close

Вход

Забыли?

вход по аккаунту

?

The growth pattern and daily rhythm of the incisor of the rat.

код для вставкиСкачать
THE GROWTI3 P 9 T T E R N AND DAILY RHYTHM O F
THE: INCISOR O F THE RAT1
ISAAC SCUOUR AND SHERWOOD R. STEADMAN
Department of Histology, University of Illinois, College of Dentistry, Chicago,
I1Zinok
FOUR FIGURES
INTRODUCTION
There is an increasing number of experimental studies in
which the incisor of the rat is being used as a test object.
This fact calls for a better appreciation of the morphology and
physiology of this tooth type and for emphasis on dynamic
rather than static aspects.
THE FORNATIVE EPITHELIUM
The rat incisor grows continuously throughout the life of
the animal. It develops primarily from an epithelial base
which is structurally and functionally different on its labial and
lingual aspects. On the labial and slightly overlapping lateral
surfaces, this base resembles in structure and function the
enamel organ of the human tooth. On the greater part of the
lateral surfaces and on the lingual it resembles Hertwig’s
epithelial sheath. On the labial aspect, t h i s epithelial base is
situated more proximally and inferiorly than on the lingual
aspect. The various portions of this epithelial a d a g e differentiate into ganoblasts (ameloblasts) or stimulate the formation of dentin and possibly that of cementum. The term
‘odontogenic epithelium’ may therefore be used to denote this
germinal tissue.
This investigation vas aided in part by a grant from the Graduate School
Research Board of the University of Illinois.
325
TIIE ANATOMICAL RECORD, VOL. 63, NO. 4
NOVEMBBE.
1935
326
ISAAC S C H O U R A N D SHERWOOD R. STEADMAN
The proximal budding base of the incisor in cross section
approximates an elliptical egg-shaped outline as shown in
figure 1, A. If these cells grew along the entire proximal
border in a distal direction at a uniform rate there would develop a straight tube (fig. 1, B). The wall of such a tube
would then consist of epithelial cells which enclose the connective tissue. The boundary between the epithelium and the connective tissue would represent the future dentino-enamel ( d e )
and dentino-cementa1 ( d c ) junctions (fig. 1, B). The connective tissue which is surrounded by the epithelial cells is comparable to the dental papilla of the human tooth germ and
represents the future pulp. Since the cellular activity is most
rapid on the labial surface, the straight tube becomes curved
as indicated in figure 1, C. This curved tube is hollow in the
diagram throughout its length and thus diflers from the rat
incisor which is hollow only at its proximal end but is almost
solid at its distal end (fig. 1,D and E).
THE DENTIN PATTERN
The peripheral cells of the connective tissue that fills the
curved epithelial tube become columnar and are called odontoblasts. Not only do they migrate toward the distal end of
the tooth but at the same time they recede centrally, i.e.,
toward the pulp (fig. 1, D).
Direction a d rate of moveme& of odontoblasts. The
pupal recession of a given odontoblast is in proportion to the
amount of dentin that is laid down. Experimental studies
have shown that dentin is laid down at the rate of 16 p in 24
hours (Schour and Smith, '34). The forward movement of
the odontoblasts is in proportion t o the rate of eruption,
which is about 2 mm. a week in the upper incisor.
Thus in a mature rat, an odontoblast 5 days old has moved
approximately 80 p (5 x 16 p ) centrally to position 5 p (fig.1,
D ) and about 1500 ~1 (5 X 300 p) anteriorly to position 5a.
This figure shows the course and position of a given odontoblast or a given point at the dentino-pulpal surface at various
times, eg., 5, 10 and 30 days after its origin a t the base ( 5 p ,
GROWTH PATTERN AKD RHYTHM OF RAT INCISOR
327
Fig.1 Schematic description of t,he pattern of the upper incisor of t ie rat;
bm, beginning of enamel matrix; c, cementum; dc, dentino-cements1 junction;
dc, dentino-enamel junction; e, enamel; fm,full width of enamel matrix; i, incremental lines in labial dentin; ,'i incremental lines i n lingual dentin; p , labial
surface of pulp; p', lingual surface of pulp; pc, pulp chamber; ps, pulpal slit in
distal portion of tooth.
328
ISAAC SCHOUR AND SHERWOOII R. STEADMAN
JOp, a d 3 0 ~ ) .The number of days was chosen arbitrarily to
indicate the given positions.
T h e daily 16 p rhythm in. the normal incremeiatal stratification.. A histologic study of the dentin and enamel of the incisor of the rat reveals an incremental stratification in form of
a succession of pairs of dark (well calcified) and light (less
calcified) layers. The width of each pair approsimates very
closely 16 p (fig. 4,D, and D3). An analysis of this stratification based on 3999 micrometer measurements of enamel and
dentin of man and other mammals is given in a separate report by Schour and Hoffman ('35).
The significance of this constant rhythm is indicated by the
fact that it is quantitatively identical with the amount of
dentin or enamel laid down in 24 hours. It appears therefore
that the 16 p stratification rhythm is a constant 24-hour
physiological phenomenon.
There arc indications that the dark and light components of
each pair of stripes are in turn diurnal and nocturnal phenomena. Further studies are being made to investigate this
phase of the problem.
The applicat,ioiz of our knowledge of the incremental stratificatiolz i+z, hist dogie aizalysis. The uniformly rhythmic
process of growth and migration of dentin and enamel gives
us an accurate chronological record which facilitates histologic
analysis (Erdheim, U). Thus in sagittal sections of the incisor of a rat killed at a given time, a given layer of dentin is
situated next to the pulp or midway in the dentin substance,
o r near the dentino-enamel or dentino-cementa1 junction depending on whether this dentin stripe was laid down approximately 5, i 5, 20, i 20, o r 40 days, i 40, before the death of the
animal (fig. 1, E and fig. 2). The extent of the record in any
given tooth is limited insofar as the incisor of the adult rat
erupts and wears off as fast as it grows and renews itself
about every 40 to 50 days. Similarly in transverse sections,
as in figure 1,F and G, cut at levels indicated in figure 1,E, the
dentin shows stratification in form of concentric rings, i 5, i 10,
i 20, and i 40, at various distances from the pulp. This process
GROWTH PATTERN
AND RHYTHM
OF RAT JNCISOR
329
can be seen more clearly in material obtained by vital staining
with alizarin or by injections of sodium fluoride (Schour and
Smith, '34) (fig. 4, D2). The width between two successive
experimental rings indicates the amount of dentin laid down
during the interval between two successive injections.
The caZcificatioPz rhythm. The calcification of the dentin
proceeds in a rhythmic manner similar to that of formation.
The dentin matrix is calcified in the form of globules that are
normally small and numerous and so close together that there
results a uniformly calcified tissue. But even in the normal
dentin the successive layers of dentin are not equally well
calcified. Well calcified layers alternate more o r less regularly and rhythmically wit,li less calcified layers so that there
arises a distinct stratification in the dentin, especially toward
the distal end.
The shape of the distal end of the incisor. The continuous
apposition of dentin in form of layers which move both
distally and centrally (pulpally) would naturally result in an
increasing approximation of the pulpal surfaces of the dentin
in the distal portion of the tooth. The distal end actually
tends to become entirely solid. Rut the activity of the odontoblasts slows down and practically stops in the most distal portion of the tooth so that normally a very narrow pulpal slit
persists and extends to the distal or incisal surface (fig. 1, E
and G ; figs. 2 and 3).
I n the normal presence of an opposing functioning incisor,
the distal end of the incisor is worn off so that it assumes the
shape of a sharp bevel as seen in figure 1, E, and figures 2 and
3. I n the adult rat, the rate of wear and the rate of eruption
are the same so that the tooth remains constant in size.
THE ENAMEL PATTERN
In a manner somewhat similar to that of the odontobIasts,
the ganoblasts (ameloblasts) move forward at a rate equal t o
that of eruption and recede labially with the apposition of
organic enamel matrix. The apposition of the latter proceeds
similarly to that of the dentin matrix, at the rate of 16 p per
330
ISAAC SCHOUR AND SHERWOOD R. STEADMSN
24 hours. Unlike the dentin, the enamel epithelium begins its
recession later than the odontoblasts because the organic
enamel matrix begins to form only after the first dentin is
laid down (fig. 1, E, b m ) . Furthermore, the recession of the
enamel epithelium stops when the enamel matrix has reached
its full and limited thickness, approximately 128 p (fig. 1, E,
fd.
The growth pattern in the enamel thus follows the same rate
as that in the dentin but is of more limited extent.
Fig. 2 Geometric construction of the incrementaa pattern of the hard tissues o f
the upper incisor of the rat. The drawing o f the longitudinal view employs a
single center, 0. Legends same as in figure 1.
CEhlENTUM
We cannot agree with Addison and Appleton (’15) who
state that “in the rat, it is not apparent that there is any
cementum at all.” The cementum is laid down along the
lingual, mesial, and distal surfaces at an apparently very slow
rate since normally its thickness gradually increases to a
width of 3 t o 4 p toward the incisal end (fig. 1, E, c ) .
GROWTH PATTERN AKD RHYTHM OF RAT INOISOE
331
INCRENENTAL P-4TTERL- O F BOTH ENAMEL AND DENTIN
Figure 2 gives a diagrammatic sketch of a midsagittal section of the entire tooth. It shows the forty daily incremental
lines that are found in the dentin in an incisor 40 days of age.
This diagram consists of lines wbich were drawn from a single
center, 0, in contrast to the diagram of Erdheim ('11) which
was based on three centers. The tlentino-enamel and dentinocementa1 junctions and the boundaries of the pupal slit are
arcs of circles drawn from this cenler. In addition, the incremental lines representing the daily deposition of enamel and
dentin are Archimedean spirals employing this same center, 0,
as an axis. Because of the more technical and mathematical
aspects involved, a detailed description of the construction of
figure 2 is not given here. The mathematical formiila and
detailed analysis are given and eqlained in another report
( Steadman and Schour, '35). T t may be pointed out, however,
that an Archimedean spiral is one which increases (or decreases) its radius vector at a constant rate, which in the rat
incisor is tlie daily rate of apposition of enamel and dentin,
namely 16 1-1 ; furthermore, this radius vector must rotate
about its fixed axis at a constant rate which in the upper incisor of the r a t is about 2 mm. per ~ e e k .
Figure 2 also represents the transverse view showing the
rings of dentin which remind one of the rings of the tree.
While the latter are in seasonal and annual sequence, the
enamel and dentin rings in the incisor of the rat are in daily
rhythm.
AGE CIIANGES
The ellipse assumed by the building base, figure 1, A,
normally enlarges in proportion to the growth of the animal
until in the adult rat whcn the final size of the tooth has been
reached, the ellipse has ceased to enlarge further, and remains
practically constant in size.
332
ISAAC SCHOUR AND SHERWOOD R. STEADMAK
Figure 3 shows a photomicrograph of a midsagittal section
of the upper incisor of a normal rat 65 days old. The iiicisor
is still growing. This is evidenced by the fact that the labiolingual width is greater in the proximal portion than in the
distal (incisal) portion.
LITERATURE CITED
w. XI. F.,A N D J. L. APPLETON1915
The structure and growth Of the
incisor teeth of the albino rat. J. Morph., vol. 56, pp. 43-96.
ERDHEIM,
J. 1911 ober die Dentinverkalkung im Nagezahn bei der Epithelkorpercheritransplantation. Frankf. Zeit. f. Pathol., Bd. 7, S.
295-342.
SCHOUR,I., AND M. C. SMITH 1934 The histologic chaiiges in the enamel and
dentin of the rat incisor i n acute and chronic experimental fluorosis,
Univ. of Arizona, Agri. Exp. Sta. Technical Bulletin, no. 52, June 15.
SCIIOUR,
I., AND M. I&. HOFFMAN1935 Demonstration of a 16 p rhythm in the
stratification of enamel and dentin in man and other mammals. J.
Dent. Res. (In press).
STEADUAN, 8, R., AND 1. SCHOUR 1935 h geometric construction of the incisor
of the rat. J. Dent. Res. (In press).
ADDISON,
Fig. 3 Photomicrograph of a midsagittal section of an upper incisor of a
normal rat 65 days old. X 8.2. h'ote the extension of the pulp t o the distal edge
and the greater labio-lingual width in the proximal than in the distal portion.
AZ. b., alveolar bone; En.ep., enamel epithelium; En.sp., space formerly occupied
by enamel lost in decalcification. P.d. ni., periodontal membrane.
Fig. 4 Photomicrograph of a transverse ground section of lower incisor of a
rat which was given four injections of 2.5 per cent sodium fluoride 48 hours apart
and killed 14 days later. X 45. To the left, the enamel covers the normal dentin,
D,, which was formed before the experiment began. Note dentin, D,, which
was formed during the experiment and which consists of four pairs of light and
dark layers, each pair 32 in width. Note the dentin, D,, formed during the 14
days that intervened between the last injection and death. This zone, D,, is
characterized by a stratification consisting of fourteen pairs of light and dark
increments, each pair 1 6 p in width. This illustration is taken from University
of Arizona Technical Bulletin no. 52 (Schour and Smith, '34).
T H E ANATOXIICAL RECORD, VOL. 6 3 , NO. 4
Документ
Категория
Без категории
Просмотров
0
Размер файла
528 Кб
Теги
daily, patterns, growth, rat, incisors, rhythms
1/--страниц
Пожаловаться на содержимое документа