**4. Tail**

origin to the tail axoneme and is reduced. The proximal centriole preserves its morphology, enters the oocyte upon fertilization, and plays a role in organizing the cleavage spindle [59].

The centriole is surrounded by striated columns, which are part of the connecting structures of the neck. The basal plate is at the base of the head nucleus. The centriole is capable of functioning as an organizing center during cell division only when having a normal morphology,

The role of the sperm centriole has come into focus of research relatively recently, with the development of ART methods. A paternal inheritance of the centriole was then demonstrated for humans and large mammals as opposed to rodents [61]. The centriole organizes microtubule assembly to produce the sperm aster, which forms around the paternal pronucleus 6 h after fertilization [62] and gives origin to the first mitotic spindle. The main function of the

Centrosome abnormalities were described as a cause of unsuccessful fertilization and abnormal embryo development [61, 62]. Decaudated or decapitated sperm is a rare syndrome in humans and includes the absence of the implantation fossa and the basal plate. Morphological features of the human syndrome were described comprehensively, and ultrastructural defects of spermatozoa with an abnormal fragility of the head-tail junction were studied by electron microscopy (**Figure 5b**). The proximal centriole/centrosome, which induces the formation of the basal plate and the implantation fossa, was assumed to play an essential role in attaching the flagellum to the nucleus. Dysfunction of the proximal centriole/centrosome may alter the formation of tail attachment structures, leading to decapitated sperm. Spontaneous fertilization is impossible with such spermatozoa because the tail easily detaches from the head because of the neck fragility. ICSI is the only way of fertilization in this case, but rarely is successful. Chemes et al. [63] observed lack of cleavage after ICSI. Porcu et al. [64] reported successful ICSI in two infertile couples where the men were brothers and produced acephalic spermatozoa or spermatozoa with abnormal head-tail attachments, and one birth was published by Gambera et al. [65].

A genetic origin is now commonly accepted for the syndrome. Baccetti et al. [66] assumed that recessive autosomal mutations account for the majority of sperm genetic defects. However, the genes affected by the mutations are unknown. Light microscopic signs of the syndrome vary. Multiple motile tails with single, if any, tailless heads are observed in semen in the majority of cases. Kamal et al. [67] described 16 cases with a variant of the syndrome wherein the spermiogram parameters were normal, while minimal ICSI-related manipulations caused decapitation and immobilization of spermatozoa. The head and tail usually separate at the head-neck junction; the connecting piece is preserved; the basal plate and implantation fossa

Several variants of decapitated sperm were described. Holstein et al. [68] reported a case where the basal plate and implantation fossa were normal in morphology, while separation occurred between the proximal and distal centrioles. Baccetti et al. [66] described a patient with sperm ruptures occurring between the nucleus and centriole region, between the anterior and caudal regions of the mid-piece, and between the mid-piece and principal piece. A

centriole is to organize a network of microtubules, which originate from the oocyte.

as was demonstrated in many somatic cell studies [60].

**3.1. Centrosome abnormalities**

82 Spermatozoa - Facts and Perspectives

are absent from the caudal pole of the nucleus.

number of variants are most likely possible for sperm decapitation.

The intact tail of a human spermatozoon is approximately 50 μm in length and consists of four regions: a connecting piece, which is attached to the head; a mid-piece, which is 3–5 μm long; a principal piece, which accounts for approximately two-thirds of the tail length; and a short end piece. In contrast to cilia, which are covered by the plasma membrane, the sperm tail has not only the axoneme but also additional structures that surround the axoneme and are known as the periaxonemal structures. A mitochondrial helix and outer dense fibers surround the axoneme in the mid-piece and a fibrous sheath in the principal piece. The axoneme has no periaxonemal structures only in the short end piece.

### **4.1. Axoneme**

The axoneme forms a core in cilia and flagella. The sperm axoneme consists of nine pairs of microtubules (doublets) that are arranged in a ring around two central singlet microtubules (9 + 2 arrangement). The doublets are numbered clockwise, starting from the site where two doublets overlay the central pair of microtubules; the right doublet is number one (**Figure 5c**, **d**). Each peripheral doublet consists of a complete microtubule (subunit A) and an adjacent incomplete microtubule (subunit B).

Two, outer and inner, arms consisting of the protein dynein (dynein arms) with ATPase activity extend from the A subunit of each doublet towards the B subunit of the next clockwise doublet. Each dynein arm is an intricate multiprotein complex and acts as a molecular motor [71]. The microtubule doublets are connected via thin bridges of the protein nexin (nexin bridges) and project radial spokes towards the two central microtubules. This sophisticated structure sustains sliding movements of the microtubules, thus providing for undulations of the tail. The axoneme is intricate molecular machinery wherein the inner and outer dynein arms generate forces to produce bending waves and the central apparatus and radial spokes play a regulatory role [72, 73].

#### **4.2. Periaxonemal structures of the sperm tail**
