Fertilization

• Surrogate Mothers: IVF embryos transferred to another mother
• Fertilization is a complex sequence of coordinated molecular events that starts with sperm-oocyte contact and ends with maternal and paternal chromosome mix at metaphase of 1st mitotic division of zygote, unicellular embryo, approximately 24hrs.
• Capacitation involves the removal of a glycoprotein coat and seminal proteins from the sperm's acrosomal surface.
• The cortical reaction prompts the release of cortical granules from the oocyte into the perivitelline space.
• The contact between the sperm and the oocyte triggers the cortical reaction.
• The acrosomal reaction involves the release of enzymes from the acrosome, aiding in the breakdown of the protective layers around the egg.
• Sperm undergoing capacitation become more functionally active without significant morphological changes.
• The acrosomal enzymes help in breaking down the proteins and glycoproteins within the zona pellucida.
• Once a sperm penetrates through the zona pellucida, it enters the perivitelline space.
• The change in resting membrane potential of the oocyte's plasma membrane prevents other sperm from binding and fusing with the oocyte.
• The cortical reaction prevents polyspermy by modifying the zona pellucida and oocyte's membrane.
• The binding of sperm to ZP-3 receptors triggers the acrosomal reaction.
• The breakdown of the zona pellucida allows for the penetration of a single spermatozoon.
• The site of fertilization is the ampulla of the uterine tube, a common site for fertilization due to its larger diameter and the presence of ciliated cells, aiding in the movement and guidance of the ovum.
• After ovulation, the secondary oocyte is released from the ovary and swept into the infundibulum, which is the funnel-shaped part of the fallopian tube by the movement of fimbriae and fluid produced by ciliated cells.
• The movement of the fimbriae, along with the ciliary action and the fluid secreted by the tubal epithelial cells, create a gentle current that assists in guiding the secondary oocyte into the infundibulum.
• Spermatozoa are expelled from male genital duct through semen, introduced into the vagina through sexual intercourse, pass through the cervix uterine lumen, and reach the ampulla.
• Spermatogenesis is the process by which male gametes (sperm) are produced from diploid stem cells called spermatogonia.
• Spermatozoa are activated in the female reproductive tract, becoming more hyperactive and having a higher affinity to bind to zona pellucida receptors.
• Capacitation is the alterations in the sperm's structure and functionality, leading to increased cAMP levels, enhanced tyrosine phosphorylation, and a rise in intracellular calcium levels within the sperm cells.
• Approximately 30 hours post-fertilization, the zygote initiates its division into blastomeres.
• The chromosomal sex of an embryo is determined by the kind of sperm that fertilizes it, with Sperm X leading to 46 XX and Sperm Y leading to 46 XY.
• The series of mitotic divisions significantly increase the number of cells within the developing embryo.
• While the number of cells increases, each individual cell, or blastomere, becomes progressively smaller with each division.
• These divisions lead to the formation of cells called blastomeres.
• Fluid begins to accumulate within the morula, creating a fluid-filled cavity called the blastocystic cavity.
• As fluid continues to accumulate, it leads to the expansion of the blastocystic cavity, resulting in the separation and organization of blastomeres into two distinct parts.
• Following the fusion of the male and female pronuclei, there is a breakdown of the membranes surrounding each pronucleus.
• Following fertilization, the zygote begins a series of rapid cell divisions known as cleavage divisions.
• These cells are pluripotent and give rise to the embryo itself, differentiating into the various cell types and tissues that form the organism.
• Trophoblastic cells secrete early pregnancy factor, which appears in the maternal serum within 24 -48 h after fertilization and is the basis of pregnancy tests.
• After a series of cell divisions, the blastomeres reach a stage where they align themselves tightly, a process known as compaction.
• Variation in human species determines the chromosomal sex of the embryo and initiates cell division of the zygote.
• The compacted ball of cells, following the process of compaction, progresses to form a structure known as the morula.
• Compaction is a critical developmental event that transforms the aligned blastomeres into a tightly packed, compacted ball of cells.
• The hatched blastocyst increases in size & nourished by uterine gland secretions.
• Fertilization induces the oocyte to complete the second meiotic division and restores chromosome number (diploid, 2n, 46) in the zygote.
• The trophoblast is essential for implantation and eventually gives rise to the embryonic part of the placenta and other extraembryonic structures necessary for supporting the developing embryo/fetus.
• Compaction is essential for the segregation of cells within the morula that ultimately contribute to the formation of the inner cell mass (ICM) of the blastocyst.
• Within the blastocyst, the inner cell mass (ICM) or embryoblast is a group of centrally located cells that remain clustered together.