1. ANIMAL MODELS IN PERIODONTICS
• INTRODUCTION
• NEED FOR THE ANIMAL MODELS
• LIMITATIONS OF ANIMAL MODELS
• CLASSIFICATION
• VARIOUS ANIMAL MODELS
• CONCLUSION
• REFERENCES
INRODUCTION
To achieve an understandining of the life process, animals are experimented since
long. This may be about the animals themselves, their physiology, their diseases and
their treatment or behavior. Much of the knowledge is sought in the hope that it may be
applicable to humans. In the field of periodontics, the first report appears to be that of
Talbott (1899), who described periodontitis in mongrel dogs. For over hundred years,
periodontal diseases have been studied in many species and a wealth of dependable data
about periodontitis in species other than human exists.
NEED FOR THE ANIMAL MODELS
1. For ethical reasons, initiation and progression of periodontal disease as well as
certain types of periodontal treatment cannot be studied in humans. Animal data
can provide us with models of biologic trends before proceeding to human
application.
2. Human periodontal disease is extremely difficult to study. The number of
cultivable bacterial species in subgingival plaque exceeds 300, and the technical
and conceptual problems involved in finding the etiologic agents among these
1
2. species are enormous. This inability to examine initiation and progression of
Periodontal Disease has led to a great interest in the use of animal models in
periodontal research. Human longitudinal studies of periodontal disease pose
many problems such as determining the level of disease activity, individuals at
risk, and susceptibility of disease progression.
3. Furthermore, periodontal disease can only be studied retrospectively in man,
since reliable clinical markers for ongoing tissue destruction (disease activity) are
not available. Therefore, an animal model in which selected microbiological and
immunological parameters can be studied prospectively is desirable.
4. Animal models have been used to evaluate various periodontal treatment
modalities like regenerative procedures like bone grafts and GTR, and implant
surgical procedures to study their safety and efficacy.
5. Eventhough, there are computer models and cell cultures, as well as other adjunct
research methods, these methods are used to screen and determine the toxic
potential of a substance in the early stages of investigation. The final test,
however, has to be done in a whole, living system. Even the most sophisticated
technology cannot mimic the complicated interactions among cells, tissues and
organs that occur in humans and animals. Scientists must understand these
interactions before introducing a new treatment or substance into humans.
6. There are striking similarities between the physiological systems of humans and
various species of animals. For example, much of what we know about the
immune system has come from studies with mice, and much of what we know
about the cardiovascular system has come from studies with dogs.
7. Research results from animals also provide the information necessary to design
human trials that must be completed for legal approval of new devices, drugs or
procedures. It is important to be able to gauge how a new drug or procedure will
affect a whole biological system before using it on humans. This is critical for
scientific as well as ethical reasons. Laboratory animals are an integral part of the
research process. In fact, virtually every major medical advance of the last
century is due, in part, to research with animals.
LIMITATIONS OF ANIMAL MODELS
2
3. 1. Animal research and its value to human experience remain controversial.
Regardless of how much data can be presented, it is, a priorly, impossible to
expect different species to respond identically or even similarly to the same
challenge except within very narrow limits.
2. There are very strong economic incentives to replace animals with computers or
other adjunct methods. Research animals are very expensive to acquire and care
for and are only used because no alternatives currently exist.
3. Features of periodontal diseases in humans and animals vary greatly depending
upon which form of the disease is present and the stage of the development.
4. Genetic background of many of the animals has not been established.
5. Animals used in research are often wild-captured animals, with heterogeneity in
age, body weight and oral and general health conditions.
CLASSIFICATION
I] Small and inexpensive rodents
Eg: Mice, Rats, Hamsters, Minks.
II] Larger animals
Eg: Dogs and sheep.
III] Non-Human primates-
Eg: Baboon, Macaque, Chimpanzee and Gorilla
IV] Various other species include
Apes, Cats, Horses, Guinea pigs, Hogs, Mongooses, Wolves, Foxes, Rabbits, Ferret etc.
ANIMAL MODELS
The most convincing animal model would be one in which all aspects of the disease are
analogous to periodontitis in human. The data make abundantly clear that while many of
the manifestations of periodontal diseases in humans are observed to some degree in
other species and no analog of human diseases exists. Varieties of mammals have been
studied in the search for satisfactory animal models.
MICE
3
4. Mice represent the primary species used in research, comprising 67% of all animals used
in biomedical research and testing. Today, the laboratory mouse is recognized as the
preeminent model for modern genetic research.
Advantages
1. Small size and short life span.
2. Proclivity for reproduction
3. Known age and genetic background.
4. Controllable microflora, resistance to other diseases.
5. Minimal expense for purchase and maintenance has made them a
desirable animal model.
They have typical rodent dentition with the formula I 1/1, C 0/0, Pm 0/0, M 3/3.
The periodontal tissues of the continuously growing incisors are rarely affected by
periodontitis. Only the molar tissues are commonly affected, bone loss, and abundant
amounts of microbial plaque, including filamentous organisms were seen at about 10
weeks of age.
As soon as functional occlusion is attained, the crown is being worn down with
relative rapidity because of the enamel free areas on the cusps. To compensate for the
occlusal wear, there is a gradual deposition of cellular cementum at the apical end of
each tooth, which keeps the teeth in occlusion. The formation of cellular cementum is
so pronounced in the mature molar, that there is a distinct hypercementosis at each
root tip.
The direction of eruption is bucco-occlusal. Accommodating this eruption, the buccal
plate undergoes resorption along the periodontal surface, and opposition along the
periosteal surface of lingual plate and along the fundus of the alveolus. Also, the
junctional epithelium shifts apically onto the root surface with age. Consequently, the
distance between the crest of the alveolus and the CEJ increases, particularly at the
lingual and palatal aspects of the mouse molars. Bear et al 1964 using periodontal
disease resistant germ free mice and found that above mentioned bone changes are
due to physiologic changes and not due to periodontal disease. The distance between
the CEJ and the alveolar bone crest, while increasing with age, was always about
twice as long on the lingual aspects of the molars as on the buccal. It is apparent that
periodontitis had not developed upto 1 year. It was only in very old animals, 365-450
days, that periodontitis appeared and even then with only moderate bone loss and
irregular, shallow pocketing.
4
5. Some strains are relatively resistant to periodontitis DBA/2JN, C57L/ HeN and Swis
albino BNL mouse is highly resistant to periodontitis.
In some strain A/LN, A/HeN the periodontal lesions were related to massive hair
impaction in to the sulcus
Only two strains STR/N, BRSUNT/N developed periodontitis regularly.
Grey lethal mouse strain seems to have genetic susceptibility to periodontal disease.
RATS (Rattus norveigicus):
Normal oral structure and physiology and pathogenesis of periodontal diseases have been
studied more extensively in rat than in any other rodent.
PERIODONTAL FEATURES OF THE RAT
Periodontal Anatomy and Physiology
Rats have 1 set of teeth consisting of 1 incisor that is rootless and 3 molars in each
quadrant (I 1/1, C 0/0, Pm 0/0, M 3/3). The incisors are rootless, continuously growing
teeth and, therefore, unsuitable as models for human periodontal disease. In contrast, the
structure and organization of the periodontal tissues of the molars (oral gingival
epithelium, oral sulcular epithelium, junctional epithelium, periodontal collagen fibres,
acellular and cellular cementum, and alveolar bone) are very similar in rats and humans.
The only major difference is that the gingival sulcular epithelium of the rat is keratinized.
Theoretically, this tissue structure could interfere with the movement of bacterial
metabolites into the gingival connective tissue, thereby preventing the initiation of an
inflammatory response. However, recent studies have shown that material placed in the
gingival sulcus swiftly enters the connective tissue via the junctional epithelium. Thus,
there is no reason to believe that the gingival barrier function is fundamentally different
in rat and man, although the extent of the affected area may be greater in man.
Keratinization could also affect the adhesion of certain bacteria to the epithelium, but
experiments have shown that a substantial number of periodontal pathogens are able to
colonize the dentogingival area of rats as well as humans. Therefore, it seems
unwarranted to discard the rat model because of the keratinized sulcular epithelium.
Usually, all molars are fully erupted when the rats are 5 weeks old. After that time a slow
passive eruption is reported in relation to attrition of the occlusal surfaces of the teeth.
With age, as interproximal attrition proceeds, the molars also drift occlusal-distal-
buccal direction and the junctional epithelium in the apical direction, which in the
normal course of aging covers coronal portions of the root cementum in germ free as
5
6. well as in conventional rats. Concurrently the interdental bone is narrowed. Along with
this migration of the teeth, the alveolar bone is continuously being remodeled. The
distance between the cemento-enamel junction (CEJ) and alveolar bone crest (ABC)
remains constant on buccal surfaces in rats without periodontal disease, whereas an age-
dependent physiological increase in the distance can be observed in some lingual and
palatal sites. In order to avoid confounding of the results by such physiological bone
remodeling processes, it is crucial that all rats in a periodontitis experiment are of similar
age.
Pathogenesis of Periodontal Disease
The most frequently used inbred strains of rats include the Wistar albino, Lewis,
Norwegian grey, Rice, Wistar, CD and CDF- Fisher 344 (fairly disease resistant) and
several strains of Sprague-Dawley rat. The most frequently used rat strain in
periodontitis studies is the Sprague-Dawley strain, but other strains have also been used
successfully. It seems probable that rat strains may differ with respect to susceptibility to
periodontitis, but no experimental data are available on the subject. Periodontal disease
may develop in rats in relation to indigenous plaque, to experimentally introduced
microorganisms, or to experimentally introduced bacterial products. In rats, however,
periodontal destruction occurs rapidly without ligatures, and there is no reason to add a
traumatic lesion to the bacterially induced lesion in the rat model. The immunological
status of the host at the time of introduction of periodontal pathogens is important for the
development of periodontitis.
The clinical and histological findings in experimental periodontal disease in rats
are similar to findings in man. Clinically, gingival bleeding upon gentle probing can be
seen in rats a few days after the introduction of periodontal pathogens. Histologically,
the junctional epithelium gradually undergoes pathologic changes, including rete peg
formation, ulceration, and apical migration of epithelial attachment. An inflammatory
cell infiltrate containing T and B-lymphocytes, macrophages, and polymorphonuclear
leukocytes (PMN) appears in the connective tissue, and PMNs migrate through the
epithelium into the gingival sulcus. Plasma cells can be inconspicuous in early stages of
the disease, but with time they become very prominent. Damage to collagen fibers and
fibroblast also occurs. Periodontal bone loss occurs rapidly in rats. Significant bone
destruction has been reported 42 days after inoculation, and the lesions progress
considerably between 60 and 90 days after infection; experiments are rarely extended
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7. beyond 100 days. Occasionally, bone loss may occur without apical migration of
junctional epithelium and loss of connective tissue attachment.
One of the most striking characteristics of the rat periodontium is the heavy impaction of
hair and feeding material that may occur in both germ-free and infected rats. Impaction
of foreign material seems closely related to loss of bone and attachment. It may be local
contributing factor; they act as a syringe effect providing direct pathway for bacteria and
their metabolites to reach deep portions of the soft periodontal tissues. A higher
frequency and greater severity of abnormality was noted in mandible than in maxilla
HAMSTERS
Hamsters account for 0.6% (approximately 500,000 used per year) of the total
number of animals used in research annually. There are more than 15 species of
hamsters, but the one used most frequently in biomedical research is the Syrian
(golden) hamster, Mesocricetus auratus.
Hamsters have the same teeth formula as rats (I 1/1, C 0/0, Pm 0/0, M 3/3) with a
continuously erupting incisor and can open their mouths almost 180 degrees wide
(Navia 1977).
The molars differ form those of rats and mice in that their crowns are completely
covered by enamel and the apical half of the molar roots has more dentin core and
less cellular cementum than in the rat. These differences imply that hamster molars
are less subject to occlusal wear and may continue their eruption to a lesser degree
that does the rat molars.
Hamsters there is molar shifting similar to that occurring in rats and mice, resulting
in an age-related change of the topographical relationship between the teeth and their
sockets, i.e., an increasing distance between the teeth and their sockets, i.e., an
increasing distance between the CEJ and the alveolar bone crest, particularly on the
palatal and lingual side of the molars.
Hamsters have been used to demonstrate the transmissibility of periodontal disease
with plaque develop is similar to rats in that there is primarily gingival retraction
with horizontal bone loss, the interdental septum being too narrow to induce
infrabony defects. Inflammation is not a prominent feature, as it is seen in humans.
In the hamster, periodontal disease is a result of experimental and highly artificial
conditions and, in general, it does not seem to occur in animals living in a natural
habitat. Albino hamsters remain essentially disease-free while the golden and cream-
7
8. colored hamsters develop spontaneous periodontal disease when fed a high
carbohydrate diet (King & Rowles 1955). They naturally harbor an infectious agent
capable of inducing the disease when experimental conditions are favorable. The
disease can be induced in non-infected albinos by inoculating subgingival plaque
from affected hamsters, and can be transmitted from generation to generation (Keys
& Jordan 1964). Subepithelial inflammatory response characteristic of human
gingivitis has not been identified for periodontal disease in the hamster.
Hamsters have been used primarily for caries research due to the capability of the
cariogenic microorganisms to form profuse amount of plaque and quickly develop
carious lesions. Actinomyces viscosus is prominent bacteria in diseased hamsters.
Hamsters do not exhibit the wide spectrum of spontaneous overt and latent diseases
common to rats and mice. Their good general health, their susceptibility to induced
disease conditions, the low cost of production and maintenance, and literature
available on the biology and physiology of these species make them useful animal
models. In conclusion, periodontal disease in the hamster resembles in type, features,
and pathogenicity very much what has been observed in the rat.
MINKS
The dentition of the adult mink is typical of the order Carnivora and is represented by
the formula I 3/3, C 1/1, Pm 3/3, M 1/2. Characteristic dental features of mink
include the presence of diastema distal to the maxillary third incisors, which
accommodate the mandibular canines when the jaws are closed and unusually large
maxillary third premolars and mandibular first molars. The vestibule of the upper jaw
is deep and has a wide band of attached gingival anteriorly which gradually narrows
posteriorly; the upper lip is flexible. The lower lip is firmly attached near the gingival
margins at the mandibular incisors, canines and premolars, so that there is a very
shallow vestibule and the band of gingiva is less than 1mm wide. However, the
vestibular depth and the zone of attached gingival in the third premolar and molar
regions are comparable to those of the upper jaw. Severe abrasion and numerous
tooth fractures, probably associated with cage chewing, are often noted. Frequently,
the palatal surface of the maxillary canines and the occlusal surfaces of the maxillary
and mandibular first and second premolars are worn away to the gingival margin.
Exposing the pulps.
8
9. The alveolar crest around the posterior teeth is almost level with the closely
approximates the CEJs. The interdental contour of the alveolar crest in the posterior
segment of the maxilla is usually flat, while in the mandible the crest rises to a slight
peak between the premolars and molars. The alveolar margin of the mandibular
canines and third incisors is slightly scalloped, but towards the midline it tapers to
create a V-shaped groove between the central incisors. The pulp chambers come very
close to the occlusal surfaces of the teeth and slight amounts of tooth wear result in
exposure, pulpal death and periapical lesions.
Minks exhibit variety of periodontal alterations extending from a clinically normal
appearance to one of massive inflammation of the marginal and attached gingiva
with tooth exfoliation. Minks many times carry Chediak Higashi Syndrome (C-HS),
a genetically transmitted autosomal trait. Minks with C-HS develop early-onset
periodontitis at approximately the age of sexual maturity and the disease progresses
rapidly to tooth exfoliation. Some C-HS minks present clinical manifestations of
disease so extensive that they cannot be quantitatively assessed. Occasionally, the
animals lose all of their teeth except for a few root tips.
Only very mild manifestations of PDS disease are seen in normal animals even at 5-6
years of age, whereas advanced lesions with tooth exfoliation are observed in the
animals with C-HS even at young age.
In both normal and affected minks, bone resorption seems to be age-related. There is
no evidence of bone loss in either group at age 6 months, and the normal animals
generally remain free of manifestations of bone loss at 18 months. There is a
considerable inter-animal and inter-tooth variation in the extent of bone loss in
normal animals. In both groups of animals, the debris and inflammation scores are
highest for the maxillary premolars and lowest for the incisors, canines, and molars.
In normal animals, there is a strong and positive correlation between debris scores
and inflammation scores (r = 0.95), whereas these values do not correlate for animals
in the C-HS group (r = 0.45). These data indicate that factors other than plaque
accumulation may exert-an overriding effect in the C-HS affected animals.
The chemotactic response of the C-HS neutrophils is only 40% that of normal cells.
Relative to normal cells, C-HS lysosomes remain intact after phagocytosis for
prolonged periods unable to deliver peroxidase and other enzymes to the phagosome.
Neutrophils from some C-HS animals have a defect in their utilization of glucose via
the hexose monophosphate shunt pathway and therefore in energy utilization.
9
10. Intracellular killing of certain bacteria by human C-HS neutrophils is greatly
reduced, at least during the first 20 min of contact with the microorganism. This
shows that these defects in neutrophils are greatly reduced, at least during the first 20
min of contact with the microorganism.
DOGS
There is general agreement that the dog is an excellent animal in which to study gingival
and periodontal disease [Anderson, 1970; Navia, 1977].
PERIODONTAL FEATURES OF THE DOGS
The most frequently used and well-characterized species is the beagle [Anderson,
1970], but mongrels, German pointers, and others have been studied as well. Beagles,
like all dogs, have a deciduous and a permanent dentition, the respective formulas for
which are I 3/3, C 1/1, M 3/3 and I 3/3, C 1/1, Pm 4/4, M 2/3, i.e the first premolars
erupt without a deciduous precursor and there are two maxillary and three
mandibular molars [Shabestari et al., 1967]. Permanent teeth of the dog consist of 3
incisors, 1 canine, 4 premolars and 3 molars in the mandible and 2 molars in the
maxilla (Navia 1977).
The deciduous teeth begin to appear at about 3 weeks after birth and their eruption is
complete by the end of the 5th
week, in the sequence of C, I3, I2, I1, M2, M3, and M1.
Exfoliation of these temporary teeth begins at about 110 days of age. The first
premolar, Pm1, the first permanent tooth to appear, erupts at about 106days in the
upper jaw and about 130 days in the lower. This tooth is followed by the incisors (I1-
3), the first molar, the canine, and the remaining premolars (Pm4, Pm2 and Pm3). The
second and third molars, the last permanent teeth to appear, erupt at about 150 and
175 days, respectively. Thus by the end of the 6th
month, the permanent dentition is
complete and ready to function [Shabestari et al., 1967]. All teeth are completely
covered by enamel but, because of the interdigitating type of intercuspidation,
surface wear is commonly observed, i.e., on the sides of the cusps.
Dogs maintained plaque free by repeated scaling and meticulous plaque control can
develop clinically healthy gingival. The etiologic factors of periodontal disease seem
to be identical in humans and dogs (Ericsson et al. 1975). Dogs may therefore be of
value as a model for experimental gingivitis. The fact that it is possible to maintain
periodontal health in sites where plaque accumulation was prevented confirms
similar reports in humans (Lindhe & Nyman 1975).
10
11. Originally, it was reported that periodontal disease began slowly in young dogs and
increased with age (Gad 1968), progressing about 5 times faster in dogs than humans
(Ericsson et al. 1975). Later, it was documented that the range and severity of
gingivitis and periodontitis varied in both young and older dogs and the gingivitis in
younger dogs did not necessarily progress into periodontitis (Hull 1974). Gingival
recession is an outstanding feature in dogs with periodontitis (Ericsson et al. 1975).
Gingivitis did not necessarily progress into periodontitis and a variety of factors may
influence this conversion (Lindhe et al. 1975). But it has been demonstrated that
plaque induced gingivitis in young beagle dogs can progress into periodontitis,
simply by allowing additional plaque to accumulate (Soames & Davies 1980). This
process can be accelerated by the placement of ligatures (Lindhe & Ericsson 1978).
Histologic features are characteristic of an advanced lesion, with the majority of
tissue destruction occurring within the 1st
4 weeks following ligature placement
(Schroeder & Lindhe 1975).
The pattern and progression of chronic periodontitis in the colony dogs described by
Page and Schroeder [1981] is markedly different from that seen in domestic dogs
[Hamp and Lindberg, 1977]. In the colony dogs, all the teeth are involved. The third
and fourth premolars and then the first molars are late and only partially involved.
The bifurcation regions than interdental regions showed severe bone loss. In case of
domestic dogs periodontitis was a strictly localized event, unpredictably affecting
single roots of either from teeth or premolars or even of the second molars. The
reason why some dogs are susceptible and others, even of the same breed, are
resistant to periodontitis is unexplained at present. It seems likely that the differences
are of an infectious or a genetic nature rather than related to diet.
- To induce short-term periodontitis of a type very similar to that observed to occur
spontaneously, cotton floss ligature is applied to the neck of single, selected teeth.
The one-time installation of a subgingivally positioned ligature, even in the presence
of subgingival microbial plaque, does not ensure maintenance of the highly acute
tissue reaction, since the initial ulceration may regress. Such regression would
explain the discontinuity of the acute inflammatory tissue reaction. However, if bone
degradation is to be a continuous process, the ligature would have to be replaced
periodically in an increasingly more apical position.
SHEEP
11
12. Sheep are diphyodont; their deciduous dentition of 20 teeth has the formula I 0/3, C
0/1, M 3/3, and their permanent dentition of 32 teeth has the formula I 0/3, C 0/1,
Pm 3/3, M 3/3. In the place of upper incisors and canines, sheep have a very broad,
thick pad of tissue (the upper dental pad) against which the lower incisors and
canines occlude [Weinreb and Sharav, 1964]. Lingual to the lower incisors, there is
another but less broad and bulky shelf, the lower dental pad, which contacts the
upper dental pad when the upper and lower jaws are brought into occlusion. Lingual
to the lower incisors, there is another but less broad and bulky shelf, the lower dental
pad, which contacts the upper dental pad when the upper and lower jaws are brought
into occlusion. The lower dental pad actually represents an extension of the lingual
gingival tissues, about 8 mm in width. Due to this tissue elevation next to the teeth,
the lingual clinical sulci are of 1.6-3.6 mm in probing depth.
Neither the anterior nor the posterior teeth cease eruption when the occlusal plane is
reached. The roots of all teeth continue to grow and the teeth erupt throughout the
life of the sheep and, by so doing, compensate for the continuous and extensive
occlusal wear, which eventually wears away most of the crown length. In addition,
there is a considerable amount of mesial drift in the premolar and molar region,
which compensates for attrition and wear of the contact surfaces between adjacent
teeth.
Shortly after eruption, enamel completely covers the crowns of the incisors and
extends over approximately one third of the roots, especially on the labial and
interdental sides. As the teeth wear, the dentin becomes exposed at the incisal edge.
The enamel on the surface of the crowns and roots is covered by a thin layer of
cementum to which collagen fibers from the gingival and those of the periodontal
ligament are attached. This relationship particularly applies on the lingual side of
anterior teeth, as lingually the gingival margin is much closer to the incisal surfaces
than labially.
The anterior teeth are anchored in a very shallow socket by a wide and loosely
organized periodontal ligament. On the lingual side, there is a complex supra-
alveolar apparatus comprising a concentration of large bundles of collagen fibers
filling and bridging a wide, triangular area between the teeth and the alveolar bone,
and connecting the former to the latter. These fibers run at about right angles to the
tooth axis and are ‘ideally situated to accept the forces’ placed upon the incisors.
12
13. Nevertheless, because of the shallowness of their bone sockets, all anterior teeth are
extremely mobile; a ‘movement of incisors of up to 2 mm was found to be normal.
The posterior teeth present a peculiar form and shape in that the enamel has folds or
embrasures of varying depths, on both the buccal and the lingual surfaces and deep
invaginations or pits on the occlusal surfaces. With occlusal function, as in the case
of the anterior teeth, abrasion wears away the enamel and the dentin becomes
exposed. The enamel extends over large parts of the root dentin, covering about one
third of the premolar and three quarters of the molar root surfaces. All enamel
surfaces are, in turn covered by a thin layer of cementum. Weinreb and Sharav
[1964] explained the presence of coronal cementum as follows: ‘At the time of
eruption, the crown of the tooth is not quite fully formed, and no roots exist at that
time. These will be lacking for a considerable period. The crown, therefore, serves as
a clinical root for a long period and provides the necessary means of attachment of
the tooth to the alveolar bone. This explains the necessity and importance of the
coronal layer of cementum’. At the gingival margin, the junctional epithelium
provides epithelial attachment to the cementum surface. However, because of
degradation of the reduced enamel epithelium preceding the formation of coronal
cementum, the junctional epithelium arises directly from the surrounding gingival
epithelium as the tooth emerges into the oral cavity. It is also noteworthy that sheep
teeth, i.e., parts of their anatomical crowns and their roots, are surrounded by a very
prominent network of epithelial cell rests of Malassez.
Spontaneous periodontitis in sheep occurs in two different forms. One of these
affects chiefly the incisors, while the other affects both incisors and molars but is
largely confined to the latter.
In the type of periodontitis that affects mostly the anterior teeth, there is ‘premature loss
or shedding ‘ of the permanent incisors. This condition is termed ‘”broken-mouth” and
develops early in a sheep’s life while the incisors are still erupting and their roots are still
growing.
Because of the lower dental pad the coronal two thirds of pockets of upto 6 mm in
depth on the lingual side of the incisors are linked by a thick, keratininzing stratified
Squamous epithelium; only the apical one third of such pockets is lined by pocket
epithelium and only connective tissues underlying this portion contain a dense
inflammatory cell infiltrate. The density of the infiltrate increases with increasing
pocket depth. Probing depth on the labial side of the incisors is usually only about
13
14. half that observed on the lingual side. Probing depth varies greatly from one flock to
another, reflecting the large interlock variability in the prevalence and severity of
‘broken-mouth’ periodontitis. Pockets deepen the proportion of the wall lined by
pocket epithelium increases.
At 2 years of age the CEJ is deep subgingivally, but with age this landmark shifts
coronally as the tooth continues to erupt, and eventually, it advances to a
surpragingival level. In 5-year-old periodontitis-resistant sheep the alveolar socket is
still very narrow and the interdental bone septa are extremely thin, while the
periodontal ligament is still very wide.
All sheep, independent of age and farm origin, had a plaque-related, mild, local
chronic gingivitis around both the incisors and the premolars. In the incisor region,
subgingival plaque occurred in greater amounts and with higher incidence and
periodontal pockets were deeper and more frequent on the lingual side than on the
labial. As seen under polarized light, pocket formation resulted in the disorganization
and separation from the lingual root surfaces of most of the collagen fiber bundles
spanning the wide, V-shaped trough between the incisors and the alveolar bone.
‘Fibers from the supra-alveolar, alveolar crest and transverse functional fiber groups
lost their incisor attachment when lingual pockets were deep. Labially, the pockets
were much shallower and fewer functional fiber groups were affected’ [Spence et al.,
1980]. Thus, on the lingual side of lower incisors, deep but still largely supra-
alveolar pocketing results in the disorganization and detachment of ‘fiber groups
within the incisor periodontal ligament, which are important for the support of the
incisors against grazing forces. Pocketing and subsequent fiber loss, particularly on
the lingual aspect, will lead to a mechanically unstable tooth that may be torn out by
dietary trauma, e.g. root feeding’
In the posterior region, particularly around first premolars, Spence et al. [1980]
observed histopathological changes similar to those around incisors. In this region,
the labial and lingual sides of the tooth were equally affected.
The premolars remained in place rather than becoming exfoliated, thus, ‘cheek teeth’
also lose functional fiber groups but since they have a large proportion of their length
surrounded by alveolar bone and cudding forces push the tooth downwards and
laterally against the socket wall, very few of them are lost.
Most of the clinical and histopathological data support the idea that ‘broken-mouth’
periodontitis is a clinically and histopathologically chronic disease, with little
14
15. evidence of involvement of even temporarily acute inflammatory phenomena.
Incisors migrate and are lost early in the life of the sheep because deep, narrow
pockets lingually undermine the most essential fiber support. Because of the very
shallow alveolar sockets and because of the response of the connective tissue to
plaque within a very narrow zone only, deep pockets, particularly on the lingual side,
develop without affecting the bone. Bone degradation appears to be a late if not
terminal event, which occurs immediately prior to tooth exfoliation. For all these
reasons, Cutress and Ludwig [1969] initially classified ‘broken-mouth’ periodontitis
as a form of ‘periodontosis’ i.e. a noninflammatory degenerative condition.
FERRETS
• The dentition, wear patterns, calculus formation, salivary glands, and periodontal
lesion of the ferret have been studied, although not to the extent of the previously
discussed species. The domestic ferret (mustela putorius furo) is believed to have
been derived from the wild (European) polecat . Use of the domestic ferret as an
animal study model in periodontics was originally described in the 1940s by king et
al., who documented that the occurrence of periodontal disease in ferrets was similar
to that occurring in humans.
• The ferret has both a deciduous and a permanent dentition. The permanent dentition
consists of incisors, canine. 2nd
, 3rd, 4th PM and first and 2nd
molar.
• Ferrets have been used as a medical and dental model. Harper et al (1990) and Mann
et al (1990) have found ferrets to be a suitable model for the study of calculus.
Calculus in ferrets has a physical structure similar to hydroxyapatite. The main
difference between the ferret and human calculus is a lesser degree of calcification in
the ferret deposits. Diet did influence the rate of formation, but not as much as in rats.
Calculus in fettets can be scored while the animal is alive, whereas this is not
possible in rats. A study compared calculus accumulation in ferrets fed a mineral
supplemented softened cat food and a 2*daily application of toothpaste containing
pyrophosphate toothpaste produced significantly less calculus.
• The tissues responded by characteristic inflammatory reactions, which are identical
in all respects to those found in human gingivitis. Calculus and plaque deposition and
impingement on the gingival crest leads to loss of keratin and splitting of the
junctional epithelium with pocket formation. Ferrets can develop ligature-induced
15
16. periodontitis within 28 days where the ligated sites lost 50- 75% of attachment, and
lesions exhibited large populations of PMNs adjacent to the ligatures. Plasma cells
and lymphocytes were observed apical to the lesions.
• The ferret is a suitable model for the study of calculus because of its resemblance to
human calculus and the act that formation of calculus is not diet dependent as in the
rat and hamster. Further research is still needed to ascertain the role of ferrets as a
model in the pathogenesis of periodontal disease.
NON - HUMAN PRIMATES
In designing any medical or dental animal study, it is often advantageous to select an
animal that is phylogenetically similar to humans. The wide range of non-human
primate species allows appropriate selection for different investigations.
Each species has unique similarities and dissimilarities to humans. The structure and
features of the dental and periodontal tissues of non-human primates closely
resemble those of man; all species are diphyodont and closely related dental anatomy
although the teeth size is dramatically smaller
Non-human primates have similar oral structures to humans and have naturally
occurring dental plaque, calculus and gingivitis, but small increase in pocket depths.
The structure of the oral epithelium, gingival sulcus, junctional epithelium,
underlying bone, and connective tissue is similar to that seen in humans, but with
some exceptions. For example, in the bush baby (Galago senegalensis bratticus) the
root surfaces have an unusually thick layer of cementum, which is unusually labile
[Grant et al., 1973]. In addition, in some marmosets (Saguninus Oedipus and
Callithrix jacchus) the oral sulcular epithelium may be keratinized in part
[Schectman et al., 1972].
The taxonomic order "primate" is extremely heterogeneous. Some are predominantly
herbivorous, whereas others are predominantly carnivorous.
Available laboratory facilities, presence of a breeding colony, cost, ease of handling,
and ease of housing. Furthermore, relatedness to humans and limitations imposed by
the size of oral structures, as well as the availability of appropriately sized
periodontal instruments, must be considered. As well be discussed later, periodontal
disease in some non-human primates is histologically very different from human
16
17. periodontal disease. Therefore, results from studies in such species may be only
slightly related to the human condition.
Age is another important factor: However, there are currently no reliable means for
determining the age of adult, sexually mature wild-captured primates.
MACACA
With respect to the permanent dentition, old world monkeys (e.g. macaques,
baboons, bush babies and chimpanzees) have the same dental formula as man: I 2/2,
C 1/1, Pm 2/2 and M 3/3, while new world monkeys (i.e. marmosets) present the
formula I 2/2, C 1/1/, Pm 3/3 and M 2/2.
Clinically, healthy monkey gingiva is histologically indistinguishable from human
gingiva. A shift in composition of plaque flora from an early gingivitis to a late stage
is also comparable to humans. The inflammatory infiltrate associated with
periodontal disease is microscopically similar to humans in some species such as
cynomolgus monkeys (Macaca fascicularis). But the squirrel monkeys (Saimiri
sciureus) and marmosets have limited numbers of lymphocytes and plasma cells,
making them inappropriate models for studying pathogenesis of periodontitis.
Monkeys have been used widely as an animal model for studying periodontal
surgical procedures. Large non-human primates have a naturally occurring
periodontitis, but it occurs later in life and the lesions are asymmetrical (Caton et al.
1994). Therefore, if osseous lesions are needed for clinical studies, they are usually
experimentally induced. Similar dental anatomy, periodontal wound healing (Caton
& Kowalski 1976), suitability of furcation sites (Giannoble et al. 1994) and
experimentally induced defects that do not spontaneously regenerate, make mature
adult cynomolgus and rhesus (Macaca mulatto) species good models for studying
ligature-induced periodontitis. Periodontal lesions in these animals are also suitable
for evaluating periodontal regenerative procedures (Shou et al. 1993), especially
since histometric analysis needed to quantify the amount of new cementum,
periodontal ligament and alveolar bone formed as the result of regenerative
periodontal surgery (Caton et al. 1994), can only be done with animals, usually
monkeys or dogs.
The most significant microbial differences between macaque species and humans are
a lower proportion of Actinomyces species, the presence of a catalase-producing
17
18. Prevotella melaninogenica strain, and the high carrier rate for Actinobacillus
actinomycetemcomitans in subgingival plaque of macaque species.
Due to the possibility of obtaining block biopsies, the rhesus monkey, cynomolgus
monkey, and baboons have been used to study osseointegrated oral implants. A
recent article (Fritex et al. 1997) suggested that ligature-induced peri-implantitis
follow similar destructive patterns, namely alteration of microbiological flora.
There are significant differences among the three species of Macaca studied with
regard to the progress of gingival inflammation and the composition of the
inflammatory infiltrate. In M. fascicularis the gingival lesion appears to evolve
very slowly and is dominated by cells reported to be macrophages, and
lymphocytes not by plasma cells. Proliferation of macrophages and fibroblasts
is a prominent feature. In contrast, in M. Speciosa and M. Mulatta, neutrophils
and acute inflammation together with a predominance of macrophages are seen at
the beginning stages of gingivitis.
Radiographs have routinely been used in macaque species but custom film packs
may be necessary in the maxilla of the macaque species due to the flat palatal
vault and they are a necessity in both the maxilla and mandible of very small
primates like squirrel monkeys.
Advantages
1. Anatomical similarities to humans.
2. Proven similarities to humans.
3. Class II furcations can be readily studied.
4. Well characterized.
Disadvantages
1. Monkeys can expensive to purchase.
2. Low prevalence of natural disease.
3. Difficult to maintain (ferocious).
4. Low frequency of Class III furcations.
5. Lengthy time necessary to induce attachment loss.
6. Difficult to maintain oral hygiene.
MARMOSETS
18
19. Marmosets demonstrate severe occlusal abrasion not seen in humans and their
bony lesions are mostly due to plunger cusps and open contacts.
Gingivitis and periodontitis have been studied extensively in wild-caught and
colony-maintained marmosets. Generally, cotton-tops (S.oedipus) and cotton-
ears (C.jacchus) have been used. Early reports indicated that marmosets do get
chronic periodontitis [that the prevalence is high, and that the lesions closely
resemble those seen in humans [Levy, 1963].
Transient gingivitis is observed in most wild-caught animals. The lesions are
more prevalent in the interproximal than in the buccal and lingual gingiva, and
the tissues around the anterior teeth are more affected than those around the
posterior teeth. Gingivitis is manifested clinically as a vasculitis in which the
small blood vessels of the marginal gingival become dilated and engorged and
can be seen by the unaided eye through the tissue. Masses of supragingival
plaque are observed, and when assessed histologically calculus is found at all
interproximal sites.
In marked contrast to the disease in humans, dogs, and most other primates,
plasma cells are not prominent. Vascular proliferation in the gingival tissues and
the periodontal ligament is a prominent feature. Radiographs and defleshed jaws
revealed extensive destruction of the alveolar bone, which is mostly of a
horizontal type. This pattern of bone destruction might be expected to occur,
because in the marmoset, a very small animal, the alveolar bone is too thin to
accommodate an intra-alveolar pocket and may preclude their used for certain
periodontal procedures.
Due to the small oral cavities of several non-human primates necessitate selection
of special examination methods. For e.g. in marmosets, magnified photographs,
magnifying glass, or stereomicroscopy must be used.
BABOONS
Gingivitis is characterized by the presence of subgingival plaque and calculus and apical
displacement and proliferation of the junctional epithelium but without ulceration of the
epithelia or suppuration. A dense inflammatory infiltrate consisting mostly of plasma
cells and lymphocytes is located in the connective tissues lateral to the pocket. The
19
20. investigators concluded that periodontitis with typical pocket is not commonly observed
in baboons (7-10 years).
The junctional epithelium is converted to pocket epithelium, which is invaded, by
neutrophils and lymphocytes, including T cells [Simpson and Avery, 1974]. Although
few neutrophils are present within the connective tissues, the inflamed blood vessels are
engorged with these cells and many invade the junctional and pocket epithelium. In most
cases the infiltrate is dominated by plasma cells, but in others by lymphocytes. In biopsy
specimens taken from animals with periodontal pockets, the inflammatory infiltrate
consists mainly of plasma cells, small and medium-sized lymphocytes and macrophages.
Vascular proliferation is also prominent. Periodontal disease in the baboon is thus
strikingly similar to the disease in humans, both clinically and microscopically.
CHIMAPANZEES
Clinical examination of the old chimpanzees revealed several interesting features
[Page et al., 1975]. The gingiva was more heavily stippled than in man and
frequently had a corrugated appearance, indicative of the fibrosis and thickening
which is sometimes associated with long-standing chronic gingival inflammation.
Many of the interdental papillae exhibited enlarged cauliflower-like growths,
presumably resulting from vascular and connective tissue proliferation. Generally,
the marginal gingival presented a picture of acute exudative inflammation,
superimposed upon thickened, fibrotic, abnormally contoured gingiva. Many of the
teeth had granular, partly calcified deposits, while in other areas; a more luxuriant,
gelatinous form of plaque was seen. Microscopically, the deposits contained bacteria,
degenerating leukocytes, and sloughed-off epithelial cells. There was subgingival
calculus on virtually all teeth in all animals.
All the aged chimpanzees had severe occlusal wear. In most cases, as seen in the
gorilla, one half to two thirds of the anatomical crowns of the posterior teeth had
been lost. Wear was most extensive on the palatal occlusal surface of the maxillary
molars and on the buccal and lingual cusps of the mandibular molars. In some
animals, the mesial occlusal surface of the mandibular first premolar was worn to the
CEJ.
Vascular proliferations, especially in the tissue lateral to the pocket epithelium, are a
major feature of periodontitis in the chimpanzee. Capillary loops, frequently
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21. engorged with neutrophils, extend almost to the epithelial surface and there are many
of these cells in the pocket epithelium. In some cases the soft deposits are separated
form the connective tissues and blood vessels by an epithelial layer only one or two
cells thick. Thus plaque-derived substances appear to have ready access to the
connective tissues and to the vascular system.
CANINE MODEL
Advantages
1. Natural occurrence of disease.
2. Low cost and ease of handling.
3. Extremely cooperative during experimentation.
4. Proven similarities (immunological) in response to humans.
5. There are gross anatomical, topographical, and physiological differences.
6. In view of their docile temperament and natural susceptibility to periodontal
disease, dogs, particularly beagles, are used in dental research for the study of
periodontal disease progression, guided tissue regeneration, tissue wound healing,
and dental implants.
7. The anatomy of the premolar teeth in dogs is such that even moderate bone loss
results in Class III furcation involvement. The pattern of bone loss in the dog
displays predominantly horizontal type osseous lesions; however, a vertical or
intrabony component of the osseous defect is present in approximately 25% of
the sites. Therefore Class III furcation can be readily studied.
Disadvantages
1. Decreased number of Class II furcations.
2. Differences from human dental anatomy.
3. Availability of animal.
4. Limited number of bony defects available.
5. Expense (but less than monkey).
The full cycle is 13 weeks in the canine and 18 weeks in the primate. While the exact
rate of bone formation may vary depending on the anatomical site (e.g., periosteal,
cortical, trabecular, etc.), this suggests that studies evaluating bone formation in the
nonhuman primate may need to assess endpoints at later times compared to similar
studies in the dog. Also, the somewhat shorter sigma in the dog may explain the more
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22. rapid bone healing response in the dog as compared to the primate. Skeletal remodeling
in the dog is about one-third more rapid than in the primate.
CONCLUSION
In conclusion it appears that a number of variables beyond complete control of the
investigator are inherent in animal models currently available. Consequently variability
in results may be expected. In no case is periodontitis in animals identical to that seen in
humams. No true analog of human periodontitis has yet been found and it is unlikely that
one will be found.
REFERENCES
1. Periodontitis in man and animals – Page and Schroeder
2. Lab animal models in periodontologyp- JCP 1999; 26:335
3. Non –Human primates used in studies of periodontal disease
pathogenesis: Review- JP 1993; 64; 947
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