Human Sperm as an In Vitro Model to Assess the Efficacy of Antioxidant Supplements during Sperm Handling: A Narrative Review

Abstract

Spermatozoa are highly differentiated cells that produce reactive oxygen species (ROS) due to aerobic metabolism. Below a certain threshold, ROS are important in signal transduction pathways and cellular physiological processes, whereas ROS overproduction damages spermatozoa. Sperm manipulation and preparation protocols during assisted reproductive procedures-for example, cryopreservation-can result in excessive ROS production, exposing these cells to oxidative damage. Thus, antioxidants are a relevant topic in sperm quality. This narrative review focuses on human spermatozoa as an in vitro model to study which antioxidants can be used to supplement media. The review comprises a brief presentation of the human sperm structure, a general overview of the main items of reduction-oxidation homeostasis and the ambivalent relationship between spermatozoa and ROS. The main body of the paper deals with studies in which human sperm have been used as an in vitro model to test antioxidant compounds, including natural extracts. The presence and the synergic effects of different antioxidant molecules could potentially lead to more effective products in vitro and, in the future, in vivo.

Keywords: antioxidants; human sperm in vitro; natural extract; oxidative stress; pathological role of ROS; physiological role of ROS; phytocomplexes; polyphenols.

Figure 1

Spermatozoon structure. The figure shows the various regions of a human spermatozoon. Starting from the head region, it is possible to distinguish the acrosome and the nucleus. The sperm flagellum contains the axoneme and periaxonemal structures such as outer dense fibres and a fibrous sheath.

Figure 2

The double face of reactive oxygen species (ROS). Under physiological conditions, ROS regulate the sperm mechanisms involved in capacitation, hyperactivation, the acrosome reaction and fertilisation. On the contrary, some pathophysiological conditions can increase ROS levels over the physiological threshold, generating a condition of oxidative stress that leads to lipid peroxidation, DNA damage and protein oxidative damage. The stars represent ROS.

Figure 3

Human sperm as an in vitro model to test various compounds. Sperm endpoints such as motility, vitality, mitochondrial membrane potential, the acrosome status and DNA integrity can be evaluated when human spermatozoa are used as an in vitro model to test effects of toxic compounds, natural extracts and molecules.