Chlamydial structure.

The Reticulate Body.

Introduction

The reticulate body is the stage of the chlamydial developmental cycle responsible for intracellular replication. Typically, reticulate bodies have a diameter of 1 micron [Fig 1] or more and they are non infectious. Reticulate bodies are metabolically active, so their cytoplasm is rich in ribosomes, which are required for protein synthesis. Their nucleic acid appears diffuse and fibrillar. They are bounded by two sets of tri-laminar membranes, an inner cytoplasmic membrane and an external outer envelope, whose surface is covered with projections and rosettes [Fig 4] similar to those seen on elementary bodies, but at higher density. These projections can be seen extending from the chlamydial surface into the inclusion membrane [Fig 2]. As the reticulate body begins to differentiate into an elementary body, sites of re-condensation of nucleic acid appear in its cytoplasm [Fig 5]. In the maturing inclusion, chlamydial particles appear to be packed around the inclusion membrane [Fig 6].

Ultrastructure of the RB

Fig 1. An immature inclusion of C. trachomatis LGV 404 consisting of a mixture of small, "black" elementary bodies ( E) and much larger, "grey" reticulate bodies ( R). Note the homogenous and granular nature of the cytoplasm of the reticulate body, due to the presence of many ribosomes. Note also the dividing reticulate body ( DR). The bar represents 1 micron. [ Figure provided by M. E. Ward. Modified from: Ward, M. E. The chlamydial developmental cycle. In: Microbiology of Chlamydia, (Barron, A. L. ed). CRC Press, (1988)].	Fig 2. Reticulate bodies of C. psittaci Cal 10 showing the connection between reticulate bodies and the inclusion membrane of isolated inclusions. The inclusions have been treated with tannic acid to reveal the projections, from the chlamydial surface, that contact and penetrate the inclusion membrane. The bar represents 0.1 microns. [ Electron micrograph kindly provided by A. Matsumoto from: Matsumoto, A. (1988). In: Baron, A. L. (ed). Microbiology of Chlamydia. CRC Press].
Fig 3. Carbon replica of a freeze-fractured face of a C. psittaci Cal 10 inclusion at 18 hours post infection. Note the clusters of projections studding the inclusion membrane. [ Electron micrograph courtesy of A. Matsumoto. Modified from: Rockey, D. D. & Matsumoto, A. (1999). The chlamydial developmental cycle. In: Prokaryotic development (Brun & Shimkets, eds.) ASM Press].	Fig 4. Portion of envelope of C. psittaci Cal 10, negatively stained, showing the rosettes with electron dense centres.
Fig 5. An intermediate body of C. trachomatis LGV 404 undergoing differentiation. In this figure there are two points of condensation of DNA into electron dense nucleoids ( n). Note the inner cytoplasmic membrane and the outer envelope (env). The bar represents 0.25 microns. [ Figure provided by M. E. Ward. Modified from: Ward, M. E. The chlamydial developmental cycle. In: Microbiology of Chlamydia, (Barron, A. L. ed). CRC Press, (1988)].	Fig 6. A freeze-fractured inclusion of C. abortus at 30 hours post infection showing how chlamydiae pack around the edge of the inclusion on the inclusion membrane. Such chlamydiae are well placed to interact via their projections or their tts system with the host cell cytoplasm. [ Unpublished electron micrograph of M. Ward and C. Inman, Southampton.]

[MEW] May 2004

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References

Louis, C., Nicolas, G., Eb, F., Lefebvre, J. F. & Orfila, J. (1980). Modifications of the envelope of Chlamydia psittaci during its developmental cycle: freeze-fracture study of complementary replicas. Journal of Bacteriology 141, 868 - 875.

Matsumoto, A. (1973). Fine structures of cell envelopes of Chlamydia organisms as revealed by freeze-etching and negative staining techniques. Journal of Bacteriology 116, 1355 - 1363.

Matsumoto A, Fujiwara E, Higashi N. (1976). Observations of the surface projections of infectious small cell of Chlamydia psittaci in thin sections. Journal of Electron Microscopy (Tokyo). *25, 169 - 170.

Matsumoto, A. (1982b). Electron microscopic observations of surface projections on Chlamydia psittaci reticulate bodies. Journal of Bacteriology 150, 358 - 364.

Matsumoto, A. (1988) Structural characteristics of chlamydial bodies. Pages 21-45. In: Microbiology of Chlamydia. (Baron, A. L. ed.). CRC Press., Boca Raton, Fl., USA ISBN 0-8493-6877-4

Miyashita, N., Kanamoto, Y. & Matsumoto, A. (1993). The morphology of Chlamydia pneumoniae. Journal of Medical Microbiology 38, 418 - 425. [ C. pneumoniae EBs often not pear shaped].

Rockey, D. D. & Matsumoto, A. (2000). The chlamydial developmental cycle. Pages 403-425. In: Prokaryotic Development (Brun, Y. V. & Shimkets, L. J. eds.). ASM Press, Washington D. C.

Soloff, B., Rank, R. G., & Barron, A. L. (1982). Ultrastructural studies of chlamydial infection in guinea-pig urogenital tract. Journal of Comparative Pathology 92, 547.

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