Keep your answer brief. 1) FAST BLOCK = electrical barrier, preventing fusion of sperm p.m. and egg p.m.; Slow Block is mechanical block to polyspermy, i.e. the fertilization membrane., which hardens and is impervious to further sperm.

The slow block to polyspermy in the sea urchin embryo consists of a physical barrier to further sperm penetration into the egg. The fertilization envelope is formed by the lifting of the vitelline envelope away from the egg plasma membrane.

The fast block to polyspermy, which has been best studied in sea urchins, consists of a rapid electrical depolarization of the plasma membrane of the egg. The resting membrane potential of the egg changes from about − 70 to + 10 mV within 2–3 s after fusion of the spermatozoon with the egg.

Water rushes into the space, causing the vitelline membrane to rise up and become the fertilization envelope. This produces a more permanent block to polyspermy (the slow block).

The slow block to polyspermy. The eggs of sea urchins (and many other animals) have a second mechanism to ensure that multiple sperm do not enter the egg cytoplasm (Just 1919). The fast block to polyspermy is transient, since the membrane potential of the sea urchin egg remains positive for only about a minute.

Fast block to polyspermy occurs early in the acrosomal reaction and involves depolarization of the egg cell membrane during the sperm/egg cell membrane fusion. Slow block to polyspermy occurs later in the cortical reaction and involves receptor clipping.

Together, the fertilization envelope and other changes in the egg’s surface impede the entry of additional sperm nuclei and thus act as a longer-term slow block to polyspermy. calcium ions (Ca2+) in the egg.

The fast block to polyspermy. The fast block to poly-spermy is achieved by changing the electric potential of the egg plasma membrane. This membrane provides a selective barrier between the egg cytoplasm and the outside environment, and the ionic concentration of the egg differs greatly from that of its surroundings.

During fertilisation, the fusion of sperm depolarises the membrane of egg cell and causes fast block to polyspermy.

At fertilization, eggs are exposed to multiple sperm but entry of more than one sperm causes polyploidy, polyspermy, which results in abnormal embryo development. To ensure fusion of a single sperm, the egg becomes rapidly intolerant to additional sperm.

Fusion of the sperm and oocyte cell membranes initiates a wave of intracellular Ca2+ that passes throughout the oocyte. This results in a transient hyperpolarization of the oocyte plasma membrane which may represent a potential block to polyspermy.

The slow block to polyspermy in the sea urchin embryo consists of a physical barrier to further sperm penetration into the egg. Cortical granule exocytosis results in the formation of the fertilization envelope (often called the fertilization “membrane”, even though the structure is not a true membrane)

The slow block to polyspermy in the sea urchin embryo consists of a physical barrier to further sperm penetration into the egg. Cortical granule exocytosis results in the formation of the fertilization envelope (often called the fertilization “membrane”, even though the structure is not a true membrane) The fast block…

Note: Polyspermy is a significant problem for in vitro fertilization of humans (IVF). The polyspermy rate in IVF is about 10%. Polyspermyis prevented by: modest sperm number; Fast block; Slow block. Acrosomal Reaction The acrosomeis the tip of the sperm head.

A “fast” block acts at the “zona pellucida” (equivalent to the jelly coat or vitelline membrane). The molecular mechanisms are less well understood than those of sea urchins. The blocking is incomplete. In in vitro fertilization, the rate of polyspermy is as high as 10%.