Reproductive System

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Reproductive System

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 21 Current Lecture Male Reproductive System So why did the testes end up all the way down where they are, if they started up in the abdominal cavity? Well, in order for proper spermatogenesis to occur, the testes must be 1-2 degrees colder than the core body temperature (up in the abdomen). By being in the scrotum, the testes are external to the warm abdominal cavity - if the testes were retained in there, they would not produce viable sperm. In a baby boy whose testes don’t descend (cryptorchidism), surgical intervention has to be done or he will be sterile. Inguinal hernia – the testes pass through the inguinal canal, which then closes. If it doesn’t close completely, it represents a potential weak spot through which intestine and other organs could herniate later in life if unrepaired. There are two mechanisms that maintain temperature at the proper level. 1.) The arrangement of the blood vessels to the testes (to the right). You see a single red artery in the middle that branches into two, as well as the highly branched blue vein that wraps around the incoming artery. The reason for this arrangement is for heat transfer: blood coming in to the testes starts in the artery at body temperature, and as it descends, it is cooled by transfer of heat from the artery to the veins. The returning, cool venous blood is warmed by the heat transfer such that it is back to normal warmth when it hits the abdominal cavity. This special arrangement is called a countercurrent heat exchanger. 2.) There is a spinal reflex involving temperature sensitive nerves in the scrotum and motor neurons to the levator muscles that contract and pull the testes up closer to the body. Whether the testes are hanging by their own weight or being pulled and scrunched up, these muscles are continually being adjusted and regulated to ensure the testes are at the proper temperature by moving them closer or further from the abdominal body wall. Evolutionary considerations: Looking at the mating patterns of humans compared to other primates is illuminating. Compare the mating habits of two of the great apes – chimpanzees that have really large testes and produce large amounts of sperm, and gorillas, who have small testes and produce much less sperm. Female chimpanzees mate with several different males simultaneously, whereas gorillas tend to be more monogamous and mate with one male at a time. So looking at chimpanzees, the increased size of their testes probably have to do with increased sperm competition – for a given male to improve his chance of leaving his genes, he has to produce a massive number of sperm. This is not necessary in monogamous gorillas. So where do humans fall in this spectrum? We are a little more on the not-so-monogamous side. Just in terms of testis size and sperm production, we are about halfway in between the gorillas and the chimpanzees. From an evolutionary perspective, ...

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