Epithelialtissuesarecomposedofcloselyaggregatedpolyhedralcellswithverylittleextracellularsubstance.Animportantfeatureofepitheliaistheirpolarity:theyhaveafreesurfaceandabasalsurfacethatrestonthebasalmembrane.Sincebloodvesselsdonotnormallypenetrateanepithel (共 1229 字) [阅读本文] >>