Molecular Mechanism of Fertilization (PDF)

MOLECULAR MECHANISM OF FERTILIZATION UNIVERSITY OF KYRENIA MEDICAL SCHOOL DR. İDİL ASLAN FERTILIZATION Fertilization is the fusion of the male and female gamete. The process involves the fusion of an oocyte with a sperm ; creating a single diploid cell, the «zygot...

MOLECULAR MECHANISM OF FERTILIZATION UNIVERSITY OF KYRENIA MEDICAL SCHOOL DR. İDİL ASLAN FERTILIZATION Fertilization is the fusion of the male and female gamete. The process involves the fusion of an oocyte with a sperm ; creating a single diploid cell, the «zygote» from which a new individual organism will develop  During sexual intercourse, millions of sperm are deposited into the vagina. A number of these will die in the acidic environment. However, many will survive due to the protective elements provided in the fluids surrounding them. Soon afterwards, the sperm have to swim through the cervical mucus, towards to the uterus and then on to the fallopian tubes.  As they swim towards these, they decrease in number, in an attempt to make it through the mucus. Inside the uterus, the contractions of the uterus assist the journey of the sperm towards the egg. Fertilization takes place in the ampulla of the oviduct. Sperm can live inside the female reproductive tract for up to 72 hours. Secondary oocyte will be die within 24 hours if the fertilization does not occur. If the oocyte is not fertilized here, it slowly passes to the uterus, where it becomes degenerated and is absorbed.  Sperm is the male gamete and is derived from the Greek word 'sperma' (meaning 'seed’). It consists of three parts, a head (containing DNA and enzymes in order to penetrate the oocyte), a midpiece (containing cellular elements, centrioles, microtubules and a number of mitochondria for energy production required to promote sperm) and a tail (enabling sperm movement)  The head of normal sperm has an oval shape, a length of 3–5 µm, a width of 2–3 µm and is 1.5 µm thick. It consists of the nucleus and the acrosome, which are covered by the nuclear membrane The nucleus contains the male genetic material. The DNA of the sperm has a slightly different structure from the DNA of a diploid (somatic) cell, meaning that the DNA is concentrated in the smallest possible volume, so as to save space. The acrosome contains proteolytic enzymes, such as acrosin, trypsin, hyaluronidase and proteases, which are released during the acrosome reaction and hydrolysis in granular cells and the zona pellucida of the egg in order to assist the penetration of the sperm and the merger with the egg  The neck or mid-section of the sperm connects the head to the tail (length, 7.5 µm; and width, 1 mm). It contains the axial filament surrounded by fibrils with circular layout and one to two centrosomes. The neck contains mitochondria, enzymes of glycolysis and oxidation systems and provides the necessary energy for survival and mobility. The tail (length, 40–50 mm) consists of a total of ten pairs of fibrils (a central and nine peripheral) and is responsible for the typical sperm motility.  The oocyte is an extremely large cell that has a large, highly transcriptionally active nucleus also known as the “germinal vesicle”. It is rich in cytoplasm with numerous organelles and proteins  Essential cytoplasmic components include ribosomes, mitochondria, and maternal mRNAs that are stored for later translation into proteins required for maturation and embryo development. Oocyte mitochondria are unique in their structure and DNA content relative to somatic cells in that they are spherical, have few cristae, and contain only one to two copies of mitochondrial DNA each. Oocytes also contain far more mitochondria than somatic cells. Primordial germ cells begin with about 10 mitochondria, and the numbers of mitochondria increase rapidly throughout germ cell migration to the gonad, entry into meiosis, and primordial follicle formation when the primordial oocytes each contain ~6000 mitochondria. With the onset of oocyte growth, mitochondrial replication continues, with an estimated 300,000 to 400,000 mitochondria in fully grown human oocytes. STEPS OF MOLECULAR MECHANISM 1-Sperm-Oocyte Encounter Chemotaxis and Thermotaxis: When the oocyte reaches the fallopian tubes and is ready for fertilization, it releases chemical signals that attract sperm. These signals help guide the sperm towards the zona pellucida. In humans, hormones such as progesterone act as chemotaxic factors, aiding in, directing sperm. Additionally, temperature differences in the fallopian tubes can influence sperm movement, a process known as thermotaxis. Sperm guidance mechanism in mammalian female genital tract, which shows the relative range of action of thermotaxis and chemotaxis. 2- Sperm Capacitation What is capacitation? Capacitation is a biochemical maturation process that sperm undergo in the female reproductive tract, which is essential for the sperm to penetrate the oocyte’s plasma membrane. During capacitation, changes occur in the sperm’s plasma membrane, increasing its mobility and preparing it for acrosome reaction. Membrane cholesterol: During capacitation, cholesterol and glycoproteins in the sperm membrane are removed increasing membrane fluidity and permeability. This allows the entry of calcium and bicarbonate ions into the sperm. Calcium Influx and cAMP: Calcium and bicarbonate ions activate the enzyme adenylate cyclase in the sperm, which increases the production of cyclic AMP(cAMP). cAMP is a critical second messenger that enhances sperm motility, enabling the sperm to swim more efficiently toward the oocyte 3- Binding to the Zona Pellucida The zona pellucida, which surrounds the oocyte, plays a key role in sperm recognition and binding. The zona pellucida is composed of three main glycoproteins: ZP1, ZP2, ZP3 ZP3 and ZP2 Receptors: Proteins on the sperm head interact with ZP3 glycoproteins in the zona pellucida. ZP3 essential for binding the sperm and initiating the acrosome reaction. This interaction helps the sperm penetrate through the zona pellucida. Interactions with ZP2: Following the acrosome reaction, the sperm binds to ZP2, allowing the sperm progress deeper through the zona pellucida 4- The Acrosome Reaction What is the Acrosome? The acrosome is a specialized vesicle at the head of the sperm, filled with hydrolytic enzymes. Binding to the zona pellucida triggers the acrosome reaction. Acrosomal Enzymes: During this reaction, enzymes such as hyaluronidase and acrosin are released from acrosome. These enzymes degrade the structure of the zona pellucida, allowing the sperm to reach the oocyte’s plasma membrane Preparing for Fusion: The structure of the sperm membrane changes during the acrosome reaction, enabling it to fuse with oocyte’s plasma membrane 5. Binding to the Oocyte Plasma Membrane: Specific Molecular Interactions: After the acrosome reaction, the sperm reaches the oocyte’s plasma membrane and binds to it. IZUMO1 protein on the sperm surface and JUNO receptor on the oocyte membrane. Sperm Entery: The interactions between IZUMO1 and JUNO facilitates the fusion of sperm membrane with oocyte plasma membrane. The membranes merge, allowing the sperm to enter the oocyte. At this stage, only the sperm’s nucleus and certain organelles are incorporated into the oocyte. 6. Cortical Reaction and Prevent of Polyspermy: Polyspermy Block: Only one sperm should fertilize the oocyte, otherwise, excess genetic material would lead to abnormal embryonic development. Once the sperm enters the oocyte, cortical granules in the oocyte’s membrane are activated. These granules release enzymes into the zona pellucida. Cortical Reaction: The enzymes released from cortical granules modify the Sperm- binding receptors in the zona pellucida, hardening it and preventing further sperm from binding. This is called the zona reaction or block to polyspermy. This process occurs immediately after the fusion of sperm and oocyte plasma membranes, ensuring that only one sperm contributes to fertilization. 7- Oocyte Activation and Completion of The Second Meiosis: Calcium Waves: Once the sperm enters the oocyte, a wave of calcium ions is triggered inside the oocyte. This calcium wave signals the oocyte to complete second meiotic division, resulting in a mature egg. Second Polar Body: The completion of meiosis results in the oocyte halving its chromosome number and a second polar body is expelled 8. Pronuclear Fusion: Formation of Sperm and Oocyte Pronuclei: Inside the oocyte, the sperm’s nucleus forms a nuclear envelope around itself, becoming the sperm pronucleus. Simultaneously, the oocyte nucleus becomes the oocyte pronucleus. Fusion of Pronuclei: The sperm and oocyte pronuclei move toward each other and fuse, creating the diploid (n)genome of zygote. This marks the beginning of a new individual’s genetic information. 9- Zygote Formation and First Division: After the fusion of pronuclei , the zygote prepares for its first cell division. The zygote undergoes mitotic division, creating blastomers and embryonic development begins. Summary 1-Sperm capacitation: Removal of cholesterol and proteins from the plasma membrane and activation of ion channels 2-Binding to the zona pellucida: Interacting with ZP3 and ZP2 glycoproteins 3-Acrosome reaction: Release of hydrolytic enzymes and alteration of sperm membrane 4-Fusion: Interaction between IZUMO1 and JUNO proteins 5-Polyspermi Prevention: Release of enzymes from cortical granules and zona hardining  Review Int J Mol Med.2016 Oct;38(4):979-86. doi: 10.3892/ijmm.2016.2723. E pub 2016 Aug 31. The molecular basis of fertilization (Review) Katerina Georgadaki 1, Nikolas Khoury 1, Demetrios A Spandidos 2, Vasilis Zoumpourlis 1 PMID: 27599669 PMCID: PMC5029953 DOI: 10.3892/ijmm.2016.2723 Free PMC article Review The molecular mechanisms mediating mammalian fertilization Hanisha H. Bhakta, Fares H. Refai and Matteo A. Avella