EL FUNCIONALISMO Y LA TEORÍAS ARQUITECTÓNICAS

Once the requisite m aturational changes have taken place, the gametes become functionally com petent and capable of fertilisation. At fertilisation the diploid chrom osom e com plem ent is restored and the developm ent program m e of the oocyte is activated, resulting in protein and DNA synthesis.

Site of fertilisation

In the hum an, fertilisation takes place in the am pulla of the fallopian tube. A lthough millions of sperm atozoa are deposited in the vagina at

intercourse, only a small proportion m ake their w ay to the site of

fertilisation. The cervical glands m ay act as a reservoir for steady release of sperm , to increase the likelihood of some sperm being in the fallopian tube at the time of ovulation. The isthmic p art of the fallopian tube m ay also act as a second reservoir, and another im portant functional change in the sperm , called capacitation, appears to occur here (H arper 1982).

Sperm capacitation and the acrosome reaction

C apacitation involves a change in the p attern of m ovem ent of the sperm tail, which becomes m ore vigorous and is com bined w ith a w ider am plitude of flagellation to produce a 'w hiplash' m ovem ent of the head (Suarez, et al. 1991). This hyperactivation is necessary for fertilisation, and m ay both increase the chance of a sperm atozoa colliding w ith a cum ulus oophorus complex in the tube and aid penetration through the cum ulus cells and zona pellucida. The other sperm atozoal change that is required for fertilisation is the acrosome reaction, w here channels develop in the acrosome cap, by fusion of the inner and outer acrosomal m em branes, allowing release of its contents. The acrosome contains a num ber of enzym es, particularly hyaluronidase and acrosin (Bedford 1982). It w as originally thought th at these were necessary for passage through the cum ulus cells, how ever there is som e evidence in other species that this is not the case (Cherr, et al. 1986). The acrosome reaction is calcium dependent, and can be induced by exposing sperm to the calcium ionophore A23187 (Crozet 1993).

Once the sperm has negotiated the cum ulus complex surro u n d in g the oocyte, it m ust penetrate the zona pellucida, for which it m ust be both capacitated and acrosome reacted (Crozet 1993). The zona pellucida consists of three glycoproteins Z P l, ZP2, and ZP3, and the ZP3 molecule is likely to

play a key role in the species specific interaction betw een sperm and ligand structures in the zona pellucida (van Duin, et al. 1992). Leaving the acrosome m em brane on the zona surface, the sperm then penetrates the zona, leaving sharply defined slits which suggest that mechanical rather than enzym atic action is involved (Bedford 1982).

Sperm-oocyte fusion

The next step is sperm -oocyte fusion, w hen the plasm a m em brane over the equatorial segm ent of the sperm atozoa fuses to the oolem m a (Johnson and Everitt 1988). The sperm nuclear envelope disintegrates and in m ost species the entire sperm is incorporated into the oocyte (Crozet 1993). At sperm -oocyte fusion, oocyte activation is triggered resulting in cortical

granule exocytosis, resum ption of meiosis w ith extrusion of the second polar body and form ation of the female pronucleus. Oocyte activation is discussed in m ore detail in the next section. Synchronously, the sperm head

décondenses to form the male pronucleus, w hich is followed by pronuclear m igration and syngamy.

Sperm head decondensation

After incorporation of the sperm head into the oocyte cytoplasm , the nuclear envelope disintegrates and the nuclear chrom atin décondenses, as a result of reduction of the disulphide bonds of protam ines, the basic proteins attached to DNA. The ability of oocytes to induce sperm chrom atin

decondensation is a function of their m aturity, and m ay be due to a factor released from germ inal vesicles at germ inal vesicle breakdow n (GVBD) (Lopata and Leung 1988). A role for cytoplasmic glutathione (a reducing agent) has been suggested in sperm chrom atin decondensation, by experim ental w ork w hich dem onstrates that inhibition of glutathione synthesis during oocyte m aturation renders them incapable of inducing

Sperm head decondensation (Perreault, et al. 1988). After second polar body extrusion the sperm chrom atin recondenses prior to m ale pronucleus form ation (Adenot, et al. 1991).

Pronucleus formation, migration and syngamy

Pronuclei form by the addition of nuclear envelopes around the male and female chrom atin. Male pronucleus developm ent is u n d er the control of a cytoplasmic factor called male pronucleus grow th factor (MPGF), which is only present in m ature oocytes, and is lost w ithin a few hours of oocyte activation (Crozet 1993). Active DNA synthesis takes place in both pronuclei and has been reported to start at 9-10 hours after insem ination and be

com pleted 3-5 hours later (Balakier, et al. 1993a). It is essential for the two pronuclei to m igrate tow ards each other in the centre of the cell, enabling the tw o sets of chrom osomes to come together on the first mitotic spindle.

Pronuclear m igration depends on cytoskeletal activity, and in the m ouse is dependent on both actin filaments and m icrotubules (Schatten and Schatten 1987). After m igration, pronuclear m em branes breakdow n and the

duplicated chrom osomes align them selves across the equator of the m itotic spindle (syngamy) in preparation for the first mitotic division.