Sunday, March 27, 2011
Hernias and Testes (Thanks A Lot Fish.)
On page 193, Shubin talks about humans' propensity for hernias. Why is it that humans have a propensity for hernias near the groin? Discuss the arrangement of the reproductive system in males, including the testes, penis, scrotum and sperm. How did this propensity for hernias result from taking a fish body and morphing it into a mammal? Compare the organization of fish gonads and mammal gonads and explain how evolution has led to hernias developing in the groin area of humans. Hannah Kay (hgkay@aol.com)
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To understand why humans have a propensity for hernias near the groin, I think it is important to first address the developmental and evolutionary history of the human gonads. As Shubin addresses, human gonads begin development near the liver, much like how shark gonads develop. As both male and female human gonads “grow and develop, [they] descend” (Shubin 195). Female gonads descend to lie near the uterus and fallopian tubes to ensure that the egg does not have to travel far to be fertilized by a sperm. However, in human males, the gonads descend farther and into the scrotum. The descent of the human gonads, unlike fish gonads, is necessary for reproduction (193). First, as already stated, egg travel is lessened. But, more importantly, males need to produce sperm, and the production of healthy sperm can only happen under ideal temperatures. Without a scrotum, if the body/outside temperature becomes too high, then the sperm are malformed, and if the body/outside temperature becomes too low, then the sperm die. So in evolutionary history, when animals started to live on land, the scrotum was necessary for these animals’ survival. If the temperature of an animal’s body or environment becomes too high, then the scrotum will descend to help reach optimal sperm production temperatures. If the temperature of the body of the animal or the surrounding environment becomes too low, then the scrotum will “tuck close to the body” (193). At some point in evolutionary history, a distant relative of the human developed a scrotum, and when land was inhabited, it had the selective advantage of compensating for the temperature swings on land that other animals could not amount for. Overtime, animals on land without scrotums or a system to compensate for temperature changes, died off because they were not producing enough healthy sperm, while human descendents lived and produced healthy sperm that could fertilize.
ReplyDeleteHowever, the downside to the development of the scrotum is the possibility of a hernia. In inguinal hernias, because of the extra space created from the creation of the scrotum, a piece of the gut can travel down with the testes as the testes descend into the scrotum (195). An acquired inguinal hernia is due to abdominal muscle contraction that pushes the gut against the body wall, and since the scrotum has led to a weaker body wall in that area, the gut can escape the body cavity and then lie next to the spermatic cord (195). Fortunately for females, their abdominal muscle walls are much stronger than males, so they do not get hernias and they have a strong enough wall to hold a baby during pregnancy.
Symptoms of hernias include pain/aching in the abdominal and/or groin region, the appearance of a new “lump” in the groin area, and sometimes bowel obstruction may occur (i.e. nausea and vomiting). If you have any signs of a hernia, it is highly advised to go to the doctor. If you do have a hernia, surgery is most likely the treatment unless you have a medical condition that will prevent you from surgery. Also activities such as “lifting, coughing, or straining,” which “increase intra-abdominal pressure” should be avoided (http://www.emedicinehealth.com/hernia/page6_em.htm#Hernia%20Treatment).
Although your title suggests that we should be mad at fish, it was essential for the development of the scrotum because without it we would have a difficult time reproducing with unhealthy sperm that would have to travel far to find an egg.
(Bobby Muttilainen, rmuttilainen@gmail.com)
I agree with Bobby in his statements that the development of the scrotum had far greater long-term evolutionary advantages that relatively balanced out the onset of hernias. He mentioned previously that scrotums could compensate for drastic temperature changes on terrestrial environments; in addition, scrotums have also appeared to allow the testes (the male sex glands located in the scrotum pouch that produce/store sperm cells and regulate testosterone levels) and epididymis (a narrow, tightly coiled tube connecting the rear sides of the testicles to the vas deferens) to function at an exposed temperature that can be slightly lower than the core body temperature, which effectively regulates sperm production and gonad health through changing environmental temperatures (Source 1). Thus, this is a more detailed elaboration on the points Bobby made about the necessity of scrotum regulation on land environments, where the weather/temperature is subject to change more easily than in a marine environment, because water’s specific heat threshold (or the amount of heat per unit mass required to raise the temperature by one degree Celsius) at 1 calorie/gram °C = 4.186 joule/gram °C is significantly higher than that of air (Campbell 48).
ReplyDelete(continued) Moreover, the fact that scrotums are unique to warm-blooded mammals show that there must be some important selective advantage to the body part; previous research holds the theory that spermatogenesis naturally requires optimum temperature lower than core abdominal temperature and scrotums evolved simply because they were more effective at fulfilling this prerequisite than different gonad structures (Source 1). While exact reasons for scrotum development in humans is still unclear, scientists believe that a plausible advantage is that lower temperatures lead to reduced rates of oxidative DNA damage and fewer genetic mutations in produced sperm, as well as reduced metabolic rate and oxidative damage (Source 1). Also, apart from the inguinal hernias that Bobby discussed in detail, other types of developed hernias include femoral hernias (developed in the femoral canal through which the femoral artery, vein, and nerve leave the abdominal cavity to enter the thigh), epigastric hernias (formed between the navel and the lower part of the rib cage, targeting weak abdominal areas) and many others, which are significantly more rare and potentially painless/non-threatening but share the characteristic of forming directly on the abdominal wall (Source 4.) Thus, even though hernias may target “a weak spot in the body wall” (Shubin 195) in males, they may well be worth the cost of maintaining the evolutionary and reproductive benefits of having a temperature-controlled scrotum.
ReplyDeleteSources:
1. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1863418/
2. Campbell Textbook
3. Neil Shubin
4. http://www.emedicinehealth.com/hernia/article_em.htm
Christine Lin
choco_cat11@comcast.net