucleus of the adult rat sertoli cell.

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THE ANATOMICAL RECORD 225:21-25 (1989)
Ultrastructural Study of the Coiled Body and a
New Inclusion, the “Mykaryon,” in the Nucleus of
the Adult Rat Sertoli Cell
MICHAEL C. SCHULTZ
Department of Anatomy, McGill University, Montreal, Quebec, Canada H3A 2B2
ABSTRACT
Random and serial thin sections of the nucleus in adult rat Sertoli
cells were examined by electron microscopy. Besides the previously reported nucleoli and heterochromatin masses, the nuclei contain a coiled body and a new
structure, the “mykaryon.”
The coiled body is 835 nm in maximum diameter. It is composed of distinct
elements referred to as “coils.” They are 32 nm wide on average and resemble the
nucleolar pars fibrosa in their intense staining with heavy metal salts and their
composition of narrow filaments. The coiled body is often close to a nucleolus,
though no direct contact is established, and it sometimes exists at a distance from
the nucleolus.
The mykaryon is spherical, 460 nm in maximum diameter, and composed of a
tridimensional network of 7-26 nm-wide electron-opaque “cords” separated by
slightly thinner spaces. It has not been observed in the vicinity of a nucleolus. A
literature survey showed no previous mention of a structure similar to the
mykaryon. The coiled body and the mykaryon are interpreted as normal constituents of the Sertoli nucleus in the adult rat.
The “coiled body” is a spherical nuclear structure from three adult Sherman rats fixed by vascular performed of an aggregation of short, moderately to highly fusion with 5% glutaraldehyde and immersion in reelectron-opaque coils that are similar in appearance to duced osmium tetroxide according to the methods outcords of the nucleolar pars fibrosa. Though known to be lined in Schultz et al. (1984). The tissue pieces were
composed of RNA and protein (Fakan et al., 1984; embedded in Epon. Serial sections were mounted on
Schultz, 19891, to date its function has only been a Formvar-coated, slotted grids as described (Schultz et
matter of speculation (Schultz, 1989). It is clear, how- al., 1984). Random and serial sections were stained
ever, that the coiled body is of widespread occurrence in with aqueous uranyl acetate and Reynold’s lead citrate
the somatic cells of plant (Moreno Diaz de la Espina et and observed in a Philips 300 or 400 electron microal., 1982) and animal species (see references in Lafarga scope. The maximum diameter of the coiled body and
et al., 1983; Schultz, 1989). It has also been described in mykaryon was taken as the widest profile observed in
the spermatogonia (Schultz, 1986) and primary sper- random sections, since the widest sectional profile of a
matocytes of the rat (Schultz, 1989). While examining sphere represents its equatorial diameter. Statistical
the coiled body in rat spermatogenic cells, it was noted abbreviations used are X, mean; SD, standard deviathat an equivalent structure is present in the nuclei of tion; n, sample size.
Sertoli cells. This report characterizes the coiled body
in adult rat Sertoli cells.
RESULTS
It was also found that the nucleus of rat Sertoli cells
Nucleolus
contains a spherical structure formed of thin, electronopaque, and branched cords that are separated by narIn random sections the nucleus of the Sertoli cell in
row electron-lucent spaces. A comparison with other the adult rat contains one or two roughly spherical
inclusions reported in eukaryote nuclei suggests that nucleoli. They occur either free in the nucleoplasm or
this is a new type of nuclear structure. Its location and in close proximity to the nuclear envelope. The nucleapparent composition of interwoven fibrils prompted olus (Figs. 1, 2) is reticulated, with the pars fibrosa
the name “mykaryon” (myc, from Gk. mylzes fungus, as
in “mycelium”; caryon, from Gk. karyon nut, as in
“karyotype”).The fine structure of the mykaryon in rat
Sertoli cells is described and its possible function is
discussed.
MATERIALS AND METHODS
The form and dimensions of structures present in the
Sertoli cell nucleus were examined in testicular tissue
0 1989 ALAN
R. LISS. INC.
Received December 5, 1988; accepted February 1, 1989.
Dr. Michael C. Schultz’s present address is Developmental Biology
Program, Fred Hutchinson Cancer Research Center, Seattle, WA
98104.
22
M.C. SCHULTZ
Figs. 1-4.
23
NUCLEAR STRUCTURES IN SERTOLI CELLS
distributed in cords that enclose diffuse fibrillar centers and intermingle with cords of the pars granulosa
(terminology reviewed in Leblond, 1981). It is usually
flanked by one or two condensed masses of heterochromatin and often is also closely related to a coiled body.
Coiled Body
The coiled body of Sertoli cells (Figs. 1-3) has a
roughly spherical outline. The widest profile in random
sections is 835 nm. One coiled body examined in serial
sections has an actual diameter of 480 nm. The coiled
body is formed of a group of dense, distinct elements
referred to as “coils” (Fig. 3) in other cell types (Kinderman and LaVelle, 1976; Schultz, 1989) that show
moderate to high affinity for heavy metal salts. The
more electron-opaque coils are 13-49 nm wide (X = 32,
SD = 9, n = 34) and up t o 120 nm long. They can appear
as slightly elongated granules, often with irregular
surface contours, or as branched structures. The coils
are composed of filaments that are 2.5 nm wide on average (SD=0.4, n=33). Filaments of the same width
(X=2.6 nm, SD=0.5, n=33) are present in the pars
fibrosa of the nucleolus. The coils are separated by electron-lucent channels similar to, and in continuity with,
the nucl.oplasm (Fig. 3). The channels are 5-38 nm in
width (X= 17, SD=7, n=35) and up to 171 nm long.
Sertoli cells with a coiled body were observed at all 14
stages of the cycle of the seminiferous epithelium defined by Leblond and Clermont (1952).
A coiled body found in close proximity to the nucleolus may be located near the pars fibrosa or pars granulosa (Fig. 2), but it never establishes contact with either. In random sections only one coiled body is ever
encountered in a nuclear profile; some such coiled bodies are at a considerable distance from the nucleolus,
occasionally against the nuclear envelope. The coiled
body traced in a complete set of serial sections (not
shown) is indeed free in the nucleoplasm and therefore
not connected to a nucleolus or a heterochromatin
mass.
c
CB
f
fc
g
H
i
M
N
NE
Abbreviations
coil
coiled body
pars fibrosa
fibrillar center
pars granulosa
heterochromatin mass
interstitial space
mykaryon
nucleolus
nuclear envelope
Fig. 1. Nuclear profile of a Sertoli cell at stage I of the cycle of the
seminiferous epithelium. The large nucleolus is approached by heterochromatin masses and a coiled body. A mykaryon is present in the
vicinity of the nuclear envelope. X 27,300.
Fig. 2. Ultrastructural components of the nucleolus and their
structural relationship to the coiled body in a stage I Sertoli cell. The
nucleolus includes diffuse fibrillar centers in direct continuity with
cords of pars fibrosa. Cords of pars granulosa occur throughout the
nucleolus but rarely associate with fibrillar centers. The coiled body
approaches both pars fibrosa (small arrowhead) and pars granulosa
(large arrowhead) a t the border of the nucleolus. x 46,700.
Fig. 3. Detailed view of part of the coiled body in a stage IV Sertoli
cell; it is composed of more electron-opaque coils separated from
TABLE 1. Dimensions (nm) of elements forming
the coiled body and mykaryon in the rat Sertoli
cell nucleus’
Cell type
Coiled body
Sertoli
Spermatocyte
Mykaryon
Sertoli
Coil
width
Max. coil
length
Max. gap
width
Filament
width
32
34
120
144
38
40
2.5
2.5
15
84
17
2.8
‘Coil, cord, and filament widths are given as means. The first 2 values
given for the mykaryon refer to its cords. Dimensions of the coiled
body in early pachytene spermatocytes of the rat (“spermatocyte”) are
provided for comparison (from Schultz, 1989).
Mykaryon
The nuclei of Sertoli cells also contain a small, regularly spherical structure designated the “mykaryon”
(Figs. 1,4).In random sections the mykaryon is 460 nm
in maximum diameter. It is composed of a tridimensional network of elements referred to as “cords.” The
cords (Fig. 4) have quite regular outlines, especially
compared to the coils of the coiled body (Fig. 3). They
are 7-26 nm wide with a mean of 15 nm (SD = 3, n = 33)
and are up to 84 nm long. Cords are separated by electron-lucent intercordal spaces 5-17 nm in width with a
mean of 10 nm (SD = 2.5, n = 33);they are up to 63 nm
long. The profiles of cords show frequent interconnections indicative of simple and regular branching (Fig.
4; compare with the less frequently branching coils of
the coiled body-Fig. 3). The cords are composed of
electron-opaque filaments with an average width of
2.8 nm (SD = 0.5, n = 33).
In random sections the mykaryon is found free in the
nucleoplasm or against the inner leaflet of the nuclear
envelope. The mykaryon has never been seen in direct
association with heterochromatin, nucleolus, or coiled
body, and is not surrounded by membrane, an electronlucent halo, or a fibrillar capsule. It is presumably embedded in the euchromatin occupying the rest of the
nucleoplasm.
The dimensions of the structural components of the
coiled body and mykaryon in the rat Sertoli cell are
given in Table 1, which includes, for comparison, similar data for the coiled body of early pachytene spermatocytes of the rat (Schultz, 1989).
DISCUSSION
The architecture of the Sertoli cell nucleus has been
examined in many mammalian species and is well
characterized for man (Bustos-Obreg6n and Esponda,
1974; Schulze, 1974, 1979; de Kretser and Burger,
1972; Nistal et al., 1982; Devictor et al., 1984;
one another by less-opaque interstitial spaces. The labeled coil is irregular in outline, with a narrow stalk (arrow) branching to a more
evenly contoured projection. The interstitial spaces may open into
regions outside the coiled body occupied by nuclear sap (arrowhead).
x 118,000.
Fig. 4. Detailed view of the mykaryon in a stage VII Sertoli cell.
Its constituent cords (arrows) have quite regular surface contours.
The cords often appear to branch (note the Y-shaped cord with arms
arrowed) and have a circular outline in cross section (central arrowhead). Narrow spaces separate adjacent cords and may open into the
nucleoplasm at the periphery of the mykaryon (bottom arrowhead).
x 144,000.
24
M.C. SCHULTZ
Paniagua et al., 1986),various ruminants (see Fawcett,
1975, and references therein), hamster (Barcellona and
Brinkley, 1973; Fawcett, 1975; Sinha Hikim et al.,
19881, and mouse (Flickinger, 1967; Krimer, 1977;
Mirre and Knibiehler, 1982, 1984; Jean et al., 1983;
Brinkley et al., 1986). It has also been briefly investigated in rat (Fawcett, 1975; Soderstrom, 1981). Thus,
electron microscopic studies have established the existence of nucleoli in the Sertoli cells of all species, of
heterochromatin bodies in the Sertoli cells of mouse
(Krimer, 1977; Mirre and Knibiehler, 1982,1984;Jean
et al., 1983; Brinkley et al., 1986) and Chinese hamster
(Barcellona and Brinkley, 1973; Fawcett, 19751, of nuclear bodies in human Sertoli cells (Bustos-Obregon
and Esponda, 1974; Devictor et al., 1984) (the
“sphaeradia” of Schulze, 1974, 19791, and of membrane-bound inclusions in the Sertoli cells of ruminants (see Fawcett, 1975, and references therein).
When the organization of the Sertoli cell nucleus was
reexamined in rat, we observed nucleoli and heterochromatin bodies as well as two structures, the coiled
body and mykaryon, which have not previously been
reported in this cell type.
Coiled Body
The coiled body of the Sertoli cell in rat is morphologically related to coiled bodies in other cells by virtue
of its overall shape, formation by electron-opaque coils
separated by electron-lucent interstitial spaces, and
lack of a limiting membrane. The maximum diameter
observed in random sections, 835nm, is within the
range of sizes reported for the coiled body of other somatic cells (300 nm minimum-Monneron and Bernhard, 1969; 900 nm maximum-Seite et al., 1982) and
spermatogenic cells (310-820 nm-Schultz, 1989). As
in other cells (references in beginning of this paper),
the Sertoli coiled body reacts positively to cytochemical
tests for RNA and protein (Schultz, unpublished observations).
The resemblance of its coils to the cords of the nucleolar pars fibrosa is an additional features shared with
coiled bodies elsewhere (reviewed in Schultz, 1989). Direct interconnection of the coiled body and nucleolus,
however, was not observed. Serial sectioning indeed
established that the coiled body of an interphase animal somatic (Sertoli) cell may exist in situ without
structural relationship to the nucleolus. A similar situation pertains in rat spermatocytes from their birth
until the end of midpachytene (Schultz, 1989).
Mykaryon
A literature survey indicated that the mykaryon is a
new type of nuclear inclusion. The mykaryon most
likely is not derived from, or ever incorporated into, the
nucleolus, since 1) there was no evidence for physical
association of the two structures, 2) the mykaryon did
not resemble any nucleolar element, and 3) known nucleoplasmic preribosomal particles (Hugle et al.,
1985a,b) are granular whereas the mykaryon was composed of branched cords. Nor was there evidence for
any relationship between the mykaryon and the coiled
body or heterochromatin masses. It was not enclosed by
a fibrillar capsule or an electron-lucent halo. Since nuclear bodies (as defined by Bouteille et al., 19741,on the
other hand, are surrounded by a capsule or halo and
partly resemble nucleolar ribonucleoprotein elements
(cf. mykaryon, above), it can be concluded that the
mykaryon is not an example of this type of structure.
Furthermore, the (rare) nuclear bodies in adult rat Sertoli cells (Schultz, unpublished observations) do not include material resembling the mykaryon. The origin
and composition of the mykaryon therefore are unknown at present.
Among conceivable functions for the mykaryon, the
data exclude two major possibilities, leaving two others
for future consideration. Since rRNA synthesis takes
place in the nucleolus and the mykaryon does not interact with the nucleolus or resemble any of its derivatives, the mykaryon probably is not involved in rRNA
metabolism. The adult rat Sertoli cell is nondividing
(Steinberger and Steinberger, 1971), so a role in DNA
replication is unlikely. The mykaryon more likely then
is a site for the storage of proteins (which it does contain-Schultz, unpublished observations), or metabolism of RNAs, besides those associated with nucleolar
function.
In summary, the coiled body and mykaryon are normal components of the nucleus in Sertoli cells of the
adult rat. While the coiled body has been reported in
other cell types, the mykaryon appears to be a new
class of nuclear structure.
ACKNOWLEDGMENTS
The author was supported by a David Stewart Memorial Fellowship from McGill University and a Foundation Travelling Scholarship from the University of
Queensland, Australia. Funding for the project was
provided through a Medical Research Council of Canada grant to Dr. Charles P. Leblond, who is gratefully
acknowledged for his critical reading of the manuscript.
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