Created by CK-12 Foundation/Adapted by Christine Miller
This false-colour image (Figure 18.4.1) shows real human sperm. The tiny gametes are obviously greatly magnified in the picture, because they are actually the smallest of all human cells. In fact, human sperm cells are small, even when compared with sperm cells of other animals. Mice sperm are about twice the length of human sperm! Human sperm may be small in size, but in a normal, healthy man, huge numbers of them are usually released during each ejaculation. There may be hundreds of millions of sperm cells in a single teaspoon of semen. Producing sperm is one of the major functions of the male reproductive system.
A mature sperm cell has several structures that help it reach and penetrate an egg. These are labeled in the drawing of a sperm shown in Figure 18.4.2.
- The head is the part of the sperm that contains the nucleus — and not much else. The nucleus, in turn, contains tightly coiled DNA that is the male parent’s contribution to the genetic makeup of a zygote (if one forms). Each sperm is a haploid cell, containing half the chromosomal complement of a normal, diploid body cell.
- The front of the head is an area called the acrosome. The acrosome contains enzymes that help the sperm penetrate an ovum (if it reaches one).
- The midpiece is the part of the sperm between the head and the flagellum. The midpiece is packed with mitochondria that produce the energy needed to move the flagellum.
- The flagellum (also called the tail) can rotate like a propeller, allowing the sperm to “swim” through the female reproductive tract to reach an ovum if one is present.
The process of producing sperm is known as spermatogenesis. Spermatogenesis normally starts when a male reaches puberty, and it usually continues uninterrupted until death, although a decrease in sperm production generally occurs at older ages. A young, healthy male may produce hundreds of millions of sperm a day! Only about half of these, however, are likely to become viable, mature sperm.
Where Sperm Are Produced
Spermatogenesis occurs in the seminiferous tubules in the testes. Spermatogenesis requires high concentrations of testosterone. Testosterone is secreted by Leydig cells, which are adjacent to the seminiferous tubules in the testes.
Sperm production in the seminiferous tubules is very sensitive to temperature. This may be the most important reason the testes are located outside the body in the scrotum. The temperature inside the scrotum is generally about 2 degrees Celsius cooler than core body temperature. This lower temperature is optimal for spermatogenesis. The scrotum regulates its internal temperature as needed by contractions of the smooth muscles lining the scrotum. When the temperature inside the scrotum becomes too low, the scrotal muscles contract. The contraction of the muscles pulls the scrotum higher against the body, where the temperature is warmer. The opposite occurs when the temperature inside the scrotum becomes too high.
Events of Spermatogenesis
Figure 18.4.3 summarizes the main cellular events that occur in the process of spermatogenesis. The process begins with a diploid stem cell called a spermatogonium (plural, spermatogonia), and involves several cell divisions. The entire process takes at least ten weeks to complete, including maturation in the epididymis.
- A spermatogonium undergoes mitosis to produce two diploid cells called primary spermatocytes. One of the primary spermatocytes goes on to produce sperm. The other replenishes the reserve of spermatogonia.
- The primary spermatocyte undergoes meiosis I to produce two haploid daughter cells called secondary spermatocytes.
- The secondary spermatocytes rapidly undergo meiosis II to produce a total of four haploid daughter cells called spermatids.
- The spermatids begin to form a tail, and their DNA becomes highly condensed. Unnecessary cytoplasm and organelles are removed from the cells, and they form a head, midpiece, and flagellum. The resulting cells are sperm (spermatozoa).
As shown in Figure 18.4.4, the events of spermatogenesis begin near the wall of the seminiferous tubule — where spermatogonia are located — and continue inward toward the lumen of the tubule. Sertoli cells extend from the wall of the seminiferous tubule inward toward the lumen, so they are in contact with developing sperm at all stages of spermatogenesis. Sertoli cells play several roles in spermatogenesis:
- They secrete endocrine hormones that help regulate spermatogenesis.
- They secrete substances that initiate meiosis.
- They concentrate testosterone (from Leydig cells), which is needed at high levels to maintain spermatogenesis.
- They phagocytize the extra cytoplasm that is shed from developing sperm cells.
- They secrete testicular fluid that helps carry sperm into the epididymis.
- They maintain a blood-testis barrier, so immune system cells cannot reach and attack the sperm.
Maturation in the Epididymis
Although the sperm produced in the testes have tails, they are not yet motile (able to “swim”). The non-motile sperm are transported to the epididymis in testicular fluid that is secreted by Sertoli cells with the help of peristaltic contractions. In the epididymis, the sperm gain motility, so they are capable of swimming up the female genital tract and reaching an ovum. The mature sperm are stored in the epididymis until ejaculation occurs.
Sperm are released from the body during ejaculation, which typically occurs during orgasm. Hundreds of millions of mature sperm — contained within a small amount of thick, whitish fluid called semen — are propelled from the penis during a normal ejaculation.
How Ejaculation Occurs
Ejaculation occurs when peristalsis of the muscle layers of the vas deferens and other accessory structures propel sperm from the epididymes, where mature sperm are stored. The muscle contractions force the sperm through the vas deferens and the ejaculatory ducts, and then out of the penis through the urethra. Due to the peristaltic action of the muscles, the ejaculation occurs in a series of spurts.
The Role of Semen
As sperm travel through the ejaculatory ducts during ejaculation, they mix with secretions from the seminal vesicles, prostate gland, and bulbourethral glands to form semen (see Figure 18.4.5 ). The average amount of semen per ejaculate is about 3.7 mL, which is a little less than a teaspoonful. Most of this volume of semen consists of glandular secretions, with the hundreds of millions of sperm cells actually contributing relatively little to the total volume.
The secretions in semen are important for the survival and motility of sperm. They provide a medium through which sperm can swim. They also include sperm-sustaining substances, such as high concentrations of the sugar fructose, which is the main source of energy for sperm. In addition, semen contains many alkaline substances that help neutralize the acidic environment in the female vagina. This protects the DNA in sperm from being denatured by acid, and prolongs the life of sperm in the female reproductive tract.
Besides providing a way for sperm to leave the body, the main role of the penis in reproduction is intromission, or depositing sperm in the vagina of the female reproductive tract. Intromission depends on the ability of the penis to become stiff and erect, a state referred to as an erection. The human penis, unlike that of most other mammals, contains no erectile bone. Instead, in order to reach its erect state, it relies entirely on engorgement with blood of its columns of spongy tissue. During sexual arousal, the arteries that supply blood to the penis dilate, allowing more blood to fill the spongy tissue. The now-engorged spongy tissue presses against and constricts the veins that carry blood away from the penis. As a result, more blood enters than leaves the penis, until a constant erectile size is achieved.
In addition to sperm, the penis also transports urine out of the body. These two functions cannot occur simultaneously. During an erection, the sphincters that prevent urine from leaving the bladder are controlled by centres in the brain so they cannot relax and allow urine to enter the urethra.
The final major function of the male reproductive system is the production of the male sex hormone testosterone. In mature males, this occurs mainly in the testes. Testosterone production is under the control of luteinizing hormone (LH) from the pituitary gland. LH stimulates Leydig cells in the testes to secrete testosterone.
Testosterone is important for male sexual development at puberty. It stimulates maturation of the male reproductive organs, as well as the development of secondary male sex characteristics (such as facial hair). Testosterone is also needed in mature males for normal spermatogenesis to be maintained in the testes. Follicle stimulating hormone (FSH) from the pituitary gland is also needed for spermatogenesis to occur, in part because it helps Sertoli cells in the testes concentrate testosterone to high enough levels to maintain sperm production. Testosterone is also needed for proper functioning of the prostate gland. In addition, testosterone plays a role in erection, allowing sperm to be deposited within the female reproductive tract.
Feature: My Human Body
If you’re a man and you use a laptop computer on your lap for long periods of time, you may be decreasing your fertility. The reason? A laptop computer generates considerable heat, and its proximity to the scrotum during typical use results in a significant rise in temperature inside the scrotum. Spermatogenesis is very sensitive to high temperatures, so it may be adversely affected by laptop computer use. If you want to avoid the potentially fertility-depressing effect of laptop computer use, you might want to consider using your laptop computer on a table or other surface rather than on your lap — at least when you log on for long computer sessions. Other activities that raise scrotal temperature and have the potential to reduce spermatogenesis including soaking in hot tubs, wearing tight clothing, and biking. Although the effects of short-term scrotal heating on fertility seem to be temporary, years of such heat exposure may cause irreversible effects on sperm production.
- Parts of a mature sperm include the head, acrosome, midpiece, and flagellum. The process of producing sperm is called spermatogenesis. This normally starts during puberty, and continues uninterrupted until death.
- Spermatogenesis occurs in the seminiferous tubules in the testes, and requires high concentrations of testosterone. Sertoli cells in the testes play many roles in spermatogenesis, including concentrating testosterone under the influence of follicle stimulating hormone from the pituitary gland.
- Spermatogenesis begins with a diploid stem cell called a spermatogonium, which undergoes mitosis to produce a primary spermatocyte. The primary spermatocyte undergoes meiosis I to produce haploid secondary spermatocytes, and these cells in turn undergo meiosis II to produce spermatids. After the spermatids grow a tail and undergo other changes, they become sperm.
- Before sperm are able to “swim,” they must mature in the epididymis. The mature sperm are then stored in the epididymis until ejaculation occurs.
- Ejaculation is the process in which semen is propelled by peristalsis in the vas deferens and ejaculatory ducts from the urethra in the penis. Semen is a whitish fluid that contains sperm and secretions from the seminal vesicles, prostate gland, and bulbourethral glands. These alkaline secretions are important for sperm survival and motility.
- Besides ejaculating sperm, another reproductive role of the penis is intromission, which is depositing sperm in the female vagina. This requires the penis to become stiff and erect, a state referred to as an erection. Erection usually occurs with sexual arousal as the columns of spongy tissue inside the penis become engorged with blood.
- Leydig cells in the testes secrete testosterone under the control of luteinizing hormone (LH) from the pituitary gland. Testosterone is needed for male sexual development at puberty, and to maintain normal spermatogenesis after puberty. It also plays a role in prostate function and penis’s ability to become erect.
- Compare and contrast the terms: erection, ejaculation, and intromission.
- Describe semen and its components.
- Explain how erection occurs.
How You’re Destroying Your Sperm! Seeker, 2014.
Human Physiology – Reproduction: Spermatogenesis, Janux, 2015.
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 27.5 Spermatogenesis [digital image]. In Anatomy and Physiology (Section 27.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/27-1-anatomy-and-physiology-of-the-male-reproductive-system
Brainard, J/ CK-12 Foundation. (2016). Figure 4 Cross-section of a testis and seminiferous tubules [digital image]. In CK-12 College Human Biology (Section 20.4) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/20.4/
Janux. (2015, January 10). Human physiology – Reproduction: spermatogenesis. YouTube. https://www.youtube.com/watch?v=krSMZDsjLuU&feature=youtu.be
Seeker. (2014, June 16). How you’re destroying your sperm! YouTube. https://www.youtube.com/watch?v=gNHSTa0Yct4&feature=youtu.be
The part of the sperm that contains the nucleus.
An organelle covering the head of animal sperm and containing enzymes that digest the egg cell coating, thus permitting the sperm to enter the egg.
The central part of the sperm cell between the head and the tail.
A whip-like structure that allows a cell to move.
The production or development of mature spermatozoa.
A period during which humans become sexually mature.
One of the many tiny tubes contained within the testes where sperm are produced.
The male sex hormone secreted mainly by the testes.
A type of cell found between seminiferous tubules in the testes that produces and secretes testosterone.
A diploid stem cell in a testis that undergoes mitosis to begin the process of spermatogenesis.
One of two male reproductive organs where sperm mature and are stored until they leave the body during ejaculation.
A part of the cell cycle when replicated chromosomes are separated into two new nuclei and then subsequent cell division gives rise to genetically identical cells in which the number of chromosomes is maintained.
Describes a cell that contain two copies of each chromosome.
The male reproductive cell.
The term used when a cell has half the usual number of chromosomes.
Deoxyribonucleic acid - the molecule carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.
A type of cell that lines the seminiferous tubules in the testes and plays several roles in sperm production.
A hormone is a signaling molecule produced by glands in multicellular organisms that target distant organs to regulate physiology and behavior.
A special type of cell division in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome.
The process by which a cell uses its plasma membrane to engulf a large particle, giving rise to an internal compartment called the phagosome.
A distinctive pattern of smooth muscle contractions that propels foodstuffs distally through the esophagus and intestines.
The process in males in which muscle contractions propel sperm from the epididymes and out through the urethra in semen.
Fluid containing sperm and glandular secretions, which nourishes sperm and carries them through the urethra and out of the body.
One of a pair of thin tubes that transports sperm from an epididymis to an ejaculatory duct during ejaculation; also called sperm duct.
The process in which a male’s penis deposits sperm in a female’s vagina.
A state in which the penis becomes stiff and erect, usually during sexual arousal, as its columns of spongy tissue become engorged with blood.
The male reproductive organ containing the urethra, through which semen and urine pass out of the body.
A hormone secreted by the anterior pituitary gland that stimulates ovulation in females and the synthesis of androgen in males.
The master gland of the endocrine system that secretes many hormones, the majority of which regulate other endocrine glands.
Two male reproductive organs that produce sperm and secrete testosterone; male gonad.
A hormone secreted by the anterior pituitary gland which promotes the formation of ova or sperm.
A gland in the male reproductive system that secretes fluid into semen and provides nourishing substances to sperm.
One of two tubes in the male reproductive system that joins the vas deferens with the urethra and carries semen during ejaculation.
A tube-like organ of the urinary system that carries urine out of the body from the bladder and, in males, also carries semen out of the body.
One of a pair of glands of the male reproductive system that secretes fluid into semen.
One of a pair of glands in the male reproductive system that secretes a fluid to help lubricate the urethra and neutralize any urine it may contain before ejaculation occurs (also called Cowper’s gland).
The female reproductive organ that receives sperm during sexual intercourse and provides a passageway for a baby to leave the mother’s body during birth.