fertilization (beginning a New organism )
FERTILIZATION IS THE PROCESS: where by THE GAMETES—sperm and egg—fuse together to begin the creation of a new organism. Fertilization accomplishes two separate ends: sex (the combining of genes derived from two parents) and reproduction (the generation of a new organism). Thus, the first function of fertilization is to transmit genes from parent to offspring, and the second is to initiate in the egg cytoplasm those reactions that permit development to proceed. Although the details of fertilization vary from species to species, it generally consists of four major events: 1. Contact and recognition between sperm and egg. In most cases, this ensures that the sperm and egg are of the same species. 2. Regulation of sperm entry into the egg. Only one sperm nucleus can ultimately unite with the egg nucleus. This is usually accomplished by allowing only one sperm to enter the egg and actively inhibiting any others from entering. 3. Fusion of the genetic material of sperm and egg. 4. Activation of egg metabolism to start development. This chapter will describe how these steps are accomplished in two groups of organisms: sea urchins (whose fertilization we know the best) and mammals. structure of the gametes A complex dialogue exists between egg and sperm. The egg activates the sperm metabolism that is essential for fertilization, and the sperm reciprocates by activating the egg metabolism needed for the onset of development. But before we investigate these aspects of fertilization, we need to consider the structures of the sperm and egg—the two cell types specialized for fertilization sperm Sperm were discovered in the 1670s, but their role in fertilization was not discovered until the mid-1800s. It was only in the 1840s, after Albert von Kölliker described the formation of sperm from cells in the adult testes that fertilization research could really begin. Even so, von Kölliker denied that there was any physical contact between sperm and egg. He believed that the sperm excited the egg to develop in much the same way a magnet communicates its presence to iron. The first description of fertilization was diagram
Each sperm cell consists of a haploid nucleus, a propulsion system to move the nucleus, and a sac of enzymes that enable the nucleus to enter the egg. In most species, almost all of the cell’s cytoplasm is eliminated during sperm maturation, leaving only certain organelles that are modified for spermatic function.. During the course of maturation, the sperm’s haploid nucleus becomes very streamlined and its DNA becomes tightly compressed. In front or to the side of this compressed haploid nucleus lies the acrosomal vesicle, or acrosome. The acrosome is derived from the cell’s Golgi apparatus and contains enzymes that digest proteins and complex sugars. Enzymes stored in the acrosome can digest a path through the outer coverings of the egg. In many species, a region of globular actin proteins lies between the sperm nucleus and the acrosomal vesicle. These proteins are used to extend a fingerlike acrosomal process from the sperm during the early stages of fertilization. In sea urchins and numerous other species, recognition between sperm and egg involves molecules on the acrosomal process. Together, the acrosome and nucleus constitute the sperm head. The means by which sperm are propelled vary according to how the species has adapted to environmental conditions. In most species, an individual sperm is able to travel by whipping its flagellum. The major motor portion of the flagellum is the axoneme, a structure formed by microtubules emanating from the centriole at the base of the sperm nucleus. The core of the axoneme consists of two central microtubules surrounded by a row of nine doublet microtubules. These microtubules are made exclusively of the dimeric protein tubulin. Although tubulin is the basis for the structure of the flagellum, other proteins are also critical for flagellar function. The force for sperm propulsion is provided by dynein, a protein attached to the microtubules. Dynein is an ATPase—an enzyme that hydrolyzes ATP, converting the released chemical energy into mechanical energy that propels the sperm.1 This energy allows the active sliding of the outer doublet microtubules, causing the flagellum to bend (Ogawa et al. 1977; Shinyoji et al. 1998). The ATP needed to move the flagellum and propel the sperm comes from rings of mitochondria located in the midpiece of the sperm (see Figure 7.1B). In many species (notably mammals), a layer of dense fibers has interposed itself between the mitochondrial sheath and the cell membrane. This fiber layer stiffens the sperm tail. Because the thickness of this layer decreases toward the tip, the fibers probably prevent the sperm head from being whipped around too suddenly. Thus, the sperm cell has undergone extensive modification for the transport of its nucleus to the egg. In mammals, the differentiation of sperm is not completed in the testes. Although they are able to move, the sperm released during ejaculation do not yet have the capacity to bind to and fertilize an egg. The final stages of sperm maturation, cumulatively referred to as capacitation, do not occur in mammals until the sperm has been inside the female reproductive tract for a certain period of time. The Egg cytoplasm and nucleus All the material necessary to begin growth and development must be stored in the egg, or ovum.2 Whereas the sperm eliminates most of its cytoplasm as it matures, the developing egg (called the oocyte before it reaches the stage of meiosis at which it is fertilized) not only conserves the material it has, but actively accumulates more. The meiotic divisions that form the oocyte conserve its cytoplasm rather than giving half of it away; at the same time, the oocyte either synthesizes or absorbs proteins such as yolk that act as food reservoirs for the developing embryo. Birds’ eggs are enormous single cells, swollen with accumulated yolk (see Figure 12.2). Even eggs with relatively sparse yolk are large compared to sperm. The volume of a sea urchin egg is about 200 picolitres (2 × 10–4 mm3 ), more than 10,000 times the volume of sea urchin sperm So even though sperm and egg have equal haploid
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The early embryonic cells must have a supply of energy and amino acids. In many species, this is accomplished by accumulating yolk proteins in the egg. Many of these yolk proteins are made in other organs (e.g., liver, fat bodies) and travel through the maternal blood to the oocyte Ribosome and tRNA . The early embryo must make many of its own structural proteins and enzymes, and in some species there is a burst of protein synthesis soon after fertilization. Protein synthesis is accomplished by ribosomes and tRNA that exist in the egg. The developing egg has special mechanisms for synthesizing ribosomes; certain amphibian oocytes produce as many as 1012 ribosomes during their meiotic prophase Messenger RNAS The oocyte not only accumulates proteins, it also accumulates mRNAs that encode proteins for the early stages of development. It is estimated that sea urchin eggs contain thousands of different types of mRNA that remain repressed until after fertilization. Morphogenetic factors . Molecules that direct the differentiation of cells into certain cell types are present in the egg. These include transcription factors and paracrine factors. In many species, they are localized in different regions of the egg and become segregated into different cells during cleavage. Protective chemicals The embryo cannot run away from predators or move to a safer environment, so it must be equipped to deal with threats. Many eggs contain ultraviolet filters and DNA repair enzymes that protect them from sunlight, and some eggs contain molecules that potential predators find distasteful. The yolk of bird eggs contains antibodies that protect the embryo against microbes. Within the enormous volume of egg cytoplasm resides a large nucleus . In a few species (such as sea urchins), this female pronuclus is already haploid at the time of fertilization. In other species (including many worms and most mammals), the egg nucleus is still diploid—the sperm enters before the egg’s meiotic divisions are completed . In these species, the final stages of egg meiosis will take place after the sperm’s nuclear material—the male pronucleus—is already inside the egg cytoplasm. Cell membrane and extracellular envelope The membrane enclosing the egg cytoplasm regulates the flow of specific ions during fertilization and must be capable of fusing with the sperm cell membrane. Outside this egg cell membrane is an extracellular matrix that forms a fibrous mat around the egg and is often involved in sperm-egg recognition (Wasserman and Litscher 2016). In invertebrates, this structure is usually called the vitelline envelope . The vitelline envelope contains several different glycoproteins. It is supplemented by extensions of membrane glycoproteins from the cell membrane and by proteinaceous “posts” that adhere the vitelline envelope to the cell membrane (Mozingo and Chandler 1991). The vitelline envelope is essential for the species-specific binding of sperm. Many types of eggs also have a layer of egg jelly outside the vitelline envelope. This glycoprotein meshwork can have numerous functions, but most commonly it is used either to attract or to activate sperm. The egg, then, is a cell specialized for receiving sperm and initiating development Lying immediately beneath the cell membrane of most eggs is a thin layer (about 5 μm) of gel-like cytoplasm called the cortex. The cytoplasm in this region is stiffer than the internal cytoplasm and contains high concentrations of globular actin molecules. During fertilization, these actin molecules polymerize to form long cables of actin microfilaments. Microfilaments are necessary for cell division. They are also used to extend the egg surface into small projections called microvilli, which may aid sperm entry into the cell . Also within the cortex are the cortical granules (see Figures 7.4B). These membrane-bound, Golgi-derived structures contain proteolytic enzymes and are thus homologous to the acrosomal vesicle of the sperm. However, whereas a sea urchin sperm contains just one acrosomal vesicle, each sea urchin egg contains approximately 15,000 cortical granules. In addition to digestive enzymes, the cortical granules contain mucopolysaccharides, adhesive glycoproteins, and hyaline protein. As we will soon describe, the enzymes and mucopolysaccharides help prevent polyspermy—that is, they prevent additional sperm from entering the egg after the first sperm has entered—while hyaline and the adhesive glycoproteins surround the early embryo, providing support for cleavage-stage blastomeres. In mammalian eggs, the extracellular envelope is a separate, thick matrix called the zona pellucida. The mammalian egg is also surrounded by a layer of cells called the cumulus , which is made up of the ovarian follicular cells that were nurturing the egg at the time of its release from the ovary. Mammalian sperm have to get past these cells to fertilize the egg. The innermost layer of cumulus cells, immediately adjacent to the zona pellucida, is called the corona radiata. Recognition of egg and sperm The interaction of sperm and egg generally proceeds according to five step (1) Chemoattraction of the sperm to the egg by soluble molecules secreted by the egg (2) Exocytosis of the sperm acrosomal vesicle and release of its enzymes (3) Binding of the sperm to the extracellular matrix (vitelline envelope or zona pellucida) of the egg (4) Passage of the sperm through this extracellular matrix (5) Fusion of the egg and sperm cell membranes After these steps are accomplished, the haploid sperm and egg nuclei can meet and the reactions that initiate development can begin. In this chapter, we will focus on these events in two well-studied organisms: sea urchins, which undergo external fertilization; and mice, which undergo internal fertilization. Some variations of fertilization events will be described in subsequent chapters as we study the development of particular organisms.
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