Alkaloid from Papaver somniferum (opium poppy) and Annona reticulata (custard apple) (S)-reticuline is an endogenous precursors of morphine. (PMID 15383669); (S)-Reticuline is a key intermediate in the synthesis of morphine, the major active metabolite of opium poppy. ; 'Endogenous morphine' has been long isolated and authenticated by mass spectrometry in trace amounts from animal and human specific tissue or fluids. (PMID: 15874902); Recently, human neuroblastoma cells (SH-SY5Y) were shown capable of synthesizing morphine as well. (S)-reticuline undergoes a change of configuration at C-1 during its transformation to salutaridinol and thebaine. From thebaine, there is a bifurcate pathway leading to morphine proceeding via codeine or oripavine, in both plants and mammals. (PMID 15937106); Reticular fibers or reticulin is a histological term used to describe a type of fiber in connective tissue composed of type III collagen. Reticular fibers crosslink to form a fine meshwork (reticulum). This network acts as a supporting mesh in soft tissues such as liver, bone marrow, and the tissues and organs of the lymphatic system.