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Scott F. Gilbert-Developmental Biology, 9Th Edition

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Scott F. Gilbert-Developmental Biology, 9Th Edition Fertilization Beginning a New Organism FERTILIZATION IS THE PROCESS whereby the sperm and the egg—collectively Urge and urge and urge, called the gametes—fuse together to begin the creation of a new individual Always the procreant urge of the world. 1 ose genome is derived from both parents. Fertilization accomplishes two sep- Out of the dimness opposite equals arate ends: sex (the combining of genes derived from two parents) and repro- advance, duction (the generation of a new organism). Thus, the first function of fertiliza- Always substance and increase, don is to transmit genes from parent to offspring, and the second is to initiate in always sex, die egg cytoplasm those reactions that permit development to proceed. Always a knit of identity, always Although the details of fertilization vary from species to species, conception distinction, _ _ nerally consists of four major events: WAIT WHITMAN (1855) L Contact and recognition between sperm and egg. In most cases, this ensures that the sperm and egg are of the same species. The final aim of all love intrigues, be 2. Regulation of sperm entry into the egg. Only one sperm nucleus can ultimate- ly unite with the egg nucleus. This is usually accomplished by allowing only they comic or tragic, is really of more one sperm to enter the egg and actively inhibiting any others from entering. importance than all other ends in 3. Fusion of the genetic material of sperm and egg. human life. What it turns upon is 4. Activation of egg metabolism to start development. nothing less than the composition of the next generation. A. SCHOPENHAUER Structure of the Gametes (QUOTED BY C. DARWIN, 1871) A complex dialogue exists between egg and sperm. The egg activates the sperm metabolism that is essential for fertilization, and the sperm reciprocates by acti- vating the egg metabolism needed for the onset of development. But before we investigate these aspects of fertilization, we need to consider the structures of die sperm and egg—the two cell types specialized for fertilization.*' sperm It is only within the past 135 years that the sperm's role in fertilization has been known. Anton van Leeuwenhoek, the Dutch microscopist who co-discovered sperm in the 1670s, first believed them to be parasitic animals living within the semen (hence the term spermatozoa, meaning "seed animals")- Although he orig- kially assumed that they had nothing to do with reproducing the organism in which they were found, he later came to believe that each sperm contained a pre- *\lany courses begin with gametogenesis and meiosis- This author believes that meiosis and gametogenesis are the culminating processes in development, and that they cannot be prop- erly appreciated without first understanding somatic organogenesis and differentiation. Also, having gonad formation and gametogenesis in the last lectures completes a circle. 122 CHAPTER 4 formed embryo. Leeuwenhoek (1685) wrote that sperm FIGURE 4.1 The human infant were seeds (both sperma and semen mean "seed"), and that preformed in the sperm, as the female merely provided the nutrient soil in which the depicted by Nicolas Hartsoeker seeds were planted. In this, he was returning to a notion of (1694). procreation promulgated by Aristotle 2000 years earlier. Try as he might, Leeuwenhoek was continually disap- pointed in his attempts to find preformed embryos with- the union of the two cells' in spermatozoa. Nicolas Hartsoeker, the other co-discov- nuclei. Hertwig had been erer of sperm, drew a picture of what he hoped to find: a seeking an organism suitable miniscule human ("homunculus") within the sperm (Figure for detailed microscopic obser- 4.1). This belief that the sperm contained the entire embry- vations, and he found the onic organism never gained much acceptance, as it implied Mediterranean sea urchin an enormous waste of potential life. Most investigators (Paracentrotus lividus) to be regarded the sperm as unimportant.* perfect for this purpose. Not only was it common through- See WEBSITE 4.1 out the region and sexually Leeuwenhoek and images of homunculi mature throughout most of the The first evidence suggesting the importance of sperm year, but its eggs were avail- in reproduction came from a series of experiments per- able in large numbers and formed by Lazzaro Spallanzani in the late 1700s. Spallan- were transparent even at high zani induced male toads to ejaculate into taffeta breeches magnifications. and found toad semen so filtered to be devoid of sperm; When he mixed suspen- such semen did not fertilize eggs. He even showed that sions of sperm together with semen had to touch the eggs in order to be functional. egg suspensions, Hertwig However, Spallanzani (like many others) felt that the sper- repeatedly observed sperm matic "animals" were parasites in the fluid; he thought the entering the eggs and saw embryo was contained within the egg and needed sper- sperm and egg nuclei unite. matic fluid to activate it (see Pinto-Correia 1997). He also noted that only one The combination of better microscopic lenses and the sperm was seen to enter each egg, elucidation of the cell theory (that all life is cellular, and all and that all the nuclei of the resulting embryo were derived mitm cells come from preexisting cells) led to a new apprecia- ically from, the nucleus created at fertilization. Fol made simi- tion of sperm function. In 1824, J. L. Prevost and J. B. lar observations and also detailed the mechanism of sperm Dumas claimed that sperm were not parasites, but rather entry. Fertilization was at last recognized as the union m the active agents of fertilization. They noted the universal sperm and egg, and the union of sea urchin gametes existence of sperm in sexually mature males and their remains one of the best-studied examples of fertilization." absence in immature and aged individuals. These obser- See WEBSITE 4.2 vations, coupled with the known absence of sperm in the The origins of fertilization research sterile mule, convinced them that "there exists an intimate relation between their presence in the organs and the Each sperm cell consists of a haploid nucleus, a propul- fecundating capacity of the animal." They proposed that sion system to move the nucleus, and a sac of enzymes thai the sperm entered the egg and contributed materially to enable the nucleus to enter the egg. In most species, almost the next generation. all of the cell's cytoplasm is eliminated during sperm mat- These claims were largely disregarded until the 1840s, uration, leaving only certain organelles that are modified when A. von Kolliker described the formation of sperm for spermatic function (Figure 4.2). During the course c4 from cells in the adult testes. He ridiculed the idea that the maturation, the sperm's haploid nucleus becomes verj semen could be normal and yet support such an enormous streamlined and its DNA becomes tightly compressed. In number of parasites. Even so, von Kolliker denied there front of this compressed haploid nucleus lies the acroso- was any physical contact between sperm and egg. He mal vesicle, or acrosome. The acrosome is derived frora believed that the sperm excited the egg to develop in much the same way a magnet communicates its presence to iron. It was not until 1876 that Oscar Hertwig and Herman Fol •"Hertwig and Hoi were actually not the first persons to report fertiH independently demonstrated sperm entry into the egg and ization in the sea urchin. At least three other astute observers— Adolphe Dufossc, Karl Ernst von Baer, and Alphonse Derbes— observed sperm-egg contact in 1847. Briggs and Wessel (2006) "Indeed, sperm was discovered around 1676, while the events of suggest that the convulsions of Europe during 1848, the low opin- fertilization were not elucidated until 1876. Thus, for nearly 200 ion German scientists had of French biology, and the tenuousness ol years, people had no idea what the sperm actually did. See Pinto- these results (given poor microscopy arid the lack of a theory in Correia 1997 for details of this remarkable story. which to place them) may have confined these papers to obscuritv FERTILIZATION 123 Microtubules ~- ad Centriole Acrosomal apparatus vesicle and granule Acrosomal vesicle 1GURE 4.2 Modification of i serm cell to form a mam- a an sperm. (A) The centri- ie produces a long flagellum t what will be the posterior nd of the sperm, and the fioigi apparatus forms the cosomal vesicle at the uture anterior end. The mito- r^ondria collect around the iasellum near the base of the Mc.oid nucleus and become racorporated into the mid- iece ("neck") of Ihe sperm. fee remaining cytoplasm is eisoned, and the nucleus nndenses. The size of the mature sperm has been enlarged rela- r.e to the other stages. (B) Mature bull sperm. The DNA is stained and the acrosomal vesicle. These proteins are used to iue with DAPI, the mitochondria are stained green, and the tubu- extend a fingerlike acrosomal process from the sperm dur- in of the flagellum is stained red. (Q Acrosome of mouse sperm, ing the early stages of fertilization. In sea urchins and sev- E:ned green by the fusion protein proacrosin-GFR (A after Cler- nont and Leblond 1955; B from Sutovsky et al. 1996, courtesy of eral other species, recognition between sperm and egg 1 Schattcn; C, courtesy of K.-S. Kim and G. L. Gerton.) involves molecules on the acrosomal process. Together, the acrosome and nucleus constitute the sperm head. The means by which sperm are propelled vary accord- ing to how the species has adapted to environmental con- ditions. In most species (the major exception is, once again, me cell's Golgi apparatus and contains enzymes that digest the nematodes, where the sperm is formed at the sites proteins and complex sugars; thus, the acrosome can be where fertilization occurs), an individual sperm is able to considered a modified secretory vesicle.
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