What is the clear cut distinction between fibers, fibrils, and microfibrils of connective tissue fibers (size, shape, etc.)?
It’s a good thing you asked! This is not easy to figure out. (I recommend careful study of RR&K first.) • First: consider the synthesis, structure and appearance of collagen molecules, fibrils, fibers, and fiber bundles. Be sure to distinguish among LM-visible features, TEM-visible features, and biochemical features. Compare the sizes and arrangements of the collagenous fibrils and fibers. (Don’t forget about the collagens that do not form fibrils.) • The collagen fibrils discussed in T&B and RR&K are EM Fibrils (also called EM microfibrils or unit fibrils… but be sure to specify collagen). The collagen fibrils are formed (in Type I, II, and III collagen) by packing staggered, aligned tropocollagen molecules (each is about 1.5 nm in diameter). [Review the reason for the 68 nm cross banding pattern.] The EM Fibrils of collagen vary in diameter from 15-200 nm. Confusion arises because of the use of terms such as “LM Fibrils” to refer to those thick fibrils of tendon that are thick enou
