Mandible measurements are available. Data collection is
ongoing; more data are expected.
Sampling procedure
Ten males and ten females of each strain (except SPRET/EiJ
where only females were tested) at 48 - 64 days of age (57 ± 4 days)
were accessioned and assigned a unique randomized
identification number. There was no period of
acclimatization. Upon receipt or within 24 h each mouse was
euthanized using CO2 gas and weighed (± 0.1 g)
and that data recorded along with date of
birth and age (days) at the time of euthanasia.
Measurements were performed on samples in ascending numeric
order and each measurement was taken at least twice and a
third time if the differences between measurements were
>10%.
Skull preparation
Following euthanasia, the heads were removed, cleaned of
skin, fur, loose musculature, and the tongue was removed.
Radiographs were taken prior to bleaching which required
soaking the head in 1-2% sodium hypochlorite (5.25% sodium
hypochorite diluted in 0.9% NaCl) for 14-16 h at room
temperature. The cleaned skulls were thoroughly rinsed in
fresh 0.9% NaCl and allowed to air dry (2-3 d) prior to
varnishing with clear polyurethane spray. The foramen magnum
of each bleached, varnished skull was filled with Protech
brand glass ionomer band cement (Ormco Corp., Glendora, CA).
A lingual button (G&H Wire Co., Greenwood, IN) was then
glued to the most posterior aspect of the occiput and used
to mount the skull to a vise. Mounting the skulls to a
moveable vise permits optimum and consistent orientation of
each skull for measurements. The processing of the skulls
results in disarticulation of both hemi-mandibles. The hemi-mandibles
were varnished as well with no other
modifications.
Mandible Analysis
Fifteen variables for each hemi-mandible were analyzed (see
Table 1
). These variables were
chosen a priori from many possibilities in order to
represent a reasonable mix of non-redundant measurements
from all parts of the hemi-mandible (see Figure 1
). Most variables are
Euclidean distances between recognizable landmarks and
others are defined by the area between specified landmarks.
Each hemi-mandible was placed buccal side up along with a
scale bar (stainless steel rectangular arch wire cut to
length and measured to ± 0.01 mm using a micrometer).
Hemi-mandibles were imaged using a Leica GZ6 stereomicroscope
equipped with a Nikon DX1200 digital camera and Dell
computer workstation. The digital images were acquired at
3600 by 2880 pixels and saved as high-resolution jpeg files
(72 pixels per inch at 50 x 40 inches image size, or 300
pixels per inch at 12 x 9.5 inches image size). Images
generated from hemi-mandible samples were subjected to
digitizing using Didge software, (Image Digitizing Software,
version 2.20) a digitizing program written and designed by
Alistair Cullum (Department of Biology, Creighton
University, Omaha, NE) to help quickly mark and record
coordinates on a series of images. (This program was
developed originally for the study of animal locomotion.)
Selected points, as pixels, were identified as landmarks and
the (X,Y) coordinates were then transferred to a Microsoft
Excel spreadsheet. Landmarks were digitized at least two
times. Basic geometry formulae were applied to determine
distances and areas. Each suite of variables was determined
in duplicate and the mean for each hemi-mandible submitted
(data for hemi-mandibles are available in the supplemental data set).
Measurement averages for right and left hemi-mandibles were
computed for each mouse and are used in the project data set.
Table 1. Mandible Variables
| short name | description | units |
definition using landmarks
(see Figure 1) |
| ant_length | mandible, infradentale to menton | mm | length 4 - 6 |
| post_length | mandible, menton to gonion | mm | length 1 - 4 |
| mid_length | mandible, infradentale to posterior ramus | mm | length 6 - 20 |
| mid_height | mandible, posterior molar to menton | mm | length 4 - 11 |
| condyle | mandible, condylar process | mm | length 15 - 18 |
| incisor_condyle | mandible, infradentale to posterior condyle | mm | length 6 - 18 |
| incisor_gonion | mandible, infradentale to gonion | mm | length 1 - 6 |
| molar_condyle | mandible, posterior molar to posterior condyle | mm | length 11 - 18 |
| molar_arch | mandible, anterior molar to posterior molar | mm | length 10 - 11 |
| incisor_region | mandible, diastema to pogonion | mm | length 5 - 9 |
| ramus_height | mandible, antegonion to sigmoid | mm | length 3 - 14 |
| ant_area | mandible, anterior area | mm2 | area 3, 4, 5, 6, 7, 8, 9, 10, 11 |
| post_area | mandible, posterior area | mm2 | area 1, 2, 3, 11, 14, 15, 16, 17, 18, 19, 20 |
| teeth_space | mandible, concavity of incisor to anterior molar | mm2 | area 8, 9, 10 |
| ramus_space | mandible, concavity of posterior ramus | mm2 | area 1, 19, 20 |
Figure 1. Comparison of mouse and human mandibles.
Upper panel: buccal view of mouse left hemi-mandible with landmarks 1-20 indicated
Lower panel: human cephalometric tracing with selected corresponding landmarks
1 = goniona
2 = inferior gonion
3 = antegonion
4 = mentonb
5 = pogonion
6 = infradentalec
7 = incisor alveolus, dorso-posterior point
8 = incisor process
9 = diastema, posterior point
10 = first molar alveolus, dorso-anterior point
11 = molar, posterior
12 = coronoid process, dorsal point
13 = sigmoid, posterior point
14 = sigmoid, ventral point
15 = condylar process, dorsal point
16 = (landmark not named)
17 = condylion
18 = condylar process, ventral point
19 = (landmark not named)
20 = posterior ramus
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Notes:
a mouse, posterior point of angular
process; human, midpoint of coutour
connecting ramus and body of mandible
b mouse, posterior tuberosity of
insertion site of mandibular transverse
muscle
c human, termed 'B point'
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Craniometric Analysis (data not yet available)
Thirteen paired variables and one midline variable for each
skull were analyzed. These variables have been shown to
discriminate face shape in a study involving genetic
determinants of oral clefting in mice (Everett et al.,
unpublished). All variables are Euclidean distances between
recognizable anatomic landmarks. All measurements were
performed using a Leica GZ6 stereomicroscope equipped with a
calibrated reticle in the micrometer scale. The suite of
variables were measured in triplicate.
Radiography (data not yet available)
Radiographs (either dorsal to ventral or cephalogram with
left side of head facing up) were performed using an in-
house designed cephalostat to maintain consistent
orientation to the film. Heads prepared for bleaching were
placed onto an individually wrapped size 2 Kodak ekta-speed
plus periapical dental film. Each head was subjected to a
voltage output of 18 kV (endpoint energy) with an exposure
time set for 24 s using a Hewlett-Packard Faxitron B Model
43855-A02 (Hewlett-Packard-McMinnville Division,
McMinnville, OR) with a film to source distance of 61 cm, X-
ray tube current 3 mA, with continuously variable voltage
output 10-110 kV (endpoint energy). (We have determined
that the image on the periapical dental film corresponds to
nearly 1:1 with the actual skull.) Radiopaque size
calibrators were included with each X-ray. Measurements of
Euclidian distances between landmarks and angles were
performed on digitized images. Many of the landmarks are
homologous to those used for standard orthodontic
cephalometry in humans.
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