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 why do the testes always have to be ready to go Why is the maintenance of temperature so critical Why do testes create sperm throughout life even at advanced age And why can they produce sperm 24 hours a day The ability of sperm to be produced throughout life and be stored in the epididymis structure that stores sperm is an important adaptation because male humans have to be able to mate whenever females are available So constant and prolonged spermatogenesis is necessary Some researchers have looked carefully at sperm production and there are a lot of sperm in ejaculated semen Some of them however are not capable of fertilizing an oocyte Nonfertilization sperm in semen in humans are there to attack and kill sperm from other individuals This way a man would be more successful in leaving his genes to the next generation In ejaculated semen from men in monogamous relationships the volume of semen depends on two things 1 Time since last ejaculation the longer the time interval the more the ejaculate 2 How long the partners have been apart independently of when the last ejaculation was This suggests a mechanism in which a man who has been away from his partner for a long period of time has an adaptation to produce more sperm than usual just as an insurance policy to compete against any other sperm that might have shown up while he was gone Structure of the Testes The testes consist mostly of seminiferous tubules Upper right a testis with the top cut off It consists of large numbers of long tubules which produce sperm Upper left cross section of seminiferous tubule Bottom left Detailed view of above You can see leydig cells which secrete testosterone The walled up tubule is made up of two main cell types sertoli cells surround and nourish the developing sperm and stem cells that divide by mitosis to give rise to sperm precursors Those are called spermatagonia which are located on the outside of the profile of tubule One will remain as spermatagonium for the next generation of sperm production and the other will enter meiosis to begin its process of differentiation towards being a sperm The first meiotic division will result in a primary spermatocyte The second will result in a 2ndary spermatocyte which is a haploid cell And that will differentiate into spermatids which are precursors of sperm Because the division of stem cells is happening at the outside periphery of the tubule as more and more get produced they get pushed up to the lumen so there is a constant movement of cells towards the lumen of the seminiferous tubule shown in yellow on the top left and the differentiation occurs as they move so that by the time they arrive at the margin of the lumen they are differentiated into sperm A sperm have 3 parts a head which includes a nucleus and large vesicle called an acrosome which contains enzymes and carbs that swell when they contact water They are necessary to penetrate the layers of granulosa cells analogous to sertoli cells that surround the oocyte In order for the spermatocyte to fuse and inject its material into the oocyte it has to make membrane to membrane contact Release of digestive enzymes from acrosomes as well as glycosaminoglycans and other molecules that swell when they contact water loosen up the granulosa cells and push them apart to allow the sperm to reach the oocyte It takes about 70 days after the mitotic division of a spermatogonium before you get a mature sperm this is happening every day all the time Several hundred million sperm are produced daily They slough off into the lumen of the seminiferous tubule as they get produced and