We report an observation of backward motile human spermatozoa in the neat sample, that is, fresh ejaculate of a patient who had come for infertility treatment to our hospital. The patient was a 35 year old marketing executive with an infertility duration of 54 months. The sample was collected through masturbation and was normozoopermic according to WHO 2010 criteria (Sperm concentration: 71 million/ml; total motility: 68%; normal morphology: 4% and normal physical examination (testis volume: left- 10.5 cc; right- 11 cc), vasa felt on both sides and both cords were bulky. The patient had a history of smoking and alcohol consumption for the last 10 years. In this patient, backward motile spermatozoa were observed in two consecutive ejaculates obtained within a gap of 2 months. The spermatozoa were observed in the wet preparation at 40X high power field (HPF); of the 68% motile spermatozoa, 10% showed reverse motility. All of the backward motile spermatozoa were seen to have structural defects in the tail region, where it was bent back on itself in a sharp fold, forming a “deep U shaped” tail (Fig. 1A, B, D, E). In some spermatozoa, the fold was seen in the mid-piece region, while in a few others it was in the principal piece region. The motility, however, was not compromised in spite of such a structural defect (Normal: 4%; Head Defects: 59%, Mid-piece defect: 2%; Tail defect: 0%; multiple defect: 35%). On the contrary, it was comparable to that of structurally normal motile spermatozoa, only it appeared to be moving backward (S1: Video 1). The couple came with secondary infertility; the first pregnancy was a spontaneous conception, but had a miscarriage in 50 days of conception.
Hypotheses: The structure of the flagellum, including the finer details, has been elucidated in detail, but the mechanism of spermatozoa motility is still not completely understood. The identification of dynein, the motor protein, on the microtubule doublets of the axoneme led to further studies attempting to understand the molecular mechanisms of motility. It has been accepted that the dynein proteins causes sliding of outer microtubule doublets, but due to the resistance provided by the nexin links, the sliding is converted into a bend formation in the tail, which when repetitive propagates a wave from base to tip. This unidirectional wave propagation in most species is because of the alternate activation and inactivation cycles along the length of the axoneme of the dynein motor protein- a minus (-) end- directed protein that pushes the microtubules to the plus end (+) (tip of the flagellum). In cases of bent tails such as the ones reported in this study, we put forth two hypotheses for the mechanism of reverse motility:
1. The wave propagation is base to tip (Fig. 1C1), but as it is the tail, and not the head, that is seen leading the cell (due to structural deformity), it appears that the sperm is moving in the reverse direction. In actuality, the spermatozoa could be progressing forward, only the head is at the side of the tail and not at the front (Fig. 1C2).
2. Due to structural defect, the wave may propagate in a tip-to-base manner akin to that seen in kinetoplastid spermatozoa. In these spermatozoa, the axonemal assembly is the same as that found in other organisms whose wave propagation is unidirectional, yet the direction the wave propagates can be changed based on the circumstances. Similarly, in the spermatozoa that we have observed the structural defect may have caused the reversal of wave propagation (tip to base), but the plausibility of this needs an in-depth molecular study of these bent flagella (Fig. 1C3).
However, the explanation put forth by Phillips and Kalay(1983) cannot be ignored. They state that in such bent-tail spermatozoa the flagella at the bent portion are damaged and therefore do not propagate a wave; instead, the distal end takes up that function. This autonomous functional capacity by any given part of the flagellum has been demonstrated by Brokaw(1961) and also by Lindemann and Rikmenspoel (1972), in spermatozoa of other invertebrate organisms. Several studies have been conducted to confirm that this characteristic phenomena may be valid to axonemes of all species. However, whether the backward motility is because of the “hairpin shape” cannot be ascertained as not all spermatozoa with such structural defects move backward, even within the same sample. At the same time, the absence of reports in the literature of backward swimming human spermatozoa with normal structure makes it more intriguing. Also worth mentioning is the rarity of this phenomenon in other patients with similar structural defects.