Acessibilidade / Reportar erro

Sensory nerve endings in the rat cheek mucosa: an electron microscopic study

Terminações nervosas sensitivas na mucosa da bochecha de ratos: estudo ao microscópio eletrônico

Abstracts

The sensory nerve endings of the rat cheek mucosa were studied using the transmission electron microscopy. The specimens were fixed in modified Karnovsky solution and embedded in Epon resin. The sensory nerve endings showed a central terminal axon containing numerous mitochondria, neurofilaments, microtubules and clear vesicles. The proximal part of corpuscle revealed the cytoplasmic extensions of lamellar cells and the perineural cells. The fine bundles of collagen fibers are identified in the interlamellar spaces and the external part of corpuscle. Numerous concentric lamellae showed caveolae, interlamelar spaces filled with amorphous material, desmosome-type junctions between adjacent lamellae and the inner lamellar cells and the axoplasmic membrane. These fine structures are important to recognise and understand the morphological characteristics in the oral mucosa.

Nerve endings; Lamellated corpuscle; Cheek mucosa; Transmission electron microscopy; Rat


As terminações nervosas sensitivas da mucosa da bochecha de ratos foram estudadas utilizando-se microscopia eletrônica de transmissão. As amostras foram fixadas em solução de Karnovsky modificada e inclusas em resina do tipo Epon. As terminaçãos nervosas sensitivas mostraram um axônio central terminal contendo numerosas mitocôndiras, neurofilamentos, microtúbulos e vesículas claras. A porção proximal dos corpúsculos revelaram projeções citoplasmáticas de células lamelares e células perineurais. Delgados feixes de fibras colágenas foram identificados nos espaços interlamelares e na região externa dos corpúsculos. Numerosas lamelas concêntricas apresentavam caveolas, espaços interlamelares preenchidos por material amorfo, desmossomos do tipo juncional entre lamelas adjacentes e células lamelares internas e membrana axolplásmatica. Essas estruturas delgadas mostram-se importantes na identificação e estudo das características da mucosa oral.

Nervos sensitivos; Corpúsculos lamelares; Mucosa; Microscopia eletrônica de transmissão; Rato


Sensory nerve endings in the rat cheek mucosa: an electron microscopic study

Terminações nervosas sensitivas na mucosa da bochecha de ratos: estudo ao microscópio eletrônico

Ii-sei Watanabe; Marcelo Cavenaghi Pereira da Silva; Marcia Consentino Kronka

Departamento de Anatomia do Instituto de Ciências Biomédicas da Universidade de São Paulo, São Paulo - SP

Correspondence Correspondence to Ii-Sei Watanabe Departamento da Anatomia Instituto de Ciências Biomédicas da USP Av. Prof. Lineu Prestes, 2415 05508-900 - São Paulo - SP E-mail: watanabe@usp.br

SUMMARY

The sensory nerve endings of the rat cheek mucosa were studied using the transmission electron microscopy. The specimens were fixed in modified Karnovsky solution and embedded in Epon resin. The sensory nerve endings showed a central terminal axon containing numerous mitochondria, neurofilaments, microtubules and clear vesicles. The proximal part of corpuscle revealed the cytoplasmic extensions of lamellar cells and the perineural cells. The fine bundles of collagen fibers are identified in the interlamellar spaces and the external part of corpuscle. Numerous concentric lamellae showed caveolae, interlamelar spaces filled with amorphous material, desmosome-type junctions between adjacent lamellae and the inner lamellar cells and the axoplasmic membrane. These fine structures are important to recognise and understand the morphological characteristics in the oral mucosa.

Keywords: Nerve endings. Lamellated corpuscle. Cheek mucosa. Transmission electron microscopy. Rat.

RESUMO

As terminações nervosas sensitivas da mucosa da bochecha de ratos foram estudadas utilizando-se microscopia eletrônica de transmissão. As amostras foram fixadas em solução de Karnovsky modificada e inclusas em resina do tipo Epon. As terminaçãos nervosas sensitivas mostraram um axônio central terminal contendo numerosas mitocôndiras, neurofilamentos, microtúbulos e vesículas claras. A porção proximal dos corpúsculos revelaram projeções citoplasmáticas de células lamelares e células perineurais. Delgados feixes de fibras colágenas foram identificados nos espaços interlamelares e na região externa dos corpúsculos. Numerosas lamelas concêntricas apresentavam caveolas, espaços interlamelares preenchidos por material amorfo, desmossomos do tipo juncional entre lamelas adjacentes e células lamelares internas e membrana axolplásmatica. Essas estruturas delgadas mostram-se importantes na identificação e estudo das características da mucosa oral.

Palavras-chave: Nervos sensitivos. Corpúsculos lamelares. Mucosa. Microscopia eletrônica de transmissão. Rato.

INTRODUCTION

The peripheral sensory nerve endings are mainly localized in the deep portion of the connective tissue and they are responsible for the monitoration of internal and external stimuli. These sensory nerve endings can be classified according to the ultrastructural characteristics of the terminal nerve and surrounding structures arrangement. In the oral mucosa, the first part of the digestive system, special attention was taken to the fine structure of nerve endings on periodontal structures like 1, 10, 17,18 and periodontal ligament3, 2, 7, 6. On the other hand, Watanabe and Yamada 19 and Halata et al. 4 reported the fine structure of nerve endings in the hard palate, Watanabe and Yamada20 in the soft palate, Tachibana et al.13 and Watanabe and Ide 16 in the rat lip.

The present paper describes the fine structure of sensory nerve endings in the rat cheek mucosa with special attention to the lamellated corpuscles, their terminal axon and surrounding cells.

MATERIAL AND METHOD

Six Wistar rats were anaesthetized with sodium pentobarbital and perfused through the left ventricle of the heart with modified Karnovsky fixative solution containing 2.5% glutaraldehyde and 2% paraformaldehyde in a 0.1 M sodium phosphate buffer (pH 7.4). The cheek mucosa was removed and immersed in the same fixative during 3 h at 4°C. The specimens were rinsed with 0.1 M sodium phosphate buffer and postfixed in 1% OsO4 during 2 h at 4°C. Then, the samples were immersed in 5% uranyl acetate aqueous solution for 2 h at room temperature, dehydrated with a graded series of ethanol and embedded in Epon resin 812.

Semi-thin sections were cut in LKB ultramicrotome using glass knives, mounted and stained with 2% toluidine blue for light microscopy study. The ultra-thin sections of the selected areas were cut with Reichert Ultracut Ultramicrotome using diamond knife. The grids were counterstained with uranyl acetate 22 and lead citrate11 and examined in a JEOL 1010 transmission electron microcopy.

RESULTS

In the rat cheek mucosa, several myelinated neurons and organized sensory corpuscles beneath to the epithelium were observed. The myelinated sensory axons lost their myelin sheet and entered to the organized corpuscles. They were separated from other elements like fibroblasts and capillaries by bundles of collagen fibers (Fig. 1).


The organized sensory corpuscles found called lamellar corpuscles because the terminal axon was involved by numerous concentric cytoplasmic lamellae (Fig. 2).


The terminal axon was located in the central portion of the corpuscles and in the lateral part of the terminal axon may be observed axoplasmic protrusions which continue laterally as lateral clefts to the periphery (Fig. 2).

At the periphery, a capsule was formed by cytoplasmic extensions of perineural cells and bundles of collagen fibers disposed in various directions (Fig. 3).


At higher magnification, the axoplasm of axons, presented scattered mitochondria (Fig. 4 and 5) whose cristae mitochondriales were clearly seen (Fig. 4 and 5). The axoplasm also contained neurofilaments and microtubules (Fig. 4 and 5). In the axoplasmic protrusions were observed several clear vesicles and scattered mitochondria (Fig. 5).



The cytoplasm of lamellar cells contained several caveolae (Fig. 6 and 7) which are invaginations of the cytoplasmic membrane and the interlamelar spaces were filled with amorphous material and fine collagen fibers filaments. Desmosome-type junctions were observed between lamellar cells (Fig. 6) and between the terminal axon and adjacent lamellae (Fig. 7). Multivesicular corpuscles were also observed in Fig. 7.



DISCUSSION

The fine structure of lamellated corpuscles observed in our study were similar to those reported by Martinez and Pekarthy 10; Watanabe and Yamada18, in the rat gingiva and Toyoshima et al.14, in primate fungiform papillae. They were composed by several concentric lamellae and a terminal axon in the center.

This corpuscle is an organized sensory nerve ending. Recent articles classified this corpuscle according to the ultrastructural and functional differences. Malinovsky8, included lamellar corpuscles in the class III which comprised rapidly adapting endings whose dendritic zone are associated with elements of ectoderm origin.

Our study revealed a single unmyelinated terminal axon in the center of the corpuscle, which was also reported by Jarayaraj et al.5 in the sheep oral mucosa. On the other hand, Watanabe 15 revealed in the gingiva of man and Cebus apella monkey and Tachibana et al.13 in the vermilion border and mucosal areas of the rat lip, observed accessory unmyelinated axons in addition to the main axon among the inner core. The terminal axon also presented a large number of mitochondria and small clear vesicles particulary in the base of axoplasmic protrusion 5, 21, 13, 16.

It is believed that the axoplasmic protrusion of Paccinian corpuscles play a significant role in the process of transduction12. Tachibana et al.13, in the fungiform papillae of monkeys observed the presence of distinct finger like projections of the terminal axons which may provide advantageous information regarding the position and shape of food in the oral cavity during mastication.

The cytoplasm of lamellar cells contained several invaginations (caveolae) or clear vesicles. Based on the incidence of small clear vesicles, Malinovsky8 classified corpuscles with abundant incidence of vesicles, medium incidence of vesicles and low incidence of vesicles. According to that, the cytoplasmic projections of lamellae could be classified as abundant incidence of vesicles.

Between lamellae , there is an interlamellar space filled with amorphous material and fine collagen fibers filaments. Watanabe and Ide16, in the rat lip, demonstrated cholinesterase activity in the periaxonal spaces between the terminal axon and adjacent lamellae as well as in the interlamellar spaces which may be related to metabolic or nutricional aspects, the trophic effects of lamellar cells on receptors or some relationship with the process of transduction. Although, it was also reported that the clear vesicles were only stationary pictures of the dynamic metabolic action involving transport, intake and elimination of substances onto the surface of cytoplasm9.

Several authors, Martinez and Pekarthy10; Watanabe and Yamada 18, 20; Tachibana et al. 13 and Halata et al. 4 reported the presence of desmosome-type junctions between lamellar cell and the terminal axon and adjacent lamellae which may be related to the mechanical stability of the corpuscle or/and connect adjacent lamellae.

Received:21/01/2002

Accepted: 31/07/2002

  • 1
    BARKER, D.S. The dendritic cell system in the human gingival epithelium. Arch Oral Biol, n.12,:p.203-208, 1967.
  • 2
    BERKOVITZ, B.K.B.; CHORE, R.C.; MOXHAM, B.J. 1983 The occurence of a lamellated nerve terminal in the periodontal ligament of the rat incisor. Arch Oral Biol, n.28, p.94-101, 1983.
  • 3
    GRIFFIN, C.J.; HARRIS, R. Innervation of the human periodontium III. Fine structure of compound receptor. Aust Dent J, n.19, p. 255-260, 1974.
  • 4
    HALATA, Z.; COOPER, B.Y.; BAUMANN, K.I.; SCHWEGMANN, C.; FRIEDMAN, R.M. Sensory nerve endings in the hard palate and papilla incisiva of the goat. Exp Brain Res, n.129, p.218-228, 1999.
  • 5
    JAYARAJ, P.; QUILLIAM, T.A.; TILLY, R. Oral nerve endings in the sheep. Journ Physiol, n.232, p.7-9, 1973.
  • 6
    LAMBRICHTS, I.; CREEMERS, J.; VAN STEENBERGHE, D. Morphology of neural endings in the human periodontal ligament: An electron microscopic study. J Periodont Res, n.27, p.191-196, 1992
  • 7
    MAEDA, T.; KANNARI, K.; SATO, O.; IWANAGA, T. Nerve terminals in human periodontal ligament as demonstrated by immunohistochemistry for neurofilament protein (NFP) and S-100 protein. Arch Hist Cytol, n.53, p.259-265, 1990.
  • 8
    MALINOVSKY, L. Classification of sensory nerve endings in vertebrates brought up to date. Folia Morphol, n.34, p.261-266, 1986.
  • 9
    MALINOVSKY, L.; PAC, L.; DUBOVY, P. On the problems of clear vesicles in sensory corpuscles. Z mikrosk anat Forsch, n.100, p.213-223, 1986.
  • 10
    MARTINEZ, Z.R.; PEKARTHY, J.M. Ultrastructure of encapsulated nerve endings in rat gingiva. Am J Anat, n.140, p.135-138, 1974.
  • 11
    REYNOLDS, E.S. The use of lead citrate at high pH as on electron opaque stain in electron microscopy. J Cell Biol, n.17, p.208, 1963.
  • 12
    SPENCER, P.S.; SCHAUMBURG, H.H. An ultrastructural study of the inner core of the Pacinian corpuscle. J. Neurocytol,n. 2, p.217-235, 1973.
  • 13
    TACHIBANA, T.; SAKAKURA, Y.; ISHIZEKI, K.; NAWA, T. Nerve endings in the vermilion border and mucosal areas of the rat lip. Arch Histol Jap, n.50, p.73-85, 1987.
  • 14
    TOYOSHIMA, K.; MIYAMOTO, K.; SHIMAMURA, A. The ultrastructure of encapsulated sensory corpuscles in the fungiform papillae of monkeys. Arch Histol Jap, n.50, p.385-392,1987.
  • 15
    WATANABE, I. Fine structure of lamellated nerve endings in the gingiva of man and the Cebus apella monkey. Okajimas Folia Anat. Jpn, n.59, p.181-198, 1982.
  • 16
    WATANABE, I.; IDE, C. Cholinesterase activity of lamellated sensory corpuscles in the rat lip. Archs Oral Biol, n.32, p.129-134, 1987.
  • 17
    WATANABE, I.; YAMADA, E. Light and electron microscopic observations on the palatine mucous membrane. Acta Anat Nip, n.53, p.117, 1979 (Abstract).
  • 18
    WATANABE, I.; YAMADA, E. The fine structure of lamellated nerve endings found in the rat gingiva. Arch Histol Jap, n.46, p.173-182, 1983.
  • 19
    WATANABE, I.; YAMADA, E. Unusual nerve endings found in the rat palatine mucosa. Arch Histol Jap, n.47, p.187-196, 1984.
  • 20
    WATANABE, I.; YAMADA, E. Fine structure of lamellated and free nerve endings of the rat soft palate mucosa. Micr Eletr Biol Cel, n.13, p.139-150, 1989.
  • 21
    WATANABE, I.; SEMPRINI, M. Light and electron microscopy observations in the rat gingiva nerve endings. A silver impregnation and a transmission electron microscopy study. Rev Cienc Morfol, n.2, p.10-18, 1985.
  • 22
    WATSON, M.L. Staining of tissue sections for electron microscopy with heavy metals. II. Application for solutions containing lead and barium. J Biophys Biochem Cytol, n.4, p.475-479, 1958.
  • Correspondence to
    Ii-Sei Watanabe
    Departamento da Anatomia Instituto de Ciências Biomédicas da USP
    Av. Prof. Lineu Prestes, 2415
    05508-900 - São Paulo - SP
    E-mail:
  • Publication Dates

    • Publication in this collection
      06 June 2003
    • Date of issue
      2002

    History

    • Accepted
      31 July 2002
    • Received
      21 Jan 2002
    Faculdade de Medicina Veterinária e Zootecnia / Universidade de São Paulo Av. Prof. Dr. Orlando Marques de Paiva, 87, Cidade Universitária Armando de Salles Oliveira, 05508-270 São Paulo SP Brazil, Tel.: +55 11 3091-7636, Fax: +55 11 3031-3074 / 3091-7672 / 3091-7678 - São Paulo - SP - Brazil
    E-mail: brazvet@edu.usp.br