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The egg

All the material necessary for thebeginning of growth and development mustbe stored in the mature egg (the ovum).Whereas the sperm has eliminated most of its cytoplasm, the developing egg (calledthe oocyte before it reaches the stage of meiosis at which it is fertilized) not onlyconserves its material, but is actively involvedin accumulating more. The meiotic divisionsthat form the oocyte conserve its cytoplasm(rather than giving half of it away), and the oocyteeither synthesizes or absorbs proteins, such as yolk,that act as food reservoirs for the developing embryo.

Thus, birds eggs are enormous single cells, swollen with their accumulated yolk.!ven eggs withrelatively sparse yolk are comparatively large. The volume of a sea urchin egg is

about "##picoliters (" $ %# & mm', more than %#,### times the volume of the sperm)(igure .&). *o,while sperm and egg have e+ual haploid nuclear components, the egg also has aremarkablecytoplasmic storehouse that it has accumulated during its maturation. Thiscytoplasmic troveincludes the following-Proteins. t will be a long while before the embryo is able to feed itself or obtainfood from itsmother. The early embryonic cells need a supply of energy and amino acids. nmany species, thisis accomplished by accumulating yolk proteins in the egg. /any of the yolkproteins are made inother organs (liver, fat body) and travel through the maternal blood to the egg.Ribosomes and tRNA. The early embryo needs to make many of its own proteins,and in somespecies, there is a burst of protein synthesis soon after fertilization. 0roteinsynthesis isaccomplished by ribosomes and t12A, which e3ist in the egg. The developingegg has specialmechanisms to synthesize ribosomes, and certain amphibian oocytes produce asmany as %#%"

ribosomes during their meiotic prophase.Messenger RNA. n most organisms, the instructions for proteins made during earlydevelopment are already packaged in the oocyte. t is estimated that the eggs of sea urchinscontain "4,### to 4#,### di5erent types of m12A. This m12A, however, remainsdormant untilafter fertilization (see 6hapter 4).Morphogenetic factors. /olecules that direct the di5erentiation of cells into certaincell typesare present in the egg. They appear to be localized in di5erent regions of the eggand becomesegregated into di5erent cells during cleavage (see 6hapter 7).

Protective chemicals. The embryo cannot run away from predators or move to asafer



environment, so it must come e+uipped to deal with threats. /any eggs containultraviolet 8ltersand 92A repair enzymes that protect them from sunlight: some eggs containmolecules thatpotential predators 8nd distasteful: and the yolk of bird eggs even containsantibodies.Within this enormous volume of cytoplasm resides a large nucleus. n somespecies (e.g.,sea urchins), the nucleus is already haploid at the time of fertilization. n otherspecies (includingmany worms and most mammals), the egg nucleus is still diploid, and the spermenters before themeiotic divisions are completed. The stage of the egg nucleus at the time ofsperm entry indi5erent species is illustrated in igure .4.!nclosing the cytoplasm is the egg plasma membrane. This membrane mustregulate the

;ow of certain ions during fertilization and must be capable of fusing with thesperm plasmamembrane. <utside the plasma membrane is the vitelline envelope (igure .=),which forms a8brous mat around the egg. This envelope contains at least eight di5erentglycoproteins and isoften involved in sperm>egg recognition (6orreia and 6arroll %??).

t is supplemented by e3tensions of membrane glycoproteins from the plasmamembrane and by proteinaceous vitelline posts that adhere the vitellineenvelope to the membrane. The vitelline envelope is essential for the species>speci8c binding of sperm. n mammals, the vitelline envelope is a separate and thick e3tracellular matri3called the zona pellucida.

The mammalian egg is also surrounded by a layer of cells called the cumulus

(igure .), which is made up of the ovarian follicular cells that were nurturingthe egg at the time of its release from the ovary.

The innermost layer of cumulus cells, immediately ad@acent to the zonapellucida, is called the corona radiata.

ying immediately beneath the plasma membrane of the egg is a thin shell(about 4 Bm) of gel>like cytoplasm called the cortex. The cytoplasm in this region

is sti5er than the internal cytoplasm and contains high concentrations of globularactin molecules. 9uring fertilization, these actin molecules polymerize to formlong cables of actin known as microfilaments. /icro8laments are necessary for celldivision, and they also are used to e3tend the egg surface into small pro@ectionscalled microvilli, which may aid sperm entry into the cell (see igure .=C: alsosee igure .%?).Also within the corte3 are the cortical granules (see igures .& and .=C).

These membrane>bound structures, which are homologous to the acrosomalvesicle of the sperm, are Dolgi>derived organelles containing proteolyticenzymes. Eowever, whereas each sperm contains one acrosomal vesicle, eachsea urchin egg contains appro3imately %4,### cortical granules. /oreover, in

addition to digestive enzymes, the cortical granules containmucopolysaccharides, adhesive glycoproteins, and hyalin protein. The enzymes



and mucopolysaccharides are active in preventing other sperm from entering theegg after the 8rst sperm has entered, and the hyaline and adhesiveglycoproteins surround the early embryo and provide support for the cleavage>stage blastomeres.

/any types of eggs also have anegg jelly

outside the vitelline envelope (seeigure .&).

This glycoprotein meshwork can have numerous functions, but most commonly isused either to attract or to activate sperm. The egg, then, is a cell specialized forreceiving sperm and initiating development.

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