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Fine structure of a fifth type of epithelial cell in the thyroid gland of the C3H mouse.

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Fine Structure of a Fifth Type of Epithelial Cell in the
Thyroid Gland of the C,H Mouse
Laboratory of Physiology, National Cancer Institute,
National Institutes of 'Health,
Bethesda, Maryland 20014
A new cell with relatively little cytoplasm has been identified in
the second kind of follicle in the CIH mouse thyroid gland. It has as ultrastructural characteristics the presence of clusters of fiber in the cytoplasm, vesicles
near the basal plasma membrane, and half desmosomes in the basal plasma
membrane. It resembles the U cell found in the ultimobranchial follicle of the
Fischer rat thyroid, but has a somewhat larger amount of granular reticulum.
It is found in the follicle wall between other epithelial cells and the basement
membrane, but occasionally is in contact with the lumen. It also occurs as a
double layer in the follicle wall. Cell debris in the lumen of the follicle contains
fibrils and may be the result of desquamation of this fibril-containing cell.
In addition to the usual functional thy- similar to the U cell in the ultimobranchial
roid follicle, a second kind has been recog- follicle in the rat thyroid ( N b e and Wollnized in the mouse (Dunn, '44; Gorbman, man, '71). The present paper reports on
'47a,b). The second kind of follicle in the the ultrastructural characteristics of this
thyroid gland (Wetzel and Wollman, '69) fifth type of cell.
contains a lumen filled with a heteroMATERIALS AND METHODS
geneous or foamy colloid and some cell
debris, and is lined by at least four easily
Male C3H/HeN mice, raised on Purina
distinguishable types of cells : typical thy- Laboratory Chow, were used at three to
roid epithelium and parafollicular, ciliated five months of age. Whole thyroid lobes
and AR cells. Of these, the typical thyroid were removed from these animals under
epithelium and parafollicular cells appear light ether anesthesia and fixed in 4.2%
indistinguishable from those in the usual glutaraldehyde in 0.1 M Millonig's phosfollicle. The AR cells which possess an phate buffer for 90 minutes. After rinsing
abundance of agranular reticulum and the with Millonig's fluid containing 0.54% of
ciliated cells are characteristic of the glucose, the lobes were postfixed for 30
second kind of follicle. This follicle is gen- minutes in phosphate buffered 2% osmium
erally lined by a single layer of cells tetroxide. Tissue was then prestained for
(made up of 3 of the cell types) each of three hours with uranyl acetate solution at
which appears to be in contact with both 4°C. Specimens then were dehydrated in a
basement membrane and lumen. In places graded series of ethanol solutions and emthe follicle wall is made up of more than a bedded in Epon. Thin sections were cut on
single layer where parafollicular cells a Porter-Blum MT-2 ultramicrotome with a
(which do not appear to occur in contact diamond knife, stained sequentially with
with the lumen) occur between one of the uranyl acetate (Watson, '58) and then
other cell types and the basement mem- with lead citrate (Venable and Coggeshall,
' 6 5 ) . They were examined with an AEI
It was noted that there was a trace of EM6B electron microscope.
possibly a fifth type of cell which was small
and contained relatively little cytoplasm
Received Jan. 18, '71. Accepted July 6, '71.
1 Now at Laboratoire de MBdecine Exphimentale.
(see fig. 14 in Wetzel and Wollman, '69). Universit6
Libre de Bruxelles, 115 Boulevard de WaterThis cell has now been recognized as being loo, Brussels, Belgium
ANAT. REC.,172: 37-24.
The newly recognized type of cell (I?
cell) found in the second kind of follicle
in the thyroid gland of the C,H mouse is a
small cell, frequently flat, with relatively
little cytoplasm (figs. 1-4). The cell con-
tains the usual complement of organelles,
including a nucleus and mitochondria; occasionally, a Golgi zone can be seen. There
is relatively little granular reticulum although the outer membrane of the nuclear
envelope frequently has areas studded with
Fig. 1 Two U cells lying between ciliated cells ( C ) and the basement membrane (BM) in a thyroid follicle of the second kind i n a CIH mouse. The U cells have a relatively small amount of relatively
dense cytoplasm and peripheral vacuoles ( V ) which are probably pinocytotic vesicles, especially near
the basal plasma membrane. The relatively dark parts of the basal plasma membrane appear to be half
desmosomes. There is a cilium (arrow) on one of the U cells. Note the presence of cilia in contact
with the lumen ( L ) and of fibrils ( F ) in the cytoplasm of the ciliated cell on the right. x 10,000.
ribosomes. Distinguishing characteristics
include the presence of half desmosomes
in the basal plasma membrane (figs. 2, 3 ) ,
vesicles (possibly pinocytotic) near the
periphery of the cell, most often at its base,
and clusters of fibrils in the cytoplasm,
some obviously connected to desmosomes
(fig. 4). Although the clusters of fibrils
are frequenBy found in the U cells they are
not always abundant or obvious.
The U cell is found in the wall of the
follicle. It is ordinarily between other kinds
of cells in the follicle, or between other
kinds of cells and the basement membrane
Fig. 2 Oblique sections of two U cells next to a n AR cell. The section is probably near
the base of the cell at the bottom of the figure, as evidenced by the presence of basement
membrane (BM) on two sides of the cell. Note the relatively small amount of cytoplasm,
peripheral vacuoles, the half desmosome (arrow) 2nd the ribosomes on the nuclear membrane. Insert: A portion of the base of the lower U cell showing a half desmosome (arrow).
x 25,000;Insert, x 37,000.
Fig. 3 Cluster of four U cells. Two cells form a double layer in the follicle wall, and one is i n
contact with both lumen ( L ) and basement membrane ( B M ) . Half desmosomes c a n be seen on the
basal plasma membranes of two cells (arrows). A Golgi zone ( G ) is visible, apical to the nucleus
of one cell. Granular reticulum is relatively rare but there are numerous free ribosomes. A few
lysosomes (Ly) are seen in one cell. Note the interfoldin= of the cells in this cluster. X 13,300.
Fig. 4 Higher power view of a U cell from tlie last figure. The cell contains many fibrils in clusters ( F ) and some pinocytotic vesicles ( V ) . x 37,000.
(fig. 1). Occasionally the cell occurs in cells when too small a sample of their
clusters forming either a single or a double cytoplasm was present.
layer bounding the follicle (figs. 3, 4).
Cell debris was observed twice in lumens
of the second kind of follicle. The debris
We are now reporting the ultrastrucwas characterized by having fibers dis- tural characteristics of a fifth type of cell
persed throughout the cytoplasm (figs. 5, in the second kind of follicle in the thyroid
6 ) . The debris did not possess many formed gland of the CsH mouse. The U cell differs
organelles or vacuoles. It was surrounded from the previously described types of cells.
by a low density area separating it from It does not have the abundant dilated cisobvious colloid.
ternae of the typical thyroid epithelium,
Statistics. The relative frequency of the the secretory granules of the parafollicular
five cell types recognized in the second cells, the abundant agranular reticulum
kind of follicle has been determined in of the AR cells, nor the numerous cilia so
sections of five follicles. Although the rela- prominent on the ciliated cells. Finally,
tive incidence of the five cell types varied those U cells in contact with the basement
considerably from follicle to follicle, ap- membrane contain half desmosomes on
proximately 25% of the cells were typical their basal plasma membrane; half desmothyroid epithelium. There was an approxi- somes have not been observed on the
mately equal incidence of AR cells. The plasma membranes of the other four cell
incidence of the other three cell types types. The U cell in the C,H mouse revaried from 10 to 20%. Completely reliable sembles that in the ultimobranchial follicle
figures could not be obtained, however, be- in the thyroid gland of the rat. Cytologicause of difficulties in identifying some cally it differs primarily in having some-
Fig. 5 Cell debris in the lumen of the second kind of follicle. The cytoplasm is filled with a loose
matting and some clusters of fibrils, and residues of cell organelles ( ? ) . x 12,500.
Fig. G Higher power view of a portion of the debris in figure 5. x 32,OGO.
what more granular reticulum, and intracellular fibrils are a less constant feature.
Its growth pattern also seems to be different since it frequently occurs as a single
layer in contact with the basement membrane, or underlying another cell type, of
possibly different lineage, such as an AR
cell or a ciliated cell (fig. 2), and in contact with the basement membrane. More-
over, when i t grows as a double layer, the
more apical U cell is not as strikingly different from the basal U cell as in the rat.
In general, the U cell in the mouse tends
to resemble that in the very young rat.
The peculiar differences in growth pLttern of this cell in rats and mice may be
due to the presence of the variety of cell
types in the mouse. It is possible, however,
that the cell itself has remarkably different
prcpertics in the two species. For example,
the frequent occurrence of the cell between AR cells or ciliated cells and the
basement mcmbrane raises the question
of whether this is possibly a basal cell
which can give rise to more than one type
of progeny. No information is available on
this as yet.
The cell of origin of the debris in the
lumen of the second kind of follicle is uncertain. The similarity of the debris to that
in the ultimobranchial follicle of the rat
(Nkve and Wollman, '71). both in its content of fiber and its being surrounded by a
relatively low density area, suggests that
it might be derived from the U cell. However, there is no evidence as yet that U cells
in the C,H mouse mature and desquamate
as in the Fischer rat. In addition, in the
I strain mouse the debris in the lumen of
the second kind of follicle was observed
to be of ciliated cells (unpublished observation). This might not be inconsistent with
the present observations in the C,H mouse
because a good deal of fiber is somstimes
found in the cytoplasm of the ciliated cells
(fig. 1 ) .
It might be suspected that the U cell is
the cell of origin of the heterogeneity, or
the foam substance, in the colloid since
the ultrastructure of the U cell and of the
foam substance resemble that in the rat.
However, in contrast to the rat, no vesicles
containing a material resembling the foam
substance have been observed either in
U cells or in the debris. In addition, in the
C,H mouse cell debris is not a prominent
feature of the lumen of the second kind of
follicle although the foam substance is
The relative frequencies of the various
cell types differ in some respects from a
Frevious survey (Wetzel and Wollman, '69).
Although the frequency of typical thyroid
epithelium equals that of AR cells, as before, these two cell types make up a somewhat smaller proportion of the total number of cells, the difference being made up
of increased numbers of ciliated cells and
U cells. It is likely that some of this discrepancy is a consequence of the marked
difference in incidence in the cell types in
djff erent follicles.
Dunn, T. B. 1944 Ciliated cells of the thyroid
of the mouse. J. Nat. Cancer Inst., 4: 555-557.
Gorbman, A. 1947a Functional and morphological properties in the thyroid gland, ultimobranchial body, and persisting ductus pharyngiobranchialis 1V of an adult mouse. Anat. Rec.,
S8: 93-101,
19471, Thyroidal and vascular chanzes
in mice following chronic treatment with goitrogens and carcinogens. Cancer Res., 7: 746-758.
NPve, P., and S. H. Wollman 1971 Fine structure of ultimobranchial follicles in the thyroid
gland of the rat. Anat. Rec., 171: 259-272.
Venable, J. A., and R. Coggeshall 1965 A simplified lead citrate stain for use in electron
microscopy. J. Cell Biol., 25: 407-408.
Watson, M. L. 1958 Staining of tissue sections
for electron microscopy with heavy metals. J.
Biophys. Biochem. Cytol., 4: 475-478.
Wetzel. B. K.. and S. H. Wollman 1969 Fine
structure of a second kind of thyroid follicle
in the CaH mouse. Endocrinology, 84: 563-578.
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structure, epithelium, c3h, gland, mouse, fifty, typed, thyroid, fine, cells
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