TABLE 2.

Morphological correlates in Rett syndrome and Fragile X syndrome.

Source	Region/Sample	Experiment	Findings	References
Mecp2tm1.1Jae, Mecp2308/Y, Mecp2tm1.1Bird, Patients with RTT	Hippocampus, cortex, cerebellum	Imaging studies	↓ Neuronal size; ↓ Dendrite numbers; ↓ Dendrite length; ↓ Dendrite spines	Armstrong, 2005; Belichenko et al., 2008
Mecp2–/y, Patients with RTT	Cerebral cortex, total brain	Imaging studies	↓ Volume	Chen et al., 2001; Armstrong, 2005
Fmr1 –/Y	Hippocampus (25 weeks)	Immunohistochemistry, imaging studies	↑ Spine density	Levenga et al., 2011; Hodges et al., 2017
Fmr1–/Y, Patients with FXS	Cortex (16 weeks), temporal cortex	Immunohistochemistry, imaging studies	↑ Filopodia; ↑ Dendrite length; ↑ Dendrite spines	Comery et al., 1997; Irwin et al., 2000
Fmr1 –/Y	CA1 and layer 5 neurons (P14-P37), hippocampal slices (P25-P35)	Imaging studies	Altered morphology may not correlate with abnormal function	Wijetunge et al., 2014; Thomazeau et al., 2020
Fmr1 –/Y	Layer 5 neurons (P7-P28)	Imaging studies	Synaptogenesis is mediated by FMRP in a time-dependant manner	Nimchinsky et al., 2001

Main findings of studies reporting on morphological phenotypes in Rett syndrome and Fragile X syndrome. Mecp2, methyl-CpG binding protein 2 gene; RTT, Rett syndrome; Fmr1, fragile X mental retardation 1 gene, FXS, Fragile X syndrome.