Understanding Male Reproduction can help us pinpoint the Causes of Infertility
Male factor infertility is defined as an abnormality in either sperm production or function. Although this condition is very common – accounting for approximately 50% of all cases of infertility – a proper evaluation of the male partner is frequently overlooked during the routine infertility evaluation.
The purpose of this section is to educate men about their role in the fertility process, as well as introduce the concept and importance of a thorough male fertility evaluation.
Sperm Development and Passage into the Female Reproductive System
Unlike women, who are born with all of the eggs that they will ever have, men produce sperm from the time they enter puberty until late in life. Sperm begin their development in the testicles. While sperm production is relatively rapid, it typically takes between 90 and 108 days from the time that a sperm is produced in the testicles until it is eventually ejaculated.
Once the sperm leave the testicles, they enter a convoluted series of tubules that lead eventually to the penis. During their journey through these tubules, sperm acquire the ability to swim as well as the structural components necessary for them to be able to negotiate their way to the female egg, bind to it, and eventually cause fertilization.
Semen Contains Sperm & Other Chemicals that help with Fertilization
In nature, when a man ejaculates during intercourse, seminal fluid is deposited at the top of a woman’s vagina. In addition to sperm, this fluid contains many chemicals that come from the other male sexual organs – primarily the seminal vesicles and the prostate gland. These chemicals perform important functions and provide support to the sperm along their journey as they continue to mature.
After ejaculation, the sperm bind to the mucus produced by the female cervix. The sperm use the mucus like a ladder, ascending through it into the uterine cavity. The mucus also has another very important function, as its high pH protects the sperm from the rapid death they would face in the very acidic environment of the vagina.
Once inside the uterus, the flagella (tail) of the sperm propels the sperm to the top of the uterus and then into the fallopian tube where it will hopefully encounter the egg. As the sperm travel down the fallopian tube, they undergo a series of reactions that prepare them for the tasks they will have to perform if successful fertilization is to result. These reactions primarily cause alterations to occur in the membrane cap that covers the sperm head during development.
Ovulation and Fertilization of the Egg
When ovulation occurs, the egg leaves the ovary and it is drawn into the fallopian tube. At this time, the human egg is surrounded by millions of cells called granulosa cells. These cells support the egg as it develops. In order for fertilization to occur, the sperm have to release enzymes found on the head of the sperm to clear a pathway to the egg.
Hundreds, if not thousands of sperm release their enzymes during this process. Eventually, one sperm travels through this pathway and binds to the surface of the egg. This single sperm is then brought into the egg itself where the sperm tail detaches and the sperm head – which contains the chromosomes (DNA) – forms a structure called a “pronucleus”.
The egg forms a similar structure and then the two pronuclei merge and fuse, forming a single nucleus inside the now fertilized egg. Once the pronuclei fuse, each chromosome from the sperm seeks out its corresponding chromosome from the egg, and the chromosomes pair off.
Normal Eggs and Sperm contain 23 chromosomes
For example, each sperm contains 23 chromosomes just like each egg. Twenty two of these chromosomes are identified by their specific number (#1-22), based on the specific genes that that chromosome contains, while the twenty third chromosome is called the “sex chromosome”. The sex chromosome that comes from the egg is always an X, while the sex chromosome that comes from the sperm can be either an X or a Y. If the sperm that fertilizes the egg has a Y sex chromosome, the resulting child will be male; if it is an X, the child will be female.
After each chromosome finds its partner, all of the chromosomes replicate before they are pulled apart and the cell divides. In other words, the one cell embryo becomes a two cell embryo. This process continues as the embryo travels back down the fallopian tube into the uterus. Once inside the uterus, it continues to divide, attaching to the uterine wall once it contains at least 128 cells. This attachment – or implantation – occurs approximately seven days after ovulation.
A woman can typically detect a pregnancy with a blood test approximately seven days after implantation, or fourteen days after ovulation.