Fertilization & Cleavage PDF

Fertilization & Cleavage Anatomy Unit201 Anatomy Unit  Objectives 1. Define the term ‘fertilization’. 2. Describe the site,  3. Explain phases of fertilization and its outcome. 4. Explain the process of cleavge  Case sc...

Fertilization & Cleavage Anatomy Unit201 Anatomy Unit  Objectives 1. Define the term ‘fertilization’. 2. Describe the site,  3. Explain phases of fertilization and its outcome. 4. Explain the process of cleavge  Case scenario Q. When does a secondary oocyte complete its second meiotic division to become a mature ovum?  A. At ovulation B. Before ovulation C. At fertilization D. At puberty E. Before birth  Fertilization  It is the process by which the sperm & 2ry oocyte meet and fuse.  Normal site: AMPULLA of the uterine tube (widest part of the tube).  Morphology of the 2ry oocyte  large sized, immotile.  Covered by zona pellucida and corona radiata.  Abundant cytoplasm (containing yolk granules….why?).  Nucleus contains (22 chromosomes + one X chromosome).  How do the 2ry oocyte reach the site of fertilization?  By the movement fimbria of the uterine tube.  By peristaltic movement of the uterine tube.  N.B: Oocyte cannot be fertilized after 24 hours as it degenerates shortly thereafter.  How do the sperms reach the site of fertilization?  By movement of their tail.  By the movement of uterine cilia.  By chemo attraction (signals from 2ry oocyte).  N.B: Human sperms do not survive for more than 48 hours.  Capacitation  Def: the glycoprotein coat is removed from the surface of the sperm's acrosome.  Freshly ejaculated sperms are unable to fertilize.  Site: in the female reproductive tract.  It takes about 7-8 hours.  Acrosome reaction  Definition: the process by which the sperms release the contents of the acrosomal cap.  Mechanism: substances are released to penetrate the zona pellucida:  Hyaluronidase enzyme (for passage through corona radiata).  Trypsin-like substances.  Zona lysine (for penetration of zona pellucida).  Phases of fertilization Passage of sperm through the corona radiate.  Penetration of the zona pellucida.  Fusion of the oocyte and sperm cell membranes. Completion of 2ry meiotic division. Syngamy: Formation of male and female pronuclei to form the zygote (the unicellular embryo).  Zona reaction  Once the sperm penetrates the zona pellucida, cortical granules present under the cell membrane of the ovum release their contents which make the zona pellucida firm and impermeable to other sperms.  RESULTS OF FERTILIZATION 2ry oocyte complete its 2nd meiotic division. Restoration of the diploid number of chromosomes (46 chromosomes). Determination of the embryo's chromosomal sex (46,XX and 46,XY). Stimulation of cleavage.  Abnormalities of Normal Fertilization: 1. Parthenogesis: The unfertilized oocyte starts cleavage without sperm penetration. 2. Dispermy and triploidy: Ovum fertilized by two sperms (zygote with 69 chromosomes). IVF (invitro fertilization)  Cleavage Definition: It is the repeated mitotic divisions of the zygote, resulting Site: occurs as the zygote passes along into a rapid increase in the number the uterine tube toward the uterus. of cells (blastomeres).  Cleavage  Time: about 30 hours after fertilization.  First the zygote divides into two blastomeres.  When there are 12 to 16 blastomeres, is called a morula. It is formed about 3 days after fertilization.  Cleavage  The blastomeres of the morula undergo repeated mitotic division within the zona pellucida.  The zona pellucida keeps the cells together during cleavage and prevents adhesion of these cells to the uterine tubes.  Cleavage  Small cavities appear between the cells that unite together to form larger cavities.  About 4 days after fertilization, a fluid - filled cavity called the blastocele appears inside the morula.  The fluid passes from the uterine cavity, through the zona pellucid to fill this space.  Cleavage  The blastocele separates the blastomeres into two groups: 1. A thin, outer cell layer called the trophoblast, which gives rise to the embryonic part of the placenta. 2. An inner group of blastomeres called the inner cell mass which gives rise to the embryo.  Cleavage  The morula is, now, called a blastocyst (blastulla) and enters the uterus.  The zona pellucid a gradually degenerates, disappears, and become ready for implantation.  Clinical application  Clinical application:  Embryonic stem cells (ES cells) are derived from the inner cell mass.  These cells can give any type of tissue (pluripotent cells).  Embryonic stem cells (Langman's Medical Embryology)  Case scenario  Q. When does a secondary oocyte complete its second meiotic division to become a mature ovum? A. At ovulation B. Before ovulation C. At fertilization D. At puberty E. Before birth