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The ‘tail’ of two cells: sperms wiggle out while avoiding head-on collision from cell enclosure

Urvashi Bhattacharyya

Coiled sperm bundles inside an adult Drosophila testis. Tails marked in cyan and the heads marked in magenta.
Coiled sperm bundles inside an adult Drosophila testis. Tails marked in cyan and the heads marked in magenta.   (Photo: Dubey and Ray, TIFR, Mumbai)

Science is fun, especially when it offers researchers newer opportunities to venture out to hidden, unexplored territories. Often though, interesting details lie hidden behind what may be a seemingly known and simple biological process. Indian biologists looking at the well-known event of sperm release in male fruit flies, have now discovered new rules of the phenomenon in a recent study published in the journal Development Cell. The research, carried out by Pankaj Dubey, Seema Shirolikar and Krishanu Ray from Tata Institute of Fundamental Research, Mumbai described the first ever live recording of the process using Drosophila testis ex vivo. According to the authors, the sperm release mechanism, which at present is suggested to occur in fruit flies, could be followed in mammalian reproductive systems as well due to their topological similarities.

In this study, the authors sought out to observe how sperms move out of testis once they mature. Ray explains the problem in simple terms, “Imagine a balloon growing within another balloon and then the one inside comes out without bursting the outer one.” A well-travelled commuter in a crowded train might be able to sympathise with the problem. Pushing your way out through the crowd, pokes and jabs with fellow passengers becomes a common occurrence. Turns out, the interaction between the sperm and cyst cells is no different. 

Spermatogenesis or generation of sperms from germ line cells occurs in an enclosure of two somatic cells in Drosophila. These are known as the head cyst cell (HCC) and tail cyst cell (TCC) covering the heads and tails of the developing sperm bundles respectively. Upon development, the mature sperm needs to find ways to get out of the enclosure, and pass through a narrow duct lined with closely packed epithelial cells in order to get into the seminal vesicle for storage. The authors used time-lapse imaging, a technique of capturing slower events at reduced frame rates and playing them back at faster speeds. This collapses hour long events into a few minutes of viewing. Using fluorescent labels, which differentially stain the somatic and the sperm cells, the researchers documented that it was the tails of the sperm that first pushed out from the enclosure. This finding is surprising and alters the leading hypothesis of sperm heads being forced out of the enclosure through the surrounding tissue. 

The authors further investigated what prohibits the sperm heads from escaping the enclosure. To do this, they labelled a protein network that is known to assemble within HCCs and observed changes as they unfolded. This led to their next interesting discovery. They found that the HCCs defend intrusions from the sperm heads through rapid assembly of the protein network. These filamentous proteins, known as actin molecules, gather near the poking sperm and resist the push of the intruding spermatids’ head. Further investigation of other protein factors involved in actin assembly was carried out using molecular genetics. The authors observed that disruption of any of these factors involved in the process leads to abnormal release of sperm, a potential source of infertility among the male member of the species. 

The study demonstrates the potential of actin dynamics as first line of defence against mechanical invasion in certain types of cells. “The idea that a cell can react to physical poking using molecular machines is a very novel one and exciting”, noted Ray. Although the research is in its early stages to be effectively used for contraceptive screens or for fertility treatment, the ability to image the testis live can help in testing the effects of small molecule candidates for contraceptives at different stages of sperm release. The results also stress the importance of observing biological phenomenon live over building models using indirect experimental analysis.