Sperm Cell (Spermatozoon): Structure, Parts, Spermatogenesis, Functions, and Male Infertility Disorders

Introduction

Sperm cell, also called spermatozoon, is the male haploid gamete produced in the testes. It unites with the female egg to form a diploid zygote that develops into a new individual.

In this article you will learn the unique structure of sperm cell with head, neck, midpiece and tail, the process of spermatogenesis in seminiferous tubules, how sperm acquire motility, main functions including fertilization and genetic contribution, and common diseases like azoospermia, oligozoospermia, asthenozoospermia, teratozoospermia, varicocele and testicular cancer.

What is Sperm Cell?

The sperm cell is a microscopic, flagellated, motile male reproductive cell produced by the male reproductive organ. Like egg cells, sperm cells are haploid, containing only half the number of chromosomes. Human sperm nucleus contains 23 chromosomes. Fusion with ovum forms a diploid zygote containing 46 chromosomes.

Sperm cells are formed through spermatogenesis in seminiferous tubules of testes. Once matured, they are released into epididymis where they gain motility and become ready for fertilization.

Structure of the Sperm Cell

General Organization

The sperm cell is about 55-65 µm long and is covered by a continuous plasma membrane. It has four main regions: head, neck, middle piece or body, and tail or flagellum. Each region is highly specialized for fertilization.

1. Head

Nucleus

The head region is usually a flat, pear-shaped structure with a pointed tip and broad base. It contains highly condensed genetic material within the cell nucleus. Human sperm nucleus is haploid with 23 chromosomes, including either X or Y chromosome determining sex.

Acrosome

The head is covered by a cap-like structure called acrosome, derived from Golgi apparatus. It covers front half of nucleus and contains hydrolytic enzymes like hyaluronidase and acrosin essential for penetrating layers of egg during fertilization. This enzyme release is called acrosome reaction.

2. Neck

Centrioles

Neck is a short connecting region between head and middle piece. It contains proximal and distal centrioles. Distal centriole organizes microtubule structure of tail forming axoneme. Proximal centriole contributes to zygote's first mitotic spindle after fertilization.

3. Body or Midpiece

Mitochondrial Sheath

The body, also called midpiece, is middle portion about 5-7 µm long. It is energy production center containing mitochondria helically wrapped around axoneme forming mitochondrial sheath. These mitochondria produce ATP required for flagellar movement via oxidative phosphorylation.

Annulus or Terminal Disc

Terminal disc or annulus is a dense ring structure located at junction of middle piece and principal piece of tail. It prevents mitochondria from sliding down flagellum and provides structural stability.

4. Tail or Flagellum

Axoneme with 9+2 Arrangement

Tail makes up about 80% of entire length, around 45-50 µm. It propels sperm. Cytoskeletal core is axoneme with 9+2 arrangement: two central singlet microtubules surrounded by nine microtubule doublets. Movement is powered by dynein motor proteins that use energy from ATP hydrolysis to cause sliding of doublets, generating whip-like beating.

Correction: Original text stated tail contains axoneme surrounded by mitochondria. Scientifically, mitochondria are confined to midpiece; tail contains only axoneme, outer dense fibers, and fibrous sheath.

Principal Piece and End Piece

Tail is subdivided into principal piece containing fibrous sheath and outer dense fibers, and end piece containing only axoneme covered by plasma membrane. This organization allows progressive motility through female reproductive tract.

Plasma Membrane

Plasma membrane is continuous lipid bilayer enclosing entire sperm cell. It protects internal components, regulates ion exchange essential for motility and capacitation, and houses specific surface receptors needed to bind with egg's zona pellucida during fertilization.

What is Spermatogenesis?

Location and Supporting Cells

Spermatogenesis is formation of sperm cells in testes, male organs producing sperm and testosterone. They contain tightly coiled structures called seminiferous tubules within which sperm cells are produced. Specialized Sertoli cells scattered among tubules provide nutrients, structural support, and form blood-testis barrier for developing germ cells.

Stages of Spermatogenesis

Spermatogonia to Primary Spermatocytes

Process starts with spermatogonia, diploid stem cells derived from primordial germ cells that migrate into testis during early embryogenesis and proliferate continuously by mitosis. Half remain as stem cells for self-renewal, while others differentiate into primary spermatocytes containing 46 chromosomes. These migrate towards Sertoli cells for further development.

Meiosis I and Meiosis II

Primary spermatocytes undergo meiotic division I to form secondary spermatocytes containing 23 chromosomes, haploid, similar to egg in chromosome number but still duplicated chromatids. Secondary spermatocytes proceed through meiotic division II to produce haploid spermatids with 23 single chromatids.

Spermiogenesis

Haploid spermatids mature and differentiate into mature spermatozoa through spermiogenesis. This involves specific changes: nucleus condenses and assumes oval shape forming head, acrosome forms from Golgi, centriole forms flagellum from cytoplasm, excess cytoplasm is shed as residual body phagocytosed by Sertoli cells, and mitochondria arrange in midpiece. After fully matured, sperm are stored and gain motility in epididymis.

The whole process in humans takes about 64-72 days.

Functions of the Sperm Cells

• Fertilization: Main function is to fertilize female egg cell to form new individual. Fusion forms zygote which divides to form embryo.
• Genetic contribution: Contributes haploid set of chromosomes determining genetic traits of offspring and sex via X or Y chromosome.
• Motility: Highly specialized for motility with flagellum allowing swimming through female reproductive tract to reach egg in fallopian tube. Requires capacitation and hyperactivation in female tract.
• Penetration of egg: Acrosome contains enzymes that help penetrate protective layers of egg including cumulus oophorus and zona pellucida. Enzymes released during acrosome reaction.
• Activation of egg: Sperm entry triggers completion of oocyte meiosis and initiates embryonic development.

Diseases and Disorders of Sperm Cell

Azoospermia

Absence of sperm in ejaculate, main cause of male infertility. Causes include blockages in reproductive tract or obstructive azoospermia, hormonal imbalances, testicular failure or non-obstructive azoospermia, and genetic conditions like Y-chromosome microdeletions. Diagnosis includes hormonal tests, semen biomarkers, ultrasonography, testicular biopsy, vasography. Treatment includes surgery for obstruction, hormonal therapy, and sperm retrieval using assisted reproductive technologies like in vitro fertilization (IVF) and Intracytoplasmic Sperm Injection (ICSI).

Oligozoospermia

Low sperm count below normal range. According to WHO 6th edition 2021, normal concentration is ≥15 million per ml and total count ≥39 million per ejaculate. Factors include genetic disorders like Klinefelter syndrome (47, XXY), infections, hormonal abnormalities, blockages, exposure to toxins, varicocele, and certain medications. Diagnosis includes semen analysis, sperm function tests, imaging, hormone measurements, and genetic testing.

Asthenozoospermia

Reduced sperm motility. Normal progressive motility ≥32% and total motility ≥42% per WHO. Sperm with reduced motility have difficulty reaching and fertilizing egg. Causes include mitochondrial dysfunction, varicocele, infections, anti-sperm antibodies.

Teratozoospermia

Abnormal sperm morphology. WHO normal forms ≥4% by strict Kruger criteria. Abnormal morphology causes difficulty penetrating egg barriers and reduces fertilization chances. Often coexists with oligozoospermia and asthenozoospermia as OAT syndrome.

Varicocele

Swelling of pampiniform plexus veins within scrotum draining blood from testicle. Causes low count, reduced motility, abnormal morphology due to increased scrotal temperature and oxidative stress. Affects testicular growth and function. Varicocelectomy surgery often improves motility and morphology.

Testicular Cancer

Abnormal growth of cells in testes, most begin in germ cells. Rapid growth may spread to other parts. Causes involve changes in DNA of germ cells; risk factors include cryptorchidism, family history. Diagnosis includes physical examination, scrotal ultrasound, serum tumor markers AFP, hCG, LDH, and imaging. Treatment involves orchiectomy, radiation therapy, and chemotherapy.

Infections of Reproductive System

Infections like epididymitis and sexually transmitted infections such as chlamydia and gonorrhea can affect sperm production, cause inflammation, and block passage of sperm, leading to infertility.

3. Key Takeaways

• Sperm cell or spermatozoon is male haploid gamete 55-65 µm long with head, neck, midpiece and tail covered by plasma membrane, produced by spermatogenesis in seminiferous tubules.
• Head contains haploid nucleus with 23 chromosomes and acrosome cap with hyaluronidase and acrosin enzymes for acrosome reaction to penetrate zona pellucida.
• Neck contains proximal and distal centrioles organizing tail microtubules; midpiece contains helical mitochondrial sheath producing ATP and annulus preventing mitochondrial sliding.
• Tail flagellum has 9+2 axoneme with two central singlets and nine doublets, dynein motors using ATP for motility; mitochondria are only in midpiece, not in tail principal piece.
• Spermatogenesis takes 64-72 days: spermatogonia (2n=46) → primary spermatocytes (2n) → meiosis I → secondary spermatocytes (n=23 duplicated) → meiosis II → spermatids (n=23) → spermiogenesis → spermatozoa stored in epididymis gaining motility.
• Functions include delivering paternal genome and sex chromosome, motility via capacitation and hyperactivation, acrosome-mediated egg penetration, and triggering egg activation.
• WHO 6th edition normal values: concentration ≥15 million/ml, total ≥39 million, progressive motility ≥32%, total motility ≥42%, normal forms ≥4% strict.
• Disorders: azoospermia (no sperm), oligozoospermia (low count), asthenozoospermia (low motility), teratozoospermia (abnormal morphology), OAT syndrome, varicocele, testicular cancer, and infections causing obstruction and infertility.
• Assisted reproduction like ICSI overcomes severe male factor infertility, but diagnosis requires semen analysis, hormones, ultrasound, tumor markers, and genetic testing.
• Prevention includes avoiding toxins, heat, STIs, early treatment of varicocele and cryptorchidism, and lifestyle measures.

4. Scientific References

1. Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P → Molecular Biology of the Cell → 6th Edition → Garland Science → Chapter 21 Sexual Reproduction, Meiosis, Gametogenesis.
2. Lodish H, Berk A, Kaiser CA, Krieger M, Bretscher A, Ploegh H, Amon A, Scott MP → Molecular Cell Biology → 8th Edition → W.H. Freeman → Chapter 19 Meiosis and Gametogenesis.
3. Cooper GM, Hausman RE → The Cell: A Molecular Approach → 8th Edition → Oxford University Press → Chapter 14 The Eukaryotic Cell Cycle, Meiosis and Spermatogenesis.
4. Karp G, Iwasa J, Marshall W → Karp's Cell Biology → 8th Edition → Wiley → Chapter 14 Meiosis and Gametogenesis – Spermatogenesis and Spermiogenesis.
5. Carlson BM → Human Embryology and Developmental Biology → 6th Edition → Elsevier → Chapter 2 Gametogenesis, Spermatogenesis and Spermiogenesis.
6. Moore KL, Persaud TVN, Torchia MG → The Developing Human: Clinically Oriented Embryology → 11th Edition → Elsevier → Chapter 2 Gametogenesis and First Week.
7. NCERT → Biology Textbook for Class XII → Reprint 2023-24 → National Council of Educational Research and Training, India → Chapter 3 Human Reproduction – Gametogenesis, Structure of Sperm.
8. Hermo L, Pelletier RM, Cyr DG, Smith CE → Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 1: Background to spermatogenesis, spermatogonia, and spermatocytes → Microscopy Research and Technique → 2010 → Volume 73, Issue 4, Pages 241-278 → DOI: 10.1002/jemt.20783
9. Neto FTL, Bach PV, Najari BB, Li PS, Goldstein M → Sperm morphology: history, challenges, and impact on natural and assisted fertility → Andrology → 2022 → Volume 10, Issue 4, Pages 751-761 → DOI: 10.1111/andr.13150
10. World Health Organization → WHO Laboratory Manual for the Examination and Processing of Human Semen → 6th Edition → WHO → 2021 → Reference values for semen analysis.
11. Barratt CLR, Mansell S, Beaton C, Tardif S, Oxenham SK → Diagnostic tools in male infertility – the question of sperm dysfunction → Asian Journal of Andrology → 2011 → Volume 13, Issue 1, Pages 53-58 → DOI: 10.1038/aja.2010.63 (Related to morphology review: 2010 Human Reproduction Update 16:645-660)
12. Inaba K → Sperm flagella: comparative and phylogenetic perspectives of protein components → Molecular Human Reproduction → 2011 → Volume 17, Issue 8, Pages 524-538 → DOI: 10.1093/molehr/gar034
13. National Center for Biotechnology Information → StatPearls – Histology, Spermatozoa and Spermatogenesis → URL: https://www.ncbi.nlm.nih.gov/books/NBK563235/ → Accessed 2025-2026
14. American Society for Reproductive Medicine → Male Infertility – Azoospermia, Oligozoospermia, Varicocele → URL: https://www.asrm.org/topics/topics-index/male-infertility → Accessed 2025-2026
15. Centers for Disease Control and Prevention → Infertility FAQs – Male Factors and Semen Analysis → URL: https://www.cdc.gov/reproductivehealth/infertility/index.html → Accessed 2025-2026