|Purpose||Deliver haploid genetic information from male to the female's egg.||Provide haploid genetic information from the female, nourish the embryo until its placental or feeding stage, and provide cytoplasmic determinants that interact with nuclei.|
|System||A closed system except for water (H2O), gas (O2 and CO2) and limited ion exchange.|
|Contents||Haploid genome; propulsion system; enzymes that allow sperm to enter the egg.||Haploid genome; yolk; cytoplasmic determinants.|
The egg must provide a nucleus (genetic informatino), specific regulatory molecules (mRNAs, cytoplasmic determinants) that interact with nuclei, and building materials for development (protein, carbon source) until the embryo's feeding or placental stage.
The volume of the egg can range from 103 (mammals, sea urchin) to 106 (amphibian) to 1011 (ostrich egg) times larger than a somatic cell. A normal-size oogonium gives rise to such an enormous ova by conserving cytoplasm during meiosis and also undergoing a massive growth phase. Except for gas and limited ion and water exchange, the egg is a closed system. Thus, embryogenesis uses only macromolecules from within the egg itself (formed during oogenesis) until the embryo is developed enough to feed itself or can draw from the mother via a placenta.
Common features of oogenesis are: the uptake of yolk from a source outside the oocyte; a high level of transcription of mRNA; and the presence of follicle cells. Features specific to oogenesis in only some organisms are: nurse cells; and DNA amplification. The components of the egg that must be synthesized during oogenesis are:
|Egg shell||Follicle cells (insects, mammals)|
|The egg shell -- or egg membrane or vitelline membrane -- is synthesized by follicle cells (the cells surrounding the egg) in most organisms and by cells of the oviduct in birds. In some insects, including Drosophila, follicle cells contain amplified quantities of egg shell genes -- thus, there are hundreds as opposed to just two copies of the genes required for egg shell formation. Amplification allows faster synthesis and assembly of the egg shell.|
|Yolk||Vertebrate liver secretion.|
Insect fat body secretion.
|Yolk is a heterogeneous mix of phospholipoproteins that serves as an energy- and carbon-source. It is synthesized in the vertebrate liver and insect fat body, transported through blood and uptaken by the egg to form membrane-enclosed yolk platelets. The amount of yolk in an egg has tremendous inter-species variability: primitive mammals contain massive amounts of yolk; placental mammalian eggs contain very little yolk.|
Fertilization is usually immediately followed by rapid cell division that requires quick synthesis of DNA, mitotic spindles and cleavage rings. Thus, the following are usually present in the egg cytoplasm before fertilization:
|mRNA||Early zygotic development relies on mRNA stored in the egg, known as stored maternal mRNA. This mRNA is stable, untranslated and mostly encodes proteins required for all rapidly dividing cells, such as cytoskeletal, membrane, metabolic, histone and other proteins. However, a small fraction of these mRNAs are localized to certain regions of the cell and control embryonic polarity -- these mRNAs determine which daughter cells will eventually become which body regions (examples include bcd, nos and others).|
A sperm's role is to deliver a male's nucleus (namely, the genetic information within) to the egg. Mature sperm lack ribosomes and mRNA; they are transcriptionally and translationally inactive.
|Haploid Nucleus||The sperm nucleus is very condensed, thus reducing its size for faster swimming. The sperm nucleus contains only DNA and positively charged proteins, which are different from somatic histones -- all enzymes and non-histone chromosomal proteins are removed during spermatogenesis.|
|Propulsion System||The flagellum contains an axoneme, composed of nine microtubules arranged in a circle, two microtubules in the center and dynein arms (which hydrolize ATP) accompanying the microtubules. At the nuclear-proximal end (near the 'head') is a cluster of mitochondria which provide metabolic energy for flagellar movement.|
|Centriole||In addition to organizing the flagellum, the sperm centriole may organize the microtubules of the mitotic spindle of the fertilized egg. This is a controversial point, but the egg may not have functional centriole, hence the presumed importance of the sperm centriole.|
|Acrosomal Vesicle||Formed from the Golgi Apparatu, the acrosomal vesicle is a membrane-bound cap on the sperm nucleus which contains digestive enzymes that allow the sperm to penetrate the outer layers of the egg and reach the egg membrane.|
Gametogenesis is the development of germ cells into gametes. Gametes are large nonmotile oocytes in females, and small motile sperm in males. Gametogenesis occurs in the gonads: ovaries in females and testes in males. The two main differences between oogenesis and spermatogenesis: prophase arrest; and unequal division.
|Germ Cell||Germ Cell||Germ cells originate in the earliest embryonic cell divisions and remain distinct.|
|Mitosis 1||Germ Cell||2N Gamete||Germ cells migrate to newly formed gonads and proliferate mitotically into diploid gametes.|
|Mitosis 2||2N Gamete||1° Gamete||Diploid gametes divide mitotically into diploid primary oocytes and primary spermatocytes.|
|Meiosis||1° Gamete||1N Gamete||Meiosis reduces the chromosomes to haploidicity.|
|Meiosis 1||Primary spermatocytes undergo the first meiotic division into haploid secondary spermatocytes.|
|Meiosis 2||Secondary spermatocytes under the second meiotic division into four haploid spermatids per primary spermatocyte.|
Spermatogonium → → Primary spermatocyte → → Secondary spermatocyte → → Spermatid → → Sperm
In spermatogenesis: microtubule-based flagellum are built (for motility); ribosomes and mRNA are lost; and the nucleus is condensed (to stop transcription).
Mammalian spermatocytes are connected by cross-bridges of cytoplasm whilst dividing. This is due to asymmetry of sex chromosomes in males. Half of secondary spermatocytes receive an X chromosome, and the other half receive a Y chromosome.
However, some gene products essential for spermatocyte development are found only on the X chromosome. Cytoplasmic contact allows all 4 secondary spermatocytes to share X chromosome gene products.
|Arrest||Oogenesis begins the first meiotic division but is arrested in prophase for days, months or years.|
|Growth||As Prophase 1 arrest ends, the primary oocyte uptakes yolk from blood and synthesizes proteins, maternal mRNAs, ribosomes, organelles and localized cytoplasmic determinants. This stocks all RNA needed for the first embryonic divisions, and all the embryo's nutrients until the placenta forms or it self-feeds.|
|Meiosis 1||The primary oocyte divides meiotically such that one daughter cell receives most cytoplasm (the secondary oocyte) and the other daughter cell receives almost none (1st polar body).|
|Arrest||In many species, the 2nd meiotic division does not occur until the egg is fertilized.|
|Meiosis 2||The secondary oocyte undergoes a second asymmetrical meiosis divides to produce a large haploid ootid and a 2nd polar body.|
|Mature||The polar bodies degenerate. The large haploid ootid is a mature egg.|