Tissue engineering done in vivo is capable of recruiting local cellular populations into a bioreactor space. Indeed a range of neotissue growth has been shown: bone, cartilage, fat, and muscle. In theory, any tissue type could be grown in this manner if all necessary components (growth factors, environmental and physical ques) are met. Recruitment of stem cells require a complex process of mobilization from their niche, though research suggests that mature cells transplanted upon the bioreactor scaffold can improve stem cell recruitment. These cells secrete growth factors that promote repair and can be co-cultured with stem cells to improve tissue formation.