Sperm in a Nutshell

The Modern Man's Sperm Guide

The conversation of fertility often revolves around female reproductive health. However, men are responsible for up to half of all couple infertility cases. It is not only crucial for procreation, but sperm quality is also an indicator of good reproductive health. That is why sperm testing is the cornerstone of investigating male fertility.

What Is Sperm?

Sperm, also called spermatozoan, is the reproductive cell or gamete of a human male. It is a haploid cell, which means it contains half of the DNA compared to other cells of the body. The DNA is the genetic material that parents transfer to their children. Sperm contributes half of it, and the other half comes from the maternal egg.

Humans have 46 chromosomes in each of their body cells. In sperm, genetic material is present in the form of 23 chromosomes. This is because the female reproductive cell also has chromosomes to give. So, the other 23 chromosomes contributed by the mother complete the set of 46 chromosomes. That is why children take some characteristics from the mother and others from the father.

This mingling happens when the sperm cell combines with the oocyte (female reproductive cell also referred to as egg or ovum) through fertilization. The fertilization results in the formation of a zygote that has 46 chromosomes. The zygote turns into an embryo, the first cell of the human body that makes the fetus.

What Is the Structure Of Sperm?

Sperm are not visible to the naked eye because they are very small in size. The approximate length is 1/6th of an inch. Under a microscope, the sperm cell reveals a peculiar shape and various structures. Each of these structures performs their respective functions to facilitate fertilization. The shape and structure of the sperm is referred to as sperm morphology.

Sperm structure determines the health and ability of sperm to fertilize the egg. So, male fertility testing includes the analysis of sperm morphology along with other features. A healthy sperm cell has the following structures:

The head of mature sperm is flat and oval. It is the only part of sperm that interacts with the oocyte during fertilization. The rest of the structures are to support the head’s penetration through the walls of the oocyte. For comparison, imagine the shape of an egg in your kitchen. The slightly pointed end represents an acrosome in the sperm˜—a cap-like structure that covers the head. The acrosome allows the sperm to fertilize the oocyte through a specialized cellular reaction. It fuses the outer layer of both cells and makes them one. The head contains genetic material in coiled chromosomes, which are transferred during fertilization.

Mid-piece

The mid-piece is longer than the head with an axoneme. The axoneme has multiple longitudinal microfilaments, which form a skeletal structure of the sperm. Think of it as the spine of the sperm, which runs throughout the tail. In the mid-piece, it is surrounded by a series of mitochondria on both sides. The mitochondria are cellular organelles that generate energy using sugar. This energy is required for the sperm to be active and reach the oocyte.

Tail

The tail is the last piece in sperm cell structure. It is also the longest part of the sperm cell and is responsible for sperm motility or movement. The flagellar movement of the tail causes propulsion of sperm through the female reproductive tract. Sperm motility is a crucial indicator of male fertility because it enables the sperm to swim to their destination. It is also one of the parameters measured in male sperm testing.

How Are They Produced In The Body?

Sperm are produced in the testes. Testes are pairs of small oval structures that hang in the scrotum below the penis. Each testis has numerous tiny tubules called seminiferous tubules. These tubules house cells called “spermatogonia” which divide to producesperm. Spermatogonia are cells that form in a fetus at the time of sex determination in the mother’s womb. Male children have these cells in the body at the time of their birth. Their conversion to sperm consists of several steps and maturation phases that begin at the onset of puberty.

Some of these occur in the epididymis, which is responsible for the storage and transport of sperm. The epididymis is a coiled tube that connects the rear of the testes to the ejaculatory duct of the male reproductive tract. The ejaculatory duct travels through the pelvis and reaches the penis. The process of sperm formation is known as spermiogenesis. It involves various cellular divisions and differentiations.

It is essential to understand the process of sperm maturation because it significantly impacts the features of sperm. Each of these features indicates the health of sperm in sperm testing. Impairment in the formation or development of sperm can lead to abnormal shape and low quality of cells.

Formation of spermatids

Spermatogonia are cells with 46 chromosomes. They replicate to form primary spermatocytes with the same number of chromosomes. The word replication is the key concept here. It means spermatogonia make copies with no difference in the genetic material.

These primary spermatocytes then divide to form two cells each. As the process of division takes place, the primary spermatocytes divide their genetic material in half. The resultant cells are two “sister” cells, each containing half of the genetic material from their parent cells. These cells are named spermatids, and they are immature sperms with no ability to move. Sex hormones in the male body drive this process. This is why hormone levels are also a part of male fertility testing because they affect the development of sperm.

Maturation of spermatids

Spermatids have cellular organelles like cytoplasm, vacuoles, and nucleus, and more.  Each of these spermatids matures to form a sperm with a head, mid-piece, and tail. This process involves the rearrangement of cellular organelles and the formation of the tail. The DNA material condenses and confines into the nucleus of the head. The tail grows from the centrioles (a cellular component of spermatid), and mitochondria shift into the mid-piece. Excess cytoplasm dries up, but the resultant sperm are immotile.

From here, they move into the epididymis for further maturation. These changes introduce the propelling action of the tail and make sperm cells motile.

What Are The Conditions That Affect The Production Of Sperm?

Various factors affect the formation and maturation of sperm cells:

Hormones

Testosterone is the dominant sexual hormone in men. It matures sperm by rearranging their cellular components and forming distinct structures. Deviations from normal testosterone levels can interupt spermatogenesis and sperm maturation.

Temperature

The sperm formation in the seminiferous tubules of the testes is a temperature-sensitive process. It is time to refresh your biology class knowledge. If you remember, female ovaries are located inside the body, but that is not the case with men. Ovaries are the female equivalent of testes. They make an egg/ovum/oocyte and release it within the body to meet sperm.

Testes are located outside the body. That’s because the body’s core temperature is too warm for sperm production. The site and structural arrangement of the testes outside keeps them about  5°F lower than the inside of the body.

The scrotum between the legs acts as a warm bag for testes. Your nerves also regulate the position of the scrotum to regulate the temperature. In cold climates, the scrotum shrinks to keep the testes warm. In moderate or warm weather, it relaxes to keep the testes cooler.

Dietary elements

Vitamins and minerals such as vitamin A, E, and selenium are imperative for the metabolic reactions involved in spermatogenesis. There’s evidence that deficiencies in these vitamins and minerals could affect  sperm concentration or quality.

What Is The Frequency Of Sperm Production?

Each ejaculation releases 20 to 300 million sperm per milliliter of semen. Spermatogenesis is a continuous cycle that contributes to the ongoing supply of sperm. The testes produce millions of sperm cells per day. The approximate estimation is that male testes can produce 1500 sperm per second but the fullmaturation process takes about 64 days to complete.

From the onset of puberty at the age of 9 to 12 years, sperm production begins.

Are Semen and Sperm The Same?

The answer is No.

Sperm are male reproductive cells, while semen is a fluid that carry the sperm. Semen is whitish or translucent discharge that comes out of the penis during ejaculation. It also nourishes sperm and carries them through the ejaculatory duct. The production of semen is not the responsibility of a single organ. Instead, it comes from various glands in the male reproductive system. The following structures contribute to the formation of semen:

  • Prostate gland
  • Bulbourethral glands
  • Seminal vesicles

Seminal vesicles provide the most significant contribution. Each of these glands is responsible for producing different components of seminal fluid. Some make substances that give smell to the sperm. Others form components that provide energy and protect sperms. They send their respective secretions to the ejaculatory duct, where sperm get mixed with it. Sperm make up 2% to 5% of the semen content.

Why Do Sperm Need Semen?

The semen keeps sperm viable. Until these cells reach the female tract, they need semen. That is why sperm cannot survive for extended periods of time in the air or outside the body. In the female tract, cervial mucus aids with viability. The cervix is the lower part of the uterus and connects the vagina to the main uterine body. The sperm deposit in the upper vagina during sexual intercourse. From there, they cross the cervix to travel through the uterus and reach the female ovum located in the fallopian tubes. The benefits derived by sperm from the semen are:

#1 It Provides Energy To Sperm Cells

Semen has fructose that provides energy to the sperm. Remember those mitochondria in the mid-piece? They are the powerhouse of sperm, but they need fuel for the production of energy. Semen provides that fuel in the form of fructose, which is a type of sugar. The amount of fructose in semen is 2 to 5 mL per ejaculation. This energy is crucial for sperm motility.

#2 It Protects Sperm From Female Immune System

Semen has prostaglandins that keep sperm alive by suppressing an immune reaction. The female body identifies sperm as foreign cells. The immune system may act to kill them, but prostaglandins prevent this. They also loosen up the cervical mucus and make it more receptive to sperm transport from the vagina to the uterus.

#3 It Increases The Chances Of Sperm Survival

The female vagina has an acidic pH because the naturally occurring bacteria there produce lactic acid. These are friendly bacteria in the female vagina to prevent the entry of hostile bacteria into the body. The acidic environment can reduce the entry of pathogens, such as those that cause sexually transmitted infections. Sperm can also become collateral damage in this as they cannot withstand the acidic environment.

Even if the sperm survive, DNA content is susceptible to damage from an acidic environment. Semen has amine compounds, which are basic in nature. The experts suggest that these compounds buffer the pH of the vagina within 6 to 8 seconds. As a result, the sperm get a favorable environment for survival.

#4 It Protects Genetic Material In The Head Of Sperm

Semen has another chromatin protective substance. It contains zinc, which prevents mutation or degradation of DNA. The sperm stay robust with healthy genetic material ready for fertilization.

#5 It Facilitates Sperm Motility

The liquid in semen, which gives it a jelly-like texture, aids sperm motility and allows it to swim through the cervix.

#6 It Regulates The Function Of Sperm

The cholesterol content of semen prevents the premature capacitation of sperm. Capacitation is a preparation process for fertilization and happens in the female reproductive tract. But before the sperm cells get there, they have a lot of distance to cover. Premature capacitation diminishes male fertility, and the cholesterol in of semen prevents that.

Apart from these compounds, semen contains several others. All these have their respective roles in maintaining the texture and consistency of semen. They also control sperm concentration and volume to enhance the possibility of fertilization.

How Does The Sperm Travel to The Egg?

Testicular cells secrete a fluid that facilitates sperm transport from the testes to the epididymis. The cilia (thread-like structures that can propel cells with their organized movement) present in tubules also participate in this process. Once they reach the epididymis, they stay there for 12 days. During this period, they go through maturation and gain the ability to swim.

Testes→ Epididymis → Ejaculatory duct →Urethra→Ejaculation through the penile opening

The next transit point is the ejaculatory duct, where sperm get seminal fluid. From here, the sperm, along with semen, move to the urethra in the penis. The ejaculation happens through pelvic muscular contractions resulting from sexual stimulation. After ejaculation through the urethra, sperm cells deposit on the upper part of the vagina.

Vagina→Cervix→ Uterus→ Fallopian tubes→Fertilization

The cervical mucus allows it to pass through and reach the uterine tubes. These tubes connect the uterus to the ovaries. Ovum releases from ovaries and meets sperm cells in these tubes. As the ovum has no tail, it moves due to cilia in fallopian tubes. Halfway through these tubes, sperm reaches and fertilizes the ovum. This transport is mainly dependent on sperm motility. Only several hundred sperm get the uterine tubes out of significant sperm volume per ejaculate.

How Does Sperm Fertilize The Egg?

Capacitation reaction

To fertilize the ovum, sperm must undergo some adjustments. These changes happen in a series and are collectively known as a capacitation reaction.

The capacitation of sperm prepares it to bind and penetrate the ovum for fertilization. It constitutes the removal of cholesterol and glycoprotein molecules from the surface of sperm. As discussed earlier, cholesterol comes from seminal fluid and prevents premature capacitation.

Glycoprotein molecules, however, attach to sperm during their maturation in the epididymis. The surfaces involved are the head and acrosome of sperm. In short, capacitation is like unclothing the sperm because it exposes receptors of sperm for binding. Receptors are like small glue patches on the surface of sperm which can adhere to the ovum’s membrane.

Acrosome reaction

After capacitation, the sperm become hyperactive and attempt to penetrate the ovum. The contact of sperm with the outermost layer of the ovum triggers an acrosome reaction.

The acrosome has several enzymes that digest the wall of the ovum and allow sperm entry. Many sperm cells attach to the ovum wall, but typically only one succeeds to burrow through it. The plasma membrane of the sperm cell fuses with that of the ovum, and the head eventually transfers the genetic material.

How Long Does Sperm Last?

The lifespan of sperm cell varies in the male and female bodies. When sperm are in the male body, they last up to 74 days from the day they’re produced. If they are not ejaculated, they die and are reabsorbed by the body. After ejaculation in the vagina, sperm can survive for  5 days or more in the female reproductive tract.

Should I Test and Freeze My Sperm?

Unlike women, men do not experience cessation of reproductive cell formation as per se. The spermatogenesis lasts throughout a man’s lifetime, but sperm count and quality decline with age. Therefore, although the production of sperm continues, male fertility is impacted by the aging process.  You can determine the quality of sperm cells through sperm testing. Early and frequent sperm testing will help to identify abnormalities, and monitor overall sperm and fertility health. In particular, as men age, the quality of sperm deteriorates, and it becomes more important to consider freezing sperm at an earlier age to ensure future fertility.

One way to avoid potential fertility concerns is opting for sperm freezing. When male fertility is at its zenith at a young age, you can freeze healthy sperm for future use.

This information is not intended or implied to be a substitute for professional medical advice, diagnosis or treatment. Dadi Inc. makes no representation and assumes no responsibility for the accuracy of information contained herein, and such information is subject to change without notice. You are encouraged to confirm any information obtained from or through this web site or article with other sources, and review all information regarding any medical condition or treatment with your physician.

References:

1. “Gamete transport”, Reference Module in Biosciences. Elsevier. 2014.

2. Carlson, Bruce M. "Chapter 14 - The Reproductive System." The Human Body. Academic press, 2019. 373-396

3. Susan A. Rothmann, Anna-Marie Bort. "Sperm Morphology". Encyclopedia of Reproduction (Second Edition). Academic Press, Pages 85-95. 2018.

4. Max M. Houck, Frank Crispino, Terry McAdam. Chapter 6.2 - Biological Evidence." The Science of Crime Scenes (Second Edition). Academic Press, 2018. Pages 259-282.