Transit Amplifying Cells (TACs): a still not fully understood cell population

Area of Science:

• Cell Biology
• Developmental Biology
• Regenerative Medicine

Background:

• Adult stem cells (SCs) maintain tissue homeostasis and regeneration by proliferating slowly in niches.
• During development and regeneration, SCs produce highly proliferative transit-amplifying cells (TACs).
• TACs are not just a transitional phase but actively regulate SCs and their differentiated progeny.

Purpose of the Study:

• To review the generation and function of TACs in high-turnover lining epithelia (epidermis, hair follicles, ocular surfaces, intestinal mucosa).
• To compare TAC roles and molecular pathways across different epithelial tissues.
• To discuss cultured mesodermal cells (mesenchymal stem cells, dedifferentiated chondrocytes) and their potential reversion to a TAC stage.

Main Methods:

• Literature review focusing on TACs in epithelial development and regeneration.
• Comparative analysis of TACs across epidermis, hair follicles, ocular surfaces, and intestinal mucosa.
• Discussion of common and tissue-specific molecular pathways (Wnt, Notch, Hedgehog, BMP) in TACs.
• Exploration of cultured mesenchymal stem cells and dedifferentiated chondrocytes in relation to TACs.

Main Results:

• TACs play a critical role in controlling stem cell populations and directing differentiation in various tissues.
• Common molecular pathways (Wnt, Notch, Hedgehog, BMP) are involved in TAC regulation, but their responses vary by tissue.
• TACs are crucial for the development and regeneration of high-turnover epithelial tissues.
• Dedifferentiated cells in culture may revert to a TAC-like state, relevant for cell therapy.

Conclusions:

• TACs are essential regulators of tissue development, regeneration, and homeostasis across diverse epithelial systems.
• Understanding TACs and their molecular signaling provides insights into regenerative processes and cell therapy.
• The potential for dedifferentiated cells to act as TACs warrants further investigation for therapeutic applications.