Tooth, hair and claw: Comparing epithelial stem cell niches of ectodermal appendages

• Mouse incisor, hair and claw utilize stem cell-based continuous growth and have similar embryonic origins and architectures.
• Comparing the stem cell niches of these organs will improve our understanding of the mechanisms that enable renewal or regeneration in mammals.
• Gli1 is potentially a common stem cell marker for mouse tooth, hair and nail.

The vertebrate ectoderm gives rise to organs that produce mineralized or keratinized substances, including teeth, hair, and claws. Most of these ectodermal derivatives grow continuously throughout the animal׳s life and have active pools of adult stem cells that generate all the necessary cell types. These organs provide powerful systems for understanding the mechanisms that enable stem cells to regenerate or renew ectodermally derived tissues, and remarkable progress in our understanding of these systems has been made in recent years using mouse models. We briefly compare what is known about stem cells and their niches in incisors, hair follicles, and claws, and we examine expression of Gli1 as a potential example of a shared stem cell marker. We summarize some of the features, structures, and functions of the stem cell niches in these ectodermal derivatives; definition of the basic elements of the stem cell niches in these organs will provide guiding principles for identification and characterization of the niche in similar systems.