Organ Regeneration Based on Developmental Biology

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6.1 Introduction

The hair organ plays biologically important roles in thermoregulation, physical
insulation from UV radiation, waterproofing, tactile sensation, protection against
noxious stimuli, camouflage, and social communication (Hardy 1992 ; Schneider
et al. 2009 ). The initiation of hair follicle organogenesis is induced in the hair germ
through reciprocal epithelial-mesenchymal interactions and, ultimately, develop-
ment into the hair follicle (Hardy 1992 ; Stenn and Paus 2001 ) (Fig. 6.1a). The hair
follicle is composed of an upper permanent region and a lower variable region that

Skin and hair follicle development stages

a

b

Epithelial

stem cell

niche

Mesenchymal

stem/progenitor cells niche

ALP

SG

Melanocyte
stem cell
niche

Peripheral

nerve

Subdermis

Dermis

HM

HS

Anagen

Anagen

Anagen

Telogen

Hair

cycle

Catagen

Body surface Placode Hair germ Hairpeg Hairfollicle
epithelium

Skin mesenchyme

Dermal

condensate

DP

ALP

Fig. 6.1 Organogenesis of the hair follicle during embryogenesis and the hair cycle of the adult
hair follicle. (a) Hair follicle development during embryonic skin development. Red round arrows
and black round arrows represent epithelial-mesenchymal interactions and hair signaling, respec-
tively. (b) Various stem cells and their niches in the adult hair follicle and hair cycle. DP dermal
papilla, HM hair matrix, SG sebaceous gland, ALP arrector pili muscle, HS hair shaft

K.-e. Toyoshima and T. Tsuji