  Given an $n\times n$ matrix $c$ over a unitary ring $R$, the centralizer of
$c$ in the full $n\times n$ matrix ring $M_n(R)$ is called a principal
centralizer matrix ring, denoted by $S_n(c,R)$. We investigate its structure
and prove: $(1)$ If $c$ is an invertible matrix with a $c$-free point, or if
$R$ has no zero-divisors and $c$ is a Jordan-similar matrix with all
eigenvalues in the center of $R$, then $M_n(R)$ is a separable Frobenius
extension of $S_{n}(c,R)$ in the sense of Kasch. $(2)$ If $R$ is an integral
domain and $c$ is a Jordan-similar matrix, then $S_n(c,R)$ is a cellular
$R$-algebra in the sense of Graham and Lehrer. In particular, if $R$ is an
algebraically closed field and $c$ is an arbitrary matrix in $M_n(R)$, then
$S_n(c,R)$ is always a cellular algebra, and the extension $S_n(c,R)\subseteq
M_n(R)$ is always a separable Frobenius extension.
