What are sperm?
Sperm are the male reproductive cells. In order to produce a child, the genetic material from the sperm must combine with the genetic material from an egg, in a process called fertilisation. Healthy, fully developed sperm are very small (0.05 millimetres long) and cannot be seen by the human eye. These mature sperm are highly specialised cells and are made up of three parts: a head, neck and tail. In the head is a structure called the nucleus, which contains 23 tightly packed chromosomes (genetic material). The head is designed to stick to and then enter (penetrate) the egg. The neck joins the head to the tail. The part of the tail nearest the neck contains the mitochondria, which provides the energy for the sperm to move. The tail moves in a whipping motion to push the sperm towards the egg.
A man’s fertility and sexual characteristics depend on the normal functioning of the male reproductive system. Where are sperm made?
The male reproductive tract is made up of the testes, a system of ducts (tubes) and other glands opening into the ducts. The testes (testis, singular) are a pair of egg-shaped (ovoid) glands (each 15 to 35 ml in volume) that are suspended in the scrotum. The testes are needed for the normal functioning of the male reproductive system. The testes have two related but separate roles:
Production of sperm. Production of the male sex hormone, testosterone.
Before birth, the developing testes move down from the abdomen into the scrotum. Successful descent of the testes is important for fertility as a cooler temperature in the scrotum is needed for sperm production and normal testicular function. In humans, this location seems important by keeping the testes between 3 to 5oC below normal body temperature.
Sperm are made in the testes, in a number of small, tightly packed, fine tubes called seminiferous tubules. These tubules have a total length of 150 metres. Between the seminiferous tubules lie another cell type, Leydig cells, which produce the male sex hormone, testosterone.
Where are sperm transported?
Leading from each testis is a long, highly-coiled tube called the epididymis that lies at the back of the testes. The epididymis connects the seminiferous tubules to another single tube called the vas deferens.
When released from the testis, the sperm spend two to 10 days passing through the epididymis. During this journey, the sperm mature and gain the ability to move (swim or become motile).
When ejaculation starts, sperm are transported from the tail of the epididymis, via the vas deferens, to the urethra in the penis. As sperm pass through the different tubes, fluid from the seminal vesicles, Cowper’s and prostate glands is added to the sperm. The mixture is then called semen. Over 90% of the semen is produced from the prostate and seminal vesicles and not from the testis.
How are sperm made?
Sperm production (spermatogenesis) is a lengthy, but continuous, process. Sperm are at different stages of their development in the testis. Within one testis, some sperm are at early stages and others are at later stages of development. It takes about 70 days for germ cells (precursors of sperm) to develop into the mature sperm found in semen that can fertilise an egg.
In the testes, cells in the lining of the seminiferous tubules divide over and over again to produce sperm. The process of sperm production starts with the earliest germ cell, called a spermatogonium (precursor of sperm). These cells divide and pass through many stages as they divide, including a major change in shape, from a round cell to the familiar ‘tadpole-like’ sperm structure. At this stage, sperm are released from the lining of the sperm-producing tubes and pass out of the testes into the epididymis.
How do hormones control sperm production?
Hormones from the pituitary, a small gland at the base of the brain, control the testes. The pituitary gland releases both luteinising hormone (LH) and follicle stimulating hormone (FSH) into the bloodstream. These hormones act as “keys” that turn on the testes.
The levels of FSH and LH in the blood rise during the early stages of puberty, which make it possible for the testes to grow and mature. With the help of LH, the Leydig cells in the testis make the male sex hormone, testosterone. Testosterone is responsible for the general growth that occurs in young boys at puberty including development of the genitals, beard and body hair and other characteristics of the adult male.
Together with testosterone, FSH from the pituitary gland acts on the seminiferous tubules in the testes to stimulate the production of sperm.
How are sperm ejaculated?
At the start of ejaculation, waves of muscle contractions transport the sperm, with a small amount of fluid, from the epididymis through to the vas deferens.
At the back of the bladder, the vas deferens becomes the ejaculatory ducts, which pass through the prostate gland (prostatic urethra) and into the urinary tract (penile urethra). The prostate gland produces fluid that mixes with the sperm in the prostatic urethra. Extra fluid from the seminal vesicles enters the urethra via the ejaculatory duct. The passage of fluid along the urethra is further helped by the lubricating fluid made by the Cowper’s glands. This mixture of fluid from the testes and other accessory glands, travels along the urethra to the tip of the penis where it is discharged (ejaculated) at the time of sexual climax (orgasm).
What is semen or seminal fluid?
Sperm, suspended in semen (seminal fluid), are ejaculated from the penis. Semen is the mixture of fluids from the different organs of the male reproductive tract. About 90 per cent of the ejaculated semen comes from the prostate gland and seminal vesicles. Only a small amount of fluid, which also contains the sperm, comes from the epididymis.
During ejaculation, usually the sperm and the prostatic fluid come out first and the seminal vesicle fluid follows.
How many sperm are needed to achieve pregnancy?
The ejaculate of fertile men contains tens of millions of sperm. However, men with much lower numbers of sperm can still achieve pregnancies. Men who never produce any sperm in their ejaculate are sterile and cannot help a partner become pregnant without some form of treatment.