Introduction

Cellular interactions are known to be essential not only in mammalian developing tissues and organs, but also during steady-state maintenance phases. In the mammalian ovary, cellular interactions are particularly important since follicle development is continuously initiated, during childhood and adulthood, until the follicle pool drops below a poorly understood threshold in menopause. In this context, the germ cell–soma interface is of major relevance since it is at this level that a fine tuning must take place to allow for correct ovarian follicle development and the full oocyte functionality, known as oocyte competence acquisition. This includes complex differentiation processes of the somatic cells, fine tuning in response to growth factors and hormones, and the regulation of meiotic arrest and metabolism to the demand of the growing and maturing oocyte.

Although the zona pellucida, an oocyte-specific extracellular layer, intercalates between oocytes and the innermost layer of cumulus granulosa cells (corona radiata), unique cellular structures are present at this level that emanate from the granulosa cell, extend across the zona pellucida, and reach direct contact with the oocyte's plasma membrane. These structures, known as the transzonal projections (TZPs), may contain cytoskeletal components, such as tubulin and/or actin, and possess membrane junctions such as gap and adhesion junctions, and cell organelles such as mitochondria. Between neighboring granulosa cells similar specialized junctions exist that allow direct cell–cell signaling but also permit nutrient access to the oocyte.