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30min, and yet be unyielding in the face of a compression test done over
Specific protocadherins, axial protocadherin (AXPC) and paraxial proto-
cadherin (PAPC), are expressed at the RNA level in the notochord and
somitic mesoderm, respectively, and they appear to have adhesion or
signalling roles, and may function in recognition and separation of these
tissues prior to or during convergence and extension (Kim et al., 1998;
Yamamoto et al., 1998; Kuroda et al., 2002). It is not known how these
molecules function, and whether they are associated with specific aspects of
cell polarity or with specific types of protrusions during convergent extension.
The messages are expressed in the notochord and somitic mesoderm,
respectively, but the expression pattern of the protein is not known. The
message for AXPC appears quite late, during mid-neurulation, and much later
than that for PAPC, which appears at mid-gastrula. When ectopically
expressed in two cell populations, an AXPC-like construct and PAPC appear
to mediate cell recognition and sorting out, and expression of PAPC appears
to increase adhesion and limit cell mixing during epiboly (Kim et al., 1998).
Whether these molecules have a signalling function, an adhesive function, or
both, in convergence and extension is not known.
Extracellular matrix and cell intercalation
It is also possible that adhesion to extracellular matrix may also function in
mediolateral intercalation despite the fact it seems unlikely to be used as a
substrate. A fibronectin-containing, fibrillar matrix forms on the blastocoele
roof during gastrulation. This matrix lies on the ventral surface of the neural
plate and between the prospective somitic mesoderm and the overlying
prospective neural tissue (Nakatsuji and Johnson, 1982, 1983; Komazaki,
1988; Darribere et al., 1990; Winklbauer and Stoltz, 1995). Inhibiting integrin-
mediated cell interactions with matrix slows blastopore closure and embryo
elongation (Ramos and DeSimone, 1996). This manipulation stops the
migration of the leading edge mesendoderm of the embryo toward the animal
pole (Davidson et al., 2002), and this alone may slow convergence and
extension by mechanical interference. However, inhibition of cell-matrix
interactions also blocks radial intercalation and explant extension (Marsden
and DeSimone, 2001, 2003), which drives the initial thinning and extension
step of convergence and extension. The effect of blocking cell-matrix
interactions could be solely due to blocking radial intercalation, or, in
addition, it could be that mediolateral intercalation also depends on cell-
matrix interactions. Also, radial intercalation may be necessary for the
subsequent mediolateral intercalation, and blocking the first may indirectly
inhibit the latter. It remains unknown whether integrin-mediated interactions
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