lication ▁of ▁extr amed ull ary ▁h emat opo ies is ▁is ▁po ik il oc yt osis , ▁or ▁the ▁presence ▁of ▁ab norm ally ▁sh aped ▁red ▁blood ▁cells . ▁ ▁My el of ib ros is ▁can ▁be ▁a ▁late ▁comp lication ▁of ▁other ▁my el op rol ifer ative ▁dis orders , ▁such ▁as ▁poly cy th emia ▁ver a , ▁and ▁less ▁commonly , ▁essential ▁th rom b oc yth a emia . ▁In ▁these ▁cases , ▁my el of ib ros is ▁occurs ▁as ▁a ▁result ▁of ▁som atic ▁evolution ▁of ▁the ▁ab normal ▁h emat opo iet ic ▁stem ▁cell ▁clone ▁that ▁caused ▁the ▁original ▁dis order . ▁In ▁some ▁cases , ▁the ▁development ▁of ▁my el of ib ros is ▁following ▁these ▁dis orders ▁may ▁be ▁acceler ated ▁by ▁the ▁or al ▁chem other apy ▁drug ▁hydro xy ure a . ▁ ▁The ▁cause ▁and ▁risk ▁factors ▁for ▁primary ▁my el of ib ros is ▁are ▁unknown . ▁ ▁S ites ▁of ▁h emat opo ies is ▁The ▁principal ▁site ▁of ▁extr amed ull ary ▁h emat opo ies is ▁in ▁my el of ib ros is ▁is ▁the ▁s ple en , ▁which ▁is ▁usually ▁marked ly ▁en larg ed , ▁sometimes ▁we igh ing ▁as ▁much ▁as ▁ 4 0 0 0 ▁g . ▁As ▁a ▁result ▁of ▁massive ▁en larg ement ▁of ▁the ▁s ple en , ▁multiple ▁sub c aps ular ▁inf arct s ▁often ▁occur ▁in ▁the ▁s ple en