Regeneration of Skin - Tissue and Organ Regeneration in Adults - page 126

athymic mice. The controls used were uncultured dissociated murine-only skin cells
(positive control), or cultured human-only KC and fibroblasts without dermal pa-
pilla cells (negative control). Only the second group of grafts (scaffolds seeded with
murine-only cells) formed hairs and sebaceous glands. The first group (scaffolds
seeded with chimeric cells) formed hairs without sebaceous glands, while the third
group (scaffolds with human-only cells) formed no follicles or glands. The study
showed that sebaceous glands are not required for hair formation when using this
protocol (Sriwiriyanont et al. 2013).
The regeneration of hair follicles and sebaceous glands described above, used
in combination with DRT scaffolds that lead to regeneration of an epidermis and a
dermis (Sriwiriyanont et al. 2012, 2013), appears to provide the missing elements
from earlier efforts to obtain a complete regeneration of skin.

5.6 Summary of Protocols for Synthesis of Tissues in Skin

We summarize below the conditions under which each of the tissues of skin, as
well as skin itself, have been synthesized. The available data make it possible to
reach rather definitive conclusions about the choice of simplest conditions that are
required to induce regeneration of tissue components of skin. Nevertheless, it is es-
sential to heed a few warnings.
First, the data summarized below have been recorded during a timescale that
has been selected in a very arbitrary manner by different investigators. In certain
cases, the duration of the study was probably insufficient; a lengthier study might
have yielded a regenerated tissue or organ that would certainly have been more
mature and perhaps closer to normal. Second, the requirements discussed below
are based on currently available data; future studies may show that some of these
requirements can be relaxed. There is no absolute requirement, for example, that
skin should be synthesized in vivo, even if the available evidence show that all in-
stances reported in which skin has been synthesized to date have been based on in
vivo protocols. Third, although significantly different from the products of repair,
the tissues and organ synthesized by induced regeneration are not completely physi-
ological. For example, skin regenerated using DRT, even when seeded with KC as
described above, lacked appendages. Later studies have shown that follicles and
sebaceous glands can also be regenerated by incorporating into DRT a preparation
of cultured dermal papilla cells.
KC cultured in vitro can form a partly differentiated epidermis . Previously, it had
been shown that epidermal cells proliferate and are differentiated in vitro, forming
an immature epidermis, in the presence of collagenous substrates (Freeman et al.
1976; Lillie et al. 1988), and in the absence of either viable fibroblasts or a fibro-
blast-conditioned medium (Rheinwald and Green 1975a; Eisinger et al. 1979). In
a particularly simple protocol, it sufficed to control conditions of pH, cell seeding
density, and incubation temperature to synthesize a fully differentiated epidermis in
vitro (Rosdy and Clauss 1990; Rosdy et al. 1993). It has also been shown that the
presence of a BM is not an essential substrate for epidermal differentiation in vitro;