The role of transmission electron microscopy analysis in different cases of reduced motility in human spermatozoa

The aim of this review is to highlight the pivotal role of transmission electron microscopy (TEM) analysis in the diagnosis of human sperm pathologies related to flagellum and affecting motility. The analysis of sperm motility plays a central role in the evaluation of male fertility, as it is known that a high percentage of poorly motile or immotile sperm will not be able to fertilize. The study of sperm flagellum and its abnormal forms is generally carried out by light microscope with obvious technical limitations. Only the use of TEM allows for the performance of an ultrastructural evaluation of sperm flagellar assembly with a precise characterization of anomalies, also extrapolating the functional aspect. Severely reduced or completely absent sperm motility in subfertile or infertile men is associated with submicroscopic alterations in the cytoskeletal structure of sperm flagellum, in the mitochondria structure and assembly, in the axonemal pattern and periaxonemal accessory structures, such as the outer dense fibres and fibrous sheath. Sperm anomalies are classified as non-specific, or non-systematic sperm defects, or as systematic sperm defects. The first and most frequent type is related to a heterogeneous combination of randomly distributed alterations affecting the head and the tail organelles in a varied percentage in sperm ejaculate. These alterations can be related to andrological pathologies or to other endogenous or exogenous factors. Necrozoospermia, an extreme manifestation of asthenozoospermia, is, a rather rare phenomenon characterized by an increase in non-viable sperm up to the persistent presence of 100% dead spermatozoa in different ejaculates from the same patient. TEM has proved to be the only tool able to specifically characterize the morphological features of the systematic sperm defects in which an identical and specific alteration affects the vast majority of the sperm population in sterile patients. These defects tend to show family clustering and are significantly more frequent in individuals with a history of consanguinity. The most documented sperm defects of supposed genetic origin affecting the tail structures include: "Dysplasia of Fibrous Sheath", "Primary Ciliary Dyskinesia ", "Detached tail", "Absence of fibrous sheath ", "Absent axoneme".