Tomes' process

Abstract: The shape of Tomes' process and its relationship to enamel precursors, the growing enamel, and the apical terminal bars were studied with light and electron microscopes in the enamel organs of the lower incisors of adult rats. It was found that the proximal part of Tomes' process has a complex cross section which is described here as consisting of head, body, and large and small foot processes. The shape of the cross section is related to the direction of tooth eruption and the direction of the presumed sideways motion of the ameloblast. At the level of the apical terminal bars the ameloblast cross section is rectangular, with the long axis in the direction of the axis of the tooth. The major terminal bars have a large number of filaments showing little recognizable order, and an almost uninterrupted zonula adherens. The minor terminal bars have fewer filaments which often form well-defined bundles, and their zonula adherens is more frequently interrupted by membrane invaginations, maculae occludentes, and distensions of the extracellular space. Stippled material was seen first in irregularly shaped vesicles in the ameloblast apex just proximal to the terminal bar level. It appears to be secreted into the extracellular space at the level of the terminal bars and distal to them. It is incorporated into the interrod enamel in an erratic fashion and remains uncalcified for a period of time. The anomaly of its distribution may indicate that it is not an essential enamel component in the rat incisor. No new stippled material was seen to be secreted in rod enamel formation. The first enamel to be formed is a broad band of interrod enamel between Tomes' processes of the same row. It is formed in close proximity to, and thus presumably to a large extent by, Tomes' processes of the neighboring (in apical direction) row. A thin band of interrod enamel between rows is then deposited by ameloblasts of the adjacent rows, while the enamel rod is formed by one ameloblast. At the enamel growth fronts, the three enamel components (crystallites, pericrystal membrane, background matrix) appear almost simultaneously in the enamel space. Secretion granules seem to participate in rod enamel formation by a type of exocytosis.

Abstract: Characters from enamel microstructure have not been used in recent phylogenetic analyses of Mesozoic Mammalia. Reasons are that enamel characters have been perceived as (A) variable without regard to systematic position of taxa, (B) inconsistently reported within the literature, and (C) simply scored as either prismatic or not prismatic in earlier mammals. Our work on Mesozoic mammals such as Sinoconodon, Gobiconodon, Triconodontidae, Docodon, Laolestes, and others suggests that synapsid columnar enamel (SCE) structure was easily transformed into plesiomorphic prismatic enamel (PPE) and that PPE may be described with at least five independent character states. Two PPE characters—a flat, open prism sheath and a planar prism seam—were present in the cynodont Pachygenelus and in several Jurassic and Cretaceous mammals. We propose that appearance of a prism sheath transforms SCE into PPE and that reduction and loss of a prism sheath reverse PPE into SCE, in both phylogeny and ontogeny. We further propose that no amniote vertebrates other than the trithelodontid cynodont, Pachygenelus, plus Mammalia have ever evolved an ameloblastic Tomes process capable of secreting PPE and that the genetic potential to secrete PPE is a synapomorphy of Pachygenelus plus Mammalia, whether or not all lineages of the clade have expressed that potential.

Abstract: The three major proteins in the enamel matrix of developing teeth are amelogenin, enamelin, and ameloblastin (1). These proteins are expressed from related genes, all belonging to the secretory calcium-binding phosphoprotein (SCPP) family (2). Intact enamelin is a large, proline-rich, phosphorylated glycoprotein. Secreted human enamelin has 1,099 amino acids. Unlike amelogenin and ameloblastin, the enamelin primary RNA transcript does not undergo alternative splicing, so there is only a single isoform of enamelin. In pigs, secreted enamelin is a 186-kDa protein that localizes to the mineralization front along the secretory surface of ameloblasts (3). Matrix metalloproteinase 20 (MMP20) rapidly degrades it into many fragments, one of which is a stable 32-kDa domain that accumulates within the matrix (4, 5). Expression of the enamelin gene (ENAM, 4q13) is largely restricted to developing teeth. The human enamelin expressed sequence tag (EST) profile ({"type":"entrez-nucleotide","attrs":{"text":"Hs667018","term_id":"317654759","term_text":"HS667018"}}Hs667018) from normal tissues (which does not include developing teeth) lists only 10 enamelin cDNAs out of 3,360,307 characterized. No tissue has more than two enamelin ESTs, which is a pattern that is characteristic of trace expression. Defects in ENAM contribute to the aetiology of non-syndromic forms of amelogenesis imperfecta (AI), a collection of inherited diseases featuring enamel malformations as the only phenotype (6–8). ENAM mutations cause AI that generally follows an **autosomal-dominant pattern of inheritance (9), with variations in both penetrance and expressivity. When a single ENAM allele is defective, the phenotype can be non-penetrant (10), or be manifested as enamel pits (11), horizontal grooves (12), or generalized thin enamel (13). There is also a dose effect: when both ENAM alleles are defective the enamel is extremely thin or non-existent (11). Enamelin gene and deduced amino acid sequences are available from species representing the main groups of tetrapods (14). There are 25 unchanging amino acid positions when the 36 known mammalian enamelin protein sequences are aligned with the few non-mammalian sequences that go back to amphibians (frog). Sites of enamelin post-translational modifications are highly conserved, including several potential and known phosphorylation and N-linked glycosylation sites, as well as six cysteines believed to form disulphide bridges (14). Evolutionary analyses have not only identified functionally critical positions in the enamelin protein, but they have also demonstrated that all of the major secretory-stage enamel proteins serve necessary functions only in tooth formation and specifically in dental enamel formation. Genes encoding enamel matrix proteins have degenerated into pseudogenes in multiple toothless or enamel-less species that descended from ancestors with teeth covered by enamel. The genes encoding amelogenin, ameloblastin, and enamelin have degenerated in birds (14, 15) and in toothless (baleen) whales (16). A functional enamelin gene is absent in enamel-less (Kogia) whales (16) and in four different orders of placental mammals with toothless and/or enamel-less taxa (17). Genetic studies in humans strongly corroborate the evolutionary findings. ENAM defects in 10 different kindreds with non-syndromic AI have been reported. One of the cases is caused by the substitution of leucine with a highly conserved phosphoserine (p.S216L) (9). Enamelin knockout mice provide still more support for the tooth-specificity of enamelin function, as the phenotype of these mice is restricted exclusively to defects in the enamel layer (18–20). The genetically engineered Enam knockout/lacZ knockin mouse model has also provided strong support for ameloblast specificity of enamelin expression through X-gal histostaining for nuclear-localized β-galactosidase activity to mark tissues that normally express enamelin. To date, nuclear staining has been detected only in ameloblasts in developing teeth (18). Enam knockout mice also provide a means to investigate normal and pathological enamel formation. Enamelin knockouts demonstrate that enamelin is not only required for the deposition of tooth enamel, but it is also necessary to maintain the ameloblast phenotype, as is the case for ameloblastin (21). Degeneration of the ameloblast layer and the subsequent appearance of cyst-like enamel organ-based structures suggested that enamelin and ameloblastin are cell-adhesion proteins. However, as no enamel layer forms in the Enam and ameloblastin (Ambn) knockout mice, ameloblasts could fail to adhere to the unnatural underlying surface even if their attachment apparatus was intact and did not normally contain enamelin or ameloblastin. RGD sequences are found in enamel proteins from some species, but they are not a conserved feature, and mouse enamelin and ameloblastin both lack these potential integrin-binding motifs. Recombinant ameloblastin shows some affinity for heparin (22) and fibronectin (23) in vitro, but these molecular partners have not been localized in ameloblasts or shown to be important for ameloblast adhesion in vivo. The observations that enamelin is critical for dental enamel formation and for maintenance of the ameloblast phenotype emphasize the dynamic roles played by ameloblasts besides simply creating an extracellular space and environment conducive for mineral deposition. In this study we characterized ameloblasts in Enam heterozygous and null mice by documenting the extent of apoptosis in maxillary first molars starting in the secretory stage at day 5 and continuing until tooth eruption at day 17. We demonstrated that ameloblasts in restricted areas of developing teeth undergo apoptosis when enamelin is completely missing, and that reducing enamelin production by one-half, as occurs in heterozygous mice, may result in more apoptosis later in crown development, despite the fact that normal enamel eventually forms on these teeth.

Abstract: Ultrastructural features of secretory amelogenesis during selachian tooth development show several similarities to mammalian amelogenesis. However, the following critical differences were noticed: 1) subcellular organelles associated with merocrine-type protein synthesis and secretion were located in both the infranuclear as well as supranuclear regions of the selachian ameloblasts; 2) no evidence for Tomes process formation was found; 3) the basal lamina was not removed during epithelial differentiation into ameloblasts in the selachian model, and the structural features of the basal lamina were significantly altered during amelogenesis in rows III, IV, and VI; and 4) no dentine-enameloid junction was detected. It is suggested that enameloid is an extracellular matrix which is derived from the selachian inner enamel epithelium and appears to be secreted from both the lateral and apical surfaces of ameloblasts.