REVIEW ARTICLE Effect of low-level laser therapy on seminiferous epithelium: a systematic review of in vivo studies Everton Lira Façanha 1 & Everton Freitas de Morais 2 & Juliana Campos Pinheiro 2 & Dennys Ramon de Melo Fernandes Almeida 2 & Danielle Barbosa Morais 1 & Carlos Augusto Galvão Barboza 1 Received: 14 May 2020 /Accepted: 10 August 2020 # Springer-Verlag London Ltd., part of Springer Nature 2020 Abstract Laser therapy has proved effective in the treatment of different tissue injuries but little is known about its effect on the testis. The aim of this review was to synthesize research on the in vivo effect of low-level laser therapy on the seminiferous epithelium. A search was performed in the PubMed/Medline, Scopus, Web of Science, and LILACS databases. The initial search retrieved 354 references, and five articles that met the eligibility criteria were selected. In general, the studies showed that laser therapy exerted a positive effect on the germ cell population; however, there was considerable variation in the laser parameters, as well as in the experimental models and methods of tissue analysis used. In conclusion, further studies determining the biostimulation param- eters of laser therapy in the testis are necessary in order to provide a basis for the possible application of this technique to the restoration of the human seminiferous epithelium and consequent treatment of some male reproductive disorders. Keywords Laser therapy . Testis . Seminiferous epithelium . Spermatogenesis Introduction Infertility is generally defined as the inability of a man or a woman to conceive after 12 months of unprotected inter- course. This condition affects up to 15% of the world’s pop- ulation, and a male factor contributes to 50% of cases [1]. According to Agarwal et al. [2], at least 30 million men world- wide are considered infertile, with a higher incidence in Eastern Europe and Africa. However, due to factors such as the arbitrary attribution of infertility to women and the refusal of many men to undergo fertility assessment, it is speculated that this number is an underestimation of the true rate of global male infertility. Several factors are related to impaired male fertility, includ- ing urinary tract infections, epididymal or ductal obstruction, hormonal imbalances (FSH/LH and testosterone), testicular trauma, immunological factors, chemotherapy/radiotherapy, and diabetes. Exposure to chemical products—especially nematocides, organophosphates, estrogen, benzene, welding fumes, zinc, lead, cadmium, and mercury—can have antispermatogenic effects. Several social drugs such as tobac- co, alcohol, marijuana, and narcotics are potentially antispermatogenic, although their frequent use is generally necessary for the manifestation of adverse effects [3, 4]. In addition, various genetic causes responsible for 10 to 15% of cases of male infertility have been well established [5], includ- ing chromosome anomalies and mutations in a single gene that, at different levels, influence many physiological process- es involved in male reproduction such as hormone homeosta- sis, spermatogenesis, and sperm quality [6]. Finally, there are cases in which the cause of infertility is not identified. These cases are classified as idiopathic [7]. An increase in scrotal temperature, although within the physiological range, can also negatively affect sperm quality [8]. It is estimated that 1 °C above the ideal temperature range can reduce spermatogenesis by 14% and, consequently, the production of spermatozoa [9]. Several environmental and occupational events can cause heating of the testes [10, 11]. Warm environments, tight clothing, and the use of laptops near the testes and cycling, among others, are events that cause occasional increases in testicular temperature [12]. Conditions such as varicocele, cryptorchidism, episodes of fever, and * Carlos Augusto Galvão Barboza cbarboza@cb.ufrn.br 1 Center of Biosciences, Federal University of Rio Grande do Norte, Natal, Brazil 2 Center of Health Sciences, Federal University of Rio Grande do Norte, Natal, Brazil Lasers in Medical Science https://doi.org/10.1007/s10103-020-03122-y