Final answer:
The structure that specializes in protecting the fetus from microorganisms is the placenta, a spongy structure that facilitates nutrient and waste exchange between mother and fetus while providing immune protection.
Step-by-step explanation:
The highly specialized structure that protects the fetus from microorganisms is the placenta. The placenta is a spongy structure that consists of membranes and blood vessels from both the mother and embryo. It not only passes oxygen and nutrients from the mother to the fetus but also removes carbon dioxide and other wastes from the fetus to the mother. An important function of the placenta is to act as a barrier to protect the developing fetus from the mother's immune system, which could recognize it as a foreign entity. Unlike the uterus, which provides the environment for the embryo and fetus to develop, or the vagina, which is the passage for birth, the placenta actively engages in the exchange of substances while simultaneously providing protection against external biological threats.