Fertilization and First Week of Development PDF

st Fertilization and week of 1 Development By SULAIMON,F.A. (Ph.D) Fertilization Fusion of spermatozoon and secondary oocyte Occurs at the ampulla of the uterine tube Fertilization proceeds for about 24 hours. It occurs in 3 phases Phase 1, pen...

st Fertilization and week of 1 Development By SULAIMON,F.A. (Ph.D) Fertilization Fusion of spermatozoon and secondary oocyte Occurs at the ampulla of the uterine tube Fertilization proceeds for about 24 hours. It occurs in 3 phases Phase 1, penetration o f the corona radiata Phase 2, penetration o f the zona pellucida Phase 3, fusion of the oocyte and sperm cell membranes for the spermatozoon to penetrate the oocyte, it has to become fully matured by undergoing capacitation and acrosome reaction.  Capacitation It is the sperm condition in the female reproductive tract It occurs for a period of 7h Mediated by oocyte fertilizin and sperm antifertilizin It is required for sperm to pass through the corona cells and undergo acrosomal reactionn acrosome reaction occurs after binding to the zona pellucida induced by zona proteins. culminates in the release of enzymes needed to penetrate the zona pellucida, including acrosin neuraminidase, zona lysins (proteases) Out of about 300 million spermatozoa released, only 300-500 reach the site of fertilization and only one eventually fertilizes the oocyte. penetration o f the corona radiata Passage of capacitated spermatozoon through the corona radiata. enhanced by the release of hyaluronidase from acrosome of the sperm. and motion of their flagellum penetration o f the zona pellucida mediated by acrosome enzymes such as acrosin, neuraminidase, esterases, and ZP3 ligand (sperm receptor). When the capacitated sperm comes in contact with zona pellucida, acrosome reaction occurs. This creates perforations in the acrosome of the sperm, resulting in the release of its enzymes and the penetration of the zona pellucida. Once the zona is penetrated, zona reaction occurs. Zona reaction is mediated by the release of lysosomal enzymes from the cortical granules of the oocyte. These enzymes cause changes in the composition of zona pellucida and the plasma membrane of oocyte, thus making them impermeable to other sperms fusion of the oocyte and sperm cell membranes The initial adhesion of sperm to the oocyte is mediated in part by the interaction of integrins on the oocyte and its ligands, disintegrins on sperm. After adhesion, the plasma membranes of the sperm (posterior region of the head) and egg fuse because the plasma membrane covering the acrosomal head cap disappears during the acrosome In human, both the head and the tail of the spermatozoon enter the cytoplasm of the oocyte, but the plasma membrane is left behind on the oocyte surface. Response of egg to spermatozoon entry As soon as the spermatozoon has entered the oocyte, the egg responds in three ways Zona and cortical reactions: Owing to the release of the cortical granules of the oocyte (with their contents of lysosomal enzymes), the composition of the zona membrane is altered (zona reaction), making it impermeable to other sperms. The plasma membrane of the oocyte also becomes impermeable. Second meiotic division of the oocyte resumes, and is soon completed, yielding a mature ovum and a 2nd polar body. The ovum receives all the cytoplasm of the oocyte. Its chromosomes (22+X) constitute the female pronucleus. Formation of the male pronucleus: The nucleus of the sperm enlarges to form the male pronucleus. Metabolic activation of the oocyte: Penetration of the oocyte by the spermatozoon is one of the factors that activate the egg to undergo post-fusion development, including cleavage, Results of fertilization Restoration of the diploid number of chromosomes, half from the father and half from the mother. Hence, the zygote contains a new combination of chromosomes different from both parents. Determination of the sex of the new individual. An X-carrying sperm produces a female (XX) embryo, and a Y-carrying sperm produces a male (XY) embryo. Therefore, the chromosomal sex of the embryo is determined at fertilization. Initiation of cleavage. Without fertilization, the oocyte usually degenerates 24 hours after ovulation. Applied anatomy In early pregnancy, the presence of an immunosuppressant protein called early pregnancy factor, secreted by trophoblast cells, allows detection as early as the first week. Dispermy, a rare condition where two sperm cells fertilize an egg, can lead to a triploid embryo with 69 chromosomes, often resulting in spontaneous abortion or stillbirth. Polyspermy is prevented through cortical and zona reactions. Parthenogenesis, the development of an unfertilized egg, is rare in humans but occurs in certain animals such as rabbit Infertile couples may choose assisted reproductive techniques like in vitro fertilization (IVF), gamete intrafallopian transfer, intracytoplasmic sperm injection, and zygote intrafallopian transfer. contraceptive methods employed for family planning Oral Contraceptive Pills (OCPs): Containing estrogen and progestin, these pills inhibit pituitary gonadotropins (FSH and LH), preventing ovulation while allowing menstruation. Administered for 21 days, they are then stopped to facilitate menstruation. Barrier Methods: Male and female condoms, diaphragms, contraceptive sponges, and cervical caps act as physical barriers, preventing the passage of sperm. Surgical Procedures: Vasectomy involves excising or tying the vas deferens in males, while tubal ligation entails tying up the uterine tubes in females. Intra-Uterine Devices (IUDs): These devices are placed in the uterine cavity, potentially interfering with implantation. Depo Provera and RU 486: Depo Provera: Administered through injection, it contains progestin, preventing ovulation for 2-3 months. It can also be subdermally implanted for contraception up to 5 years. RU 486: An antiprogesterone agent, it induces abortion by initiating menstruation. Administered within 2 months of the previous menses. Cleavage Cleavage Process: Mitotic divisions follow fertilization, reducing the cytoplasm-nucleus ratio in resulting blastomeres (cells). First Mitotic Division: About 30 hours after fertilization, the zygote undergoes its first mitotic division, yielding two blastomeres. Compaction at 8- or 9-Cell Stage: During the 3rd cleavage, blastomeres form a compact ball of cells through compaction. Tight junctions enhance cell-cell interaction, leading to the segregation of inner cells that form the inner cell mass, communicating via gap junctions. Morula Formation: Around three days post-fertilization, the embryo reaches the morula stage with 12-16 cells, resembling a mulberry. Cells are divided into the central inner cell mass (embryoblast) and the surrounding outer cell mass (trophoblast). Uterine Entry: At the end of the 3rd day, the morula enters the uterine cavity for further development.  The process of blastogenesis transforms the morula into a fluid-filled blastocyst with the following key points: Blastocyst Formation: After the morula enters the uterine cavity (end of the 3rd day), fluid from the uterine cavity passes through the zona pellucida into the morula's intercellular spaces. This results in the formation of a single fluid-filled cavity, known as the blastocyst cavity, transforming the embryo into a blastocyst. Blastocyst Structure: The blastocyst consists of the blastocyst cavity (blastocoele), an inner cell mass or embryoblast (at the embryonic pole), and an outer cell mass or trophoblast. The trophoblast surrounds the inner cell mass and blastocyst cavity, while the zona pellucida lies external to the trophoblast. Zona Pellucida Degeneration: Following blastocyst formation, the zona pellucida begins to degenerate, enabling direct contact between the blastocyst and the endometrium, facilitating implantation. Implantation Initiation: Implantation of the blastocyst typically starts around the 6th day of development. Applied anatomy Monozygotic Twins Formation: If the two cells resulting from the first mitotic division of the zygote develop independently, monozygotic twins are formed. Mosaicism in Cleavage: During early cleavage, non-dysjunction or anaphase lag of chromosomes may occur, leading to the production of an embryo with two or more cell lines, each having a different number of chromosomes. This condition is termed mosaicism, and individuals with this condition are referred to as mosaics (e.g., mosaic Down syndrome).