Additional Table 1.

Summary of age-related morphological and cytoskeletal changes covered in this review

Neuronal compartment	Summary of age-related changes
Dendrites	Morphological changes
	Changes in dendrite length A 13% reduction in layer 3 pyramidal cells, 37% reduction in level 5 basal dendrites of humans (Nakamura et al., 1984; Nakamura et al., 1985b), and 83% reduction in rodent dentate gyrus dendrite length (Wang et al., 2017) and dendrite length within rodent hippocampus (Aguilar-Hernández et al., 2020). Dendrite length reduction is also reported in PFC dendrites of monkeys and rodents (Kabaso et al., 2009; Sotoudeh et al., 2020). However, no dendritic regression is seen in the human prefrontal cortex (de Brabander et al., 1998), monkey layer 3 pyramidal neurons (Duan et al., 2003), or superior temporal cortex pyramidal dendrites (Luebke et al., 2015). Conversely, dendrite length is seen to increase in the human parahippocampal gyrus (Buell and Coleman, 1979) and rodent amygdala (Sotoudeh et al., 2020).
	Reduction in dendrite volume/diameter Dendrite surface area/volume is reduced in long projecting PFC dendrites (Kabaso et al., 2009)) and diameter is reduced in lumbar spinal cord motor neuron dendrites (Castro et al., 2023).
	Reductions in dendrite branching/space A 50% reduction in dendritic branching is seen in rodent dentate gyrus neurons (Wang et al., 2017) and a 20% reduction in synaptic coverage of dendrites in rodent spinal cord motor neurons (Castro et al., 2023).
	Cytoskeletal changes
	Changes to microtubule integrity Up to a 55% reduction in microtubule density between the ages of 62 and 80 in humans (Cash et al., 2003).
	Reduction in tubulin CacyBP/SIP and beta-tubulin staining reduces with age in rodents (Filipek et al., 2008).
	Reductions in Tau Phosphorylated tau (Niewiadomska and Baksalerska-Pazera, 2003; Niewiadomska et al., 2005; Filipek et al., 2008) and non- phosphorylated tau (Niewiadomska, 2003) staining is reduced in processes (some thought to be dendrites) with age in the rodent hippocampus and cortex.
	Reductions in MAP2 MAP2 immuno-staining is reduced in dendritic processes in the rodent dentate gyrus and CA1 (Di Stefano et al., 2001; Di Stefano et al., 2006).
Soma	Morphological changes
	Changes to soma size/volume A 33% reduction in soma volume reported in human super frontal gyrus cells between the ages of 52 and 94 years (Uemura and Hartmann, 1978), soma size reduces in the cerebral cortex and PFC of rodents (Ueno et al., 2018; Sotoudeh et al., 2020) and spinal motor neurons of cats (Liu et al., 1996). Additionally, soma size decreases by 30% in touch receptor neurons between 1 and 8 days of age (Hess et al., 2019). Conversely, soma size remains consistent in human and rodent spinal cord motor neurons (Castro et al., 2023), monkey visual cortex neurons (Luebke et al., 2015) and rodent amygdala (Sotoudeh et al., 2020).
	Appearance of lipofuscin granules Lipofuscin granules are seen to accumulate in many regions of the aged brain (Brody, 1960; Robles, 1978; Gray and Woulfe, 2005; Pannese, 2011; Merlo et al., 2015) although this is not reported in hypothalamic neurons (Brody, 1970).
	Cytoskeletal changes
	Increases in staining for cytoskeletal proteins in the soma Usually found to be an axonal MAP, both tau and phosphorylated tau are shown to mislocalize to the soma in rodents whilst soma tubulin staining is reduced (Filipek, Schneider 2008, Niewadomska 2003, 2006).
Axon	Morphological changes
	Changes to axonal diameter A 1.5-1.6 fold increase in axon diameter is seen in human male skin sensory neurons (Metzner et al., 2022)) and an increase in axonal diameter index is shown in the corpus callosum of aged humans (Fan et al., 2019)) whilst rodent optic nerve axon volume increases by 50% and diameter increases between 1 and 12 months (Stahon et al., 2016). However, no change in axon diameter is seen in the corpus callosum, cingulate gyrus, and visual cortex of monkeys (Peters et al., 2001; Bowley et al., 2010).
	Appearance of axonal swellings “Torpedo” axonal swellings are seen in aged human (Kato and Hirano, 1985) and rodent (Bäurle and Grüsser-Cornehls, 1994) Purkinje cells. Thickened fibers and ballooned terminals are also seen in the anterior cingulate cortex, entorhinal cortex, parietal lobe, and temporal gyrus of aged humans (Geula et al., 2008).
	Cytoskeletal changes
	Changes to microtubule integrity In the aged human retina, up to 30% of rods have microtubules with altered morphology such as non-uniformed alignments and wavy appearances (Nag et al., 2020).
	Changes to microtubule staining Staining for tubulin, tau, and P-tau (phospho-tau) are all reduced within processes of aged rodents (Filipek 2008, Niewiadomska 2003, 2005) but NFH, tubulin and actin are shown to increase within human sensory axons with age (Metzner et al., 2022).
	Changes to axonal transport A reduction in axonal transport is reported in the CNS and PNS of rodents (Niewiadomska and Baksalerska-Pazera, 2003; Milde et al.,
	2015; Takihara et al., 2015) and the Drosophila wing sensory neurons (Vagnoni et al., 2016).
Synapses	Morphological changes
	Reductions in spine density In humans, spine density/volume in pyramidal neurons and the frontal cortex does not change into old age (Jacobs et al., 1997; Scheff et al., 2001) although this is argued as more recent work shows an age-related reduction in spine density within the human prefrontal cortex (Boros et al., 2019). In rodents, overall spine count/density is lost in multiple regions (Bloss et al., 2011; Mostany et al., 2013; Calì et al., 2018; Aguilar-Hernández et al., 2020). In monkeys, a reduction of spine density is also observed in the prefrontal and visual cortices (Kabaso et al., 2009; Dumitriu et al., 2010; Young et al., 2014; Luebke et al., 2015).
	Change to spine type In rodents a loss of thin and stubby spines is widely documented (Bloss et al., 2011; Mostany et al., 2013) but conversely, an increase in stubby spines (Aguilar-Hernández et al., 2020) and 13% increase in spine head volume is also reported (Chen and Hillman, 1999). In monkeys, a loss of thin spines alongside reductions in spine density is observed in the prefrontal and visual cortices (Kabaso et al., 2009; Dumitriu et al., 2010; Young et al., 2014; Luebke et al., 2015).
	Cytoskeletal changes
	• Reduced membrane-association of Rho GTPases in mice (critical to actin remodeling) (Afshordel et al., 2014); • Reduced synaptic levels of actin and synaptophysin in rodents (Ve et al., 2020); • Reduction and increased inactivation of cofilin in the aging mouse brain (cofilin is important for actin remodeling) (Barone et al., 2014).
Biochemical changes to MT cytoskeletal components (not clear what compartment as difficult to determine this from biochemical fractions)	Tubulin levels Soluble tubulin decreases by up to 75% in aged human cortex between the ages of 0-90 years (Yan et al., 1985), decreases also shown by (Labisso et al., 2018). In rodents, there is evidence for either a reduction (Shimada et al., 2006) or a trend for a decrease (Fifkova and Morales, 1992; Kneynsberg and Kanaan, 2017) in tubulin.
	Tubulin PTMs Age-related decreases in acetylated tubulin (suggesting decreased MT integrity) reported by some in human studies (Larrayoz et al., 2017); no change reported by others in rodents (Larrayoz et al., 2017). Interestingly, some report increases in acetylated tubulin in rodents (Marton et al., 2010).
	MAP2 levels Reduction in MAP2 in human retina (Nag et al., 2020) and rodent brain (Chauhan and Siegel, 1997; Di Stefano et al., 2001; Himeda et al., 2005).
	MAP1 and 5 levels MAP1 and MAP5 reduced in rodent hippocampus (Chauhan and Siegel 1997).
	Total Tau levels
	Shown to decrease with age in humans (Mukaetova-Ladinska et al., 1996).
Biochemical changes to MT cytoskeletal components (not clear what compartment as difficult to determine this from biochemical fractions)	P-Tau levels Phosphoproteomic studies show an increase in phosphorylated cytoskeletal proteins (Andrés-Benito et al., 2023); significant increases shown in rodents (Larrayoz et al., 2017). Evidence for increases in monkeys (Datta et al., 2021; Leslie et al., 2021).
Overall neuronal loss:	No more than 10% overall neuronal loss in healthy physiological aging (Pannese, 2011) though there are regional differences with cerebellar Purkinje cells more vulnerable and showing up to 40% neuronal loss in some regions (Zhang et al., 2010).

CNS: Central nervous system; MAP: microtubule associated protein; MTs: microtubules; NFH: neurofilament-heavy; PFC: pre-frontal cortex; PNS: peripheral nervous system; P- Tau: phosphorylated-Tau.