Comparative dental development and microstructure of ... - UCL

176 A. D. BEYNON ET AL.
(mesiobuccal cuspal times in molars) with
average perikymata counts made on the
buccal (incisors, canines and premolars) or
mesiobuccal (molars) cusps. In this way
estimates for canines and P 3 s, for example,
could be included, and a more realistic estimate
of the average lateral enamel formation
time for several teeth of each tooth type used
in the composite reconstruction. In the two
more complete sections of P. nyanzae it was
possible to estimate enamel formation times
in more than one cusp. It was also possible
to use daily lines in dentine to estimate
crown formation times for each tooth. These
data are presented in full together with those
for P. heseloni.
Root extension rates
Three things must be measured in order to
estimate the rate at which the crowns and
roots of teeth grow in length. (i) The daily
rate at which cells produce matrix. (ii) The
direction of cell movement and (iii) the
number of mature secretory cells active at
any one time (their rate of differentiation).
Shellis (1984) has expressed the ‘‘extension
rate’’ of teeth at the enamel–dentine junction
in the crown or at the cement–dentine
junction (CEJ) in the root mathematically.
In the equation c=d{sin I/tan D)cos I},
‘‘c’’ is the extension rate, ‘‘d’’ the daily rate
of dentine secretion, Angle ‘‘I’’ is the angle
the dentine tubules make with the root surface
and Angle ‘‘D’’ is the angle between an
incremental or accentuated line and the root
surface. These variables are illustrated with
respect to the root dentine of the P. heseloni
P 4 in Figure 7. The equation defines how
each of these variables can be used to estimate
the rate of tooth root extension. In
order to calculate the rate of extension of
tooth roots in Proconsul, three things need to
be measured from photomontages made
using high power reflected or transmitted
light images of tooth roots. These are: (i)
The amount of tissue secreted in a day
which is equivalent to the spacing between
daily lines in dentine, (ii) the direction of
travel of the odontoblast relative to the EDJ
or CEJ (which can be inferred from the
alignment of a dentine tubule) and (iii) the
angle that the active cell sheet subtends to
the EDJ (which is a reflection of the number
of active secretory cells). It was possible to
measure each of these variables in the dm 2
of the juvenile specimen and in the M 1 ,M 2
and P 4 of the adult Proconsul specimen.
Estimates of the rate at which roots
extended (the extension rate) were therefore
possible in these teeth, in more than one
position in some teeth.
Sequence of dental development
In order to reconstruct a chronology of
dental development in P. heseloni, the positions
of homologous accentuated lines in
each individual (that represent a single
event) were identified in ground sections of
both the adult and juvenile specimens. This
allowed the parts of teeth forming at the
same time in each individual to be crossmatched.
To provide additional evidence for
a sequence of dental development in P.
heseloni, linear hypoplastic markings, visible
on the resin replicas of all of the permanent
upper and lower teeth of the exquisitelypreserved
specimen KNM-RU 7290 were
studied across all teeth. On the basis of the
combined evidence from accentuated lines
in the ground sections and from the distribution
of linear hypoplasia in KNM-RU
7290, a sequence of tooth development was
proposed. Details of the histological procedure
for doing this in the ground sections
are detailed here.
Examination of the dm 2 ,M 1 ,I 1 and I 2
germs of the juvenile specimen revealed neonatal
lines in the dm 2 and M 1 that allowed
their dental development to be registered to
birth. An additional accentuated marking,
with a constant number of cross striations
between it and the neonatal line in the M 1 ,
I 1 and I 2 also allowed these teeth to be
securely registered with each other. Since