Hair follicles are mini-organs from which new hair constantly grows. The basis for new hair growth is the proper function of hair follicle stem cells (HFSCs).
HFSCs undergo cyclic symmetric and asymmetric cell divisions (SCDs and ACDs). SCDs generate two identical cells that go on to have the same fate, while ACDs generate a differentiating cell and a self-renewing stem cell.
“For proper tissue function, symmetric and asymmetric cell divisions have to be in balance,” says corresponding author of the study Emi Nishimura. “Once stem cells preferentially undergo one of either or, worse yet, deviate from the typical process of either type of cell division, the organ suffers. In this study, we wanted to understand how stem cell division plays into hair grows during aging.”
To achieve their goal, the researchers investigated stem cell division in HFSCs in young and aged mice by employing two different types of assays: Cell fate tracing and cell division axis analyses.
The researchers were able to show that while HFSCs in young mice underwent typical symmetric and asymmetric cell divisions to regenerate hair follicles, during aging they adopted an atypical senescent type of asymmetric cell division.
But why does the mode of cell division change so drastically during aging? The researchers found that during aging both hemidesmosomal and cell polarity proteins become destabilized, resulting in the generation of aberrantly differentiating cells during division of HFSCs. As a result, HFSCs become exhausted and lost (leading to hair thinning and hair loss) over time.
“These are striking results that show how hair follicles lose their ability to regenerate hair over time,” says first author of the study Hiroyuki Matsumura. “Our results may contribute to the development of new approaches to regulate organ aging and aging-associated diseases.”
Hiroyuki Matsumura, Nan Liu, Daisuke Nanba, Shizuko Ichinose, Aki Takada, Sotaro Kurata, Hironobu Morinaga, Yasuaki Mohri, Adèle De Arcangelis, Shigeo Ohno, Emi K. Nishimura. Distinct types of stem cell divisions determine organ regeneration and aging in hair follicles. Nature Aging, 2021; 1 (2): 190 DOI: 10.1038/s43587-021-00033-7
Tokyo Medical and Dental University. “The bald truth: Altered cell divisions cause hair thinning.” ScienceDaily. ScienceDaily, 16 March 2021. <www.sciencedaily.com/releases/2021/03/210316093424.htm>.
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