Q. I have a quick question about the basement membrane that we went over today in class. I know it is composed of two parts: the basal lamina (directly connected to epithelium) and the reticular lamina (below the basal lamina). Is the reticular lamina itself considered connective tissue, or would connective tissue be below that? I see many sources saying that the basal lamina connects the epithelium to connective tissue, but I’m wondering if this is a case of mistakenly substituting “basal lamina” with “basement membrane.”
A. GREAT question! First, just to confirm what you said about the two parts of the basement membrane – yes, the basement membrane consists of the basal lamina (which is directly connected to the epithelial cells) and the reticular lamina (which sits below the basal lamina). And there is connective tissue underneath the reticular lamina (the composition of that connective tissue varies depending on where you are in the body – we looked at pictures of skin, which has big bundles of collagen underneath the lamina reticularis). We’ll talk more about connective tissue tomorrow, so that will hopefully clear things up a bit.
I think some of the confusion arises from the fact that the reticular lamina is composed of some components (like collagen) that also exist in connective tissue – so sometimes, it’s described as a specialized form of connective tissue. However, it’s best to simply think of it as part of the basement membrane, and to think of the connective tissue beneath it as a separate thing.
I’ve seen the same thing – there are sources that say the basal lamina connects the epithelium to connective tissue. In fact, as I was lecturing, I noticed that exact statement on one of my slides (weird, I’ve never caught it before) – it’s on slide 14:
What I meant to convey was that the basal lamina is important because it provides a place for the epithelial cells to attach themselves to the underlying tissue (without the basal lamina, the epithelial cells would just be floating in space). What I SHOULD have said is that the basal lamina attaches the epithelial cells to the reticular lamina (which is then attached to the underlying connective tissue).
There are definitely some sources that (mistakenly) say that the basal lamina attaches the epithelial cells to the underlying connective tissue. This is from the University of Leeds histology website, which is otherwise a fairly good resource:
In fact, this University of Leeds source just skips right over the reticular lamina and equates basal lamina with basement membrane:
That’s not correct! Even Wikipedia gets it right:
So the bottom line is this: while it is true that the reticular lamina contains some connective tissue components, it is properly considered to be part of the basement membrane. It’s inaccurate to just lump it together with the connective tissue that lies underneath it.