Final answer:
Microvilli on epithelial cells are filled with 4) microfilaments composed of actin to maintain their structure and maximize absorption.
Step-by-step explanation:
Microvilli on the apical ends of epithelial cells are filled with microfilaments. These microfilaments are primarily composed of actin, a type of protein that helps maintain the structure and shape of the microvilli. The extended surface area of the cell's plasma membrane created by the microvilli is essential for maximizing nutrient absorption in the small intestine. Within the cylindrical extensions of these cells, the actin microfilaments support the structure, allowing the microvilli to form a brush border that greatly enhances the ability to absorb nutrients. Unlike the Golgi apparatus, mitochondria, and endoplasmic reticulum, which reside within the cytoplasm, the actin filaments are an integral component of the microvilli's composition.