Histomorphometric analysis of epithelial changes in chemically induced oral mucosal carcinogenesis in rats

Ultrastructural changes of the palatal mucosa following application of 4-nitroquinoline-l-oxide (4NQO) in rats subjected to major salivary gland excision

OBJECTIVE

It has been suggested that saliva exerts a protective role against the carcinogenic effect of various substances in the oral cavity. The objective of this study was to examine the ultrastructural changes of the palatal mucosa caused by the application of 4-nitroquinoline-l-oxide (4NQO) in the presence or absence of saliva.

STUDY DESIGN

Wistar-Furth rats subjected and not subjected to total bilateral excision of the major salivary glands were either painted with an aqueous solution of 4NQO or with propylene glycol only (controls). Two animals of each group were humanely killed periodically. The areas of the palatal lesions were immediately sliced and processed for TEM examination.

RESULTS

Ultrastructurally, the progressive changes to squamous cell carcinoma were observed in the animals painted with 4NQO. In the desalivated animals group, the ultrastructural alterations appeared earlier than in the group with salivary glands.

CONCLUSIONS

Saliva appeared to delay but not hinder tumor induction by 4NQO.

The role of ascorbic acid in oral cancer and carcinogenesis

L-ascorbic acid is an essential dietary vitamin in humans, primates and certain mammals and is endogenously synthesised in some species. Epidemiological and ecological studies have shown that L-ascorbic acid has a protective effect against cancer, in particular non-hormone-dependent malignancies, such as oropharyngeal neoplasms. Experimental in vivo and in vitro studies, however, have yielded more controversial results, suggesting that the effects of L-ascorbic acid are dose- and perhaps, time-dependent with different effects depending on the species or organ studied. An update of the epidemiological and experimental evidence linking L-ascorbic acid to oral cancer and carcinogenesis is discussed together with a brief review of the possible mechanisms of action of L-ascorbic acid.

Photofrin-mediated photodynamic therapy of chemically-induced premalignant lesions and squamous cell carcinoma of the palatal mucosa in rats

Photodynamic therapy (PDT), an experimental cancer therapy, was studied in an animal model of chemically-induced epithelial dysplasia and squamous cell carcinoma. PDT was performed 24 hours after i.v. injection of 2.5 mg/kg bw Photofrin, and using 100 J/cm2 incident light at two activation wavelengths (514.5 nm or 625 nm). Two days after PDT, the majority of rats macroscopically showed a marked erythema of the entire palatal region. Microscopically all the rats showed oedema, haemorrhage, and necrosis of the epithelium of the intermolar area. The long-term results were not so favourable. No evidence of disease was found in 6 out of 20 rats in the 514.5 nm group and in 2 out of 20 rats in the 625 nm treated group. Epithelial dysplasia was found in 14 out of 20 rats in the 514.5 nm group, and in 18 out of 20 rats of the 625 nm treated group. Squamous cell carcinomas were found in 4 out of 20 rats treated with 514.5 nm and in 7 out of 20 rats in the 625 nm treated groups. Comparing both treatment wavelengths, better results were obtained in the 514.5 nm groups as this wavelength gave less normal tissue damage. Based on the results of this study the application of PDT for the treatment of field cancerization and squamous cell carcinoma of the oral cavity, is discussed.

In vivo photo-detection of chemically induced premalignant lesions and squamous cell carcinoma of the rat palatal mucosa

Photo-detection using in vivo fluorescence was studied for different stages of chemically induced premalignant lesions and squamous cell carcinoma (SCC) of the Wistar rat palatal mucosa. It was found that the epithelial dysplasia (numerically expressed in the epithelial atypia index (EAI) of the rat palate, induced by repeated application of the carcinogen 4-nitroquinoline 1-oxide (4NQO), showed an increase approximately proportional to the duration of the application period. Photo-detection of the lesions using Photofrin-induced fluorescence was studied with dual-wavelength excitation and the subtraction of images, in an attempt to reduce the autofluorescence. The Photofrin dose was 2.5 mg kg-1. This was based on a dose-response study for normal tissue damage by photodynamic therapy (PDT) in this animal model, because the underlying rationale was to study photo-detection as a method of locating additional (early) malignancies in patients treated by PDT. Fluorescence intensities 24 and 48 h after injection of Photofrin were shown to increase with the duration of 4NQO application and with increasing EAI. For an EAI greater than 15, there was a statistically significant difference (p < 0.01) between the fluorescence signals obtained with and without the injection of Photofrin. Fluorescence signals of these lesions without the use of Photofrin (autofluorescence) also showed an increase with increasing stages of epithelial dysplasia of the rat palate. However, the fluorescence signals obtained with Photofrin were always higher than those of the autofluorescence. From this study, we conclude that photo-detection with Photofrin has potential in distinguishing chemically induced premalignant lesions and squamous cell carcinomas from the normal rat palatal mucosa. Photofrin (2.5 mg per kg of body weight) certainly adds to the sensitivity of photo-detection, but autofluorescence alone also has promising features for detecting premalignant and malignant lesions of the oral mucosa.

Epithelial dysplasia and squamous cell carcinoma of the Wistar rat palatal mucosa: 4NQO model

BACKGROUND

The carcinogen 4-nitroquinoline 1-oxide (4NQO) has been used in several studies concerning experimental oral carcinogenesis to induce squamous cell carcinoma in the palatal mucosa of rats, whereas limited attention has been paid to preceding premalignant mucosal changes. The aim of this study was to describe the macroscopic and microscopic changes of the rat palatal mucosa treated with 4NQO as a function of the application time of this carcinogen.

METHODS

The carcinogen 4NQO was applied to the palatal mucosa of male Wistar rats three times a week for 2-26 weeks. The Epithelial Atypia Index (EAI) was used to quantify the degree of dysplasia induced by the application of 4NQO for various time intervals.

RESULTS

This study shows that the EAI increases as a function of the application time. As soon as 2 weeks following application of the carcinogen, dysplastic alterations in the mucosa were observed. The extent and grade of the dysplastic changes increased with time. At week 26, squamous cell carcinoma developed. In rats in which the application of 4NQO was discontinued earlier, the application period appeared to be in inverse proportion to the latency period i.e., the period between the last application of the carcinogen and the first clinical sign of squamous cell carcinoma. This emphasizes the premalignant character of the induced epithelial changes, because in all the rats squamous cell carcinoma eventually developed.

CONCLUSIONS

From this study, we concluded that the 4NQO rat palate model is a proper model in which both premalignant epithelial lesions and squamous cell carcinoma of the oral mucosa can be studied.

Nuclear morphometry in experimental oral mucosal carcinogenesis

The area, perimeter and diameter of basal cell nuclei of rat palatal epithelium were measured and the deviation of the basal cell nuclear profile from the form of a sphere was assessed after the application of the carcinogen 4-nitroquinoline-1-oxide (4NQO). After a 24-week treatment-free period, designed to eliminate the irritant effect of the carcinogen, the rats were killed, the palatal mucosa was recovered and processed and the nuclear histometry was assessed with image analysis techniques. The basal cell nuclear area increased as the epithelium became dysplastic and then decreased as carcinoma developed, but there were significant variations in this parameter in the control groups. Basal cell nuclei from moderately or severely dysplastic epithelium, and from epithelium adjacent to areas of invasive carcinoma, were significantly less regular in profile by comparison with control nuclei. Variations in nuclear profile, but not nuclear area, perimeter or diameter, might reflect fundamental nuclear alterations of significance during the process of carcinogenesis.

Induction by 4-nitroquinoline-1-oxide of oral epithelial dysplasia and neoplasia in scurvy-prone osteogenic disorder Shionogi (ODS) rats

Carcinogenesis by 4-nitroquinoline-1-oxide (NQO) in the oral mucosa is a reliable method of obtaining oral mucosal squamous cell carcinoma (OMSCC) and allows examination of various stages of oral cancer development. In vivo and in vitro studies have indicated that L-ascorbic acid (AA) may have a role in cancer prevention. The Wistar "scurvy-prone" osteogenic disorder Shionogi (ODS) rat of the od/od substrain is unable to synthesize AA and requires supplementation for its survival. This study examined the effects of NQO on the oral mucosa of ODS and outbred Wistar rats. NQO (0.5%) was applied topically to the palatal mucosa of 72 male ODS and 36 outbred Wistar rats three times weekly for 4, 8, 12, 16, 20, and 24 wks. The ODS rats were divided so that 36 rats were given 2.5 g/l AA in the drinking water and 36 rats were given 0.33 g/l AA. Vehicle-treated and untreated control animals were included. The rats were killed two weeks after the final NQO application, and the tissues were examined. Epithelial dysplasia was assessed using a modified Smith and Pindborg (1969) index. The ordered categorical scores were analyzed appropriately. Plasma AA levels were checked in ODS and outbred rats at the start and end of the experiment. The results indicated that the oral mucosa of the ODS and outbred rats were susceptible to NQO but that the rate of dysplasia and OMSCC development differed between them, with more rapid changes being found in the ODS rats (p < or = 0.05). No significant difference was found in the dysplasia scores and in the rate of OMSCC development between ODS rats given 2.5 g/l of AA and ODS rats given 0.33 g/l of AA (p > 0.05). No epithelial changes were observed in the palatal mucosa of vehicle-treated and untreated controls. The plasma AA level mean (+/- SEM) was 56 +/- 6 microM for the outbred rats, 8 +/- 1 microM for the ODS rats given 0.33 g/l AA supplementation, and 29 +/- 2 microM for the ODS rats given 2.5 g/l AA. It was concluded that the chronic AA-deficient state in ODS rats played an insignificant role in oral carcinogenesis and that other factors, for example, genetic differences in susceptibility to NQO, contributed to the present findings.

Preliminary assessment of the epithelial nuclear-cytoplasmic ratio and nuclear volume density in human palatal lesions

We have analysed both the nuclear-cytoplasmic (N/C) ratio and nuclear volume densities (VVN) in defined strata from human hard palate lesions with and without malignant potential to determine the prognostic reliability and/or validity of this parameter. Measurements of cellular and nuclear areas of basal and spinous cells from normal (N) and pathological palatal epithelium were made on histological sections using an image analyser. The lesions comprised fibrous hyperplasia (FH), traumatic inflammation (INF), benign hyperkeratosis (HK), squamous cell papilloma (PP), dysplastic epithelium adjacent to invasive carcinoma (CE) and islands of invasive squamous cell carcinoma (CI). In basal cells, no significant differences were detected in comparisons of N/C and VVN between all pathological groups and the N control group. The mean value for CE was lower than that obtained for N. In spinous cells, the only statistically significant comparison was between IF and FH for both N/C and VVN. Both parameters were lower in CE than in N. Of all groups analysed except CI, the CE group is the only one likely to possess an increased malignant potential. The N/C ratio therefore seems to be of no value as a predictor of malignancy in palatal epithelial lesions.

Comparison of epithelial dysplasia--the 4NQO rat palate model and human oral mucosa

Epithelial dysplasia in the rat palatal mucosa was induced by application three times a week of the carcinogen 4-nitroquinoline 1-oxide (4NQO). With the Epithelial Atypia Index (EAI), the successive stages of 4NQO-induced epithelial dysplasia were compared with specimens of human oral epithelial dysplasia. It appeared that there was a close similarity between the histologic features of 4NQO-induced dysplasia in the rat palatal mucosa and human oral epithelial dysplasia. Thus, the 4NQO rat palate model seems to be appropriate to study and assess new treatment modalities of premalignant epithelial lesions of the oral mucosa in man.

The effect of intraperitoneal N-methyl-N-nitrosourea on hamster palatal gingiva and intermolar mucosa

Fifty 4- to 6-week-old male random-bred golden hamsters were injected intraperitoneally with a weight-related dose (12.5 mg/kg body weight) of N-methyl-N-nitrosourea (NMU) three times a week for 4 weeks. Groups of seven animals were killed 10, 16 and 22 weeks after the first injection. The palatal gingiva from six animals and the intermolar mucosa from 21 animals was examined. Seven male age-matched untreated control animals were killed at each period. Although all NMU-treated hamsters showed dysplastic and neoplastic changes similar to those in human oral squamous-cell carcinoma, other changes such as acantholytic dyskeratosis, invading cysts, duct-like structures and basaloid islands and cords were not. The extent and severity of the changes increased with time so that by 22 weeks there was extensive involvement of the palatal bone and marrow spaces, the molar periodontal ligament and the greater palatine neurovascular bundle by neoplastic epithelium. The invading epithelium was derived from the junctional, crevicular and palatal gingival and intermolar epithelium. The latent period for the crevicular and junctional epithelia was shorter than that for the palatal gingival and intermolar epithelium. The invasive changes from the latter epithelium were often preceded by exophytic changes such as epithelial projections, papillae and papillomas. Such changes were infrequent for the gingival, crevicular and junctional epithelia. The study shows that intraperitoneal NMU acts as a complete carcinogen on the palatal gingival and intermolar epithelium in hamsters.

Histoquantitative study of inflamed tongue mucosa

A quantitative, semiautomatic analysis of histologic features of the specific inflammatory changes occurring on the dorsum of human tongue was made for 37 tongues. Well-documented histologic differences between clinically normal appearing tongue (control), atrophic, geographic and fissured tongue were shown quantitatively. The epithelium of atrophic tongue was thinner compared to the controls. Reduction of the thickness was due to the shortened or totally missing rete pegs. Epithelium of geographic and fissured tongue showed irregular rete pegs. In fissured tongue heavy inflammation seemed to shorten the rete pegs. In atrophic tongue the connective tissue was thicker compared to the controls. This could be due to mild, chronic inflammation causing loose connective tissue described in atrophic tongue. Muscle cells of geographic tongue appeared to be as in the controls, whereas in atrophic and fissured tongue the uppermost muscle cells were small and separated compared to the controls.

Basal cell nuclear size in experimental oral mucosal carcinogenesis

It has been suggested that the size of the nuclei of epithelial basal cells can be used in predicting the likelihood of malignant transformation of epithelium. This proposition was assessed in rat palatal epithelium after the carcinogen 4-nitroquinoline-1-oxide had been applied to the epithelium for varying periods of time. No consistent alterations in basal cell nuclear size, including area, perimeter, diameter and regularity of form were found with routine light microscopy as the epithelium passed through various stages of dysplasia to carcinoma. This finding casts doubt on the value of using a variation of basal cell nuclear size as a predictor of malignant transformation.