Spermatozoon

Head ultrastructure (Caput)

The sperm head has a flattened shape and comprises three essential functional components responsible for DNA preservation and the fertilization process:

1. Nucleus (Nucleus): Contains a single (haploid) set of genes. Chromatin here is condensed to the limit—its volume is 30 times smaller than that of immature precursor cells. The classic nucleosomal organization is modified: conventional histones are replaced by protamines (proteins rich in arginine and cysteine). Such compaction protects the genetic material from any damage while traversing the oocyte envelopes.
2. Acrosome (Acrosome): A membrane-bound vesicle derived from the Golgi apparatus that covers the anterior two-thirds of the nucleus like a "double cap." Essentially, it is a large specialized lysosome. Inside it lies a complex of lytic enzymes (acrosin, hyaluronidase, collagenase, penetrase) that dissolve the protective layers of the female gamete.
3. Plasmalemma: The cell membrane containing specific receptor proteins. It imparts a negative electrical charge to the cell, preventing spermatozoa from sticking together, and participates in the directed search for the ovum (taxis). Membrane proteins are strictly species-specific—this is a biological barrier preventing interspecies hybridization.

Tail morphology (Cauda)

The tail, or flagellum, is the organ of locomotion and consists of four consecutive sections, each with its own ultrastructural features:

• Neck (Cervix): The shortest initial segment. Two centrioles with a typical structure of $(9 \times 3) + 0$ are located here perpendicular to each other. The proximal centriole lies adjacent to the nucleus, while the distal centriole initiates the formation of the flagellar motor shaft, the axoneme.
• Middle piece (Pars intermedia): The thickest segment. The axoneme runs through the center, organized in a $(9 \times 2) + 2$ pattern. It is surrounded by 9 dense outer cytoskeletal fibrils. This entire assembly is enclosed by a mitochondrial sheath—a spiral of approximately 15 turns of mitochondria generating energy for movement.
• Principal piece (Pars principalis): The longest section. It retains the axoneme and outer fibrils, but mitochondria disappear. Their place is taken by a finely fibrillar fibrous sheath.
• End piece (Pars terminalis): Outer fibrils and accessory sheaths are absent here. The axoneme microtubules lose their strict organization and break down into a bundle (about 20 in total) covered only by the plasma membrane. This zone performs active whip-like movements.

Physiology of movement

The flagellar plasmalemma does not merely cover the cell—it is excitable similar to nerve or muscle tissue membranes and contains ion channels for sodium ($Na^+$) and calcium ($Ca^{2+}$). The interaction of axonemal microtubules (sliding of doublets via dynein arms) is triggered by a complex molecular cascade:

1. Generation of acetylcholine within the flagellum and its effect on cholinergic receptors.
2. Opening of calcium channels and massive influx of $Ca^{2+}$ ions into the cell.
3. Activation of the enzyme adenylate cyclase followed by synthesis of cyclic AMP (cAMP).
4. cAMP activates protein kinases that phosphorylate axonemal proteins.
5. Mechanical beating of the tail is initiated.

Briefly: answers to topical questions

What else to study on this topic

• Mechanism and stages of the acrosome reaction
• Properties and species specificity of plasmalemma membrane receptors
• Chemotaxis and electrotaxis phenomena during ovum searching
• Structure and functions of axonemal dynein arms
• Comparative characteristics of ovum envelopes at the time of fertilization

Other topics in the section "Germ cells, fertilization, cleavage, blastula"

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