phoblast-associated HA secretion during outgrowth in vitro but significant synthetic activity by the endoder- mal derivatives of differentiating inner cell masses. To identify the cell lineages responsible for secretion of HA into the embryonic cavities and to investigate the origin

The extracellular matrix (ECM) glycosaminoglycan (GAG) hyaluronan (HA) and its binding proteins, the hyaladherins, have long been implicated in the regulation of a variety of morphogenetic processes associated with embryogenesis (reviewed, Toole, 1981, 1990; Toole et al., 1984). Due to the structure of the molecule and its hydrophilic nature, HA forms hydrated gels even at very low concentrations (Comper and Laurent, 1978). This property results in expansion of the ECM by the generation of a high osmotic pressure, permitting cell proliferation and migration, and, by reducing contact-mediated cell-cell communication, postponing differentiation (Toole and Trelstad, 1971; Toole et al., 1972; Underhill and Toole, 1981; Kujawa and Tepperman, 1983; Brecht et al., 1986). Removal or reduction of ECM HA in later development is often associated with the onset of differentiation (Toole and Gross, 1971; Orkin et al., 1985; Kulyk et al., 1987; Toole et al., 1989; Van Straaten et al., 1990). Since HA lacks an integral protein core and is not known to exhibit any species-specific structural differences, in situ detection of the molecule has not been possible using standard immunohistochemical techniques. Early in situ studies of HA in embryogenesis at the light microscope level relied on comparative analyses of Alcian blueor Ruthenium redstained tissues, following selective digestion of ECM components with specific GAG-degrading enzymes (Kvist and Finnegan, 1970; Bernfield et al., 1972; Derby, 1978; Morriss and Solursh, 1978; Pintar, 1978). However, use of the hyaluronan-binding region of cartilage proteoglycan as a high-affinity HA-specific probe has greatly facilitated the detection of HA in vivo and in vitro (Ripellino et al., 1985; Green et al., 1988). Whilst investigating the role of HA in mouse uterine differentiation and trophoblast invasion during implantation, using the biotinylated proteoglycan 483 Development 117, 483-492 (1993) Printed in Great Britain © The Company of Biologists Limited 1993