Fig. 4. Reconstruction of the nigrostriatal pathway using microtissue-engineered neural networks (micro-TENNs).

a Immunocytochemical image of the axonal segment of a micro-TENN showing the robust outgrowth of dopaminergic axons in c, as labeled using an antibody for tyrosine hydroxylase (TH; red). The hydrogel shell is highlighted with a dotted line. b Immunocytochemical image of the somatic end of a uniaxial micro-TENN showing a large cluster of aggregated neurons in c, labeled with a Hoechst nuclear counterstain (blue) and using antibodies for all neurons/axons (β-tubulin III; green) and dopaminergic neurons/neurites (TH; red), with an overlay of all three. c The cartoon (left) and actual (right) unidirectional micro-TENN show the long-distance axonal outgrowth. The bolus of neurons is at the bottom with axonal outgrowth projecting upwards. The actual micro-TENN has the same staining as b, and the hydrogel shell is highlighted with a dotted line. d A diffusion tensor imaging representation of the long-distance axonal tracts (lilac) that connect discrete populations of neurons in the human brain. This conceptual rendition shows how a unidirectional micro-TENN—consisting of a population of dopaminergic neurons extending long, aligned processes—can be used to recreate the nigrostriatal pathway (green) that degenerates in PD. The magnification inset in the lower right depicts axons (blue) in the substantia nigra functionally integrating with the transplanted dopaminergic neurons in the micro-TENN (green). The magnification inset in the upper left depicts transplanted dopaminergic axons (green) functionally integrating with neurons in the striatum (red). The micro-TENN implant theoretically recreates the full motor feedback circuit by receiving the stereotypical inputs in the SNpc while projecting axons to the striatum to release regulated amounts of dopamine in that structure.