Introduction

The egg, or ovum, is the female gamete that contains the haploid maternal genome and all the material necessary to begin growth and development. Here we will learn about the structure of egg. The egg is a large cell that is surrounded by a membrane and an extracellular matrix that regulate the entry of sperm and prevent polyspermy (the entry of more than one sperm). The egg cytoplasm contains various components that provide nutrition, protein synthesis, morphogenesis, and protection for the early embryo. Its nucleus undergoes meiosis to produce a haploid pronucleus that fuses with the sperm pronucleus during fertilization. The interaction of sperm and egg involves five steps: chemoattraction, acrosome reaction, binding, penetration, and fusion.

Egg cytoplasm and nucleus

• The egg cytoplasm is the cellular material that surrounds the nucleus and occupies most of the egg volume.
• The egg cytoplasm contains the following components:
  • Nutritive proteins: These are proteins such as yolk that act as food reservoirs for the developing embryo. They are either synthesized by the oocyte or absorbed from other organs through the maternal blood.
  • Ribosomes and tRNA: These are molecules that are involved in protein synthesis. They are produced by the oocyte in large numbers during meiotic prophase.
  • Messenger RNAs: These are molecules that encode proteins for the early stages of development. They are stored in the oocyte and remain repressed until after fertilization.
  • Morphogenetic factors: These are molecules that direct the differentiation of cells into certain cell types. They include transcription factors and paracrine factors. They are localized in different regions of the egg and become segregated into different cells during cleavage.
  • Protective chemicals: These are molecules that protect the embryo from environmental threats. They include ultraviolet filters, DNA repair enzymes, antibodies, and antipredator substances.
• The egg nucleus is the organelle that contains the haploid maternal DNA that is tightly packed and condensed for efficient transport.
• The egg nucleus is derived from the primary oocyte that undergoes two meiotic divisions to produce one haploid oocyte and three polar bodies (small cells with little cytoplasm).
• The egg nucleus can be either haploid or diploid at the time of fertilization, depending on the species. In some species (such as sea urchins), the egg completes meiosis before fertilization and becomes a haploid pronucleus. In other species (including many worms and most mammals), the egg remains diploid until fertilization and completes meiosis after the sperm enters.

Egg membrane and extracellular envelope

1. The egg membrane is the outermost layer of the egg cell that separates it from the external environment. It regulates the flow of specific ions during fertilization and must be capable of fusing with the sperm cell membrane.
2. The egg membrane is composed of a phospholipid bilayer with embedded proteins that mediate cell signaling, transport, adhesion, and recognition.
3. Outside the egg membrane is an extracellular matrix that forms a fibrous mat around the egg and is often involved in sperm-egg recognition. It can be called different names depending on the species:
   • Vitelline envelope: This is the term used for invertebrate eggs. It contains several different glycoproteins that are secreted by the oocyte or its surrounding follicle cells. 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.
   • Zona pellucida: This is the term used for mammalian eggs. It is a thick matrix composed of four glycoproteins (ZP1, ZP2, ZP3, and ZP4) that are synthesized by the oocyte. It mediates species-specific binding of sperm to the egg and triggers acrosome reaction.
4. Many types of eggs also have a layer of egg jelly outside the vitelline envelope or zona pellucida. This glycoprotein meshwork can have numerous functions, but most commonly it is used either to attract or to activate sperm.
5. 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. These molecules polymerize to form long cables of actin microfilaments during fertilization. Microfilaments are necessary for cell division and for extending the egg surface into small projections called microvilli, which may aid sperm entry into the cell.
6. Also within the cortex are the cortical granules. These membrane-bound, Golgi-derived structures contain proteolytic enzymes and other substances that help prevent polyspermy by modifying the extracellular matrix of the egg after fertilization. They also secrete hyalin and adhesive glycoproteins that surround the early embryo and provide support for cleavage-stage blastomeres.
7. In mammalian eggs, the extracellular envelope is surrounded by a layer of cells called the cumulus. This layer 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.

Summary

• The structure of egg is very interesting and unique.
• The egg is a specialized cell that contains the maternal genome and all the material necessary to begin development.
• The egg has a large cytoplasm that contains various components such as nutritive proteins, ribosomes, mRNAs, morphogenetic factors, and protective chemicals.
• The egg has a haploid nucleus that undergoes meiosis to produce a pronucleus that fuses with the sperm pronucleus during fertilization.
• The egg has a membrane and an extracellular matrix that regulate the entry of sperm and prevent polyspermy. The extracellular matrix can be called vitelline envelope or zona pellucida depending on the species. The egg also has a cortex that contains actin microfilaments and cortical granules that are involved in fertilization.
• The interaction of sperm and egg involves five steps: chemoattraction, acrosome reaction, binding, penetration, and fusion.