Development and terminal differentiation of pulp and periodontal nerve elements in subcutaneous transplants of molar tooth germs and incisors of the rat.

Ectopic tooth transplants are known to receive rich innervation of local neurons, but the precise location and structural features of neurites in the pulp and periodontal ligament (PDL) of such transplants are unclear. In this experiment, the molar tooth germs of rat embryos and incisors of young rats were subcutaneously transplanted into the dorsal regions of rats and processed, at various time intervals, for immunohistochemical demonstration of neural elements. Teeth with periodontal tissue elements developed in most of the molar transplants in 6 or 8 wk and received rich innervation, including some autonomic fibres, in the pulp. Nerve elements were also confirmed to be present in the PDL of these transplants, including specialized nerve ending-like structures reminiscent of the periodontal Ruffini endings. Mechanoreceptor-like structures were also induced in the regenerated PDL of similarly transplanted incisors, although the success rate was low. We conclude that rich and highly ordered innervation of the pulp, and occasional development of mechanoreceptors in the regenerated PDL of ectopic dental transplants, imply a high probability of successful induction of teeth with both nociceptive and mechanical sensations in the ectopic tooth and/or tooth germ transplant systems, although differentiation of mechanoreceptor-like nerve endings occurred in only a few rare cases.

Dorsal longitudinal stretch receptor of Drosophila melanogaster larva - fine structure and maturation.

Insects possess two types of sensory neurons: ciliated type I sensory neurons that innervate external sensory organs and chordotonal organs, and type II sensory neurons that form a subepidermal plexus or innervate stretch receptors. Among stretch receptors, a dorsel longitudinal stretch receptor is highly conserved in insects, being found in all insect orders investigated. Here we describe the topology and anatomical structure of this receptor in the fruit fly embryo and larva using transmission electron microscopy and single cell staining for fluorescence microscopy. The receptor is composed of the dorsal bipolar dendrite neuron, which arises from an archetypal cell lineage, its sister glial cell and the peripheral glial cell accompanying the nerve. The neuron is situated among the muscles in the dorsal body wall on the intersegmental nerve. Its two dendrites stretch the length of the segment to the segmental folds. The neuron is wrapped by both glial cells and surrounded by a common basal lamina, which fans out at the dendritic tips to attach them to the epidermal cells at the segmental borders.

Central and peripheral anatomy of slowly adapting type I low-threshold mechanoreceptors innervating trunk skin of neonatal mice.

Despite intensive study, our understanding of the neuronal structures responsible for transducing the broad spectrum of environmental energies that impinge upon the skin has rested on inference and conjecture. This major shortcoming motivated the development of ex vivo somatosensory system preparations in neonatal mice in the hope that their small size might allow the peripheral terminals of physiologically identified sensory neurons to be labeled intracellularly for direct study. The present report describes the first such study of the peripheral terminals of four slowly adapting type I low-threshold mechanoreceptors (SAIs) that innervated the back skin of neonatal mice. In addition, this report includes information on the central anatomy of the same SAI afferents that were identified peripherally with both physiological and anatomical means, providing an essentially complete view of the central and peripheral morphology of individual SAI afferents in situ. Our findings reveal that SAIs in neonates are strikingly adult-like in all major respects. Afferents were exquisitely sensitive to mechanical stimuli and exhibited a distinctly irregular, slowly adapting discharge to stimulation of 1-4 punctate receptive fields in the skin. Their central collaterals formed transversely oriented and largely nonoverlapping arborizations limited to regions of the dorsal horn corresponding to laminae III-V. Their peripheral arborizations were restricted entirely within miniaturized touch domes, where they gave rise to expanded disc-like endings in close apposition to putative Merkel cells in basal epidermis. These findings therefore provide the first direct confirmation of the functional morphology of this physiologically unique afferent class.

Effect of BDNF depletion on the formation of Ruffini endings in vibrissa follicles and the survival of their mechanoreceptive neurons in trigeminal ganglion.

We examined the influence of BDNF depletion in peripheral tissues on the formation of Ruffini endings and their neuronal survival by injections of neutralizable anti-BDNF antibody into mouse mystacial pads for periods of 5 days at different developmental stages of the Ruffini endings (the pre-formation stage from the 2nd to 6th day after birth, the formation stage from the 4th to 8th, or the post-formation stage from the 10th to 14th). The treatment at the pre-formation and formation stages caused a significant decrease in the number of Ruffini endings in vibrissa follicles. This decrease in Ruffini endings was accompanied with a significant increase in neuron apoptosis in the trigeminal ganglion (TG) in both stages. However, at the post-formation stage, the anti-BDNF injection showed no effect on the formation of the mechanoreceptors nor their neuronal survival. In the post-formation stage, the axoplasmic spins of Ruffini endings were circumferentially embraced with the cytoplasmic processes of terminal Schwann cells. The present study indicates that target-derived BDNF is essential for survival of mechanoreceptive nerves in the pre-formation and formation stages, but not in the post-formation stages of their development. It seems that Schwann cells participate in this switch-over of neuronal dependency on brain-derived neurotrophic factor.

Multiple mechanosensory modalities influence development of auditory function.

Sensory development can be dependent on input from multiple modalities. During metamorphic development, ranid frogs exhibit rapid reorganization of pathways mediating auditory, vestibular, and lateral line modalities as the animal transforms from an aquatic to an amphibious form. Here we show that neural sensitivity to the underwater particle motion component of sound follows a different developmental trajectory than that of the pressure component. Throughout larval stages, cells in the medial vestibular nucleus show best frequencies to particle motion in the range from 15 to 65 Hz, with displacement thresholds of <10 mum. During metamorphic climax, best frequencies significantly increase, and sensitivity to lower-frequency (<25 Hz) stimuli tends to decline. These findings suggest that continued sensitivity to particle motion may compensate for the considerable loss of sensitivity to pressure waves observed during the developmental deaf period. Transport of a lipophilic dye from peripheral end organs to the dorsal medulla shows that fibers from the saccule in the inner ear and from the anterior lateral line both terminate in the medial vestibular nucleus. Saccular projections remain stable across larval development, whereas lateral line projections degenerate during metamorphic climax. Sensitivity to particle motion may be based on multimodal input early in development and on saccular input alone during the transition to amphibious life.

Involvement of GDNF and its receptors in the maturation of the periodontal Ruffini endings.

Our recent study revealed an intense immunoreaction for GDNF and its receptors in the Ruffini endings, primary mechanoreceptors in the periodontal ligament, of young rats. However, no information is available for the expression of GDNF and its receptors during their development. The present study aimed to reveal postnatal changes in the immuno-expression of GDNF, GFRalpha1 and RET in the periodontal Ruffini endings of the rat incisors by double immunofluorescent staining. At postnatal day 3 (PO 3d), no structure with GDNF-, GFRalpha1-, or RET-immunoreaction existed in the periodontal ligament. The PGP 9.5-positive nerve fibers without GDNF- and RET-immunoreaction displayed a dendritic fashion at PO 1w, with a GFRalpha1-reaction found around these nerves. At PO 2w, GDNF-positive terminal Schwann cells occurred near the thick and dendritic axons, a part of which showed a RET-reaction, with no reactive cells near the thin nerves. The terminal Schwann cells became positive for GFRalpha1, but lacked RET-immunoreaction. At PO 3w, when the formation of the periodontal Ruffini endings had proceeded, GDNF-positive terminal Schwann cells began to increase in number. This stage-specific immuno-expression pattern suggests that GDNF is a key molecule for the maturation and maintenance of the periodontal Ruffini endings.

Structure and development of free neuromasts in barramundi, Lates calcarifer (Block).

This study was conducted to clarify the development of free neuromasts with growth of the barramundi, Lates calcarifer. A pair of free neuromasts was observed behind the unpigmented eyes in newly hatched eleutheroembryos with a mean total length of 1.93 mm, and two-hour-old eleuthero-embryos could respond to an approaching pipette. At 2 days after hatching, the egg yolk sac was mostly consumed, the eyes were pigmented, and the larvae commenced feeding on rotifers. Free neuromasts increased in number with growth and commenced developing into canal neuromasts in barramundi 15 days old with a mean total length of 8.07 mm. The average length of the major axis of the trunk free neuromasts attained approximately 12.9-15.5 microm, and the number of sensory cells was 15.4-17.5 at 15-20 days old. Developed cupulae of free neuromasts were observed in 1-day-old eleutheroembryos. The direction of maximum sensitivity of free neuromasts, determined from the polarity of the sensory cells, coincided with the minor axis of the lozenge-shaped outline of the apical surface of the free neuromasts. The polarity of trunk neuromasts was usually oriented along the antero-posterior axis of the fish body, but a few had a dorso-ventral direction. On the head, free neuromasts were oriented on lines tangential to concentric circles around the eye.

Ontogeny of air-motion sensing in cricket.

Juvenile crickets suffer high rates of mortality by natural predators that they can detect using extremely sensitive air-sensing filiform hairs located on their cerci. Although a huge amount of knowledge has accumulated on the physiology, the neurobiology and the biomechanics of this sensory system in adults, the morphological and functional aspects of air sensing have not been as well studied in earlier life history stages. Using scanning electronic microscopy, we performed a survey of all cercal filiform hairs in seven instars of the wood cricket (Nemobius sylvestris). Statistical analyses allowed us to quantify profound changes in the number, the length and the distribution of cercal hairs during development. Of particular importance, we found a fivefold increase in hair number and the development of a bimodal length-frequency distribution of cercal hairs from the second instar onwards. Based on theoretical estimations of filiform hair population coding, we found that the cercal system is functional for a wide range of frequencies of biologically relevant oscillatory flows, even from the first instar. As the cricket develops, the overall sensitivity of the cercal system increases as a result of the appearance of new hairs, but the value of the best tuned frequency remains fixed between 150 and 180 Hz after the second instar. These frequencies nicely match those emitted by natural flying predators, suggesting that the development of the cercal array of hairs may have evolved in response to such signals.

Requirement of proper occlusal force for morphological maturation of neural components of periodontal Ruffini endings of the rat incisor.

The present study examined the effect of reduced occlusal force on morphological maturation of periodontal Ruffini endings, primary mechanoreceptors in the periodontal ligament, of the rat incisor. The reduction of occlusal force was induced by grinding the cutting edges of unilateral incisors of the rat from postnatal day 14 (PN14d), when periodontal Ruffini endings are immature. Under normal development, the axon terminals of Ruffini endings gradually ramified with the passage of time, and showed ruffled outlines having numerous dot-like structures around PN28d. When the mechanical stimulation was reduced, appearance of dot-like structures at the axon terminals delayed. Quantitative analysis elucidated that the percentages of immunoreactive areas for protein gene product 9.5, a marker protein of neural elements, at ground side were significantly smaller than those at non-ground side 14 days following the initial grinding. The distribution and morphology of terminal Schwann cells was not apparently affected. The present results indicate that the proper mechanical stimulation to the ligament contributes to the morphological maturation of the periodontal Ruffini endings.

Lateral line hair cell maturation is a determinant of aminoglycoside susceptibility in zebrafish (Danio rerio).

Developmental differences in hair cell susceptibility to aminoglycoside-induced cell death has been observed in multiple species. Increased sensitivity to aminoglycosides has been temporally correlated with the onset of mechanotransduction-dependent activity. We have used in vivo fluorescent vital dye markers to further investigate the determinants of aminoglycoside induced hair cell death in the lateral line of zebrafish (Danio rerio). Labeling hair cells of the lateral line in vivo with the dyes FM 1-43, To-Pro-3, and Yo-Pro-1 served as reliable indicators of hair cell viability. Results indicate that hair cell maturation is a determinant of developmental differences in susceptibility. The age dependent differences in susceptibility to aminoglycosides are independent of the onset of mechanotransduction-dependent activity as measured by FM 1-43 uptake and independent of hair cell ability to take up fluorescently conjugated aminoglycosides.

Effects of neurotrophin-3 gene mutation in the expression of neurocalcin.

Neurocalcin (NC) is a neuron-specific "EF-hand" calcium-binding protein present in a non-fully characterized subpopulation of dorsal root ganglion (DRG) neurons, some kinds of mechanoreceptors and proprioceptors, and in motor end-plates. In the present study we have characterized NC expression in spinal sensory and motor neurons, and their endings in newborn mouse. Because the neurotrophic factor neurotrophin-3 (NT-3) appears to plays a major role in the development and maturation of sensory and motor neuronal populations, we have studied NC immunoreactivity in newborn NT-3 null mutant. In NT-3 deficient animals the overall number of NC-immunoreactive DRG neurons was reduced by as much as 70% including all large neurons, but subpopulations of NC expressing small and intermediate-sized neurons survived. As expected no muscle spindles were found in NT-3 mutant mice while they were present and normally innervated by NC-positive nerve fibers in wild-type animals. On the other hand, NC immunoreactivity was dramatically decreased in motoneurons of the spinal cord, ventral root nerves and motor end-plates in the absence of NT-3. The present results demonstrate that NC-containing DRG neurons include all proprioceptive, and a subset of mechanoreceptive and proprioceptive. Furthermore, they strongly suggest that NT-3 is involved in the maturation of motor end-plates.

BDNF, but not NT-4, is necessary for normal development of Meissner corpuscles.

Meissner corpuscles are rapidly adapting cutaneous mechanoreceptors depending for development on TrkB expressing sensory neurons, but it remains to be established which of the known TrkB ligands, BDNF or NT-4, is responsible of this dependence. In this study we analyze Meissner corpuscles in the digital pads of mice with target mutations in the genes encoding for either BDNF or NT-4, using immunohistochemistry and transmission-electron microscopy, and they were identified based on their morphology and expression of S100 protein. All wild-type animals as well as NT-4(-/-) animals and BDNF and NT4 heterozygous animals have Meissner corpuscles that are normal in number and size. However, Meissner corpuscles are absent the BDNF(-/-) mice. These results suggest that BDNF is the only TrkB ligand involved in the development of Meissner corpuscles in murine glabrous skin, and it probably regulates the development of the sensory neurons that innervate Meissner corpuscles.

Lateral line receptors: where do they come from developmentally and where is our research going?

The lateral line system is composed of both mechanoreceptors, which exhibit little variation in structure between taxonomic groups, and electroreceptors, which exhibit considerably more variation. Cathodally sensitive ampullary electroreceptors are the primitive condition and are found in agnathans, chondrichthyans, and most osteichthyans. Aquatic amphibians also have ampullary electroreceptors for at least part of their life cycle. The more recently evolved anodally sensitive ampullary electroreceptors and tuberous electroreceptors are only found in four groups of teleost fishes. The basic ontogenetic unit of lateral line development is the dorsolateral placode. Primitively, there are six pairs of placodes, which pass through sequential stages of development into lateral line receptors. There is no question about the origin of primitive mechanoreceptors or electroreceptors, however, we do not have a good understanding of the origin of teleost mechanoreceptors and their ampullary or tuberous electroreceptors; do they come exclusively from dorsolateral placodes or from neural crest or even general ectoderm? A second intriguing lateral line question is how certain teleost fish groups evolved tuberous electroreceptors. Electroreception appears to have re-evolved at least twice in teleosts after being lost during the neopterygian radiation. It has been suggested that the development of tuberous electroreceptors might be due to changes in placodal patterning or a change in the general ectoderm that placodes arise from. Unfortunately, our understanding of lateral line origins in fishes is very sketchy, and, if we are to answer such an evolutionary question, we first need more complete information about lateral line development in a variety of fishes, which can then be combined with gene expression data to better interpret lateral line receptor development.

Keeping sensory cells and evolving neurons to connect them to the brain: molecular conservation and novelties in vertebrate ear development.

The evolution of the mechanosensory cellular module and the molecular details that regulate its development has included morphological modifications of these cells as well as the formation of larger assemblies of mechanosensory cell aggregates among metazoans. This has resulted in a wide diversity of mechanosensory organs. The wide morphological diversity of organs, including the associated morphological modifications of the mechanosensory cells, suggests parallel evolution of these modules and their associated organs. This morphological diversity is in stark contrast to the molecular conservation of developmental modules across phyla. These molecular data suggest that the evolution of mechanosensory transduction might have preceded that of distinct cellular differentiation. However, once a molecular network governing development of specialized cells involved in mechanosensory transduction evolved, that molecular network was preserved across phyla. Present data suggest that at least the common ancestor of triploblastic organisms, perhaps even the common diploblastic ancestor of bilaterian metazoans, had molecular and cellular specializations for mechanosensation. It is argued that the evolution of multicellular organs dedicated to specific aspects of mechanosensation, such as gravity and sound perception, are evolutionary transformations that build on this conserved molecular network for cellular specialization, but reflect distinct morphological solutions. We propose that the sensory neurons, connecting the craniate ear with the brain, are a derived feature of craniates, and possibly chordates, that came about through diversification of the lineage forming mechanosensory cells during development. This evolutionarily late event suggests a heterochronic shift, so that sensory neurons develop in mammals prior to mechanosensory hair cells. However, sensory neuron development is connected to hair cell development, likely in a clonal relationship. The theme of cellular conservation is reiterated in two examples of chordate otic diversification: the evolution of the horizontal canal system and the evolution of the basilar papilla/cochlea. It is suggested that here again, cellular multiplication and formation of a special epithelium predates the functional transformation to an 'organ' system for horizontal angular acceleration and sound pressure reception, respectively. Overall, evolution of the vertebrate ear needs to be understood as an interplay between and utilization of two gene networks or modules. One is at the level of the molecularly and developmentally conserved mechanosensory cellular module. The other is an increased complexity in the morphology of both adult mechanosensory cells and organs by the addition of end-stage and novel features and associated gene networks to detect specific aspects of mechanosensory stimuli.

A tissue specific cytochrome P450 required for the structure and function of Drosophila sensory organs.

Cytochrome P450s have generally been acknowledged as broadly tuned detoxifying enzymes. However, emerging evidence argues P450s have an integral role in cell signaling and developmental processes, via their metabolism of retinoic acid, arachidonic acid, steroids, and other cellular ligands. To study the morphogenesis of Drosophila sensory organs, we examined mutants with impaired mechanosensation and discovered one, nompH, encodes the cytochrome P450 CYP303a1. We now report the characterization of nompH, a mutant defective in the function of peripheral chemo- and mechanoreceptor cells, and demonstrate CYP303a1 is essential for the development and structure of external sensory organs which mediate the reception of vital mechanosensory and chemosensory stimuli. Notably this P450 is expressed only in sensory bristles, localizing in the apical region of the socket cell. The wide diversity of the P450 family and the growing number of P450s with developmental phenotypes suggests the exquisite tissue and subcellular specificity of CYP303a1 illustrates an important aspect of P450 function; namely, a strategy to process critical developmental signals in a tissue- and cell-specific manner.

Myf5 expression in satellite cells and spindles in adult muscle is controlled by separate genetic elements.

The myogenic regulatory factor Myf5 is integral to the initiation and control of skeletal muscle formation. In adult muscle, Myf5 is expressed in satellite cells, stem cells of mature muscle, but not in the myonuclei that sustain the myofibre. Using the Myf5(nlacZ/+) mouse, we now show that Myf5 is also constitutively expressed in muscle spindles-stretch-sensitive mechanoreceptors, while muscle denervation induces extensive reactivation of the Myf5 gene in myonuclei. To identify the elements involved in the regulation of Myf5 in adult muscle, we analysed reporter gene expression in a transgenic bacterial artificial chromosome (BAC) deletion series of the Mrf4/Myf5 locus. A BAC carrying 140 kb upstream of the Myf5 transcription start site was sufficient to drive all aspects of Myf5 expression in adult muscle. In contrast, BACs carrying 88 and 59 kb upstream were unable to drive consistent expression in satellite cells, although expression in muscle spindles and reactivation of the locus in myonuclei were retained. Therefore, as during development, multiple enhancers are required to generate the full expression pattern of Myf5 in the adult. Together, these observations show that elements controlling adult Myf5 expression are genetically separable and possibly distinct from those that control Myf5 during development. These studies are a first step towards identifying cognate transcription factors involved in muscle stem cell regulation.

Development of the lateral line system in the shovelnose sturgeon.

The lateral line systems of aquatic amphibians and all chondrichthyan and osteichthyan fish present a similar array of mechanoreceptors. However, electroreceptors, the second major component of the lateral line system, have clearly undergone more significant evolutionary change. Chondrichthyans and non-neopterygian fish possess primitive ampullary organ electroreceptors, whereas significantly different 'new' ampullary organs and tuberous electroreceptors are found in a few groups of teleosts (mormyrids, gymnotids and some catfish). The pairing of mechano- and electroreceptors in the lateral line system, as well as the morphologically and physiologically distinct electroreceptors of teleosts have inspired several recent studies on the origin and evolution of the lateral line receptors. We described the development of the lateral line system in sturgeon (Scaphirhynchus platorynchus) as part of an outgroup analysis of lateral line development in three taxa: vertebrates that have both mechanoreceptive neuromasts and primitive electroreceptors; neopterygian fish that only have mechanoreceptors; and teleosts that have re-evolved new electroreceptors. Development in Scaphirhynchus was consistent with previously studied taxa in that the lateral line system developed from a series of six dorsolateral placodes. Interestingly, we found that the octaval placode was bound rostrally and caudally by large placodal fields, out of which the six lateral line placodes arose. This finding supports recent suggestions for a common placodal primordium for all placodes. Each of the six placodes gave rise to the lateral line nerves before elongating into sensory ridges, which contained neuromast primordia. The ampullary organ fields of Scaphirhynchus arose from the lateral zones of the anterodorsal, anteroventral, otic and supratemporal sensory ridges, which is also consistent with recently studied taxa. Comparisons of the lateral line system of Scaphirhynchus and close relatives, Acipenser and Polyodon, indicate that variation in some aspects of lateral line receptor numbers and distribution are related to changes in head morphology and feeding strategy, whereas other changes, such as a reduction in receptor number without a change in placode field size, indicate changes in placode development.

Patent ductus arteriosus remodels cardiac sensory neuronal chemotransduction.

We sought to determine the capacity of neonatal ventricular sensory nerve endings (neurites) to transduce the cardiac milieu in the presence of cardiovascular pathology. The spontaneous activity generated by nodose ganglion cardiac afferent neurons was identified in situ using extracellular recording techniques in two groups of piglets approximately 2 weeks old: (i). controls that underwent sham operations (n=19 piglets) 2 weeks earlier and (ii). a pathological model of patent ductus arteriosus stented open for about 2 weeks (n=16 piglets). The capacity of ventricular sensory neurites associated with nodose ganglion afferent neurons to transduce local mechanical (including alterations in right or left ventricular volumes) or chemical stimuli was studied in both groups. The average conduction velocity of afferent axons associated with identified neuronal somata was estimated to be 1.5+/-0.6 or 2.9+/-1.3 m s(-1). Ventricular afferent neurons transduced mechanical stimuli similarly in both groups. In control animals, ventricular afferent neurons transduced the following chemicals: the sodium channel modifier veratridine (delta 23+/-7 impulses min(-1)), the P(1)-purinoceptor agonist adenosine (Delta 24+/-8 impulses min(-1)), and the beta-adrenoceptor agonist isoproterenol (delta 18+/-7 impulses min(-1)). On the other hand, patent ductus arteriosus cardiac afferent neurons did not transduce these chemicals. It is concluded that neonatal cardiac afferent neuronal chemosensory-as opposed to mechanosensory-transduction remodels in the presence of a patent ductus arteriosus. The reduced capacity of neonatal cardiac afferent neurons to transduce chemicals in the presence of a patent ductus arteriosus should be taken into account when considering neonatal cardiovascular control in such a state.

Long continuous actin bundles in Drosophila bristles are constructed by overlapping short filaments.

The actin bundles essential for Drosophila bristle elongation are hundreds of microns long and composed of cross-linked unipolar filaments. These long bundles are built from much shorter modules that graft together. Using both confocal and electron microscopy, we demonstrate that newly synthesized modules are short (1-2 microm in length); modules elongate to approximately 3 microm by growing over the surface of longitudinally adjacent modules to form a graft; the grafted regions are initially secured by the forked protein cross-bridge and later by the fascin cross-bridge; actin bundles are smoothed by filament addition and appear continuous and without swellings; and in the absence of grafting, dramatic alterations in cell shape occur that substitutes cell width expansion for elongation. Thus, bundle morphogenesis has several components: module formation, elongation, grafting, and bundle smoothing. These actin bundles are much like a rope or cable, made by overlapping elements that run a small fraction of the overall length, and stiffened by cross-linking.

Involvement of brain-derived neurotrophic factor (BDNF) in the development of periodontal Ruffini endings.

The periodontal Ruffini ending has been reported to show immunoreactivity for tyrosine kinase B (trkB), the high-affinity receptor for brain-derived neurotrophic factor (BDNF), in the periodontal ligament of the rat incisor. Furthermore, adult heterozygous BDNF-mutant mice showed malformation and reduction of the periodontal Ruffini endings. To investigate further roles of BDNF in these structures, the development, distribution, and terminal morphology of Ruffini endings were examined in the incisor periodontal ligament of heterozygous and homozygous BDNF mutant mice, as well as in the wild-type littermate by immunohistochemistry for protein gene product (PGP) 9.5, a general neuronal marker. A similar distribution and terminal formation of PGP 9.5-immunoreactive nerve fibers was recognized in the periodontal ligament of all phenotypes at postnatal week (PW) 1. At this stage, the nerve fibers had a beaded appearance, but did not form the periodontal Ruffini endings. At PW2, the heterozygous and wild-type mice started to show ramified nerve fibers resembling the mature shape of periodontal Ruffini endings. At PW3, the Ruffini endings occurred in the periodontal ligament of the wild-type and heterozygous mice. While the Ruffini endings of the wild-type mice appeared either ruffled or smooth, as reported previously, most of these structures showed a smooth outline in the heterozygous mice. The homozygous mice lacked the typical Ruffini endings at PW3. In the quantitative analysis, homozygous mice had the smallest percentages of PGP 9.5-immunoreactive areas at the same postnatal periods, but there were no significant differences between wild-type and heterozygous mice during PW1-3. These findings suggest a possible involvement of BDNF during the postnatal development and, in particular, the maturation of periodontal Ruffini endings. Furthermore, other neurotrophins may play a role in the development and/or early maturation of the periodontal nerve fibers, as indicated by the presence of nerve fibers in the BDNF-homozygous mice.