Adhesion Molecules in Skin
Development: Morphogenesis of
Feather and Hair.
CHENG-MING CHUONG,b HAI-MING CHEN,
TING-XING JIANG, AND JENNIFER CHIA
Department of Pathology
School of Medicine
University of Southern California
Los Angeles, California 90033
INTRODUCTION
One of the key issues in the development and maintenance of skin is the formation of skin appendages such as hair, nail, horn, and feather. The process is formed
by complex interactions between the epithelium and the mesenchyme, resulting in
the conversion of a flat piece of ectoderm into cutaneous appendages with unique
structure and keratin subtypes (reviewed in Ref. 1).The failure of this process leads
to various disorders, ranging from congenital malformation to tumors to alopecia? In
order to correct these disorders, the mechanism underlying this process must be
understood.
Cell interaction plays a central role in the formation of skin appendages, and it
is essential to identify cell surface molecules involved in this interaction. Recent
work has led to the identification of several new adhesion molecule families that
mediate cell-cell and cell-substrate adhesion, and they are likely candidates for this
process. There are neural cell adhesion molecules (N-CAM), which belong to the
immunoglobulin gene superfamily (reviewed in Ref. 3); cadherins, which mediate
adhesion in the presence of calcium (reviewed in Ref. 4); tenascin, which is a unique
matrix molecule composed of domains homologous to epidermal cell growth factor
(EGF); fibronectin type I11 repeat and fibrinogen (reviewed in Ref. 5); and integrins,
which serve as cellular receptors for fibronectin, collagen, and other extracellular
matrix molecules (reviewed in Ref. 6). These molecules have been shown to be
essential during embryonic development and play important physiological roles in
the mature adult as well as in regeneration (reviewed in Ref. 7). We have previously
shown that N-CAM is expressed during feather induction8v9and that antibodies
applied to liver CAM (L-CAM) can alter feather pattern formation.'O
In this communication, we demonstrate our continuing study of the expression
and function of adhesion molecules in ski development. Because of the distinct
pattern and accessibility to experimentation, the feather is a classical model for
This work is supported by NIH HD 24301 and the Council for Tobacco Research. C.-M.
Cis a recipient of American Cancer Society Junior Faculty Research Award. J. C. is supported
by the Edmonson Fellowship, Department of Pathology, University of Southern California.
Address for correspondence: Dr. Cheng-Ming Chuong, HMR 204, Department of Pathology, School of Medicine, University of Southern California, 201 1 Zonal Avenue, Los Angeles,
California 90033.
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studying the induction and morphogenesis of cutaneous appendages. Therefore, we
have been studying feather. We have also checked the roles of adhesion molecules
in hair for comparison. In this study, we examined the expression of N-CAM,
tenascin, integrin, peanut agglutinin (PNA), and platelet-derived growth factor
(PDGF) receptor during the morphogenesis of hair and feather. We used feather
explant culture and dermal papilla cell cultures to further analyze the function of
these adhesion molecules.
MATERIALS AND METHODS
Chicken embryos were obtained from Red Wing Farm (Los Angeles, CA) and
staged according to Hamburger and Hamilton.” Antibody to PDGF receptor beta
chain was prepared against synthesized peptide and provided by Jung-Sun
Huang.12 PDGF BB was from R and D Systems (Minneapolis, MN). Antibody to
tenascin was from the Developmental Studies Hybridoma Bank. Antibody to integrin beta subunit (CSAT) was a kind gift from Dr. Clayton A. Buck (Wistar
Institute, PA). Immunostaining was prepared according to previous publication.7.8 Skin explant cultures were prepared according to Gallin et aL1OWe used a
Biorad con-focal microscope. Feather dermal papilla cultures were prepared according to Messenger et ~ 1 . ~ 3
RESULTS
Expression of Adhesion Molecules in the Development of the Feather
During the morphogenesis of the feather bud and follicle, remarkable molecular
heterogeneity defined by the expression pattern of adhesion molecules is observed.
In FIGURE
lA, the mesodermal cells inside the feather bud appear similar to one
another, but different regions in the bud express different types of molecules. These
cellular domains probably have special functions, although their actual purpose has
yet to be determined. At stage 34, there is an anterior (defined as the side of the bud
that forms an obtuse angle to the body surface)-posterior gradient of N-CAM in the
feather bud (FIG.1B). In contrast, there is a posterior-anterior gradient of fibronectin
(FIG.1C and Ref. 14). Tenascin is present on both the anterior and posterior bud
around the bending region where the surface ectoderm evaginates to form the feather
bud (FIG.1D). Chondroitin sulfate was shown to be enriched in the anterior bud.I4
By the time a chicken has hatched, its feather follicles have already formed (FIG.
2). The dermal papilla is enriched with N-CAM and tenascin (FIG.2A and B), but
is negative for fibronectin (FIG.2C). It is also negative for neuro-glia (Ng)-CAM and
L-CAM.8 Above the dermal papilla is a zone of epithelial cells called the “collar,”
FIGURE 1. Differential expression of adhesion molecules in developing feather buds.
Immunofluorescent staining of stage 34 chicken dorsal skin. A, phase contrast; B, N-CAM;
C, fibronectin; D, tenascin. Note that N-CAM is enriched with the anterior bud (solidarrows),
and fibronectin is enriched in the posterior bud (open arrows), whereas tenascin is present in
both anterior and posterior buds. Therefore, there are remarkable molecular heterogeneities in
the apparently similar mesenchymal cells within the feather bud. Antifibronectin also stains
blood vessels. Magnification: 50x. Bar: 100 wm.
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FIGURE 2. Expression of adhesion molecules in feather follicles. Sections from newly
hatched chicken skin. A, N-CAM; B, tenascin; C, fibronectin; D, PDGF beta receptor. Dermal
papilla: both N-CAM and tenascin are enriched in the dermal papilla, but the other two
molecules are not. Collar epithelium: PDGF beta receptors are highly expressed; fibronectin
is also present; a low level of N-CAM can be detected. Feather filament: N-CAM is present
on the marginal plate epithelia; tenascin is present on the basement membrane and the pulp;
fibronectin is all over the pulp. A-C, immunofluorescencepictures. D, alkaline phosphatase
secondary antibodies were used. Abbreviations: cl, collar; dp, dermal papilla; ff, feather
filament; fs, feather sheath. Magnification: 50x. Bar: 100 pm.
CHUONG et af.: ADHESION MOLECULES
267
which surrounds and comes in close contact with the dermal papilla. The feather
collar is equivalent to the matrix region of the hair follicle and represents the
epithelial cells undergoing active cell proliferation. We found the PDGF receptor
beta subunit to be enriched in this collar region (FIG.2D). We examined the distribution of PDGF receptor because it was shown to be a member of the immunoglobulin superfamily and homologous to N-CAM.ISThe complete analysis of PDGF
in feather is to be presented elsewhere (Chuong and Huang, unpublished data). The
collar stained positive for fibronectin and L-CAM, weakly positive for N-CAM, and
negative for tenascin (FIG.2). The feather filament is generated from the collar, and
its core contains pulp that is composed of nerves and blood vessels. The pulp is
enriched with fibronectin (FIG.2C). Tenascin is also present in the pulp but is limited
to the basal lamina underneath the feather filament epithelia (FIG.2B).
In the feather filament, N-CAM is expressed on the longitudinal rows of epithelial
cells forming the marginal plate. Later, these cells die away, creating a space between
the branches of feather barbs that arise from the barb plate. This zebra-stripe staining
pattern of N-CAM can be seen clearly in the longitudinal section (FIG.3A) and cross
section (FIG.3B) of feather filaments. L-CAM is present on all the epithelia at this
stage? Interestingly, the PDGF receptor beta chain is present on the barb plate
epithelia that survive and differentiate, thus forming a complementary pattern with
N-CAM that stains only the marginal plate epithelia (FIG.3C and D).
Expression of Adhesion Molecules in the Development of the Hair
We also examined the expression of adhesion molecules during different stages
of hair development.16 At the hair placode/peg stage, N-CAM is positive on the
mesenchyme immediately surrounding the growing hair placode (FIG.4A). N-CAM
was also positive on the epithelial placode in a membrane-staining pattern (arrow),
which is more obvious in the enlarged panel (FIG.4D). Tenascin is present on the
mesenchyme surrounding the placode but appeared to be traced farther outward than
N-CAM. Tenascin is also present in the basal lamina underneath the epithelium (FIG.
4B). Because the tenascin-associated proteoglycan contains the PNA binding site
and PNA binding cell surface molecules have recently been shown to induce collapse
of the growth cone, we also carried out PNA staining. PNA is present on the
mesenchyme surrounding the hair placode, but the expression is restricted to the
superficial dermis only. This mesenchymal expression is dynamic and disappears
soon. PNA is also present on the epithelia and is enriched in the apical surface of
placode epithelia (FIG.4C).
In the forming hair follicles, N-CAM is present on the dermal papilla, the hair
sheath as well as the connective tissue (e.g., muscle) that surround the hair follicles
(FIG.5A). In the epithelia, PNA is present in the hair matrix, the inner hair root
sheath, the basal lamina of the outer root sheath, and the non-hair forming ectoderm.
In the mesenchyme, most of the early PNA expression has disappeared FIG.^^), but
it is highly enriched in the region where the surface epithelia invaginate to form the
follicle (FIG.5C, arrow). Tenascin is present on the dense connective tissue surrounding the hair follicle sheath. At this stage, tenascin is also enriched in the developing
dermis where muscle, tendons, and myotendinous junctions are forming (FIG.5D, E).
In the adult, N-CAM remains enriched in the dermal papilla and is also present
on the hair sheath. N-CAM is absent in the dermis (FIG.5B). Tenascin is present
mainly in the dense connective tissue surrounding the hair follicle (not shown). PNA
remains on the hair matrix and root sheath (not shown).
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Function of Adhesion Molecules in the Development of Feather Buds
The specific spatial and temporal expression pattern of adhesion molecules during skin appendage development prompted us to look into their function. Due to our
establishment of a reproducible culture system, feather explant cultures were used as
a model enabling the slightest perturbation to be detectable. We started from stage
32 of the embryonic chicken dorsal skin. At day 0, the condensations were barely
visible (FIG.6A), but in 4 days they developed into conical-shaped feather buds (FIG.
6B). This change was particularly impressive when viewed with a confocal microscope. In an optic section, explants cultured for 12 hours showed dermal condensations (FIG.6C). After 4 days in culture, the condensation was transformed into feather
buds protruding from the skin surface (FIG.6D).
We have tested the perturbation effect of specific antibodies to various adhesion
molecules. Interestingly, the reaction of antibodies with several adhesion molecules
inhibit feather growth, and the aborted feather patterns show marked differences. The
detailed results will be published elsewhere. Here we show the inhibition of N-CAM,
tenascin, and the integrin beta subunit by a combination of antibodies. Unlike the
control, the buds did not grow but remained as small bumps. The buds also became
heterogeneous in size (compare FIG.7A and C).
Because of the expression of PDGF receptor in the collar and the barb plate
epithelia in feather filament (FIGS.2D and 3C), we hypothesize that PDGF plays a
role in feather development. To test this, PDGF BB was added to the culture media.
It showed a remarkable effect in enhancing the growth of feather buds. With control,
feather germs form cone-shaped buds after 4 days in culture (FIG.7A) and elongate
to form slender feather filaments after 8 days in culture (FIG.7B). With PDGF,in 4
days the long, slender feather filaments had already appeared (FIG.7D).
C u b r e of Dermul Papilla Cells
Dermal papilla cells have the unusual ability of inducing epithelial cell growth.
The molecular basis of this property is analyzed through the culture of dermal papilla
cells. We have adapted methods for culturing human hair dermal papilla13to culture
feather follicle dermal papilla cells. The cells grew out from the dermal papilla
slowly and required 2-3 weeks to form a sheet of cells. Clusters of small, tightly
packed cells formed that were surrounded by larger, fibroblast-shaped cells (FIG.8
A, C). N-CAM was positive on all the cells inside the cluster but not on cells outside
the cluster (FIG.8 A, B). Enhanced staining of N-CAM was frequently observed at
the cellular interface. Within the cluster, tenascin showed an extracellular fibrillar
staining pattern. Outside the cluster, tenascin was mostly negative. Although further
FIGURE 3. Expression of adhesion molecules in feather filaments. A, B, C, N-CAM; D,
PDGF beta receptor. A, B, N-CAM from newly hatched chicken is visualized by
immunofluorescence; C, D, adjacent sections from stage 38 embryo are visualized by alkaline
phosphatase. A is a longitudinal section of the feather filament showing that the marginal plate
cells (mp) are positive for N-CAM. B is a cross section of a filament showing that the barb
plate cells (bp) are negative for N-CAM, while the mp cells and axial plate cells (ap) are
positive for N-CAM and appear in regular periodicity. C shows that mp cells stained with
anti-N-CAM are positive for the alkaline phosphatase reaction. D shows the opposite staining
pattern obtained by staining the PDGF receptor beta subunit with antiserum. Magnifications:
A, B, 240x; C, D, 60x. Bars: 100 pm.
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FIGURE 4. Expression of adhesion molecules in developing hair placode. Immunofluorescent staining on E 13 mouse whisker pad. A, N-CAM; B, tenascin; C, P N A D,
N-CAM. N-CAM, tenascin, and PNA all are expressed in mesenchyme surrounding the
developing hair placode but are distributed differently. N-CAM can be seen on the lateral
surface of placode epithelium (arrows in A and D), whereas PNA is more restricted to the
apical surface of the placode epithelia. N-CAM can also be seen in nerves (n), which contain
larger amounts of N-CAM. Magnifications: A-C, 50x; D, 125x. Bar: 100 pm.
CHUONG et al.: ADHESION MOLECULES
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characterization of these cells is required, the in vitro expression pattern of adhesion
molecules is consistent with the in vivo situation in which dermal papilla is positive
for both N-CAM and tenascin (FIG.2 A, B).
DISCUSSION
Comparative Morphogenesis of Hair and Feather
The formation of hair and feather parallel each other in that they both involve
induction between epithelium and mesenchyme, cell proliferation, epithelial folding,
and mesenchymal condensation. Both follicles contain dermal papilla in the base.
New epithelial cells are added to the proximal end and become more differentiated
towards the distal portion of the appendages. The end results are skin appendages
anchored in follicles and made of specialized keratin.
In terms of morphogenesis, hair and feather differ in two major ways. The first
is that feather germs form by growing upwards and form buds protruding above the
body surface, whereas hair germs begin by forming “epithelial pegs” that grow inside
the dermal region. The second difference is that hair ends up as a keratinized cylinder
structure, whereas further morphogenetic events take place in feather to generate
branched structures.
The expression of adhesion molecules in the morphogenesis of hair and feather,
however, are fundamentally similar.
(1) Both feather and placode epithelia are positive for N-CAM8 (FIG.4D).This
transient expression of N-CAM is fundamental and has also been observed
in other placodes, including lens placode, otic placode, and the apical ectodermal ridge of limb.17J8
(2) Mesenchymal condensation in feather germs and those surrounding the hair
pegs are both positive for N-CAM. Again the presence of N-CAM in mesenchymal condensation is fundamental and has also been observed in precartilaginous condensation and kidney tubule condensation.17J9
(3) As the mesenchymal component expands, it shows heterogeneity in the
expression of N-CAM, tenascin, fibronectin, etc., each in different restricted
regions. The functional significance has not been determined, but some
interesting speculation can be contemplated. For example, both N-CAM and
homeoproteins XIHox 1 form an anterior-posterior distribution gradient in
the feather bud and may be involved in setting up the anterior-posterior axis
of the feather.20In the feather, tenascin is seen in the flanking region of
feather buds (FIG.1D); in hair, tenascin and PNA are seen in the “ring”
surrounding the hair follicle (FIG.4C and D). These are the regions underneath the bending of epithelia, although one is evaginated and the other is
invaginated. Adhesion molecules might exert some mechanical force with
their adhesive property during the topological transformation of the epithelial
sheet.
(4) Both dermal papilla are enriched with N-CAM, even in the adult.
(5) Both hair sheath and feather sheath are positive for N-CAM and tenascin,
even in the adult.
It is compelling to speculate on the evolutional significance of these
findings.2l The scale, the skin appendage of the reptile, is a flat plaque on the skin
surface and appears similar to an overgrowth epithelial placode. During evolution,
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one more morphogenetic process was evolved in development to form the hair in
mammals. To achieve the longer appendage, the placode cells continued to proliferate and grew into the dermis to form the hair peg. In contrast, when the prototype
feather evolved in the avian, the newly added morphogenetic process was for the
placode epithelia to proliferate and protrude to form the feather bud. Later, the
epithelium flanking the feather bud invaginated into the dermis to form a follicle. The
follicle forms in hair morphogenesis, too, and is probably the result of convergent
evolution. The follicular structure has many advantages: it is a well-protected sac
where epithelial-mesenchymal interactions can take place, new epithelial cells can
be added to its base, and it provides good anchorage for the longer cutaneous
appendage.
Our results suggest that adhesion molecules are used repeatedly in similar key
morphogenetic steps underlying different developmental processes. In terms of phylogeny, N-CAM appears early and can be detected in the shark. The binding function
is highly conserved; frog N-CAM can bind mouse N-CAM. Indeed, N-CAM is also
expressed in the scale and is distributed in a more diffuse pattern (unpublished
observation). These results are consistent with the hypothesis that during evolution
adhesion molecules such as N-CAM were used in different scenarios when developmental mechanisms were evolved to generate novel structures.
Identijicatton of Molecules Involved in the Formation of Skin Appendages
To demonstrate that a molecule is involved in a morphogenetic process, we have
to show that the molecule and its receptor is expressed during that process, that
overexpression or underexpression of that molecule perturbs the end results, and that
we can reconstitute the molecular sequence in terms of its upstream regulation and
downstream events. We have been using the feather explant culture system as a
model for these analyses. In this explant culture, small dermal condensations develop
into feather buds in 4 days and become feather filaments in 8 days. This provides an
excellent model by which many different cellular processes can be analyzed.
Having shown that several adhesion molecules were indeed expressed in feather
morphogenesis, we tried to perturb feather development with the addition of antibodies to adhesion molecules. We added antibodies to N-CAM, tenascin, and integrin beta subunit. Notably, feather development was inhibited in each instance.
Antibody to integrin had more overall inhibition, but antibody to N-CAM led to
feather buds of different sizes, with the distortion of the hexagonal pattern. These
results suggest that the adhesion molecules are involved in different parts of the
morphogenetic process. The most profound inhibition, however, occurred when a
combination of all three antibodies were used.
FIGURE 5. Expression of adhesion molecules in hair follicles. A, B, N-CAM; C, PNA, D,
tenascin; E, phase contrast. A, C, D, E, from E 17 mice; B, from adult mouse skin. Note that
N-CAM is always present in the dermal papilla (dp) and part of the hair root sheath (hs); mx,
hair matrix. N-CAM is also highly expressed in other connective tissues during development
but has disappeared in the adult (compare A and B). The keratinized hair in the upper left
comer of panel B shows hair shaft autofluorescence. The mesenchymal region flanking the
invagination point, described as the region in which the surface epithelium folds in to form the
follicle, appears to be a special region enriched for PNA and tenascin (C and D, solid arrows).
This region is actually sleeve shaped when seen in three dimensions, as seen by the section
tangential to this “sleeve” (C, open arrow). Magnification: 50x. Ear: 100 pm.
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FIGURE 6. Formation of feather buds on embryonic chicken skin explant cultures. A, C,
stage 32, beginning of cultures; B, D,4 days after culture. A and B are views using a stereo
dissection microscope; C and D are views using a confocal microscope. Note that in the
beginning of cultures there are flat small dermal condensations (dc), which developed into
feather buds protruding out of the surface of the skin. Magnifications: A, B, 25x; C, D, 175x.
Bar: 100 pm.
CHUONG ef al.: ADHESION MOLECULES
275
FIGURE 7. Perturbation of feather bud formation. Stage 32 embryonic chicken dorsal skin
(as shown in FIG.3A) were cultured for 4 days (A) or 8 days (B). C, 4-day culture in the
presence of antibodies to integrin beta chain, N-CAM, and tenascin. D, 4-day culture in the
presence of PDGF BB chain. Note the inhibition of feather development when the activity of
adhesion molecules was reduced. Also note the acceleration of feather growth in the presence
of PDGF. Magnification: 25x. Bar: 100 pm.
In our previous work, antibodies to L-CAMled to the disruption of the hexagonal
pattern and to the fusion of dermal condensations into horizontal stripes.I0 Goetinck
showed that proteoglycan is also involved in this process by using xyloside to obtain
a similar perturbed pattern in the feather explant.= In mouse whisker pad cultures,
use of antibodies to P cadherin and E cadherin separately inhibits whisker growth,
while the two combined have the most notable inhibition.23These studies suggest
that multiple adhesion molecules and extracellular matrix components are involved
in the moxphogenesis of feather and hair. The sequence of events in which they are
involved will have to be determined in the future. Similar multiple involvement of
adhesion molecules in cell migration has been reported in cerebellum granule cell
migration, neurite fasciculation, and neural crest cell migration (reviewed in Ref. 24).
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Growth factors such as EGF have been shown to increase hair growth. Although
previous work has shown mainly the effect of PDGF on mesenchymal ~ e l l s , 2 ~ ~ ~ ~
recent work has shown that PDGF also regulate the proliferation and differentiation
of neural ti~sues.2~
Here our observation of enhancement of feather growth by PDGF
and the presence of PDGF receptor in feather collar represent other examples of
PDGF acting on ectodermal cells. The factors exchanged between dermal papilla and
epidermal collar are most interesting; multiple factors may be involved. To further
explore this area, dermal papilla cultures are required for molecular analyses.
Adhesion Molecules in Dermal Papilla Cells
Dermal papilla cells are unique because they can induce epithelia to grow out a
new feather or hair continuously in the adult. The molecular characterization of this
ability is of central importance to embryonic induction as well as to growth control
of epithelial cell proliferation. We therefore sought to culture these cells. The behavior of dermal papilla cells from feather appears to be similar to that of human hair
dermal papillaI3 and mouse whisker.28They grow slowly and form cellular clusters.
The cells within clusters were small, round, tightly packed, and positive for both
N-CAM and tenascin. The cells outside the clusters were of different shapes and
were mostly negative for N-CAM and tenascin. Further molecular and cellular
characterization with different markers is obviously required. Jahoda and Oliver
have cultured dermal papilla from the whisker of rats and found that cells tend to
form clusters. The loss of the ability to form clusters after several generations in
culture correlates with the loss of these cells to induce new hair growth.28 This is
consistent with our observation that the adhesion molecule N-CAM is involved in
the formation of these clusters. It will be interesting to find out whether N-CAM is
indeed essential for the induction of new hair.
SUMMARY
FIGURE
9 summarizes the morphogenetic process of feather and hair. Hair of
feathers are formed from a layer of homogeneously distributed mesenchymal cells.
The mesenchymal cells start to condense to form foci in response to some
unidentified induction signal (FIG.9B). Several adhesion molecules, including LCAM, N-CAM, integrin, tenascin, as well as proteoglycan, are involved. These
adhesion molecules appear to have different roles in this process, because perturbation with specific antibodies leads to different aborted patterns. Hair or feather
follicles then form following cell proliferation and epithelial invagination (FIG.9C).
FIGURE 8. Dermal papilla cultures. A, B, immunofluorescence stained for N-CAM; C,
phase contrast; D, stained for the presence of tenascin. A, low-power, showing cells forming
clusters (c) that were N-CAM positive. Magnification of part of one cluster double stained for
N-CAM (B, fluorescein) and tenascin (D, Texas Red). Note that the cells inside the cluster
appeared small, round, tightly packed, while the cells surrounding the clusters were flat and
dispersed. N-CAM staining was positive on the cells inside the cluster and appeared to be on
the cell membrane, but was negative on the outside fibroblastic cells. Tenascin was also
enriched inside the cluster with an extracellular matrix staining pattern. Magnification: A, 20x;
B-D,100x. Bur: 100 pm.
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MESENCHYMAL
CONDENSATION
CELL
PROLIFERATION
EPITHELIAL
INVAGI NATlON
DIFFERENTIATION
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CHUONG et al.: ADHESION MOLECULES
The dermal papilla is enriched with N-CAM and tenascin, whereas the feather collar
(equivalent of hair matrix) is enriched with L-CAM and PDGF receptor. Epithelial
cells in the feather collar receive a signal from the dermal papilla and are able to
continue to divide. Several growth factors, such as PDGF and EGF, may be involved.
As epithelial cells are pushed upwards, they differentiate and keratinize in a cylindrical structure into hair. In feather, another morphogenetic event takes place to form
the branched structure. The epithelial cylinder of the feather shaft invaginates to form
rows of cells that die to become space and create the secondary branch or barbs (FIG.
9D). N-CAM is enriched in the cells destined to die and appears to form the border
of cell groups within which the “death signal” is transmitted. In some, but not all,
feathers the same process is repeated, in a way analogous to fractal formation, to
form the tertiary branches or the barbules (FIG. 9E). Thus, in each step of the
morphogenesis of feather and hair, different adhesion molecules are expressed and
are involved in different functions: induction, mesenchymal condensation, epithelial
folding, and cell death, depending on different scenarios. We have just begun to
elucidate these molecular events.
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~
FIGURE 9. Schematic drawing showing adhesion molecules in the development of skin
appendages using N-CAM as an example. A, a piece of ectoderm with mesenchyme underneath. B, the circles represent dermal condensation. N, N-CAM expressing dermal condensation. C, following cell proliferation and epithelial invagination, the major shaft of the skin
appendages, hair, or feather rachis is formed. Large N in panels C-E represents N-CAM
expressing dermal papilla. D, occurring in feather only, the epithelial cylinder forms alternating rows of cells that either die or are keratinized to generate branches (barbs) inserted on the
rachis. E,this process repeats in a way similar to the formation of a fractal. The result is the
tertiary branch (barbules) inserted on the secondary branch. In panels D and E, the smaller and
smallest Ns represent epithelial cells that express N-CAM (marginal and axial plate, respectively) and die, to become spaces between barbs and barbules, respectively. The barb plate and
barbule plate epithelia cells differentiate to express special kinds of keratin and become feather
proper.
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