|
The ova of P. semisulcatus undergo a series of activational
changes upon spawning into normal seawater. These changes
include: (1) release of abundant jelly rods from the cortical
crypts and breakdown of the vitelline envelope; (2)
transformation of the jelly rods into a homogeneous jelly layer
that forms around the egg; (3) resumption and completion of
meiosis; (4) formation of an extracellular hatching envelope
between the extrusion of two polar bodies. The jelly rods of
the eggs were originally composed of dense matrices and bottle-
brush elements and then were transformed to network material
by proteases to form a homogeneous jelly layer covering the egg
surface after spawning. Ca2+ was required for jelly
transformation. The vitelline envelope was also dissipated by
proteases, and the reactive process of these proteases may need
Mg2+. The vitelline envelope was dissipated very quickly when
eggs were spawned in Ca2+-free ASW and this breakdown was not
triggered by the proteases. The resumption of meiosis and the
formation of the hatching envelope were triggered by exposure
to Mg2+ in the seawater. The activating pathway of the meiotic
resumption and hatching envelope formation had no relation to
the pathways of the vitelline envelope breakdown and the jelly
layer formation. The formation of the hatching envelope
required external Ca2+. In Ca2+-free ASW, the cortical
vesicles of eggs can exocytose, however, the reactions were
delayed and the formation and elevation of the hatching
envelope were incomplete. Protease inhibitors, SBTI and PAB,
inhibited formation of the hatching envelope. The exocytosis of
the cortical vesicles was not inhibited by SBTI, thus SBTI may
act on the process after exocytosis to affect the hatching
envelope formation.
|