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Total protein and organic iodine in the colloid and cells of single follicles of the thyroid gland.

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TOTAL PROTEIN AND ORGANIC IODINE IN THE
COLLOID AND CELLS O F SINGLE FOLLICLES
O F THE THYROID GLAND1
I. GERSR" AND T. CASPERSSON
Department of Chemistry, Caroline Institute, Stockholm, Sweden
THREE TEXT FIGURES AND T W O PLATES (TEN FIGURES)
Thyroglobulin has a characteristic absorption curve in the
ultraviolet region of the spectrum. The two absorption maxima of this curve are in the neighborhood of 2800B and of
3200 A. The former is due chiefly to the absorption peaks of
tyrosin and tryptophane. The latter is caused almost entirely
by the absorption of thyroxin and diiodotyrosin. Methods
developed by Caspersson (to be published) make it possible
to measure directly and quantitatively with the aid of a photoelectric cell the absorption of ultraviolet light in a small,
definitely circumscribed area of about 0.5 sq. p of a thin section of an organ such as the thyroid. Even smaller areas can
be studied by his photographic methods ( ' 3 6 ) . These procedures permitted us to take advantage of the specific ultraviolet light -absorbing proper ties of thyr oglobulin to determine
quantitatively the concentration of total protein in the colloid,
and of thyroxin
diiodotyrosin in both colloid and cells.
Attempts were made to localize the iodine-containing substances in the gland cells. The tyrosin and tryptophane content of the colloid is regarded as a measure of its protein
concentration. This assumption is justified by the analyses of
Cavett, who showed that the amino acid composition of thyroglobulin (except for its iodine-containing moieties) remains
+
'Thanks are due to the Rockefeller Foundation and the Th6rBse and John
Anderason Memorial Fund for the optical instruments used.
a Fellow of the John 8irnon Guggenheim Memorial Foundation.
303
304
I. GEHSH AND
'r.
CASPEILSSON
relatirely unaltered, in spite of great variations in the physiological state of the glands from which the protein was extracted. Similarly, repeated analyses by numerous workers
diiodotyrosin
have shown that protein-bound thyroxin
should be regarded as a measure of the total organic iodine in
the thyroid gland ( Harington).
By the methods of Caspersson, analyses were made of the
protein and organic iodine content of about 100 follicles. These
were located in the middle portion of thyroid glands removed
froiii five guinea pigs weighing 150 t o 300 gm. The animals
were in different physiological states : normal (1),potassiuni
iodide (1%)administered in food for 2 weeks (1),anterior
lobe extract administered, resulting in a weak (2) and in a
strong (1)stimulation of the thyroid gland. The glands were
immersed in liquid air, dried in a vacuum at -30"C., and
embedded in paraffin. Sections 20 p thick were used for analyses of the colloid; f o r measurements on cells, sections were
5 or 10 p thick. The sections were placed in chloroform, transferred to absolute alcohol and mounted in glycerine, care
being taken t o dissolve completely all the paraffin and to remove almost all of the alcohol. The subsequent procedures will
be described shortly by Caspersson.
I n figure A are given typical absorption curves obtained by
measurement of the colloid in the follicle of the normal animal
(A), the guinea pig treated with potassium iodide (B), of a
guinea pig with mild stimulation of the thyroid gIand (C).
For comparison, a fourth curve (D) is given showing the
absorption of ultraviolet light by extracted thyroglobuliii
(Ginsel). It is clear that in all cases, the shape of the absorption curre, and the positions of the absorption maxima are
very nearly identical.
I n these curves, the height of the absorption maxima (from
which may be calculated the relative concentration of protein
and of total organic iodine) are comparable. The sections were
of the same thickness, the area of light transmission mas identical, and the colloid appeared homogeneous. No evidence whatever could be uncovered indicating significant in1iomogeneit;v
+
'
305
IODINE CONTENT O F THE THYROID GLAND
of the colloid. Its uniform nature is indicated by low and high
power photograph of follicles in different physiological states
(figs. 1 4 , 9 ) , preserved by freezing and drying. These are to
be compared with figure 10, showing the numerous peripheral
colloid vacuoles and other grosser inequalities observed in
control material from one of the same glands fixed in formalin.
1.0
I
0.0
I
I
3400
3600
B S O R P T I O N C U R V E S OF C O L L O I D A =NORMAL
B = PO TAS 5 I U M I OD IDE
C=ANT.PIT.- MILD
0.6
t
0.4
4YROGLO
-.
'.
0.2
t
Z
W
6
0.1
I
,0.08
b.
W 0.06
0
u
Z 0.04
0
k
u
z 0.02
k
X
W
2400
2600
WAVE L E N G T H
2800
-i
3000
3200
Fig. A Ultraviolet absorption curves of colloid of three different individual
follicles (A, B, C) and of extracted thyroglobulin ( D ) reproduced from Ginsel
(Biochem. J., vol. 33, p. 428, 1939). Note that the absorption peaks in all four
curves are i n the neighborhood of 2800 A and 3200 A.
Further study of a large number of follicles emphasized the
marked variability in the protein and organic iodine concentration in the colloid of different follicles. This is clearly
shown in figure B, which gives comparable extinction coefficients of the colloid in the follicles of the animals already
considered at the critical wave lengths of 2800 and 3200 d.
Measurements at the longer wave lengths were corrected for
errors due to diffraction and for the non-specific absorption
306
I. GERSH AND T. CASPERSSON
of protein substances which was calculated as 2 EBao,
in view
of the higher values for the extinction values. The error
caused by these factors at 2800 A is negligible. The mean and
the standard deviation are also indicated in the figures.
Fig. B Absorption peaks at 2800 A and 3200 A of colloid of individual follieles
from thyroid glands i n different physiological states, showing their fluctuations
in each gland. While the mean values of the protein peak (at 2800 A) vary
depending upon the experimental condition of the animal, those for the organic
iodine peak ( a t 3200 A ) show no significant differences, except in the case of the
colloid in the gland stimulated markedly with anterior pituitary extract, where
this latter peak is entirely absent.
I n all these cases, the absorption curve gave clear evidence
of the existence of organic iodine in the colloid. In striking
contrast are the results obtained by study of the colloid in the
follicles of the gland stimulated markedly by the administration of anterior lobe extract. I n no case was there any recog-
IODINE CONTENT OF THE THYROID GLAND
307
nizable indication of an absorption peak at 3200 A, even though
such a peak was clearly obtainable from the cytoplasm of the
gland cells. The shape of the curve obtained from the colloid
was clearly that of a “non-specific” protein, showing no trace
of the presence of organic iodine compounds (see fig. C, curve
t
Fig.C Absorption curves of colloid, cytoplasm and nucleus in a section of a
thyroid gland stimulated markedly by anterior pituitarp extract. The slight peak
at 3000 A in curve B was not observed in other absorption curves of the Cytoplasm.
A). Direct measurement of the absorption curve of cytoplasmic
regions of the gland cells was made on sections 5 or 10 p thick.
For the measurements to be significant, it was necessary to be
certain that cytoplasm extended throughout the whole thickness of the point measured, with no overlay or underlay by
colloid, connective tissue or nucleus. These requirements were
308
I. GERSH A N D T. CASPEHSSON
satisfied only in measurements of the cuboidal or columnar
gland cells of the slightly or markedly stimulated follicles. A
typical curve is reproduced in figure C, B. It shows a small
but distinct peak at 3200 A which is absent from the absorption curve of an adjacent nucleus (fig. C, C), as well as from
that of the colloid in the markedly stimulated follicle (fig.
C, A ) . Curves demonstrating the presence of organice iodine
were obtained when the point measured in the cytoplasm was
in a supra-, inter-, and, in one case, infra-nuclear position.
Attempts to localize morphologically the iodine-containing
elements required the use of photographic methods developed
by Caspersson ( '36). It was clearly demonstrated in all favorable sections of all glands that there were small, unequal,
irregularly disposed areas in the cytoplasm that absorbed
light in much the same way as the colloid (see figs. 5-8).
Quantitative measurements of these areas in photographs of
several cells (taken at 2800 A and at 3200
clearly indicated
that these areas contained organic iodine (thyroxin
diiodotyrosin). A more positive identification awaits the development of finer methods that will enable one to make a complete
absorption curve of such minute objects surrounded as they
are on all sides by other cytoplasmic constituents.
a)
+
DISCUSSION
For a long time attempts have been made to establish differences in the protein concentration in colloid by its reaction
to stains, or by the degree of shrinkage that it undergoes
during fixation. While the results are suggestive, they lack
the precision and validity that is afforded by the use of ultraviolet absorption methods. Not even the deductions of Williams ('37) on possible changes in the viscosity of colloid in
follicles in different functional states necessarily reflect actual
changes in protein density. Apart from the methods employed
here, no one can lap claim to the accurate estimation and
localization of thyroxin
diiodotyrosine in cells and colloid
in sections.
+
IODINE CONTENT O F T H E THYROID GLAND
309
I n the case of both groups of substances, comparable measurements in the colloid of different follicles would have been an
enormously coinplicated procedure without the preliminary
fixation of the material by freezing and drying. This procedure results in the preservation of the colloid as a uniform,
homogeneous substance presenting none of the inequalities obtained with almost all other methods of fixation.
There were no visible or measurable indications of structure
in the colloid. The absence of peripheral colloid vacuoles in
sections subjected t o no more radical treatment than immersion in paraffin oil is especially significant (see figs. 9, 10).
These observations on the absence of peripheral colloid vacuoles supplement the in vivo observations to the same effect
by Vonwiller and Wigodskaya, Hartoch, and Bucher. The
former workers emphasized the absence of colloid vacuoles ;
tlie last author pointed out that no or very few peripheral
vacuoles were observed at the beginning of a period of observation, and that as the experiment continued they increased
in number. It is quite possible that the small number of vacuoles observed initially also might be the result of the manipulation of the gland. The vacuoles described in vivo by Andersson may have been due to such mistreatment of the tissue. It
is difficult to evaluate in terms of the normal gland the description of occasional colloid irregularities observed by
Williams (’39) in grafts of thyroid gland in transparent moat
chambers in the rabbit’s ear.
A casual inspection of the extinction coefficient of the colloid
in different follicles of the normal gland (fig. B) shows that
there is a great spread in the concentration of protein and of
organic iodine. The former may be niore than twice as great
in some follicles as in others ; the latter shows even a greater
disparity in different follicles, being seven times more concentrated in some units than in others. It is clear also that
there is no correlation between the concentration of the two
groups of substances-in some follicles, the colloid may be
rich in protein and poor in organic iodine o r vise versa. This
310
I. GERSH AND T. CASPERSSON
is of importance in interpreting the existing studies on the
chemical and physical analyses of thyroglobulin.
The ratio of the total protein extinction coefficient with that
of the organic iodine is particularly significant. A comparison
of this ratio f o r extracted thyroglobulin (Ginsel) with that
for the colloid in follicles, shows that in almost all cases
the ratio is larger in the former than in the latter. This
observation indicates that the extracted thyroglobulin used
by Ginsel may contain difficultly separable contaminating protein substances. Indeed, thyroglobulin may be looked on as
the pooled colloid of numerous heterogeneous follicles, the
contents of many of which are different from the others. The
physical and chemical properties of thyroglobulin may well be
regarded as representing average values derived from a crude
mixture of large numbers of individually variable units rather
than specific constants.
The rharked variability which has been noted in the concentration of protein and of organic iodine in the colloid of
the normal gland is somewhat reduced in the colloid of the
animals treated with potassium iodide. I n the follicles of this
animal, the average concentration of protein falls below the
normal, while the concentration of organic iodine is not significantly altered. I n other words, the administration of these
drugs results in the formation of a more nearly uniform colloid showing a higher ratio of iodized to uniodized protein.
The follicles behave differently when the thyroid gland is
stimulated mildly with anterior lobe extract. Inspection of
figure 3 clearly shows that the amount of light (at 2750 A)
absorbed by the colloid of different follicles varies much more
than in the normal gland (fig. 1). The actual measurements
of the light absorbed (fig. B), are from follicles selected to
show the range of variability. No attempt was made t o establish average values by random selections a-s in the case of
those from the normal animal or from the guinea pig treated
with potassium iodide. Direct observation (or the use of
photographs) sufficed to show that the concentration of the
total protein in the colloid of many follicles is considerably
IODINE CONTENT O F THE THYROID GLAND
311
less than in the normal. The measurements show also (fig.B)
that in spite of this great variability in the protein concentration, the concentration of organic iodine is relatively unaffected. Thus it is clear that mild stimulation with anterior
lobe extract results in an accentuation of the normal variability in the concentration of the total protein in the colloid,
together with an increase in the efficiency of the iodization of
the protein.
These changes in the chemical composition of the colloid
of the glands of experimental animals are highly significant.
It might be supposed that a mechanism which could explain
how it comes about that the chemical components vary in different physiological states might also interpret the similar
variability in the chemical composition of the colloid of f ollicles in the normal animal. The changes induced by the
administration of potassium iodide or anterior lobe extract
~
indicate that there is a constant secretion of “ ~ o l l o i d ’(that
is, of iodized and uniodized protein). The relative concentration of iodized protein differs from time to time, and is
determined by at least two factors : (1)the blood iodide level,
and (2) the physiological state of the gland cells. This “new”
colloid is mixed with the “old” colloid which may be of different composition. At the same time, the mixed colloid is
reabsorbed. This constitutes the essential elements of a mechanism for altering not only the concentration of colloid, but
also its composition.
The mechanism proposed above is the simplest one capable
of explaining the changes in the composition of the colloid
after the administration of either potassium iodide or anterior
lobe extract. It is highly probable that this mechanism applies
equally well to the variabilities in the composition of the
colloid of different follicles in the normal animal. Changes
in the blood iodide level, superimposed on cyclic or accidental
fluctuations in the capillary bed of individual follicles such as
have been described in the frog (Vonwiller and Wigodskaya)
and on the state of activity of the gland cells may be factors
at work. By altering the composition of the “new” colloid
312
I. GEBSH A N D T. CASPERSSON
continually produced by the activity of the cells, and as a
result of a persistent absorption of the altered colloid through
the intermediation of cells, the composition of the colloid may
be made to vary. The observed variations in the composition
of the colloid in different follicles of the same gland thus lead
to the concept that, in the normal animal, the production of
colloid (for storage and use) is a continuous process. Whether
such a constant absorption and secretion of this stored colloid
is supplemented by direct secretion of the active principles
into the blood in these conditions is not known.
The basis for such an absorption mechanism exists in published reports. The colloid in the lumen may be digested by
the action of enzymes secreted there by the gland cells. That
such hydrolysis may take place is known, for the effects of
the process as postniortem phenomena are well known. The
hydrolytic products (polypeptides or peptones) may he capable of rapid absorption across the cell membrane. I n the
cytoplasm it may be reconverted to products of protein dimensions by the protease which is known to exist in the thyroid
gland (Solter). These substances a r e then capable of passing
directly into the blood stream and of affecting the metabolism
of the organism (Lerman and Solter).
Evidence for an alternative explanation was offered by
McClendon. This author found that when fresh thyroid gland
is ultracentrifuged at a rate of 400,000 times the force of
gravity, colloid leaves the follicle, in spite of the absence of
any visible intercellular spaces. The mechanism proposed
above would seem to be simpler than the mustering of such
enormous forces. The intercellular passage of colloid as described by Uhlenhuth and many others, is obviously a special
process which has not been found to take place in mammals.
The method of trans-cellular release of colloid described by
Williams ( '37) as taking place in some follicles may also be
explained in terms of the theory proposed in the previous
paragraph.
These conditions a r e clearly altered when the gland is
placed in a state of marked activity. I n this phase, the con-
IODINE CONTENT O F THE T H Y R O I D G L h K D
313
centration of the protein in the colloid is markedly decreased.
The values given in figure B a r e from follicles selected to show
the extreme ranges of variability in the protein concentration,
rather than their true average. Though the protein concentration in any one of these follicles may lie below the lowest
values found in the normal animal, it may also reach the normal
range. Yet in spite of this there was no evidence of the presence o i organic iodine in the colloid. This is especially significant, since organic iodine could be demonstrated to be
present in the cytoplasm of tlie gland cells of similar follicles,
and in high concentration in the circulating blood (see AlcClendon’s monograph). These findings constitute a demonstration
of the concept expounded by Bensley that in a state of extreme
activity the thyroid gland cells secrete organic iodine directly
into the blood stream. The problem of whether this mode of
secretion takes place normally also cannot be approached by
the methods employed.
That iodine is present in tlie supra-, inter-, and inf ra-nuclear
parts of the cytoplasm of active gland cells was demonstrated
hy direct measurement of the absorption cnrves of these
regions. No evidence could be uncovered for the existence of
such compounds in the nucleus. Supplementary, less specific,
photographime measurements made it possible to localize the
iodized substances, at least tentatively, to small structures,
irregularly disposed as to their size, shape, and position i n
the cytoplasm. Their appearance resembles closely descriptions made by Andersson in fixed material and Uhlenhuth in
fresh cells. The possibility exists still that some organic
iodine compounds may be present also in the ground substance
of the cytoplasm.
A comparison of the absorption curves of the colloid and of
the cytoplasm and nucleus of gland cells emphasizes the
marked difference in their shape. This has a direct bearing
on the nature of the secretory process that results in the
formation of stored colloid. Earlier workers in the field, and
some more recent ones, believed that the colloid appeared as
a result of a transformation (which took place before o r after
314
I. GERSH A N D T. CASPERSSON
sloughing off froin the follicle wall) of the whole cytoplasm,
or smaller segments of it, with or without the nucleus. This
concept was developed in the course of numerous studies on
the nature of the “colloid” cells of Langendorff. If the colloid
were in fact formed in this way, the absorption curves of colloid and cell cytoplasm should resemble each other closely,
differing only in the height of their absorption maxima. The
absence of such an identity in the experimental curves shows
clearly that the colloid is a true merocrine secretion, which is
produced in the cytoplasm as a result of its activity, and
extruded from it to be stored in the lumen. The nature of
these curves demonstrates clearly that nucleoprotein is not a
regular constituent of colloid, contrary to the claims of Oswald.
I n most follicles of the glands stimulated mildly with anterior lobe extract, and occasionally in the other animals already
discussed, free cells are present in the colloid. From their
variegated shape and rich cytoplasm, it is possible that these
cells are ameboid and active. They are commonly highly vacuolated. The material included in the vacuoles resembles colloid in its light-absorbing properties, and may perhaps be
iodized or uniodized colloid protein. Further work is necessary to determine whether these free cells are engaged in the
“softening” or lysis of colloid (Loeb) or in the phagocytosis
of colloid.
SUMMARY
Frozen-dried sections of the thyroid gland were studied with
the ultraviolet microscope. It was possible to obtain the absorption curves of minute volumes of colloid, cell cytoplasm,
and nucleus, and to analyze these in terms of the characteristic absorption curves of thyroglobulin, protein-bound iodine
(thyroxine diiodotyrosine), and cyclic components of protein (tyrosine
tryptophane). The two latter groups of
substances were found to be homogeneously distributed in the
colloid. Quantitative determinations showed that their concentrations in the colloid varied independently of each other
to a very marked degree in different follicles .in the normal
animal. Similar analyses of the colloid led to the conclusion
+
+
IODINE CONTENT OF THE THYROID GLdND
315
that in animals treated with potassium iodide or with anterior
lobe extract colloid is continually secreted into the lumen to
be stored and, subsequently, reabsorbed. Study of glands
markedly stimulated led to the conclusion that in this condition, secretion takes place directly toward the blood vessels.
Photographic measurements of gland cells tentatively localized the protein-bound iodine in structures resembling closely
the chromophobic droplets of Andersson and the colloid droplets of Uhlenhuth. Organic iodine compounds were found to
be absent from the nucleus.
LITERATURE CITED
ANDERSSON,
0. A. 1894 Zur Keiintiiisx der Morphologie dcr Schilddriise. &4rch.
f. Anat. u. Physiol. (Anat. Abth.), pp. 177-221.
BENSLEY,R. R. 1916 The normal mode of secretion of the thyroid gland. Am.
J. Anat., vol. 19, pp. 37-55.
BUCHER,0. 1938 Untersuchungen iiber den Einfluss vorschiedener Fixationsmittel auf das Verhalten des 8childdriisenkolloids. Zeit. f. Zellf. u.
niikr. Anat., vol. 28, pp. 359-381.
CASPERSSON,
T. 1936 Uber den eheinischen Aufbau der Strukturen des Zell73, Suppl. 8, pp. 1-151.
kernes. Skand. Arch. f . Physiol.,
J. Roy. Micr. Soe., in press.
J. W. 1936 Thyroglobuliii studies. 11. The van Slyke nitrogen distribuCAVETT,
tion and tyrosine and tryptophane analyses for normal and goitrous
human thyroglobulin. J. Biol. Chern., vol. 114, pp. 65-73.
GINSEL,L. A. 1933 The ultraviolet absorption of sheep thgroglobulin. Biochem.
J., VOI.33, pp. 428-434.
HARINGTON,
C. R. 1933 The Thyroid Gland. London: Oxford University Preas.
HARTOCH,
W. 1933 hfikroskopische Beobachtungen a11 lebenden Organen. Speicheldriise nnd Schilddriise. Klin. Wochenschr.. 1-01. 12, pp. 942-944.
HIRSCHLXXOU-A,
Z. 1927 Mikroskopisch-anatomische Untersuchungen an der Amphibienschilddriise mit besonderer Beriicksichtigung ihres Golgi-apparates. Zeit. f. Zellf. u. mikr. Anat., vol. 6, pp. 234-256.
LERMAN,
J., AND W. T. SALTER 1936 The behavior of natural and artificial thyroid protein. Trans. Am. Ass. Study of Goiter, pp. 143-154.
LOEB,L. 1929 The structural changes which take place i n the thyroid glands
of guinea pigs during the process of compensatory hypertrophy under
the influence of iodine administration. Endocrin., vol. 13, pp. 49-62.
MCCLENDON,
J. F. 1939 Iodine and the Incidence of Goiter. Minneapolis:
University of Minnesota Press.
1939 The release of colloid from the thyroid gland by centrifugal
force. Endoerin., rol. 24, pp. 82-86.
OSWALD,A. 1899 Die Eiweisskorper der Schilddriise. Zeitschr. f . physiol.
Chem., 1-01. 27, pp. 14-49.
1701.
316
I. GERSH A N D T. CASPERSSON
SALTER,
W. T. 1936 In Lerinan and Salter.
UHLENHUTH,
E. 1936 The thyreoactivator hormone. Trans. Am. Ass. Study of
Goiter, pp. 25-50.
VOSWILLER,
P., AND R. WIGODSKAYA1934 Etudes sur les barrieres histoh6matiques. 11. La thyrkoscopip. Observation de 18 glande thyroids, de ses
61Bments histologiques, de ses vaisseaux sanguins et dc son produit de
s6crCtion sur l’animal vivant. Bull. d’Histol. appl., vol. 11, pp. 20-31.
WILLIAXS,R. G. 7937 Microscopic studies of living thyroid follicles implanted
in transparent chambers installed in the rabbit’s ear. Am. J. Anat.,
VOI. 62, pp. 1-29.
1939 Further observations on the microscopic appearance and behavior of living thyroid follicles in the rabbit. J. Morph., vol. 6.5,
pp. 17-51.
PLATE 1
EXPLANATION OF FIQURES
Photomicrographs of sections of thyroid gland in different physiological states
taken with monochromatic light having a wave length of 2750 A. Glands prepared
by freezing-drying method; sections 20 p thick, mounted in glycerine, unstained;
X 110. These photomicrographs are not quantitative ; f o r actual measurements
see figures A and B. Note differences in the amount of light absorbed by the
colloid in diff went follicles of each preparation, the homogeneous appearance of
the colloid, and the absence of peripheral colloid vacuoles.
1
2
3
The
Kormal guinea pig thyroid gland.
Thyroid gland of guinea pig treated with potassium iodide.
Guinea pig thyroid gland stimulated mildly with anterior pituitary extract.
cells in the colloid of some of the follicles are shown more clearly in figure 5.
4 Guinea pig thyroid gland stimulated markedly with anterior pituitary extract.
1OI)INR CONTENT OF THE THYROID GLAKD
PLATE 1
I. QlCRSH AND T. CASPERSSON
317
PLATE 2
EYPLAN4TION OF F I G U R E S
5 Photoiiiicrogi apli of a single follicle TI hose gland cells contain cytoplasmic
droplets whose light absorptioii properties are like those of colloid in the lumen.
Glaiid stiinulateci mildly with anterior pituitary extract. Photographed with
monochromatic light with a wave length of 27.50 A ; X 1455.
6 Pliotomicrograph of two follicles of a glaiid stimulated niaikedlg n-ith
anterior pituitary extract. Similar colloid droplets are visible in tlie gland cells.
Photographed with nioiiochromntic light with a wavelength of 2750 A ; X 1435.
7 Dra\ring of gland cells in same follicle as in figure 5, with cniphasis on the
colloid droplrts which they contain. X 1455.
8 Drawiiig of gland cells in same follicles a s in figure 6, n i t h rmphasis o n
the colloid droplets present in tlie cytoplasm. X 1455.
9 Photoniicrogixph of seJeral follicles of a section of n thyroid gland stimulated mildly uitli anterior pituitary extinct and fixed by freezing and drying.
Paraffin replaced I)? mineral oil. Photographed with nioiiochromntic light having
a w a r e length of 2750 A ; X 723. Notr the absence of peripheral colloid vacuoles.
10 I’1iotomicrogr:cph of a scctioii of the opposite gland of tlie same auimal as
in figure 9, under the same coiiditions except that the organ mas fixed in formalin.
Kote that peripherical colloid vacuoles and other inlioinogeneities appear here as
a resnlt of tlie mode of fixation.
318
IODINE CONTEXT OF THE THYROID GLAND
I. GEBSH AND l'.
CASPEBSBOK
319
PLATE 2
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