A spermatozoon is a haploid male gamete highly specialized for the preservation and delivery of genetic material to the oocyte. It is species-specific in form and consists of two key structural components: the head and the flagellum, which provides active motility.
Chromosome complementHaploid. DNA is maximally condensed using specialized basic proteins called protamines.
AcrosomeA modified lysosome containing lytic enzymes necessary for penetration of the oocyte.
MotilityHuman sperm velocity reaches 30–50 µm/s, with a flagellar beat frequency of approximately 5 Hz.
Tail structureComposed of four segments: the neck, middle piece, principal piece, and end piece.
Head Ultrastructure (*Caput*)
The sperm head is flattened and contains three essential functional components responsible for safeguarding DNA and executing fertilization:
Nucleus (Nucleus): Contains a single (haploid) set of chromosomes. Chromatin is condensed to its limit, occupying a volume 30 times smaller than that of immature precursor cells. The classic nucleosomal organization is modified: typical histones are replaced by protamines (proteins rich in arginine and cysteine). This tight compaction protects the genetic material from any damage while traversing the oocyte investments.
Acrosome (Acrosome): A membrane-bound vesicle derived from the Golgi apparatus that covers the anterior two-thirds of the nucleus like a "double cap." Functionally, it is a large specialized lysosome. It contains a complex of lytic enzymes (acrosin, hyaluronidase, collagenase, penetrase) that digest the protective layers of the female gamete.
Plasma membrane: The cell membrane containing specific receptor proteins. It imparts a negative electrical charge to the cell, preventing spermatozoa from agglutinating and participating in directed chemotaxis toward the oocyte. The membrane proteins are strictly species-specific, serving as a biological barrier that prevents cross-species fertilization.
Tail Morphology (*Cauda*)
The tail, or flagellum, is the organ of locomotion and consists of four sequential segments, each with distinct ultrastructural features:
Neck (Cervix): The shortest proximal segment. Two centrioles with a typical $(9 \times 3) + 0$ structure lie perpendicular to each other here. The proximal centriole lies adjacent to the nucleus, while the distal centriole gives rise to the motor core of the tail—the axoneme.
Middle piece (Pars intermedia): The thickest segment. The center contains the axoneme, organized in a $(9 \times 2) + 2$ pattern. Surrounding it are 9 dense outer cytoskeletal longitudinal fibers. This entire assembly is enclosed by a mitochondrial sheath—a helix of approximately 15 turns of mitochondria generating energy for movement.
Principal piece (Pars principalis): The longest segment. It retains the axoneme and outer dense fibers, but mitochondria disappear. Their place is taken by a finely fibrillar fibrous sheath.
End piece (Pars terminalis): Outer dense fibers and accessory sheaths are absent here. The axonemal microtubules lose their strict organization and break down into a bundle (about 20 microtubules) covered only by the plasma membrane. This zone performs active whip-like movements.
Physiology of Motility
The flagellar plasma membrane is not merely a passive covering—it is excitable, much like nerve or muscle cell membranes, and contains ion channels for sodium ($Na^+$) and calcium ($Ca^{2+}$). The interaction of axonemal microtubules (sliding of doublets driven by dynein arms) is triggered by a complex molecular cascade:
Acetylcholine production within the flagellum and its action on cholinergic receptors.
Opening of calcium channels and a massive influx of $Ca^{2+}$ ions into the cell.
Activation of adenylyl cyclase and subsequent synthesis of cyclic AMP (cAMP).
cAMP activates protein kinases that phosphorylate axonemal proteins.
Mechanical beating of the tail is initiated.
Mnemonic
To remember the internal structure of the axoneme, picture a wheel: 9 pairs of spokes (peripheral microtubule doublets) in a circle and 2 central singlets in the very center. Formula: (9×2)+2.
Frequently asked questions
Which proteins mediate the binding and sliding of microtubules in the axoneme?
Microtubule sliding in the axoneme is driven by dynein. Dynein arms project from each peripheral doublet toward the adjacent one; the cyclical attachment and detachment of dynein cross-bridges causes the sliding displacement of adjacent doublets, resulting in axonemal bending. Radial spokes, which extend from the peripheral doublets to the central sheath, are also important structural elements of the axoneme.
What stages does spermatogenesis include?
Spermatogenesis proper proceeds through four main phases:
Proliferation phase — mitotic division of diploid spermatogenic cells.
Growth phase — DNA replication and enlargement of spermatogonia (preparation for meiosis).
Maturation phase — two meiotic divisions resulting in the formation of haploid spermatids.
Spermiogenesis (differentiation phase) — morphological maturation without cell division, during which spermatids transform into mature spermatozoa.
Additionally, post-testicular maturation occurs during transit through the epididymis.
What is the essence of sperm capacitation and where does it occur?
Capacitation is the functional activation of spermatozoa induced by the female reproductive tract environment, occurring primarily in the uterus.
The process involves the removal of surface glycoproteins and cholesterol from the sperm head plasma membrane. This triggers several molecular changes:
Marked enhancement of motility and metabolism;
Increased membrane lability (preparing for the acrosome reaction);
Acquisition of the ability to bind to the zona pellucida.
What is the average lifespan of a spermatozoon in the female reproductive tract?
Sources typically cite two timeframes: spermatozoa retain fertilizing capacity in the female tract for 1–2 days; however, for calendar-based natural family planning methods, the average lifespan of sperm is estimated to be up to 4 days.
Go deeper
Mechanism and stages of the acrosome reaction
Properties and species-specificity of plasma membrane receptors
Chemotaxis and electrotaxis phenomena during oocyte tracking
Structure and function of axonemal dynein arms
Comparative characteristics of oocyte investments at the time of fertilization