How many mitochondria are in a cell?
Counts by cell type, from red blood cells to egg cells
Matic Broz, PhD8 min
A liver cell holds about 1,000 to 2,000 mitochondria, and many other human cells hold hundreds. There is no single average for the body. Mature red blood cells have none, while a mature egg cell holds about 100,000, more than any other human cell.
Among the cells that make up the body's tissues, heart muscle carries the most mitochondrial material. Mitochondria fill nearly a quarter of each heart muscle cell, and heart tissue holds more copies of mitochondrial DNA per cell than brain or skeletal muscle. Counting separate mitochondria in those cells is harder than it sounds, because the organelles pack tightly and join into networks.
How many mitochondria are in a cell?
Many human cells contain hundreds of mitochondria, while a liver cell contains about 1,000 to 2,000 that together take up about a fifth of its volume.[1] The often quoted range of 100 to 2,000 is a rule of thumb, not a measured average across every human cell type.
One biochemical study of five mammalian cell types estimated 83 to 677 “virtual mitochondria” per cell, with an eightfold spread in mitochondrial DNA content between the cell types.[2] One average hides more than it explains, so a useful number has to name the cell.
| Human cell type | Mitochondria per cell | Other measure |
|---|---|---|
| Mature red blood cell | 0 | Removed as the cell matures |
| Resting platelet | 5 to 8 | Can divide when the platelet is activated |
| Sperm cell | 50 to 75 | Wound around the midpiece of the tail |
| Liver cell | 1,000 to 2,000 | About one-fifth of cell volume |
| Heart muscle cell | No reliable direct count | About 23% of cell volume and about 9,200 mtDNA copies per cell |
| Mature egg cell | About 100,000 | About 800,000 mtDNA copies on average |
Which cells have the most mitochondria?
By count, the mature egg cell, or oocyte, has the most mitochondria of any human cell, about 100,000.[8] That supply supports egg maturation, fertilization and the first divisions of the embryo, which share out the mitochondria inherited from the egg instead of building a full new set after each division.
Egg cells also carry hundreds of thousands of mtDNA molecules. One qPCR study of 87 human eggs at metaphase II, the stage at which an egg is ready to be fertilized, measured an average of 795,000 ± 243,000 copies, with large differences between eggs from the same patient.[9] Some sources give egg-cell figures far above 100,000 for this reason: they report copies of mitochondrial DNA, not separate mitochondria.
Among the body's working cells, heart muscle is the clearest answer. In biopsies of human heart muscle, mitochondria made up 23% of cell volume, beside 59% for the contractile fibers.[6] They sit in rows between those fibers, where the ATP that powers each heartbeat is used, and their inner membranes carry more folds, or cristae, than liver mitochondria.[1] Liver cells, with mitochondria filling about a fifth of the cell, are close behind.
Heart muscle cells are often credited with about 5,000 mitochondria each, but we could not trace that figure to a direct count. Packed rows of mitochondria are hard to separate in micrographs, so heart studies report mitochondrial volume and mtDNA instead. In a study of five tissues from 50 people, heart muscle held the most mtDNA, about 9,235 copies per cell, roughly twice the level in skeletal muscle or frontal cortex.[7]
Energy demand sets this ranking. Across 10 mammal species, mitochondria filled 22% to 37% of heart muscle cells, and the share rose with heart rate and resting oxygen consumption, so the smallest species had the most.[10] A mouse heart muscle cell is about 32% mitochondria, against 23% in a human one.[6]
Which human cells have few or no mitochondria?
Mature human red blood cells contain no mitochondria. Their precursors remove mitochondria and other organelles as they mature, leaving more room for hemoglobin and preventing the cells from consuming the oxygen they carry.[3]
Red blood cells dominate the number of cells in the human body, so a whole-body mitochondrial total cannot be found by multiplying one cell-level average by the body's cell count.
Platelets retain mitochondria despite having no nucleus. A resting platelet has about 5 to 8 mitochondria.[4] During platelet activation, those mitochondria can divide, so the number also depends on when the cell is measured.
A human sperm cell has about 50 to 75 mitochondria, arranged end to end in a sheath around the midpiece of the tail.[5] That is a small supply for a cell that has to swim, and far below the 100,000 in the egg it fertilizes.
How many copies of mitochondrial DNA are in a cell?
The mtDNA copy number per cell commonly ranges from hundreds to several thousand, while mature human egg cells can contain hundreds of thousands of mitochondrial genomes.[9]
A study of five tissues from 50 people reported mean mtDNA copy numbers of 1,259 per cell in cerebellar cortex, 4,198 in frontal cortex and skeletal muscle, 4,617 in the caudate nucleus, and 9,235 in heart muscle.[7] These are tissue-study means. Individual cells can differ, and each tissue contains more than one cell type.
A separate PCR-based analysis estimated 6,970 ± 920 mtDNA copies per diploid nuclear genome in heart muscle and 3,650 ± 620 in skeletal muscle.[11] The two studies used different samples and assays, so their figures are not directly interchangeable, but both put heart muscle at roughly twice skeletal muscle.
Mitochondrial DNA copy number and mitochondria count measure different things. A mitochondrion can contain several mtDNA molecules, and connected mitochondria can share one network. In the five mammalian cell types studied by Robin and Wong, mtDNA ranged from 220 to 1,720 molecules per cell, while the estimated number of “virtual mitochondria” ranged from 83 to 677.[2]
MtDNA copy number qPCR estimates the ratio between a mitochondrial target and a nuclear reference sequence. The muscle study measured a segment of mitochondrial cytochrome b against the single-copy nuclear β-globin gene, then reported mitochondrial genomes per diploid nuclear genome.[11]
Why is the number of mitochondria hard to count?
Mitochondria constantly divide, fuse, move and change shape, so a cell can shift between many separate organelles and a few connected networks without a comparable change in total mitochondrial mass.[12]
This makes the boundary of one mitochondrion less stable than a textbook drawing suggests. In some cells, interconnected tubules can behave as one continuous network. A microscope and its image-analysis settings may count that network as one object, several branches, or many fragments.
Mitochondrial DNA copy number is easier to measure than the number of separate organelles. Dividing genome copies by an assumed number per mitochondrion still produces an estimate rather than a direct count.
The distinction also matters in sequence analysis. Human mitochondrial genes use a different genetic code from most nuclear genes, so translating mitochondrial DNA requires the vertebrate mitochondrial code. The mitochondrial genome itself is only 16,569 base pairs long, compared with billions of base pairs in the human nuclear genome.[13]
For comparisons across tissues or conditions, mitochondrial volume, membrane area, DNA copy number and respiratory capacity are often more reproducible than a count of separate objects. A mitochondrial number is most useful when the cell type, measurement method and biological state are stated beside it.
The same choice of measure decides which cell has the most mitochondria. The egg cell wins by count, and heart muscle by the share of the cell given over to them.
