FGF2 deficit during development leads to specific neuronal cell loss in the enteric nervous system

The largest part of the peripheral nervous system is the enteric nervous system (ENS). It consists of an intricate network of several enteric neuronal subclasses with distinct phenotypes and functions within the gut wall. The generation of these enteric phenotypes is dependent upon appropriate neurotrophic support during development. Glial cell line-derived neurotrophic factor (GDNF) and fibroblast growth factor-2 (FGF2) play an important role in the differentiation and function of the ENS. A lack of GDNF or its receptor (Ret) causes intestinal aganglionosis in mice, while fibroblast growth factor receptor signaling antagonist is identified as regulating proteins in the GDNF/Ret signaling in the developing ENS. Primary myenteric plexus cultures and wholemount preparations of wild type (WT) and FGF2-knockout mice were used to analyze distinct enteric subpopulations. Fractal dimension (D) as a measure of self-similarity is an excellent tool to analyze complex geometric shape and was applied to classify the subclasses of enteric neurons concerning their individual morphology. As a consequence of a detailed analysis of subpopulation variations, wholemount preparations were stained for the calcium binding proteins calbindin and calretinin. The fractal analysis showed a reliable consistence of subgroups with different fractal dimensions (D) in each culture investigated. Seven different neuronal subtypes could be differentiated according to a rising D. Within the same D, the neurite length revealed significant differences between wild type and FGF2-knockout cultures, while the subclass distribution was also altered. Depending on the morphological characteristics, the reduced subgroup was supposed to be a secretomotor neuronal type, which could be confirmed by calbindin and calretinin staining of the wholemount preparations. These revealed a reduction up to 40 % of calbindin-positive neurons in the FGF2-knockout mouse. We therefore consider FGF2 playing a more important role in the fine-tuning of the ENS during development as previously assumed.