Luana and I have been trying to get an accurate figure for the difference in VOLUME between a human sperm and egg, which of course reflects a difference in metabolic investment in making them. I’m talking now about cytoplasmic volume, not length, as the disparity in length isn’t that relevant (in some flies, the sperm are longer than the fly!).
The figure bandied about is a volume difference of ten million, but I don’t believe that. All these figures trace back to one assertion on a Northwestern University site, but the paper it cites doesn’t give any such figure (or any figure). Everybody quotes that figure, but it seems way too large for me. However, it might be accurate.
So, I’m crowdsourcing the answer. I have no prize here except for approbation (and I’ll put the winner and correct answer below).
Question: What is the difference in cytoplasmic volume between a human sperm and a human egg? A reference to a respectable source must be included.
Thank you!
Very roughly, the human egg cell has 25,000 times the volume of a sperm cell. (Using measurements given on Wikipedia along with some formulae I learned in junior high.) I assumed the egg to be a sphere (120 microns in diameter, for ~905,000 cubic microns), but the sperm (4.3 x 2.9 microns, not including the tail, for ~36 cubic microns) is a flattened disk (according to Wikipedia), so my calculation for the sperm is very rough (I assumed the shape of a rectangular box, which probably overestimates the volume to some extent.)
Is Wikipedia a respectable source?
No, I don’t think so. I’m looking for a refereed paper. But a game try!
Wikipedia usually lists references.
A thought:
Molecular crowding ; water content ;
I think these properties would be important to note in the side-by-side.
I mean I’d need to look them up – maybe not. Just occurred to me is all.
Other thoughts:
Density
Specific gravity
Cell membrane – how are these structurally different – because ideally, this region defines the volume, but is not a line.
Of course, I know they are vastly different in size, just the definitions are important. Maybe the extracellular regions are different too.
Very interesting literature search to make.
I found two citations with egg and sperm sizes. The one for egg size didn’t reference how they got the sizes, the but lead author is also the author of textbook on this (cited in the paper) and I’m assuming she drew her numbers from that book.
The sperm sizes were calculated from length and width measurements and I used the formula for an ellipsoid to calculate volume, assuming the two width measurements were equivalent (and even if not, this would not greatly alter the estimated volume calculation).
For eggs, the volume given was 900,000 um3, and the nucleus had a volume of 65,000 um3.
For sperm, average lengths were 4.5 um and 2.8 um, giving a volume of just 146 um3.
The ratio of egg to sperm volume is 6,164. Egg volumes are thousands of times larger than sperm volumes.
Citations: Burrill Stem Cell Report October 2007; AA. Kiessling, Human Eggs: The Need, the Risks, the Politics.
Xu et al. 2022. Effects of six kinds of sperm staining methods on human sperm size and evaluation of their staining effects. J. Clin. Lab Analy. https://doi.org/10.1002/jcla.24794
I get a related question all the time in astronomy discussions (and it’s cognate, planetary science). People see that (say) the Moon is 1/4 the diameter of the Earth, and walk away thinking that “the moon is a quarter the size of the Earth. In fact, the Moon is close to 1/64th the volume of the Earth, and because of differing densities (not likely to be a large factor in [unmineralised] biological materials – they’re all within a few % of 1.0 tn/cu.m), the ratio of masses is closer to 1/81.
The same confusion happens all the time with most other bodies. People get unduly upset to discover that the surface area of the asteroid belt may exceed that of Mars, while it’s mass is less than 1/1000th that of Mars.
That’s a relatively simple situation : the bodies involved are all of similar shape – spheres (half of the asteroid belt is Ceres). The egg too is, I understand, a fairly good approximation to a sphere. But how close to an ellipsoid of rotation a sperm cell is, and what it’s dimensions are, and an approximation to the dimensions and shape of the sperm tail … some(biologist)one is going to have to search the biological databases for.
Are the cell walls of sperm (head and tail) and egg similar? Because if you’re comparing cytoplasmic volume, you need to correct for that. The egg cell wall has a behaviour that the sperm cell wall doesn’t have, so I wouldn’t assume the same answer for both. (The parallel question in astronomy/ planetary science relates to core/ mantle/ crust volumes – people guess, rather than calculate.)
The approximation I hear is that “sperm are 1/1000 the size of eggs” – which I would expect to be a linear (radius or diameter) comparison. That would give a volume (without correction for cell walls) ratio of one (short) billion to one. Adding shape and wall-thickness corrections, ten million to one doesn’t surprise me at all.
A volume ratio of ten million to one would imply a diameter ratio of 215(egg):1(sperm), with no shape or cell wall corrections.
At least in animals, there are no cell walls. The plasma membrane consists of a lipid bilayer in both eggs and sperms and that won’t differ much, if at all, in thickness.
Animal ova are also enveloped in a vitelline membrane (zona pellucida in mammals), however, so there’s a slightly thicker rind to the ovum. I don’t think that this outer membrane is very thick either, however, and I doubt that it contributes much to the ovum’s volume.
Jerry was specifically asking about the cytoplasmic volume, not the cytoplasmic+(cell-boundary-structure, however named) volume. So you need to at least consider that factor, even if it finally turns out to be negligible.
(I just did the comparable calculation for the Earth and it’s crust ; radially, the crust is about 1/4 of 1% of the whole-Earth measurement ; volumetrically it’s about 3/4 of 1% ; you may or may not count that as negligible. I don’t know Jerry’s purpose, so I leave that question for him to consider.)
Howard up-thread gives dimensions for sperm heads of “4.5 [] and 2.8” µm (microns). That’s an eccentricity of 0.804, if it were an ellipse. Assuming the sperm to be an ellipsoid of rotation about the longer of those two measures, … I get 147.8 cu.microns – obviously we’re using similar calculations and assumptions. That’s about just under 40% of the volume of the “spherical sperm” first approximation. (Volume of a triaxial ellipsoid = (4/3)*π*axis-a*axis-b*axis-c). Does anyone care to calculate the “cylindrical sperm” volume second approximation, and we can call the “triaxial ellipse sperm” the third approximation?
Now, if someone more biologically sophisticated than me can come up with thicknesses for the “cell bounding structures, however named”, we’ll have pretty much all the needed figures, unless we want to go to a fourth approximation and consider the volume of the sperm’s tail.
Or fifth approximation, if you want to define the shape of a sperm as something other than an ellipse. It may be more important than the “cell boundary structure, however named” correction ; I’m not going to guess.
I asked Gemini, Claude, and GPT-4. I found that:
• They all hallucinate references, meaning that a given reference exists, but does not provide the claimed information.
• It seems to work better to ask separate questions for egg and sperm volumes.
Gemini provided this link for sperm volume:
https://pubmed.ncbi.nlm.nih.gov/321264/#:~:text=The%20mean%20value%20of%20spermatozoa,15.2%2B%2F%2D1.27%20cu%20micronm
which says that
“The mean value of spermatozoa from 25 normal human seminal fluid specimens is found to be 17.4+/-1.46 cu micronm and the modal volume, 15.2+/-1.27 cu micronm.”
This source says that a human egg is 0.12mm in diameter. https://www.apricityfertility.com/uk/blog/human-egg-cell-explained
So the volume of a human egg would be: 4/3 * pi * 0.06^3 * 1e9 = 904,779 cubic micrometers, assuming it’s perfectly spherical.
The ratio would then be 904,779 / 15.2 = 59,525.
BTW, we’re also assuming that the materials used to create each are equivalent in cost. I would guess that the great volume of the egg is essentially ‘food’ which is much cheaper to produce than the genetic machinery in the nucleus. Of course, both egg and sperm have machinery and energy, so it’s primarily the energy storage that’s the difference. Still, that could be a tiny part of the cost to produce the egg.
The Northwestern University ratio is several orders of magnitude larger than I have seen before. Unhelpfully, I can’t find where I’ve seen the 100,000 figure that I’ve previously relied on.
Nature has a paper from 1999, so predating the NWU one, but I don’t have access. Gee, H. “Size and the single sex cell” (1999): https://doi.org/10.1038/news991125-4
A Wikipedia search gives human egg cell size as 120 μm. Reference is Alberts et Al Molecular Biology of the cell. Resulting volume, assuming omit is a perfect sphere, is 7.24 x 10^6 μm^3. Sperm cell volume is given in a quite old paper by Laufer et al 1977 Fertility and Sterility 28: 456-458 (Full text available at Elsevier) as 17.4 μm^3. thus, it results in a ratio of approx 400,000.
Sorry, need to correct a mistake. Volume of the egg cell is smaller: 9.05×10^5. this, the ratio is approx 50,000
“The analysis revealed that the mean value for volume in normal sperm is 8.03 ± 0.72 μm3.”
Coppola et al., Zygote , Volume 22 , Issue 4 , November 2014 , pp. 446 – 454
DOI: https://doi.org/10.1017/S0967199413000026
I asked Gemini, Claude, and GPT-4. I found that:
• They all hallucinate references, meaning that a given reference exists, but does not provide the claimed information.
• It seems to work better to ask separate questions for egg and sperm volumes.
Gemini provided this link for sperm volume:
https://pubmed.ncbi.nlm.nih.gov/321264/#:~:text=The%20mean%20value%20of%20spermatozoa,15.2%2B%2F%2D1.27%20cu%20micronm
which says that
“The mean value of spermatozoa from 25 normal human seminal fluid specimens is found to be 17.4+/-1.46 cu micronm and the modal volume, 15.2+/-1.27 cu micronm.”
This source says that a human egg is 0.12mm in diameter. https://www.apricityfertility.com/uk/blog/human-egg-cell-explained
So the volume of a human egg would be: 4/3 * pi * 0.06^3 * 1e9 = 904,779 cubic micrometers, assuming it’s perfectly spherical.
The ratio would then be 904,779 / 15.2 = 59,525.
All we need is a picture of the two together; we don’t even need a scale since the question was about relative volumes. We can all figure it out ourselves, no authorities needed.
Here is one:
https://media.sciencephoto.com/image/c0369821/800wm/C0369821-Human_egg_and_sperm,_SEM.jpg
Answer: diameter of egg in pixels = 536 pixels
Average diameter of sperm in pixels = 49 pixels
Ratio of their volumes is the ratio of their diameters cubed,which is approx 1300
Did you notice that the image you used is a composite, where the egg and sperm appear to have been magnified by different amounts, if I understand right?
No, I didn’t notice that! Are you sure? The shadow of the tail gives the impression that it is not a composite, though of course that’s easy to make in Photoshop.
I see from another photo that the result is closer to 6000x, just like Comment 3 says:
https://media.sciencephoto.com/image/p6480195/800wm/P6480195-Human_Sperm_and_Egg.jpg
Wild / rhetorical question – how far are each from the .. image capturing apparatus .. is it as you say below EM?
Or are the images scaled together?
I disregarded perspective effects because I think they are small; I have never noticed perspective effects on SEMs.
Here is a light microscope photo (original magnification 400X) of an ovum surrounded by spermatozoa. Because the flagella are very thin and whipping in and out of the focal plane you don’t see them well. But I think this is a fair representation of the relative diameters of the two gametes. This appears to be a wet mount of live cells so no shrinkage artifact from drying and fixation.
https://www.gettyimages.ca/detail/photo/human-ovum-surrounded-by-sperm-royalty-free-image/523741126
Thanks. That one gives about 7000x assuming roughly spherical sperm, so the real ratio might be closer to 10000x as given by one of the references above and in Comment 11.
You’re implying that sperm and egg are the same shape – cubes, spheres, 4th stellation of the icosahedron (as long as you’re comparing geometrically similar diagonals or edges) … it doesn’t matter.
You need at least two measurements of the sperm – and more is better until the changes from each additional measurement stops making much difference.
See my Comment 12.
In this reference, it just gives the difference as eggs 10,000 > sperm.
https://news.mit.edu/2021/study-reveals-how-egg-cells-get-so-big-0304
That seems close to right, but a little high; see my comments above and below yours.
This is an addition to my Comment 10. I was editing it and it took me too long, losing all edits. I needed to take into account that the sperm is a disk, not a sphere. This might increase the ratio by a small factor, probably not more than a factor of two or three. Also, the sperm’s tail has volume not included in the calculation, but the egg may shrink more than the sperm when prepared for the SEM. So overall I think Comment 3 has the closest estimate, 6000x, but this still seems high based on what we can observe ourselves in this SEM image. The much higher estimates in many of the comments have to be way off if this SEM is correct.
I did the same thing with three different photos I found: 2 electronmicrographs and one from a light microscope with a protein stained in the sperm. I calculated ratios ranging from about 600 to 3000, assuming the sperm is spheroid. The electron micrograph of hamster egg and sperm gave the lowest ratio but I found it in a review article which discussed all the problems of cell shrinkage etc during preparation and how that is now being overcome. Unfortunately I can’t access the original paper.
I feel sure that our estimates are the right order of magnitude.
Hmmmmm. Curiouser and curiouser. I would have thought this was an easy figure to look up but so far the estimates here, even with citations, are all over the place. It’ll be interesting to see which one is correct. Or even in the same ball park.
This is one of the many reasons I love this website!
+1
As regards the volume of a human egg cell, I have found this:
https://pubmed.ncbi.nlm.nih.gov/2394794/
“The metaphase II oocyte was an irregular 3.5 x 10^6 cubic microns sphere of 1.05 coefficient of form. The ooplasmic volume of 1.4 x 10^6 cubic microns was reduced by 10% by fertilization.”
As regards human sperm, the situation is complicated by the variety of irregular shapes – eg:
https://pubmed.ncbi.nlm.nih.gov/3384107/
As regards the average volume, however this paper:
https://pubmed.ncbi.nlm.nih.gov/321264/
quotes a mean total volume of 17.4 cubic microns, of which around 45% is in the head section.
As regards external volume, the ratio of the figure for the oocyte to 45% of that of the sperm is around 450,000.
A further paper discusses the particular difficulties in accurately measurng the volume of human sperm, and comes up with a suggested “net volume” (aqueous content) of 25 cubic microns. Dividing this into the ooplasmic volume of 1.4e6 cubic microns gives a ration of around 56,000.
https://pubmed.ncbi.nlm.nih.gov/1338068/
So, there are very big uncertainties, especially for sperm, but 10 million seems definitely wrong.
I wonder what that volume difference will turn out to be when comparing the total volume of eggs and the total volume of sperm that can be produced in a given timeframe.
+1 also
Is there a reason you’re asking about volumes and not masses? I’d hazard that mass is easier to get a handle on.
Most unmineralized biological materials have a density within a few percent of 1 – be that g/cc, tn/cu.m, relative density (to water) …
So a volume comparison is pretty close to a mass comparison.
In terms of investment made for each conception, I’d say you have to consider the investment in all the spermatozoa in the ejaculate that results in conception. No male has the option of firing single rounds.
That’s a good point and it totally changes the way I look at the question. The total investment in making gametes is enormously larger for the male than for the female. It is so profligately excessive that they can be spent recklessly for mere enjoyment without loss of reproductive fitness or survival ability.
But here’s the thing. In mammals, the investment to make gametes as cells is such a small fraction of the total metabolic needs over the lifetime of the individual of either sex that it hardly seems to matter to them what the size ratio is. The anatomic and physiologic demands of gestation, lactation, and protection of the newborn from predatory males is of course much larger for the female but this doesn’t seem to relate to gamete size per se. For the new individual formed as a zygote, yes the female contribution to survival to implantation is much larger. Perhaps the idea Jerry and Luana are developing looks at relative sizes from the perspective of the zygote and not of the parents?
Yes, the metabolic investment is trivial (even as we agree males expend more energy on making billions of sperm over a lifetime, whilst ovaries contain a reproductive career’s worth of ova at birth) compared to the investment both parents will make after conception, but which is far and away more costly to the female.
Finally the answer!
+1
The lifespan of each gamete class as well.
Also wondering how volume* changes with conditions. I’m not sure they stay in the same conditions their entire lifespan.
Or different species too – that’s probably worth noting (not that I know).
*volume and anything used to estimate it.
“Study reveals how egg cells get so big: Oocyte growth relies on physical phenomena that drive smaller cells to dump their contents into a larger cell.
Egg cells are by far the largest cells produced by most organisms. In humans, they are several times larger than a typical body cell and about 10,000 times larger than sperm cells.
There’s a reason why egg cells, or oocytes, are so big: They need to accumulate enough nutrients to support a growing embryo after fertilization, plus mitochondria to power all of that growth. However, biologists don’t yet understand the full picture of how egg cells become so large.
A new study in fruit flies, by a team of MIT biologists and mathematicians, reveals that the process through which the oocyte grows significantly and rapidly before fertilization relies on physical phenomena analogous to the exchange of gases between balloons of different sizes. Specifically, the researchers showed that “nurse cells” surrounding the much larger oocyte dump their contents into the larger cell, just as air flows from a smaller balloon into a larger one when they are connected by small tubes in an experimental setup.”
Source: https://www.eurekalert.org/news-releases/765600
Here’s the paper (2021): “Dynamics of hydraulic and contractile wave-mediated fluid transport during Drosophila oogenesis”: https://www.pnas.org/doi/full/10.1073/pnas.2019749118
According to this study the ovum is 10 million times the volume of spermatozoa. But not being a biologist I’m not sure if “cytoplasmic volume” you have asked for makes a difference here. I will say I know more than I did before, so that’s one thing and the more technical ones I failed to understand in any depth.
The latter cites the former and they are not the formal paper but journalistic reports.
https://news.northwestern.edu/stories/2021/04/gametes-egg-and-sperm-cell-size-evolved-from-competition/
https://www.miragenews.com/egg-and-sperm-cell-size-evolved-from-competition-545203/
Also :
Error estimate on each figure would be nice.
And :
The assumption is :
The size/shape/volume of sperm carrying the X sex chromosome are identical to sperm carrying the Y chromosome.
Is that in fact demonstrated? I imagine with modern technology, tiny discrepancies might get picked up (even if not related to the volume question).
In the estimates of the sizes of the two gametes, it might be helpful for reference to recall that the diameter of a human red blood cell is about 8 micrometres, which is on the small side for body cells. Like sperm, they are discoid. A spermatozoon’s head is just over half as long as an rbc. The ovum is 15 times the diameter of an rbc and is spherical, making it by far the largest cell in the body, so they say.
But tarry a moment. Recall that lower motor nerve cells extend all the way from their cell bodies in the spinal cord to the muscle fibres each one innervates, which can be about 1 metre (or more if you play basketball) to reach the small muscles in the far end of the foot that spread the toes. (The spinal cord ends about two-thirds the way down your spine, at L1.) Even though the axons are very small in diameter, they are, for large nerves that must conduct rapidly, still of the same order of magnitude as a red cell and I wouldn’t want to bet that the volume of such a long cell is less than an ovum.
Using values from Norman@1 and Howard@3, three credible shapes for spermatozoan heads (cylinders, ellipses of rotation and triaxial ellipses with a median axis between the quoted measurements), including the only values I can get for dimensions of sperm tails (modelled as cylinders of as cones), and applying the minimum and maximum thicknesses I can find for the cell membranes, I get egg cytoplasm volume to sperm cytoplasm volume ratios from 43291 to 28945. I’d believe the first couple of significant digits of those numbers.
The difference between “thin” membranes (3.3nm) and “thick” (23nm) leads to about 3.5% change in the volume ratio. Which may or may not be negligible, depending on purposes.
I’ve only got one real estimate for the dimensions of an egg cell, and I think that’s the biggest source of uncertainty in the final ratio.
Having set up the volume calculations per shape, I can now crunch the numbers for other sources fairly rapidly.
Working is available, but forum posts aren’t very amenable to spreadsheets.
https://www.exploratorium.edu/sites/default/files/files/SizingUpObjects_v2.pdf suggests the volume ratio would be about 53000. 150 micrometers for Egg cell diameter, so radius would by 75. 4 micrometers for sperm head diameter, so ignoring the flagellum, the ratio would be 75 cubed divided by 2 cubed (aka 8) giving about 53,000 as the volume ratio.
This is complicated…but then what isn’t? Both human gametes regulate volume although sperm do not do this quite as robustly. So, you will get different answers depending on the conditions (medium osmolarity, temperature, etc…) and a lot of the published values are not physiologically relevant. That is particularly the case for morphometric calculations. The best place to look for water content and volume is in the cryopreservation literature.
Human sperm have a total volume of ~34 femtoliters and a free water volume of ~20 femptoliters (Kleinhans et al, PMID: 1338068). Human eggs (actually, oocytes arrested in meiosis II) have a reported volume of 800 picoliters, with around 80% being osmotically active. That was quoted as personal communications in Newton et al, PMID: 10645242. If the 800 pL value is correct then the volume ratio is ~23,000:1.
If you are looking at the metabolic investment for gamete production then you are probably already familiar with some of the Drosophila data. D. bifurca does invest a great deal of resources in making those gigantic sperm. However, they don’t make many sperm. Bjork and Pitnick report a sperm:egg production rate there of only 5.8:1 (PMID: 16760976).