During cell division, the mitotic spindle apparatus directs the localization of the actomyosin contractile ring and cleavage furrow ingression; thus, spindle positioning serves as an essential determinant of cell division orientation. Two fundamental aspects of animal development arise from this principle. First, spindle orientation directs the asymmetric segregation of cell fate determinants during stem cell divisions, providing a means of balancing self-renewal and differentiation. For example, uncoupling of spindle orientation from the cortical polarity axis in Drosophila neuroblasts can contribute to an overproliferation of these neural stem cells, disrupting proper CNS development and resulting in severe tissue overgrowth phenotypes [
,
]. Second, the establishment and maintenance of complex tissue structures relies on spindle orientation in order to balance cell divisions that lead to tissue expansion versus stratification. For example, spindle orientation defects in the mouse epidermis result in defective stratification, yielding tissue structures that are incapable of proper fluid and electrolyte regulation [
]. Despite being linked to several developmental disorders and having recently emerged as a possible contributor to tumorigenesis [
], the molecular details of spindle orientation process remain incomplete.
Common Design 