AsianScientist (Mar. 20, 2026)–For sperm cells, swimming ability is everything. The long, whip-like tail, known as a flagellum, generates the propulsion needed to reach and fertilise an egg. But before the tail can form, a pair of tiny cylindrical protein barrels inside developing sperm cells known as centrioles must transform into specialised structures that support the moving tail.
The transformation has been recognised for decades, but two questions remained unresolved: which of the two centrioles becomes which, and what molecular machinery drives the process. Both questions have now been answered by a team at the RIKEN Centre for Biosystems Dynamics Research (BDR) in Japan, led by Hiroki Shibuya, whose findings appear in Science Advances.
During spermatogenesis, two canonical centrioles present in early germ cells gradually adopt distinct identities. One becomes the distal centriole (DC), which will anchor the emerging flagellum, while the other becomes the proximal centriole (PC), which attaches to the sperm nucleus. The researchers discovered that the transition involves a rearrangement in the spatial relationship between the two structures, a process they describe as geometry switching.
“While the causes of female infertility have been studied extensively,” said Shibuya, team director at the Laboratory for Gametogenesis, RIKEN BDR, “the mechanisms underlying male infertility, which are known to account for about half of all infertility cases, remain poorly understood.”
Using ultrastructure expansion microscopy adapted for mouse germ cells, the team was able to visualise how the transformation unfolds as sperm develop. The technique embeds cells in a swellable gel that expands several times its original size, enlarging cellular structures while preserving their spatial arrangement. This allowed fluorescent markers to reveal molecular components that would otherwise remain hidden.
As germ cells progressed through meiosis and entered the spermatid stage, the researchers observed two concurrent molecular changes. At the distal tips of both centrioles, proteins including SFI1 and the tip-localised pool of centrin were stripped away. At the same time, a distinct pool of centrin, together with a binding partner called POC5, was accumulating in the inner scaffold of the DC. This protein lattice runs through the centriole’s interior, forming the structural backbone of the DC during flagellar assembly.
To test the importance of the scaffold, the team used CRISPR gene editing to generate mice lacking the Poc5 gene. The mice grew up healthy and viable, and the females remained fertile, though the males produced no viable sperm. Without POC5, the inner scaffold failed to form properly, causing the DC to split or disintegrate before the flagellum could assemble, leaving sperm unable to swim.
The function of centrioles in normal body cells was unaffected by the deletion. This finding therefore emphasises the centrin-POC5 inner scaffold as a sperm-specific structural element that is dispensable for normal development but crucial for reproduction.
The team attributes this specificity to an unusual structural feature of the DC, which, unlike other centriole types, appears to lack A-C linkers—the lateral connectors that normally help maintain the microtubule triplet wall. In their absence, the inner scaffold may serve as the primary load-bearing structure during flagellar assembly, which would explain why its loss is catastrophic in sperm but inconsequential elsewhere.
“Our modified expansion microscopy protocol can be extended to other analyses, including human sperm, opening new possibilities for investigating fine structural abnormalities that account for male infertility,” said Shibuya. “In the long-term, this could lead to novel diagnostic and therapeutic approaches in reproductive medicine.”
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Source: RIKEN; Image: mdsultanahmad95/Freepik
This article can be found at: Centrin-POC5 inner scaffold provides distal centriole integrity for sperm flagellar assembly
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