The tendency to assign traits perceived as masculine and feminine to sperm and egg cells is both sexist and factually wrong. Sperm and Eggs do not “think”, but instead follow a program written in their DNA. Both of them have evolved to perform their respective tasks, delivering DNA or developing into a new organism, and attempting to force them into a narrative role does this process a great disservice. Personally I find that bad metaphors are one of the great banes of science, because someone will read to much into or completely misunderstand it. Unfortunately, it is difficult for people without a good grounding on a topic- any topic, not just scientific ones- to understand many concepts without the use of metaphors. Unfortunately, this including the people trying to learn about a topic.
Please please ask any questions you have on this topic in the comments. I may at a later date find a good video (or video series) that shows this process. For now, this should give you a good idea of scale.
More after the jump.
Sperm only have one purpose: the delivery of a payload of genetic material to an unfertilized egg of the same species. Sperm are extremely small cells, only about 5 μm (micrometers, which are one millionth of a meter) wide at the head and 60 μm long, with most of the length made up of a long flagellum. Most of the sperms “head” is taken up by a nucleus containing one set of chromosomes for its species. Thus sperm are packed with what little space they have left in full of mitochondria. The “crown” of the sperm contains proteins that will allow them to, in sufficient numbers, break down the barriers surrounding the egg cells.
Sperm are mass produced in the testis, by the continual meiotic devision of stem cells. The name of the game here is quantity, not quality. The malformed sperm tend to be weeded out en route to the egg. Common problems include multiple heads, tails, missing tails, and the inability to swim in a strait line. However, some sperm carrie the wrong number of chromosomes due “mistakes” that occur during the distribution of chromosomes between daughter cells. For instance, a sperm cell may contain both an X and a Y chromosome, or neither.
Eggs, meanwhile, are enormous. During egg production, the stem cells forming the gametes undergo meiosis. Unlike normal cell division, the contents of the daughter cells are not divided equally, and one forms a tiny polar body. The Eggs need to contain all of the organelles that the stem cells need to function, as well as a large amount of extra strands of RNA, proteins, and nutrients that the developing blastocyst will need until it successfully implants. These extra proteins and RNA come from the mother, and are needed because the new embryo will not manufacture any using its own DNA for the first few cell divisions. Thus, eggs are extremely expensive to produce.
Eggs remain in stasis after the first meiotic division, literally pausing midway through the process. Once a cycle several egg cells begin to ripen, usually only in one ovary. Eggs that do not function properly are pruned, and the remaining egg(s) form large cysts. Once a hormonal signal is received, the cysts rupture, propelling the eggs out of the ovary and into the fallopian tube. Thus the ovary becomes scared over successive cycles, although this has little effect on its ability to function.
Relatively speaking, the sperm are quite fragile while the eggs are extremely hardy. Eggs can be cryogenically frozen relatively easily, while special preparation is required to preserve sperm, which require a supply of nutrients.
Sperm are incredibly fast cells- however, that is what they are designed to do, and they are extremely fragile, only lasting about seven days after being produced. They requirer an external supply of nutrients, contained within the semen, to survive the journey to the egg. Semen also neutralizes the slightly acidic PH of the vagina and uterus, which would destroy sperm. An egg, meanwhile is like a fortress- it has layers of guard cells that prevent viruses, bacteria, and even incompatible sperm from entering. This barrier renders the eggs nonmotile, meaning that they cannot move themselves. Thus eggs, after being literally exploded out of the ovary, are swept down the fallopian tube by a lining of cilia. However, the egg is not passive in the fertilization process. Sperm are entirely dependent on eggs for navigation. The egg releases a hormonal signal that sperm attempt to follow by swimming towards areas of higher concentration. The sperm that reach the egg swarm around it, and use their enzyme caps to break thorough the guard cells. As soon as one reaches the egg, its enzyme cap causes a reaction with the cell membrane of the egg, merging it with its own and literally locking out all other sperm. The sperm then releases the chromosomes it caries, and dies. The egg then picks up meiosis with its own chromosomes where it left off, creating another poler body, before beginning regular cell devision with both the fathers and the mothers DNA.
A note on gamete deterioration:
While the stem cells that create sperm literally ware down over time, female ovaries loose their ability to regulate egg ripening and pruning. Thus, its not so much that the eggs themselves are deteriorating, but that the ovaries are becoming worse at choosing eggs to release. Hence the higher percentage of fraternal twins born to older mothers.
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Now, I think a good metaphor for the process of fertilization would need to encompass both the tasks preformed by both gametes, and the relative fragility of them.
So say we have a space ship that is supposed to be terraforming a planet and starting a new colony as our egg. It has everything it needs to do so- except for the other half of its blueprints. However, a small shuttle is supposed to meet up with it en rout and give it the other plans. The people sending the shuttle do not know when the colony ship is going to arrive, and everyone in the colony ship is in stasis until the messenger arrives. So thousands upon thousands of one-man shuttles are sent on suicide missions to the route the colony ship is supposed to take.
If the ship is en route, it has a beacon which the shuttles can follow. Once located, the shuttles have to work together to get through the ships defenses- but after the first shuttle docks, the colony ship locks out the others.
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