Fluorescent nanodiamond labels: Size and concentration matters for sperm cell viability.

Nanodiamonds are increasingly popular in biomedical applications, including optical labelling, drug delivery and nanoscale sensing. Potential new applications are in studying infertility or labelling sperm cells. However, for these applications, it is necessary that nanodiamonds are inert and do not alter sperm properties. In this article, we assessed the biocompatibility of nanodiamonds in detail. We investigated different sizes and concentrations of nanodiamonds and sperm preparation methods. We evaluated if the metabolic activity, membrane integrity, morphology and formation of reactive oxygen species were altered. These parameters were tested for sperm cells in their uncapacitated and capacitated states. Unfortunately, FNDs are not universally biocompatible. Generally, cells in the capacitated state are more prone to stress. Additionally, larger particles and lower concentrations are tolerated better than smaller and higher concentrated particles.

Nanoscale MRI for Selective Labeling and Localized Free Radical Measurements in the Acrosomes of Single Sperm Cells.

Free radicals play a major role in sperm development, including maturation and fertilization, but they are also linked to infertility. Since they are short-lived and reactive, they are challenging to detect with state of the art methodologies. Thus, many details surrounding their role remain unknown. One unknown factor is the source of radicals that plays a role in the sperm maturation process. Two alternative sources have been postulated: First, the NADPH-oxidase system embedded in the plasma membrane (NOX5) and second, the NADH-dependent oxidoreductase of mitochondria. Due to a lack of localized measurements, the relative contribution of each source for capacitation remains unknown. To answer this question, we use a technique called diamond magnetometry, which allows nanoscale MRI to perform localized free radical detection. With this tool, we were able to quantify radical formation in the acrosome of sperm heads. This allowed us to quantify radical formation locally in real time during capacitation. We further investigated how different inhibitors or triggers alter the radical generation. We were able to identify NOX5 as the prominent source of radical generation in capacitation while the NADH-dependent oxidoreductase of mitochondria seems to play a smaller role.