EM 393 Spermatozoa

Spermatozoon

Scanning electron microscopy (SEM) of spermatozoa. The head, neck, and midpiece are observed in longitudinal sections (A and B), whereas the mid, principal, and end pieces are shown in cross sections (C to H).

A spermatozoon (or sperm cell) is the male gamete that transfers the male’s genetic material to the female’s egg (oocyte) for fertilization. It is a motile cell with a tail-like flagellum (about 65 um long) designed for rapid and efficient motility.

• Head (A) - flat, disc-shaped (5 µm long and 3 µm wide)
  • Nucleus (dark blue) / Nuclear Envelope (purple) - contains haploid genetic material (23 chromosomes in humans)
  • Acrosome (cyan) - cap-like structure covering the anterior two-thirds of the nucleus
    • Contains digestive enzymes that help disperse the cells of the corona radiata and penetrate the zona pellucida during fertilization
• Neck (B) - short segment (about 1 µm in length) that contains the centrioles (light blue)
  • One centriole forms the axoneme that forms the core of the flagellum
• Tail (B to H) - long flagellum that contains the axonemal complex that forms its core
  • Middle Piece (B, C, and D) - initial region about the same length as the head
    • Axoneme (light blue) - core structure composed of nine pairs of microtubules surrounding two central tubules
    • Dense Fibers (yellow) - surround the axoneme
    • Surrounded by a helical sheath (around 10 to 15 spirals) of tightly packed mitochondria (red)
    • Annulus - ring-shaped structure that forms a diffusion barrier between the midpiece and principal piece
  • Principal Piece (E and F) - makes up most of the length of the tail (abount 40 µm)
    • Axoneme - extends through the middle piece
    • Fibrous Sheath (cyan) - surrounds the axoneme and provides structural support
  • End Piece (G and H) - narrowing segment of the tail (about 5 µm in length)
    • Only the axoneme is present

The tail produces whip-like movements that propel spermatozoa through the female reproductive tract

Courtesy of Gary E. Olson, Department of Cell and Developmental Biology, Vanderbilt University, Nashville, Tennessee.