Fibrous Sheath

Structure and/or Key Feature(s)

The fibrous sheath is a cylindrical cytoskeletal structure that determines the scope of the principal piece of the sperm tail and is adherent to the plasma membrane.

The fibrous sheath takes the shape of a tapering cylinder, which constitutes two longitudinal filamentous columns affixed by circumferential ribs. The longitudinal columns course along the edges of microtubule doublets three and eight of the axoneme and are affixed to the outer dense fibers three and eight in the proximal aspect of the principal piece (Plant & Zeleznik, 2014).

The circumferential ribs of the fibrous sheath are made up of tightly packed circumferentially arranged filamentous material. Each rib measures approximately 50nm in width and lies 10-20nm away from the adjacent ribs. The ribs widened distally as they fuse with the longitudinal columns and with themselves.

At the distal aspect of the fibrous sheath, the thickness of the ribs is significantly reduced. The ribs, as well as the outer dense columns terminate at the distal margin of the principal piece (Plant & Zeleznik, 2014).

Anatomical Relations

The fibrous sheath is located in the principal piece of the tail of the sperm. It is situated outside of the axoneme and the outer dense fibers. It lies deep to the plasma membrane and is adherent to it.

Function

The fibrous sheath regulates the directionality of the sperm’s tail beat, thereby restricting the sliding of the microtubule doublets in the axoneme during tail motion, as well as regulating the bending of the tail (Plant & Zeleznik, 2014).

List of Clinical Correlates

- Fibrous sheath dysplasia

- Flagella dyskinesia

- Asthenozoospermia

References

Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Plant, T. M. & Zeleznik, A. J. (2014) Knobil and Neill's Physiology of Reproduction. Elsevier Science.