Comparative dental development and microstructure of ... - UCL

DENTAL DEVELOPMENT IN PROCONSUL
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Table 5
Proconsul root extension rate data
Tooth
Angle I
(degrees)
Angle D
(degrees)
Daily rate (d)
(μm/day)
Extension rate
(μm/day)
dm2 112 4·5 2·9 34·8
M1 (apical 1/3) 107 16 3·5 12·7
105 8 2·0 14·3
98 7 2·0 16·4
M1 (apex) 105 10 3·2 18·1
100 5·5 1·9 20·4
P4 (cervix) 115 20 2·1 6·1
126 16 1·8 6·1
108 15 1·7 6·5
112 15 1·8 6·8
M2 (cervix) 91 14 1·6 6·4
101 15 1·6 6·2
M2 (apex) 115 7 3·3 25·7
Data used to calculate root extension rates using the formula from Shellis (1984)
described in the text. All angular measurements and spacings between daily lines were
measured on montages constructed at 500 original magnification. The daily rates
that appear here are the mean values for as many measurements as possible in each
field of view under consideration and do not correspond to those given for different
regions in Table 4.
the measurements required to estimate
extension rates in the same fields of view.
Importantly, in the M 2 and P 4 it was possible
to make these measurements in two
widely spaced positions in the root. Close to
the enamel cervix and also low in the apical
third of the root. The measurements and the
calculated extension rates appear in Table 5.
Extension rates in the cervical third of the
M 2 and P 4 were on average 6·5 μm per day. In
the apical third of the root they were on average
14·5 μm per day and close to the apex
21·5 μm per day. The completed root lengths
for these teeth are not all known but are
approximately 7 mm in the P 4 and 8 mm in
the M 1 and M 2 . Given the state of preservation
of these teeth and the fact that there is
cementum deposition apically, these root
lengths are only likely to approximate the true
lengths at apical closure. However, they do
allow some broad estimates of the time taken
to form roots. Root extension rate in the dm 2
is estimated at 35 μm per day. This is equivalent
to 200 days for a tooth root of just under
6 mm long. Deciduous teeth contain incremental
lines with a constant angulation to the
root surface from cervix to apex implying a
constant extension rate. However, permanent
teeth in Proconsul begin to form roots slowly
and then speed up towards apical closure.
The times for root formation can therefore be
derived in different ways. If half the root
formed at the slower rate and half at the faster
rate, root formation would have taken around
2·4 years in the premolar and molar teeth. If
the middle third of the root is assumed to have
formed at an intermediate rate of 10·5 μm per
day then the sum of the time for each third of
root formation is equal to 2·3 years.
Sequence of dental development
Figure 14 is a diagrammatic summary of
histological evidence for the sequence of
tooth development in P. heseloni. Table 6
is a summary of all the crown formation
times and root formation times calculated
using the different approaches adopted in
this study. In addition to these there are
three other lines of evidence that provide
information on the sequence of dental