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.
Highlights
Mammalian gametes represent extremes of cellular organization
Among the most prominent structural distortions that result from conventional electron microscopy (EM) sample preparation are those associated with aggregation, which can be caused by fixation or by dehydration
Our modern understanding of cell biology is built on foundations laid by EM-based structural work [136,137,138]
Summary
Mammalian gametes represent extremes of cellular organization. The male gamete—the sperm—has lost most of its cytoplasm and many of the organelles present in its somatic counterparts, becoming a small, streamlined cell. The light microscope played a central role in the first descriptions of oocytes [2] and provided direct evidence that fertilization involves gamete fusion [3]. With recent developments such as expansion microscopy and super-resolution light microscopy, specific proteins can be localized in cells to within tens of nanometres [4,5,6]. Following the invention of the electron microscope in 1931 and the first attempts to study biological samples by transmission electron microscopy (TEM) in 1934 [9], the technique made it possible to image cells and tissues at much higher resolutions than light microscopy.
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