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The extrusion of membraneous threads by avian and mammalian spermatozoa, in vitro
1Department of Physiology, School of Medical Sciences, University of Bristol, U.K.
Abstract:
Extremely fine threads have been seen, under dark-field illumination, extending from the tips of the sperm flagella of four avian and two mammalian species. They occur after a variable period of incubation in balanced salt solutions. The threads, never more than one per flagellum, can be as long as the flagellum itself. Often they are attached to the substrate by their remote end, tethering the cell and showing elastic deformation. When not stressed, they remain threadlike and undergo the Brownian motion characteristic of fine filaments. Shedding of the threads has been seen, as has their apparent retraction. When cockerel sperm are broken into proximal and distal segments which are respectively motile and immotile, the threads develop at the tips of only the proximal segments. This proves that the formation of the thread is not dependent on structures normally present at the natural tip of the flagellum. Each thread is a regular tubule of membrane, with a standard minimum diameter of 28 nm, and having blebs of membrane, containing vesicles, at irregular intervals. The tubular membrane is assumed to be plasmalemma and is equivalent, maximally, to less than 10% of the cell's calculated surface area. It is most likely that the thread is formed by extrusion, as a stalk bearing fluid-filled pinocytotic vesicles. Thread formation is prevented by adding bovine serum albumin to the diluent. The phenomenon may be relevant to understanding how spermatozoa are impaired by the 'dilution effect'.
Insights
Sperm flagella can extrude extremely fine membrane threads, tethering cells and exhibiting elastic properties. This novel finding offers insights into sperm function and potential impairment from the dilution effect.
Area of Science:
- Cell Biology
- Reproductive Biology
- Biophysics
Background:
- Spermatozoa possess flagella essential for motility.
- The structural integrity and behavior of flagella under various conditions are crucial for fertilization.
Purpose of the Study:
- To investigate the formation and characteristics of extremely fine threads observed extending from sperm flagella tips.
- To elucidate the origin and composition of these flagellar threads and their potential functional implications.
Main Methods:
- Dark-field microscopy was used to observe sperm from avian and mammalian species.
- Sperm were incubated in balanced salt solutions, and observations were made under varying conditions.
- Cockerel sperm were mechanically separated into proximal and distal segments to assess thread formation sites.
Main Results:
- Extremely fine, tubular membrane threads (minimum diameter 28 nm) were observed extending from sperm flagella tips after incubation.
- These threads could be as long as the flagellum, attached to the substrate, and showed elastic deformation and Brownian motion.
- Thread formation occurred only on proximal, motile sperm segments, indicating independence from the natural flagellar tip.
- Thread formation was inhibited by bovine serum albumin and may relate to the 'dilution effect' impairing sperm.
Conclusions:
- Sperm flagella can actively extrude membrane tubules, suggesting a novel mechanism of cell-surface modification.
- The observed threads, potentially formed by plasmalemma extrusion, may play a role in sperm-substrate interaction or be indicative of cellular stress.
- Understanding this phenomenon could provide insights into factors affecting sperm viability and function, particularly in diluted semen.