A little anatomy to get us started
The urethra is the duct through which urine passes outside the body from the bladder during urination, and it also fulfils a reproductive function in men as it allows semen to pass outside the body from the seminal vesicles which lead to the prostate.
The prostate is a glandular organ which is part of the genitourinary system. It is shaped like a chestnut and lies below and at the exit to the urinary bladder. It contains cells which produce part of the seminal fluid which protects and nourishes the spermatozoa contained in semen.
The ejaculatory ducts begin at the end of the vasa deferentia and end at the urethra. During ejaculation, semen passes through these ducts and is subsequently expelled from the body through the penis.
The seminal vesicles secrete a viscous alkaline liquid which neutralises the acidic environment of the urethra.
The vas deferens or ductus deferens: these are two muscular tubes surrounded by smooth muscle. Each approximately 30 cm in length, they connect the epididymis (which stores sperm) with the urinary tract (through which sperm are expelled). Although the urinary tract is also for removing urine, there is a valve which regulates the flow of semen and urine. During ejaculation, spermatozoa flow out of the testicles through the vasa deferentia and along the urinary tract. As the semen leaves the body through the urethra, the prostate and other glands add seminal fluids.
The epididymis is a long, narrow, tightly coiled duct located to the rear of the upper part of the testicle, which connects the vas deferens to the back of each testicle. Approximately 5 cm long by 12 mm wide, if it were uncoiled the epididymis would measure around six metres in length. The spermatozoa must pass through this for their final development and maturation and they are stored here in order to be ready for ejaculation. From the epididymis, the spermatozoa move along to the vas deferens, or spermatic cord.
The testicles (testes) normally each measure four to five centimetres in length, and they are located in the scrotum (a muscular structure with various layers which protect the testicles and contribute to temperature regulation). The testicles have two very important functions, both of which are essential for normal fertility in men: the first is the production of the male hormone, testosterone, and the second is the production of spermatozoa. The development of spermatozoa begins in the testicles. From there they move to the epididymis where they mature and are stored (until ejaculation).
Ejaculation and Spermatozoa
Ejaculation is the emission of semen, an off-white viscous liquid which is expelled through the penis. Semen is composed of:
- Seminal plasma (which is composed of contributions from the testicles, the epididymis, the seminal vesicles, the prostate, the Cowper’s glands, the Littre’s glands and the vasa deferentia).
- Millions of spermatozoa (the male gamete). These are microscopic in size (about 60 Microns long), and have the following structure:
- The head, which contains the genetic material which will condition the characteristics of the new individual produced. This has the acrosome at the front, a corpuscule which is of great importance when it comes to fertilisation.
- The body, mid-piece and tail are responsible for moving the spermatozoon.
Did you know that…?
Both the quantity and the quality of the spermatozoa are decisive in determining the chances of fertilisation.
Production of Spermatozoa
The production of spermatozoa (known as spermatogenesis) is a complicated process which begins at puberty (age 12-14) and continues, in a healthy man, until death.
The pituitary gland in the brain is responsible for producing and regulating the hormones which control the sequence of events which lead to the production of spermatozoa. The two hormones which govern this process are:
- Follicle-stimulating hormone (from here on FSH)
- Luteinizing hormone (from here on LH)
LH activates the production of testosterone (a hormone required for the production of sperm), while FSH also activates hormones which help with sperm production. In addition to sperm production, testosterone is also responsible for virility, the growth of masculine hair, and muscular development.
From start to finish, the production of spermatozoa takes around 72 days. Spermatozoa spend the first 50 days in the testicles and the last 22 – 24 in the epididymis. In the epididymis the sperm reach maturity and acquire motility (the ability to move).
During sexual activity, semen is ejaculated into the female reproductive tract via the vagina, and the spermatozoa begin their journey (of approximately 12 cm) through the cervix and the uterus to the Fallopian tubes, which is where fertilisation will take place. During the journey there are many cavities, folds, etc., which prevent many of the spermatozoa from reaching their final destination. That is why so many sperm are needed for the fertilisation process.