The people behind the papers – Elena Popa and Abigail Tucker
While many vertebrates have multiple sets of teeth over their lifetime,
some, like humans, have just a single set of replacement teeth
(diphydonty), while others, like mice, manage with a single set
(monophydonty). This diversity raises both evolutionary questions –
how did different tooth replacement strategies evolve? – and
developmental ones – what mechanisms prevent replacement teeth
in animals that have lost them? A new paper in this issue of
Development tackles these questions with a molecular analysis of
mouse tooth development. We caught up with first author Elena Popa
and her supervisor Abigail Tucker, Professor of Development and
Evolution at King’s College London, to find out more about the work.
Abigail, can you give us your scientific biography and the
questions your lab is trying to answer?
AT My lab is interested in how bodies are formed during
development, both from a clinical perspective of understanding
birth defects, but also from the point of view of understanding how
evolution has shaped our bodies. I started out investigating tadpole
tail development for my PhD with Jonathan Slack and then
swapped ends and moved to the head for my first postdoc with Paul
Sharpe. Here, I investigated how the face and dentition are
patterned. This is where I first encountered tooth development, and
although I have moved on to study the cranial neural crest, the jaw,
cranial glands and ear, I have always kept some experiments going
to understand more about the tooth. Teeth are often the only thing
left preserved in the fossil record, so they have a central importance
to our understanding of evolution. There are still lots of
unanswered questions, such as what regulates tooth number,
tooth size and tooth shape? It’s a great model for understanding
epithelial-mesenchymal interactions, as both tissues are integral to
the formation of the final tooth but take it in turns to play the
Elena, how did you come to join the Tucker lab and what
drives your research?
EP Developmental biology was by far my favourite subject during
my undergraduate years at Royal Holloway. I didn’t know it was
going to be tooth development in particular back then, but as soon as
I read about Professor Abigail Tucker’s research I was completely
captivated. Her lab provided the opportunity to perform a
comparative study of molecular interactions and developmental
processes that allow or disrupt tooth replacement in awide variety of
vertebrates. Why can’t we have more than two sets of teeth but
snakes can? This was essentially the question that drove my research
in the Tucker lab.
What initially led you to tryand ‘reawaken’ tooth replacement
in the mouse?
EPWhy the potential for tooth replacement varies so much across
vertebrates is an intriguing question. Having performed an in-
depth study of the development of the dental lamina in many
different animal models, we suspected that this structure retains the
capacity to give rise to a subsequent generation of teeth, even in the
molar region of the mouse (the mouse only has one generation of
teeth and molars, in general, do not replace). The dental lamina
next to the first molar can be seen protruding at E16.5 but
disappears soon after birth, and has been termed the rudimentary
successional dental lamina (RSDL). We compared the RSDL with
the successional lamina of the minipig (which gives rise to a
second generation tooth), and found that both expressed the
epithelial stem cell marker Sox2; however, Wnt activity was only
present in the minipig lamina. Knowing that stimulating Wnt
signalling by means of different transgenic lines leads to the
formation of supernumerary teeth, we based our experimental
design on these comparisons, aiming to recapitulate tooth
replacement in the mouse.
Can you give us the key results of the paper in a paragraph?
EP & ATOur results show that, although the mouse normally does
not form a second replacement set of teeth, it still has the potential to
do so if given the right signals. Stimulation of Wnt signalling in the
rudimentary replacement lamina in transgenic mice or isolation of
the lamina in culture both led to the formation of a new tooth. We
started by showing that the RSDL exhibits molecular similarities to
the competent dental lamina in a diphyodont mammal and retains
odontogenic capacity, which we were able to reawaken by
selectively inducing Sox2+ cells to activate canonical Wnt/β-
catenin signalling.Wewere able to confirm the dental identity of the
structures that arose from the RSDL in the mutant mice by
performing in situ hybridization for genes known to be expressed
during normal tooth development. The mutant RSDL was also
highly proliferative and gave rise to multiple ectopic teeth, many of
which were complex in shape and mineralised after transplantation
in kidney capsule. We also uncovered an inhibitory relationship
betweenWnt signalling and Sox2, where ectopic stimulation ofWnt
signalling leads to downregulation of Sox2 expression.
Elena Popa (L) and Abigail Tucker (R).
Centre for Craniofacial and Regenerative Biology, Department of Craniofacial
Development and Stem Cell Biology, King’s College London, London SE1 9RT, UK.
E-mail: [email protected]
© 2019. Published by The Company of Biologists Ltd | Development (2019) 146, dev176313. doi:10.1242/dev.176313
When you remove the first-generation tooth, this frees the
RSDL to form a tooth bud. How is this potential inhibited in
the context of normal development?
EP&AT Tooth number has previously been shown to be controlled
by a balance between activators and inhibitors, creating an
inhibitory cascade. For example, in many mammals three molars
form at the back of the mouth by serial addition from a single molar
placode. If the primordium for the subsequent molars is separated
from the murine first molar in culture, the second molar initiates
development faster and grows to a larger size than if left intact. The
first molar therefore appears to be controlling the development of
the next tooth in the series. In the shrew, the first generation of teeth
initiate but then regress and are replaced by the permanent set of
teeth. Here, again, it has been suggested that early formation of the
permanent teeth might inhibit the development of the first set:
timing and spatial arrangement of tooth germs is therefore clearly
important in the control of final tooth number.
In our paper, we show that the RSDL has the potential to form a
tooth and speculate that the adjacent molar sends a Wnt-inhibitory
signal to the surrounding dental tissue. This then prevents Wnt
activity in the RSDL, and leads to its regression. This is relevant to
human tooth replacement, as structures similar to the RSDL have
been identified next to the permanent teeth during development. In
normal development of our dentition, therefore, the permanent tooth
may inhibit the generation of a third set of teeth.
Wnt signalling leads to a reduction in Sox2 in the dental
epithelium but not associated epithelia – what makes this
relationship context dependent?
AT The context-dependent nature of the relationship between Wnts
and Sox2 was very striking. This fits, however, with the literature,
which has shown similar context-dependent interactions. For
example, in the airway submucosa, Sox2 has both an inductive
and a repressive effect on Wnt signalling that is dependent on the
presence of other factors, whereas in the lungWnts inhibit Sox2 but
only at early stages of development. In the tooth, the repression of
Sox2 by Wnts might be dependent on other factors with dynamic
temporal and spatial expression patterns. It will be very intriguing to
work out what these factors might be.
Do you know of any evolutionary scenarios where
monophyodonty transitioned to diphyodonty, and if so does
this involve a similar revitalisation to that you have
discovered in the mouse?
EP&AT Throughout evolution, the general trend is one where animals
reduce the number of tooth generations in favour of more-complex tooth
shapes and better occlusion. As in themouse, there is often evidence of a
rudimentary structure, which points to this reduction in number. For
example,wehave shown that in thediphyodont fruit bat, the canine shows
evidence of a third generation as it displayed a vestigial structure
homologous to the mouse RSDL next to the second-generation tooth. In
nature, there are rare cases proposed where teeth have been lost and then
reappear, e.g. in the frogGastrotheca guentheri, where teeth are found on
the lower jaw but are absent in all other frogs. This would suggest that
rudimentary tooth structures can be reawakened not just in the lab.
Did the research include any particular result or eureka
moment that has stuck with you?
AT For me the eureka moment was when we generated a tooth germ
from the RSDL by simply cutting off the main tooth. Really it’s a
simple experiment but has a key message, which is that the reason a
mouse doesn’t have a second set of teeth is that the first generation
of teeth inhibits this from happening. This has important
consequences, as it means that if this inhibition could be lifted an
extra set of teeth might be possible.
And what about the flipside: any moments of frustration
EP For my PhD I worked with a lot of non-model organisms (bats,
geckos, guinea pigs, opossum) in addition to the minipig and mouse
shown here. These samples were always much more difficult to
obtain and every time you wanted to look at gene expression it