  This review paper presents the gluon, sea, and valence quark distributions of
the pseudoscalar (PS) mesons. The calculations of the parton structure of PS
mesons are performed using the Bethe-Salpeter equation-Nambu--Jona-Lasinio
(BSE-NJL) model, which offers a clear description of the dynamical chiral
symmetry breaking (D$\chi$SB) of the low-energy nonperturbative quantum
chromodynamics (QCD), with the help of the Schwinger proper-time regularization
scheme that simulates the color QCD confinement. Our results for the dynamical
quark mass -- which emerged from the spontaneous chiral symmetry breaking
(S$\chi$SB) -- are generated via the chiral condensate in the chiral limit and
beyond. Results for the valence parton distribution functions for the PS meson
at the factorization scale $Q = 2$ GeV are in excellent agreement with
experimental data and the gluon distribution for the pion fitted well with the
lattice QCD and Jefferson Lab Angular Momentum (JAM) QCD global fit analysis.
