What cell types comprise the median eminence and the tuber cinereum?

I have tried pretty hard to get a detailed description of what exactly the median eminence and the tuber cinereum are but to no avail. I am very familiar with their anatomical relationships (spatially) with the hypothalamus, 3rd ventricle, mammillary bodies, etc; however, I still do not quite understand what they are or what cell types comprise them.

Are they areas of grey matter? Are they some sort of modified ependymal cell layer of the 3rd ventricle floor?

Or are they areas of connective tissue that merely “house” white matter projections from the hypothalamus to the pituitary? So, to repeat the inquiry from above:

What cell types comprise the median eminence and the tuber cinereum?

And, moreover, what’s the actual function of the cell types that comprise them?