Porterplasm" and the microtrabecular lattice - 《cell》细胞杂志干细胞之家



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发表于 2009-3-6 09:03 |只看该作者 |倒序浏览 |打印

The latter stages of Keith Porter's eminent cell biology career began with the installation of a 1-MeV high voltage electron microscope (HVEM) at the Molecular, Cellular, and Developmental Biology department at the University of Colorado, Boulder, in 1972. Porter had high hopes that the HVEM would allow him to pursue a long-term goal of defining the structure of the cytoplasm by looking at whole cell mounts. Although success did come in imaging whole cells, in the end, the ambitious goal of defining the definitive structure of the cytoplasm was not matched by the technology of the day (Heuser, 2002).
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Structure in the void
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As early as the mid-1950s, Porter had a strong hunch that there must be some structure to the "optically ＆empty＊ parts of the protoplasm" that gave the cell its "elastic framework" (Porter, 1956). With the HVEM he could study a variety of types of cultured cells without the interference of embedding resins. Under a myriad of conditions, he saw a scaffold, or spongework, that encased all the then-known components of the cell. The scaffold consisted of fine, interconnected fibrils, which Porter named microtrabeculae (trabeculae roughly translates from Latin to tiny beams or girders).2 c& {4 X; k- n& U

The concept of the microtrabecular lattice was first described in 1976 (Wolosewick and Porter, 1976). The lattice backed up the idea that the cytosol was not all liquid, but rather contained a structured, linking framework. Porter also extended the responsibilities of the lattice beyond a mere scaffold, to include directing intracellular movements (Byers and Porter, 1977), giving shape and rigidity to cells, and even perhaps possessing information for cellular organization (Porter, 1978).* b& U* l9 i# g. O" {3 U
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Skeptics weigh in
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But critics soon voiced concern that the lattice might simply represent a fixation artifact of condensed soluble components of cytosol. Cytoskeletal components had just recently been identified, and many cell biologists still struggled with changing their concept of the cytoplasm from "soup" to "scaffold." Until the Porter studies, investigators had mostly concentrated on the visible cell structures〞membranes, organelles, and filaments〞and had ignored any possible cytoplasmic matrix. It was hard to believe that this empty space was highly organized. To address "the artifact controversy," Wolosewick and Porter used combinations of the best possible, least distorting EM techniques of the day to show relatively unchanged microtrabeculae (Wolosewick and Porter, 1979).

The study explored chemical (glutaraldehyde and osmium tetroxide) and nonchemical (freeze drying and freeze substitution) fixation techniques. It compared cells dehydrated by conventional alcohol/acetone dehydration and those that were critical point dried〞a method favored by Porter because it caused less damage to specimens. Finally, they examined two model systems as negative controls: red blood cells〞presumably a membrane filled with hemoglobin protein〞and a bovine serum albumin solution. They saw no lattice structures in those latter preparations and concluded, "the microtrabecular lattice must exist in a form not too dissimilar" from that depicted in the article.

Glomming but structured& b6 O0 I  o( ^; l# E3 p: I

Unfortunately, the best EM techniques of the day were giving what some consider to be a consistent artifact. Two later papers showed that aldehyde fixation and water contamination in critical point drying caused the soluble, hydrophilic proteins in the cytoplasm to glom onto the insoluble cytoskeletal filaments (Heuser and Kirschner, 1980; Ris, 1985). The resulting cytoskeleton, overdecorated with gooey protein, looked like a microtrabecular meshwork.

The microtrabecular lattice (arrows and arrowheads) as seen by Wolosewick and Porter.
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WOLOSEWICK

But Porter's basic idea that the cytoplasm is structured still holds true, although what it "looks like" in a live cell is still up for discovery. "If Professor Porter were alive today, he would still defend the concept of a nonrandomly organized cytoplasm," says John Wolosewick (University of Illinois, Chicago). He says the work sparked an ongoing debate about how cell structures are linked. He contends the lattice was not necessarily an artifact, but rather the best representation at the time of the numerous molecular cytoskeletal cross-linkers that have since been identified.. L7 @5 I) R) ]4 A; v9 k

The latest EM images and tomograms of cytoplasm from the labs of John Heuser and Richard McIntosh (a student of Porter's) confirm that Porter was correct in his intuition, if not his details. "The cytoplasm is ＆Porterplasm＊〞a beautiful spongework with organelles suspended in it," says Heuser of his latest freeze-dried, frozen thin sections of cells. However, the soluble components are so densely packed that the overall structure is still difficult to discern.& ]" `9 O- j( o) ]

Heuser's more modern view of cytoplasmic structure.

HEUSER
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Porter himself best described the EM conundrum: "In the strictest sense, of course, the content of the images is all artifact where the usual procedures are employed. The question is one of equivalence. To what extent do the images represent what was in the  when the fixative was applied, and to what extent may these images be used to investigate the form and function of this part of the cell?" KP( z" W8 a. p% J9 H+ n1 ], q
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Byers, H.R., and K.R. Porter. 1977. J. Cell Biol. 75:541–558.

Heuser, J.E. 2002. Biol. Cell. 94:561–596.
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Heuser, J.E., and M.W. Kirschner. 1980. J. Cell Biol. 86:212–234.
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Porter, K.R. 1956. Harvey Lect. 51:175–228.
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Porter, K.R. 1978. Lecture at Marine Biological Laboratory, Woods Hole, MA (as recorded by J. Heuser).0 `' g8 \0 w( ~3 _2 t
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Ris, H. 1985. J. Cell Biol. 100:1474–1487.+ S' P3 B# i9 F
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Wolosewick, J.J., and K.R. Porter. 1976. Am. J. Anat. 147:303–323.

Wolosewick, J.J., and K.R. Porter. 1979. J. Cell Biol. 82:114–139.(Keith Porter was to many the father of b)

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