/. Embryol exp. Morph. Vol. 24, 1, pp. 173-186, 1970
173
Printed in Great Britain
Tissue interactions in embryonic mouse
tooth germs
II. The inductive role of the dental papilla
By EDWARD J. KOLLAR 1 AND GRACE R. BAIRD 2
From the Department of Anatomy and Zoller Dental Clinic,
The University of Chicago
SUMMARY
The response of embryonic mouse dental epithelium and mesoderm to tissues of ectopic
origin was examined. Isolated molar or incisor mesoderm was confronted with epithelium
isolated from the plantar surface of the embryonic mouse foot plate or from the snout.
Harmoniously structured teeth were formed from the foot epithelium and incisor or molar
mesoderm. These data are interpreted as an unequivocal demonstration of the inductive role
of the dental mesenchyme.
Teeth were absent in confrontations of dental mesenchyme and snout epithelium. The
presence of hair follicles in these explants is described and discussed with reference to other
integumental epithelio-mesenchymal interactions.
Dental epithelium forms keratinizing surface-like epithelium and invading bands of
epithelium in association with foot mesoderm; definitive structures are not formed.
On the other hand, when incisor or molar epithelium is associated with snout mesoderm,
hair follicles are seen in addition to keratinizing surface-like epithelial configurations.
The roles of the epithelial and mesenchymal tissues and the nature of epithelio-mesenchymal interactions in the developing mouse integument are discussed.
INTRODUCTION
Previous studies of tooth development have provided a number of important
conclusions. (1) Both epithelium and mesenchyme must be present if tooth
development is to proceed (Huggins, McCarroll &Dahlberg, 1934; Koch, 1967;
Kollar & Baird, 1969). (2) The epithelium and mesoderm will develop typical
matrices if physically separated by Millipore filters (Koch, 1967). (3) The structural specificity for the shape of the tooth germ resides in the mesoderm; typical
incisiform and molariform patterns of cytodifferentiation are directed by the
mesoderm (Kollar & Baird, 1969, 1970a, b). (4) The enamel organs from embryonic mice (Kollar & Baird, 1970#) and rabbits (Glasstone, 1952; Slavkin &
1
Author's address: Department of Anatomy, The University of Chicago, Chicago,
Illinois 60637, U.S.A.
2
Author's address: Zoller Dental Clinic, The University of Chicago, Chicago, Illinois
60637, U.S.A.
174
E. J. KOLLAR AND G. R. BAIRD
Bavetta, 1968) remain plastic and will regulate far into the developmental
period. (5) The lip-furrow epithelium, an epithelium temporally and spatially
related to the incisor enamel organ, is able to regulate into harmonious tooth
germs in the presence of incisor or molar mesoderm. Thus, the dental papilla
can elicit new developmental expressions from the lip-furrow epithelium. These
data imply an inductive interaction between the epithelium and the dental
papilla (Kollar & Baird, 1970a).
These conclusions indicate that tooth development is the product of an
epithelio-mesenchymal tissue interaction similar to interactions described for
many other structures. However, an unequivocal demonstration that dental
papillae induce tooth germs has been lacking.
This investigation describes experiments in which dental papillae from
embryonic mice are associated with epithelium from embryonic snout and foot
plate (plantar surface). The dental papillae induce recognizable tooth structures
in the plantar surface epithelium from the foot plates of embryonic mice;
matrix synthesis is induced in this surface epithelium. In addition, the previously
described versatile developmental performance of the enamel organ (Kollar &
Baird, 1970«) is confirmed and is extended to include the ability of the enamel
epithelium to reorganize into0 surface epithelium and hair follicles when snout
mesoderm is present.
MATERIALS AND METHODS
Tissue sources
Molar and incisor tooth germs, snout skin (upper lip) containing the mystacial vibrissae primordia, and hairless plantar surface of posterior foot plates
were dissected from embryonic C57/10 mouse embryos. Embryonic stages,
spanning 12-16 days of gestation, were determined by the appearance of a
vaginal plug and confirmed by the staging criteria of Griineberg (1943).
Molar and incisor tooth germs from the mandibles of 14- and 16-day-old
embryonic mice were used in this investigation. The in vivo and in vitro development of these tooth germs have been repeatedly described (Cohn, 1957; Hay,
1961; Glasstone, 1967; Kollar & Baird, 1968, 1969, 1970a). The reader should
consult these papers.
The structure and development of mystacial vibrissae follicles (Davidson &
Hardy, 1952; Hardy, 1949, 1951, 1968; Kollar, 1966) as well as the response of
these follicles in tissue culture (Hardy, 1968; Kollar, 1966) have been described
in detail. At 12 and 13 days of embryonic development, the upper lip contains
a number of mystacial follicle primordia. The epithelium thickens to form
placodes in association with mesodermal condensations. The epithelium continues to invade the mesoderm during subsequent days of development and
eventually gives rise to hair follicles with keratinizing hair shafts. The mesodermal condensations are incorporated into the epithelial follicles as dermal
papillae.
Mouse tooth germs. II
175
The plantar surfaces of posterior foot plates of 14- and 15-day-old embryos
were chosen because this epithelium is hairless, and, at this developmental
period, the mesodermal condensations associated with foot pads are beginning
to develop. The dissection of this integumental site was performed so that only
central portions of the plantar surfaces were excised. The tissue was trimmed
so that contaminating skin containing hair follicle primordia was excluded.
Tissue separation and recombination
Trypsinization was accomplished by incubating excised tissue fragments
with a 1 % solution of trypsin (Bacto-Difco, 1:250) in Tyrode's solution at
4 °C for 1-2 h. The 12-day snout skin required 1 h of incubation; 16-day
incisor germs required a minimum of 2 h incubation. This procedure is described
in detail elsewhere (Kollar & Baird, 1969). The epithelial and mesodermal components separate at the basement membrane and can be handled as uncontaminated sheets of epithelium and mesoderm (Kollar, 1966; Kollar, 1970; Kollar
& Baird, 1969, 1970a).
The isolated components were recombined as control and experimental
combinations. The ability of the isolated tooth components to develop in
culture and to produce type specific structures has already been described
(Kollar & Baird, 1968, 1969). Control recombinants of snout epithelium
(SE12_13) and snout mesoderm (SM12_13) as well as the control combinations of
foot epithelium (FE14_15) associated with its homologous foot mesoderm
(FM14_15) were reconstructed. These data are described elsewhere (Kollar, 1970).
Culture methods
The isolated tissue fragments were stored in a solution of foetal bovine
serum and Tyrode (1:1, v/v) and were transferred to Falcon organ culture
dishes containing 1 ml of Eagle's basal medium containing 10 % foetal bovine
serum, 1 % glutamine, 0-4 % agar and 50 units/ml each of penicillin and
streptomycin. The explants were incubated at 37 °C in a humidified 5 % CO 2 95 % air-gas mixture and were allowed to cohere for 1 or 2 days on this medium.
When the explants had re-established a stable junction between the epithelium
and mesoderm and could be transferred safely, they were grafted into the
anterior chambers of homologous mice eyes. The grafted explants were allowed
to grow for 1-2 weeks before harvesting.
Histology
The explants were fixed in Zenker's acetic acid and decalcified with Versene
(Schmidt, 1956). The tissue was paraffin-embedded, serially sectioned at 7 /i,
and stained with hematoxylin and eosin (H and E) or Masson's trichrome
(MT).
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E. J. KOLLAR AND G. R. BAIRD
RESULTS
Recombinations of dental mesoderm and foreign epithelium
(1) Dental mesenchyme combined with foot epithelium
The plantar surface of the posterior foot plate does not produce structurally
complex epithelial adnexa. Thus, this set of experiments provided a crucial test
of the inductive capabilities of the dental mesenchyme. The foot epithelium consistently produced surface epithelium characterized by extensive surface keratinization. The presence of tooth structures in these explants provided an
unequivocal and dramatic demonstration of the inductive properties of the
mesenchyme and the developmental plasticity of the epithelium.
When 15-day-old molar mesoderm was confronted with foot epithelium
from 15-day-old embryos (MM15FE15) and allowed to grow in the eye for 2
weeks unmistakable teeth were produced (Fig. 1A-C). Similarly, when younger
tissue components were combined (IM14FE14) tooth structures were recovered
after 2 weeks of growth (Fig. ID).
The tooth structures could be serially traced to the heavily keratinizing
surface-like epithelial cysts and did not occur in isolation from the grafted epithelium. The deposition of matrix and the formation of dentin and enamel were
harmoniously structured.
(2) Dental mesenchyme combined with snout epithelium
The development of snout epithelium confronted by dental mesenchyme was
restricted to a more characteristic surface-like epithelial pattern. Stratified
Scale line represents 50/* throughout except Fig. 1E.
FIGURE 1
(A) An harmonious tooth developing after 2 weeks in the anterior chamber from
an explant of molar mesoderm and plantar surface foot epithelium (MM15FE15)
from a 15-day-old embryo. Note the keratinizing epithelium (k) in close association
with the tooth germ. MT. x 300.
(B) A tooth-germ construction formed after 2 weeks in the anterior chamber. This
graft was composed of molar mesenchyme and foot epithelium (MM15FE15) from
a 15-day-old embryo. H and E. x 600.
(C) An harmonious tooth germ contiguous with a heavily keratinizing epithelium
(k). This explant (MM15FE15) was grown in the anterior chamber for two weeks.
MT. x300.
(D) A tooth primordium developing in an explant (IM14FE14) after 2 weeks in
the anterior chamber. Note the keratinizing epithelium (k) and an additional area
suggestive of a tissue interaction (/). H and E. x 300.
(E) Integumental adnexa developing in a graft of dental mesenchyme and snout
epithelium (IM14SE12) 2 weeks after explantation. Note the surface-like epithelium
(se), the hair follicle with papilla (p) and keratinizing hair shaft (ks), and the
sebaceous gland (sg). H and E. x 1000. Scale line = 15//.
(F) Hair follicles developing in an explant of molar mesenchyme and snout epithelium (MM14SE12) after 1 week in culture. MT. x 750.
111
Mouse tooth germs. II
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E. J. KOLLAR AND G. R. BAIRD
keratinizing epithelium and hair follicles were present in these grafts (Fig. 1E,
F). Sebaceous glands were associated with the hair follicles. Because the presence
of hair follicles in grafts composed of snout epithelium and dental mesenchyme
were unexpected and of unusual interest, these experiments were repeated. The
dental mesenchyme was severely trimmed in order to ensure that no other
mandibular mesenchyme was present except the papilla and a narrow strip of
surrounding mesenchyme. These experiments confirmed our earlier finding;
hair follicles were present in these explants as well.
Recombinations of dental epithelium and foreign mesenchyme
(1) Dental epithelium combined with foot mesenchyme
The dental epithelium developed surface-like epithelial sheets; stratification
and heavy keratinization were present. In addition, the basal layer of these
epithelial sheets underwent extensive downgrowths into the foreign foot mesoderm (Fig. 2 A, B). The pattern of collagen deposition was of interest in these
grafts. The invading epithelium appeared to be walled off by heavy collagen
deposition around the invading epithelium (Fig. 2C, D).
Several grafts provided insights into the source of the epithelial proliferation
(Fig. 2C, E). When the plane of section was favorable, it was possible to trace
the epithelial downgrowths serially to a section that appeared to be the original
grafted enamel organ. For example, extensive epithelial proliferation appeared
to originate from a tissue configuration similar to the enamel organ (Fig. 2E).
Note that in the enamel organ-like configuration the cellular pattern includes
a stellate reticulum. Similar observations were made when the incisor enamel
organ (IE15FM15) was the source of the epithelium confronted by ectopic foot
mesoderm (Fig. 2C).
Of unusual interest in these observations was the source of the proliferation
from these enamel organ-like tissue patterns. The proliferation was most
extensive from the outer enamel epithelium. Similar proliferations were seen
along the inner enamel epithelium, but they were not as extensive as those
apparently derived from the outer enamel epithelium.
(2) Dental epithelium combined with snout skin mesenchyme
The dental epithelium confronted with snout mesenchyme behaved in a
fashion similar to its response to foot mesenchyme. Incisor or molar enamel
organ epithelium from 14- and 15-day embryonic tooth germs displayed remarkable proliferative and invasive properties when associated with mesenchyme from 12- or 13-day-old embryonic snout skin. The dental epithelium
produced keratinizing surface-like epithelium and deeply invaginating tongues
of epithelium. The pattern of epithelial invasion into this ectopic mesenchyme
resembled abortive enamel organ formations (Fig. 3A-C). At no time, however, did the dental epithelium or the snout mesenchyme suggest cellular
Mouse tooth germs. II
179
FIGURE 2
(A) This explant consists of molar epithelium and foot mesoderm and displays the
extensive and random invasion of the mesoderm by dental epithelium. MT. x 750.
(B) Incisor enamel organ epithelium invades the foot mesoderm incorporating
small clusters of mesodermal cells. MT. x 750.
(C) Heavy deposition of collagen (c) at the interface between incisor epithelium
and foot mesoderm. MT. x 750.
(D) Collagen deposition (c) at the interface between molar epithelium and foot
mesoderm. MT. x 500.
(E) An enamel organ-like epithelial configuration. Note the surface epithelium
(se), the stellate reticulum-like (sr) epithelium and the overall shape of the epithelium.
Epithelial proliferation can be seen budding from the enamel organ-like structure.
This explant (ME15FM15) was grown for 2 weeks. MT. x 750.
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E. J. KOLLAR AND G. R. BAIRD
patterns typical of the inner enamel organ or an odontoblast layer of the mesenchyme; tooth structures were never seen in these explants. However, hair
follicles were observed (Fig. 3D-F). The follicles were both pelage- and vibrissallike. Occasionally, the follicle structure appeared aberrant, but, even in these
cases, keratinizing hair shafts were present (Fig. 3E, F). In addition to the
hair follicles, occasional small clusters of sebaceous glands were observed.
DISCUSSION
Induction by the dental papilla
The murine dental papilla elicits new developmental expressions from
ectopic embryonic mouse epithelium. The deposition of dentin and enamel
matrices in histotypic patterns clearly recognizable as tooth constructions indicates that the dental papillae act inductively during the ontogeny of teeth. Thus,
these data provide a stringent test in that an unrelated non-dental epithelium in
the presence of mesodermal tissue produces structural equivalents of the epithelium homologous to the inducing mesoderm. These data extend the observations of Lillie & Wang (1941, 1944), Wang (1943), Cairns & Saunders (1954),
Gomot (1958), Rawles (1963), Sengel (1964), and others who have demonstrated
the inductive nature of the local mesodermal components in the integument,
[ndeed, studies of epithelio-mesenchymal interactions in general confirm the
notion that the mesodermal component is necessary for the induction and
maintenance of ectodermal or endodermal structures during the early stages of
development (see Wessells (1967), Grobstein (1967) and McLoughlin (1968)
for reviews of the properties of epithelio-mesenchymal interactions in embryonic systems).
These data confirm our earlier data (Kollar & Baird, 1970 a) which showed
that a more closely related epithelium, lip-furrow epithelium, can participate
with the dental mesoderm to produce perfectly harmonious teeth. It is of interest
FIGURE 3
(A) This explant (ME15SM12) demonstrates the invasion of the molar epithelium
into snout mesoderm. MT. x 625.
(B) Incisor epithelium incorporating snout mesoderm in an explant (IE14SM12)
grown for 2 weeks. H and E. x 625.
(C) Incisor epithelium invading snout mesoderm (IE14SM12) in a fashion reminiscent of an enamel organ. H and E. x 750.
(D) Hair follicles and surface-like keratinizing epithelium derived from an explant of incisor epithelium and snout mesoderm (IE14SM12). H and E. x 300.
(E) An aberrant hair follicle developing in an explant (ME15SM12) composed of
molar epithelium and snout mesoderm. Note the papilla (p) and keratinizing hair
shaft (ks). H and E. x 750.
(F) A detail from Fig. E demonstrating the keratinized hair shaft. H and E. x 2800.
Mouse tooth germs. II
181
182
E. J. KOLLAR AND G. R. BA1RD
that the structural harmony of the tooth germs induced in the lip-furrow epithelium is often more obvious than that produced in the foot epithelium. There
is no doubt, however, that tooth structures are produced from the ectopic surface epithelium of the foot plate, although dental constructions easily scored
as incisiform or molariform are less obvious in these latter experimental teeth.
None the less, the production of dentin and enamel matrices in normal configurations, cytodifferentiation of the epithelium into a basal layer of cells with
tall columnar cells that undergo a reversal in nuclear polarity, and specialized
secretory activity are compelling demonstrations of mesodermal induction in
interacting embryonic systems.
In contrast, the absence of tooth structures in experimental confrontations
consisting of dental mesoderm and snout epithelium requires cautious interpretation. Despite the optimal culture conditions afforded by the intraocular site,
the snout epithelium remained refractory to the inductive influence of the dental
mesoderm. Certainly, the data of Rawles (1963) that clearly establish the importance of tissue age and the establishment of developmental stability in the
epithelium must be considered here. It may be inferred from the present data
that snout epithelium at 12 and 13 days of gestation has already stabilized and
cannot respond to the inductive activity of dental mesoderm. Clearly, these
negative data do not diminish the impact of the demonstration of the inductive
role of dental mesoderm; rather, this series of experiments suggests that snoutskin epithelium must be examined at earlier ages.
In addition, this series of experiments can be viewed as an additional control
for other combinations of dental mesoderm and ectopic epithelium. Since the
dental mesoderm from a single litter was combined with snout and foot epithelium, the ectopic teeth formed by the foot epithelium cannot be the result of
random contamination of the dental mesoderm with dental epithelial fragments. The complete absence of tooth structures or developing fragments of
contaminating dental epithelia in the snout epithelium-dental mesoderm confrontations provides additional confirmation of the effectiveness and reliability
of tissue separations after tryptic digestion.
The development of hair follicles in those explants composed of snout epithelium and dental mesenchyme provides further insights into the developmental performance of these two tissues. The persistence of hair follicles in
carefully controlled experiments that excluded mandibular mesenchyme with
potential hair-follicle-inducing qualities and the absence of hair follicles in
other combinations of dental mesenchyme with homologous or heterologous
epithelia confirm the view that snout epithelium and dental mesenchyme can
participate in hair follicle development. At first view these data appear contradictory; however, these data are supported by similar observations in developing
chick skin.
Rawles (1963) demonstrated that in the closely related avian feather-bearing
dorsal skin and scaled metatarsal skin, developmentally advanced and stabilized
Mouse tooth germs. II
183
back skin produces typical feathers when confronted with mesoderm from the
scale-bearing mesoderm from the metatarsal region. In contrast, metatarsal
mesoderm induces scales in younger feather-producing back skin. These data
lead to the conclusion that a stabilized epithelium can produce type-specific
structures when associated with an ectopic but related mesoderm.
The similarities between the snout hair follicle and dental structures must be
considered in this context. The developmental origins of the snout and mandibular mesenchyme from the cranial neural crest, the similar trigeminal innervation to vibrissae follicles and teeth, and the anatomically similar developmental sequence in the initial development of the primordia of these dissimilar
structures suggest that the phenomenon reported here may be similar to that
described in avian skin. Thus, if 12- and 13-day-old embryonic snout epithelium
has indeed stabilized and lost its regulative capabilities, the response of this
epithelium to dental mesenchyme would be to produce type specific structures:
hair follicles. This view must await confirmation from the response of 11-day
snout epithelium confronted by dental mesenchyme. Younger, and therefore
more plastic, epithelium should respond in predictable fashion; in the presence of
dental mesenchyme, the more labile epithelium should produce dental structures.
These data demonstrate the inductive and supportive qualities of the mesenchyme in developing integumental systems and, once again, emphasize the
importance of examining the temporal and spatial parameters that influence
epithelio-mesenchymal interactions.
The response of the dental epithelium to foreign mesoderm
The dental epithelium does not produce recognizable structures when associated with foot mesoderm. Instead, the epithelium invades the mesenchyme
and proliferates as crenulated epithelial bands. The observation that the epithelial proliferation originates, in part, from the outer epithelium of the enamel
organ supports the view that this epithelium retains the properties of the stratum
germinativum, and adds further support to the notion that during reconstruction of harmonious teeth from fragments of the enamel organ (Kollar &
Baird, 1970tf, b) or in heterologous combinations of dental mesenchyme and
enamel organs (Kollar & Baird, 1969) the outer enamel epithelium may contribute to the reorganization of the explanted dental epithelium.
The dental epithelium can participate in hair-shaft production when confronted by snout mesoderm containing the mystacial vibrissae dermal condensations. This ability of the dental epithelium to organize into new histotypic
patterns, to suppress enamel matrix synthesis, and, instead, to produce
organized keratinizing hair shafts and sebaceous glands, corroborates the
inductive role of the vibrissae dermal condensations (Kollar, 1966, 1970) and
the plasticity of the enamel organ epithelium. Our previous findings (Kollar
& Baird, 1970 a) that the enamel organs of incisor germs from 16-day-old
embryos can reconstruct complete tooth germs and keratinizing surface epithelia
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E. J. KOLLAR AND G. R. BAIRD
are supported by these new observations of regulative ability in the dental
epithelium.
The pattern of epithelial invasion into an ectopic mesoderm also provides
some insight into the nature of the interactions between the epithelium and the
mesoderm. In those cases in which the dental epithelium is confronted with
snout mesoderm the topography of the epithelium resembles an enamel organ
rather than the basal layer of surface epithelium with follicle adnexa. The epithelium invades the mesoderm as crenulated tongues of epithelium incorporating mesodermal cells into the invaginations of the epithelium. This pattern of
epithelial invasion is suggestive of the invasion pattern of the enamel organ.
Often these structures were induced to form keratinizing hair shafts and
sebaceous glands. Although some of these tissue configurations are not normal
hair follicles, many other follicle structures are harmonious in all respects.
Similarly, the dental epithelium invades the mesoderm of the foot plate in a
fashion reminiscent of the enamel organ.
These data suggest that the invasiveness of the epithelium in relation to the
mesoderm may be determined, in part, by inherent properties of the epithelium.
On the other hand, the random nature of the epithelial invasion in these heterologous combinations suggests that the usual incisiform, molariform and follicle
patterns are stabilized by some properties of homologous or closely related
mesoderm. Thus, appropriate structural relationships are established by an
interplay between the invasive properties of the epithelium and the modeling
properties of the local inductive mesoderm. The depth of epithelial invasion, the
definitive shape of the invading epithelium, and the spatial relationship of the
epithelium to the inductive papilla appear to be determined by the mesoderm.
The data discussed here suggest that, as suspected, the development of teeth
is the result of an epithelio-mesenchymal interaction not unlike many other
developing systems in the avian and mammalian embryo. Once again, the lack
of information concerning the processes involved in the inductive event is the
most intriguing aspect of this problem.
RESUME
Interactions tissulaires des germes dent aires de Souris.
If. Le role indueteur de la pap i lie dent a ire
La reponse de l'epithelium dentaire de l'embryon de Souris ainsi que du mesoderme a
l'egard de tissus d'origine ectopique a ete examinee. Du mesoderme molaire ou incisif isole
a ete combine a de l'epithelium isole de la surface de la plaque plantaire d'embryon de souris
ou a partir du museau.
Des dents harmonieusement constitutes ont ete formees a partir de l'epithelium de la patte
combine a du mesoderme incisif ou molaire. Ces resultats sont interpretes comme une demonstration non equivoque du role inducteur du mesenchyme dentaire.
Des dents ne se sont pas formees a la suite de la combinaison de mesoderme dentaire et de
l'epithelium du museau. La presence de follicule pileux dans ces explants est decrite et discutee en relation avec les autres interactions epithelium-mesenchyme.
Mouse tooth germs. II
185
L'epithelium dentaire forme un epithelium a keratinisation de surface et des travees
epitheliales profondes en reponse a la combinaison avec le mesoderme du pied; des structures
deflnies ne sont pas formees.
Par ailleurs, lorsque l'epithelium incisif ou molaire est associe a du mesoderme du museau,
on voit des follicules pileux s'ajouter aux formations epitheliales keratinisees en surface.
Les roles des tissus epitheliaux et mesenchymateux et la nature des interactions epitheliummesenchyme dans le developpement du tegument de la souris sont discutes.
The authors wish to thank Dr Benson E. Ginsburg who generously made available his
animal colony and animal care facilities. This research was supported by a grant from the
American Cancer Society (ACS-IN-41-H). Finally, we wish to thank the Inland SteelRyerson Foundation for a Faculty Fellowship to E.J.K.
REFERENCES
CAIRNS, J. M. & SAUNDERS, J. W., JR. (1954), The influence of embryonic mesoderm on the
regional specification of epidermal derivatives in the chick. /. exp. Zool. 127, 221-248.
COHN, S. A. (1957). Development of molar teeth in the albino mouse. Am. J. Anat. 101,
295-320.
DAVIDSON, P. & HARDY, M. H. (1952). The development of mouse vibrissae in vivo and in
vitro. J. Anat. 86, 342-356.
GLASSTONE, S. (1952). The development of halved tooth germs; a study in experimental
embryology. J. Anat. 86, 12-15.
GLASSTONE, S. (1967). Morphodifferentiation of teeth in embryonic mandibular segments in
tissue culture. J. dent. Res. 46, 611-614.
GOMOT, L. (1958). Interaction ectoderme-mesoderme dans la formation des invaginations
uropygiennes des Oiseaux. /. Embryol. exp. Morph. 6, 162-170.
GROBSTEIN, C. (1967). Mechanisms of organogenetic tissue interaction. Natn. Cancer Jnst.
Monogr. no. 26, 279-299.
GRUNEBERG, H. (1943). The development of some external features in mouse embryos.
/. Hered. 34, 89-92.
HARDY, M. H. (1949). The development of mouse hair in vitro with some observations on
pigmentation. /. Anat. 83, 364-384.
HARDY, M. H. (1951). The development of pelage hairs and vibrissae from skin in tissue
culture. Ann. N.Y. Acad. Sci. 53, 546-551.
HARDY, M. H. (1968). Glandular metaplasia of hair follicles and other responses to vitamin
A excess in cultures of rodent skin. /. Embryol. exp. Morph. 19, 157-180.
HAY, M. F. (1961). The development in vivo and in vitro of the lower incisor and molars of
the mouse. Archs oral Biol. 3, 86-109.
HUGGINS, C. B., MCCARROLL, H. R. & DAHLBERG, A. A. (1934). Transplantation of tooth
germ elements and the experimental heterotopic formation of dentin and enamel. J. exp.
Med. 60, 199-210.
KOCH, W. E. (1967). /// vitro differentiation of tooth rudiments of embryonic mice. I. Transfilter interaction of embryonic incisor tissues. /. exp. Zool. 165, 155-170.
KOLLAR, E. J. (1966). An in vitro study of hair and vibrissae development in embryonic
mouse skin. /. invest. Derm. 46, 254-262.
KOLLAR, E. J. (1970). The induction of hair follicles by dermal papillae./, invest. Derm.
(In the Press.)
KOLLAR, E. J. & BAIRD, G. R. (1968). Effect of beta-2-thienylalanine on developing mouse
tooth germs in vitro. J. dent. Res. 47, 433-443.
KOLLAR, E. j . & BAIRD, G. R. (1969). The influence of the dental papilla on the development
of tooth shape in embryonic mouse tooth germs. /. Embryol. exp. Morph. 21, 131-148.
KOLLAR, E. J. & BAIRD, G. R. (1970a). Tissue interaction in embryonic mouse tooth germs.
I. Reorganization of the dental epithelium during tooth germ reconstruction. J. Embryol.
exp. Morph. 24, 159-171.
186
E. J. KOLLAR AND G. R. BAIRD
E. J. & BAIRD, G. R. (19706). Tissue interactions in developing mouse tooth
germs. In Studies in Dental Morphology (ed. A. Dahlberg), The University of Chicago
Press. (In the Press.)
LILLIE, F. R. & WANG, H. (1941). Physiology of development of the feather. V. Experimental morphogenesis. Physiol. Zool. 14, .103-133.
LILLIE, F. R. & WANG, H. (1944). Physiology and development of the feather. VII. An
experimental study of induction. Physiol. Zool. 17, 1-31.
MCLOUGHLIN, C. B. (1968). Interaction of epidermis with various types of foreign mesenchyme. In Epithelial-Mesenchymal Interactions (ed. R. Fleischmajer & R. E. Billingham),
pp. 244-251. Baltimore: The Williams and Wilkins Co.
RAWLES, M. E. (1963). Tissue interactions in scale and feather development as studied in
dermal-epidermal recombinations. J. Embryol. exp. Morph. 11, 765-789.
SCHMIDT, R. W. (1956). Simultaneous fixation and decalcification of tissue. Lab. Invest. 5,
306-307.
SENGEL, P. (1964). The determinism of the skin and cutaneous appendages of the chick
embryo. In The Epidermis (ed. Wm. Montagna & W. C. Lobitz, Jr.), pp. 15-34. New York
and London: Academic Press.
SLAVKIN, H. C. & BAVETTA, L. A. (1968). Odontogenic epithelial-mesenchymal interactions
in vitro. J. dent. Res. 47, 779-785.
WANG, H. (1943). Morphogenetic functions of the epidermal and dermal components of the
papilla in feather regeneration. Physiol. Zool. 16, 325.
WESSELLS, N. K. (1967). Differentiation of epidermis and epidermal derivatives. New Engl. J.
Med. 211, 21-33.
KOLLAR,
(Manuscript received 11 August 1969)