Henson, who endured an injury and suspension-hit campaign, will remain at home this summer to concentrate on a full pre-season's training, while Jenkins is nursing a shoulder problem.
Wales attack coach Nigel Davies said: "We are looking at players who have the potential to compete for a place in the World Cup squad in 18 months' time, and both John and Nathan fit that bill.
"Nathan has been in outstanding form for the Dragons in the latter part of this season after returning from injury, and we have had a good close look at him.
"He is primarily selected as an outside centre or wing, as we've concluded we have adequate cover for the inside centre position vacated by Gavin (Henson) elsewhere in the squad."
Jenkins, whose shoulder was operated on after last year's British & Irish Lions tour of New Zealand, received an injection this week to help settle inflammation.