P14. Extracellular matrix deposition and function in the early chick embryo

Abstract

Cells within all multicellular organisms are surrounded by a heterogeneous mixture of macromolecules, collectively known as the extracellular matrix (ECM). The ECM is synthesised and organised by cells, providing them with a substrate to interact with, spaces to migrate through and cell–ECM engagement also directly impacts cell polarisation, proliferation, survival and differentiation. ECM molecules are present from the earliest stages of embryo development and the developmentally regulated dynamics of ECM assembly and degradation play key roles in many embryological processes. In this study we addressed how fibronectin, laminin and chondroitin sulphate proteoglycans are deposited in the early (HH12–15) chick embryo, focusing on the developing mesodermal regions and the neural tube. We find that fibronectin is the first ECM molecule to form a complex ECM. Although present in the interstitial space, fibrillar fibronectin matrices are more prominent lining epithelioid (somites, notochord and intermediate mesoderm) and epithelial (neural tube) tissues, before any assembled basement membrane is detected. However, as these epithelioid/epithelial tissues develop, laminin is progressively assembled on their surface. Assembly starts in small patches localized in fibronectin-poor areas and with developmental time, these patches grow in size until the tissues are lined with a laminin basement membrane. Inhibition of fibronectin assembly leads to abnormal cell polarization and containment, suggesting that the fibronectin matrix is essential for the initial epithelial development of these tissues, before the appearance of the basement membrane. Although the interstitial spaces of HH12–15 embryos are only partially filled with a fibronectin matrix, we find that chondroitin sulphate proteoglycans are progressively deposited in these spaces and appear to play a role in separating the early tissue units. In agreement with this, treatment of cultured chick embryo explants with chondroitinase ABC leads to the collapse of all extracellular space and to mesodermal tissue deformations. We conclude that fibronectin matrices play an important role in polarizing cells and containing cell movements in young epithelia before a laminin-containing basement membrane forms, while chondroitin sulphate proteoglycans are crucial for maintaining interstitial spaces. The developmental deposition of these ECMs is thus essential for the correct morphogenesis of the early embryo.