Enamel Deposition Research Articles

EMMPRIN/CD147 deficiency disturbs ameloblast–odontoblast cross-talk and delays enamel mineralization

Tooth development is regulated by a series of reciprocal inductive signaling between the dental epithelium and mesenchyme, which culminates with the formation of dentin and enamel. EMMPRIN/CD147 is an Extracellular Matrix MetalloPRoteinase (MMP) INducer that mediates epithelial–mesenchymal interactions in cancer and other pathological processes and is expressed in developing teeth. Here we used EMMPRIN knockout (KO) mice to determine the functional role of EMMPRIN on dental tissue formation. We report a delay in enamel deposition and formation that is clearly distinguishable in the growing incisor and associated with a significant reduction of MMP-3 and MMP-20 expression in tooth germs of KO mice. Insufficient basement membrane degradation is evidenced by a persistent laminin immunostaining, resulting in a delay of both odontoblast and ameloblast differentiation. Consequently, enamel volume and thickness are decreased in adult mutant teeth but enamel maturation and tooth morphology are normal, as shown by micro-computed tomographic (micro-CT), nanoindentation, and scanning electron microscope analyses. In addition, the dentino-enamel junction appears as a rough calcified layer of approximately 10±5μm thick (mean±SD) in both molars and growing incisors of KO adult mice. These results indicate that EMMPRIN is involved in the epithelial-mesenchymal cross-talk during tooth development by regulating the expression of MMPs. The mild tooth phenotype observed in EMMPRIN KO mice suggests that the direct effect of EMMPRIN may be limited to a short time window, comprised between basement membrane degradation allowing direct cell contact and calcified matrix deposition.

Comparison of tetrachromic VOF stain to other histochemical staining techniques for characterizing stromal soft and hard tissue components

The components of hard tissues including dentin, enamel, cementum, bone and other calcified deposits, and mature and immature collagen pose problems for identification in routine hematoxylin and eosin (H & E) stained sections. Use of combinations of stains can demonstrate the components of hard tissues and soft tissues distinctly. We assessed the efficacy of the Verde Luz-orange G-acid fuchsin (VOF) stain for differentiating hard and soft connective tissues and compared results with other histochemical staining techniques. Eighty tissue sections comprising developing tooth (30), ossifying fibroma (30) and miscellaneous pathologies (20) expected to contain varying types of calcified tissues were stained with H & E, VOF, and Masson's trichrome (MT). In developing tooth, VOF demonstrated better differentiation of hard tissues, while it was comparable to MT for ossifying fibroma and miscellaneous pathologies. The intensity of staining was greater with VOF than with the other stains studied. VOF stains hard tissue components distinctly and gives good contrast with the surrounding connective tissue. VOF is comparable to MT, but has added advantages including single step staining, rapid and easy procedures, and it distinguishes the maturity of the tissues.

Brief Communication: An enigmatic enamel alteration on the anterior maxillary teeth in a prehistoric North Italian population.

In this paper we describe a hitherto undocumented modification of the dental enamel surface observed in an Early Bronze Age population from northern Italy. The defect, which can be described as a curvilinear groove, is located on the lingual surface of incisors and canines in the upper jaw. This groove, documented both in the permanent and deciduous dentition, is located at approximately 1 mm from the cervix and extends from the mesiolingual to the distolingual surface. The occurrence of the groove is not related to the sex of the affected individuals, but its degree of expression is related to age at death. Because of its morphology, the groove cannot be considered as a result of disruptions in the process of enamel deposition. At the present stage of research we suggest that the groove might have been the result of some kind of dental erosion caused by as yet unidentified chemical factors.

Patterns of morphological variation in enamel-dentin junction and outer enamel surface of human molars.

Tooth crown patterning is governed by the growth and folding of the inner enamel epithelium (IEE) and the following enamel deposition forms outer enamel surface (OES). We hypothesized that overall dental crown shape and covariation structure are determined by processes that configurate shape at the enamel-dentine junction (EDJ), the developmental vestige of IEE. This this hypothesis was tested by comparing patterns of morphological variation between EDJ and OES in human permanent maxillary first molar (UM1) and deciduous second molar (um2). Using geometric morphometric methods, we described morphological variation and covariation between EDJ and OES, and evaluated the strength of two components of phenotypic variability, canalization and morphological integration, in addition to the relevant evolutionary flexibility, i.e. the ability to respond to selective pressure. The strength of covariation between EDJ and OES was greater in um2 than in UM1, and the way that multiple traits covary between EDJ and OES was different between these teeth. The variability analyses showed that EDJ had less shape variation and a higher level of morphological integration than OES, which indicated that canalization and morphological integration acted as developmental constraints. These tendencies were greater in UM1 than in um2. On the other hand, EDJ and OES had a comparable level of evolvability in these teeth. Amelogenesis could play a significant role in tooth shape and covariation structure, and its influence was not constant among teeth, which may be responsible for the differences in the rate and/or period of enamel formation.

Ameloblasts Express Type I Collagen during Amelogenesis

Enamel and enameloid, the highly mineralized tooth-covering tissues in living vertebrates, are different in their matrix composition. Enamel, a unique product of ameloblasts, principally contains enamel matrix proteins (EMPs), while enameloid possesses collagen fibrils and probably receives contributions from both odontoblasts and ameloblasts. Here we focused on type I collagen (COL1A1) and amelogenin (AMEL) gene expression during enameloid and enamel formation throughout ontogeny in the caudate amphibian, Pleurodeles waltl. In this model, pre-metamorphic teeth possess enameloid and enamel, while post-metamorphic teeth possess enamel only. In first-generation teeth, qPCR and in situ hybridization (ISH) on sections revealed that ameloblasts weakly expressed AMEL during late-stage enameloid formation, while expression strongly increased during enamel deposition. Using ISH, we identified COL1A1 transcripts in ameloblasts and odontoblasts during enameloid formation. COL1A1 expression in ameloblasts gradually decreased and was no longer detected after metamorphosis. The transition from enameloid-rich to enamel-rich teeth could be related to a switch in ameloblast activity from COL1A1 to AMEL synthesis. P. waltl therefore appears to be an appropriate animal model for the study of the processes involved during enameloid-to-enamel transition, especially because similar events probably occurred in various lineages during vertebrate evolution.

Craniofacial and dental manifestations of triple X syndrome associated with congenital hypothyroidism: a case report

Triple X syndrome (47,XXX) is a numerical chromosomal alteration that affects 1/1,000 women, in which the woman is born with an extra X chromosome. Some oral changes have been reported in the literature, as hypodontia, influence on deposition of crown enamel and discrepancies in cephalometric measurements. Other systemic complications may lead to oral abnormalities similar to those seen in triple X patients, such as congenital hypothyroidism (CH). This paper reports a triple X syndrome case associated with CH later treated. Besides delay in cognitive and intellectual development, the patient had changes in teeth development and in cephalometric measurements with deficiencies in the maxilla and mandible. This is the first report of a triple X syndrome associated with CH. Both conditions may result in changes in dentofacial development.

Prospective in (Primate) Dental Analysis through Tooth 3D Topographical Quantification

The occlusal morphology of the teeth is mostly determined by the enamel-dentine junction morphology; the enamel-dentine junction plays the role of a primer and conditions the formation of the occlusal enamel reliefs. However, the accretion of the enamel cap yields thickness variations that alter the morphology and the topography of the enamel–dentine junction (i.e., the differential deposition of enamel by the ameloblasts create an external surface that does not necessarily perfectly parallel the enamel–dentine junction). This self-reliant influence of the enamel on tooth morphology is poorly understood and still under-investigated. Studies considering the relationship between enamel and dentine morphologies are rare, and none of them tackled this relationship in a quantitative way. Major limitations arose from: (1) the difficulties to characterize the tooth morphology in its comprehensive tridimensional aspect and (2) practical issues in relating enamel and enamel–dentine junction quantitative traits. We present new aspects of form representation based exclusively on 3D analytical tools and procedures. Our method is applied to a set of 21 unworn upper second molars belonging to eight extant anthropoid genera. Using geometrical analysis of polygonal meshes representatives of the tooth form, we propose a 3D dataset that constitutes a detailed characterization of the enamel and of the enamel–dentine junction morphologies. Also, for the first time, to our knowledge, we intend to establish a quantitative method for comparing enamel and enamel–dentine junction surfaces descriptors (elevation, inclination, orientation, etc.). New indices that allow characterizing the occlusal morphology are proposed and discussed. In this note, we present technical aspects of our method with the example of anthropoid molars. First results show notable individual variations and taxonomic heterogeneities for the selected topographic parameters and for the pattern and strength of association between enamel–dentine junction and enamel, the enamel cap altering in different ways the “transcription” of the enamel–dentine junction morphology.

Transcription factor BCL11B enforces asymmetric enamel‐secreting cell development in the mouse incisor by bidirectional regulation of gene expression

Dental caries is one of the most common diseases, affecting 92% of adults, and is a primary reason for tooth loss. The current state‐of‐the‐art approach to replace a missing tooth is by means of a dental implant, which is, however, prone to mechanical and biological failure. Therefore, the ultimate goal of the therapeutic sciences is to develop dental tissue and whole tooth bioengineering.Molecular mechanisms of tooth organogenesis and enamel formation can be studied using mouse incisor development. Asymmetric deposition of enamel only on the outer side of the tooth makes mouse incisor an attractive model system.We show that the transcription factor BCL11B strongly enforces asymmetric localization of enamel‐secreting cells (ameloblasts) in the mouse incisor. BCL11B does so by promoting development and differentiation of the epithelial stem cells into the ameloblasts on the outer side while suppressing these processes on the inner side of the tooth. Underlying these spatio‐specific functions of BCL11B is the bidirectional regulation of a large gene network, comprised of genes encoding members of the FGF, TGFβ, and Sonic hedgehog signaling.These studies are of significance because they will contribute to a broader understanding of how the activities of transcription and growth factors are regulated during tooth organogenesis and how they can be modulated as an approach to therapy and tooth bioengineering.

Tooth development in a model reptile: functional and null generation teeth in the gecko Paroedura picta.

This paper describes tooth development in a basal squamate, Paroedura picta. Due to its reproductive strategy, mode of development and position within the reptiles, this gecko represents an excellent model organism for the study of reptile development. Here we document the dental pattern and development of non-functional (null generation) and functional generations of teeth during embryonic development. Tooth development is followed from initiation to cytodifferentiation and ankylosis, as the tooth germs develop from bud, through cap to bell stages. The fate of the single generation of non-functional (null generation) teeth is shown to be variable, with some teeth being expelled from the oral cavity, while others are incorporated into the functional bone and teeth, or are absorbed. Fate appears to depend on the initiation site within the oral cavity, with the first null generation teeth forming before formation of the dental lamina. We show evidence for a stratum intermedium layer in the enamel epithelium of functional teeth and show that the bicuspid shape of the teeth is created by asymmetrical deposition of enamel, and not by folding of the inner dental epithelium as observed in mammals.

Nicotinic Receptor Alpha7 Expression during Tooth Morphogenesis Reveals Functional Pleiotropy

The expression of nicotinic acetylcholine receptor (nAChR) subtype, alpha7, was investigated in the developing teeth of mice that were modified through homologous recombination to express a bi-cistronic IRES-driven tau-enhanced green fluorescent protein (GFP); alpha7GFP) or IRES-Cre (alpha7Cre). The expression of alpha7GFP was detected first in cells of the condensing mesenchyme at embryonic (E) day E13.5 where it intensifies through E14.5. This expression ends abruptly at E15.5, but was again observed in ameloblasts of incisors at E16.5 or molar ameloblasts by E17.5–E18.5. This expression remains detectable until molar enamel deposition is completed or throughout life as in the constantly erupting mouse incisors. The expression of alpha7GFP also identifies all stages of innervation of the tooth organ. Ablation of the alpha7-cell lineage using a conditional alpha7Cre×ROSA26-LoxP(diphtheria toxin A) strategy substantially reduced the mesenchyme and this corresponded with excessive epithelium overgrowth consistent with an instructive role by these cells during ectoderm patterning. However, alpha7knock-out (KO) mice exhibited normal tooth size and shape indicating that under normal conditions alpha7 expression is dispensable to this process. The function of ameloblasts in alpha7KO mice is altered relative to controls. High resolution micro-computed tomography analysis of adult mandibular incisors revealed enamel volume of the alpha7KO was significantly reduced and the organization of enamel rods was altered relative to controls. These results demonstrate distinct and varied spatiotemporal expression of alpha7 during tooth development, and they suggest that dysfunction of this receptor would have diverse impacts upon the adult organ.

BCL11B Regulates Epithelial Proliferation and Asymmetric Development of the Mouse Mandibular Incisor

Mouse incisors grow continuously throughout life with enamel deposition uniquely on the outer, or labial, side of the tooth. Asymmetric enamel deposition is due to the presence of enamel-secreting ameloblasts exclusively within the labial epithelium of the incisor. We have previously shown that mice lacking the transcription factor BCL11B/CTIP2 (BCL11B hereafter) exhibit severely disrupted ameloblast formation in the developing incisor. We now report that BCL11B is a key factor controlling epithelial proliferation and overall developmental asymmetry of the mouse incisor: BCL11B is necessary for proliferation of the labial epithelium and development of the epithelial stem cell niche, which gives rise to ameloblasts; conversely, BCL11B suppresses epithelial proliferation, and development of stem cells and ameloblasts on the inner, or lingual, side of the incisor. This bidirectional action of BCL11B in the incisor epithelia appears responsible for the asymmetry of ameloblast localization in developing incisor. Underlying these spatio-specific functions of BCL11B in incisor development is the regulation of a large gene network comprised of genes encoding several members of the FGF and TGFβ superfamilies, Sprouty proteins, and Sonic hedgehog. Our data integrate BCL11B into these pathways during incisor development and reveal the molecular mechanisms that underlie phenotypes of both Bcl11b−/− and Sprouty mutant mice.

Root length in the permanent teeth of women with an additional X chromosome (47,XXX females)

Objective. Previous studies have demonstrated differential effects of the X and Y chromosomes on dental development. The expression of sexual dimorphism in terms of tooth size, shape, number and developmental timing has been explained especially by Y chromosome influence. The Y chromosome promotes enamel, crown and root dentin development. The X chromosome has an effect on enamel deposition. The aim of this research is to study the influence of the extra X chromosome on the development of permanent tooth root length. Material and methods. The study subjects (all of whom were from the Kvantti Dental Research Project) were seven 47,XXX females, five female relatives and 51 and 52 population control men and women, respectively. Measurements were made from panoramic radiographs on available permanent teeth by a digital calliper according to established procedures. Results. The results showed that the maxillary root lengths of the 47,XXX females were of the same magnitude as those in normal women, but the mandibular root lengths were longer in 47,XXX females than in normal men or women. Conclusions. Increased enamel thickness in the teeth of 47,XXX females is apparently caused by the active enamel gene in all X chromosomes having no increased influence on crown dentin formation. These results in 47,XXX females indicate an increase in root dentin development, at least in the mandible, which together with the data on crown formation reflects a continuous long-lasting effect of the X chromosome on dental development.

Brief communication: Contributions of enamel‐dentine junction shape and enamel deposition to primate molar crown complexity

Molar crown morphology varies among primates from relatively simple in some taxa to more complex in others, with such variability having both functional and taxonomic significance. In addition to the primary cusps, crown surface complexity derives from the presence of crests, cuspules, and crenulations. Developmentally, this complexity results from the deposition of an enamel cap over a basement membrane (the morphology of which is preserved as the enamel-dentine junction, or EDJ, in fully formed teeth). However, the relative contribution of the enamel cap and the EDJ to molar crown complexity is poorly characterized. In this study we examine the complexity of the EDJ and enamel surface of a broad sample of primate (including fossil hominin) lower molars through the application of micro-computed tomography and dental topographic analysis. Surface complexity of the EDJ and outer enamel surface (OES) is quantified by first mapping, and then summing, the total number of discrete surface orientation patches. We investigate the relative contribution of the EDJ and enamel cap to crown complexity by assessing the correlation in patch counts between the EDJ and OES within taxa and within individual teeth. We identify three patterns of EDJ/OES complexity which demonstrate that both crown patterning early in development and the subsequent deposition of the enamel cap contribute to overall crown complexity in primates.

Current knowledge of tooth development: patterning and mineralization of the murine dentition

The integument forms a number of different types of mineralized element, including dermal denticles, scutes, ganoid scales, elasmoid scales, fin rays and osteoderms found in certain fish, reptiles, amphibians and xenarthran mammals. To this list can be added teeth, which are far more widely represented and studied than any of the other mineralized elements mentioned above, and as such can be thought of as a model mineralized system. In recent years the focus for studies on tooth development has been the mouse, with a wealth of genetic information accrued and the availability of cutting edge techniques. It is the mouse dentition that this review will concentrate on. The development of the tooth will be followed, looking at what controls the shape of the tooth and how signals from the mesenchyme and epithelium interact to lead to formation of a molar or incisor. The number of teeth generated will then be investigated, looking at how tooth germ number can be reduced or increased by apoptosis, fusion of tooth germs, creation of new tooth germs, and the generation of additional teeth from existing tooth germs. The development of mineralized tissue will then be detailed, looking at how the asymmetrical deposition of enamel is controlled in the mouse incisor. The continued importance of epithelial-mesenchymal interactions at these later stages of tooth development will also be discussed. Tooth anomalies and human disorders have been well covered by recent reviews, therefore in this paper we wish to present a classical review of current knowledge of tooth development, fitting together data from a large number of recent research papers to draw general conclusions about tooth development.

Vascularization of Engineered Teeth

The implantation of cultured dental cell-cell re-associations allows for the reproduction of fully formed teeth, crown morphogenesis, epithelial histogenesis, mineralized dentin and enamel deposition, and root-periodontium development. Since vascularization is critical for organogenesis and tissue engineering, this work aimed to study: (a) blood vessel formation during tooth development, (b) the fate of blood vessels in cultured teeth and re-associations, and (c) vascularization after in vivo implantation. Ex vivo, blood vessels developed in the dental mesenchyme from the cap to bell stages and in the enamel organ, shortly before ameloblast differentiation. In cultured teeth and re-associations, blood-vessel-like structures remained in the peridental mesenchyme, but never developed into dental tissues. After implantation, both teeth and re-associations became revascularized, although later in the case of the re-associations. In implanted re-associations, newly formed blood vessels originated from the host, allowing for their survival, and affording conditions organ growth, mineralization, and enamel secretion.

Differential Expression of the Tight Junction Proteins, Claudin‐1, Claudin‐4, Occludin, ZO‐1, and PAR3, in the Ameloblasts of Rat Upper Incisors

Tight junctions (TJs) create a paracellular permeability barrier to restrict the passage of ions, small solutes, and water. Ameloblasts are enamel-forming cells that sequentially differentiate into preameloblasts, secretory, transition, and ruffle-ended and smooth-ended maturation ameloblasts (RAs and SAs). TJs are located at the proximal and distal ends of ameloblasts. TJs at the distal ends of secretory ameloblasts and RAs are well-developed zonula occludens, but other TJs are moderately developed but incomplete zonula occludens (ZO) or less-developed macula occludens. We herein examined the immunofluorescence localization of TJ proteins, 10 claudin isoforms, occludin, ZO-1, and PAR3, a cell polarity-related protein, in ameloblasts of rat upper incisors. ZO-1 and claudin-1 were detected at both ends of all ameloblasts except for the distal ends of SAs. Claudin-4 and occludin were detected at both ends of transition and maturation ameloblasts except for the distal ends of SAs. PAR3 was detected at the proximal TJs of all ameloblasts and faintly at the distal TJs of early RAs. These results indicate that functional zonula occludens formed at the distal ends of the secretory ameloblasts and RAs consisted of different TJ proteins. Therefore, the distal TJs of secretory ameloblasts and RAs may differentially regulate the paracellular permeability to create a microenvironment suitable for enamel deposition and enamel maturation, respectively. In addition, PAR3 may be principally involved in the formation and maintenance of the proximal, but not distal, TJs.

An FGF signaling loop sustains the generation of differentiated progeny from stem cells in mouse incisors

Rodent incisors grow throughout adult life, but are prevented from becoming excessively long by constant abrasion, which is facilitated by the absence of enamel on one side of the incisor. Here we report that loss-of-function of sprouty genes, which encode antagonists of receptor tyrosine kinase signaling, leads to bilateral enamel deposition, thus impeding incisor abrasion and resulting in unchecked tooth elongation. We demonstrate that sprouty genes function to ensure that enamel-producing ameloblasts are generated on only one side of the tooth by inhibiting the formation of ectopic ameloblasts from self-renewing stem cells, and that they do so by preventing the establishment of an epithelial-mesenchymal FGF signaling loop. Interestingly, although inactivation of Spry4 alone initiates ectopic ameloblast formation in the embryo, the dosage of another sprouty gene must also be reduced to sustain it after birth. These data reveal that the generation of differentiated progeny from a particular stem cell population can be differently regulated in the embryo and adult.

Regulation of epithelial stem cells in tooth regeneration

Teeth form as epithelial appendages and the mechanisms regulating their development share similarities with other organs such as hairs, glands, and gut. However, the regenerative potential of mammalian teeth is generally limited. Stem cells have been identified in the epithelium of continuously growing incisors of mice. We have identified a network of signalling molecules that regulates the proliferation and differentiation of these stem cells, and that thereby influences the incisors' growth and enamel formation. The signals, including FGFs, BMPs, and Activin, mediate interactions between the mesenchymal and epithelial cells within the stem cell niche and form an integrated network. Follistatin antagonizes the functions of BMPs and Activin, and is a key regulator of the asymmetry of the incisor structure. The evolutionary variation in the growth capacity of teeth and the extent of enamel deposition may have resulted from fine-tuning of this signal network. In addition, subtle variations in this or in related regulatory networks may explain the different regenerative capacities of various organs and animal species. To cite this article: I. Thesleff et al., C. R. Biologies 330 (2007).

Nature of laminations and mineralization in rhinoceros enamel using histology and X-ray synchrotron microtomography: Potential implications for palaeoenvironmental isotopic studies

Incremental features of rhinoceros molar enamel were studied quantitatively on histological slices. The different kinds of incremental features of enamel were related to one another and counted to determine the rate and duration of crown formation and to clarify the status of the laminations. The mineralization and the microstructure of the enamel were examined in a rhinoceros molar germ using X-ray synchrotron microtomography respectively using absorption and phase contrast mode at different levels of resolution, from 45.71 μm for the largest scale up to 0.28 μm for the finest one. The different microtomographic data were combined to obtain a multiscale integrated overview of the microstructure and mineralization in order to understand better the relationship between these two aspects. Additional human and herbivorous mammal teeth were examined qualitatively using histological sectioning to obtain a better understanding of laminations in mammal tooth enamel. Laminations, regularly spaced features parallel to the Retzius lines, represent regular isochrons of enamel deposition. This study shows that laminations and cross-striations are equivalent daily features in enamel of rhinoceros, humans, and probably of middle sized and large herbivorous mammals in general. Laminations are shown to be three-dimensional alignments of cross-striations. In addition to the fact that cross-striations are visible only in prismatic enamel, observation of histological slices, and high resolution phase contrast X-ray synchrotron microtomography demonstrate three main parameters influencing the dominant aspect of daily features in rhinoceros and human enamel: the angle between prisms and Retzius lines, the enamel deposition rate and the thickness of the histological slices. Given their daily nature, laminations can be used to determine the crown formation time of middle sized and big herbivorous mammals. Quantitative study of the enamel mineralization in the rhinoceros germ fragment using X-ray synchrotron microtomography suggests that the innermost 20 μm of the enamel near the enamel–dentin junction are highly mineralised (77–90% of the final enamel mineralization) during, or just after the deposition of the corresponding enamel matrix. Integrated microtomographic study shows that this area corresponds to the thin aprismatic enamel layer at the EDJ level plus a thin layer of radial enamel directly adjacent. It appears that, in some parts of the enamel, there may be a considerable time lag between matrix deposition and full maturation, which could have important implications for geochemical studies. This study reinforces the previously suggested hypothesis that isotopic measurements derived from the innermost enamel may represent the period most closely related to matrix secretion. By using information derived from incremental features (laminations and Retzius lines) and restricting the isotopic measurements to the innermost part of the enamel, precise temporal calibration of isotopic signals should be possible, and the complication of time averaging due to enamel maturation could be avoided or strongly reduced.

Humans are evolutionarily adapted to caloric restriction resulting from ecologically dictated dietary deprivation imposed during the Plio–Pleistocene period

Humans are evolutionarily adapted to chronic undernutrition as a consequence of ecologically dictated dietary restriction. Increased aridity, cooler temperatures and increased climatic oscillation effected an alteration of the quantity and quality of vegetation upon which hominids depended for food during the Plio-Pleistocene period. Hominids responded physiologically to climate-induced caloric curtailment in the same way organisms respond to experimentally imposed caloric restriction: by reducing the rate and/or altering the manner in which they metabolized fuel. Such metabolic alterations are mediated principally by the hypothalamus and it is herein hypothesized that the human hypothalamus was subjected to substantial selective pressure, promoting an energetically conservative hypometabolic state. Moreover, the most salient phenotypic characteristics typifying the human species - long lifespan, low reproductive potential, lengthy development and high brain/bodyweight ratio - are effectuated in organisms undergoing caloric restriction. These phenotypic/physiological characteristics - herein termed the quadripartite complex - can be modulated by metabolic rate, which is, in turn, modulated by the hypothalamus. An appreciable alteration in climate occurred between 2.0 and 1.5 million years ago, a juncture at which one hominid lineage (Paranthropus) went extinct. Paranthropus was characterized by such external adaptations as robust cranio-facial morphology and pronounced enamel deposition, indicative of subsistence on tough, low-quality vegetal foods. Conversely, the Homo lineage responded to its marginal dietary repertoire through internal means, centering on metabolic suppression. It is herein hypothesized that this adaptive metabolic alteration, enacted in response to ecologically imposed caloric restriction, produced the defining morphologic attributes of Homo and enabled the evolutionary success of the human species. Among the implications of this line of thinking is that modern humans may be particularly sensitive to the deleterious effects of excess energy intake and, concomitantly, particularly amenable to the ameliorative effects of caloric restriction.