Directions: In the following passage, some words have been deleted. Select the most appropriate option to fill in each blank.

In recent years, cell biology has made significant progress in understanding the roles of various cells in the human body. One of the most intriguing discoveries is related to a specific type of cell found predominantly in females. This cell, known as the follicular cell, plays a critical role in the reproductive system. Follicular cells surround and nourish the developing oocyte, providing the necessary signals and nutrients for its growth. As a result, these cells are essential for (A) _________ fertility.

Scientists are also exploring the potential of follicular cells in vaccine development. Given their unique properties, researchers believe that these cells can be (B) _________ to produce specific immune responses. This approach could lead to the creation of more efficient and targeted vaccines.

Recent studies have shown that follicular cells can be engineered to express (C) _________ that are part of a pathogen, thereby training the immune system to recognise and combat the real pathogen more effectively. This method is (D) _________ from traditional vaccine development techniques, which often rely on weakened or inactivated forms of the virus.

One of the primary challenges in this area of research is ensuring the long-term stability and safety of the engineered cells. Scientists are employing advanced techniques, such as CRISPR, to (E) _________ the genetic modifications precisely. The success of these modifications is crucial for the cells to function as intended without causing unintended side effects.

Moreover, the regulatory aspects of utilising genetically modified cells in humans present another layer of complexity. Scientists must ensure that their research complies with (F) _________ guidelines and ethical standards to gain approval from regulatory bodies.