Acer palmatum branches are dying

Small inner branches on my nine-year-old Acer palmatum ‘Bloodgood’ have been dying and by last autumn had dropped off. No new inner branches have grown and the tree has bare trunks. Does it need pruning to encourage it to thicken up?

‘Bloodgood’ is one of the best of the A. palmatum range, with dark, almost black young branches and deep colour to the palmate leaves which also fire up nicely in autumn. The cool, damp climate of coastal Lancashire suits it to a tee!
The small side branches probably died as a result of die-back disease and if you see this returning, take great care to remove every infected shoot, cutting back into healthy, living tissue where possible. You need to be vigilant with this disease which, in some maples, can be fatal. There is no fungicide treatment available, so just keep a watching brief.
Your tree could be simply growing up. It is natural for the trunks to be bare in mature specimens as the canopy develops and spreads. Get used to being able to see through the limbs to view the crown of the tree from below and enjoy its grace and beauty. You could prune to make it bushyor let it grow naturally. If you do decide to prune, late winter is a good time, but pick a mild spell.