Ovary Structure

Oogenesis results in a large non-motile ovum (Fig.5). This cell is spherical and many times larger than a spermatozoon. Ovum has no centrioles, so this cell is not capable to divide itself. The cytoplasm of the ovum is packed with food reserve. Membrane-bound cortical granules accumulate in the cytoplasm, just beneath the plasma membrane. Cortical granules contain enzymes releasing a cortical reaction.

Fig.5. Human egg cell: 1) nucleus; 2) egg cell microvilli; 3) follicular cells; 4) follicular cells processes; 5) cortical granules; 6) yolk inclusions; 7) zona pellucida; 8) receptors.

In matu re follicles the oocyte is enveloped by the zona pellucida, a thick coat, composed of at least 3 different glycoproteins (ZP1, ZP2, ZP3). ZP3 is the ligand in the zona pellucida which binds with the acrosome of the sperm in the acrosome reaction.

The oocyte with the zona pellucida is surrounded by several layers of follicular cells. Long apical processes of follicular cells and microvilli of the oocyte penetrate the zona pellucida and make contact with one another via gap junctions. The follicular cells of the first layer around the ovum, that are, therefore, in close contact with the zona pellucida form the corona radiata, which accompanies the ovum when it leaves the ovary in the process of ovulation. The corona radiata is still present when the spermatozoon fertilizes the ovum; it is retained for some time during the passage of the ovum through the oviduct.

Fertilization

Fertilization (also known as conception), the process by which male and female gametes fuse, occurs normally in the in the ampulla of the uterine tube.

During ovulation the secondary oocyte, together with surrounding the zona pellucida and corona radiata cells, is discharged from the ovary and is carried into the tube by sweeping movements of the fimbriae and by motion of cilia on the epithelial lining.

Human ejaculate contains 3x10⁸ spermatozoa. These cells remain viable in the female reproductive tract for approximately 24 hours. Only 300-500 spermatozoa reach the site of fertilization.

Fertilization is more a chain of events than a single, isolated phenomenon and may be subdivided into 3 stages:

• distant interaction;

• contact interaction;

• penetration of the sperm cell into the oocyte and egg activation;

• fusion of male and female pronuclei, and formation of a zygote.

The process of fertilization lasts 12-24hours.

The distant interaction is based on phenomenon of chemotaxis: it was demonstrated that the surface membrane of human sperms possess receptors for specific chemoattractant substances (such as formyl Met-Leu-Phe) which have the ability to induce migration of sperm. Contraction of the musculature of the uterine tube promote this ascent.

Freshly ejaculated sperm are unable to fertilize. Sperm cells must undergo capacitation.

Capacitation occurring in the female reproductive tract, is a complex process, which renders the sperm cell capable for specific interaction with the oocyte. This process takes place under influence of mucous secretion of uterine tube epithelial cells, pH in the female reproductive tract etc. This final step in the maturation of spermatozoa is biochemical event. It associated with removal of adherent seminal plasma glycoproteins, reorganization of plasma membrane lipids and proteins, rendering it more fusigenic. The result is a more fluid membrane with an increased permeability to Ca^2+. An influx of Ca²⁺ produces increased intracellular cAMP levels and thus, an increase in motility. Hyperactivation is also part of capacitation and is the result of the increased Ca²⁺ levels.

Capacitation lasts approximately 7 hours. Spermatozoa used for in vitro fertilization are artificially capacitated.

The contact interaction includes:

• attachment and binding. The sperm binds to the zona pellucida of the egg. A special complementary molecule on the surface of the sperm head then binds to ZP3 glycoproteins in the zona pellucida. Binding of sperm to the zona pellucida is a receptor-ligand interaction with a high degree of species specificity: only sperm of same species may enter.

• the acrosomal reaction. This binding triggers the acrosomal reaction: the acrosome vesicle membrane fuses with the sperm plasma membrane at multiple sites, releasing acrosomal enzymes, such as hyaluronidase and acrosin to degrade the zona pellucida and, in that way, facilitate penetration through it. After the acrosome reaction, it is believed that the sperm remains bound to the zona pellucida through exposed ZP2 receptors.

Penetration of spermatozoon and egg activation and the cortical reaction. Upon binding of a sperm cell, the egg rapidly undergoes a number of metabolic and physical changes that collectively are called egg activation. Prominent effects include a rise in the intracellular concentration of calcium, completion of the second meiotic division and the cortical reaction. Once the sperm cells find their way past the zona pellucida (owing to 2 factors: motility and zona-digesting enzymes), the cortical reaction occurs: cortical granules inside the ovum fuse with the plasma membrane of the cell, causing enzymes (including several proteases) inside these granules to be expelled to the zona pellucida. This in turn causes the glycoproteins in the zona pellucida to cross-link with each other, making the whole matrix hard and impermeable to sperm (sperm receptors are destroyed also). So, the zona pellucida transforms into the fertilization membrane (or the zona reaction) around a fluid-filled perivitelline space. This membrane serves as a barrier and prevents completely further penetration of other sperms (block to polyspermy).

Fusion and formation of a zygote

Both the oocyte and the sperm go through transformations, as a reaction to the fusion of cell membranes, preparing for the fusion of their genetic material.

Entry of the sperm triggers completion of the oocyte second meiotic division. This results in a mature ovum. The nucleus of the oocyte is called a female pronucleus in this process, to distinguish it from the nuclei that are the result of fertilization.

The head of engulfed sperm swells, forming a male pronucleus. The sperm’s tail and mitochondria degenerate with the formation of the male pronucleus. This is why all mitochondria in humans are of maternal origin.

This process ultimately leads to the formation of a diploid cell called a zygote. The zygote is the fertilized ovum, and is the first form of a new human being.

The male pronucleus, its centrioles and female pronucleus migrate towards the center of the oocyte, rapidly replicating their DNA as they do so to prepare the new human for its first mitotic (cleavage) division.

The male and female pronuclei don’t fuse, although their genetic material do so. Instead, their membranes dissolve, leaving no barriers between the male and female chromosomes. During this dissolution, a mitotic spindle forms around them to catch the chromosomes before they get lost in the egg cytoplasm. The first cleavage division follows, producing two identical daughter cells.