Which of the following characteristics apply to endothelial cells?

Endothelial cells, which line the blood vessels and lymphatic vessels, have several distinct characteristics that play significant roles in their function.

The presence of nuclei is a universal feature of all nucleated cells, including endothelial cells, and while it's true that these cells contain nuclei, this characteristic is not unique or defining for endothelial cells. Similarly, although endothelial cells have varying levels of tight junctions that contribute to the permeability and barrier functions of blood vessels, this specificity is what makes option B particularly important.

Endothelial cells do not originate from bone marrow; instead, they are derived from mesodermal progenitors during development. This means while the first characteristic may apply to them, and the incorrect third characteristic does not, it's the high levels of tight junctions that are a more defining trait of endothelial cell functionality. Tight junctions enhance the selective permeability of the endothelial layer, allowing for controlled exchange of substances between the bloodstream and surrounding tissues.

Therefore, the correct choice regarding the specific defining trait of endothelial cells in this context is the presence of high levels of tight junctions, highlighting their critical role in vascular biology.