RANKL pretreatment plays an important role in the differentiation of pit-forming osteoclasts induced by TNF-α on murine bone marrow macrophages

•We examined the condition of acceleration of osteoclast formation by TNF-α.•We checked whether TNF-α after RANKL pretreatment could promote resorbing activity.•TNF-α after 10ng/mL RANKL pretreatment significantly promoted osteoclast formation.•TNF-α after 10ng/mL RANKL pretreatment dynamically generated pit-forming osteoclasts.•The effects of TNF-α after RANKL pretreatment might be independent of RANKL.