I'd suggest looking at the help page a little more for PCA:
"COEFF = princomp(X) performs principal components analysis (PCA) on the n-by-p data matrix X, and returns the principal component coefficients, also known as loadings. Rows of X correspond to observations, columns to variables. COEFF is a p-by-p matrix, each column containing coefficients for one principal component. The columns are in order of decreasing component variance."
by first component of the 1-10 analysis, do you mean the first row of the coefficients? If so, I think this will work for you:
for i = 1:size(x,1)/10
[COEFFnew,SCOREnew,latentnew] = princomp(x((i-1)*10+1,i*10));
coeff(i,:) = COEFFnew(1,:);
coeff should be a 5x50 for your example of a 50x50 matrix, but the same code will also expand to a 1000x1000