The circaseptan interval in human dental enamel

The circaseptan interval in human dental enamel.

A survey of past scholarship with suggestions for

future research

Bram Mulder s1190997

Advanced Human Osteology

Dr. Andrea Water-Rist, Leiden University

Abstract.

Incremental growth lines in dental enamel are recognized to follow a circaseptan rhythm

which is uniform within the dentition of a single individual. Spaced at about eight to ten

cross-striations, these ‘striae of Retzius’ are generally considered to be a deviation in the level

of mineralization, caused by some process that affected all ameloblasts at the same moment.

Even though a circaseptan interval is a recurring feature throughout the biological realm,

scholars have not reached consensus on the origins of the regularly appearing lines. The

present paper aims at presenting previously suggested causes of the incremental markings, as

well as selecting the most promising of these for future research. By elaborating on both

functional and causal explanations, a better view on Retzius lines will be acquired. Since the

mechanisms behind the interval are poorly understood, most attention will be drawn towards

potential causal connections with the interrelation between biorhythms, correlations with

body mass, and the interaction with melatonin level fluctuations. Since much of the

circaseptan interval can be related to changes in melatonin concentrations, research into the

hormonal intervals in humans is expected to yield the most fruitful results. Even though the

outcomes of this study are definitely not conclusive, they may provide guidance to the

directions for future research that seem to be most promising.

The circaseptan interval in human dental enamel

1

1. Introduction

There are many ways to estimate the age at death of an individual, with varying degrees of

accuracy. One of the main issues in such estimations is the need for a reliable reference

standard to which to compare the uncovered remains. Particularly when dealing with extinct

ancestors of Homo sapiens, it is questionable, if not doubtful, whether a modern reference

population would render very reliable age at death estimations (esp. Bromage and Dean

1985). Much of the problems involved can be overcome by the use of dental microstructures

in age estimations, at least in cases concerning younger individuals. The advantage of this

method lies in the appearance of periodic markers, at circadian and circaseptan intervals, that

provide the osteoarchaeologist with a fairly accurate record from the start of tooth

development up to tooth completion. However, since the processes involved are poorly

understood, and thus the actual meaning of their demarcations, the usefulness remains quite

limited in practice. It is agreed upon that the circadian interval represents a more or less

diurnal process indeed (Smith 2006), while the circaseptan interval is said to commonly vary

from eight to ten days among individuals (FitzGerald and Rose 2008:244), creating large

difficulties in understanding its patterning. The enigmatic character of the circaseptan interval

in teeth requires a reevaluation of research on the matter and a selection of the most promising

directions for future study.

Dental enamel and dentine exhibit comparable periodic markers, whose mutual

relationship has partly been pointed out (Dean and Scandrett 1996). However, there are

several reasons for limiting this paper only to their occurrence in enamel. First, dental enamel

is the most durable mammalian tissue and as such much less prone to taphonomic alterations

than dentine. Second, whereas dentine can remodel during lifetime to some extent, enamel

secretory cells die off after tooth completion, locking the developmental record forever. Third,

dentine formation is, perhaps due to the previous reasons, much more complex and much less

understood. Consequently, this paper has the limited aim of selecting the most plausible

causes for the appearance of time dependent features in enamel only. It will appear that the

two different periodic markers are closely related, yet the main focus will be the circaseptan

interval.


2

2. Theoretical background

For this paper it is essential to come to an understanding of how periodic markers are formed

in human enamel. Therefore, a short outline of the biological and academic context of the

research question is helpful. This section will provide a basis for later findings.

Biological context Enamel is formed by secretory cells, ameloblasts, which travel from the enamel-

dentine-junction towards the tooth surface, secreting enamel matrix at their distal end. Each

cell has two separate secretory aspects: the first is the Tomes’ process, a cytoplasmic

projection from the distal surface of the cell, and the second is a small rim from the distal end

which is not covered by the Tomes’ process (Boyde 1964; Risnes 1998). Accordingly, the

combination of a protruding and a relatively retracted secretory aspect creates relief on the

front of the enamel matrix. Both aspects give distinct orientation to the deposited crystals,

creating so-called prism enamel from the Tomes’ process, and interprism enamel from the

non-projecting rim. As an ameloblast proceeds, the

rod that marks the cell’s track steadily increases in

length. Thus, a longitudinal cross-section of

enamel allows the observer to recognize the exact

path of each single ameloblast. Since this

amelogenesis is a continuous process, enamel

should not be regarded a layered tissue, as it might

appear under the microscope (FitzGerald and Rose

2008:245).

In a transverse cross-section of a tooth,

dental enamel resembles a felled tree: from the

inner core to the outer surface, it is made up of

concentric rings. In longitudinal cross-section,

however, it becomes clear that these ‘rings’ are

more cap-like, marking events that affected all the

ameloblasts simultaneously at a particular stage of

their route, ‘recording’ the extent of the crystallite

rods at each of these moments. (See figure 1,

where these recordings are clearly visible for

Fig. 1. Laser confocal scanning microscope image of a deciduous third premolar. The dentine exhibits five lines: three minocycline injections on days 1, 8, and 14 (lines 1-3), one xylenol injection on day 25 (line XO), and one DCAF injection (line DCAF) on day 39. (Smith 2006: fig 2.)


3

odontoblasts in dentine. The principle in enamel is the same.) These lines are called “striae of

Retzius”, sometimes abbreviated as SR, or Retzius lines, or even perikymata when pointing at

the linear expressions on surface enamel. Even though generally referred to as occurring

about every seven days (“circaseptan”), their interval varies among individuals (e.g. Reid and

Dean 2005). The interpretation of their interval relies on the meaning of smaller lines, the so-

called cross-striations, of which multiple appear within one circaseptem interval. These

smaller lines follow a circadian rhythm as will appear below, so the periodicity of striae of

Retzius is usually expressed in a number of days between two lines.

Academic context Even though Asper (1916) already recognized that cross-striations mark daily

increments, unequivocal substantiation of such observations was not published before Smith

(2006). By injecting living macaques with particular fluorescent dyes at particular moments,

Smith (2006) was able to link the colored striations in dentine to corresponding days of

injection. Comparing the amount of striations between two colored lines to the amount of

days between two injections, she showed the 1:1 similarity. Afterwards she matched enamel

lines to those in dentine and counted the number of cross-striations between each of the

matched lines. Unsurprisingly, this rendered the same results. The evidence has proven to be

conclusive: the cross-striations appear once a day.

Translating this primate-based study to the reality in human teeth, Antoine et al.

(2009) undertook a histological age assessment on five children with known ages at death.

Based on a very small sample (n=5) from an excavation at Christ Church in Spitalfields,

London, the researchers compared parish records of the children’s age to the number of cross

striations in dental enamel. They found clear resemblances between counted striations and the

days the individuals had lived. This deductive study showed that histological observations in

enamel provide a highly reliable and very accurate age determination (Antoine et al 2009).

This conviction of the regularity of the striations was uncommon in earlier times, even

though the connection between the so-called perikymata on surface enamel and the internal

brown lines was made for years already (see Boyde 1964:10-30). When Asper (1916) noticed

the more-or-less daily rhythm of the smaller intervals that constituted the circaseptan

intervals, the regularity of lines, and thus their value, slowly started to be acknowledged. The

duration of these regular striations, however, was only considered a serious issue from the

presentation of the study by Bromage and Dean (1985) onwards. The aim of those researchers

was to demonstrate that Plio-Pleistocene hominids were far more comparable to modern apes


4

in tooth development than to modern humans (Bromage and Dean 1985).They had counted

the perikymata, and concluded that if the mean periodicity, which is the number of cross-

striations between adjacent striae of Retzius, was maintained, the hominids must have had far

fewer recordings of daily increments. Thus, so they thought, enamel was completed within a

shorter time frame (Bromage and Dean 1985). Mann et al. (1987) questioned this

methodology, arguing that the number of cross-striations between adjacent striae of Retzius

widely varied and that the mean could thus not be bluntly applied to dentition of a single

individual. Moreover, as Bromage and Dean (1985:525-526) themselves already noticed, not

all striae of Retzius reach the surface enamel, so estimations based on perikymata inevitably

are underestimations of age. Histological methods of ageing was thus questioned, and not

considered unequivocally reliable until the work of FitzGerald (1998). Avoiding the pitfalls

his predecessors stepped into, FitzGerald (1998) measured both the circaseptan interval and

the associated circadian interval, and found a constant ratio between the two. This approach

rendered results that proved the uniformity of periodicity anywhere in the dentition of a single

individual. To specify, this showed that the number of daily cross-striations between two

consecutive Retzius lines was uniform within a person’s dentition, regardless of tooth type or

place of counting within the tooth. This work expressed that the intervals were periodic

indeed, and that the length of this periodicity must somehow be determined by a systemic

factor.

3. Discussion of proposed explanations

From available research, a few supposed relationships between incremental lines of Retzius

and other processes can be deduced. Some of these are more functional explanations, others

try to elucidate causal processes. Those are not necessarily mutually exclusive.

Functional

1. Reciprocal reduction and expansion of prism size cause varicosities and linings

subsequently.

2. Striae occur as a result of changes in mineralization.

Causal

3. The striae of Retzius is the oscillation of two separately running cycles.

4. The lines of Retzius occur due to frequent resynchronization of two related

processes.


5

5. Both the circaseptan and circadian rhythm are the result of fluctuations in the level

of melatonin.

6. Enamel growth is somehow affected by body mass or related factors, such as

metabolic activity.

The first step is to consider each of the explanations separately. After a short elaboration on

each of the given explanations, a more integrating discussion will follow.

Prism varicosities Prisms varicosities are a still unexplained phenomenon in enamel growth. The term denotes

the regular expansion of a prism, simultaneously forcing the adjacent enamel to dilate. The

result is a wave-like pattern of more or less thickened prisms, in which the thicker parts

constitute the daily cross-striations.

Boyde (1964) argued that a slower growth rate of enamel would result in relative

growth of interprismatic substance at the expense of the prisms. He suggested, and Risnes

(1998) later observed, that a slower growth, in which the relative growth rate between prism

and interprism was maintained, caused overhanging interprism enamel (Fig 4). Such

overgrowth of interprism enamel constricts the Tomes’ process, which is subsequently

squeezed through the opening. That forced throughput results in a short boost of the Tomes’

process relative to the rest of the ameloblast, which compensates for the restriction. This

thickening and dilution of prisms could then be seen as cross-striations in the tracks of

ameloblasts. Risnes (1998) expanded these ideas to

incorporate the phenomenon of striae of Retzius.

However, he only mentions that the striae of Retzius

may be seen as a result of the same phenomenon,

but fails to explain why Retzius lines usually run

oblique with regard to the mineralizing front of the

enamel, whereas cross-striations appear to run

perpendicularly. So far, this theory is considered

plausible, but remains an incomplete hypothesis.

Fig. 2. The pit floors (Pi), which correspond to the underlying prims (P) are obstructed by overhanging interprism (IP) near the arrow. (Risnes 1998:fig.17)


6

Changes in mineralization Multiple scholars hypothesized that a rhythmic change in mineral content caused striae to

appear (Schour and Hoffmann 1939; Gustafson and Gustafson 1967). How the mineral

content changes remains obscure, though. Gustafson and Gustafson (1967) argued that the

appearance of cross-striations and Retzius lines in “microradiographs” evidences a variation

in mineral content. They left the reader in ignorance about whether the lines constituted hypo-

or hypermineralized areas (Boyde 1964). Boyde contested a discontinuity in the level of

mineralization, since the same amount of mineral could be present, though just as larger

crystals. Even in the most recent authoritative publications, authors are forced to express this

endless uncertainty. “Little is known about the discontinuity itself.” (Hillson 2014:95)

However, a few decades before, Driessens et al. (1984) used an electron microprobe to

analyse the concentrations of Ca, Na and Mg in human dental enamel. An electron

microprobe emits an electronic beam, subsequently analyzing the photon-emitted x-rays,

thereby identifying the emitters by comparing emitted frequencies to known frequencies for

elements (Jansen and Slaughter 1982). Even though this technique was relatively unreliable

by that time (Jansen and Slaughter 1982), Driessens et al. (1982) were able to recognize an

interval of 25 ± 4 μm in the fluctuations in the concentration of Mg, which they identified as

the circaseptan interval of enamel. Since the band width in which the high values were

recorded measured about 3 μm, the researchers proposed Retzius lines to be identified on the

basis of this high Mg content. Their reference data are not mentioned, however. They

furthermore explained that higher Mg values increase the solubility of the minerals (Driessens

et al. 1982), which would explain for the commonly spread of carious lesions via Retzius

lines (Darling 1963). Their method is somewhat problematic, since Retzius lines have no

particular bandwidth or interval measurement, and their identification of the interval is

expressed without any references.

Interference of cycles Newman and Poole (1974) faced the same problem as all others before them, which was the

inability to match Retzius lines to known biological rhythms. Their suggestion was that two

distinct processes with circadian rhythms must run alongside each other, of which one is

exactly 24 hours and the other a few hours more. After a certain number of periods, the

interference of the two frequencies causes an enhanced stria to occur (Newman and Poole

1974). However, the authors already remarked that the transition from ‘normal enamel’ to a


7

Retzius line seems to occur quite suddenly, whereas a beats frequency of two interfering

waves would exhibit more gradient changes (Newman and Poole 1974:1139).

‘Slipping’ The same researchers also suggested that if a timing incongruence between two almost-daily

patterns would render cumulatively increasing separation, the system would recover at some

point and start the processes anew, as a sort of ‘recalibration’ (Newman and Poole 1974). So,

when ameloblasts rely on two separate circadian cycles, of which one accumulates an

increasing lag respective to the other, the body would restart the process once in a while. It

remains obscure why the interesting suggestions they did for further research (Newman and

Poole 1974:1139) seem to have never been carried out.

Melatonin levels Since there is a daily rhythm in enamel growth, the ameloblasts possibly are affected by a

rhythm of day and night. The biological day/night clock is thought to be maintained by the

pineal gland, which secretes melatonin. Mainly based on signals from the photoreceptors,

melatonin levels increase at night and decrease during the day (Hastings 1997; Turek and

Gilette 2004). This might give a hint of the origins of the periodic markings.

Recently, Mishimaa et al. (2012) used rats to investigate the relationship between

melatonin and incremental lines in dentine. They found that darkstained layers in dentine

were formed at night, when melatonin levels were high, whereas lighter layers were formed

during daytime, when melatonin levels were lower. The seeming ‘lines’ thereby appearing

were spaced at an interval of 15.50±1.15 μm, which is comparable to the circadian interval for

rats (Schour and Hoffman 1939).

Body mass Dean and Scandrett (1996) saw that large mammals usually had longer intervals between two

adjacent striae of Retzius. They suggested that body size might explain for variation in

Retzius lines between animal groups, although they immediately realized that this would still

not explain for wide variability of the interval within each species (Dean and Scandrett

1996:234).


8

Tab. 1.

Body mass and periodicity in hominoids

(Smith 2008:tab 2)

Smith (2008) further explored this

probable relationship and found a

positive correlation (p<0.001) between

body mass and the Retzius line

periodicity when she studied

hominoid dentition. She added six

hominin species to samples from

earlier publications, thus obtaining a

total of 26 different hominoids.

Logically her sample, as presented

here in table 1, was not evenly

distributed. For each of the species,

Smith used the mean periodicity in

dental enamel. The limitations to

such studies become immediately

clear, since only for extant species

the sample was very large. The

result is that for the species where

n=1 or n=2, the dentition of a single

individual is treated as a population mean. But, as is visible in the cases of the extant species,

periodicity may widely vary. The statistical results should consequently be treated with care.

It remains interesting to note that calculations for the top twenty hominoids rendered

significant correlation between periodicity and body mass, despite the wide variation among

extant apes. For the hominins separately this relationship was not significant (Smith 2008).

This should not be surprising, since the samples were too small. Statistical analysis on 6

different data is of little use, as it would still tell us very little if significance actually was

obtained.

The researcher moreover acknowledged that, regarding the limited enamel growth rate

of large lemurs for example, positive correlations of body mass and periodicity might not hold

true for the primates as a whole (Smith 2008).

A backdrop of her methodology lies in the estimation of body masses. This obviously

is the only way to get some data, but, combined with the earlier mentioned sample size issues,

her results might be heavily skewed. Furthermore, cross-striation is not always unequivocally


9

Fig. 4.

Lineair regression of

periodicity against log body

mass.

● platyrrhine primates

□ catarrhine primates

○ and ∆ strepsirrhine primates

( Bromage et al. 2009: fig 1)

observable, due to which observer errors easily occur. So Smith (2008), counting the number

of cross-striations within intervals at multiple positions in teeth, used the average count as

mean periodicity. This results in that she carried out statistical analysis on mean periodicities

from individuals of whom she already had calculated averages.

The results of her research are depicted in figure 3.

Smith’s publication led Bromage et al. (2009) to continue that line of thought, in which they

included other primates in the sample, as Smith (2008) proposed. By extending their research

beyond hominoids, and even beyond catarrhine primates, Bromage et al. (2009) proved the

very strong relationship between body mass and the repeat interval of Retzius lines (which is

the periodicity). Contrary to Smith (2008), the authors here did not averaged the periodicities

of individuals within a species. It remains, however, that one individual is treated as

stereotype of its species, yet this is the best one can do in case of fossil primates. All

individuals within this sample are at least equally weighed. The regression analysis is

presented in figure 2, obtained from their own report (Bromage et al. 2009:fig 1). Noteworthy

is the relative deviation of the strepsirrhine primates, to which lemurs belong, from the

regression line, as Smith (2008) expected.

Fig. 3.

Relationship between body

mass and striae periodicity

among hominoids.

(Smith 2008:fig 3)


10

Similar research was recently carried out by Hu et al (2012), who investigated the

correlation between body mass and striae periodicity in fossil pongo, discriminating between

male and female. For both sexes, this too revealed a significant correlation.

4. Integration of results and preliminary conclusions

Prism varicosities and changes in the mineral content of the matrix have been proposed as

functional explanations for the appearance of regular striae in enamel. Causal relationships are

sought for in body mass, melatonin levels, interference of two circadian cycles, and the

process of ‘slipping’.

Functional explanations The theory of prism varicosities as an explanation for the appearance of ‘lines’ has wide

support. As explained before, the theory is quite incomplete and lacks firm evidence. No

wonder most scholars stay away from any elaboration on the combined Boyde/Risnes

amelogenesis theory. What remains particularly unclear is the cause for the cross-striations to

run perpendicularly to the path of the ameloblast, while the striae of Retzius form cap-like

features which clearly run in an oblique way. It does not follow from the literature how those

two different features could ever result from a similar event.

There are multiple relevant arguments for explaining striations as a result of alterations

of mineral content. So far, however, little is known about the relation between the enamel

within and outside the striae. Since striations do appear in radiographs, it is quite probable that

there is a change in mineral content. Whether this constitutes hypomineralization or the

opposite seems relevant, but has still not been researched, or say published, properly.

To sum up, it is clear that very little is known about the physical mechanism behind

the appearance of ‘lines’, whether daily cross-striations or near-weekly Retzius lines. It seem

safe to believe that varicosities do account for cross-striations, but the exact underlying

process is only hinted at though hardly investigated. The solutions proposed are so far

unsatisfying. The same is valid for the supposed deviating mineral content of Retzius lines, of

which, again, very little is known. Faculty-overarching research programs might shed new

light on these aspects. Particularly from the field of chemistry and geology serious

contributions to our knowledge could be expected.


11

Causal explanations

• Two different rhythms

As Newman and Poole (1974) themselves implied, an interference of two distinct cycles is not

very likely to be the underlying cause of effects. The result of interference can in a simplistic

way be seen as the sum of two superposed cycles. Accordingly, when two sinusoidal waves

approach each other’s maximum, their interference starts to result in a gradual amplification

of processes. Even with different frequencies, the so-called “beats” should have a gradient

scale (Newman and Poole 1974). This is incompatible with the sudden occurrence and the

clear demarcation of the striae of Retzius, even though some scholars disagree in the

distinctiveness of the striae when studied under the microscope (Fitzgerald 1996:382).

Scholars that still adhere to this explanation for the existence of these lines (e.g. Dean and

Scandrett 1996; FitzGerald and Rose 2008; Antoine et al. 2009), give no literary references to

support this idea.

Whereas the former theory put forward by Newman and Poole was undermined by

themselves (Newman and Poole 1974), but still yields support, their theory of ‘slipping’,

which they thought to be more plausible, is hardly ever referred to. Due to the limited extent

of this paper, further examination of the occurrence of such slipping in other physical rhythms

is not possible. However, their own example of the electronic oscillator that must be

recalibrated once in a while (Newman and Poole 1994), remains a fitting example. Future

research must explore this hypothetical relationship with enamel striation.

If research would indeed confirm that recalibration of cycles underlies the occurrence of

striation in enamel, it would still not explain for the wide variability of periodicity among

individuals. The circaseptan interval only contains full circadian intervals, so their periodicity

is expressed in full days. That means that recalibration does not occur sometime during the

daily growth of enamel, whenever it is necessary, but at fixed moments at the end of a

circadian cycle only. For example, slipping could occur after exactly 7 days or exactly 8 days,

but apparently nowhere in between. This seems unlikely, since ‘slipping’ simply occurs when

the separation between two cycles has become too large, which can happen at any time.

• Bodily variables

It seems to be firmly established that there is a strong correlation between periodicity and

body mass. However, since the periodicity remains the same throughout the dentition of one


12

individual, it is obviously not individual body size that matters, which is partly determined by

circumstantial factors, but rather the estimated size of the species in general. After all, the

studies here mentioned did not examine periodicity variation within populations, but between

species only. The body mass of Homo sapiens, for example, was estimated at 62 kilograms by

Smith (2008), but is by no means the weight of each of the sampled individuals. As far as

body size is genetically determined, there might be an intra-species correlation with

periodicity as well, but this should be investigated in future research.

The question here is whether adult body size influences periodicity, which seems

problematic at this point, or whether the correlation is more or less ‘coincidental’, perhaps due

to a single process that influences both features. In case of such a common cause, comparison

would yield positive correlation, while body size and periodicity were just different results

from the same process. Here one could think of metabolic rates, for example, which may be

linked to both enamel growth and body size.

The fact that the strepsirrhine primates (Bromage et al. 2009) and hominins (Smith

2008) seem to deviate from the other sampled primates in their relative periodicity suggests

that the correlation with body size cannot be taken as a general rule. The question that needs

to be answered is why those branches deviate from the general trend, because that might

simultaneously reveal the origins of periodicity. The next step would undoubtedly be to take

representative samples with appropriate statistical analysis. Outliers can only be treated as

such if the basics of statistics are observed in examination.

One other limitation of the mentioned body mass studies is that these barely are

performed on extant mammal species beyond primates. If there is a relationship between body

mass and the Retzius line repeat interval, the same observations should be done in dentition of

other animals. Bromage et al. (2009) made the first steps in that direction, but performed such

research on seven species from the Proboscidea were investigated, which clearly is far too

small a sample.

More success can be expected from research in the field of relationships with

melatonin. As explained above, increased levels of melatonin seem to cause dark stained

layers, at least in rat dentin (Mishimaa 2012). If this works the same in enamel, which is most

likely, striations occur at night, as the photoreceptors give the stimulus to the pineal gland to

increase melatonin secretion. This implies that the longer one is exposed to daylight, the

longer the cross-striation interval becomes. So, Southern Africans would be likely to have less

melatonin than North Europeans, for example, and their circadian interval would thus become


13

larger. Fortunately, Reid and Dean (2005) explored variations in the formation time of dental

enamel. Most of their work focused on the comparison of North Europeans and South

Africans, in which they found that the South Africans completed their teeth systematically

faster (Reid and Dean 2005). This is completely in line with the ideas that melatonin is lower

during the day, that South Africans are longer exposed to light, so that South Africans form

more enamel per daily cycle. Moreover, FitzGerald (1998) noted that not only the daily

increments were relatively large, but that South Africans have relatively large periodicities in

comparison to Europeans and Amerindians. As explained earlier, he divided circaseptan

interval by the circadian interval, which eliminates circular reasoning. In sum, African daily

increments appear to be larger, but the number of days between Retzius lines is larger as well.

This means that far fewer of these lines could fit within a single tooth. The total number of

striae should thus be significantly smaller in Africans when compared to Europeans.

Reid and Ferrell (2006) explored the relationship between periodicity and enamel

formation times from a different angle. They used 49 mandibular canines from medieval

Denmark to test the relationship between periodicity and the total number of Retzius lines.

Their study revealed a strong inverse relationship between the two. This means in practice

that the larger the periodicity of the circaseptan interval is, the fewer Retzius lines exist in

sum within the enamel. We may expect the African sample to have a mean number of Retzius

lines which is significantly lower than the European mean. If this could be confirmed, the

abovementioned studies would be in support of each other and constitute seriously promising

evidence for a melatonin-related enamel growth.

These conclusions give a good indication of studies that might render positive results.

First of all, comparative studies of daily increments between South Africans and North

Europeans, in which not only periodicities are recorded, but metric measurements of the

intervals as well, could strengthen the hypothesis of melatonin-related enamel growth.

Secondly, an investigation in both the number of cross-striations and the number of Retzius

lines in the African and European groups could confirm earlier mentioned assumptions, and

could eliminate false impressions due to the use of population means.

Furthermore, the impact of melatonin on dental growth in general should really be

studied. It is not recommended to lock up newborn children in complete darkness in order to

observe their enamel increments, but researchers might actually do this with rats. Instead of

observing daily increments one might see a complete delayal of enamel growth. This would

really boost scholarly knowledge about dental growth.


14

If a relationship with melatonin levels would eventually be confirmed, two

fundamental questions regarding enamel increments must subsequently be answered: 1) why

is there such wide variability of periodicites among individuals?, and 2) how is it possible that

periodicity remains uniform within the dentition of an individual?

5. Conclusions

The purpose of this paper was to address the proposed explanations for the existence of long

period markers in human dental enamel. Scholarly research in that field was separated into

functional explanations and causal explanations, which are not mutually exclusive.

A survey of available literature yielded two functional and four causal relationships of

the occurrence of striae. The two functional explanations appeared to remain mere hypotheses

at this stage, since very little is known about the exact mechanisms underlying periodic

markings. Moreover, scholars fail to explain the difference in angle between the respective

linings in respect to the enamel rod.

Of the four causal relationships examined, only two seem promising at this point. Two

free-running cycles that interfere was not considered a plausible cause, since such beats do not

occur suddenly, but rather gradually, whereas the borders of Retzius lines are clearly defined

and point at a sudden change of events. Neither was the idea of frequent resynchronizations

regarded possible, since this suggests relatively random recalibration, whereas Retzius’ striae

always appear at the end of a daily cycle and never in the middle.

The significant correlation between body mass and periodicity of the circaseptan

interval is more likely to be related to the cause of striations. Since periodicity remains the

same throughout life, body mass is unlikely to be a determining factor in itself, but rather a

result of a common cause.

By far, the most successful results can be expected from melatonin-studies. There is

serious reason to see its levels as being related to dental growth. Of all the proposed

explanations for the occurrence of linings in dental enamel, the link with melatonin appears to

be the most plausible. A lot is known about melatonin secretion and its effects, so integrative

research which incorporates neurobiology expertise would enhance our understanding of

dental enamel growth.


15

6. References cited

Antoine D, Hillson S, and Dean MC (2009) ‘The developmental clock of dental enamel: a test for the periodicity of prism cross-striations in modern humans and an evaluation of the most likely sources of error in histological studies of this kind’ Journal of Anatomy 214:45–55.

Asper H von (1916) ‘Über die Braune Retziusche Parallelstreifung im Schmelz der menschlichen Zahne’ Schweizerische Vierteljahrsschrift für Zahnheilkunde 26:275–314.

Boyde A (1964) The Structure and development of mammalian enamel. PhD dissertation: University of London. Pp.186-207.

Bromage TG, Lacruz RS, Hogg R, Goldman HM, McFarlin SC, Warshaw J, Dirks W, Perez-Ochoa A, Smolyar I, Enlow DH and Boyde A (2009) ‘Lamellar Bone is an Incremental Tissue. Reconciling Enamel Rhythms, Body Size, and Organismal Life History’ Calcified Tissue International 84:388-404.

Darling AI (1963) ‘Resistance of the enamel to dental caries’ Journal of Dental Research 42:488-496.

Dean MC and Scandrett AE (1996) ‘The relation between long-period incremental markings in dentine and daily cross-striations in enamel in human teeth’ Archives of Oral Biology 41:233-241.

FitzGerald CM (1998) ‘Do enamel microstructures have regular time dependency? Conclusions from the literature and a large-scale study’ Journal of Human Evolution 35:371-386.

FitzGerald CM and Rose JC (2008) ‘Reading between the lines: dental development and subadult age assessment using the microstructural growth markers of teeth’ in: Katzenberg MA and Saunders SR (eds.) Biological anthropology of the human skeleton. Hoboken: John Wiley & Sons, Inc.

Gustafson G and Gustafson AG (1967) ‘Microanatomy and histochemistry of enamel’ in Miles AEN (ed.) Structural and Chemical Organisation of Teeth vol.2 pp. 135-162. New York: Academic Press.

Hastings MH (1997) ‘Circadian clocks’ Current Biology 7:R670-672. Hillson S (2014) Tooth Development in Human Evolution and Bioarchaeology, Cambridge

University Press. Hu R, Zhao LX, Wu XZ (2012) ‘Periodicity of Retzius lines in fossil Pongo from South

China’ Chinese Science Bulletin 57: 790-794. Jansen W and Slaughter M (1982) ‘Elemental mapping of minerals by electron microprobe’

American Mineralogist 67:521-533. Mishimaa H, Hattori A, Suzuki N, Tabata MJ, Kakei M, Miake Y and Suzuki M (2012) ‘The

connection between the periodicity of incremental lines in the tooth dentin and the regulation by melatonin’ Bone 50 suppl. 1: Abstracts of ECTS 2012, 39th Annual Congress, Stockholm, Sweden (May 19-23, 2010):session 102.

Newman HN and Poole DGF (1974) ‘Observations with scanning and transmission electron microscopy on the structure of human surface enamel. Archives of Oral Biology 19:1135-1143.


16

Reid DJ and Dean MC (2005) ‘Variation in human enamel formation times’ Journal of Human Evolution 50:329-346.

Reid DJ and Ferrell RJ (2006) ‘The relationship between number of striae of Retzius and their periodicity in imbricational enamel formation’ Journal of Human Evolution 50:195-202.

Risnes S (1998) ‘Growth tracks in dental enamel’ Journal of Human Evolution 35:331-350. Schour I and Hoffman MM (1939) ‘Studies in Tooth Development: I. the 16 Microns

Calcification Rhythm in the Enamel and Dentin from Fish to Man’ Journal of Dental Research 18:91-102.

Smith TM (2006) ‘Experimental determination of the periodicity of incremental features in enamel’ Journal of Anatomy 208:99–113.

Smith TM (2008) ‘Incremental dental development: methods and applications in hominoid evolutionary studies’ Journal of Human Evolution 54:205–224.

The circaseptan interval in human dental enamel

Documents

Transcript of The circaseptan interval in human dental enamel