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Max Planck Institute for Evolutionary Anthropology

DEPARTMENT OF HUMAN EVOLUTION

DENTAL HARD TISSUE LABORATORY

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Table of Contents

Tooth Histology and Human Evolution 3Dental Microstructure 3

Principles of Incremental Development 3

Histological Facilities Available 6Significance 9

Contact Information 11

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Tooth Histology and Human Evolution

The MPI-EVA Human Evolution Department has recently created

state of the art facilities for the study of dental development in fossil

hominids. Histological analysis of dental material facilitates understanding

of the final functional products of the processes of development and growth,

which may be understood in terms of enamel thickness (macrostructure) and

enamel microstructure. Recent studies have provided information on age at

death in hominids with developing dentitions, absolute and relative timing of

dental development, age at first molar emergence, and differences in the

developmental pathways of enamel formation. These studies have important

implications for our understanding of hominid evolution and the origin of

developmentally modern humans.

Dental Microstructure

Dental development in humans and great apes begins prior to birth

and continues throughout adolescence. Like many biological systems, hard

tissue formation is characterized by a circadian rhythm. Developmental rate

and time are permanently recorded by incremental lines in enamel and

dentine, which remain unchanged in these tissues for millions of years.

Given that dental remains are the most common, well-preserved type of

fossil evidence for extinct species of primates, examination of incremental

growth processes may shed new light on the evolutionary developmental

biology of early humans.

Principles of Incremental Development

Enamel is secreted by cells known as ameloblasts, which differentiate

at the enamel-dentine junction and migrate outward towards what becomes

the surface of the crown. The tracks left by these individual cells are known

as enamel prisms (Figure 1). The prisms show cross-striations that resultfrom the circadian rhythm of enamel secretion (Figures 1 & 2).

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The successive positions of the advancing front of forming enamel are

preserved as long-period incremental structures termed Retzius lines

(Figures 1 & 3). These lines conform to the shape of the dentine horn in the

cuspal enamel. In the lateral and cervical enamel, Retzius lines contact the

enamel surface, forming circumferential rings known as perikymata

(Figures 4 & 5). This region is referred to as lateral or imbricational enamel.

The periodicity of Retzius lines may be determined in imbricational enamel

by counting the number of cross-striations between Retzius lines (shown in

the enlarged box in the upper right of Figure 1).

Figure 1. Schematic of enamel microstructure modified from Smith et al. (2003).

Dentine is produced by cells known as odontoblasts that generatedentine tubules (similar to enamel prisms), and show daily incremental lines

known as von Ebners lines (equivalent to cross-striations) and long-period

structures known as Andresens lines (equivalent to Retzius lines).

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It has been demonstrated that both cross-striations and von Ebners lines

show a 24-hour frequency, and because they are easier to image than von

Ebners lines, cross-striations are used as a standard to determine the

periodicity of long-period features in both tissues. Long-period features

show a consistent periodicity within a single tooth and in all teeth belonging

to the same individual, although this may vary within a taxon. Counts and

measurements of these short- and long-period lines provide information on

the rate and duration of enamel and dentine secretion, which may be

combined to determine the total crown formation time and the rate and

duration of root extension.

Figure 2. High magnification polarized light image of a Paranthropus boisei molar showing

cross-striations. These light and dark bands run horizontally across the vertical enamel prisms,

and are spaced approximately 5.5 microns apart. Fifty to 55 lines can be counted from the bottom

to the top of the image, representing 50 - 55 days of tooth growth.

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Histological Facilities Available

Hard tissue preparation and analysis laboratories are available for

examination of dental material. Molding, casting, and replicating techniques

may be used to study external features or naturally fractured surfaces.Standard refluxing and embedding techniques will be used to prepare

samples, which may be sectioned with the use of a diamond wafering blade,

ground, and polished to a fine sub-micron finish.

For the preparation lab, we offer the use of two Buehler Isomet

precision saws, a Buehler Ecomet Grinder-Polisher, and a large-diameter

Logitech automated annular saw (APD1). Several forms of microscopy are

available to study casts and histological sections, including transmitted and

polarized light microscopy. We use the automated Olympus SZX9 Stereo

microscope with the 12.5 mega-pixel DP 70 camera and SIS Imaging

software for stereo microscopy, and the Olympus BX51 upright lightmicroscope for transmitted and polarized light imaging, which can be

coupled with the DP 70 and SIS software for a range of measurements, or

used to capture images to 35 mm film. The microscopes are coupled to a

high-speed computer with a 21-inch monitor and sufficient space to archive

images to DVDs or for storage on our department server.

A range of printing options is available, including local poster-sized

image generation and high quality photo printing. Additional forms of

analysis include scanning electron microscopy (SEM) and tandem scanning

reflected light microscopy (confocal) available though collaborativearrangements with Stony Brook University and the University of Leipzig

(Figures 6 & 7).

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Figure 3. Polarized light photo montage of the lateral and cervical enamel of a Paranthropus

boisei molar showing prominent accentuated growth lines and structural features known as

Hunter-Schreger bands running from the lower border towards the tooth surface. The vibrant

colors are partially due to post-mortem mineralization that is characteristic of fossil material.

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Figures 4. (above) and 5. (below) illustrating perikymata (growth lines) on hominid teeth. These

high-resolution replicas have been sputter coated with a metallic source to enhance the

appearance of horizontally oriented long-period lines.

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Significance

Early studies of fossil dentitions concentrated on aspects of crown size

and morphology, root morphology, and the mechanics of occlusal wear.

Additional studies have looked at the age of emergence and developmentalchronology in fossil and extant taxa in an attempt to understand aspects of

life history and phylogeny. Phylogenetic and functional importance has also

been attached to tooth enamel thickness in analyses of Miocene hominoid

fossils and interpretations of hominid origins. Examination of hard tissue

development from a histological perspective is a relatively new field of

odontological inquiry, particularly in relation to answering phylogenetic

questions. Recently, there has been a dramatic increase in the number of

studies on incremental dental development in hominoids. Studies on Plio-

Pleistocene hominids and Neanderthals have indicated that the relatively

slow developmental rate and prolonged duration of modern human crownformation is a fairly recent and unique development.

Figures 6. (above) and 7. (following page) illustrating prisms and incremental features on

Moroccan hominid teeth. Figure 6 is a scanning electron micrograph of naturally fractured

enamel (scale bar = 20 microns), and Figure 7 is a confocal micrograph of sub-surface enamel

from the same tooth.

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Members of the Department of Human Evolution are currently

utilizing incremental structures preserved on the surface of teeth to study the

growth processes of early humans (Figures 4 & 5). Current projects include

a study of the development of Moroccan hominid dental material, as well as

on-going work on Neanderthal growth processes. Additional projects includea study of fossil and extant Asian hominoid dental development, including a

study of temporal and geographic developmental variation in Pongo, which

may provide insight into the evolutionary developmental biology of the

genusHomo.

Over the next few years, we aim to build a substantial reference

collection of histological sections of hominid material. This will yield

additional developmental information that is not available from external

surfaces or naturally fractured teeth, including developmental rate and total

formation time. These types of data are illustrated here with images from

histological sections ofParanthropus boisei (Figures 2 & 3). Fossil sections

will be considered in the context of a large comparative collection of

histological sections of extant hominoid material. We are open to

considering proposals for collaborative work within all of these broad

themes. Please do not hesitate to contact us for more information.

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Contact Information

Tanya M. Smith

Max Planck Institute for Evolutionary Anthropology

Department of Human EvolutionDeutscher Platz 6

D-04103 Leipzig

Germany

Tel: + 49 (0) 341 - 35 50 362

Fax: + 49 (0) 341 - 35 50 399

E-mail: tsmith@eva.mpg.de

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2005

Department of Human Evolution

Max Planck Institute for Evolutionary Anthropology

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