Layer of cells immediately surrounding the ovum but outside the zona pellucida is called as

The correct option is corona radiata.

To understand why this is the correct answer, let us break down the structure of the layers surrounding a mature mammalian oocyte (ovum) from the innermost to the outermost region:

1. Oocyte (Ovum) Membrane: The innermost boundary is the plasma membrane of the oocyte, also known as the oolemma.
2. Zona Pellucida: Immediately surrounding the oolemma is a thick, transparent, non-cellular glycoprotein membrane called the zona pellucida.
3. Corona Radiata: This is the layer of follicular cells (granulosa cells) that directly surrounds the zona pellucida. These cells are radially arranged and provide vital proteins and nutrients to the oocyte during its development.
4. Other Layers: Further outwards, we find the membrana granulosa (outer layers of granulosa cells) and the protective sheaths known as the theca interna and theca externa.

Therefore, the specific layer of cells situated immediately outside the zona pellucida, surrounding the ovum, is the corona radiata.