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The regeneration of transitional epithelium.

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Depaitnwnt of Zoolog!y, Uwiversity of Kentuck!l
A review of our present-day text-books of histology shows
histologists to be quite uniformly agreed that the regeneration of stratified epithelia takes place by mitosis in the deeper
strata, and that in consequence of the divisions of the cells
of the lower layers the cells lying above them are gradually
pushed outward where they become desquamated. All cells
of such an epithelium consequently occupy successively positions in each of the several strata during their life-history.
1ii most text-books this method of regeneration is described
for stratified epithelia in general, while others also mention
it specifically as the method of rebuilding for transitioiial
In reviewing the literature 011 the transitional epithelium of
the urinary tract, one may find suggestions of regeneration
in the deeper strata in literature antedating the work of Flemming 011 mitosis of tissue cells and before it was generally
lmown that this method of cell division played the important
rile that it does iii the rebuilding of the tissues. Thus Burckhardt(1) thought that the basal cells of this epithelium had
their origin in the underlying connective tissue and that these
hasal cells, in turn, by elongating and constricting off their
upper portion, produced the cells of the next more superficial
layer. The cells thus formed would then be pushed upward
by the next generation forming beneath them.
Linck( 2) held that there was no contiiiuous regeneration
after the epithelium was once formed. He considered the
9 N ~ ' l ' 0 3 5 I C A I i IIECORD, VOI,.
.S3, NO. 2
rriucosa to 1)c a composite of several epithelia aiid that tlie
tli flereiit strata originally were formed iiidependeiitly of each
Obei.steiiier(3) and l’aneth (5) eacli described fully the
bt ~ * i i c t u r c
of 1,hc epithelium uiider diflereiit caonditioiis, bal
(lid not allude to the method of regeneration.
Robiii and Cadiat ( 3 ) , thongh not, discussing this problem,
referred to the basal layer as the ‘regenerative layer.’
Hamhurger(6) also thought jt probable that the cells of
(he lowest layer originated in the imderlyiiig coiinectivc tissiie
aiid that these cells were in tiim responsible for the more
superficial ones.
At this lime, however, new light was thrown on the prohlem
of regeneration of animal tissues by the well-lsiiourn work of
Waltlier Flemming(7, 8, 10, 11, 12) on iiidil-ect cli,jsion of
tissue cells, and his observations on the epidermis of salamanc1ei.s aiid filially on that of man, and on the st~*atifjed
qclLlilmoLls epithelium of the oesophagus led him to conclude
that tlie regeneration of stratified epithelia occurred by
miiosis in Ihe deeper strata.
Pfitziie1*(9)noted that jn the stratified epithelium of blie
nriiiary tract of salamanders “Mitotic figures were especially
large aiid numei-ous and occurred in various strata.”
Obcrtlieck( 13) found that regeiierntion occurred by jndj1-ect (dell division of the cells of the third stratum, but did
iiot find mitoses in cells of any other layers. According to
him, the epithelium of the bladder consists of the first, or
superficial stratum; a sccoiid, stratum of several rows of
cdls, and a third, or basal stratum. Regeiieratioii, according
to him, then takes place by mitosis in the basal stratum (after
Flcmming(14) found amitotic division to be eommoi) i n tlic
cells of the bladder epitlielinm of salamanders, hilt noted thai
mitosis also occurred iii tlic deeper cells.
Dogiel(15), employing improved technical methods which
had been developed by the later workers, studied the liladcler
epithelium of white rats, mice, dogs, cats, and man. Jli all
of these lie found amitotic nuclear divisioii to be commoii in
the superficial cells. In the basal layer (his fourth layer)
he fouiid mitosis to be common, but rarely found figures iii
the middle strata. His conclusion was that cell division was
chiefly amitotic in the upper stratum, but that in the basal
stratum it was by mitosis with occasional figures in the central strata.
I’racticallp all of our well-knowii text-books of histology
state that regeneration of stratified epithelia talies place by
mitosis in the basal stratum or in the ‘deeper strata.’ I shall
list some of these statements. Some authors describe this
method specifically for transitional epithelium, others discuss it under the general heading of ‘Stratified epithelia.’
Lewis and S t o h r ( l 6 ) : “ I n stratified epithelia the basal
cells a r e nsually columnar and closely crowded. They multiply by mitosis and give rise to cells which a r e pushed toward
the free surface.”
Bohm, Davidoff, H u h e r ( l 7 ) : “It is clear that all cells of
a stratified epithelium cannot be equally well nourished by
the blood supply from the vessels in the highly vascular
connective tissue beneath. The middle and outer layers of
cells accordingly safTer. The deeper layers a r e much better
nourislied, and as a consequence their cells increase more
rapidly than those above ; they push outward, replacing the
superficial cells as fast as they die or a r e thrown off. The
proliferation of cells in a siratifid epj theliam occurs therefore, chiefly in the basal layers.”
Schafer(l8) : “In stratified epithelia the deeper cells multiply by karyokiiiesis. The newly formed cells tend as they
enlarge to push those superficial to them nearer the surface
from which they are eventually thrown off .” Of transitional
epithelium iii particular lie says : “The epithelium seems
to be renewed by mitotic division in the deeper cells.”
Schafier (19) : “ Origiiially mitosis occurs in the cells of
the upper strata a s well a s in those of the lower ones and the
division planes of the mitoses a r e at right angles to the upper
surface. I n the deeper strata they may also occur parallel
t o the surface. 111 this manner a stratified epithelium is
formed; then the divisioiis in the upper strata disappear, aiid
are found only in the basal layers. This applies to all stratified
squamous epithelia in the finislied state.” Again he says:
“The many cell -forms of stratified epithelia may easily he
explaiiied genetically ; cell division occiirs only in the deeper
strata by mitosis, and after the losses of successive superficial
J ~
upward. ”
layers are S ~ O W ~shoved
Szymonowicz and ICrause(20) : “Replacemeiit of cells in
stratified epithelia is brought about by mitotic divisions of
the cells of the lomest layer so that the lower strata are slowly
shoved upward and after each superficial layer is thrown off,
the next layer beneath it becomes the superficial one.”
Jordan(21), of transitional epithelium, says: “The cells
of the deeper layers divide by karyokinesis and push toward
the surface to replace the more superficial cells which are
gradinally desquamated. Direct cell division occurs in the
large plate-like cells of the superficial layer. ”
111 1924, Thuringer(22) observed iii the epidermis of man
that regeneration was accomplished not primarily by mitosis
in the basal layer or even in the deeper layers, but by an
active mitosis of the cells throughout the stratum spinosnm.
Desiring to determine if this method of regeneration applied
to other stratified epithelia, I decided upon the epithelium of
the urinary bladder of mammals and have used for the purpose the urinary bladders of five cats aiid five sheep.
11riiiary bladders of freshly killed aiiimals were fixed in
Kouiii’s or Zenli-er ’s fluid, embedded in paraffin, sectioned a t
I t o 10 p and stained with hematoxylin and eosiii. I n order
to secure rapid penetration by the fixing fluid, the muscular
coats of the sheep bladders were first removed. The sect,ions
were searched for mitotic figures, and when these were found
they were recorded as t o phase, plane of division, and stratum
i n which they were found. The thickness of the epitheliinm in
iinmber of cell layers m7as likewise recorded, the number of
layers being counted from the basal layer outward. No figures were listed from portions where the epithelium was cut
tangentially, nor from epithelium deep in the folds. The
usual thickness of section was 7 p, and was more favorable
than thicker sections, as it lessened the chance of error in
determining the position and the plane of the figures. Sections were mounted serially from different parts of the same
bladder and the number of figures found in the different parts
was later compared.
I n the normal mucosa of the contracted bladder of the cat
we find an epithelium of about five to six nuclear rows. The
basal or first layer is composed of small more o r less ciiboidal
cells consisting of a small amount of cytoplasm and relatively
large deiisely staining nuclei. I n the four or five middle
strata the cells become larger with more cytoplasm and larger
nuclei. These cells may have long slender processes extending downward between the more basal cells. I n the contracted
bladder of the sheep these central cells may be quite tall and
coiiical with a long, pointed basal end. The superficial cells
are usually large, and rounded on their free surface. They
frequently contaiii two or three vesicular nuclei. On the
lower surface of these cells are depressions into which fit the
upper rounded surfaces of the cells of the more superficial
central layer. As the bladder becomes stretched through
various degrees of distention the more superficial cells first
hecome flattened and then the central cells become drawii
dowii between the basal cells, so that the number of strata
becomes reduced.
I n going over the results of the study listed in the accompanying table we may consider first the epithelium of the
first three bladders used. The first was from a young halfgrown female, the second from a young but mature female,
and the third from a mature male. The epithelia were very
similar in appearance and consisted of five- to six-cell layers.
I n the first two proliferation was active, but in the third only
txveiity f i g i x e s could bc found in a I a ~ g enumber of sectiniis
takeii from various parts of tlie bladder. The epithelium of
each showed a large nurnbc~of divisioiis in the s t ~ a t aof
polygonal cells. In every case o\-er 50 pel. relit of ihe
mitoses were found in the upper half of the epitlielium.
'I'hc foiirth and fiffh cats were both youiig mature aiiim;ils.
'I'lie bladder of tlie fourth mas somewhat distended a d mas
fixed in this state. The epithelium consisted of four l a y ~ r sof
wlls and was nndergoing very active regeneration. This cpithelium, in spite of thc distention, showed the minimum iiumber of figures in the basal stratum. On the other liaiid, 60
p i * cent of the figures occurred im the polygonal cells just
;Lbove the basal stratum. The disteiitioii was not enough to
affect the number of cells in the basal layer, but did i*esult
i i i a much greater number in the second stratum, lienre Ilie
greater percentage of figures there. The fifth cat bladder
was distended by filling it with Bouin's fluid arid iminersiiig
it in the same fluid, This resiilted in a much larger number
of cells in the basal as well as in the second layer, and with
this is correlated the fact, that 53 per cent of the mitotic figures were f o u n d in the first layer aiid 40 per cent in tho
The epithelium of slieep Madders was v e i y favorable material f o r this study, a s in the contracted bladder the mucosa
i s thick and the strata are quite distinct. Rladders 1,2, 3, and
Unretouched I)hotoriiicroffraphs.
X 350.
Sections of urinary bladder.
Fig. 1 Trlophase and inctaphasc. in swond a n d t h i r d lavers, respectively.
Fig. 2
Fig. 3
Fig. 4
cat no. 1 .)
Telophase j i i third cell layer. (Bladder ('at no. 2.)
Telophase in second cell layer. (Bladder cat no. 4 ) .
Telophase in third cell layer. (Rladdcr sherp n o 3.)
4 were listed together as the material was similar in every
respect aiid because there was not ail active cell proliferation
in any one of them. The epithelia usually had a thickness of
five- to six-cell strata. The fifth sheep bladder was from a
newborn lamb, but the epithelium was completely formed. I t
had five cell layers and was active mitotically. Superficial
cells had already been desquamated, as attested by a number
found in the sections free from the epithelium. This epithelium showed a larger percentage of figures iii the upper strata
than did those of the mature sheep.
The results of this study are not in accord with the accepted views on regeiieration of transitional epithelium. The
study reveals that mitosis not only is mot confined to the cells
of the deeper strata, but that in all of the epithelia studied it
took place more actively in the cells of the middle strata. Due
to the fact that the cells of the basal stratum are smaller and
more crowded, about one-third of all the cells of the
contracted bladder epithelia were located here, leaving
two-thirds for all the strata above the first. This ratio applied to the contracted slieep bladders as ell a s to those of
the cat. I n the distended bladder one-half of all the cells were
located in the basal stratum. Consequently, if these cells
were more active mitotically than the larger cells of the middle layers, more than one-third of all the divisions in cont racted bladder epithelia should have been found there.
Furthermore, it would have been expected that the division
plaiie of the majority of mitoses in the first layer would be
parallel t o the surface. Instead of this, about 70 per cent
of tlie divisioii planes of these basal cells were at right angles
to the surface. Since the larger percentage of the divisions
was found in the larger polygoiial cells, these must be considerably more active in prolifei-ation tliaii the smaller basal
ones. A gradual upward movement t o the superficial layer
must be accomplished by the growth ;%liddivisioii of the cells
of all the layers and by the subsequent crowding thus prodnced by them.
The plane of division of the central cells of the epithelium
may be largely determined by the contraction o r distention
of the bladder. In the completely contracted bladder of the
sheep the epithelium showed that the cells of the central strata
were tall, with their nuclei at different levels due to crowding.
Here the long planes of the mitotic spindles were more frequently oblique to the surface. I n the bladder of the fifth
sheep the crowding was not so apparent, and consequently
the majority of the spindles were parallel to the surface.
1. Observations were made on normal transitional epithelia
of the urinary bladders of cats and sheep, the mitotic figures
were noted and recorded as to stratum in which they occurred
and as to the plane of their division.
2. Mitotic divisions in this transitional epithelium were en
countered regularly in all cells from the basal layer up
through the central strata, until the cells showed signs of degeneration in the most superficial layers.
3. I n all cases, with the exception of one, namely, that of
a much-distended bladder, a t least 50 per cent of the divisions
were found in the central strata.
4. The long axes of the mitotic spindles were most frequently so directed that the division plane of the cells was
a t right angles to the surface and not parallel to it, so that
the divisions do not cause, directly, a movement upward of
the more basal cells.
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