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Vascular turnover of a continuously growing organThe rat incisor.

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T H E ANATOMICAL RECORD 202:203-208 (1982)
Vascular Turnover of a Continuously Growing Organ:
The Rat Incisor
S. PITARU, Y. MICHAELI, AND G. ZAJICEK
Department of Oral Biology, School of Dentistry, Tel Auiu Uniuersity, Tel
Auiu (S.l?);and the Department of Anatomy and Medicine (EM.) and the
Hubert H . Humphrey Center for Experimental Medicine and Cancer Research
(G.Z.1, The Hebrew Uniuersity-Hadassah Medical School, Jerusalem, Israel
ABSTRACT
The vascular tissue supplying the pulp of the continuously growing and developing rat incisor was quantitatively analyzed in six female rats, weighing approximately 200 gm each. One hour after a single administration of 'H-TdR, 1
pCilgm body weight, the rats were sacrificed and the lower left jaws excised and
processed histologically. Every 20th section was evaluated for venular and arterial
count, endothelial cell count, and percentage of labeled endothelial cells. I t was
found that the proliferative capacity of the vascular tissue of the rat incisor is
confined to its most apical 3.0 mm-a zone defined as the vascular progenitor compartment. Daily production of endothelial vein cells and of veins within the progenitor compartment were calculated to be 7,392 cellslday and two veinslday, respectively. Generation time of these cells was found to be 3.56 days.
Endothelial cell production within the progenitor compartment provides for the
formation of new vessels and for the enlargement and elongation of existing ones.
The vessels originating within the progenitor compartment move in an incisal and
centripetal direction, while simultaneously undergoing a continuous process of aging, until their ultimate disintegration.
The proliferation of vascular tissue has been
intensively studied during the past decades.
Most reports on this subject deal with regenerating tissues (Clark and Clark, 1939;Cliff, 1963,
1965; Schoefl, 1963) or with the influence of
solid tumors or embryonic tissues on the
process of vascular proliferation and differentiation (Cavallo et al., 1973; Huseby et al., 1975;
Crim and Huseby, 1976; Folkman and Cotran,
1976; Ausprunk and Folkman, 1977). Little is
known, however, concerning the behavior of
vascular tissue in a continuously growing adult
organ, such as the rat incisor, one of the most
complex organs in the mammal. The continuous
attrition at the distal end of the incisor is balanced by formation of odontogenic tissues at its
basal end, deep in the bony socket, and their
continuous incisal migration. During their
movement the basal cells differentiate and produce tooth substance. Thecells migrate at a rate
of approximately 450 pmlday, equal to the rate
of eruption (Michaeli and Weinreb, 1968; Zajicek, 1976).
The pulp of the rat incisor is supplied by vessels that penetrate the tooth through a foramen
situated at its apical end (Fig. 1).The pattern of
these vessels, as represented in Figure 2, was
0003-276)(182/2022-0203$02.00 0 1982 Alan R. Liss. Inc.
described in detail by Moe et al. (1979).Generally, three principle rows of vessels are observed:
a centrally located row consisting of arterioles,
and two rows of venules located at the lateral
and medial sides. Many capillaries are located
peripheral to the principle clusters of vessels,
especially near the pulpodentinal junction.
Different studies (Ness and Smale, 1959;
Robins, 1968; Zajicek, 1976)have indicated the
possible proliferative capacity of the vascular
tissue. Recently, it has been suggested (Moe et
al., 1979)that the vascular tissue, like other tissues forming the continuously growing incisor,
is continuously renewed. However, a quantitative study of the process of continual renewal in
this particular type of vascular tissue has never
been undertaken. In the present investigation
this process is quantitatively estimated in order
to elucidate some of the properties of vascular
tissue supplying a continuously growing adult
organ.
Received April 30. 1981: accepted J u n e 10, 1981
Dr Sandu Pitaru's present address is Medical Research Council
Group in Periodontal Physiology. 4384 Medical Sciences Building.
Toronto, Ontario M5S 1A8 Canada.
204
S. PITARU, Y. MICHAELI, AND G. ZAJICEK
LABIAL
Fig. 1. Diagrammatic representation of longitudinal section of lower rat incisor, showing the apical foramen (AF). the
pattern of vein supply, and the boundaries of the progenitor compartment (PC). Bar A indicates approximate level of cross
section shown in Figure 2. d, Dentine; e, enamel.
MATERIAL AND METHODS
Six female albino rats of the Hebrew University strain, each weighing approximately 200
gm, were intraperitoneally injected with tritiated thymidine (3H-TdR,1.9 CilmM), 1 pCilgm
body weight. One hour later the animals were
sacrificed, and the left mandibles were excised,
fixed in Bouine-Holland solution, and decalcified in 10% EDTA. The mandibles were embedded in paraplast and cross-sectioned serially at 6
pm. The sections were dipped into Illford L4
emulsion, exposed for 3 weeks, developed in Kodak Dektol 19 for 3 minutes, fixed in Kodak
Rapid Fixer for 10 minutes, and stained with
hematoxylin-eosin. Starting from the basal end
of the incisor, every 20th microscopic section of
each left mandible was evaluated up to a distance of 3.5 mm.
The evaluation of each section consisted of
three procedures, performed for venules and arterioles separately: (1)mapping the position of
each vessel according to its location relative to
the labial side of the tooth; thus, every vessel
was ranked on a labiolingual axis (Fig. 2); (2)
counting the total number of arterioles and venules and the number of labeled and unlabeled
cells of each vessel; and (3)measuring the diameters of each vessel with the aid of a micrometer.
Using the data obtained, the following values
were computed for venules and arterioles separately in every evaluated section: (1)the total
endothelial cell count; (2) the percentage of labeled endothelial cells; (3)the perimeter of each
vessel (computed by assuming each vessel had a
perfect elliptical form);(4) the average endothelial cell count per vessel; and (5) the mean internuclear distance, which equals the average
diameter of an endothelial cell (computed by dividing the average perimeter of arterioles or
venules by the average endothelial cell count per
vessel).
The progenitor compartment of endothelial
cells was defined according to Zajicek (1976).
The total number of endothelial cells (venular or
arterial) in the compartment was computed using the formula:
n,
N =
f
n(x) dx
(1)
n,
where N represents the total number of endothelid cells in the progenitor compartment and n(x)
the number of endothelial cells per section at a
certain distance from the basal end of the rat
incisor.
Daily endothelial cell production was estimated by determining the average diameter (D)
of an endothelid cell in a transverse section of
the progenitor compartment; on the assumption that the same value is valid in a longitudinal direction, each endothelial cell progresses
daily a distance of S cells, such that:
S=
450 pm (Daily eruption rate)
D p n (average diameter of an
endothelial cell).
Since the total number of endothelial cells at
the distal limit of the progenitor compartment
VASCULAR TURNOVER O F RAT INCISOR PULP
LABIAL
205
in the progenitor compartment by the length (in
millimeters) of the compartment, and multiplying the result by the daily rate of eruption of the
rat incisor.
RESULTS
The results obtained for arterioles and venules were almost identical. Since the majority of
studies that have investigated the development, growth, and turnover of vascular tissue
have dealt with the arterial part, it was decided
to describe in the present study the venular
part.
The observed and calculated results obtained
for the venules are graphically depicted in Figures 3-7 in which the abscissa represents the
length of the tooth, and in Figures 8 and 9 in
a
0
0
0
0
0
7 4
15
10
@
5
0
Fig. 3. Graphic illustration of distribution of the percentage of labeled endothelial vein cells along sagittal axis of the
tooth. The abscissa represents the distance from the basal
end of the tooth. Each point represents the mean, and the bar
one standard error of the mean.
LINGUAL
Fig. 2. Diagrammatic representation of cross section of
lower rat incisor, showing general pattern of venules (v)and
arterioles (a) within the pulp cavity. Vessels are ranked according to their position on the labiolingual axis of the tooth.
d, Dentine; e, enamel.
equals n,(x),daily cell production P,,, is calculated as:
Pidl= S
n,(x)
(3)
Taking into account all endothelial cells comprising the progenitor compartment cycle and
the growth fraction G F = 1, the average turnover time t,,,equals the generation time of cells
in the compartment (Michaeliet al., 1978);thus,
The daily production of veins was computed
by dividing the total number of veins produced
Fig. 4. Graphic illustration of progressive increase in total endothelial vein cell count per section along sagittal axis
of the tooth up to 3 mm from its basal end, the distal limit of
the progenitor compartment. Dotted area beneath curve represents the total number of endothelial vein cells found in the
progenitor compartment. Each point represents the mean,
and the bar one standard error of the mean.
S. PITARU, Y. MICHAELI, AND G. ZAJICEK
206
1
5
1
2
3
5
mm
Fig. 7. Graph illustrating mean number of endothelial
cells per vein along the sagittal axis of the tooth. Each point
represents the mean, and the bar one standard error of the
mean.
Fig. 5. Graph depicting mean internuclear distance of endothelial vein cells along the sagittal axis of the incisor. Average distance in the middle of the progenitor compartment is
approximately 13.4 pm.
1
2
3
mm
Fig. 6. Graphic illustration of mean number of veins per
section along sagittal axis of incisor. The increase in the number of veins per section levels off at the distal limit of the
progenitor compartment. Each point represents the mean,
and the bar one standard error of the mean.
Daily production of vein cells
The curve in Figure 4 reveals that the total
vein cell count per section increased in almost
linear progression throughout the progenitor
compartment, from 35 cells per section at its
proximal limit to 220 cells per section at its distal boundary. Further distal to this point the
total vein cell count leveled off. The internuclear
distance of the vein cells also increased progressively (Fig. 5), from 11.4 pm at the proximal end
of the progenitor compartment to approximately 15.2 pm at the distal border. The average
diameter of a progenitor endothelial cell was approximately 13.4 pm.
According to equation (2), each endothelial
cell progresses a distance S of 33.6 cells. Using
formula (3), the daily vein cell production P,,,
was found to be 7,392 cellslday.
Total cell count of the progenitor
compartment
The total cell count of the progenitor compartment N is represented graphically by the dotted
area in Figure 4. Using equation (l),N was
which the abscissa represents the ranking num- found to equal 26,305 cells.
ber of venules on the labiolingual axis of the rat
Generation time evaluation
incisor.
The great majority of labeled endothelial vein
Based upon the value determined for N and
cells was noted within the most apical 3.0 mm of for P,,,and by using equation (4),the generation
the basal end of the rat incisor (Fig. 3). The per- time was estimated to be 3.56 days. Thus, the
centage of labeled cells increased gradually in growth process requires that each endothelial
successive sections from the basal end and cell in the progenitor compartment reproduce
reached a peak at a distance of approximately every 3.56 days.
1.3 mm. It thereafter decreased, reaching the
Daily production of veins
lowest value a t a distance of 3.0 mm from the
basal end. From this point on the number of
The number of veins per section increased
labeled cells was negligible. This part of the throughout the length of the progenitor compulp, within which endothelial cell proliferation partment and levelled off at its distal end (Fig.
takes place, is defined as the progenitor com- 6). A total of 13.3 veins are produced throughpartment of the pulp vascular tissue.
out the 3.0-mm length of the compartment, a
207
VASCULAR TURNOVER OF RAT INCISOR PULP
mean of 4.4 veinslmm (13.313). Since the eruption rate of the incisor is 450 pmlday, vein p r o
duction is 4.4 X 0.45, or two veinslday. The
mean number of endothelial cells per vein (Fig.
7) increased progressively throughout the vascular progenitor compartment and leveled off at
its distal end.
The mean venular perimeter increased linearly throughout the compartment, reflecting
the increase in the number of endothelial cells
and in their diameter.
The mean cell count of all venules with identical ranking numbers was determined for all the
evaluated sections (Fig. 8).The results indicate
a relation between the position of a vein on the
labiolingual axis of the tooth and its cell count;
that is, the more centrally located the vein, the
greater the number of cells that comprise it.
The mean percentage of labeled vein cells of
all venules with identical ranking numbers was
computed in a similar manner. The results were
divided into three equal groups representing the
labial, central, and lingual zones of the tooth.
The results of each group, represented in Figure
9, were subjected to an analysis of variance and
were found to differ statistically at F = 0.025. It
is evident that the more centrally located veins
have a lower ratio of labeled cells.
I
1
2
3
4
5
6
7
8
9
Fig. 8. Graph illustrating change in mean cell count per
section of all veins with identical ranking numbers, according to their position along the labiolingual axis of the tooth
(see Fig. 2). The abscissa depicts the labiolingual axis, the
numbers represent the ranking numbers of the vessels. The
vein ranked No. 1 is the most labial. Each point represents
the mean. and the bar one standard error of the mean.
DISCUSSION
The present study indicates that the vascular
tissue supplying the rat incisor pulp is endowed
with proliferative properties which are confined
within the most apical 3.0 mm of the pulp-a
zone defined as the vascular progenitor compartment.
The new endothelial cells originating in this
compartment are utilized in different ways.
They contribute to the formation of new vessels
and to the enlargement of existing ones, as deduced from the progressive increase calculated
in the number of veins and in the number of
endothelial cells per vein throughout the progenitor compartment (Figs. 6 , 7 ) . Moreover, the
continuous eruption of the rat incisor displaces
the tissues produced at its basal end incisally, a
process requiring continuous elongation of the
supplying vessels and, obviously, the formation
of additional new endothelial cells.
Careful consideration of the data reveals that
the procedure described herein is a course of related events, all part of an overall process of
continuous renewal of the vascular tissue supplying the rat incisor. The continuous elongation of the vessels at their base enables migration of the older parts in an incisal direction.
Therefore, the more incisal the location of a
1
2
1
3
GROUP
Fig. 9. Histogram showing mean percentage distribution
of labeled endothelial vein cells arranged in three groups: 1
= three most labial vessels; 2 = three central vessels; 3 =
three most lingual vessels. Bar represents one standard error of the mean.
given segment of a vessel, the older it is. Since
the number of cells per vein (Fig. 7) also increases in a basoincisal direction, this value may
be used as an indirect index for estimating the
age of a vein at a given distance from the apex.
The curve in Figure 8 describes the relation
between the number of cells per vein and the
208
S. PITARU, Y. MICHAELI, AND G. ZAJICEK
location of the vein on the labiolingual axis of
the tooth. I t is evident that the higher the cell
count per vein-and consequently the older the
vessel-the more centrally the vein is located. It
can therefore be concluded that new vessels are
formed within the vascular progenitor compartment at theperiphery of thepulp, functioningas
suppliers for newly formed segments of the
Pulp.
Further evidence for this is provided by the
data in Figure 9, which indicate a higher percentage of labeled cells in the peripheral group
of veins than in the centrally located veins, a
fact in accordance with the low proliferative capacity of mature endothelial cells (Folkman and
Cotran, 1976).
As mentioned above, the continuous eruption
of the tooth displaces the young tissue-vessel
complex in a distal direction. Since the pulp is
cone-shaped, tapering at its incisal edge, concomitant with the vessel's incisal migration is
its centripetal displacement. Two simultaneous
and related events are therefore characteristic
of the vessels supplying the rat incisor pulp: bidirectional displacement accompanied by continuous aging.
Since the tooth is in a steady-state condition,
the same amount of vessels formed must be destroyed. The most centrally located vessels are
the oldest, so they are expected to disintegrate
together with the tissues they supply, now located in the most distal part of the tooth.
In a histologic study on the arterial part of the
vascular tissue supplying the pulp of the upper
rat incisor, Moe et al. (1979)also suggested the
continuous cycling of the arterial bed, thus providing further evidence for this concept. The
distribution of mitotic figures observed in arterial endothelialcells was generally similar to the
distribution of venular and arterial labeled endothelial cells found in our study. However, in
Moe's study the mitotic figures were observed
in arterial endothelial cells also in the middle
and incisal parts of the pulp. Since the number
of mitotic figures observed is not reported, a
comparison between the two findings is not possible.
The histopathologic findings of regression
and degeneration observed by Moe et al. (1979)
in the centrally located arterioles supplying the
most incisal part of the tooth confirm our kinetic findings regarding the relation between
the location of a vessel and its age.
The present study reveals that vascular tissue supplying the rat incisor has a progenitor
compartment, like all other tissues forming the
rat incisor (Ness and Smale, 1959; Smith and
Warchawsky, 1975; Zajicek et al., 1979). This
may serve as a good model for further investigation concerning vascular tissue proliferation,
differentiation, maturation, and disintegration.
ACKNOWLEDGMENTS
The authors wish to express their gratitude to
Prof. M.M. Weinreb for his helpful remarks during the preparation of this manuscript.
The authors thank Misses Sylvie Mayer, Jardena Mazor, and Gloria Ganzach for their help in
preparing this manuscript.
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