How is a thickening of grey matter possible?

Question

There are plenty of studies that document grey matter thickening in certain brain areas as a result of meditation or exercise. However, it's often said that the extent of neurogenesis outside of the hippocampus is negligible. So can certain activities promote significant neurogenesis in the adult brain or can grey matter thickening be explained through other mechanisms (e.g. cell migration)?

An excerpt from Wikipedia article on grey matter:

Grey matter (or gray matter) is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distinguished from white matter in that it contains numerous cell bodies and relatively few myelinated axons, while white matter contains relatively few cell bodies and is composed chiefly of long-range myelinated axons.

Sources:

Meditation-related increase in grey matter:

1. https://pubmed.ncbi.nlm.nih.gov/27983555/
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3004979/

Grey matter increase in response to exercise:

3. https://academic.oup.com/biomedgerontology/article/61/11/1166/630432

Answer 1

Except in three specific areas (olfactory bulb, dentate gyrus, SVZ) where neurogenesis continues to occur in adult organisms, it is virtually absent from everywhere else, including the cortex. However, there have been many reports that the grey matter of the cortex, i.e. the zone where cell bodies and their processes reside, can enlarge. Cortical expansion has been linked with changes in dendritic branching, neuronal soma size, synapse size, the number of dendritic spines, and the number of glial cells.

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