Axonemal doublet microtubules can split into two complete singlets in human sperm flagellum tips

Abstract

Motile flagella are crucial for human fertility and embryonic development. The distal tip of the flagellum is where growth and intra‐flagellar transport are coordinated. In most model organisms, but not all, the distal tip includes a ‘singlet region’, where axonemal doublet microtubules (dMT) terminate and only complete A‐tubules extend as singlet microtubules (sMT) to the tip. How a human flagellar tip is structured is unknown. Here, the flagellar tip structure of human spermatozoa was investigated by cryo‐electron tomography, revealing the formation of a complete sMT from both the A‐tubule and B‐tubule of dMTs. This different tip arrangement in human spermatozoa shows the need to investigate human flagella directly in order to understand their role in health and disease.

The multi‐scale architecture of mammalian sperm flagella and implications for ciliary motility

“…We found that doublets can transition into singlets by two possible arrangements, either by a doublet splitting into two independent singlets ( Fig 7D-F, left panels) or by the B-tubule terminating abruptly ( Fig 7D-F, right panels). Similar patterns have been observed in human sperm (Zabeo et al, 2019).…”

Doing more with less: The flagellar end piece enhances the propulsive effectiveness of human spermatozoa

“…(iii) the principal piece of the flagellum, typically 40-60 µm in length (although much longer in some species [8]), the core of which is the 9+2 axoneme that produces and propagates active bending waves through dynein-ATPase activity [9]; and (iv) the end piece, typically a few microns in length, which consists of disorganized singlet microtubules (sMTs) only [10].…”

The tubulin code specializes neuronal cilia for extracellular vesicle release

“…In CEM cilia, the proximal region following the transition zone has nine doublet MTs lacking Y links; these doublet MTs splay to form individual 13 protofilament A‐tubule and 10 protofilament B‐tubule singlet MTs which rejoin in the sub‐distal region to form “modified doublets” with A and B tubules (where both tubules are complete) (Silva et al., 2017) (Figure 5b). The splitting of doublet MTs into A‐tubule and B‐tubule singlets is also observed in mammalian sperm flagella, indicating that this CEM ultrastructural specialization is evolutionarily conserved (Afzelius et al., 1995; Zabeo et al., 2019).…”