Change in the metaphase/prophase ratio
435
The change in the ratio of the first two stages of mitotis appears to be a sensitive indicator of bone marrow damage as a result of exposure to low doses of ionizing radiation. The problem needs further and more comprehensive investigation in order to ascertain the destiny of the delayed metaphases. REFERENCES 1. KARPFEL, Z., Folk
Biol. (Praha)
INTURNINGS
7, 129 (1961).
OF THE OUTER
LIMITING
MITOCHONDRIAL
MEMBRANE S. K. MALHOTRA Cytological
Laboratory,
Department
of Zoology,
Oxford, England
Received June 18, 1962
THE electron
microscope structure of mitochondria is fairly well known [Z, 3, 41. It is generally accepted that the inner limiting membrane invaginates to form the cristae and the outer limiting membrane is a continuous sheath round the entire inclusion. However, the presence of a discontinuous outer mitochondrial membrane has recently been reported in electron micrographs of liver and kidney cells .of the hamster [l]. In the mitochondria of these cells, the outer membrane is at some places broken, and one edge of such a membrane is joined to the inner membrane, while the other edge is left free. A similar appearance of the mitochondrial membranes has been described in the striated duct cells of the human submaxillary .gland (see [I]). In the exocrine cells of the pancreas of the mouse the author has come across another type of variation from the generally accepted appearance of the outer limiting mitochondrial membrane. In this cell the outer membrane does not always seem to form a continuous sheath, but appears to invaginate into the mitochondrial matrix to form a crista (Figs. 1 and 2). Such a crista resembles, in its electron microscopic appearance, the common type of cristae that are formed by the inturnings of the inner membrane, except that it is open at the surface of the mitochondrion; thus its interior is continuous with the ground cytoplasm. Such an appearance of the mitochondria would be suggestive of one of the two limiting membranes as a discontinuous structure. If the crista of rare type is formed by the invagination of the outer membrane, it would then be considered that there are pores (of about the size of the width of the crista) in the inner limiting membrane to allow the crista to extend into the mitochondrial matrix. On the other hand, if this crista resembles the common type seen as an infolding of the inner membrane, then it is possible that the outer memExperimenfal
Cell Research 28
436
Experimental
S. K. Malhotra
Cell Hesearch 28
Outer limiting
mitochondrial
membrane
437
brane is broken opposite to where the inner membrane infolds into crista, and the two broken edges become continuous with the inner membrane (Text-Fig. 3). These inturnings of the outer mitochondrial membrane have not been seen very frequently, but they have been encountered in pancreatic exocrine cells that give the impression of being well-fixed, if the criteria usually accepted for judging good fixation in electron micrographs be accepted. Inner
Outer
membrane \
Ground
membrane
cytoplasm
I
7-..tl A
B
Mitochondriol
I-
C
matrix
Text-Fig. 3.-Diagrammatic representation of common type of mitochondrial crista (A) formed by iuvagination of inner membrane, as compared with rare type of crista (B and C) seen in the cxocrine pancreatic cells. In B the crista is shown to be formed by the infolding of the outer membrane. Such cristae would be suggestive of presence of pores in the inner membrane for the cristae to extend into the mitochondrial matrix. In C the outer membrane is shown broken opposite to where the inner membrane infolds to form a crista and its two edges turned in to become continuous with the inner membrane.
REFERENCES 1. CIIANDRA, S., J. Cellular Biol. 2. PALADE, G. E., J. Histochem.
12, 503 (1962). Cyfochem. 1, 188 (1953). in Proc. Fourth Neurochem. Symp.,
3. ROBERTSON, J. D., London, 1961. 4. SJBSTRASD, F. S., Nnfure
S. KETY (cd.).
Pergamon
Press,
171, 30 (1953).
Fig. 1.-A mitochoudrion from an exocrine pancreatic cell fixed in a 1 y0 solution of potassium prrmanganate in Verona1 buffer and embedded in Epikote. Arrow indicates where the outer limiting membrane appears to have infolded to form a crista. l’ig. 2.-A mitochondrion from au exocrine pancreatic cell fixed in a 2 o0 solution of osmium tetroxide in Verona1 buffer and embedded in Epikote. Arrow indicates where the interior of a crista seems to be continuous with the ground cytoplasm. Experimental
Cell Research 28