The diurnal variation of mitotic response in the epithelium at the edge of acute ulcers in the rectal mucous membrane of guinea-pigs.

The mitotic response to wounding was examined in ulcers resulting from the excision of small pieces of rectal mucous membrane in two groups of guinea-pigs. One group was wounded at 09.00 hours and a second group at 21.00 hours. Mitotic counts were carried out in the glandular epithelium at the ulcer edges at 2 hourly intervals over a period of 24 hours. Mitotic activity increased in 2--4 hours and thereafter showed a peak-and-trough pattern. The wounded rectal epithelial cells exhibited a diurnal variation with a peak of activity during the day and low activity at night. It would appear that such diurnal rhythmicity is not species-specific because it was similar in both the guinea-pig and rat. The evidence suggests that there is not a chalone-adrenaline complex in the rectal epithelium as the diurnal variation was similar in the nocturnal rat and in the diurnal guinea-pig, and was not abolished by wounding. Mitotic activity was lower in the glands at the immediate wound edge, and in the upper cells of the glands. The cells in these situations are probably designated for migration to close the defect, for migration excludes mitosis. It seems possible that the mechanisms responsible for the circadian rhythm of mitosis are different from those which induce the bursts of mitotic activity after wounding; if so this would be contrary to the chalone concept.

The mitotic response of the stratified squamous epithelium at the edge of large perforations of the tympanic membrane in guinea-pigs.

Large perforations of the tympanic membrane were made in groups of guinea-pigs. The mitotic response in the stratified squamous epithelium at the edge of the wound was examined in perforations between 1 and 28 days after injury. Peaks of activity were discovered at 2, 15-16 and 24 days after wounding. It was also found that the maximal proliferative response was not at the immediate wound edge, as is usually maintained, but as some 2 mm from the wound edge. The results correspond remarkably well with work previously done by the present author on wounded rectal mucous membrane. The results are examined in the light of the chalone hypothesis of mitotic control.

Some observations on the diurnal variation of mitosis in the stratified squamous epithelium of wounded tympanic membrane.

A study of mitosis in the stratified squamous epithelium of the tympanic membrane of the guinea-pig was made after wounding this organ at 09.00 h in one series of animals and at 21.00 h in a second series. It was found that the diurnal variation of mitosis was abolished by the injury. This finding corresponds with a basic requirement of the chalone-adrenaline hypothesis which is considered to be a factor in the mitotic control of epidermal cells. It was also found that the maximum number of dividing cells was at the wound edge in the group wounded at 09.00 h, which also agrees with the chalone concept. In the group wounded at 21.00 h, however, the initial mitotic response was in cells distally placed from the wound edge which is more difficult to explain. Perhaps these cells are inhibited mitotically by the factors normaly producing a low nocturnal mitotic rate and undergo migration to cover the defect in the first instance, and only divide at a later time when the mitotic inhibition is lifted. It appears that the factors responsible for the increased mitotic rate after wounding are different from those responsible for the diurnal variation.

An electron microscopical study of chronic ulcers of the colon in rats.

Chronic colonic ulcers in the rat were produced by the administration of a hydrogen peroxide enema. Lesions of up to 10 months' duration were obtained by this method and a selection was examined ultrastructurally. It was hoped that the study would reveal causes for the chronicity of the lesions. From previous work it appeared that there was no failure of epithelial migration from the edge of the wound. The majority of cells at the ulcer margins demonstrated features of regeneration similar to those seen in other tissues, and that there were no specific changes concomitant with chronic ulceration. The majority of epithelial cells at the ulcer edge showed features of differentiation which indicated that there was no obvious fault in this process so essential to the basic requirements of tissue repair. It is possible that chronicity is produced by a failure of the epithelio-mesenchymal interaction mechanisms which, potentiated by an ischaemia resulting from submucosal vascular damage, results in the consequential death of migrating epithelial cells. This hypothesis is supported by the appearance of a squamous metaplasia in the healing lesions, a process which is commonly attributed to a chronic inflammatory response which may be induced by a relative vascular insufficiency.

Squamous metaplasia in the healing of chronic colonic ulcers of the rat.

Chronic ulcers of the rat colon were produced by a administering a 3 per cent. hydrogen peroxide enema. Twenty-three per cent. of the animals with chronic ulcers had areas of squamous metaplasia at the wound margins which in some cases covered the defect. Electron microscopy revealed some areas similar to epidermis with the four characteristic cell layers. The cells demonstrated many filaments, membrane-coated granules, keratohyalin granules and keratin. The cells were linked by complex of cytoplasmic bridges and frequent desmosomes. Basal cells often displayed large irregular nuclei with double nucleoli. Other areas demonstrated a grossly irregular surface with marked cellular pleomorphism and defects in the basement lamina. The incidence of squamous metaplasia and squamous-cell carcinoma in the colon of man is discussed. The ultrastructural characteristics of squamous metaplasia in the trachea and bronchi is recorded and compared with those of the colon. It is suggested that as neoplasia has been observed to follow experimental metaplastic lesions in the bronchus, a similar ocurrence could take place in the colon as the squamous cells exhibited similar characteristics.

A study of mitotic activity and the diurnal variation of the epithelial cells in wounded rectal mucous membrane.

Excision ulcers of the rectal mucous membrane were made in two groups of rats. One group was wounded at 09.00 hours and the second group at 21.00 hours. Mitotic counts were carried out in the glandular epithelium at the ulcer edges at 2 hour intervals over a period of 24 hours. Mitotic activity increased in 2-4 hours and thereafter showed a peak-and-trough pattern. The wounded rectal epithelial cells exhibited a diurnal variation with a peak of activity during the day and a low period of activity at night. It would seem unlikely that the adrenaline-chalone complex acts on the rectal epithelium, as this would entail maximal mitotic activity during periods of rest, when the circulating levels of adrenaline in the rat are at their lowest. The experiments clearly showed that the diurnal variation was not abolished by wounding. The increased mitotic activity occurred in the epithelial cells in the lower and mid thirds of the colonic glands; dividing cells were rarely seen in the top twenty cells or so of the glands, or in the surface epithelium. Mitotic activity was often lower in the first one or two glands at the immediate wound edge, which is difficult to explain by present theories of mitotic control.

Metabolism of H-Gibberellin A(1) and H-Gibberellin A(4) by Phaseolus coccineus Seedlings.

[(3)H]-Gibberellin A(1) (GA(1)) and (3)H-GA(4) were applied separately to Phaseolus coccineus seedlings grown under red light. (3)H-GA(1) was converted to a compound with gas-liquid radiochromatography retention times identical to those of GA(8). (3)H-GA(4) underwent conversion to at least three metabolites, none of which corresponded to GA(1-38). The rate of metabolism of (3)H-GA(4) was significantly higher than that of (3)H-GA(1).

The effect of light on the gibberellin metabolism and growth of Phaseolus coccineus seedlings.

Light inhibits the rate of stem elongation of Phaseolus coccineus L. seedlings. Gibberellin A4 (GA4), an endogenous component of Phaseolus seedlings (Bowen et al., Phytochem. 12, 2935-2941, 1973) promotes stem growth in the light but not in darkness. Dark-grown seedlings contain larger GA pools than light-grown plants. Apically applied [(3)H]GA4 in etiolated bolised more extensively in the light. The slower rate of metabolism of [(3)H]GA4 in etiolated seedlings is not a consequence of isotopic dilution by the endogenous GA4 pool or a lack of penetration of the labelled material. While it can be concluded that the capacity of seedlings to metabolise [(3)H]GA4 is greater in the light than in darkness, it does not necessarily follow that there is a more rapid rate of turnover of endogenous GA4 in light-grown tissues. The results are discussed in relation to the involvement of GAs in the inhibitory effects of ligh on stem elongation.