Spermatozoa, commonly known as sperm cells, are highly specialized male gametes essential for sexual reproduction in animals. Their primary function is to fertilize the oocyte, female egg cell, thus leading to formation of a diploid zygote that develops into a new organism inside the female. They carry genetic information from the male and are uniquely designed to travel to and fertilize the female gamete. In fact, the human sperm has a tadpole-like shape, with a head that contains DNA and a tail which propels the cell forward [1].
What are these ‘little animals’?
They were first observed in 1677 by Antonie van Leeuwenhoek, a Dutch naturalist who is credited as the ‘father of microbiology’, and Johan Ham [2]. Leeuwenhoek used his custom-made microscopes to visualize sperm cells and microorganisms which he described as ‘animalcules’ or ‘little animals’ as he believed that sperm cells housed little babies that were incubated in women. This is known as Preformation Theory which was popular around late 17th century to late 18th century. However, the role of sperm in reproduction eluded him and other scientists for a few hundred years [3].
Cells that renew forever
Spermatozoa are produced in seminiferous tubules in the testes through a complex process called spermatogenesis that can take about 86 days to complete [4]. This process results in differentiated and undifferentiated stem cells called spermatogonia. Differentiated sperm cells can fertilize female gametes whereas undifferentiated spermatogonia have the potential to further differentiate into sperm cells, thus continuously replenishing the supply of sperm cells [5]. In human males, this process begins during puberty and continues throughout life [4]. Abnormalities in the number or morphology of sperm cells have been linked to fertility issues in men [6].
Built for speed
Evolution has created a fascinating diversity of male gametes in the animal kingdom, which has intrigued scientists for decades. The spermatozoa are unique in their morphology as their only purpose is to quickly reach the female gamete and deliver the male genetic material. This means that they have to pack light and only what’s essential. They also need a way to travel fast. As a result, the sperm cell is the smallest in the human body and has a tail that can propel the cell forward [1].
It consists of two main parts: head and tail. The spermatozoa head is a distinctive oval-shaped, flattened structure containing a large nucleus with a scarce amount of cytoplasm. The nucleus is surrounded by the acrosomal vesicle which comprises 40–70% of the volume of the head. The head is 5 to 6 μm long and 2.5 to 3.5 μm wide [7]. The head is connected to the tail by the postacrosomal region commonly described as the neck. The tail or the flagellum can be divided into four sections: the connecting piece, the midpiece, the principal piece and the endpiece. The midpiece is the powerhouse of the sperm cell. It contains mitochondria that provide the tail with energy for movement. The axoneme, composed of microtubules, is the main structural component of the tail that enables it to move. It runs throughout the entirety of the tail [6]. Any irregularities in the morphology of the sperm cell can cause fertility issues [7].
Male infertility
Abnormalities in sperm motility and count within male ejaculate are common contributors to male infertility, with around 20% of infertile men exhibiting slower sperm motility, a condition known as asthenozoospermia [6]. Lifestyle factors, environmental conditions and genetic defects can lead to infertility in men. For example, infertile men with azoospermia lack the DAZ gene on their Y chromosome, which is essential for encoding the RNA-binding protein called DAZ, needed for sperm production [8]. Environmental factors such as exposure to heat may reduce sperm motility and structure. Additionally, pesticides and heavy metals can disrupt endocrine function, thus reducing sperm motility. A multitude of environmental factors have been linked to abnormalities in sperm motility, including smoking, infections, drugs, and stress. Fortunately, not all is lost. Sometimes lifestyle modification can reverse issues with sperm motility [9].
Spermatozoa might be the most important cell type in the body from an evolutionary perspective. The diversity in the morphology and method of transmission to the female gamete is truly fascinating [1].
Recognizing and appreciating the labs working in this space
- Bruce Alberts, Department of Biochemistry and Biophysics, University of California, San Francisco, USA https://brucealberts.ucsf.edu/about-bruce-alberts/
- Melissah Rowe, Department of Animal Ecology, Netherlands Institute of Ecology, Wageningen, The Netherlands https://nioo.knaw.nl/nl/employees/melissah-rowe#tab-publicaties , X: @melissah_rowe
- Ulrich Kutschera, Professor of Biology, AK-Evolutionsbiologie, Germany & I-Cultiver, Inc., Manteca, California, USA https://www.researchgate.net/profile/Ulrich-Kutschera
- Maria Teves, Department of Obstetrics and Gynecology, Virginia Commonwealth University, Richmond, Virginia https://obgyn.vcu.edu/about/our-team/maria-e-teves-phd.html , X: @the_teves
- Eduardo Roldan, Department of Biodiversity and Evolutionary Biology, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain. https://www.researchgate.net/profile/Eduardo-Roldan
- Rute Pereira, Laboratory of Cell Biology, Department of Microscopy, ICBAS-School of Medicine and Biomedical Sciences, University of Porto, 4050–313 Porto, Portugal https://www.researchgate.net/profile/Rute-Pereira-4
- Nicola Reynolds, Cambridge Stem Cell Institute, University of Cambridge, Cambridge, UK https://www.stemcells.cam.ac.uk/directory/brian-hendrich-group
- Howard Cooke, MRC Human Genetics Unit, University of Edinburgh, Edinburgh, UK https://www.researchgate.net/profile/Howard-Cooke
- Guruprasad Kalthur, Department of Clinical Embryology, Kasturba Medical College, Manipal, India https://www.manipal.edu/kmc-manipal/department-faculty/faculty-list/guruprasad-kalthur.html
- Ricardo Bertolla, School of Medicine Obstetrics & Gynecology, Wayne State University, Detroit, Michigan https://obgyn.med.wayne.edu/profile/hr1632
References
- Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Sperm. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26914/
- Fisher, H. S. et al. “On the Origin and Evolution of Sperm Cells.” Cells, vol. 12, no. 1, 2022, doi:10.3390/cells12010159.
- Kutschera, U. “Antonie Van Leeuwenhoek (1632–1723): Master of Fleas and Father of Microbiology.” Microorganisms, vol. 11, no. 8, 2023, doi:10.3390/microorganisms11081994.
- Holstein, A. F. et al. “Understanding Spermatogenesis Is a Prerequisite for Treatment.” Reprod Biol Endocrinol, vol. 1, 2003, p. 107, doi:10.1186/1477–7827–1–107.
- Teves, M. E. and E. R. S. Roldan. “Sperm Bauplan and Function and Underlying Processes of Sperm Formation and Selection.” Physiol Rev, vol. 102, no. 1, 2022, pp. 7–60, doi:10.1152/physrev.00009.2020.
- Pereira, R. and M. Sousa. “Morphological and Molecular Bases of Male Infertility: A Closer Look at Sperm Flagellum.” Genes (Basel), vol. 14, no. 2, 2023, doi:10.3390/genes14020383.
- Puerta Suárez, J. et al. “Spermatozoa: A Historical Perspective.” Int J Fertil Steril, vol. 12, no. 3, 2018, pp. 182–90, doi:10.22074/ijfs.2018.5316.
- Reynolds, N. and H. J. Cooke. “Role of the Daz Genes in Male Fertility.” Reprod Biomed Online, vol. 10, no. 1, 2005, pp. 72–80, doi:10.1016/s1472–6483(10)60806–1.
- Dcunha, R. et al. “Current Insights and Latest Updates in Sperm Motility and Associated Applications in Assisted Reproduction.” Reprod Sci, vol. 29, no. 1, 2022, pp. 7–25, doi:10.1007/s43032–020–00408-y.
- Bertolla, R. P. “Sperm Biology and Male Reproductive Health.” Sci Rep, vol. 10, no. 1, 2020, p. 21879, doi:10.1038/s41598–020–78861–7.
About the author:
Dr. Nowrin Ahmed
Content Editor The League of Extraordinary Cell Types, Sci-Illustrate Stories
Dr. Nowrin Ahmed has a PhD in Behavioral and Neural Sciences from Rutgers University-Newark (NJ, USA) where she studied the interactions between the midline thalamus and the amygdala. She continued her studies on amygdala circuits during her post-doctoral fellowship at Rutgers University — Newark. Currently, she works as a freelance medical writer. Dr. Nowrin enjoys sharing the beauty of science with diverse audiences.
About the artists:
NELLY AGHEKYAN
Contributing Artist The League of Extraordinary Cell Types, Sci-Illustrate Stories
Nelli Aghekyan, did a bachelor’s and master’s in Architecture in Armenia, after studying architecture and interior design for 6 years, she concentrated on her drawing skills and continued her path in the illustration world. She works mainly on children’s book illustrations, some of her books are now being published. Currently living in Italy, she works as a full-time freelance artist, collaborating with different companies and clients.RENSKE HOSTE
Contributing Artist The League of Extraordinary Cell Types, Sci-Illustrate Stories
Renske is a professional medical artist who spends most of her time working on anatomy and pathology in 2D and 3D. She joined as a creator of the League of Extraordinary Cell Types to challenge herself to combine science with her love for art and all things nature and growing. When not working full-time or as a freelancer, Renske loves growing things in her garden, kicking butt doing Krav Maga, or being knee-deep in various creative projects.
About the animator:
DR. EMANUELE PETRETTO
Animator The League of Extraordinary Cell Types, Sci-Illustrate Stories
Dr. Petretto received his Ph.D. in Biochemistry at the University of Fribourg, Switzerland, focusing on the behavior of matter at nanoscopic scales and the stability of colloidal systems. Using molecular dynamics simulations, he explored the delicate interaction among particles, interfaces, and solvents.
Currently, he is fully pursuing another delicate interaction: the intricate interplay between art and science. Through data visualization, motion design, and games, he wants to show the wonders of the complexity surrounding us.
About the series:
The League of Extraordinary Cell types
The team at Sci-Illustrate and Endosymbiont bring to you an exciting series where we dive deep into the wondrous cell types in our body, that make our hearts tick ❤.
