I've noticed in the e2 community that while sex is routinely discussed (and is pretty tame by our standards), little is given as far as actual knowledge about the birds and the bees. While most people generally think of sexual reproduction in terms of our own species Homo sapiens , sexual reproduction in the animal kingdom is pretty much the same- the combination of an egg and a sperm, each of which is a single cell. Mating combines the two single cells to produce an entirely new organism. The widespread similarity of sexual reproduction at the cell level is also an argument for evolution- the embryos of many species have little morphological differences in the early stages of development.
More about the sex cells
Gametes are the sex cells. A sperm and an egg are both gametes. Each gamete contains half the number of chromosomes that are normally present in a whole organism- also referred to as a haploid cell, and is represented as "N". When a cell has a full chromosome count, it is written as "2N." This kind of cell would be referred to as diploid (to remember, think of a haploid cell being "happy" to shoulder half the load and a diploid being a dip to carry everything. Cheesy, but it works.) When the gametes join, the organism has all of the chromosomes it needs.
Gametogenesis is the process by which gametes form.
In spermatogenesis, the creation of the human gametes, I'll use human males as an example. Beginning at puberty, human males start producing millions of sperm every day. Sperm need to be continually produced as they die each day, and a certain temperature must be kept inside fo the testes for their survival. Spermatogonia are diploid, and start spermatogenesis, which occurs in the seminifurous tubules. The spermatogonia go through mitosis, and each spermatogonium produces a primary spermatocyte, which also contains 46 chromosomes. The primary spermatocyte then goes through meiosis two produce two secondary spermatocytes, which contain 23 chromosomes (still referring to humans). Secondary spermatocytes then go through meiosis again to produce four spermatids , which develop in the testes into a (the sperm that we know and love.) The spermatid matures to have a tail, a middle piece with many mitochondria, and a head containing the chromosomes.
Oogenesis (pronounecd o-o-genesis) is all about the production of eggs inside females. A human female is born with all the eggs she will ever have, numbering in the thousands. They remain dormant in the ovaries until puberty, when hormones kick-start the menstrual cycle, which continues until menopause.
Oogenesis begins in the developing fetus, in the ovaries. The oogonium is the initial cell, and it is diploid. It grows to mature into a primary oocyte . The primary oocyte begins going through meiosis, but the process is paused until the female enters puberty (that's a long wait!) . At the midpoint of the female menstrual cycle, ovulation - the release of an egg from an ovary- takes place. The primary oocyte continues meiosis, producing a daughter cell and three non-functioning polar bodies. Only the daughter cell that is available for fertilization. Eggs are produced in follicles of the ovary. Once a follicle develops and releases and egg, it is called a corpeus luteum (Latin for "yellow body"). The corpeus luteum hangs around to help with the beginning stages of pregnancy as well.
The act of mating
Obviously, the way that organisms reproduce depends on the organism. . And no, this isn't just about the obvious- there's probably some stuff going on inside your body that you don't even know about.
How birds do it
Birds copulate. The male bird deposits his sperm inside the female bird. The egg becomes fertilized, but then is deposited outside the female bird's body to continue developing until it is time to hatch. Of course, birds lay the fertilized eggs. When you crack one open, the yellow part- the yolk- is where the developing embryo would reside. The white part- the albumen- serves to nourish the embryo throughout its development.
How bees do it
Most insects reproduce through a process called parthenogenesis, I'll illustrate this with bees. Parthenogenesis literally means "virgin production." A queen bee recies all the sperm she will ever be impregnated with during her nuptial flight. She never mates again- she store all of the sperm cells in her body. Obviously, bees were the first members of Planned Parenthood- the queen bee has complete control over when she can have bee children. She lays eggs, and then releases the stored sperm to fertilize them- resulting in female worker bees and a few new queens. She can choose not to fertilize them- the bees that result become male drones. Drones are born haploid.
How educated MD's do it
While the mechanics of sexual pleasure are not documented throughout the entire animal kingdom, a definite sexual response is observed in class Mammalia. This sexual response is proposed to exist in order for the animal to want to propagate the species. Doesn't sound very romantic, but that's just part of life, eh?
Stages of sexual response
1. Excitement phase- Begins with physical or psychological stimulation, and can last from minutes to hours. In males, the penis becomes erect. In females, the erectile tissue within the vagina fills with blood, and the increased pressure causes drops of fluid to be squeezed out of the tissue. This lubrication prepares the vagina for easy penetration.
2. Plateau phase- can be prolonged for an indefinite amount of time. Excitement increases, and then plateaus.
3. Orgasmic phase- Kind of obvious what happens here :-). While no one is really sure how the nervous system or biochemical systems trigger an orgasm, what is known is that physical and psychological factors are important. Interestingly, physical stimulation of the genitals is not, in all cases, essential for orgasm.
4. Resolution phase- Everything returns back to normal. Cigarette smoking optional.
Great- you knocked her up
Barring protection, most normal, fertile women have about an 85% chance of getting pregnant during sexual intercourse. After sperm and egg fuse, the newly formed zygote travels down the fallopian tube torwards the uterus, where it can be implanted (Sometimes, the zygote will fuse in the fallopian tube, resulting in a fatal ectopic pregnancy for both the embryo and the female). They zygote begins to divide immediately, becoming a 16 cell morula , and soon begins to experience differentiation within the germ layers, developing into a blastocyst , and once implanted, a gastrula. The gastrula is basically the early embryo. In most mammals there are three germ layers- the endoderm, ectoderm, and mesoderm. Muscle and bone tissue develop in the mesoderm, the nervous system and skin in the ectoderm, and various internal organs in the endoderm. By the end of the first trimester, or three months, the embryo has fully experienced cell differentiation, and the morphology resembles that of a human. It is now a fetus, and simply grows for the next two trimesters until pregnancy is induced, a process still widely unknown by modern scientists. Some theorize that the hormones oxytocin and progesterone have something to do with it, and they are administered to induce labor.
Advantages of sexual reproduction
It's fun. Sex is one of the niftiest things you can do with a willing (or kinky, if you prefer) partner.
It's healthy for you. In women, for example, the sexual act triggers the release of oxytocin. Oxytocin promotes feelings of affection and triggers that nurturing instinct. In men, sex encourages the flow of testosterone, which strengthens bones and muscles and helps transport DHEA, a hormone that may be important in the function of the body’s immune system.
Sex can also make you happier in the long run. Like exercise, releases endorphins. Endorphins contribute to the runner’s high and diminishes pain levels. And sex sure beats running as a form of good old fashioned exercise.
And, that concludes my sexual reproduction node. Tell your parents to thank me for getting them off the hook!