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Transitional cells in the postnatal thyroid gland of the rat.

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Transitional Cells in the Postnatal
Thyroid Gland of the R a t ’
R. CALVERT
Department of Anatomy, Faculty of Medicine, University of Sherbrooke,
Sherbrooke, Canada
ABSTRACT
In two and four-week old rats three types of follicles were observed: thyroid follicles made of follicular cells and C cells; ultimobranchial
follicles (or cysts) and ducts composed of lumen-bordering cells and subjacent
cells; mixed follicles lined by follicular cells on one side and by lumen-bordering
cells on the other. Occasionally C cells were seen in the wall of mixed follicles.
On their basal side the lumen-bordering cells of mixed follicles were in direct
contact with the wall of ultimobranchial ducts or cysts. In the wall of these ducts
transitional cells were observed. One type had ultrastructural features of subjacent cells (i.e., hemidesmosomes) and of lumen-bordering cells (i.e., pale
cytoplasm and dispersed tonofilaments). Another type of transitional cell had
hemidesmosomes and a deeply indented nucleus, like subjacent cells, but had
accumulated slightly distended cisternae of rough-surfaced endoplasmic reticulum and was displaying long slender microvilli, which is typical of follicular
cells. A third type of cells were called “young follicular cells.” They contained
slightly distended cisternal profiles of rough-surfaced endoplasmic reticulum collected in one corner of the cells and few apical vesicles. However, their cisternae
were less abundant, less distended and had a more restricted distribution than
those found in mature follicular cells. Hence the subjacent cells of ultimobranchial ducts might possibly give rise to lumen-bordering cells and to some follicular cells. The latter are associated with the lumen-bordering cells in the formation of mixed follicles; at the beginning these new follicular cells seem less active
than the mature follicular cells. The origin of C cells after birth is discussed.
Three types of follicles are found in the
postnatal thyroid parenchyma of the rat.
The most numerous are the typical thyroid
follicles responsible for the iodine metabolism of the gland. The second type, the
ultimobranchial follicles, possess a wall
made of a stratified squamous epithelium
(Van Dyke, ’44; Axelrad and Leblond, ’55)
and are often referred to as ultimobranchial bodies, cysts or ducts. Their function
in the postnatal thyroid gland is unknown.
The third type, the so-called mixed follicles
have lumina lined on one side by typical
follicular cells, and on the other by ultimobranchial cells (Calvert and Isler, ’70;
N6ve and Wollman, ’71; Wollman and
N6ve, ’71a,b). The origin and fate of mixed
follicle are not clarified yet.
In the postnatal thyroid gland of the
dog, Godwin (’37) has described the formation of typical thyroid follicles from the
ANAT. REC., 174: 341-360.
ultimobranchial follicles. Although he did
not use the terms “mixed follicles” he
claimed that: “Soon all or nearly all of the
cells of the (mixed) follicle have acquired
an appearance identical with the cells of
the large (thyroid) follicles.” Also, in the
postnatal thyroid gland of the sheep, “new
growths” arise from the utlimobranchial
body and they were said to transform into
thyroid-like parenchyma (Van Dyke, ’45).
Hence, there are some indications that
mixed follicles may represent an intermediate stage in the formation of thyroid follicles from ultimobranchial follicles.
Notwithstanding these observations
made with the light microscope, it seems
that the electron microscope would permit a more accurate identification of inReceived April 18, ’72. Accepted July 24, ’72.
1 This investigation w a s supported by a grant from
the Medical Research Council of Canada.
341
342
R. CALVERT
termediate steps or transitional cells associated with mixed follicles. This paper
presents further observations on the possible origin and fate of mixed follicles in
the postnatal thyroid gland of the rat.
MATERIALS AND METHODS
Ten female Holtzman Albino rats, two
and four weeks of age, were used. They
were perfused with a glutaraldehyde-acrolein solution (Sandborn et al., '64) for 30
minutes. While immersed in the same fixative for another 30 minutes the two thyroid lobes were excised carefully. Then
the lobes in toto were post-fixed in osmic
acid (Palade, '52) for 90 minutes, stained
in 1% uranyl acetate solution for 60 minutes and embedded in Epon (Luft, '61).
Sectioning was done with a LKB Ultratome
I11 and a diamond knife. Thick Epon sections were stained with an aqueous solution of 1% toluidine blue and 1% borate
at 70°C for one minute.
OBSERVATIONS
I. CeZl types in rut thyroid gland
The fine structure of typical follicular
cells and of C cells (calcitonin-producing
cells) has been extensively described by
many authors (Monroe, '53; Ekholm and
Sjostrand, '57; Young and Leblond, '63;
Lucian0 and Reale, '64; Aoi, '66; Ekholm
and Ericson, '68). Therefore, only features
essential for the discussion will be reported
here. Furthermore, age does not seem to
be a factor influencing these cell types, at
this stage of development.
( a ) FollicuZar ce2ls. Follicular cells
are seen in the wall of thyroid follicles and
of mixed follicles (fig. 1 ) . They form a
single layer of epithelial cells surrounding
a colloid-containing lumen (figs. 1, 2, 4).
Their apices display long and slender microvilli. Laterally, junctional complexes
are observed near the follicular lumen and
few small desmosomes are present. A
basal lamina follows the contour of the
base of follicular cells. Dilated cisternae
of rough-surf aced endoplasmic reticulum
are widely distributed throughout the cytoplasm. A well-developed Golgi zone is
found in the apical region of the cell. Nuclei are usually circular to oval-shaped,
rarely indented.
(b) C cells. C cells are found in the
wall of thyroid follicles (fig. 1) and of
mixed follicle (fig. 2 ) ; none are observed
in the wall of the ultimobranchial follicles
(or cysts). These calcitonin-producing
cells represent 1 to 5% of the epithelial
cells of rat thyroid (Stux et al., '61). C
cells are typically located between follicular cells and the basal lamina (fig. 1 ) :
they never reach the colloid. With the
electron microscope, they were characterized by their richness in small granules
measuring 0.1-0.18 in diameter (Ekholm
and Ericson, '68 ). Their rough-surf aced
endoplasmic reticulum is scanty and often
localized in one part of the cell.
( c ) Ultimobranchial cells. The wall
of the ultimobranchial follicles (or cysts)
in the rat is made of a stratified squamous
epithelium in which two cell types are
described here: subjacent cells and lumenbordering cells (fig. 3 ) . A third type, thc
mucous-like cells (Calvert and Isler, ' 7 0 ) ,
was not observed in this material.
Subjacent cells are located in the wall
of ultimobranchial follicles and of mixed
follicles (figs. 3 , 8). They may constitute
one or two layers of cells (i%g.3 ) and they
never reach the lumen. These basal cells
contain a deeply indented and flattened
nucleus, numerous bundles of tonofilaments, very few profiles of rough-surfaced
endoplasmic reticulum and abundant free
ribosomes. Their cellular contacts are
characterized by intricate infoldings of the
plasma membrane and by the presence of
many large desmosomes. On their basal
side, hemidesmosomes face a slight thickening of the basal lamina (figs. 3, 8).
Lumen-bordering cells are present in the
wall of the ultimobranchial follicles; they
also line the lumen of mixed follicles (figs.
1, 2, 8). They possess some of the features
of subjacent cells: indented nucleus, infoldings of the plasma membrane, large
desmosomes and a few flat cisternae
of rough-surf aced endoplasinic reticulum
(figs. 3 , 8). However, the cytoplasm is
filled with dispersed tono filaments not
aggregated in thick bundles as in subjacent
cells (fig. 5). Scattered glycogen granules
may be present. These cells also contain
fewer free ribosomes than subjacent cells.
The apices of lumen-bordering cells display short microvilli which can be easily
TRANSITIONAL CELLS I N RAT THYROID
distinguished from those of follicular cells
(fig. 7). Near the lumen, junctional complexes are observed.
11. Transitional cells in rat thyroid
This category includes cells displaying
ultrastructural features typical of more
than one of the cell types already described
in the first section. Three types are described here. With the light microscope,
the presence of these cells was not noticed.
( a ) One type of transitional cell exhibits characteristics of subjacent cells
and of follicular cells (figs. 5, 6). Like the
former it has hemidesmosomes on the
basal plasma membrane (fig. 5), a long
flattened nucleus (fig. 5) or a deeply indented nucleus (fig. 6). Golgi apparatus
is poorly developed. In addition it contains
a few slightly distended cisternae of roughsurf aced endoplasmic reticulum accumulated in the apical and/or in the basal
portion of the cell (fig. 6). These profiles
are less numerous and less distended in
typical subjacent cells while they are more
abundant, more dilated and have a wider
distribution in follicular cells (fig. 4). Unlike subjacent cells this cell type lies on a
basal lamina and is in contact with a
lumen. Its microvilli are long and slender
(fig. 5) which is typical of those observed
on follicular cells. These cells are located
in the wall of stratified squamous ultimobranchial follicles.
( b ) Cells grouped in the second type
of transitional cells are designated “young
follicular cells” (figs. 7, 8). They are observed in the wall of ultimobranchial follicles where they line the lumina of small
mixed follicles. They contain slightly distended cisternae of rough-surfaced endoplasmic reticulum, very few apical vesicles
and a poorly developed Golgi apparatus.
When compared with follicular cells located in the wall of thyroid follicles (fig.
4 ) thev look similar to hypoactive follicular
cells. The contact between young follicular cells and lumen-bordering cells is characterized by the relatively straight path of
their plasma membranes and by the presence of small desmosomes and junctional
complexes. The nucleus and microvilli of
these cells are also typical of follicular
cells.
343
Some of these young follicular cells were
studied with serial sections. Their relative
paucity of rough-surfaced endoplasmic reticulum and of apical vesicles when they
are compared with mature follicular cells,
is not a mere sectioning artifact. Moreover,
in two and four-week old rats, these hypoactive follicular cells are frequently seen
in the wall of ultimobranchial follicles
where they are associated with small lumina of mixed follicles. Their presence is
not incidental.
(c) A third kind of transitional cell
is related only to the ultimobranchial cells
(fig. 8). Like the subjacent cells, it lies on
a basal lamina and displays hemidesmosomes. Otherwise it looks similar to lumenbordering cells with their pale cytoplasm
and dispersed tonofllaments. No microvilli
are present since the cell does not reach
the lumen. This cell is observed in the
wall of a mixed follicle at the point of
fusion with an ultimobranchial follicle or
duct.
DISCUSSION
It cannot be considered established that
all atypical cells and/or follicles of the
thyroid gland are derived from the ultimobranchial body (Wollman and Nbve, ’71b).
In the present work atypical follicles include only the mixed follicles; they are
always in a close vicinity to or in direct
contact with the wall of an ultimobranchial
follicle. Also, they are located in the central part of the gland where the ultimobranchial bodies typically lie. Other kinds
of atypical follicles not described here are
sometimes found between the thyroid
gland and the trachea or between the thyroid and the parathyroid glands (Calvert,
unpublished). In the mouse thyroid, different unusual follicles were also observed
(Gorbman, ’47; Wetzel and Wollman, ’68;
Wollman and Nkve, ’71b).
The origin of ultimobranchial follicles
The ultimobranchial follicles in two and
four-week old rats are slightly different
from those observed in the rat embryo.
The lumen-bordering cells display the
same features except that they contain
more glycogen granules before birth. The
subjacent cells also appear more differentiated in the postnatal glands, as desmo-
344
R. CALVERT
somes and dense bundles of tonofilaments
become m;re numerous. The luminal contents are comparable : cellular debris and
granular material are seen before and after
birth.
From these observations it seems that
the ultimobranchial follicles found in the
postnatal thyroid glands have derived from
or are remnants of the ultimobranchial
bodies of the embryo. Nevertheless this is
in conformity with the statement that ultimobranchial follicles are an example of
postnatal development (Wollman and
N&ve,'71a). In the twentieth day embryo,
the ultimobranchial bodies are minute
structures with a slit-like lumen. In the
four-week old rats they have grown into
large follicles. Hence, it can be stated
that the ultimobranchial bodies continue
to differentiate after birth and that they
give rise to the stratified squamous ultimobranchial follicles.
Within the wall of these ultimobranchial
follicles the subjacent cells are believed to
merge into lumen-bordering cells and finally into cellular debris seen in the
lumen (NBve and Wollman, '71). Transition between subjacent and lumen-bordering cells must be abrupt since with the
electron microscope intermediate steps
have never been reported. This is not
peculiar to these cells: in the skin the
transition between stratum granulosum
and stratum corneum is also abrupt
(Breathnach, '71). However one cell described above could represent a transitional
step between the subjacent cell and the
lumenbordering cells since it contained
fine structural features typical of both cell
types. But this kind of cell is very infrequently observed.
T h e origin of mixed fallicles
In rat thyroid gland the lumina of mixed
follicles are lined by squamous or cubcidal
follicular cells on one side and by lumenbordering cells on the other (Calvert and
Isler, '70; N6ve and Wollman, '71). The
histogenesis of these peculiar follicles is
uncertain. At least three hypotheses should
be examined: ( a ) they derive from the fusion of follicular cells originating from the
median thyroid primordium, with lumenbordering cells originating from the ultimobranchial bodies; ( b ) typical thyroid fol-
licle s are tr ansformc d into ultimobranchial
follicles or cysts (Van Dyke, '55); finally
( c ) the follicular cells in mixed follicles
could arise directly from ultimobranchial
cells.
If mixed follicles are formed simply by
the fusion of follicular cells and ultimobranchial cells, one should not expect to
find transitional cells between these cell
types. Moreover the fusion should occur
first between follicular cells and the most
peripheral cells of ultimobranchial follicles,
that is the subjacent cells. In fact, the
latter were never seen in direct contact
with the lumen of a mixed follicle, and a
transitional cell type was identified.
Hence it may be suggested that mixed
follicles originate by metaplasia of certain
thyroid follicles and/or of the ultimobranchial follicles. In rats maintained on a
vitamin A-deficient diet for 20 days the
basal cells of stratified ultimobranchial
cysts produce small vesicles resembling
young thyroid follicles. Thes,e single-layered
vesicles may undergo a transformation
again into stratified ultimobranchial cysts.
The process seems reversible (Van Dyke,
'55).
The present observations appear to support a transformation of ultimobranchial
cells into follicular cells. If thyroid metaplasia occurred, transitional cells would be
seen in the wall of large mixed follicles.
Here, all the transitional cells described
are in contact with very small lumina similar to those observed in the eighteenth day
embryos, and they are located inside the
basal lamina of ultimobranchial follicles
or ducts. It is believed that the subjacent
cells of ultimobranchial follicles may give
rise to lumen-bordering cells and also, to
follicular cells. This view is sustained by
the existence of transitional cells differentiating along both pathways.
The cell displaying herrddesmosomes,
slightly distended cisternae of roughsurf aced endoplasmic reticulum, large
mitochondria and slender microvilli would
represent the first step in the differentiation of a subjacent cell intls a follicular
cell. Since this cell is in contact with an
ultimobranchial body cell the formation of
a mixed follicle would occur. Then, by accumulation of more rough membranes
"young follicular cells" would appear. At
TRANSITIONAL CELLS IN RAT THYROID
the beginning they look less active than
mature follicular cells but as the mixed
follicle grows they become similar to the
typical follicular cells. Further evolution of
mixed follicles is not clear. Can they give
rise to thyroid follicles containing only follicular cells? More work is needed before
answering this question.
In the postnatal thyroid gland, subjacent cells located in the wall of stratified
ultimobranchial follicles can give rise to
lumen-bordering cells and follicular cells.
Therefore mixed follicles would derive
from ultimobranchial follicle metaplasia.
The origin of C cells
Since C cells were shown to migrate
from the ultimobranchial bodies into thyroid cell cords during embryonic life it is
pertinent to discuss here the origin of these
cells. One would expect to find some undifferentiated C cells in the wall of postnatal ultimobranchial bodies. Such cells
were not identified.
The identification of C cells in the wall
of the ultimobranchial bodies during the
seventeenth day of development in the embryo was taken as an evidence for the
ultimobranchial origin of these cells
(Stoeckel and Porte, '70; Calvert, '72).
However, in the mouse embryo, the last
pharyngeal pouch is colonized, at about the
ninth to tenth day, by APUD cells ( m i n e
precursor uptake and decarboxylation)
from the neural crest (Pearse and Polak,
'71). Since C cells have the same APUD
characteristics after their appearance in
the thyroid parenchyma (Pearse and Carvalheira, '67) it was concluded that they
originate from the neural crest (Pearse and
Polak, '71). The ultimobranchial bodies
would act as a carrier of C cells. In the
chick embryo it seems that C cells also
originate from the neural crest (Le Douarin
and Le Likvre, '70). The neural crest origin
of C cells was foreseen by Pag6s ('56).
After birth, in the rat, C cells were not
observed in the wall of stratified ultimobranchial follicles, and nonetheless they
have been shown to increase in number
without a parallel increase in their mitotic
rate, after treatment with somatotropic
hormone and after hypophysectomy (Sarkar and Isler, '63). In the present material,
some C cells were seen in the wall of mixed
345
follicles in four-week old rats. Thus it is
possible that they derive from unidentified
ultimobranchial cells or neural crest cells
located in the wall of ultimobranchial follicles, since mixed follicles originate from
the wall of these follicles. However, the
hypothesis that some C cells are "modul a t e d follicular cells (Sarkar and Isler,
'63) should not be overlooked since C cells
and some follicular cells might possibly
derive from the same cell type located in
the wall of ultimobranchial follicle in the
rat. It is pertinent to note that in the chick
embryo, neural crest cells also colonize
the thyroid tissue (Le Douarin and Le
Libvre, '70).
ACKNOWLEDGMENTS
Author is grateful to A. Pusterla for expert technical assistance, to G . Lambert
for the photomicrography and to Mrs. D.
Desrochers for the office work. Dr. N.
Brikre and Dr. R. L. Kidd have made many
valuable editorial suggestions.
LITERATURE CITED
Aoi, T. 1966 Electron microscopic studies of the
follicle cells and parafollicular cells in the thyroid gland of the primates. Okajimas Fol. Anat.
Jap., 42: 63-89.
Axelrad, A. A., and C . P. Leblond 1955 Induction of thyroid tumors in r a t by a low iodine
diet. Cancer, 8: 339-367.
Breathnach, A. S. 1971 Ultrastructure of Human
Skin. J. & A. Churchill, London.
Calvert, R. 1972 Electron microscopic observations on the contribution of the ultimobranchial
bodies to thyroid histogenesis in the rat. Am. J.
Anat., 133: 269-290.
Calvert, R., and H. Isler 1970 Fine structure of
a third epit!ielial component of the thyroid
gland of the rat. Anat. Rec., 168: 23-42.
Ekholm, R., and L. E. Ericson 1968 The ultrastructure of the parafollicular cells of the thyroid gland in the rat. J. Ultrastruct. Res., 23:
378402.
Ekholm, R., and F. S. Sjijstrand 1957 The ultrastructural organization of the mouse thyroid
gland. J. Ultrastruct. Res., 1 : 178-199.
Godwin, M. C. 1937 Complex IV in the dog with
special emphasis on the relation of the ultirnobranchial body to interfollicular cells in the
postnatal thyroid gland. Am. J. Anat., GO:
299-339.
Gorbman, A. 1947 Functional and morphological properties in the thyroid gland, ultimobranchial body, and persisting ductus pharyngiobranchialis IV in a n adult mouse. Anat. Rec,,
98: 93-102.
Le Douarin, N., and C . Le Li2vre 1970 Demonstration de l'origine neurale des cellules B calcitonine du corps ultimobranchial chez l'em-
346
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bryon de Poulet. C. R. Acad. Sc. (Paris), 270:
2857-2860.
Luciano, L., and E. Reale 1964 Electronmikroskopische beogachtungen a n parafollicularen
Zellen der Rattenschilddriise. 2. Zellforsch.,
64: 751-766.
Luft, J. H. 1961 Improvements in epoxy resin
embedding methods. J. Biophys. Biochem.
Cytol., 9: 409-415.
Monroe, B. G . 1953 Electron microscopy of the
thyroid. Anat. Rec., 116: 345-361.
Nbve, P., and S. H. Wollman 1971 Fine structure of ultimobranchial follicles in the thyroid
gland of the rat. Anat. Rec., 171: 259-272.
Paghs, A. 1956 Contribution B l’6tude du systBme des “cellules claires” de Feyrter. Biol.
Med. (Paris), 45: 414480.
Palade, G. E. 1952 A study of fixation for electron microscopy. J. Exp. Med., 95: 285-298.
Pearse, A. G. E., and A. F. Carvalheira 1967
Cytochemical evidence for an ultimobranchial
origin of rodent thyroid C cells. Nature (Lond.)
214: 929-930.
Pearse, A. G. E., and J. M. Polak 1971 Cytochemical evidence for the neural crest origin of
mammalian ultimobranchial C cells. Histochemie, 27: 96-102.
Reynolds, E. S. 1963 The use of lead citrate at
high pH as an electron-opaque stain in electron
microscopy. J. Cell Biol., 17: 208-218.
Sandborn, E., P. F. Koen, J. D. McNabb and
G . Moore 1964 Cytoplasmic microtubules in
mammalian cells. J. Ultrastruct. Res., 11:
123-128.
Sarkar, S . K., and H. Isler 1963 Origin of the
“light cells” of the thyroid gland. Endocrinology, 73: 199-204.
Stoeckel, M. E., and A. Porte 1970 Origine embryonnaire et differenciation s6crCtoire des cellules B calcitonine (cellules ( 2 ) dans la thyro’ide
foetale du rat. Etude au microscope klectronique. Z. Zellforsch., 106: 251-268.
Stux, M., B. Thompson, H. Isler and C. P. Leblond
1961 The “light cells” of the thyroid gland in
the rat. Endocrinology, 68: !292-308.
Van Dyke, J. H. 1944 Behavior of ultimobranchial tissue in the postnatal thyroid gland:
the origin of thyroid cystadenomata in the rat.
Anat. Rec., 88: 369-391.
1945 Behavior of ultimobranchial tissue in the postnatal thyroid gland: epithelial
cysts, their relation to thyroid parenchyma and
to “new-growths” in the thyrciid gland of young
sheep. Am. J. Anat., 76: 201--251.
1955 Experimental thyroid metaplasia
in the rat. AMA Arch. Pathol , 59: 73-81.
Wetzel, B. K., and S . H. Wollinan 1969 Fine
structure of a second kind of follicle in the
CsH mouse. Endocrinology, 84: 563-578.
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development and properties of ultimobranchial
follicles in the rat thyroid. Anat. Rec., 171:
247-258.
1971b Ultimobranchial follicles in the
thyroid glands of rats and mice. Rec. Progr.
Hormone Res., 27: 213-234.
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669-686.
PLATES
Abbreviations
A, transitional cell
B, lumen-bordering cell
bl, basal lamina
C , C cell
co, colloid
d, desmosome
er, rough-surfaced endoplasmic
reticulum
f, thyroid follicle
F, follicular cell
h, hemidesmosome
L, lumen
m, mixed follicle
S, subjacent cell
UBB, ultimobranchial body
v, microvillus
PLATE 1
EXPLANATION OF FIGURES
348
1
Thyroid gland of a four week old rat; the lumen of a mixed follicle
( m ) is lined by follicular cells ( F ) and by lumen-bordering cells ( B ) .
Subjacent cells ( S ) partially surround the wall of this follicle but
they are not in contact with the colloid. Small lumina containing a
colloid-like substance (arrows) are seen within the ultimobranchial
cell cord. However, it is impossible to identify accurately the type of
cells responsible for the synthesis of this material. A C cell ( C ) is
located in the wall of a thyroid follicle ( f ) ; it does not reach the
colloid of this follicle. Toluidine blue. x 700.
2
This mixed follicle ( m ) is the same as in figure 1, a few sections
further. A C cell ( C ) is observed in its wall; it is separated from colloid by follicular cells ( F ) . The ultimobranchial body (UBB) has ihe
aspect of a cell cord; figure 3 reveals that it is a tubule or a duct
with a slit-like lumen. Here, subjacent cells ( S ) are much darker than
lumen-bordering cells. In the lumen of the mixed follicle, note that
immediately i n front of the lumen-bordering cell ( B ) , colloid has a
spongy appearance. Toluidine blue. x 700.
TRANSITIONAL CELLS I N RAT THYROID
R. Calvert
PLATE 1
349
PLATE 2
EXPLANATION OF FIGURE
3
350
Micrograph of an ultimobranchial duct of a four-week old rat. The
slit-like lumen ( L ) is partially filled with a finely granular material.
The wall of the duct is composed of two types of cells: the lunienbordering cells (B ) characterized by their short microvilli, indented
nuclei, dispersed tonofilaments, pale cytoplasm and few flat cisternae
of rough-surfaced endoplasmic reticulum (er); the subjacent cells
( S ) contain a more electron dense cytoplasm, numerous desmosoines
and infoldings of the plasma membrane, large bundles of tono.Filaments and hemidesmosomes (h). The basal lamina follows the bery
sinuous pathway of the plasma membrane of subjacent cells.
x 9,500.
TRANSITIONAL CELLS I N RAT THYROID
R. Calvert
PLATE 2
351
PLATE 3
EXPLANATION OF FIGURE
4
352
A typical follicular cell (F) in the thyroid gland of a four-week old
rat. Distended cisternae of rough-surfaced endoplasmic reticulum are
distributed throughout the cell. Golgi apparatus is well-developed and
apical vesicles are numerous near the microvilli (v). Mitochondria
and free ribosomes are fairly abundant. The nucleus is almost circular and is located toward the basal part of the cell. Colloid (co)
reaches the apical plasma membrane. x 13,400.
TRANSITIONAL CELLS IN RAT THYROID
R. Calvert
PLATE
a
353
PLATE 4
EXPLANATION OF FIGURE
5
354
Thyroid gland of a four-week old rat; micrograph of a transitional
cell (A) displaying fine structural features of subjacent cell and o€
follicular cell. Like the subjacent cell it contains a long flattened nucleus and hemidesmosomes (h). However some slightly distended
cisternae of rough-surfaced endoplasmic reticulum (er ) are collected
i n one corner of the cell. In subjacent cells (S) these profiles are
scanty and flat and in follicular cells they are more numerous, more
distended and have a wider distribution. The mitochondria in the
transitional cell ( A ) are larger than those of ultimobranchial cells.
Moreover this cell ( A ) has long, slender microvilli (v), typical of
follicular cells. Subjacent cells never have microvilli and lunienbordering cells (B) display short microvilli. The lumen-bordering cell
( B ) shown in this picture contains numerous tonofilaments and scattered glycogen granules. x 20,400.
TRANSITIONAL CELLS I N RAT THYROID
PLATE 4
R. Calvert
355
PLATE 5
EXPLANATION O F FIGURES
6
Tangential section of a transitional cell (A) in the thyroid gland
of a two-week old rat. Without its few and slightly distended cisternae of rough-surfaced endoplasmic reticulum (er) this cell ( A )
would be identified as an ultimobranchial body cell. It lies among subjacent cells ( S ) and is in contact with a lumen-bordering cell, ( B ) .
The tangentially cut lumen ( L ) is also lined by the adjacent lumenbordering cell ( B ) , as shown by serial section. The nucleus (of the
transitional cell ( A ) is deeply indented, which is typical of ultimobranchial body cells ( B and S). Unlike the ultimobranchial body
cells this cell ( A ) touches the basal lamina (bl) and also lines a
lumen. The cisternae of rough-surfaced endoplasmic reticulum in the
transitional cells are less numerous and less distended than those of
typical follicular cell (F). x 6,800.
7 “Young follicular cells” in the thyroid gland of a two-week old rat.
The lumen ( L ) of a mixed follicle is lined by a lumen-bordering cell
( B ) and by young follicular cells (A). Microvilli projecting from the
latter are longer than those observed at the apex of the lumenbordering cell. The lumen is completely filled with a finely gr,anular
material. Young follicular cells in this picture display more disi ended
cisternae of rough-surfaced endoplasmic reticulum than the transitional cell of the preceding picture. However these cisternae are still
less dilated than those of mature follicular cells. Desmosomes ( d )
attach one of the young follicular cell to the lumen-bordering cell.
x 6,800.
356
TRANSITIONAL CELLS I N RAT THYROID
R. Calvert
PLATE 5
357
PLATE 6
EXPLANATION OF FIGURE
8
358
Transitional cells (A%,Az, A3, A4) in the wall of an ultimobranchial
duct in the thyroid gland of a four-week old rat. Cells Al, AZ and A3
are young follicular cells. Their few cisternae of granular reticulum
are slightly distended. With serial sections it can be shown that they
line the small lumen (L) of a mixed follicle. In this section the lumen
is completely surrounded by a lumen-bordering cell (Bz). Another
type of transitional cell (A4) is observed in this micrograph. This cell
(A4) lies on a basal lamina, does not reach a lumen and has hemidesmosomes ( h ) on the basal plasma membrane; this is typical of
subjacent cells (S). On the other hand, like the usual lumen-bordering cells (BI,Bz) it has a pale cytoplasm, its tonofilaments are not
aggregated in thick bundles and they bear fewer desmosomes than
subjacent cells. Here the ultimobranchial duct is in continuity with a
large mixed follicle located at the top of the micrograph. Notice that
colloid (co) in this lumen does not reach the plasma membrane of
the lumen-bordering cell (BI). x 12,100.
TRANSITIONAL CELLS IN RAT THYROID
R. Calvert
PLATE 6
359
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