Fertilization PDF

Fertilization Dr. Udeshika Yapa Bandara BSc. (Hons) Human Biology (USJP), PhD (UOC)  Fertilization Fertilization occurs when a sperm and an oocyte (egg) combine and their nuclei fuse. Each of these reproductive cells is a haploid cell containing half of the g...

Fertilization Dr. Udeshika Yapa Bandara BSc. (Hons) Human Biology (USJP), PhD (UOC)  Fertilization Fertilization occurs when a sperm and an oocyte (egg) combine and their nuclei fuse. Each of these reproductive cells is a haploid cell containing half of the genetic material needed to form a human being, their combination forms a diploid cell. This new single cell, called a zygote, contains all of the genetic material needed to form a human—half from the mother and half from the father.  Fusion of a spermatozoon with oocyte resulting in formation of one- cell embryo – zygote. Brings male and female gametes together – produces diploid zygote. Fertilization is accounted as the first day of embryogenesis.  Fertilization cont… Fertilization of the ovum occurs in the ampulla of the uterine tube Fertilization process lasts about 24 h Ejaculation and ascent of spermatozoa into the fallopian tubes  Phases of fertilization 1.Passage of the sperm through corona radiata. Hyaluronidase enzyme release from the acrosome of sperm facilitate the penetration of sperm through corona radiata layer. Movements of the tail of the sperm also help it to penetrate to corona radiata. 2. Penetration of the zona pellucida by the sperm. Pathway through the zona pellucida also results from the action of enzymes released from the acrosome. The enzymes acrosin & neuraminidase cause lysis of the zonapellucida, thereby forming a path for the sperm to follow to the oocyte. Once the sperm passes through the zona pellucida, a zona reaction occurs in this layer that makes it impermeable to other sperms. Although several sperms begin to penetrate the zona pellucida, usually only one sperm enters the ovum & fertilize it. 3. Fusion of the oocyte & sperm cell membrane The plasma membranes of the oocyte & sperm fuse and soon break down at the area of fusion. Sperm releases its nucleus to the cytoplasm of the oocyte. 4. Completion of the second meiotic division of the secondary oocyte After entry of the sperm, the secondary oocyte completes its second meiotic division, forming a mature oocyte (ovum) & second polar body. At this moment, the oocyte has become an ovum, the female haploid gamete. The nucleus of the mature oocyte is known as the female pronucleus. 5.Formation of the male pronucleus Within the cytoplasm of the oocyte the nucleus of the head of the sperm enlarges to form the male pronucleus. During this process the tail of the sperm degenerate. Morphologically the male & female pronuclei are indistinguishable. During growth of the pronuclei, they replicate their DNA. 6. The male & female proneuclei contact each other Lose their nuclear membrane & fuse to form a new cell called zygote. Fertilization is completed within 24 h of ovulation. Each pronucleous has 23 chromosomes so that the fertilized ovum now has 46 chromosomes in all Each of these 46 chromosomes split in to two. Spindle forms & mitosis occurs leading to the formation of 2 daughter cells. This is called the 2-cell stage of the embryo.  The main results of fertilization 1. Restoration of the diploid number of chromosomes 2. Determination of the sex of embryo 3. Initiation of a cleavage  Embryo An embryo is the early stage of development of a multicellular organism. From the moment two cells until the 8th week of gestation, the development of the embryo takes place. At this stage, intense cellular changes occur. When the embryo is on day 6, it is called blastocyst.  Embryo cont… The development of multi-cellular organisms begins from a single- celled zygote, which undergoes through following processes. 1.Cleavage 2.Growth 3.Differentiation After fertilization the diploid zygote undergoes cleavage divisions in the oviduct.  Cleavage of the zygote The zygote undergoes a number of ordinary mitotic divisions that increase the number of cells in the zygote but not its overall size. Each cycle of division takes about 24 hours. The individual cells are known as blastomeres. At the 32-cell stage the embryo is known as a morula , a solid ball consisting of an inner cell mass and an outer cell mass (trophoblast). The inner cell mass will eventually become the embryo and fetus, while the outer cell mass (trophoblast) will eventually become part of the placenta. Growth  Growth During growth phase blastula containing one-dimensional layer of epithelial cells reorganizes into a multilayered and multidimensional structure called the gastrula. Three germ cell layers are formed. - Ectoderm, endoderm and mesoderm  Differentiation The process of forming different kinds of cells from similar cells of the early embryo. The changing of unspecialized embryonic cells into the specialized cells, tissues and organs of a multicellular animal.  Cells in each germ layer differentiate into tissues and embryonic organs Ectoderm - Outer layer Nervous system including brain, spinal cords and nerves Lining of the mouth, nostrils and anus Epidermis of skin, sweat glands, hair, nails Mesoderm – Middle layer Bones and muscles Blood and blood vessels Reproductive and excretory systems Inner layer (dermis) of skin Endoderm - Inner layer Lining of digestive tract Lining of trachea, bronchi and lungs Liver and pancreas Thyroid, parathyroid, thymus, urinary bladder  Embryo implantation Occurs between day 7 and 8. Implantation stands for the fixation of the embryo—at blastocyst stage—in the woman's uterus, in an inner layer called endometrium. Implantation will finish (more or less) on day 14 after fertilization. On this stage, we can say the endometrium is invaded by the embryo. If this process goes as expected, one can confirm that the woman is indeed pregnant. From this moment on, production of the beta - hCG hormone begins. https://www.youtube.com/watch?v=_5OvgQW6FG4 Questions: 1. Describe the phases of fertilization with a fully labelled diagram