**2.1 Types of stress**

Although gliomas can vary by type and stage, more advanced gliomas are characterized by high rates of mitosis, hypercellularity, evidence of angiogenesis, and areas of necrosis. Gliomas are known to have relatively high cellular heterogeneity, much of which may be caused by different areas of a tumor encountering different stresses and growing conditions. Thus, stress may be a driving factor in tumor heterogeneity. Although, medical professionals, along with patients, are able to control to a certain extent the extrinsic stresses put on patients' bodies and tumors such as treatments and environmental stressors (*e.g.* smoking), intrinsic stresses still naturally affect the tumor as it progresses.

Fig. 1. Typical stresses encountered by glioma cells during tumor growth and progression.
