Figure 9. Factors that regulate the interaction of progenitor subtypes in the olfactory epithelium. HBCs can self renew (size of curved arrow indicates capacity to divide-self-renew) and exist in a tightly adherent and loosely adherent (HBC^*) mitotic phenotypes, which are differentiated by their expression levels of ICAM-1 (yellow) and and integrin complexes (orange arrows). A more restricted neuronal-sustentacular (SUS) progenitor is found in the GBC population, whereas a gliogenic precursor can also be derived from mitotic HBCs. ECM gradients for laminin, collagen, and fibronectin are indicated, along with the factors that combine with matrix stimulation to positively regulate the balance between progenitor subtypes and immature and mature cells within the OE and lamina propria. Inset simplifies the relationship between different OE progenitor subtypes, where B (GBC) and C (OEC-GRPs) reside in entirely different ECM, cell-cell, and growth factor environments within the OE and lamina propria.