Exocytosis in vitro: Ultrastructure of the isolated sea urchin egg cortex as seen in platinum replicas

Sea urchin egg cortices, isolated on polylysine-coated cover glasses by the method of V. D. Vacquier (1975, Dev. Biol. 43, 62-74), have been fixed with 2% glutaraldehyde, critical point-dried, and rotary shadowed with platinum-carbon. The replicas produced show that the isolated cortex consists of a plasma membrane having a regular array of microvillar evaginations each containing a filamentous core. Attached to the plasma membrane is a densely packed layer of cortical granules; each granule is linked to the plasma membrane and to four to six adjacent granules by fine, 6-nm-diameter filaments. Between the plasma membrane and cortical granule layer runs a tubular endoplasmic reticulum consisting of bulb-like varicosities joined in chains that are arranged in a network of polygons. Interspersed among the cortical granules are occasional spherical organelles which correspond in size to osmiophilic granules seen in the intact egg cortex. Addition of 1000 μM or 50 μM Ca²⁺ to the cortex initiates fusion of individual granules with the plasma membrane, as well as extensive fusion between granules, producing extended, branched exocytic pockets. Frequently, fusion patterns are observed that suggest a propagation of granule fusion outward from one or more foci within a single cortex.