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many respects. Not least of these is the animal/vegetal polarity present within the membrane before and after fertilisation (see 1.2b above). Comparison with other embryo membranes suggests that a polarity in the Xenopus plasma membrane may also exist at later stages of development. Analysis of mouse morulae embryos indicates polarization of the plasma membrane at compaction of the embryo, with microvilli being localised mainly on the outer surface of the embryo, and microtubules and mitochondria localised beneath areas of intercellular membrane (Ducibella et al, 1977). Antibody, enzyme and lectin studies have also revealed differences in the membrane components present in these two domains in both mice and rat embryos (Izquierdo et al, 1980;

Izquierdo and Ebensperger, 1982; Handyside, 1980; Lois and Izquierdo, 1984). Cell surface labelling studies of

Pleurodeles waltlii embryos have indicated differences in the proteins present in the outer and newly synthesised inner membranes at very early stages of development (Darribere et al, 1982). A similar investigation of Xenopus laevis embryos has been carried out as part of the work in this thesis, and comparable results are obtained (See Chapter 4).

Polarity in the Xenopus embryo membrane exists between the animal and vegetal pole membranes around fertilisation, and between intercellular and external plasma membranes at later stages of development. Cell surface polarity seen in the embryo membranes at later stages of

development can be partly ascribed to the presence of epithelial cells which develop during the formation of the blastula. Epithelial cells are polarised and a substantial amount of work has been carried out to investigate how this polarity is established and maintained. A brief discussion of this topic is included below to illustrate the nature of this specialised membrane system, and to enable comparison of the cell surface polarity in epithelial cells with that present in the Xenopus embryo around fertilisation. For a review of cell surface polarity in epithelial cells, see Simons and Fuller (1985).

Epithelial cells are organised into sheets that separate compartments within the organism (Berridge and Oschman, 1972). The cell sheets maintain concentration gradients between the compartments that they separate. Cells

in the epithelium are linked through membrane Junctions and, as such, form a selective permeability barrier. Examples of these cells include transporting epithelia (e.g. those in the renal tubule), absorptive epithelia (e.g. those of the intestine) and secretory epithelia (e.g. hepatocytes).

Epithelial cells maintain concentration gradients by localising distinct sets of cell surface components to separate cell surface domains. The plasma membrane of epithelial cells is comprised of an apical domain (which is often covered with microvilli and faces the external side of the organism or organ), and the basolateral domain (which is often attached to an extracellular matrix arranged as a basal lamina).

apical and basolateral membranes in epithelial cells differs (Forstner et al, 1968; Douglas et al, 1972; Kawai et a l. 1974; Brasitus and Schachter, 1980). This suggests that the lipids in the separate membrane domains are not able to diffuse freely. Epithelial cell membrane proteins are restricted to one surface domain or the other. A protein assigned to one domain in one epithelial cell type is usually found in the same domain in all other epithelial cells (Simons and Fuller, 1985). No proteins are known which are distributed without polarity to the two surface domains.

The polarity of the epithelial cell membrane arises during biosynthesis from the sorting of membrane components to the separate membrane domains. The sorting of membrane proteins is believed to occur in the trans compartment of the Golgi in recognition of a signal on the protein thought to be part of the amino acid sequence. No specific primary sequence responsible for directing proteins to a particular membrane domain has been identified (Matlin, 1986), but other signals involved in intracellular transport have been characterised (Schatz and Butow, 1983; Hurt et al, 1984; Kalderon _e£ a l, 1984; Lanford and Butel, 1984; Dingwall, 1985; Blobel, 1980; Walter et al, 1984). Transport of the proteins (and possibly also membrane lipids) to the correct membrane domain occurs through selective membrane transport via budding and fusion of membrane vesicles (Palade, 1975; 1982).

Polarity in epithelial cell membranes is maintained by the presence of tight junctions which act as a fence to limit the diffusion of membrane constituents (Diamond, 1977). The tight junctions are organised alongside intermediate

junctions into a junctional complex which encircles the apex of each c e ll (Farquhar and Palade, 1963). This complex inhibits diffusion of membrane proteins and lipids (Diamond, 1977).

1.2e Summary

The Xenopus embryo cell membrane is specialised to its particular role in development. This is demonstrated by the unusual nature of the membrane in terms of its

biochemical and permeability properties, and in the polarisation of the membrane into separate domains. Substantial changes occur within the Xenopus laevis cell surface membrane throughout development. These changes appear to be related to cell differentiation and to varying functions of the membrane in morphogenesis. Examples of changes in the cell membrane relating to characterised functions of the membrane in development will be discussed in section 1.4 below.

The polarity in epithelial cells was discussed because these cells are present in developing Xenopus embryos at a fairly early stage, and because membrane protein distribution between inter-cellular and external cell surface membranes is relevent to the work carried out in this thesis in Chapter 4. The polarity in epithelial cells is not similar to that seen in Xenopus eggs ( indeed the mechanism involved

in maintaining the separate membrane domains in fertilsed and unfertilised Xenopus eggs is unknown). No evidence exists for the presence of a permeability barrier to explain membrane domains in Xenopus embryos, such as the tight junction in

SECTION 1.3

One of the central problems of development concerns the nature of the control mechanisms whereby cells of a common lineage are caused to differentiate into distinct cell types. In this section aspects of developmental control will be discussed with the emphasis placed on potential areas of interaction by the cell membrane.