The word canonical in this context implies that nerve growth factor is the original, standard or prototype of some sort (in relation to a class of biological agents).
Without knowing details of growth factors I found the sentence slightly ambiguous. As I suspected, it means that NGF is either the original growth factor or the original representative of the “neurotrophin family of growth factors”, which includes BDNF (brain-derived growth factor) — see Wikipedia article. However the use of the word “related” could be interpreted incorrectly to mean that the neurotrophins are in a different family from NGF.
Relationship to the general meaning of the word in English
The excellent Oxford Dictionary, accessible from any iPhone, has this as the first meaning of the word canon:
- A general law, rule, principle or criterion by which something is judged.
The appointment violated the canons of fair play and equal opportunity
So its ecclesiastical connotation (in church law) is only one example of the application of the word, and is not intrinsic to it. The word comes from Middle English through Latin from the Greek, kanōn, meaning rule. Hence it has been applied in many spheres to things that are rules or standards.
Another examples of ‘canonical’ in biological science
Perhaps the most well-known usage is in relation to the base-pairing of nucleic acids. A definition by implication can be found in the Wikipedia entry for ‘Non-canonical base pairing’
The standard Watson-Crick base pairs (which are adenine–thymine in DNA, adenine–uracil in RNA, and guanine–cytosine in both DNA and RNA).
(The non-canonical base-pairs are those that are found outside the context of double-helical DNA–DNA or DNA-RNA duplexes, for example G–U base-pairs in dsRNA, wobble base-pairing between mRNA and tRNA, and other base-pairs in non-duplex components of RNA structure.)