Mitochondrial division in rat cardiomyocytes: an electron microscope study

Abstract

In cardiomyocytes of rats, two distinct mitochondrial division processes are in operation. The predominant process involves extension of a single crista until it spans the full width of a mitochondrion. Ingrowth of the outer membrane ultimately results in scission. The second division process involves "pinching," in which narrowing of the organelle at specific surface locations leads to its attenuation. When limiting membranes from opposite sides meet, mitochondrial fission ensues. When pinching is the operative mode, elements of sarcoplasmic reticulum always are associated with the membrane constrictions. The nuclear control mechanisms that determine which modality of mitochondrial division will prevail are unknown.

Fig. 1

Several aggregations of interfibrillar mitochondria. A partitioned mitochondrion is indicated by the arrow. 24 month Fischer 344 rat. Scale bar = 1.0 µm.

Fig. 2

A pair of subsarcolemmal mitochondria with medial partitions (arrowheads) in a 24 month-old Fischer 344 rat. Cristae on either side of the partition are seen in profile. Scale bar = 0.2 µm. Inset. The point of connection of a median partition with the boundary membrane. Note that the outer membrane spans this connection. A conventional crista is at the left. The unit membrane structure of all membranes is clearly resolved. 24 month-old Fischer 344 rat. Scale bar = 0.2 µm.

Fig. 3

An interfibrillar mitochondrion with a longitudinally-oriented median partition (arrowheads). Six month-old Fischer 344 rat. Scale bar = 0.2 µm

Fig. 4

An interfibrillar mitochondrion undergoing pinching. Note the elements of SR related to the membrane incursions (arrows). Six month-old Fischer 344 rat. Scale bar = 0.2 µm

Fig. 5

An interfibrillar mitochondrion that has almost completed the fission process; the two prospective daughter organelles are still connected by a slender isthmus. An element of SR has penetrated the constriction to its very terminus. Six month-old Fischer 344 rat. Scale bar = 0.2 µm

Fig. 6

Survey micrograph of a pellet of IFM. A partitioned mitochondrion is indicated by the arrow. This organelle is shown at higher magnification in the inset. Although some neighboring mitochondria superficially appear to possess partitions, only the organelle shown in the inset had a partition that met our criteria for such a morphology. Six month-old Fischer 344 rat. Scale bar= 1.0 µm. Inset. The partitioned mitochondria from Fig 6. The bicompartmentalized structure of this organelle is accentuated by the different density of its two halves, due in part to a different orientation of cristae in its two sides. Scale bar = 1.0 µm.

Fig. 7

An isolated IFM that exhibits a median partition. The outer membrane (open arrows) does not extend into the partition. Six month-old hybrid rat. Scale bar = 0.2 µm

Fig. 8

An isolated mitochondrion in a more advanced stage of division than that shown in the preceding figure. Continued ingrowth of the outer membrane would lead to the separation of the two daughter organelles. Six month-old hybrid rat. Scale bar = 0.2 µm