1. ^*Department of Pathology, Keck School of Medicine, University of Southern California, Los Angeles, California, USA
2. ^†Department of Dermatology, Tzu Chi Medical Center, Tzu Chi University, Hualien, Taiwan
3. ^‡Department of Dermatology, Kaohsiung Medical University, Kaohsiung, Taiwan
4. ^§Department of Dermatology, College of Medicine, National Taiwan University, Taipei, Taiwan

Correspondence: Cheng-Ming Chuong, MD, PhD, Department of Pathology, University Southern California, HMR 315B, 2011 Zonal Ave, Los Angeles, CA 90033, USA. Email: chuong@pathfinder.usc.edu

Received 14 May 2003; Revised 26 November 2003; Accepted 22 December 2003.

Abstract

Complex skin appendages are built from the epidermal cells through various cell events. Here we used TUNEL and caspase-3 immuno-localization to examine apoptosis in feather morphogenesis. We deduced three modes. In Mode 1A, apoptosis occurs within the localized growth zone (LoGZ) to regulate growth (feather buds). In Mode 1B, morphogen secreting cells are present adjacent to LoGZ and apoptosis may work to remove such signaling centers (barb ridges). In Mode 2, keratinocytes apoptosed before terminal differentiation and left spaces between branches (marginal plate). In Mode 3A, keratinocytes cornified and flaked off to free skin appendages (feather sheath, pulp epithelium). In Mode 3B, keratinized apoptosed epithelial cells became permanent structures (rachis, ramus, barbules). Thus, different apoptotic modes can have different impacts on morphogenesis. We further tested effects of imbalanced Shh on apoptosis. Shh suppression reduced marginal plate apoptosis and caused abnormal differentiation of barbule plates. Shh over-expression enhanced proliferation in barb ridges. Expression of Patched in the barbule plate epithelia implies a paracrine mechanism. The current work complements our recent work on LoGZ to show how adding and removing cell masses in temporally and spatially specific ways are coordinated to sculpt skin appendages from epidermal layers.