SUMMARY Scanning electron microscopy of the small intestine of the mouse 5 days after X‐ or neutron irradiation has revealed the formation of giant cells on the villus surface. Correlative light microscopy and transmission electron microscopy have shown that these giant cells are syncytial in nature. Characteristic features of lipid inclusions and apical microvilli suggest that these syncytia are giant enterocytes. It has also been shown that these giant cells are in contact with the connective tissue core of the villus and have a close contact with the normal enterocytes, thus maintaining mucosal integrity. It is postulated that radiation damage has caused incomplete separation during mitosis and that attempted division occurs outside the crypts of Lieberkuhn.

SUMMARY Examination of autolysed control mouse small intestine using scanning electron microscopy has revealed details of the connective tissue components of the mucosa. The cores of the villi are seen collapsed across the intervillous basin. Crypts of Lieberkuhn are seen as tubular channels stretching down from the intervillous basin. Sometimes the crypts are split in two by a connective tissue septum. The mouths of the crypts of Lieberkuhn are, in general, arranged in double rows between the single rows of villi. The ratio of number of crypts to numbers of villi was calculated as 5.01:1. This is close to the figure of 4.53:1, as quoted by Smith & Jarvis (1980) who used differential interference contrast microscopy to investigate the crypt to villus ratio. After radiation, the severe drop in the number of crypt mouths can be clearly seen by the combination of autolysis and scanning electron microscopy: the rows of crypt mouths between the villi have been lost, and many crypt mouths have been occluded by stromal tissue. The arrangement of the crypt mouths and the observation of mucosal abnormalities after irradiation have led to the postulation that cells leaving the crypt mouths move in a spiral manner towards and then up the villous surface: this postulated movement might imply an asymmetry in some properties of enterocytes. The use of scanning electron microscopy in conjugation with autolysis and irradiation has thus forced a critical re‐examination of the relationships between crypts and villi.

Scanning electron microscopy has been used to compare damage to mouse small intestinal mucosa after irradiation with different doses of photons and neutrons. Various stages of the collapse of villous structure seen after radiation include the production of conical and rudimentary villi and a flattened mucosa. A scale is proposed to relate radiation to villous damage. Points from this scale are taken to produce comparative ratios for equivalent damage produced by different radiation conditions. RBE values are quoted for neutron. X and gamma radiation given as single or fractionated irradiation doses and as whole or partial body irradiation. The relationship between the stroma in intravillous pegs and that of the pericryptal compartment is explored.

It has been observed that scanning electron microscopy is a more sensitive indicator of mucosal damage at low radiation dose levels than conventional quantitative crypt counting techniques. Three different fractionation schedules were subjected to investigation by both of these methods to try to elucidate some features of irradiation damage to the whole of the intestinal mucosa, at dose levels commonly used in clinical practice. Despite variations in the qualitative observations, there was a marked difference in two of the schedules between damage expressed as crypt counts and that described by the qualitative techniques. In the first case high crypt numbers were associated with severe mucosal damage, whereas the second schedule produced a reduced crypt count in association with low damage to the surface mucosa. A third schedule produced results in which there was a general agreement between low crypt numbers and considerable surface mucosal damage. However, observations were made of mucosal formations not previously seen on damaged mucosal surfaces. These resembled the appearance normally associated with the gut of patients suffering from coeliac disease. Variations in the qualitative observations were seen in all the schedules so that their interpretation in terms of perturbation of cellular kinetics is difficult.

The mucosal surface of human colon in 18 adults and children was examined by S.E.M. with subsequent resin histology and transmission electron microscopy of the same specimen. Results showed that this technique contributed useful information on the mucosa in health and in disease.