Fig. 3

Functional categorization of HFSC niche cells. According to the functions of niche cells, they are categorized into 3 groups: signaling modules, sensing modules, message-relaying modules. These functionally distinct modules are assembled into a multifunctional niche. Signaling modules regulate HFSC activity via cell-cell contact or paracrine secretion. Sensory modules detect environmental cues. Message-relaying modules transmit signals from remote cells/tissues to HFSCs. Sensory modules and message-relaying modules can directly signal to HFSCs or indirectly regulate HFSC activity through the signaling modules