Intestinal folds of adult Xenopus laevis are formed by the proliferation of groups of cells in the tadpole epithelium a t metamorphosis. As the folds increase in number and in height, mitoses become confined to the troughs and a gradient of decreasing mitotic activity from troughs to crests is formed. This pattern of cell proliferation was confirmed in autoradiographic studies with 13Hlthymidine. Migration of cells from the fold troughs to the fold crests was also demonstrated by thymidine autoradiography and the epithelium renewal time was approximately 16 days. Thus cell proliferation and cell specialisation in the small intestinal epithelium of adult X laevis are inversely correlated.
The cyst wall which encloses the metacercaria of Fasciola hepatica consists of four major layers, one of which is further divisible into three sublayers. These layers are numbered I to IV, the first being the external layer. Layers I and II form the outer cyst which may be separated from the inner cyst formed from layers III and IV.Layer I, the thick incomplete external layer, covers the metacercaria dorsally and laterally. It is composed of tanned protein.Layer II is a thin fibrous layer closely adherent to the inner surface of layer I and is composed of mucoprotein and acid mucopolysaccharide.Layer III is made up of three separate sublayers, the relative widths of which vary in different regions of the cyst. In general IIIa consists of mucoprotein, IIIb of acid mucopolysaccharide and IIIc of neutral mucopolysaccharide.In the dorsal and lateral regions, layer IV appears to be formed of lamellae held in a protein–lipid matrix. The lamellae are composed of protein stabilized by disulphide linkages.In the ventral region of layer IV there is a thickened mucopolysaccharide area, the ventral plug, through which the metacercaria excysts.The relative importance of these resistant layers in providing protection against desiccation, toxic substances and attack by other organisms is discussed.This work was carried out during the tenure of a Commonwealth Post Graduate Scholarship. The author is grateful to Professor J. D. Smyth and Dr J. A. Clegg for advice and encouragement during the course of this work and in preparation of the manuscript.
Excystment of the metacercaria ofFasciola hepaticais an active process and occurs in two stages—activation and emergence. Activation is initiated by high concentrations of carbon dioxide, reducing conditions and a temperature about 39 °C. The reducing conditions increase the rate of action of the other two stimuli. The carbon dioxide stimulus need only be applied for 5 min, but the exposure time to the reducing conditions has to be of the order of 30 min. Changes in the order of application of the stimuli carbon dioxide and redox potential have no effect.The second phase, emergence, is triggered by bile. Metacercariae were held in an activated condition for 24 h, and when bile was added emergence took place normally.During excystment the metacercariae exhibit a complex behaviour pattern. After activation there is an initial period of rotatory activity, but after about 20 min a quiescent phase ensues when the metacercariae contract away from the cyst wall at an imperceptible rate. This behaviour occurs in response to stimulus from a high concentration of carbon dioxide and a temperature about 39 °C. When the metacercariae are exposed to bile the second phase of activity is initiated, consisting of antero-posterior thrusting movements directed against the ventral side of the cyst wall. Within about 15 min the juvenile flukes escape through a small circular hole in the ventral surface of the cyst wall which corresponds to the ventral plug region.
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