The tooth fairy never forgets the pain in your heart

The pain may be healed and the memory may fade away, but the body will never forget the damage we have suffered.

Written by | BiPuXianren

Enamel, which came from the Western Regions, sounds mysterious and romantic, just like the "Western Regions" and as romantic as the "Franc". Of course, you can also call it "enamel", which is more pragmatic and simple, and less romantic and mysterious.

Generally speaking, enamel is a coating composed of various minerals, which is applied to objects by skilled craftsmen or professional equipment, giving them practical or beautiful features. When it is called enamel products, material engineers will describe it in rigorous terms: wear resistance, high hardness, heat resistance, corrosion resistance, insulation, gloss... If it is presented as an exquisite enamelware, the viewer will appreciate the splendor of Ming and Qing enamel craftsmanship.

A Qing Dynasty Qianlong period cloisonné enamel duck vase, the shape of which is modeled after a bronze duck vase[1]

When you are faced with a number of enamel treasures in the museum's collection, and the meticulous explanations of the guides linger in your ears, in addition to feeling the powerlessness of language in visual shock, you can also learn a few professional expressions:

Body: The utensil to which enamel is attached can be divided into gold body, copper body, porcelain body, purple clay body, etc. according to the material.

Glazing: Prepare the glaze you want to use through various operations. Here glaze = enamel = porcelain enamel.

Glazing: Apply glaze to the body through various methods.

Firing: Sinter your enamel and body at a high temperature of more than 800 degrees to obtain a dense composite...

If enamelware is classified according to the production process, there are cloisonné enamel, chiseled enamel, champlevé enamel, painted enamel, oral enamel...

Hearing this, you may feel something is wrong: "What is the process of 'oral enamel'?"

Before I explain this concept, please open your mouth wide.

Adult full mouth[2]

Pictures speak louder than words, your teeth are awesome! The upper and lower rows are neat and white, with incisors, canines, premolars, and molars. These are the 32 food weapons in your mouth. You rely on them to cut, bite, and chew, and they are not afraid of heat, cold, acid, or alkali.

It should be noted that 20 of these 32 teeth have been replaced by permanent teeth. Generally speaking, the deciduous teeth will grow before you are three years old and start to fall out when you are six or seven years old.

Obviously you have realized that what I call mouth enamel are these teeth.

But this expression is not rigorous enough.

Schematic diagram of tooth structure | Source: kocakayaali

The part of the tooth exposed in the mouth is called the crown, and its surface is enamel, which is known as the hardest tissue in the human body. As shown in the picture above, when we cut open the crown, we can see the dentin wrapped by the enamel and the pulp hidden in the dentin. If the tooth is regarded as an enamelware, then the enamel is the enamel and the dentin is the body.

In fact, tooth enamel is nicknamed enamel.

The hardest part of the tooth

Both "tooth enamel" and "enamel" are translated from the English word "enamel". The word Enamel was formed in the Middle Ages and was used by Western Europeans at that time to refer to "glassy materials coated on metal or ceramic utensils", that is, enamel/porcelain. (You can also use it as a verb, meaning "glaze".) Enamel is called "émail" in French. After entering the 18th century, the French began to use émail to refer to "the hardest part of the tooth", that is, enamel; "enamel" gradually gained this meaning.

However, humans knew about enamel two centuries before they named it enamel.

As the Renaissance brought about new ideas in Europe, a revolution began in the fields of human anatomy and physiology, including dentistry.

In 1530, the world’s first book on dental treatment was published by German dentist Artzney Buchlein. The book was titled Artzney Buchlein: wider allerlei kranckeyten und gebrechen der tzeen, which translates to “Various Dental Problems” in Chinese.[3]

Artzney Buchlein: wider allerlei kranckeyten und gebrechen der tzeen[3]

In 1563, Italian anatomist Bartolomeo Eustachio, known as the “father of dental anatomy”, introduced dentin and enamel in his first work, Libellus de Dentibus[4], comparing them to “acorns and acorn shells”. The Libellus de Dentibus has 30 chapters, recording important discoveries about tooth morphology and function, and is considered the first monograph that explains the anatomical structure and function of teeth in detail.

Leeuwenhoek—the Dutch observer who is now popular on the Internet—used a homemade microscope that could magnify hundreds of times to observe tooth structure in the 17th century. In 1677, this “godfather of microscopes” who shares the title of “father of microbiology” with Pasteur drew a microscopic sketch of tooth enamel[5].

After that, more and more scholars began to observe and draw tooth enamel, but they all had their own names for it. For example, the Italian biologist Marcello Malpighi, the founder of both microscopic anatomy and histology, called tooth enamel "substantia filamentosa" (filamentous substance), while the Scottish dentist Robert Blake called tooth enamel "cortex striatus" (linear cortex) [5].

Robert Blake's section of an elephant molar. The letter e stands for cortex striatus, or tooth enamel.[5]

Whether it is called "acorn shell" or "filament", it is far less vivid than "oral enamel".

More than 95% of tooth enamel is composed of minerals, mainly hydroxyapatite and supplemented by fluoroapatite. It has a hardness that is second only to diamond. Scientists have found that the chewing pressure on tooth enamel shares the same measurement scale as the pressure on the earth's crust. Its tolerance to sudden changes in temperature and acid-base fluctuations is amazing, and its spotless, translucent appearance is impeccable.

Enamel is the perfect enamel for the dentin that it firmly surrounds.

Of course, the process of oral enamel is not orthodox at all. Its glaze making and glazing are completed independently without external force, and it does not undergo high-temperature baking - this may sound a little less heroic, but if you really understand the process of enamel formation, you may feel the beauty of the microscopic world of life.

Before telling the story of oral enamel's self-realization, we need to spend some time introducing its organizational structure.

Glaze columns and microcrystals

According to the definition of oral histology, the basic unit of enamel is enamel rod, which is millions of slender columns; the cross section of enamel rod is keyhole-shaped, with a large head and a slender tail. Each enamel rod is composed of millions of apatite crystals.

Schematic diagram of glaze column structure[6] (click to see larger image)

There are two enamel heads on the left and right sides of a enamel tail. There is an obvious gap between the adjacent enamel heads and enamel tails, which is filled with interrod substance (enamel cement). The interrod substance is of course also microcrystalline, but its overall structure is not as dense as the enamel rod itself. The arc-shaped boundary formed by the interrod substance is called the enamel rod sheath.

Enamel prisms and enamel prism interstitium under a scanning electron microscope[7]

The enamel pillar starts from the interface between dentin and enamel, the dentinoenamel junction (DEJ), and penetrates to the tooth surface, but the growth path of the enamel pillar is not completely straight: the 1/3 of the enamel pillar near the surface is relatively straight, called straight enamel; while the inner 2/3 is relatively curved, called twisted enamel - especially at the incisal edge/tooth tip, the twisting and curvature of the enamel pillar is more obvious.

Therefore, if we only talk about the shape, it seems more appropriate to call the twisted glaze "glaze strips". (In several diagrams used in this article, the glaze columns seem to be straight lines throughout, but this is not the case.)

Enamel twist under a microscope[7]

On the other hand, the arrangement of enamel columns in different parts of the tooth enamel is different. For example, the enamel columns in the pit and fissure area of ​​the tooth start from the DEJ and converge toward the bottom of the pit and fissure, while the enamel columns near the neck of the tooth are mostly arranged horizontally.

Different arrangements of glaze pillars in different areas | Photo provided by the author

After having a general understanding of the internal structure of tooth enamel, we seem to be able to make the following summary: the formation process of oral enamel is the process of growing countless closely arranged enamel columns outward with dentin as the matrix.

The question now is——

The core technology of ordinary enamel is the three steps of glazing, glazing and sintering. The purpose of sintering is to allow the enamel, which is originally in the form of paint, to melt and crystallize at high temperature to become a high-quality crystalline enamel layer, which is tightly combined with the body.

Where does the hydroxyapatite used as the enamel material in oral enamel come from? How do these raw materials grow into crystals without high temperatures? Why do microcrystals form enamel columns? Why do so many enamel columns grow together to form the unique structural form of tooth enamel?

The answer to all these questions lies in the ameloblasts—

That’s right, they are those magical ameloblasts whose English name is “ameloblast”, those ameloblasts that are rooted in the dentin and look out of the gums, those mobile, good at secreting and very “prominent” ameloblasts, those ameloblasts that finish their work and leave without revealing their achievements.

Glaze making skills

When seeing the hand-drawn diagrams of ameloblasts by scholars, most laymen find it hard to believe that these rough patterns drawn with delicate brushstrokes are cells.

Because they are thin and long columns with hexagonal cross-sections, and adjacent cells fit seamlessly and stand in parallel, which is very different from the cell shape we imagine.

The predecessors of these cell "columns" were actually short and thick blocks (in English, they can be described as "cuboidal"), and they grew into tall and thin ones through differentiation. Of course, the complicated past of how they developed before, how they set up camp on the dentin interface, and how they prepared for "enamel making" and "enamel coating" is not mentioned here.

Schematic diagram of ameloblast structure[8]

We focus only on the ameloblasts in their prime.

Ameloblasts secrete specific matrices. They are called enamel matrix, which can be regarded as the precursor of enamel in a sense. They are composed of 30% hydroxyapatite and 70% "water + protein". As shown in the figure above, the secretion activity of the cells is carried out at the end close to the dentin, that is, the end away from the nucleus; and you can clearly see that the secretory end is a cone-shaped protrusion, which scholars call "Tomes processes". Where there are protrusions, there are concavities. The matrix secreted from the protrusions has pits. There are as many "Tomes process pits" as there are Tomes processes.

Tomstu diligently secreted matrix, and the matrix with pits accumulated higher and higher, and the ameloblasts naturally migrated to higher places - they were like a group of construction magicians, laying bricks and tiles one by one, and building up layer by layer.

Schematic diagram of the glaze making process[9]

The deposited matrix will continue to absorb calcium ions and phosphate ions from the mouth (saliva, etc.) to form mineral crystals, while continuously removing proteins and water. In this way, the matrix originally with organic matter as the core gradually turns into hard core enamel with a mineral content of up to 96% - countless hydroxyapatite microcrystals are formed by the mineral ions entering the matrix, and the microcrystals form enamel columns.

Mature ameloblasts under an electron microscope. Mineralization of the enamel matrix occurs during this stage.[10]

The unique growth path of enamel rods is actually the migration path of ameloblasts.

The above is what is mentioned in the previous article as "independent glazing and glazing" and "crystallization without the aid of high temperature".

When the magician of building enamel has finished his task of making enamel, the ameloblasts will return to their short and thick cuboidal shape and combine with other cells to form squamous epithelium, temporarily covering the surface of the enamel. They will then migrate to the gingival sulcus during the period when the crown of the tooth erupts from the gums and finally leave quietly. This is a model of hiding one's merits and fame.

I've been talking about oral enamel for so long that you may be tired of listening to it, just as I'm tired of writing about it.

This is the so-called black humor of popular science writing: when the author piles up a bunch of tedious details to clarify the "principles", "mechanisms" and "core" to realize his ideal "hard-core popular science", most readers will feel annoyed and complain, "What is the difference between reading such a pile of things and studying by myself with a textbook?" If you want to cater to the tastes of the general public, you must offer "soft and emotional" content. Obviously, "a brief description of the formation mechanism of tooth enamel" is not one of them.

As a writer, I am always torn between "providing practical knowledge" and "enhancing the readability of articles", and I can never find the answer, which makes me feel more frustrated and powerless than my readers.

That feeling of powerlessness is like putting a cross section of tooth enamel under a thousand-fold microscope in front of you. You see a dense and endless forest of enamel pillars, unfathomable. You want to pass through it, but you have a feeling that you can't find the end...

Until you discover that there are loops carved on the cross section, just like forest paths from a bird's eye view. You feel curious and walk towards the path, into the forest of glazed columns, and into the depths of the loops, where you will find a different world.

Short horizontal lines and long growth lines

These loops are the timeline of the oral enamel.

There are two scales of time lines on the enamel: horizontal lines that increase by one circle every day, and growth lines (also known as Richter's lines) that increase by one circle every 5 to 20 days. The former is a direct sign of the growth and elevation of the enamel column. Although the pattern is small, it records the longitudinal growth of the enamel column in an intuitive and detailed manner; Richter's lines have a broader vision, marking the overall growth of the enamel in both the horizontal and vertical dimensions with a longer period, and are a witness to the "interruption period" encountered at the forefront of enamel development.

Schematic diagram of enamel striations and Richter lines[11]

Let’s talk about cross-striations first.

As mentioned earlier, the growth of enamel prisms depends on the secretion of matrix by ameloblasts, and cell activity is cyclical, so the growth process of enamel prisms (or the longitudinal development of enamel) is also cyclical and will leave periodic marks.

In a day and night cycle, ameloblasts will experience rapid secretion and slow secretion stages; due to the internal microcrystal orientation problem, the enamel column grows longer and thicker in the rapid secretion stage, and grows thinner and thinner in the slow secretion period. Therefore, at the turning point from fast to slow, the enamel column is the thickest, and at the critical point from slow to fast, the enamel column is the thinnest. Every 24 hours, the enamel column adds a thinnest circle and a thickest circle.

The thinnest circle on the column is a dark line when viewed from a distance; if many columns stand in parallel, you will naturally see a different texture formed by the combination of those dark lines - this is how the regular horizontal lines under the microscope appear. They reflect the longitudinal growth rhythm of the enamel. (Note that both horizontal lines and growth lines exist inside the enamel and can only be observed on the cross section of the enamel.)

Regarding growth lines (Retzius lines), the academic community is still not very clear about their formation mechanism, but what can be determined at present is that it represents a significant slowdown/interruption at the frontier of enamel development, recording the interruption period in the enamel growth history.

Enamel formation begins at the tooth tip, spreads to the surrounding area, and then covers the entire crown. In other words, the development and secretion of ameloblasts in different areas are sequential, and the growth of enamel pillars is naturally sequential, with the tip of the tooth growing first and the area farther away from the tip growing later.

To make it easier to understand, we might as well put aside the vertical dimension and only focus on the horizontal expansion of enamel, and idealize it as the expansion of a circle - with the tooth tip as the center, the circle expands outward, and the longer the radius, the later the circumference is formed. The outermost circle formed the latest corresponds to the circle where the enamel just covers the entire crown. (The enamel on the same radius can be regarded as formed at the same time.)

Imagine the lateral expansion of enamel as the expansion of a circle | Image provided by the author

For some reason, at certain times, the originally active ameloblasts will suddenly slow down or even stop their secretion activities, and the lateral development rate of the enamel will become very slow. In the words of some scholars, "the development front of the enamel has changed, and the orientation of the formed microcrystals is unusual." This static period appears periodically, and each time it appears, it will leave a mark, which is the growth line.

If you observe the cross section of tooth enamel under a microscope, you can see circles of dark brown concentric circles, like tree rings. Through these rings, researchers can accurately retrieve the interruption period; and if you click on these interruptions, you may see the mottled past of the owner of the rings.

Idealized Richter line from a cross-sectional perspective | Image provided by the author

The line of growth carves out the pain of growth

Scientists have a consensus on growth lines: the thicker the enamel growth lines, the more severe the growth interruption period, and the more significant the interruption, the greater the stress the human body is experiencing. For this reason, many scholars call some "thick" growth lines stress lines.

The neonatal line formed during the fetal birth stage is the strongest and most scientifically significant stress line, because birth is a super stress source for the growth of tooth enamel. The neonatal line can not only be used as a dividing line before and after the birth of an individual, but also provides researchers with a reference for tracing the mother's stress history during pregnancy.

Psychiatric epidemiologist Erin Dunn and her colleagues studied the morphology of 70 naturally exfoliated deciduous teeth from 70 children aged 5-7 years [12], aiming to correlate "maternal mental stress during pregnancy" with "the thickness of the growth lines."

The results showed that children born to mothers with mental health problems tended to have thicker new lines on their tooth enamel, while mothers with good mental health were more likely to have offspring with thinner new lines. Mental health problems here include mental depression at 32 weeks of pregnancy, major depression, and a history of mental illness such as anxiety. Dunn and his colleagues found that this association still held after controlling for factors such as obesity, maternal age, and supplements taken during pregnancy.

Dunn, who works at Massachusetts General Hospital, calls herself "the science tooth fairy" and is a big fan of baby teeth. She has long been devoted to persuading ignorant children to donate their baby teeth. The free baby teeth are handed over to her like-minded academic partner, dental development expert Felicitas Bidlack. The latter transformed into a Leeuwenhoek girl, examining every groove, notch and crack of each baby tooth, using X-rays and CT scans to explore its internal structure, measuring the thickness of enamel and mineral density, cutting them into thin slices and observing them under a microscope...

Irene Dunne (left) and Felicitas Bidlack (right)[13, 14]

At the end of 2019, Dunn and Bidlack began an ambitious project[15]. They recruited hundreds of mothers and their children who had “witnessed” the 2013 Boston Marathon bombing in different ways and who had become pregnant and given birth before and after the incident. They collected deciduous teeth from these children and then used medical records to analyze stress lines related to the traumatic event. The forms of “witnessing” here include witnessing the crime scene in person, watching real-time news on TV, or living and working in the area where the crime occurred for a long time. In addition, they set up a control group that did not witness the explosion. The deciduous tooth samples of the control group came from the brothers and sisters of the children in the observation group.

In Bidlak’s words, “If it turns out that children conceived before or after the bombings do have enamel with deeper fracture lines, that would be a shocking result.”[16]

In the eyes of Dunn and Bidlack, tiny pieces of deciduous tooth enamel are tiny life archives that record the body's early contact with environmental substances and memories of stressful events. By decoding the dark lines in these archives, we can peer into the spiritual world of the mother before giving birth and trace the spiritual journey of the child in his or her early years.

Eight-year-old Nora Fong (left) participated in the study by Dunn et al. Her mother, Andrea Fong (right), followed the Boston Marathon bombings on the live news and gave birth to Nora a few weeks later. (ERIN CLARK/GLOBE STAFF)[17]

“When we go through intense mental experiences or experience strong stressors, the body responds. The response to trauma is bound to be integrated into the body’s physiology in some way. Some traumas can affect brain development and even leave marks in the DNA. Of course, the teeth will record something.”

"Tooth enamel may serve as a biomarker for childhood trauma, helping to address key questions we have in the field of childhood adversity."

"The cells build up the enamel layer by layer in a cyclical process, like a ticking clock, until the tooth is fully formed."

"That dark line of birth marks the leap from the womb into the world. From the line of birth, the growth lines that follow record the growth of the enamel in a fixed cycle; among them there may be certain thicker and darker stress lines, which signify interruptions in enamel development, indicating that something has disturbed the child."

"But surprisingly, almost no one has done this kind of association study..."

As mentioned above, Dunn and Bidlack, as pioneers in this field, have put their academic explorations into practice to focus more on the stress of the mother during pregnancy, but have not yet mentioned the trauma of the baby after birth. It is not difficult to imagine that such research involving children's adversity requires a huge investment of manpower and time, overcoming many obstacles, and it is often difficult to get ideal answers.

Longitudinal section of tooth enamel on a glass slide (JESSICA RINALDI/GLOBE STAFF) [18]

However, some researchers have focused on monkeys, which are also primates, and indirectly examined the marks left on teeth by childhood stress.

Treating monkeys doesn’t have to be as warm as spring. We can relatively freely select ideal cubs, keep them in a specific, controlled environment, distribute food to them uniformly, implement long-term monitoring, arrange physical examinations, tattoo identity numbers on their thighs, record every injury and illness, and separate them from their mothers and other community members for a period of time... (It seems that many humans have experienced such a life.)

Environmental medicine expert Christine Austin and biological anthropologist Tanya Smith “recruited” a dozen captive monkeys and investigated the stress profiles in their teeth in the same way [19]. They found that tiny stress lines formed in the monkeys’ tooth enamel during the period when they were separated from their mothers and their social groups. This result is enough to explain a lot of problems.

Witness of history

Teeth can resist the erosion of time, and their growth lines will of course remain fresh over time.

In the eyes of anthropologists and archaeologists, tooth enamel not only records the experiences of individuals, but also allows us to follow those timelines of varying depths and travel through tens or even millions of years to get a glimpse into the lives of ancient people.

From left to right, the skull fossils of Australopithecus africanus, Australopithecus afarensis and Homo erectus (Sabena Jane Blackbird / Alamy)[20]

Unfortunately, the young Homo erectus died young, leaving the first permanent molar buried in the dust, with the horizontal lines remaining and the growth lines indelible.

We are fortunate to have discovered fossils that are millions of years old. We use the neotenysis as the starting point, the age of death as the end point, and the horizontal lines and growth lines as the scale to calculate the rate of enamel development of the child. The rate of enamel development is an important reference for distinguishing human species. This calculation helps us to infer that "the development closest to Homo sapiens began after the emergence of Homo erectus."[21]

During the severe winter of the Ice Age, the Neanderthals were traveling in a hurry with their families, and their young were still growing teeth.

They suckled breast milk and also obtained 18O, calcium, barium, and lead from breast milk. The latter was concentrated in tooth enamel, and severe cold stress made the growth lines darker. Modern scholars have gained a rough understanding of the climate environment and metal element exposure they faced at the time, and even the lactation cycle, by analyzing the teeth of Neanderthals 250,000 years ago.[22]

Newborns in ancient Rome were fragile, the world outside the womb was dangerous, and countless fetuses died during childbirth.

When archaeologists discuss the stillbirth rate in ancient Rome, they often talk about the linea neoplasia, because only babies who leave their mothers alive can have a complete linea neoplasia. The linea neoplasia is deep and eye-catching, separating the enamel secretion before and after birth, and in a sense, it is a birth certificate. Of course, not every child who is born successfully has a birth certificate; if a baby dies before 7-10 days after birth, we may not be able to observe the linea neoplasia in his teeth, because it takes time for the enamel matrix to fully mineralize, and premature death may make the enamel in the linea neoplasia too fragile and lost over time [23].

Researchers analyzed Neanderthal deciduous teeth from 120,000 years ago and found that their teeth developed much earlier than those of modern humans.[24]

Tooth enamel, produced by ameloblasts, is as hard as diamond and can last for millions of years. However, as a reader, you don’t need to delve into why it is “hard and long-lasting”, because “deep study” is more likely to make you “soft and fast” (the will to explore science becomes weak, and the interest in studying things disappears quickly) than “reading”. In fact, as long as the readers can persist to read this far, the author who persists to write here will already show a weak and relieved smile.

References

[1] https://baijiahao.baidu.com/s?id=1729227604691660269&wfr=spider&for=pc

[2] https://www.seasons-of-smiles.com/your-childs-teeth-ages-6-to-12.htm

[3] https://www.deutschestextarchiv.de/book/show/nn_tzeen_1530

[4] https://pubmed.ncbi.nlm.nih.gov/11794332/

Eustachio and "Libellus de dentibus" the first book devoted to the structure and function of the teeth

[5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5995149/

[6] http://www.dent-wiki.com/dental_technology/formation-of-enamel-amelogenesis/

[7] From: Warshawsky, H. (1988) The teeth. In: Cell and Tissue Biology: A Textbook of Histology (L. Weiss) 6th edn. pp. 595–640. Urban and Schwarzenberg, Baltimore. Reproduced by permission of Elsevier.

https://pocketdentistry.com/4-enamel/

[8] https://max.book118.com/html/2012/0217/1105070.shtm

[9] Incremental Development of Primate Dental Enamel

https://www.researchgate.net/publication/272152243_Incremental_Development_of_Primate_Dental_Enamel

[10] Courtesy of V.-L. Ferrer and A. Lichter, University of Connecticut School of Dental Medicine.

https://www.muhadharaty.com/lecture/16470/%D8%AF--%D8%B3%D9%87%D9%8A%D8%B1/the-enamel-pptx

[11] Association of Maternal Stress and Social Support During Pregnancy With Growth Marks in Children's Primary Tooth Enamel

https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2785880?resultClick=3

[12] Association of Maternal Stress and Social Support During Pregnancy With Growth Marks in Children's Primary Tooth Enamel

https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2785880?resultClick=3

[13] https://www.researchgate.net/profile/Erin-Dunn-2

[14] https://www.researchgate.net/profile/Felicitas-Bidlack

[15] https://www.bostonglobe.com/2022/03/07/metro/tooth-fairy-is-calling-boston-researchers-seeking-baby-teeth-health-study/

[16] https://www.vox.com/the-highlight/22876530/baby-teeth-science-anthropology

[17] https://www.bostonglobe.com/2022/03/07/metro/tooth-fairy-is-calling-boston-researchers-seeking-baby-teeth-health-study/

[18] https://www.bostonglobe.com/2022/03/07/metro/tooth-fairy-is-calling-boston-researchers-seeking-baby-teeth-health-study/

[19] Uncovering system-specific stress signatures in primate teeth with multimodal imaging

https://www.nature.com/articles/srep18802#Sec1

[20] https://www.smithsonianmag.com/science-nature/ancient-teeth-reveal-our-roots-180969495/

[21] Dean, C., Leakey, M., Reid, D. et al. Growth processes in teeth distinguish modern humans from Homo erectus and earlier hominins. Nature 414, 628–631 (2001). https://doi.org/10.1038/414628a

[22] Wintertime stress, nursing, and lead exposure in Neanderthal children. https://www.science.org/doi/10.1126/sciadv.aau9483

[23] Siebke I, Moghaddam N, Cunningham CA, Witzel C, Lösch S. Those who died very young-Inferences from δ15 N and δ13 C in bone collagen and the absence of a neonatal line in enamel related to the possible onset of breastfeeding. Am J Phys Anthropol. 2019 Aug;169(4):664-677. doi: 10.1002/ajpa.23847. Epub 2019 May 3. PMID: 31050814.

[24] 2021Growth of Neanderthal infants from Krapina (120–130 ka), CroatiaProc. R. Soc. B.2882021207920212079http://doi.org/10.1098/rspb.2021.2079

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