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The theme of structure and function runs throughout biology, and is of the utmost importance for reproduction. Some of the strangest biological structures have been discussed in this chapter. They are each designed to perform a specific duty and require a specific shape.
Let's start with the testicles. Most mammals have testicles that hang outside of their body in a sac called the scrotum. The scrotum is a structure built for function. It likely exists to keep the testicles at an optimal temperature for sperm production. As sperm prefer a cooler temperature than the rest of the body, the testicles allow them to "hang-out" in a more comfortable climate. The structure of the sac also allows them to be pulled in closer if they get too cold.
Next, we have the penis and the vagina. As discussed above, the purpose of the penis is to deposit sperm inside the female during internal fertilization. The vagina is the receiving runway for the sperm. It is necessary that the penis and the vagina have a lock and key type of fit. They must be complementary for copulation to succeed. Argentine lake ducks even have corkscrew shaped penises and vaginas that spiral in opposite directions to make a perfect match.
The uterus of a female is a structure built for function as well. In an unfertilized female, the uterus is a small organ. Then, when necessary, it can miraculously grow and expand into a large cavity to accommodate the developing baby (or babies). It must be tough enough to protect the fetus from the outside, and tough enough to protect the mother from the fetus. Remember, some of these babies have hooves. Ouch.
Sperm must be tiny so that they can be produced cheaply in mass quantities and so that they can fit inside the egg easily. They must have a strong tail for swimming, and a head full of enzymes for breaking into the egg. Gametes must be haploid so that when they meet they can create a diploid zygote. Eggshells must be thick for protection, but thin enough to break out of. We could go on and on. The structures involved in reproduction are all perfectly suited for their functions.
Throughout reproduction, regulation is very important. From gamete production to birth, events must be carefully coordinated to make the process successful. Let's discuss a few of these events.
During meiosis, the maternal and paternal chromosomes must split and then the two copies of those must carefully split. It is important to have a full set of single chromosomes. The wrong number of chromosomes in a gamete can cause problems in the offspring. Down's syndrome is caused by an extra chromosome in either the sperm or the egg. Older mothers are at a higher risk for Down's syndrome because their eggs have more mistakes from meiosis than the eggs of younger mothers.
Egg production must be tightly regulated. In amniotes, it must be carefully coordinated with the production of the things necessary for embryo development. In mammals, this means the uterine lining must be ready. With no proper lining available, a fertilized egg is useless. Hormones are the key directors in this process. They keep both processes in synchronization. These regulating hormones can vary amongst different animals. In humans, Follicle stimulating hormone (FSH), Lutenizing hormone (LH), estrogen, and progesterone are the main players. They tell the uterus to build a lining and they tell the follicle to release the egg.
Copulation must be synchronized with the estrus or menstrual cycle. While there are some species (such as humans and bonobos) that engage in sexual behavior for pleasure, many animals do not. They only copulate for reproductive purposes, and courtship rituals are very important to promote mating at the right time. Pheromones and physical and social cues are necessary to keep these processes carefully synched so that no energy is wasted on a copulating when egg is not on the menu.
Of all the processes, embryo development is the most tightly regulated. It was said by the biologist Lewis Wolpert that:
"It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life."
This is because there are SO many important things taking place. The organization of your entire body happens at gastrulation. The setup of the embryo at this point sets the stage for the development of all of your organs. Every movement of every single cell is critical, and of course, they are all under tight regulation. The specifics of this are still being worked out, but it involves the turning on and turning off of different developmental genes such as the wnt signaling pathway.