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Studies on the thymus of the mammal. I. The distribution of argyrophil fibers from birth through old age in the thymus of the mouse

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STUDIES ON T H E THYRIITS O F T H E RIAXMAL
I. TIIE DISTILIBUTION O F ARGYROPHIL FIBERS F R O M BIRTH
T H R O U G H OLD AGE I N T H E THYMUS OF T H E MOUSE'
CHRISTIANNA SMITH ,4ND LOUISE M. IRELAND
Walter H. Merriam Laboratory, Department of Zoology,
Moztnt Holyoke College, South Hadley, Massachusetts
FOUR PLATES ( E I G H T E E N F I G U R E S )
The history of the thymus gland a s it undergoes involution
varies according to one or niore underlying factors. A primary
consideration with respect t o it is the distinction between
normal physiological or age involution and accidental involution due to inanition, toxic conditions, injection of hormones,
shock, treatment by x-ray, and disease. The former occurs
gradually; the latter, depending on the nature of the stimulus,
is much more rapid. I n both cases the cortex is more vulnerable than the iriedulla and both types are characterized by loss
of lymphocytes. The replacement of the lobules by adipose
tissue is typical of the later stages of physiological involution
in mammals. Hammar ( '27) recognizes the following differences between age and accidental involution : (1)real increase
of interstitial fatty tissue in the former and actual reduction
in the latter, ( 2 ) the retention of the boundary between the
cortex and medulla in the first and its disappearance in the
second. However Marine ( ' 3 2 ) claims that there are no definite distinguishing anatomical differences between the two
types. Both map occur simultaneously. There also may be
dissimilarities in the structural patterns of involution correlated with differences in animal species. Schaffer and Rabl
('09) note that in comparison with the mole, the connective
*This study was aided in part by a grant froin the Penrose Fund, American
Philosopliical Society.
133
TIIL A N A T O M I C A I A RRCOBD. VOL
79 NO 2
FFRRUARY,
1941
A N D SI'I'P1,RBIRIT
KO. 1
134
CIIRISTIANN.4
SMITH A N D L O U I S E M. IREL.4ND
tissue in the cortex of the involuting thymus of the shrew is
not a s plentiful or iniportant.
The present study of the involution of the thymus of the
mouse has been directed to the argyrophil fibers and it has
been found that they increase with age particularly in the
region of the medullary vessels and in those p a r t s of the cortex
being replaced by fat.
MATI+;ETAT> A N D METHODS
Thymus glands from normal albino mice were used. The
younger mice were reared in tlie laboratory of Mount Holyokc
College; the older ones were obtained from the Roscoe B.
5aclison Laboratory, Bar Harbor, Maine and were animals
from a strain with a low incidence of tumors. The first group
consisted of females whose ages were 0, 1, 7, 14, 21, 28, 35, 42,
49, 56, 60, 63, 70, 77, 171, 230, 256, 282, and 476 days. When
the first studies were made it was not possible to get timed
virgin mice for the later stages, so mated mice were used. As
involution due to pregnancy is said to be transient and since
the range of days between the last pregnancy and death was
14 to 116 days, with a n average of 47 days, it was thought that
no great confusion would arise from their use. The second
series included both males and females that were 0, 35, 62, 93,
185, 226, 407, 472, 685, 732, 798, and 826 days old. After an
animal had been etherized and weighed, the thymus was removed immediately. Except in a few of the first dissections,
the adipose tissue intimately connected with the thymus was
included with the gland. The thymuses of animals from the
first group were weighed and then the lobes of each gland were
separated. One was fixed in Helly’s fluid and the other, in a.
1:9 mixture of neutral formalin and absolute alcohol for 24
hours to prepare the material for microincineration. The
technique of Scott (’33) was used f o r this latter process.
Biiimals of the second series were not weighed. The thymuses
were sectioned a t 5 p except those intended for microincineration which were cut at 4 p. Foot’s inodification of Bielschowsky’s silver impregnation niethotl was used with variations,
HETICULAIL FIBEHS OF THYMTTS O F MOUSE
135
with and without staining in llallory 's connective tissue stain,
Alallory-azan or hematoxylin.
OBSERVATIOES
The distribution of arg-yropliil fibers in the thymus of the
iiiouse is dependent on the vascular pattern. The main arteries
and veins a i d their larger branches a r e found in the medulla.
Characteristic of the cortex a r e the radial arterial and venous
capillaries with their fairly numerous anastomoses (Smith,
Conant and Sayer, '39). However, as compared with the
nearby richly vascularized fat, this cortical supply is rather
poor. The reticular fibers which form the supporting framework of the larger vessels a r e therefore nuiiierous in the
medulla and scanty in the cortex.
I n the thymus of the newborn niouse the fibers a r e fine and
few in number (fig. 1). The blood vessels a r e supported by a
delicate network from which small twigs extend among the
neighboring cells. Here and there fine branches run parallel
to the lumen and foreshadow the future double sheaths so
characteristic of thymic veins. Although in sections taken
longitudinally through the lumen the argyrophil fibers appear
as the basal support f o r the endothelial cells, nevertheless,
when the wall is cut tangentially the network often reaches the
lumen. The outer surface of the cortex is bounded by a capsule
formed of fine interlacing reticular fibers. By the time the
mouse is a week old they a r e heavier (fig. 2 ) but there is little
change in quantity.
I n 3 t o 5 weeks the fibers thicken, increase markedly in number and extend among the thymic cells in both cortex and
medulla (fig. 3 ) . The 35 day old gland is the one that is typical
of puberty. This is tlie period when tlie absolute weight of the
thymus is the greatest although in percentage of body weight
it has passed its crest. A t this time, the diagnostic characteristics of the vessels show clearly. The doubling of the sheaths
around the medullary veins is now evident. Both of these
venous sheaths a r e networks ;the inner is next to the irregular
endothelial cells and the outer is separated from i t by thymic
136
C H R I S T I A N N A SMITH A N D LOUISE ?if. I R E L A N D
cells most of which are lymphocytes (fig. 4). Figure 5 is the
untouched photograph of the reticular fibers of five serial sections of medullary veins and adjacent territory projected on
cellophane and superiniposed (Smith, '38). Even in so few
sections the close relation of the scattered fibers to the adventitia of the blood vessels is seen. The apparently free fibers i l l
the figure connect with those from other vessels. This is shown
best in figures 8 and 11. The medullary arteries are also surrounded by two layers of argyrophil fibers but the cells
enclosed are the circularly arranged, single smooth muscle
cells (fig. 7). These two layers in the wall of the artery, the
inner subendothelial and the outer one, external to the muscle
cells, are typical of the same type of arteries (i,e., those with
a single layer of muscle cells) found elsewhere ; the sheaths of
the veins, on the other hand, are distinctive of the thymus. In
the arteries the long spindle shaped endothelial cells are evidently contracted and running parallel with these lines of
contraction are the silvered fibers which make up the well
defined subendothelial layer. On the left side of the lumen of
the thymic artery (fig. 7) of the 56 day old mouse two parallel
subendothelial fibers can be seen and, at the lower end, the
faint outlines of the elongated narrow endothelial cells. When
the section is tangential the outer layer resembles a series of
barrel hoops and when the cut is longitudinal through the
artery it appears like a row of scallops.
By 8 and 9 weeks the double sheaths (figs. 7, 8, 9) have
appeared around both larger and smaller veins and a third
network of fibers is seen around the larger arteries which is
comparable to those typical of the veins and which encloses
thymic cells. The outer lamellae may or may not be complete
and fibers from neighboring vessels extend among the cells
and may join each other. At this time the capillaries have only
the subendothelial layer hut two more or less intact ones are
present in advanced stages of involution (compare figs. 3 and
6 with 15). The casual relationship of the argyrophil fibers to
Hassal's corpuscles is shown in figures 8 and 9. I n all of these
thymuses cells with typical fibroblastic nuclei can be found
RETICULAR FIBERS O F THYMUS O F MOUSE
137
along the vascular walls but in any one section they are not
too common.
The most rapid decrease in weight of tlie thymus of the
mouse has taken place by the time the animal is 80 days old and
thereafter the decline is gradual. This is evident in the early
diminution in width of the cortex and the consequent shorteriing and approximation of the radial capillaries. Later the
medulla is also reduced so that, for example, in the 476 day old
gland (fig. 1 2 ) there is an apparent or relative increase in the
number of blood vessels in this zone (compare figs. 4, 5, 8 with
10 and 12). The nearness of the veins and arteries, the lamellated structure of their walls (figs. 10, 12, 13), and the numerous branches of reticular fibers among the cells produce a very
fibrous medulla. This may be observed particularly well if the
35 day old thymus in figure 4 is compared with the 256 and 476
day thymuses photographed a t the same magnification in
figures 10 and 12. Instead of double sheaths the medullary
veins in t.hese involuting glands often have three or more
concentric layers of fibers. The number of fibers extending
among the cells increases as does the thickness of the bundles.
In some mice (685, 798, and 826 days old) these latter may
transform into collagenous fibers. I n reduction of silver, the
smaller medullary arteries of the thymus are more variable
during age involution than any of the other vessels. At 56
days, the fibers are well defined, clear-cut and characteristic
(fig. 7). However one can find in a 62 day old mouse sections
of a few arteries of approximately the same bore where no
silver is precipitated in the subendothelial layer although the
fibers outside of the muscle cells are well blackened. I n general
the loss of ability to reduce silver takes place in much older
thymuses and is preceded by the formation of bridges of fibers
between the subendothelial layer and that outside of the muscle
cells. Where the fibers between these bridges join the parallel
subendothelial ones there are triangular nodal points. Longitudinal fibrils closely placed, and often concave in shape, bind
the circularly arranged hoops to each other. I n thymuses in
138
C H B I S T I A N N A S M I T H A N D LOUISE M. IRRLAh-D
which the fibers in the medulla a r e very numerous the next
stage in the involutional history of the arteries seems to be
the thinning and fragmentation of the subendothelial fibers
and of those between the muscle cells. First the straight
clearly outlined ones of the subendothelial layer become uneven in diameter, show thick and thin places, arid finally they
break. Those between the muscle cells become slender and
detached from the inner and outer sheaths. Following this al!
fibers except those outside the muscle cells gradually disappear
(fig. 13). But not all arteries in a thymus a r e in the same
conclition for the larger the artery, the better defined are the
endothelial and intermuscular fibers (fig. 13). I n later stages
these also may begin to fragment and disappear near the
hiluiii or the networks may remain intact until the more
peripheral branches are reached. I n the older mice (figs. 12
and 13) the lumens of the arteries a r e about the same size a s
those in the younger ones (fig. 7 ) but their walls are much less
sturdy. Also in such senile thymuses it is not possible to follow
a gradual ageing of the arteries as the successively older specimens a r e studied. I n the 226, 407, 472, 685, and 826 day old
glands, the endothelial fibers of the smaller arteries a r e either
poorly defined, irregular in diameter, fragmented or absent,
while in soiiie sections of vessels in the 256, 282, 356, 476, 732
and 798 day old ones clear parallel fibrils can still be seen.
In these silvered sections there is no evidence of the obliterated
capillaries described by some authors. The characteristic picture of these vessels with their lumens can be seen in the cortex
of a 161 day old thymus (fig. 15). Often what seems to be a
fibrous band or a dense, almost solid network is in reality a
tangential section of a wall.
The general behavior of the reticular fibers in age involution
has been described above ; the portions of the cortex which are
replaced by adipose tissue have a different history. Adipose
tissue is generously vascularized and its cells a r e encapsulated
with ar,gyrophil fibers. The reduced and comparatively much
less vascular and less fibrous cortex transforms into such a
RETICULAR FIBERS O F T H Y M U S O F NOUSE
139
tissue. The differentiation of f a t cells within the thymus begins at any time after the onset of involution and more of the
organ continues to be lost in this way with advancing age. The
thin anterior and posterior edges of the lobes are the first to
be converted; later isolated regions on the surface and parts
adjacent to the exit and entrance of the blood vessels a r e
transfornied.
A study of the thymic cells a s they are found previous to
the development of the adipose tissue and the role of the
capillaries in the formation of the latter will be reported in
a subsequent paper. However it is easily seen (figs. 16, 17, 18)
that the cells within the thymus adjacent t o the fat cells are
lighter in color, fewer in number and surrounded by argyrophi1 fibers. Sections shown in figures 17 and 18 are from the
same area, but the former one was stained in iron hematoxylin
and orange G and the latter, impregnated with silver and
stained in Rlallory-azan. At the left side of these sections a
shallow bay can be seen on the surface while the main part of
the picture shows the transformation of an edge of a lobule.
The heavy network of reticular fibers around the thymic cells
at the border or beneath the f a t stands in sharp contrast to
the lack of scattered fibers in the adjoining cortex (figs. 1.5,
16, 18). Figures 14,15, and 16 also show how a new lobulation
is imposed on the thymus by the formation of adipose tissue.
In this process the arched blood vessels (figs. 14, 16, 17) are
evidently the limiting pattern and the reticular fibers associated with them form the new capsule. A model of a small
island of f a t from the surface of a thymus injected with India
ink showed four such vessels between the adipose tissue and
the cortex. This model was based upon eleven sections cut
a t 20 p, the first and last including a row of f a t cells similar to
those shown in figure 14.
It was hoped that microincinerated material would show
changes in the mineral content of the thymus with progressivc
senility but with the methods used no definite conclusions
could bc drawn.
140
CHKISTIAX’XA S M I T H AND LOUISE M. IRELAND
DISCUSSION
It is generally agreed that the argyrophil fibers of the
thymus are most numerous in the regions of the blood vessels
and that there may be few or none of them in parts of both cortex and medulla. After a n examination of the literature from
1909 through 1935, Hammar ( ’36) decided that discrepancies in
the estimates of the relative amounts of fibers in the cortex and
medulla a r e probably due to a failure to consider accidental
and age involution. The pattern usually discussed is that
characteristic of the human gland in which there is a rich
plexus between the cortex and medulla. From this circummedullary network a scanty number of fibers penetrates the
cortex and many more enter the medulla. Hammar is also
inclined to believe with Tschassownikow (’30, after Hammar,
’36) that some of these argyrophil fibers a r e intracellular in
position. Since Tschassownikow noted that in his prepartttions the tonofibrils of the epithelial cells were not colored,
i t is evident that he was not confusing the two types of fibrils.
Although a few investigators (e.g., Jordan, ’28) see in the
precipitation of silver within Hassal’s corpuscles a suggestion
of their derivation from atrophic arteries and their reticulum,
the majority state that there is no connection between argyrophi1 fibers and these bodies. Hammar (’36) concludes his
review of the results of silver impregnation of thymic fibers
with this sentence: “In das in friiheren Fotalstadien ganz
epitheliale Thymusretikulum dringt eine postfotal immer mehr
anwachsentle Xenge von Bindegewebselementen, Fasern,
wahrscheinlich auch Zellen, mit oben erwahnter Verteilung
hauptsachlich langs der Gefasse ein.”
I n the mouse the reticular fibers a r e definitely disposed
according t o the vascular pattern. The arrangement is hidden
only when there is marked reduction in the volume of the
thymus with a consequent crowding of the fibers and cells, and
when there is a marked extension among the cells of the cortex
previous to the replacement of the latter by fat. Their relative
scarcity in the iiormal cortex is correlated with the number
RETICULAE FIBERS OF THYMUS OF MOUSE
141
of fine capillary loops found there and their greater abuiidance
in the medulla is associated with the presence of large veins.
Since they are the supporting framework of the thymic vessels,
extensibility and strength must be a function of the network
which they form because the reticular fibers are themselves
inelastic. The dense circummedullary network so often described for otlier animals is not present for there is no definite
collection of blood vessels running between the cortex and
medulla.
TABLE 1
Observations o n the argyrophil fibers during age involution of ihr thynitis
AUTHOR
DATE
ANIMAL
COWNENT
Pappenheimer
1910
Man
Increase of fibers limited to region of blood
vessels; other parts unusually free of fibers;
increase of fibers only in secondary atrophy
due to wasting disease.
Strandberg
1917
Man
Increase in size of interlobular septa and more
fibere from them into thymus; scanty fibers
in cortex throughout life history.
Canelli
1922
(Hammar ' 3 6 )
Man
Fibers prominent; less regular; numerous around
calcified bodies.
Cooper
1925
Man
Dense fibers.
Fukuchi
1928
Man
First pseudo-hypertrophy, followed by atrophy
and disappearance; in fatty degeneration
cliangc into white fibers.
Monroy
1938
(Hammar ' 3 6 )
Man
Change to white fibers, obliterate capillaries.
Hammar
Man
Increase in connective tissue elements chiefly
along the blood vessels.
1936
Most of the descriptions of the normal thymus after silver
impregnation are of the human gland, so also are the studies
on age involution. A brief survey of the data (table 1) shows
that there is little specific. information available. Almost all
investigators agree that there are few free fibers in the cortex
at any time in the life history of the thymus and that there is
an increase in the adventitia of the blood vessels accompanying involution. No inention is made of changes in the reticular
network preceding the replacement by fat.
142
CHBISTIAIS'NA SMITH AND L O U I S E M. I R E L A N D
As in any problem which deals with the process of ageing,
the chief difficulty encountered is the presence of changes due
to other factors. Even when care is taken to work with healthy
laboratory animals, examiliation of various organs often brings
pathological changes to light. I n this study pathological
changes in the thymus were seen in the following animals:
226 days, many macrophages and hypertrophied cells thought
to be of lymphocytic origin; 407 days, cortex and medulla not
well marked, many vesicles filled with colloid, a large cyst in
one lobe; 685 days, remnant mostly medulla, cortex not well
marked ; 826 days, nodules circumscribed and infiltrated with
silvered fibers. It is of interest that these glands a r e all included among those with more marked atrophy of reticular
fibers i n the smaller medullary arteries.
However, in the mouse, a certain sequence of modifications
can be traced in age involution. I n senility there is a real and
relative increase in the number of fibers and a thickening of
the bundles which extend among the cells. White fibers sometimes develop in the regions of greatest density in some of
the very old glands. The relative increase in the number of
fibers is due to the reduction in volume of the organ and the
consequent crowding of fibers and vessels already present
while the actual multiplication is taking place in two ways.
The first of these is by the doubling and tripling of the net-like
lamellae around the veins which enclose thymic cells and by
the developinelit of a similar sheath (sheaths) around the
capillaries and arteries; the second way is by the infiltration
of a larger number of fibers from these laminae among the
surrounding cells. Such double sheaths of cortical and medullary capillaries were noted by Strandberg ( '17).
In the arteries of the thymus of the mouse there is only a
single row of smooth muscle cells. These medullary arteries
a r e the most vulnerable in the senile gland as f a r a s can be
judged by the condition of the argyrophil fibers although
Rammar ( '21) found changes most often in the medium sized
veins. I n the young gland there a r e two layers of fibers, a
clear subendotlielial one and one outlining the muscle cells.
RETICULAR FIBERS OF THYMUS OF MOUSE
143
Later these a r e joined by a network which develops between
the muscle cells. With the development of the fine parallel
longitudinal fibrils connecting the circular hoops, the muscle
cells a r e closely surrounded. A thinning of the subendothelial
layer of fibers and of those between the muscle cells occurs
a s the first step in their atrophy, particularly in the siiialler
branches of the main arteries and Iater also i n the larger
ones. Next these fibers fragment and disappear and in the
very old glands either only a n indistinct subendothelial lamella
is apparent or merely the outer network of the muscle cells.
The history of the arterial fibers is similar to the observations
recorded by Fukuchi (’28) (table 1) on the human thymus.
According t o Hainmar ( ’Z),Monroy (’35 after Hammar, ’ 3 6 )
in man, and Jordan (’28) in guinea pig and man, atretic
changes a r e almost always found i n the vessels of the aged
thymus which end in obliteration of parts of them. Obliterated vessels were not observed in our material. Neither were
silver-reducing fibers found in the epithelial cells o r in Hassal’s corpuscles which were thought to be produced by them.
As f a r as could be judged no particular relationship existed
between the blood vessels or the fibers and these structures.
,4 typical senile change in the mouse is the replacement of
the diminishing thymus by the development within the gland
of subcapsular f a t cells (also true of adjacent lymph nodes)
besides the obvious increase outside of lobules of fat. In the
literature this process is most often pictured as one of invasion
o r infiltration. Cooper ( ’25, pp. 102, 103) describes the involuted thymus as “embedded in very vascular fat” aiid the
method of development a s “. . . f a t cells make their way into
the thymic masses.” And Hammar (’21, p. 5.51) thus states
the case: “After this time [referring to age of puberty] there
occurs a reduction of the bulk of the organ while at the same
time the interstitial connective tissue assumes the character
of adipose tissue and forms a greater and greater p a r t of
the organ.” I n his discussion of the morphology of the gland
he uses the term “interstitial tissue’’ in connection with inter-
144
CHHISTIANNA SMITH AND LOUISE M. IRELAND
lobular septa. F r o m Maximow-Bloom (’30, p. 714)2 the following is quoted : “After puberty the process of involution
begins. This consists of a gradual thinning out of the lymphoid cells of the cortex, the reticular cells of epithelial nature
become compressed, and the area formerly occupied by lymphocytes and reticular cells is gradually replaced by adipose
tissue, which is thought to arise in the interlobular connective
tissue. ”
The characteristics of adipose tissue a r e well defined by
Maximow-Bloom (’30,p. 96) :3 “In those places of the fetal
connective tissue in which f a t will later be present, at first,
rich networks of blood capillaries always develop.” “The
argyrophil fibers a r e well developed, especially along the blood
vessels, and form a densely reticular basket-like capsule around
each f a t cell (fig. 7 2 ) . ” Portions of the cortex of the thymus
transform into such a dissimilar tissue with tlie differentiation
of f a t cells preceded by a proliferation of argyrophil fibers
surrounding the cells. As shown by Hall ( ’39),the cells thus
encircled change in staining reactions and present a marked
contrast to the deeply colored normal lymphocytes of the cortex (figs. 17, 18). The increased number of fibers described by
earlier workers as present in the cortex may well have come
from such regions as these and not always be due to a reduction in volume as interpreted by Hammar ( ’36).
The initiation of the focal proliferation of fibers previous
to the differentiation of fat cells within the thymus presents
an interesting problem. So also does the related question of
the role of the capillaries in f a t formation. Nor is the source
of the fibers in the thymus clearly proved. That they a r e
connected with the blood vessels is clear but whether they are
all associated with the fibroblasts which a r e known to be present, even though not too abundant, is still a question. The
subcnclotlielial fibers a r e closely applied to the lining cells and
a Tlic first edition of “A text-book of histology” by Maximon,-Bloom, 1930,
lixs been used instead of the third, 1938, because the forms of the statements as
given there were preferred.
Bec footnote 2.
RETICULAR F I R E R S O F THYMUS OF MOUSE
145
are similar to those described by Corner ( '20) as produced by
the endothelium. Although Corner states that such fibers
occur only where there are no fibroblasts and where there is
a single layer of endothelium, the intimate union of the fibers
and the endothelial cells observed in the thymus material presents a basis for their inclusion in this group. The suggestion
is therefore made that the argyrophil fibers of the thymus are
associated in their origin with both the endothelial cells and
fibroblasts and that the networks so formed anastomose with
each other and are indistinguishable in silver impregnation.
SUMMARY
1. A study of the thymus (after silver impregnation) was
made in mice of various ages from birth through old age
(826 days).
2 . During the life history of the thymus there is an increase
in the argyrophil fibers chiefly in the medulla (1) by the
development of concentric networks of fibers around the blood
vessels enclosing thymic cells and (2) by the extension of
branches from these lamellae among the cells.
3. I n the very old glands white fibers may develop from the
thickened bundles among the cells.
4. Preceding the differentiation of fat cells within the thymus, there is a local proliferation of argyrophil fibers encapsulating the cells of the cortex.
5. The medullary arteries are most vulnerable during a g ~
involution. The atrophic changes a r e indicated by a thinning,
fragmentation, and final loss of the network beneath the endothelium and between the muscle cells.
6 . The suggestion is made that the argyi-ophil fibers of the
thymic vessels of the mouse are associated in their. origin with
both tlie endothelial cells and the fibroblasts.
LITERATURE CITED
COOPER,E. R. A. 1925 The histology of the more important human endocrine
organs at various ages. Oxford University Press, Edinburgh.
CORNER,G. W. 1920 On the widespread occurrelice of reticular fibrils produced
by capillary endothelium. Cont. Embryol. Carnegie Institution of
Washington, vol. 9, pp. 87-93.
146
C H R I S T I A K N A SMITH AND LOUISE 31. IRELAND
FUKUCHI,
K. 1928 Bber die Gitterfasern im Thymus unter besonderer Bcruchsiclitigung ihres Verhalteiis bei der Thymusinvolution. J. Orient. Med.,
vol. 8, German summary, pp. 67-68.
HALL, L. 1939 Histological changes in the thymus of the albino mouse during
age involution. Unpublished, Master's thesis, Mount Holyoke College.
HAMMAR,
J. A. 1921 The new virws as to the morphology of the thymus gland
end their bearing on the problem of the function of the thymus.
Endocrin., vol. 5, pp. 543-573, 731-760.
- ___
1927 O n the asserted non-existence of the age involution of the
thymus gland. Endoerin., vol. 11, pp. 18-24.
___.- 1936
Die normal morphologische Thymusforsrhung im letzten Vierteljahrhundert. Kapitel 1, V. Ergebnisse roil Silberfarbungen des
Bindegewebes der Thymus. S. 70-81.
JORDAN,
TI. E. 1928 The distribution of reticulum in the thymus. Anat. Rec.,
vol. 38, p. .50.
MARIXE, I). 1932 The thyroid, parathyroids and thymus. Vol. 3, section 22,
pp. 799-868. Special Cytology, ed. E. V. Cowdry, Paul R. Hoeber, h e . ,
New York.
N A X I M O ~ V , A. h.,A N D W. BLOOM 1930 A text book of histology. 1st ed. W.B.
Saunders Co., Philadelphia.
PAPPENREIXER,
8 . M. 1910 A contribution t o the normal and pathological
histology of the thymus gland. J. Med. Research, vol. 22, pp. 1-73.
SCHAFFER,
J., AND H. RARL 1909 Ijas thyreo-thymusehe System des Maulwurfs
und der Spitzmaus. Wiener Akademie Sitzungsberichte, MathematischNaturwissenschaftlic.Iieklasse, Bd. 118, Abt. 3, S. 217-263.
Scow, C. H. 1933 A critical study and rrview of the method of microincinera
tion. Protoplasma, rol. 20, pp. 133-151.
SXITH,C . 1938 Crllophaue used f o r projection drawings. Science, vol. 87, p. 586.
SiWTH, C., B. D. CONANT
A N D E. Q . SAYER1939 The vascular pattern of the
mousc thymus. Anat. Rec., vol. 73, Suppl. no. 2, p. 47.
STRAXDRERG, A. 1917 Zur Frage des intrathymischen Bindegewcbes. Anatoinische
Hefte, l t e Abt., 5.5, S. 171-186.
PLATE 1
F S P L A S k T I O N OF F I G r R E S
l'liymus glands of the mouse. Foot's modification of Bielschowsky 's stain
used throughout.
1 Newborn. Vein i n medulla with single layer of reticular fibers and a few
I-)ranches. X 430.
2 Seven days old. Cortex showing a thin capsular layer and the characteristic
:irrangement of reticular fibers of the septum and capillaries. X 430.
3 Twenty-one days old. Cortex showing the septum and capillaries. X 430.
4 Thirty-five days old. Medulla showing the doubling of the reticular fibers
in the wall of a large vein and a few scattered fibers. X 430.
5 Thirty-five devs old. Mrdulla. Photograph or" five serial sections projected
on cellophane and superimposed upon one another. Only reticular fibers were drawn.
On? large rein is shown and three venules. V = vein or rcnule. X 165.
KETIOUJAR FIBERS OW THYMUS OF MOUSE
CIIKISTI.\KXA S M l l ' H AYIb I A C I S Y Y. IROIANI)
I47
PLATE '7
ESPL.\N.ITION
OP FIGI'KES
Tliyuius glands of tlic iiioiise. Poot 's iilodificntio~i of Bielsr.lio\vsky 's stain
used tliroug1io:it.
S w days old. Re1)tuni :uid c*ortcss showing slightly tliickciicd capsular
layer of reticular fibcru and radial capillaries. X 430.
7 F i f t y s i x d a y s old. Iifeclulla sliowjiig oiic artery, with well devcloped siil)cndotlielinl layer of reticular fibers :Ind outer iiiuscular oiic, in upper riglit hand
region. Two reins :ire hclo\v. X 4.70.
8 F i f t y s i r d a p old. Medulla. Pliotogr:rph of six serial sections projected on
cel1oph:iiic superiiiiposed upoii oiie another, and a recoiistruct,ion of the lumens of
the vciiules added. Only rcticular fibers \vcrc! drawn. The posit.ioiis of two Hassal 's
corpuscles m'e sllowii b y c1i:ignn:il lines. X 165. V = vein or renule. H = IIassal's
corpuscle.
9 F i f t y s i x days old. Meclnlln showing vein wit.11 tlic typical double slieat.li of
reticular fibers mid it Hassnl 's corpuscle. X (i.13.
148
KETIOUJAR FIBERS OW THYMUS OF MOUSE
CIIKISTI.\KXA S M l l ' H AYIb I A C I S Y Y. IROIANI)
149
PLATE 3
EXPLAPihTIOK O F F I G U R E S
Tliyiiius glaiitls of the iiioustx.
used tliroughout.
Foot, 's lnodificatioli of Bielscllowskjr 's stain
10 Two hundred and fifty-sis days old. Medulla s h o ~ i n gveins with triple
sheatlis of reticular fibers. X 130.
11 TTTOhundred :ind fift
days old. Region of interlobular septum. Photograph of seven serial sections projected o n cellophane and superimposed upon one
another. Only reticular fibers were drawn. All fibers are located around reins;
aiid three reins are seen learing by an interlobular septum. X 16,:. S = interlobular septuiii. V = vein or vriinle.
1 2 Four hundred and seveuty six days old. Mednlla with abuudant ret,irnlar
fibers, mostly around tlie veins. Oblique section of one artery is seen. X 430.
A = artery.
13 Seven hundred a i d thirty-txro days old. Jfednlla with a lo~lgitudinalsection
of an artery iii the center in wliicli tlie subciidotlielial layer and the network between the musclo cells liare fragiiirnted. I n tlie sectiou of tlie snialler artery, above
: l i d to the right, the subendotlielial layer has disappeared. The outcr muscular
laper is intact in both. Fhdeiisire networks enclosing t h y i c cells arc seen outside
of tho artery. Msllorp-azan. X 215.
14 One hundred and seventy-one days old. C'ortex sliowing :I single row of fat
cells with a section of a capillary 1)etweeii tlieiii and the cortex. X 645.
15 One hundred and sixty-oils days old. Cortex shoiTing replnceincnt by fat
and new lohulation. Numerous capillaries are seen i n section. X 430.
16 One hundred aiid sixty-one d a p old. Cortex a t edge of region that is bcing
replaced by fat, shomiiig cells surrounded h y rcticular fibers. X 645.
I
1.50
KETIOUJAR FIBERS OW THYMUS OF MOUSE
CIIKISTI.\KXA S M l l ' H AYIb I A C I S Y Y. IROIANI)
P I L ~ T E4
EXPI..\N \TION 0)’ FIUI RES
l‘hymus glands of tlir nioiise. I’liotograplis taken I I 12.
~ Hall.
Eight huntlrrd and tveiit\-sia days old. Cortex being rrplaced by adiposcx
tissue. On the left, a b a y ; in tlie ceiitci, the edge of a lobule. Iron herrintoxyljii
and mange G . X 430. F = fat cells. B = region of light cells \vliich are encapsnlated by ieticnlar fibers. See figure 18. C = eiidotlielial cells of capillary between
cortex and lighter area. L = area of deiisely packed I~mphocgtes.
18 Eight hundred and twenty six dagq old. Same legion as nbore. Foot’s
modification of Bielsrhowsky ’s staiq and Mallory-azan. X 430.
li
152
KETIOUJAR FIBERS OW THYMUS OF MOUSE
CIIKISTI.\KXA S M l l ' H AYIb I A C I S Y Y. IROIANI)
c?
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fiber, distributions, mammal, birth, old, mouse, age, thymus, argyrophilic, studies
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