Epithelial appendages are derivatives of the epithelial sheet. They add structural and functional complexities to the otherwise flat epithelia (Fig. 1.1). Epithelial appendages can be categorized as evaginated or invaginated appendages. Evaginated epithelial appendages like feathers and...

In 1875 Charles Darwin reported a fascinating set of phenotypes associated with the “toothless men of sind”, members of a Hindu kindred which resides in the vicinity of Hyderabad in India. Darwin described a family “in which ten men, in the course of four generations, were furnished, in...

I have had a long research career centered around hair follicles. I enjoyed it tremendously and so have some of my mentors and peers. Why do we do this? In this brief autobiographical commentary, I will make a personal reflection on my research career, spanning a period of 50 years. My intention...

In amniotes, the skin is well equipped with cutaneous appendages whose morphological type and distribution show specific regional variations. In birds, the integument can give rise to two types of appendages: scales and feathers. In chick, while most of the body is covered with feathers which are...

Among the skin appendages, feathers certainly have the most elaborate structures and are richly patterned. Feathers distinguish the bird from other vertebrate classes and provide the birds with a unique niche, the sky, in which to thrive and evolve. Feathers also serve other crucial functions...

The hair follicle is the most prominent cutaneous mini-organ and one of the defining features of mammalian species. Only mammals display hair follicles, although not every mammalian species does so; whales, for example, are devoid of hair. As discussed in chapters 5 and 19, hair follicle...

Hair growth involves the unique phenomenon of cyclic regeneration: This structure forms, regresses, and regrows over a recurring period. After a brief description of cycling and regeneration in animal biology, and the introduction of an antler cycle, in this essay we will review the hair cycle...

The pigmented cells of the body, the melanocytes, have long been a favored cell type for studies of the genetic and epigenetic factors involved in cell differentiation, partly because pigment makes these cells easily recognizable without any staining, but also because of the astonishing variety...

Teeth develop from stomodeal or pharyngeal epithelium and the underlying neural crestderived mesenchymal cells in a way very similar to skin derivatives such as hair, feathers and scales (Fig. 9.1). The first morphological sign of tooth development is a thickening of the oral epithelium, which...

The epithelial-mesenchymal interaction (EM) is a critical mechanism employed during embryogenesis. The signaling between these two different tissues is required to form the functional unit of the systems in which it is used. The gastrointestinal tract (gut) is dependent on EM interactions for its...

The endodermal cells of the gut serve as progenitors for the origin of many diverse tissues including pancreas, liver, thyroid and the lung. The primordium of each tissue is thought to emerge as an endodermal appendage consisting of a small group of cells with a specific developmental fate. One...

The mammalian lung has the largest surface area of any epithelial appendage in the human body. Its intricate vaulted structure could indeed be considered a veritable “biological cathedral”. The lung develops as an epithelial appendage of the primitive foregut in the early embryo, undergoes...

Embryogenesis is composed of a series of inductive events that change the fate of responding tissues. Through induction, new tissues and organs are generated. In the beginning of embryogenesis, ectodermal-endodermal interactions produce the mesoderm by what is termed primary induction. In the...

Organogenesis is a complex process that results from sequential and reciprocal cell and tissue interactions. Tooth development, which shares similarities with the development of other embryonic organs, is characterized by a series of reciprocal epithelial-mesenchymal interactions that result in...

One fundamental question in hair follicle (HF) development is: How can a tiny cluster of seemingly uniform epithelial cells, which associates with a small cluster of apparently homogeneous mesenchymal cells, give rise to a complex mini-organ that serves as an enormously productive fiber...

The late events of keratinocyte differentiation discussed here are mainly those that occur in the hair follicle. The reason for this emphasis is that more is known about the keratinization process of hair follicles than of feather follicles. The hair shaft keratinocyte is dedicated to the rapid...

The molding of living tissues that occurs during development, regeneration, wound healing, and various pathological processes is referred to as morphogenesis. During morphogenetic events tissue masses may disperse, form internal foci of cell condensation, lengthen or shorten, or acquire lumens....

A major question in embryonic development is how cells and tissues become precisely arranged to make up the body plan. It is best to study this process in an organ with well defined morphological patterns. One of the simplest patterns observed is the maintenance of a minimum distance between...

Chicken skin development is an excellent model to study the mechanisms of morphogenesis. It has a long experimental history and has been well characterized morphologically. Chicken skin offers distinct patterns and large numbers of different cutaneous appendages, accessibility to microsurgery,...

Historically, the phenotype of anhydrotic ectodermal dysplasia (EDA) has stimulated considerable interest among investigators. A famous early description of the disorder was written by Charles Darwin, even though an earlier report exists. In his description, Darwin also accurately listed all...

The inbred laboratory mouse has long been the species of choice for biomedical research including genetics based studies. In the past decade tremendous advances in molecular biology and gene manipulation have put the mouse into the forefront of species used in biomedical research. Large numbers...

Many genes involved in development are evolutionarily conserved and interact with other genes either directly or indirectly. More recently it has become clear that gene interactions are also evolutionarily conserved. Here a model of gene interaction is presented. Many interacting genes give rise...