Looking back and looking forward: contributions of electron microscopy to the structural cell biology of gametes and fertilization

Abstract

Mammalian gametes—the sperm and the egg—represent opposite extremes of cellular organization and scale. Studying the ultrastructure of gametes is crucial to understanding their interactions, and how to manipulate them in order to either encourage or prevent their union. Here, we survey the prominent electron microscopy (EM) techniques, with an emphasis on considerations for applying them to study mammalian gametes. We review how conventional EM has provided significant insight into gamete ultrastructure, but also how the harsh sample preparation methods required preclude understanding at a truly molecular level. We present recent advancements in cryo-electron tomography that provide an opportunity to image cells in a near-native state and at unprecedented levels of detail. New and emerging cellular EM techniques are poised to rekindle exploration of fundamental questions in mammalian reproduction, especially phenomena that involve complex membrane remodelling and protein reorganization. These methods will also allow novel lines of enquiry into problems of practical significance, such as investigating unexplained causes of human infertility and improving assisted reproductive technologies for biodiversity conservation.

A Frame-by-Frame Glance at Membrane Fusion Mechanisms: From Viral Infections to Fertilization

“…Recently, with the new advances in sample preparation, imaging, and data processing, cryo-EM has been helping to elucidate the atomic details of the fertilization process. Cryo-EM has provided significant insight into gamete ultrastructure, but the harshness of past sample preparation methods has precluded observing at a truly molecular level [241][242][243][244][245][246][247]. However, SPA, such as demonstrated by Umeda et al for the CD9-EWI-2 complex [240], and cryo-electron tomography (cryo-ET) for the uromodulin (UMOD)/Tamm-Horsfall complex (a common sperm-binding region at the interface between subunits) [248], has shown how analysis of high-pressure frozen specimens is helping to understand the molecular mechanisms on the surface of gametes [242][243][244].…”

Electron Microscopy of Cells Grown on Polyacrylamide Hydrogels

“…Despite the expanding application of hydrogels in cell biology, transmission electron microscopy (TEM) has rarely been employed in these investigations (Macrí‐Pellizzeri et al., 2015). This fact is somewhat intriguing since TEM remains the morphological technique with the finest spatial resolution and is widely used to visualize fine structural features in cell biology (Ravi, Leung, & Zeev‐Ben‐Mordehai, 2020; Richert‐Pöggeler, Franzke, Hipp, & Kleespies, 2019; Winey, Meehl, O'Toole, & Giddings, 2014).…”