The intestinal proliferon

The intestinal mucosa consists of several rapidly proliferating cell populations. These may be divided according to their function into two classes: (1) parenchyma: or epithelial cells; and (2) supporting cell populations including fibrocytes and vascular endothelial cells. The kinetics of all the cells in the villus are highly co-ordinated. It is postulated that at the pericryptal progenitor region where the cells originate, they are assembled into complex functional units denominated herewith as proliferons. The proliferon consists of four basic elements; parenchyma, connective tissue, blood vessels and nerve fibers. It starts its existence in the pericryptal lamina propria where the proliferating epithelia and fibroblasts receive their own capillary buds and special nerve supply. The displacement of the whole complex towards the villus tip is accompanied by a gradual elongation of the central villus artery. As the proliferon approaches the villus tip its supporting cell populations are catabolized while the epithelium is sloughed off into the lumen.This model has been previously called upon to describe the kinetics of rodent incisor tooth cell populations.     

Early rodent incisor enamel evolution: Phylogenetic implications

Incisor enamel microstructure proved to be a very effective tool for assessment of phylogenetic relationships among the Rodentia. Pauciserial and multiserial Schmelzmuster are clearly distinct by structural characters such as orientation of interprismatic matrix, presence or absence of transition zones between Hunter-Schreger bands (HSB), inclination of HSB, enamel thickness, and others. Pauciserial HSB are structurally very close to the earliest known mammalian HSB found in Paleocene arctocyonids. Biomechanical arguments and outgroup comparison with mixodontians indicate that the pauciserial Schmelzmuster is a symplesiomorphy of the Rodentia. Transitional stages from pauciserial to multiserial Schmelzmuster were observed in middle Eocene ctenodactyloids and from pauciserial to uniserial in middle to late Eocene anomalurids. The multiserial Schmelzmuster is considered a synapomorphy of the Hystricognathi, ctenodactylids, and pedetids. Schmelzmuster evolution reflects the early dichotomy of the Rodentia: In the Asian ctenodactyloid lineage a multiserial Schmelzmuster evolved once and in the North American/European ischyromyoid lineage a uniserial Schmelzmuster developed several times convergently. The pauci- to uniserial Schmelzmuster of the anomalurids excludes a close relationship to the phiomyids, because the ctenodactyloid-phiomyid lineage is characterized by the development of a multiserial Schmelzmuster.     

Epidermal growth factor-induced precocious incisor eruption is associated with decreased tooth size

Epidermal growth factor (EGF) causes precocious eruption of incisors in vivo and is mitogenic for tooth-derived cells in vitro. These two observations lead to the hypothesis that the EGF-induced precocious eruption is the result of an increase in the size of the incisor. To test this hypothesis, neonatal mice were injected daily with various doses of EGF and, seven days after birth, were perfused with fixative. EGF causes a retardation of overall growth (as measured by body weight) and a dose-dependent thickening of the epidermis. The incisors were examined in midsagittal histological sections and in X-ray microradiographs. Contrary to our expectations, EGF causes a dose-dependent decrease in the size of the incisors. This result suggests that the stimulation of the growth of odontogenic cells seen in tissue culture is not part of the physiological response to EGF in vivo and that EGF-induced precocious eruption of incisors is not due to an increase in the growth rate of the tooth.     

Studies on dentin 2. transient vasodentin in the incisor teeth of a rodent (perognathus longimembris).

Vascular inclusions regularly occur in the lingual dentin of the constantly erupting teeth of the pocket mouse (P. longimembris). The inclusion of a capillary loop and surrounding perivascular tissues is associated with odontoblasts whose cytodifferentiation is relatively immature. These same cells produce dentinal tubules which are more irregular in their course, more arborescent, with more lateral branches, wider in diameter and less numerous than are the tubules of the labial orthodentin. The patent vascular inclusions are surrounded by a broad halo of incompletely mineralized dentin. With further maturation complete obliteration of the vessels occurs, accompanied by complete dental matrix mineralization. A literature review supports the contention that vasodentinogenesis is related operationally to lower stages of odontoblastic cytodifferentiation although the processes by which this occurs are not yet clear.     

Relationship of incisor size to diet and anterior tooth use in sympatric sumatran anthropoids

Researchers often relate anthropoid incisor size to diet and ingestive behaviors. It is suggested that primates that frequently consume large, tough foods (i.e., fruits) require large incisors to process these items. This idea has been difficult to test because of a lack of data on anterior tooth use in wild primates, and a lack of understanding concerning the relationships between food properties and ingestive behaviors. The first field study of primate ingestive behaviors has recently been completed for four species of Sumatran anthropoids: Hylobates lar, Macaca fascicularis, Pongo pygmaeus, and Presbytis thomasi [Ungar, American Journal of Physical Anthropology 95:197–219, 1994; International Journal of Primatology 16:221–245, 1995]. This paper documents both relative and absolute incisor row width differences among these taxa, and evaluates the relationships between incisor size and feeding behaviors for specific taxa. Results indicate that differences in incisor size among these species cannot all be explained by degree of frugivory, food item size, or even degree of incisor use in ingestion alone. It is therefore suggested that inferences of dietary differences based on largely or solely on differences in incisor sizes of specific fossil anthropoid taxa should be approached with caution. © 1996 Wiley-Liss, Inc.     

Structure and composition of the incisor enamel of extant and fossil mammals with tooth pigmentation

The inclusion of iron compounds in teeth, which impart a red to orange colour to them, is a phenomenon shown by several groups of vertebrates in different periods of their evolution. Incisors from fossil and extant shrews and from extant rodents were sectioned and studied with the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to compare their structure and the distribution of Fe. The enamel in white‐ and red‐toothed soricids has three layers; two of them are divided into two zones in the red‐toothed species. However, the most external layer varies among taxa; it is well defined in Sorex but difficult to identify in the Early Pleistocene genera Beremendia or Dolinasorex. In the arvicoline rodent Terricola, only two layers can be defined, the outer of which is divided into two zones depending on the presence or absence of Fe. The Fe proportions in the larger soricids reach up to 45%, but in rodents only up to 10% (weight % with respect to Fe + Ca + P). The STEM study shows that in a fossil soricid the Fe phases form clusters of nanometric particles of very poor crystalline oxides or hydroxides surrounding the apatite crystals that form the enamel.     

Shh expression in a rudimentary tooth offers new insights into development of the mouse incisor

For teeth as for any organ, knowledge of normal development is essential for the proper interpretation of developmental anomalies in mutant mice. It is generally accepted that tooth formation is initiated with a single signaling center that, in the incisor region, is exclusively related to the development of the functional adult incisor. Here, using a unique combination of computer-aided three-dimensional reconstructions and whole mount in situ hybridization of mandibles from finely staged wild-type mouse embryos, we demonstrate that several Sonic hedgehog (Shh) expression domains sequentially appear in the lower incisor region during early development. In contrast to the single Shh expression domain that is widely assumed to be present in each lower incisor area at ED12.5-13.5, we identified two spatially distinct regions of Shh expression that appear in an anterior-posterior sequence during this period. The initial anterior, more superficially located Shh expression region represented the rudimentary (so-called deciduous) incisor, whereas only the later posterior deeper situated region corresponded to the prospective functional incisor. In the more advanced embryos, only this posterior Shh expression in the incisor bud was detectable as a precursor of the enamel knot. This study offers a new interpretation of published molecular data on the mouse incisor from initiation through ED13.5. We suggest that, as with Shh expression, other molecular data that have been ascribed to the progressive development of the mouse functional incisor at early stages, in fact, correspond to a rudimentary incisor whose development is aborted.     

The immunohistolocalization of carbonic anhydrase in rodent tissues.

Carbonic anhydrase has been localized with an immunoenzyme bridge technique in the following sites in paraffin sections of fixed rodent tissues: gastric parietal cells, the brush border of enterocytes in the small intestine, superficial nongoblet cells of the colon, selective segments of the nephron, glial cells, erythrocytes and adipose cells. Immunocytochemical localizations of carbonic anhydrase isozymes I and II in different histologic sites, by means of affinity column purified antibodies, agreed with the distribution of these enzymes in the various sites, as indicated by immunologic assays. The immunocytochemical results are compared with those reported for the cobalt-bicarbonate cytochemical method and with biochemical knowledge of the occurence of carbonic anhydrase.     

The rodent preputial gland

Researches published in the last 2 years have advanced direct evidence for regarding the rodent preputial gland as a scent organ, confirming a speculation made at least as long ago as 1725. The gland is the source of olfactory stimuli which mediate behavioural responses in social and sexual interactions between conspecifics. Although the gland has received little direct attention from investigators, a considerable amount of information on it is available. Most of this was gained ancilliary to other, and very diverse, research objectives so that the literature is very scattered. These references are collated to present a comprehensive review of knowledge of the biology of the rodent preputial gland. Theoretical concepts of population structure and regulation, involving density-dependent factors, require a sensory mechanism to activate the physiological effects attributed to social interactions. The preputial gland, as a source of olfactory cues, has all the biological characteristics required of such a sensory modulator.     

Formation of the tooth enamel rod pattern and the cytoskeletal organization in secretory ameloblasts of the rat incisor

The localization of actin, myosin, tropomyosin, alpha-actinin, vinculin, and desmoplakin I/II was visualized by immunofluorescence microscopy. Antibodies against myosin, tropomyosin, and alpha-actinin and rhodamine-phalloidin labeled strongly the proximal and distal terminal webs which ultrastructurally consist of dense microfilament bundles. In the distal terminal web, the staining by these reagents occurred mostly perpendicular to the long axis of the incisor. Antivinculin stained the general area where the distal terminal web is located in the ameloblast. Anti-desmoplakin I/II labeled the junctional area associated with the proximal and distal terminal webs. The anti-desmoplakin staining was stronger along the cell border perpendicular to the long axis of the incisor. Comparison of the rhodamine-phalloidin staining pattern of the distal terminal web and the enamel secretion pattern by ameloblasts revealed that a change in the distal terminal web staining pattern preceded a change in the secretion pattern. These observations suggest that the cytoskeletal organization in the ameloblast is involved in the formation of the enamel matrix pattern in the rat incisor.     

The in vitro phosphorylation of the native rat incisor dentin phosphophoryns.

Phosphophoryns are the major non-collagenous proteins of the mineralized matrix of rat incisor dentin. Nearly half the phosphophoryn residues are serines, and 85-90% of these are phosphorylated. Since phosphorylation may be important for phosphophoryn function, it was of interest to identify the kinase(s) responsible for catalyzing their phosphophorylation. Rat osteosarcoma (ROS) 17/2.8 osteoblast-like cells were selected as the enzyme source. Native rat incisor phosphophoryns (RIPP-I, II, III) were not substrates for any of the ROS 17/2.8 messenger-dependent kinases but were phosphorylated by membrane-associated endogenous messenger-independent kinases. These were resolved chromatographically and identified as casein kinase (CK) I and II by elution properties and immunoblotting with a CKII antibody. The CKI preferentially used RIPP-III as substrate, while CKII preferred RIPP-I and II. Heparin at 100 and 500 ng/assay and NaCl at 0.25-0.4 M inhibited phosphorylation of the RIPP by CKI and CKII in parallel. At 10 mM spermine, phosphorylation of RIPP-I and II by CKII, and of RIPP-III by CKI were inhibited, but phosphorylation of RIPP-III by CKII was enhanced. Purified sea star oocyte CKII demonstrated the same substrate specificity and spermine concentration shift as the ROS 17/2.8 CKII. These data show that osteoblast-like cells are a rich source of membrane-bound CKI and CKII activity. The different patterns of phosphorylation of RIPP-I, II, and III further show that they are distinct synthetic products of the odontoblast.