Intricate Design Found in Sperm Cells
Research is proving that a supposed simple cell
is far more complex than once thought
by Jerry Bergman, PhD
Part I: Sperm Design vs Evolution
Human sperm carries the 23 male chromosomes required to fertilize the egg in the woman’s fallopian tube. The male sperm (spermatozoon, plural spermatozoa) is the smallest cell in the human body and, for much of history, was believed to be one of the simplest human cells. Its task was also viewed as simple, namely to move paternal genes from the male to the female. To do this, the transport system was streamlined for speed and efficiency. The vast majority of sperm fail in their mission: of the billions of sperm released during the reproductive life of a human male, very few manage to fertilize an egg.
Why Sperm Were Believed to Be One of the Simplest Cells
Sperm lack most cytoplasmic organelles including ribosomes, endoplasmic reticulum, and the Golgi apparatus. The DNA in sperm’s nucleus is extremely tightly packed to minimize volume. The chromosomes of many sperm, instead of using histones to pack the DNA, instead use simple, positively charged proteins called protamines. For all of these reasons it was regarded as one of the simplest cells in the body. That is, it was until recently. As is true of human anatomy as a whole, when more research is completed, it is shown to be even more complex than originally thought.
As is true of other cells, sperm use mitochondria to produce energy; specifically, approximately 50–75 mitochondria are located in the midpiece. Mitochondria provide the energy required to cause the tails to propel the sperm to its destination in the female. The structure and function of sperm mitochondria are close to identical to the mitochondria in somatic cells.
New Discoveries Reveal Sperm Is More Complex than Once Realized
Among the new discoveries that reveal sperm is far more complex than once realized is this one by Murugesu. Murugesu discovered sperm can adjust their swimming mode by regulating their flagellar waveform to adapt to the fluctuating fluid conditions encountered in their journey to the oocyte. Specifically, they must adjust to the variations in fluid viscosity that they travel in. For example, the tails beat faster when their environment resembles the upstream area of the vaginal tract, which is when the sperm begin to approach an oocyte.
The experimental design Murugesu employed involved a “testing arena” to observe the sperm’s behavior under the major physiologically relevant conditions. The device varied microfluidics to examine the response of the sperm flagellar waveform and energetics to a range of fluid flow and viscosity. The researchers quantified the flagellar dynamics observed by using high-speed high-resolution microscopy set at 200 frames per second. The research determined that sperm flagellar waveforms are primarily influenced by viscosity. The response included increased energy-efficient beating behavior. 
This adaptation is required due to the complex rheological (i.e., flow-trait) properties of the female reproductive tract. This response is necessary for the sperm to be able to navigate the female reproductive tract in order to reach, and fertilize, the oocyte in the fallopian tube. Sperm cells are guided to travel to the oocyte by a combination of thermotaxis (temperature gradients) and chemotaxis (chemical stimuli).
Human sperm are first deposited into the anterior vagina, where they contact the cervical mucus and, to avoid vaginal acid and the immune responses, immediately attempt to enter the cervix. Cervical mucus effectively filters out sperm with damaged morphology. Consequently, only a minority of sperm enter the cervix. When ovulation approaches, sperm are hyperactivated to enable them to proceed towards the tubal ampulla. Motility hyperactivation also assists sperm in penetrating the mucus in the tubes and also the cumulus oophorus and zona pellucida of the oocyte. They then fuse with the oocyte plasma membrane. Although only one sperm is required to fertilize an oocyte, several are required to attach to the outer egg shell and membrane to begin preparing the entry process before one can enter to fertilize it.
This response to fluid conditions requires a sensory system to evaluate the liquid traits, which triggers variations in the sperm tail and energetic level. At this point, research has observed the physical conditions that alter the sperm’s behavior and the specific alterations that result. The next step is to explore the genetic and physical alterations in the sperm that produce the adaptations to the fluid environment. This involves the required nerve, muscle, and chemical design.
This discovery of the sperm design that is able to adjust to fluid conditions has opened up a whole new field of research, including the mechanism that is responsible for the system. In the end, sperm may prove to be far more complex than most somatic cells. One importance of this research is partly to understand the current infertility epidemic the West is currently facing.
 Alberts, Bruce, et al. Molecular Biology of the Cell, 4th edition. W.W. Norton & Co., New York, New York, 2002.
 Hirate, Shuji, et al. Spermatozoon and mitochondrial DNA. Reproductive Medical Biology 1(2): 41–47, 2002.
 Murugesu, Jason Arunn. Sperm sense what they are swimming through and adapt their behavior. New Scientist; https://www.newscientist.com/article/2400654-sperm-sense-what-they-are-swimming-through-and-adapt-their-behaviour, 1 November 2023.
 Sperm can adjust their swimming style to adapt to fluctuating fluid conditions; https://phys.org/news/2023-11-sperm-adjust-style-fluctuating-fluid.html;
https://www.sciencedaily.com/releases/2023/11/231101134736.htm, 1 November 2023.
 Murugesu, Jason Arunn. 2023.
 Parast, Farin, et al. The cooperative impact of flow and viscosity on sperm flagellar energetics in biomimetic environments, Cell Reports Physical Science. DOI: 10.1016/j.xcrp.2023.101646; www.cell.com/cell-reports-phys … 2666-3864(23)00469-1, 2023.
 Suarez, S., and A. Pacey. Sperm transport in the female reproductive tract. Human Reproductive Update 12(1): 23-37; doi: 10.1093/humupd/dmi047, January-February 2006. Epub. PMID: 16272225, 4 November 2005.
Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,800 college libraries in 27 countries. So far over 80,000 copies of the 60 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.