Male reproductive system
The male reproductive system is made up of the male gonads called the testes, the penis, the scrotum,the reproductive tract and the seminal glands. It's primary role is to produce the male gametes also known as sperm and deliver them into the uterus of the female.
The testes are the point of sperm production and storage and therefore play a pivotal role in the function of the reproductive system. They are a pair of small oval organs situated outside the main region of the body in a sac because the optimum temperature for sperm to be produced is different to the regular internal body temperature. They are surrounded by connective tissue for protection. Inside the testes, there are nearly 300 different compartments which all contain seminiferous tubules and it is within these that sperm are produced. In between these tubules are the leydig cells and these are the source of testosterone production. Sertoli cells are found within the seminiferous tubules and control the development of sperm. Developing sperm are found in the space between the sertoli cells whereas the more mature sperm are situated in between the seminiferous tubules. In order to move the sperm into different regions of the reproductive tract there is a layer of smooth muscle which contracts in order to propel the sperm from that region.
The sertoli cells are linked together by tight junctions which essentially contribute to the formation of the blood testis barrier- this prevents the molecules diffusing into the compartments containing sperm and has a number of different roles. It protects the sperm from unwanted toxins that may be flowing in the blood. It also protects the sperm from the body's immune system. Sperm are not produced until puberty and therefore once production starts, sperm are treated as a foreign object by the immune system. The blood testis barrier stops the immune system from attacking the sperm which is a cause of male infertility in some cases. The sertoli cells also release a number of important substances for the development of the sperm. They control the amount of lumenal fluid produced and they also release a protein called the androgen binding protein. This is ensures that there are the correct amount of androgens in the fluid in order for the sperm to develop. They also produce mullerian inhibiting substance which repressed the development of the mullerian ducts during embryonic development.
The penis is the primary organ in the penetration of the female and is responsible for transporting the sperm into the uterus of the female. It is usually flaccid but upon stimulation becomes erect. This is a process whereby the penis becomes much harder and swells. During ejaculation, sperm are passed through the penis and into the vagina. The sperm pass down the urethra which is the tube a male uses to urinate. During ejaculation, a sphincter closes around the tube so that the sperm cannot enter the bladder and are hence forced up and out of the tube. The penis is surrounded by tissue called erectile tissue and this is made up of both connective tissue and smooth muscle. The amount of blood present in the tissue increases during an erection.
The scrotum is also known as the sac and is situated beneath the penis. It holds the testes and is made of connective tissue and skin. It provides a lower body temperature for the testes so that the sperm is protected from high temperatures and the scrotum is separated into two compartments by the septum.
The Reproductive Tract
There are numerous tubules within the reproductive tract. The seminiferous tubules are the site of sperm production and these leaad into tubules called the rete testis. These tubules then lead into the efferent ductules which are situated in the outer layer of the testis. Outside of the testis is the epididymis which is a large duct that leads to the vas deferens. The epididymis is split into three different compartments. The first is the caput. This recieves sperm from the efferent ducts and concentrates the sperm. The next is the body which is also known as the corpus and a site of sperm storage. The third section is the cauda where the sperm become more concentrated. As the sperm pass through the epididymis, a number of essential molecules are added to the fluid including sodium, potassium and steroids. Several proteins are also added such as albumen and gluthathione. The sperm have no cytoplasm once fully mature and therefore have to rely on substrates within the seminal fluid in order to survive.
Once the sperm have passed through the epididymis, they enter the vas deferens which connects the left and right ducts. This then leads into the prostate gland.
The prostate is a secretory gland that is located beneath the bladder. It is responsible for making up 30% of the seminal plasma. The rest of the plasma comes from the seminal vesicles. The prostate is made up of a large number of small glands that are surrounded by connective tissue. There are a number of different secretions and the prostate secretes prostasomes into the seminal fluid. These are small vesicles that contain nutrients such as citric acid and zinc as well as other proteins such as the plasminogen activator. The role of plasminogen activator is to break down the clotting of the sperm so that they are released inside the fermale reproductive tract. In clinical terms, plasminogen activator is very important because high levels in the blood can indicate prostate cancer.
The sperm are the male gametes and contain half of the genetic information that makes up an embryo. They fuse with an egg upon fertilisation and carry one of the sex chromosomes. It is the sperm that decides whether the embryo is male or female by the presence of the Y chromosome.
Development of Sperm
In terms of sperm production, there are three different stages before a sperm is fully mature. At puberty, the testicles increase in size and the increased production of hormones stimulates the production of sperm. Sperm production is an ongoing process and can go on indefinitely unlike the production of eggs in females which are limited in number.
The production of sperm is called spermatogenesis and it starts with a stage called proliferation. One spermatogonia undergoes the process of mitosis to create two B spermatogonia. These then undergo mitosis to create primary spermatocytes. Therefore at this stage there are four in number. Once the cells have divided, they do not become individual sperm because they are still connected by a cytoplasmic bridge called the syncytium. This ensures that the sperm that only contain a Y chromosome, get everything they need from the sperm with X chromosomes. The Y chromosome has an arm missing and therefore the sperm with a Y may not be making all the proteins needed.
The next stage is division. The primary spermatocytes push through the blood testes barrier and into the adluminal compartment. They then undergo their first meiotic division. Recombination can occur at this stage. The product is secondary spermatocytes which then further divide to form spermatids. At this stage there are 16 gametes in humans but this can differ amongst species because they undergo more divisions.
The final stage is differentiation. During this process, the spermatids elongate to form the unique sperm shape. They form specialised structures so that the sperm can be effienct in its job. These include the production of the flagella. This is the long tail protruding from the head of the sperm which is involved in propelling the sperm through the mucus and fluid found in the reproductive tract. It is also important in the penetration of the egg. The sperm also develops an acrosome at the tip of its head. This is a small vesicle that contains a number of enzymes responsible for digesting through the layers of the egg. When a sperm binds the egg, it releases the enzymes in the acrosome so that it can penetrate it. There are also a high number of mitochondria along the top of the tail piece. These provide the energy for sperm energetics. The cytoplasm in the sperm head is released as a residual body to make the sperm more streamlined. After this, a process called spermiation occurs whereby the bridges between all the sperm rupture and they become individual gametes. They are now fully mature and can be stored until an ejaculation. The sperm are continually produced in a spermatic wave to ensure that there are enough sperm to increase the chance of fertilisation. Males produce millions of sperm but no more than ten will ever reach an egg so therefore the chances of fertilisation are actually very slim and the more sperm produced, the more likely fertilisation will occur.
The process of producing sperm takes approximately 64 days to go from a spermatogonium to a fully mature sperm. It occurs constantly throughout a males life from puberty onwards and throughout the tubules, there are different stages of the process happening simultaneously. This ensures that there is always a constant production of sperm. This is a major barrier in the development of hormonal male contraception because there is a long delay in stopping the production of sperm.
The sperm have to travel a vast distance compared to their body size in order to reach an egg. They measure a few microns in length but have to travel at least thirty centimetres before they have a chance of fertilising an egg. It is estimated that they have to travel more than 60,000,000 times their own length and the terrain is difficult so one in a million actually complete the journey. Other processes such as capacitation need to occur before a sperm is ready to fertilise an egg. From ejaculation, the sperm travel through the urethra and into the female reproductive tract. They then have to swim through the cervical mucus which is designed to prevent anything from entering the female tract. Following this, the sperm have to traverse the uterus which is a tricky environment as the walls are made of many indents and folds. They then have to choose the correct fallopian tube to travel up to reach the egg. It is thought that they use both changes in temperature and chemical signals in order to establish the whereabouts of the egg. They can get stuck in the folds of the fallopian tubes and can wait until the egg comes near them. They then need to detach and penetrate the egg. Once a sperm has penetrated the egg, the outer layer of the egg hardens so that no more sperm can penetrate it and the egg is said to be fertilised. The sperm can survive in the environment for a number of hours.
Male Reproductive Disorders
There are a number of different male reproductive disorders that can result in male infertility. These include problems with the sperm themselves such as lack of sperm or problems with motility or problems with the transport of sperm such as erectile dysfunction, loss of male genitalia, conditions which interfere with the transport through the urethra and genetic defects. These problems can be solved by some form of IVF treatment or adoption.
Problems with Sperm
The average characteristics of a normal ejaculate are that there are more than 15 million sperm per ml, with 32% displaying normal motility and 24% displaying normal morphology. If there is a problem with fertility, it may be that one of these is abnormal. Oligozoospermia is where there is a lower number of sperm in the ejaculate on average. Athenozoospermia is where not many of the sperm are sufficiently motile and teratozoospermia is where the morphology of the sperm is abnormal. It is possible for there to be no sperm in the ejaculate which is known as azoospermia and all these conditions can lead to reduced fertility.
Failure to Produce Sperm
Sometimes, the sperm are simply not present and this can be for a number of different reasons. Some males have a congenital testicular deficiency where they may be missing a Y chromosome or have klinefelters syndrome. In this syndrome, the testes are small and there is a lack of testosterone in the body, a hormone crucial for sperm production. Maldescended testes is another condition that can lead to a failure in production and during the first year of birth, the testes usually descend. If this does not occur, there is a reduced amount of spermatogenesis. Failure to produce sperm can also be acquired throughout a lifetime from trauma for example. Testis tortis is a condition that can occur through trauma where the testes become twisted and the blood supply is cut off.
Failure in transmission
In some cases, the sperm themselves are normal but they are not efficiently transported into the female tract. Erectile dsyfunction is a common cause where the male fails to get an erection. This is usually due to the narrowing of the blood vessels in the penis and is usually a result of damage to the connective tissue through injury of diseases such as diabetes. Ejaculatory dysfunction can also prevent the transport of the sperm and one condition is known as retrograde ejaculation. This is where the sphincter that closes in the urethra to prevent sperm entering the bladder is dysfunctional. Therefore the sperm enter the bladder instead of exiting the urethra.
Failure in Transport
Male infertility can also be caused by defects in other areas of the transport system. Infections in the epididymis can lead to a blockage of sperm transport and therefore there is no sperm in the ejaculate. There us a condition called congenital bilateral absence of the vas deferens that is usually found in males that suffer from cystic fibrosis. In this condition there is a mutated gene in the male that prevents the vas deferens from forming properly and sperm cannot pass through it.