The highlighted structure separates the oocyte from the externally located . zona pellucida primary follicle corona radiata granulosa cells

Granulosa cells                          

Explanation:

Foliculogenesis is a dynamic and complex process through which the follicle goes through many stages of development in a short period (approximately 60 days in humans and some chimpanzees).

The stages classification is based on morphological changes such as follicular size or diameter, the number of follicular cell layers, etcetera. These classifications might vary among authors but could be denominated as:

Through each stage, there are certain modifications in the oocyte, follicular cells, and connective tissue. As the oocyte grows, the follicular cells might reach a size twice or three times bigger than the ones of the primordial follicles. Their shape also changes and they adopt an aspect of stratified epithelium. Their cytoplasm gets a granulous aspect, after what these cells receive the name of granulous cells. The De Graaf follicle characterizes for having a space named follicular antro that displaces the oocyte to one of the sides.    

The oocyte at this point gets surrounded by zona pellucida and corona radiata .    

The follicular antro (The highlighted structure) separates the oocyte from the externally located granulosa cells or follicular cells.    

b.have there own function.