Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos

1302 Chapter 21: Sexual Reproduction: Meiosis, Germ Cells, and Fertilization(A)(B)(C)Figure 21–36 Immunofluorescencemicrographs of human sperm and eggpronuclei coming together after in vitrofertilization. Spindle microtubules arestained in green with anti-tubulinantibodies, and DNA is labeled in bluewith a DNA stain. (A) A meiotic spindle ina mature unfertilized secondary oocyte.(B) A fertilized egg extruding its secondpolar body about 5 hours after fusionwith a sperm. The sperm head (left) hasnucleated an array of microtubules. Theegg and sperm pronuclei are still farapart. (C) The two pronuclei have cometogether. (D) By 16 hours after fusion witha sperm, the centrosome that entered theegg with the sperm has duplicated, andthe daughter centrosomes haveorganized a bipolar mitotic spindle. Thechromosomes of both pronuclei arealigned at the metaphase plate of thespindle. As indicated by the arrows in (C)and (D), the sperm tail is still associatedwith one of the centrosomes. (FromC. Simerly et al., Nat. Med. 1:47–53, 1995.With permission from MacmillanPublishers Ltd.)(C)(D)100 mm3 of the best-looking early embryos are transferred with a catheter into thewoman’s uterus; the remaining embryos are usually kept frozen in liquid nitrogen,for further implantations, if necessary. The main complication of IVF ismultiple pregnancies, which occur in over 30% of cases, compared with about2% in unassisted pregnancies.The IVF procedure just described has enabled many previously infertilewomen to produce normal children. It does not, however, solve the problem forinfertile men who usually produce too few or abnormal sperm. A second breakthrough,which occurred in 1992, provided the solution for most such men. Inthis modification of IVF, called intracytoplasmic sperm injection (ICSI), an eggis fertilized by injecting a single sperm into it (Figure 21–37). This strategy eliminatesthe need for large numbers of motile sperm and bypasses many of thehurdles that a sperm normally has to clear to fertilize an egg, including capacitation,swimming to the egg, undergoing an acrosomal reaction, burrowingthrough the zona pellucida, and fusing with the egg plasma membrane. ICSI hasa success rate of better than 50% and has produced more than 100,000 children.In addition to revolutionizing the treatment of infertility, IVF has opened upmany new possibilities for manipulating the reproductive process. It has, forexample, made it possible for parents carrying a defective gene to avoid passingthe gene on to their children, by screening IVF embryos for the gene beforeimplanting them into the uterus.As discussed earlier, in vitro techniques for handling mammalian eggs havemade it possible to produce clones of many types of mammals by transferringthe nucleus of a somatic cell from the animal to be cloned into an unfertilizedegg that has had its own nucleus removed or destroyed. It is not an easy procedure;the success rate is low, and it is still uncertain whether a human could becloned in the same way. Moreover, there are serious ethical arguments about50 mmFigure 21–37 Intracytoplasmic sperminjection (ICSI). Light micrograph of ahuman secondary oocyte being heldwith a suction pipette (on the left) andinjected with a single human spermthrough a glass needle. The zonapellucida surrounds the egg and thepolar body. (Courtesy of ReproductiveBiology Associates, Atlanta, Georgia.)