|MadSci Network: Cell Biology|
Hello Christine, I wasn't able to find your answer but I asked around, and a fellow moderator, Steve Mack, who worked on mitochondrial DNA evolution in his thesis, gave me this response: See this reference paper. Robin, E. D. and Wong, R. (1988) Mitochondrial DNA molecules and virtual number of mitochondria per cell in mammalian cells. J. Cell. Physiol. 136, 507-513. Steve says: This paper estimated the number of "virtual" mitochondria (see below) in cells from 5 different cell lines, which were derived from rat (skeletal muscle), mouse (fibroblast), rabbit (2 types of macrophage), and human (fibroblast). They found that the rat skeletal muscle cells had on average 140 "virtual" mitochondria. The overall counts ranged from ~90 mitochondria per cell in one type of rabbit macrophage to ~700 in the other type of macrophage. The authors used the term "virtual" mitochondria, because it was not clear if the mitochondria existed in the cell as discrete units, or if they were part of a single large branching network. Today, it looks like a more accurate answer to this question might be something like "just one big one," because while individual mitochondria can exist as discrete organelles, they can just as easily form a giant branched "reticulum" structure. The paper from 2002 that I list below suggests that complete mixing of the contents of the mitochondrial matrices in a human cell can occur in 12 hours, and the paper from 1994 suggests that "the actual number of mitochondria per cell is not of crucial importance, and mitochondria in a cell should be considered as a virtually single dynamic unit." The mass of mitochondria in a cell changes in response to the energy demands of the cell, but it looks like measures of mitochondrial mass are relative to the cell types being studied, so it is hard to say what the absoloute mitochondrial mass of a muscle cell is in comparison to any other cell type. However, it should be noted that the muscle cells in a cell line are not "active" in the sense that a working muscle is, so the count of 140 "virtual" mitochondria from the cell line data could be a significant undercount in comparison to muscle tissue. Here are some references for the "one big happy mitochondrion" concept : Frédéric Legros, Anne Lombès, Paule Frachon, and Manuel Rojo (2002) Mitochondrial Fusion in Human Cells Is Efficient, Requires the Inner Membrane Potential, and Is Mediated by Mitofusins. Mol Biol Cell. December; 13 (12): 4343–4354 Hayashi J, Takemitsu M, Goto Y, Nonaka I (1994) Human mitochondria and mitochondrial genome function as a single dynamic cellular unit. J Cell Biol. Apr;125(1):43-50. Kirkwood, S.P., Munn, E.A., and Brooks, G.A. (1986). Mitochondrial reticulum in limb skeletal muscle. Am. J. Physiol. 251, C395–C402. Stevens, B. (1981). Mitochondrial structure. In The Molecular Biology of the Yeast Saccharomyces: Life Cycle and Inheritance, J.N. Strathern, E.W. Jones, and J.R. Broach, eds. (Cold Spring Harbor, NY: Cold Spring Harbor Press), pp. 471–504. These references might be a bit too academic for your purposes, but I hope you get the general idea and that this helps, Sanjida
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