Fertilization

Definition of Fertilization

• is the fusion of gametes to initiate the development of a new individual organism

• In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo.

• Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside (external fertilization

External Fertilization

• Increasing the chances of fertilization, the following things must be available:

• 1. Shedding large # of gametes

• 2. Aquatic medium, to prevent gametes desiccation

• 3. Chemotaxis: one of the gametes secretes a substance like phermones to attract the other specific gamete.

Sea urchin

Mechanism of Chemotaxis in Sea urchin

• 1. In testes , sperms can’t move because of their internal pH is low(7.2) ,this is because of high concentration of CO2 in testes.

• 2. Sperms motility is acquired when sperms are spawned into the sea water . The pH of the sea water is 7.6

• This pH will activate an enzyme in the sperm tail (dynein ATPase) , splitting ATP , providing energy for the flagellum , sperm begins swimming

• 3. The ovum of the same species will secrete a chemotactic material (Resact) which is a peptide constructed from 14 amino acids.

• This material will direct the sperm toward the ovum for fertilization.

Fertilization in Sea urchin

• The unfertilized egg of sea urchin is surrounded by the following envelopes:

• 1. JELLY COAT

• 2. VITELLINE MEMBRANE (with binidin receptors)

• 3. PLASMA MEMBRANE

• 4. CORTICAL GRANULES

Unfertilized egg of Sea urchin

Steps of fertilization in Sea urchin

• 1. Gametes come close to each other by chemotaxis• 2. Sperm gets in contact with the outer most coat (jelly

coat). As a result of contact ,acrosomal reaction will be as following:

• A. Exocytosis: releasing proteolytic enzymes that digest a path through jelly coat

• B. Acrosomal process formation: actin that located behind the acrosomal vesicle will be polymerized forming a protrusion like in front of the head called Acrosomalprocess. The aim of this step to expose binibin ligands in the cell membrane of the sperm which are specifically will bind with binidin receptors on the vitelline membrane

• 3. Binding between binidin ligands on the acrosomal process and the binidin receptors on the vitelline membrane.

• RESULT: Rupturing the vitelline membrane.

• 4. Fusion between the sperm cell membrane and ovum cell membrane .

• RESULT: acrosomal cone formation.

• NOTE: ovum cell membrane before fusion is polarized, and after fusion it will be depolarized

• Fast block to polyspermy:In marine invertebrates, including the sea urchin, a fast block to polyspermy occurs within a tenth of a second of fusion. The fast block to polyspermy involves the opening of Na+ channels in the egg plasma membrane. Na+ flows into the egg cell, depolarizing the membrane. This depolarization prevents additional sperm from fusing to the egg plasma membrane. The egg plasma membrane is restored to its normal -70mV potential within minutes of fusion as the Na+ channels close, other + ions flow out of the cell, and Na+ is pumped out. If depolarization is prevented, polyspermy occurs - but how depolarization blocks polyspermy is not yet understood

• 4. Injection of the sperm nucleus and centosome through the ferilization cone into the ovum cytoplasm.

• During this step Inositol Triphosphte(IP3) will be generated as a second messenger

• Slow block to polyspermy:• The slow block to polyspermy begins within 10 seconds of

fusion of the sperm and egg plasma membranes. A compound called inositol triphosphate (IP3) causes the release of Ca++ from intracellular stores in the egg endoplasmic reticulum. Ca++ is first released at the site of sperm entry, and during the next minute, a wave of free Ca++ passes through the egg. This Ca++ results in the fusion of cortical vesicles with the egg plasma membrane, releasing their contents into the space surrounding the egg, called the perivitelline space. This raises the vitellinemembrane, and inactivates bindin receptors on the vitellinemembrane(fertilization membrane (FM)). Thus, any additional sperm will be inhibited or stuck in the FM

Sea urchin fertilized egg

Effects of Ca++ that released from Endoplasmic Reticulum

• 1. Cortical reaction and finally fertilization membrane formation.

• 2. Activation of the egg:

• A. triggers the proton pump . Cytoplasm becomes slightly alkaline(from pH6.8 – 7.3)

• B. Activation of metabolic machinery

• C. Increasing the rate of cellular respiration and protein synthesis

• D. Fusion of egg and sperm nuclei

• E. Onset of DNA synthesis

• F. First cell division( 1st cleavage ) after 90 minutes of the beginning of fertilization

Envelopes of bird egg