Actin Cell Cortex: Structure and Molecular Organization

Abstract

The actin cytoskeleton consists of structurally and biochemically different actin filament arrays. Among them, the actin cortex is thought to have key roles in cell mechanics, but remains a poorly characterized part of the actin cytoskeleton. The cell cortex is typically defined as a thin layer of actin meshwork that uniformly underlies the plasma membrane of the entire cell. However, this definition applies only to specific cases. In general, the cortex structure and subcellular distribution vary significantly across cell types and physiological states of the cell. In this review, I focus on our current knowledge of the structure and molecular composition of the cell cortex.

Keywords: actin cytoskeleton; cell cortex; ultrastructure.

Figure 1. Key Figure.. Structure of the Cell Cortex in Normal and Cancer Cells.

(A) Cytoskeleton organization in normal fibroblasts. A relatively continuous cortex (grey-red pattern) formed by long bundled actin filaments covers the dorsal surface of the cell body, but not lamellae and lamellipodia. (B–D) PREM of the cytoskeleton of a cultured REF52 cell (immortalized rat embryo fibroblast). (B) Overview of a front half of the cell. Dense actin cortex is present in the cell body (lower left). The peripheral lamella has a much sparser cytoskeleton, whereas the cytoskeleton is dense again in lamellipodia. Stress fibers extend from the dorsal cortex toward the cell periphery and terminate in the lamella. Yellow frame is enlarged in C. (C) Transitional zone between the dense dorsal cortex (lower left) and the peripheral lamella (upper right corner) with a sparse cytoskeleton. (D) Densely packed and aligned long actin filaments in the dorsal cortex. (E) Cytoskeleton organization in neoplastically transformed cells. The cell cortex (grey-red pattern) is sparse and fragmented. Fragments have meshwork-like organization of actin filaments. (F–H) PREM of the cytoskeleton of a HeLa cervical carcinoma cell in culture. (F) Overview of a front half of the cell. Lamellipodia are small; lamellae are narrow; no stress fibers are seen on the dorsal surface. Fragmented actin cortex extends almost to the leading edge of the cell. Yellow frame is enlarged in G. (G) The dorsal cortex has irregular structure with many holes. (H) The dorsal cortex has patchy structure with the internal cytoskeleton visible through numerous holes. Upper layers of the cytoskeleton that could be considered to represent the cell cortex (arrows) consist of randomly oriented actin filaments of variable lengths.