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 Table of Contents  
Year : 2017  |  Volume : 7  |  Issue : 2  |  Page : 68-75

Rat as laboratory animal model in periodontology

1 Department of Periodontics and Implantology, Bapuji Dental College, Davangere, Karnataka, India
2 Department of Periodontics, College of Dental Sciences, Davengere, Karnataka, India
3 Department of Periodontics and Implantology, Bapuji Dental College and Hospital, Davengere, Karnataka, India

Date of Web Publication8-Jan-2018

Correspondence Address:
Dr. Mallanagouda B Patil
Department of Periodontics, College of Dental Sciences, Room No. 4, Davangere - 577 004, Karnataka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijohs.ijohs_47_17

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For ethical reason, initiation and progression of periodontal disease as well as certain types of periodontal treatment cannot be studied in humans. Instead, numerous studies in this field have been carried out in laboratory animals. Animal models are needed to objectively evaluate the pathogenesis of human periodontal disease and its various treatment modalities. A number of animal models have been used in studying etiology and pathology of periodontitis. The primate model is recommended because the pathogenesis in primate model closely resembles that in humans. In addition, the dog model is used frequently because of ease of ligature placement as well as the natural occurrence of periodontal disease. However, ease of handling, inexpensive, short study time, low variation among strains and controlled microflora, relatively disease resistant, make the rat model extremely versatile and suited for a wide range of research endeavors. Rats are used mainly for research in toxicity, nutrition, behavior, and cancer. Normal oral structure and physiology and the pathogenesis of periodontal diseases have been studied more extensively in the rat than in any other rodents. Rats are used extensively to study the effects of drugs on the gingiva because their tissue overgrowth is similar to that of humans. The purpose of this review is to evaluate rats as models for studying various aspects of periodontal disease, including disease process and its treatment handling, advantages and limitations of these models.

Keywords: Animal models, periodontitis, rat, research

How to cite this article:
Guvva S, Patil MB, Mehta D S. Rat as laboratory animal model in periodontology. Int J Oral Health Sci 2017;7:68-75

How to cite this URL:
Guvva S, Patil MB, Mehta D S. Rat as laboratory animal model in periodontology. Int J Oral Health Sci [serial online] 2017 [cited 2022 Jan 27];7:68-75. Available from: https://www.ijohsjournal.org/text.asp?2017/7/2/68/222402

  Introduction Top

For very long time, periodontal diseases, in particular, the severe forms of periodontitis, have been and still are enigmatic both to those who suffer from them and to those who try to understand and prevent them. Attempts to understand these diseases through clinical and experimental research have been made continuously and with varying success for more than a 100 years.[1]

In addition to man, several other mammals, particularly those kept in captivity or in the domestic domain, spontaneously develop periodontitis with age. Therefore, the periodontal tissues of a variety of mammals/animals have been studied, some more extensively than others, in the hope of finding a true analog to human gingivitis and periodontitis. This search has been going on since about the turn of the century. We now have a great deal of reliable information about the histopathologic and pathophysiology of the normal periodontium and about the histopathologic and pathophysiologic aspects of spontaneously and experimentally diseased periodontal tissue of several animals.

In the process of searching for common denominators, Page and Schroeder introduced us to some very interesting forms of periodontitis in animals, such as “cara inchada” (swollen face), a disease found in cattle in West Central Brazil and the “broken mouth” periodontitis of sheep. Because of these observations are then related to periodontal diseases in rodents and nonhuman primates, to ligature-induced disease, to the nature of the disease in germ-free animals, to the incidence of periodontal disease in nondominated versus dominated animals and to many other aspects of comparative pathology.[1]

Animal models for testing periodontal regenerative procedure are necessary because controlled quantitative histological analysis is required to evaluate the quality and extent of the rarely formed supporting tissues. Histometric evaluation can determine the amount of new cementum, periodontal ligament, and alveolar bone formed the amount of new regenerative periodontal surgery. These studies are not possible in man because of the need to retrieve the teeth and their surrounding periodontium in large blocks appropriate for histological analysis. Furthermore, proper evaluation of a new therapy necessarily involves the use of treated and untreated controls which are difficult to obtain in the human. The testing of potentially harmful new devices and pharmaceuticals may be unethical in man before thorough evaluation in higher animals.[2]

Animal research and its value to human experience remain controversial. Animal model data can provide us with models of biologic trends before proceeding to human application. Each animal species are manifested by a wide range of clinical and histopathological features. Different species have distinct traits, habits, life spans, tissue structures, host defense mechanisms and genetic microbiological immunological, and clinical features of periodontal disease and its prevention and treatment.

Here, we have made an attempt to review to evaluate rats as models for studying various periodontal diseases, including disease process, its prevention and treatment, advantages, and limitations of these models.

  Origin Top

The laboratory rat, Rattus norvegicus, is a rodent of the family Muridae [Figure 1]. Wild rats' apparently originated, in recent times at least, in the temperate regions of central Asia, from southern U. S. S. R. through northern China. Through migration along trade and military routes, the cosmopolitan rat has spread around the world.
Figure 1: Rat

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Domesticated rats were raised by fanciers in the seventeenth century and for combat with terriers in subsequent centuries. By the mid-1800s, rats were used in scientific experimentation. Rats, such as mice, are now available in various ecologic and genetic varieties, however, most research rats are barrier-raised in specific pathogens free colonies.[3],[4]

Rat is one of the most widely used species of laboratory animals. Its popularity is next only to that of mice and found all over the world, especially in association with human habitation. R. norvegicus (scientific name) adopts readily in laboratories all over the world. Rats are used mainly for research in toxicology, nutrition, behavior, and cancer. Proportionately, a smaller number, are also used in physiological and pharmacological investigations and for teaching.

The most frequently used inbred strains of rats include the Wister albino, Lewis, Norwegian grey, Rice (Oryzomys palustris) (Leonard, 1979), Kyoto and Carworth Wistar, Osborne-Mendel, Holtzman, white Lobund, CD and CDF-Fisher 344, and several strains of the Sprague Dawley rat (Forsyth strain, CR strain, and RIC strain).[1]

Sprague Dawley rat, an albino, has a narrow head and tail longer than the body, whereas the Wistar rat has a wide head and shorter tail. The Long-Evans and other “hooded” varieties are smaller than the albino strains and have darker hair over portions of the head and anterior body.

Normal oral structure and physiology and the pathogenesis of periodontal diseases have been studied more extensively in the rat than in any other rodent. For this reason, the rat serves as an example for other rodents such as mice and hamsters. Although particular dental and periodontal tissues differ in size among various rat strains, principles of tissue structure and growth phenomena are identical in all strains.

  Anatomic Physiologic Characteristics Top

The rat is the most extensively studied rodent for the pathogenesis of periodontal disease. Typical rodent dentition is Incisor 1/1, canine 0/0, premolar 0/0/ and molars 3/3. The incisors are rootless with continuously erupting [Figure 2], [Figure 3], [Figure 4], [Figure 5]. The cheeks closes into the diastema, separating incisors from the oral cavity and have an articulated mandibular symphysis. The structure of the dental gingival area in rat is quite similar to that observed in humans. They have a tapered head with pinnae relatively smaller than those of mouse, a long tail, and an anus usually pressed on the ground. An adult rat weights about 530-900 grams and varies from species to species.
Figure 2: Maxilla and mandible of rat

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Figure 3: Mandibular molar teeth

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Figure 4: Lower incisors

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Figure 5: Upper incisor

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The gnawing apparatus, shared with other rodents, is remarkable. The continuously erupting (hypsodontic), chisel-like incisors, powerful jaw muscles, articulated symphysis, and long diastema with loose cheek skin all contribute to the rat's omnivorous habits and gnawing ability. The 12 molars, on the other hand, are permanently rooted (brachiodontic), located far back in the mouth, and are used for grinding.[3],[4]

  Life Span Top

Rats may live longer than 3 years and have a productive breeding life of approximately 9 or more months or until they are 12 or more months of age, during which time they may bear 7–10 L with 6–14 offspring per litter. After 12 months, litter size decreases and litter interval increases until sexual senescence at 450–500 days.

  Handling Top

Rats gently handled become tame and will rarely bite unless startled or hurt. Because the tail may tear, the tail should be grasped only at the base and for short periods. Rats are picked up by placing the hand firmly over the back and rib cage and restraining the head with thumb and forefinger immediately behind the mandibles. Rats held upside down are more concerned with righting themselves than with biting [Figure 6].
Figure 6: Rat holding

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  Housing Top

Rats are housed either in metal cages [Figure 7] with mesh or in plastic cages with solid floors. “Rats mesh” should have openings of 2 wires per inch. A homemade, shoebox-shaped cage with a hardware cloth top is satisfactory for pet rats, Terraria are commonly used to house pet rodents, but care must be taken to assure accessibility to water and feed. Care should be taken to prevent escape, loss of neonates through wire floors, and fractured limbs from too close a mesh. Young rats housed on the wire and deprived of the nest may become dehydrated, especially during months when ambient humidity is low. Rat cages should provide at least 259 cm square floor space per adult (300 g) rat. A female with litter requires 1000 cm square floor space. Rat cages should be at least 18 cm high. Bedding for rats, which may be paper, wood chips or shavings, ground corn cob, or sawdust, should be nonallergenic, dust free, absorptive, nontoxic, and clean. Soft shavings are more easily formed into nests.[3],[4]
Figure 7: Rat cage

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Temperatures in rat rooms should be maintained between 20°C and 25°C relative humidity should be between 40% and 70%. Light intensity is reduced within translucent or filtered cages. A 12:12 dark light cycle is usually used in rat colonies; continuous light will depress cycling. Rodents in general especially nocturnal rodents, do no very well in dimly lighted rooms. Apparently, the bright light used in most benefit than for the rodent's wellbeing. Bedding should be charged as often as necessary to keep odor minimal and the rats dry and clean, one to three bedding changes per day week are usually sufficient. Cages are washed once or twice weekly.

  Feeding and Watering Top

Rats are fed a clean, wholesome, fresh, and nutritious pelleted rodent diet free choice. The commercially available diets are complete that is they do not require supplementation. Rats should be watered from water bottles with sipper tubes or through an automatic watering system. Several varieties of special purpose rodent chows are available. Rats are cautious feeders and will avoid strange foods.

An adult rat will consume approximately 5 g food and 10 ml water/100 g body weight per day. Consumption varies with the ambient temperature, humidity, health status, breeding stage, diet, and the time of day. Rats are nocturnal and feed primarily at night.

  Public Health Concerns Top

Allergies to animal dander and urinary proteins (serum proteins) occur commonly in humans, and cutaneous and upper and lower respiratory allergies to rats (large crowded, poor air circulation, infrequent cage changing, and cleaning lead to accumulation of ammonia gas) are very common.

Zoonotic diseases carried or transmitted by rats include leptospirosis, streptococcal infections,  Salmonellosis More Details, cestodiasis, Korean hemorrhagic fever and rat bite, or Haverhill fevers. Sylvatic plague is carried by rat fleas. The mite liponyssus sylviarum can transmit the St, Louis encephalitis virus, and L bacoti attacks man directly. Rat-bite fever is caused by Streptobacillus moniliformis, which is carried in the nasopharynx of asymptomatic rats. People experiencing rat bite fever may develop recurrent fever with petechial hemorrhages, endocarditits, and polyarthritis. Although these diseases and infections may occur in wild rats brought into a laboratory, they are rare or unknown in domestic rats.[3],[4]

  Uses in Research Top

Laboratory rats include studies of aging, neoplasia, drug effects and toxicity, gnotobiology, dental caries, lipid metabolism, vitamin effects, behavior, alcoholism and cirrhosis, arthritis, phenylketonuria, jaundice, fructose intolerance, hypertension, embryology, teratology, diabetes insipidus, and infectious disease.

  Collection of Blood Top

In long-term experiments, it is essential that the same method of blood collection should be used thought for both control and experimental animals because the composition of a blood sample will depend on the method of sampling.

Small samples can be obtained from a tail vein by snipping the tip of the tail [Figure 8]. The tip of the tail is cleaned with alcohol and when this has dried the tip is snipped with a clean scalpel and the blood collected in a pipette or directly on to a slide or tube, after discarding the first few drops. The tail must not be massaged, or the sample will not be diluted with tissue fluids.
Figure 8: Blood collected from tail

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Large blood samples should be removed by cardiac puncture and rats orbital sinus with a hematocrit tube [Figure 9].
Figure 9: Blood collection from eye

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The anesthetized rat is placed on its back and the heart located under the 5th or 6th rib by palpation with the index finger of the left hand the left thumb being held on the rats' right side. A 2–5 ml capacity syringe and a 19–25 mm, 24–26 gauge needle is suitable for the purpose. The needle is inserted at 45° to the long axis of the body into thoracic cavity up to the point where the heart is felt throbbing against the needle. The needle is then pushed into the ventricle and blood sample is withdrawn. The syringe and needle can contain an anticoagulant if necessary.

In rat the total blood can be collected at one bleeding is 5.5 ml/kg body weight.

  Drug Administration Top

Drug can be administered through oral, subcutaneous, intramuscular, intraperiotoneal and intravenous routes. Convenient volumes for injections are 0.2–0.3 ml/100 g body weight concentration adjusted accordingly. The dorsal tarsal vein is useful for injection as also the saphenous vein where it crosses the leg above the hock. Intravenous injection is best given in the tail vein using a 12.7 mm, 24 gauge needle. Tail veins can be made visible by diping the tail in a saturated solution of sodium sulfide for 2 min.

  Rats as Model in Periodontal Research Top

Rats have the typical rodent dentition, and the molars are fully erupted between about the 15th and the 40th day of postnatal life. Eruption time differs from strain to strain. In the Cotton rat, the first molars begin to erupt 4–7 days after birth, whereas in the Sprague Dawley rat this occurs at about 15–17 days and in the osbeorne Mendel rat at about day 17. On the other hand, the first molars begin to emerge in the rice rat within 9–10 days after birth.[5]

The structure of the dentogingival area in the rat, including the junctional epithelium and its attachment to the tooth surface, the configuration, and topography of the epithelial tissues at the gingival margin, and the very shallow gingival sulcus with the free surface of the junctional epithelium at its bottom or at the level of the gingival margin, is very similar to that of man.[6]

Although there are occasional statements to the contrary such as, the crevicular epithelium in rats is keratinized,[7] the only significant difference in gingival tissue structure between rats is in the relationship between the oral gingival epithelium and the junctional epithelium. In the rat as the gingival epithelium bends apically and joins the coronal portion of junctional epithelium at buccal or lingual aspects of the molars, it often produces a stratum granulosum and a stratum corneum, the most superficial cells are in desmosomal contact with the nondifferentiating and nonkeratinizing cells of the junctional epithelium.[7] Nevertheless, the junctional epithelium is the pathway for the influx of foreign substances and the pathway for inflammatory cell exudation in the rat as it is in the human.

  Age Changes in Rat Top

Age-dependent phenomena in rat include a series of finely tuned and interrelated tissue changes including growth of the jaws, wear of the occlusal surfaces, continuous eruption of the teeth, and apposition of cementum and bone, etc. These changes concomitantly result with increasing postnatal age in a shifting of the molars in three-dimensional space, i.e., in the vertical (apical-occlusal) as well as in the horizontal (mesial-distal and buccal-lingual) planes and thus in a continues change of tooth position as it relates to the bony socket, the alveolar process and the entire jaw.

Hoffman and Schour [8] were the first to document many of these phenomena and reported that first mandibular molar,[1] the height of the anatomic crown decreases as a result of attrition from 1.3 to 0.7 mm with the most rapid attrition taking place within the first 100 days of postnatal life,[2] the length of the roots increases from 0.4 to about 3.7 mm, with most of this elongation resulting from apposition of cellular cementum,[3] the cementoenamel junction (CEJ), which is located at the alveolar bone crest at about 35 days of age, continues to move in an occlusal direction more rapidly than bone is deposited at the alveolar crest, resulting in an increasing distance of up to 0.8 mm.[4] The distance between the root apices and the bone in which the roots are embedded increases with age as well, reflecting an occlusal drift of the entire tooth. As this drift occurs, bone is deposited at the fundus of the alveolus [Figure 10].
Figure 10: Radiograph of molars

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  Eruption Rate Top

The eruption of rat molars, accompanies by rapid occlusal wear and hard tissue apposition, continues through life. The rate of eruption, at least in young animals, is faster than the concomitant increase in bone height. During the period from 21 to 35 days, the first mandibular molar erupts at a rate of 590 μm/week, its root elongates at a rate of 540 μm/week and alveolar bone apposition at the fundus occurs at a rate of 54 μm/week and at the alveolar crest, at 160 μm/week.[8]

Sicher and Weinmann [9] calculated the rate of distal molar drift to be 60–80 μm/week and concluded that the continuous occlusal distal buccal movement of rat molars is the physiological expression of adaptive changes required by growth changes of the jaws and by rapid occlusal wear. This physiological, age-related drifting of erupting rat molars in the buccal-distal direction has, at present, become a fascinating rat model for studying the dynamics of bone remodeling. With the use of stereological methods, we determine the exact remodeling cycle and also the bone balance. A period of 1.5 days is required for resorption, 3.5 days for reversal, and 1 day for bone deposition: the exact volume of bone resorbed on the mesial side of the tooth is deposited on the distal side. The molars erupt early in life in the growing jaws; continuous vertical eruption is not only compensating for attrition but correlated to the growth of the jaws in height. The molars drift distally through life in correlation with the growth of the maxilla and the mandible in length at their posterior end.[10]

Belting et al. 1953[10] using sections cut in the mesiodistal plane found that the distance between the CEJ and the alveolar bone crest increases with age, more so at lingual and distal than at buccal and mesial surfaces, but the width of the supraalveolar interdental space and of the entire interdental bony septum becomes greatly reduced with age. While continuing (Belting et al.) to erupt in an occlusal direction throughout the life of the animal, the molar teeth tilt mesially while migrating distally and also tilt lingually while shifting slightly in a buccal direction. Accompanying these movements is a shift of the junctional epithelium in the apical direction, which in the normal course of aging covers coronal portions of root cementum in germ-free/conventional rats.[10]

  Periodontal Disease in Rats Top

One of the most successful approaches to studying oral disease in rats appears to be the utilization of the gnotobiotic or germ-free rat. Gnotobiotic rats of the Sprague Dawley strain have been used to demonstrate the ability of various filamentous bacteria to form plaque and induce periodontal disease in the absence of other bacteria.[11]

Several Gram-positive species of bacteria isolated from the human oral cavity were used as monocontaminations in rats, causing periodontal destruction in 84 days.

When gnotobiotic rats were monoinfected with a Gram-negative anaerobic rod isolated from a localized JP patient [12] or Eikenella corrodents, plaque adhering to the tooth surface was not formed. Once initiated, bone resorption occurred continuously rather than sporadically as in humans.

In germ-free rats, however, there is a considerable amount of impacted hair and bedding material between the teeth whose role in the disease process remains unclear. Lesions induced by Gram-negative bacteria showed minimal inflammation. The computed tomography infiltrate contained primarily neutrophils, few lymphocytes, and no plasma cells. Thus, the destructive process in response to Gram-negative bacteria can occur in the absence of a cell-mediated immune response which is not similar to humans.[12]

Periodontal disease in rats is different from that of humans. After inoculation of microorganisms into germ-free rats, periodontal destruction occurs very rapidly, so there is no need for inducing disease with ligatures. The rat is relatively resistant to periodontal disease and is therefore used mostly for oral microflora research. Another difference between the rat and human periodontal disease is that, instead of the lesion extending along the root surface as in man, the most apical extent of the lesion is located along the central part of the interdental tissues. Bone loss could occur without apical migration of the junctional epithelium.[13] The gingival response involved is an acute, not a chronic, immune infiltrate.

Periodontitis occurs spontaneously in selected strains of inbred animals such as a particular disease susceptible strains of rice rat. However, disease is diet dependent.[1]

The disease prone rats develop rampant periodontal lesions within their 1st year of postnatal life. Gupta and Shaw 1956[5] stated that the earliest evidence of abnormalities in the periodontium was noted in 16-day-old rats whose second molars were only partially erupted. Marginal gingivitis was accompanied by edema and ulceration, with the formation of deep pockets that often were filled with food debris and hair, a higher frequency and greater severity of abnormalities was noted in the mandible than in the maxilla. With varying degrees of severity, these lesions affect both interdental and inter-radicular spaces. They are characterized by deep crater-like pockets filled with large bacterial masses which are often attached to the root surfaces, varying amounts of impacted hair, extensive alveolar bone resorption, and 50% or more denudation of the molar roots. In final stages, the teeth become extremely loose and eventually exfoliate. In few animals, the entire alveolar process carrying the molars may become completely degraded.

In spite of a statement to the contrary, periodontitis does not occur in germ-free Sprague Dawley, White Lobound the CDF Fisher rats, all of which, even under conventional conditions, are periodontal disease-resistant animals.

When silk ligature is tied around the mandibular molars of germfree rats, the gingival tissue becomes displaced but do not react in any way except for an enhanced transmigration of polymorphonuclear leukocytes a similar transmigration through the junctional epithelium also occurs in untreated germ-free control animals.[14]

A similar response is obtained when stainless steel wire ligatures are applied to the maxillary molars of conventional rats systemically treated with either tetracycline or cortisone. Experimentally, induced chronic traumatic injury of the gingival tissues does not lead to periodontitis in germ-free animals.[15]

  Conclusion Top

The modern era of periodontal research began in the mid and late 1960s with documentation of the fact that gingivitis and periodontitis in humans are caused by bacteria. Since that time, information has been accumulating at an astounding rate.

The diversity among animal species in susceptibility, progression, and features of periodntitis provides rich research possibilities and opportunities. On the other hand, this diversity necessitates that the questions asked be defined with precision and the animal to be used selected with great care. Human periodontitis can be dissected into several components with regard to research. Excellent animal models do exist for some of these components and in many cases, the unusual features of the disease in a particular species make it especially useful.

Because rat has been the most widely used rodent in periodontal research, features of its normal periodontium, in particular, the dentogingival area and the physiological age-dependent changes in the molar regions are especially well documented and understood.

Animal research and its value to human experience remain controversial. Regardless of how much data can be presented, it is impossible to expect different species to respond identically or even similarly to the same challenge except within very narrow limits. Animal data can provide us with models of biologic trends before proceeding to human application.

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  References Top

Page RC, Schroeder HE. Periodontitis in Man and Other Animals: A Comparative Review. 1st edition, S. Karger; 1982.  Back to cited text no. 1
Weinberg MA, Bral M. Laboratory animal models in periodontology. J Clin Periodontol 1999;26:335-40.  Back to cited text no. 2
Baker HJ, Lindsey HJ, Weisboth SH. Comprehensive Works on the Rat are The Laboratory Rat. Vol. 2. Orlando, FL: Academic Press; 1979.  Back to cited text no. 3
Poole TB. The UFAW Handbook on the Care and Management of Laboratory Animals. 6th ed. Wiley-Blackwell;1987. p. 309-30.  Back to cited text no. 4
Gupta OP, Shaw JH. Periodontal disease in the rice rat. I. Anatomic and histologic findings. Oral Surg 1956;9:592-603.  Back to cited text no. 5
Listgarten MA, Mayo HE, Tremblay R. Development of dental plaque on epoxy resin crowns in man. A light and electron microscopic study. J Periodontol 1975;46:10-26.  Back to cited text no. 6
Thilander H. Periodontal disease in white rat, experimental studies with special reference to some etiologic and pathogenetic features. Trans R Sch Dent Stockh Umea 1961;6:1-99.  Back to cited text no. 7
Hoffman MM, Schour I. Quantitative studies in the development of the rat molar. II Alveolar bone, cementum and eruption. Am J Orthod 1940;26:854-74.  Back to cited text no. 8
Sicher H, Weinmann JP. Bone growth and physiologic tooth movement. Am J Orthod 1944;30:109-32.  Back to cited text no. 9
Belting CM, Schour I, Weinmann JP, Shepro MJ. Age changes in the periodontal tissues of the rat molar. J Dent Res 1953;32:332-53.  Back to cited text no. 10
Socransky SS, Hubersak C, Propas D. Induction of periodontal destruction in gnotobiotic rats by a human oral strain of Actinomyces naeslundii. Arch Oral Biol 1970;15:993-5.  Back to cited text no. 11
Irving JT, Newman MG, Socransky SS, Heely JD. Histological changes in experimental periodontal disease in rats mono-infected with a gram-negative organism. Arch Oral Biol 1975;20:219-20.  Back to cited text no. 12
Heijl L, Wennström J, Lindhe J, Socransky SS. Periodontal disease in gnotobiotic rats. J Periodontal Res 1980;15:405-19.  Back to cited text no. 13
Labelle RE, Schaffer EM. The effects of cortisone and induced local factors on the periodontium of the albino rat. J Periodontol 1966;37:483-90.  Back to cited text no. 14
Fitzgerald RJ, Jordan HV, Stanley HR. Experimental caries and gingival pathologic changes in the gnotobiotic rat. J Dent Res 1960;39:923-35.  Back to cited text no. 15


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]

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