I am reading the textbook Neuroscience (6th ed.) by Dale Purves and colleagues. In one of the chapters (Chapter 7, Molecular Signaling within Neurons), I am reading about the different types of signaling molecules.
The book states that signaling molecules can be grouped into 3 classes: cell-impermeant, cell-permeant and cell-associated signaling molecules. Unlike the first 2 classes, cell-associated signaling molecules are membrane-found, found on the extracellular surface of the plasma mmebrane. Examples of such molecules are cell adhesion molecules such as the integrins and neural cell adhesion molecules (NCAMs).
There is something that the book says which I am unclear about. They say that "Membrane-bound signaling molecules are more difficult to study, but are clearly important in neuronal development and other circumstances where physical contact between cells provides information about cellular identities".
Why is it the case that membrane-bound molecules are more difficult to study compared to secreted/soluble molecules? Any advice is appreciated.