Reproduction, Fertility and Development 

Abstract: New research on the cooling and cryopreservation of mammalian spermatozoa is reviewed in the context of the older literature. Cryoinjury to a variety of cell organelles is regarded as being due to the two major stresses of cryopreservation, i.e., the change in temperature, and the formation and dissolution of ice and its consequences. Since the cryopreservation process involves departure of the cells from and return to body temperature, both cold shock and warm shock are included as potential stresses to be considered, as well as the stages involving cooling below the freezing point of the medium. The causes of cryoinjury are reconsidered and new concepts concerning the influence of osmotic stress are presented. Heterogeneity of the sperm population is discussed in the context of the success with which spermatozoa can be cryopreserved between and within ejaculates and individuals. The functional state of frozen and thawed spermatozoa is examined on the basis of published results of structural and functional tests of sperm competence. The hypothesis is advanced that cryopreserved mammalian spermatozoa are in a state resembling capacitation, which accounts for their relatively reduced longevity and their readiness to undergo egg penetration without incubation. The importance of this to the utilization of cryopreserved spermatozoa is examined, and proposals are made for new avenues of research to overcome these problems.

Abstract: The cellular generation of reactive oxygen species was first observed in mammalian spermatozoa in the late 1940s. The field then remained dormant for 30 years until Thaddeus Mann and Roy Jones published a series of landmark papers in the 1970s in which the importance of lipid peroxidation as a mechanism for damaging mammalian spermatozoa was first intimated. The subsequent demonstration that human spermatozoa produce reactive oxygen species and are susceptible to peroxidative damage has triggered intense interest in the role of oxidative stress in the aetiology of male infertility. Moreover, data have recently been obtained to indicate that, although excessive exposure to reactive oxygen species may be harmful to spermatozoa, in physiological amounts these molecules are of importance in the control of normal sperm function. This review considers the dualistic role of reactive oxygen species and sets out the current understanding of the importance of oxidative processes in both the physiology and the pathology of the human spermatozoon. Extra keywords: human spermatozoa, reactive oxygen species.

Abstract: The glucocorticoids, cortisol and corticosterone, have a unique function in the fetus in inducing a wide range of enzymes before birth that have little or no function during fetal life but on which survival after birth is dependent. The loss of the placenta at birth deprives the fetus of a source of oxygen, glucose and heat (among many other things) for which alternatives must be available immediately if survival is to be assured. In anticipation of these needs several organs undergo maturational changes in late pregnancy aimed at meeting these requirements. The lungs mature structurally and functionally, becoming distensible and capable of coping with high surface tension when air enters the alveoli with the first breath. In the liver, glycogen accumulates and gluconeogenesis is initiated to meet the demands for glucose until feeding begins. There is an increase in the production of tri-iodothyronine and catecholamines in preparation for the sharp increase in metabolic rate and thermogenesis associated with breathing and the cold environment. All these dramatic maturational events are regulated by cortisol as are numerous others in most organ systems that contribute to neonatal well-being but on which survival is less dependent. Pharmacological manipulation of these systems before birth has made a substantial contribution to improving human health.

Abstract: When sperm of the ram, bull, boar and stallion are cold-shocked by rapid cooling to near freezing point, motility and metabolic activity are irreversibly depressed and the acrosome and plasma membrane disrupted. Ram sperm become susceptible to cold shock in the proximal corpus region of the epididymis when the cytoplasmic droplet has moved backwards to the distal portion of the sperm midpiece. The membrane constituents phospholipids and cholesterol are important in cold shock which causes loss of lipid from sperm. The susceptibility of sperm to cold shock is linked with a high ratio of unsaturated:saturated fatty acids in the phospholipids and a low cholesterol content. The high unsaturated fatty acid content of sperm also makes them susceptible to damage from peroxidation which adversely affects motility, metabolism, ultrastructure and fertility. Hydroxynonenal, a product of fatty acid peroxidation, depresses the motility and oxygen uptake of ram sperm in vitro and may react with the -SH groups of the axonemal microtubules. High calcium concentrations in the external medium may decrease the motility and metabolism of sperm and 'calcium intoxication' may be a factor in cold shock. Lowering the environmental temperature increases calcium uptake by sperm and the effect is aggravated if the rate of cooling is rapid. Phospholipids, particularly those in egg yolk, protect sperm to some extent from cold shock and also prevent increased calcium flux into the sperm. Suggestions are made for increasing the life span of sperm during preservation and microencapsulation by adding agents that may stabilize membranes, counter peroxidation and decrease calcium uptake.

Abstract: As oocytes near the end of their growth phase, they become competent to undergo two aspects of maturation, cytoplasmic and nuclear. Both are essential for the formation of an egg having the capacity for fertilization and development to live offspring. Nuclear maturation encompasses the processes reversing meiotic arrest at prophase I and driving the progression of meiosis to metaphase II. Cytoplasmic maturation refers to the processes that prepare the egg for activation and preimplantation development. This review focuses on the developmental programmes whereby oocytes at the germinal vesicle (GV) stage acquire competence to undergo nuclear and cytoplasmic maturation, the coordination of programmes regulating the acquisition of these competencies in GV-stage oocytes, and the coordination of the maturational processes themselves. Although the developmental programme of the GV-stage oocyte for acquiring competence to complete preimplantation development does not appear to be tightly linked to the acquisition of competence to complete nuclear maturation, GV breakdown (GVB) is probably essential for activating some critical aspects of cytoplasmic maturation, particularly those related to fertilization and activation. Nuclear and cytoplasmic maturation are normally coordinated by this mechanism requiring the mixing of the GV contents with the cytoplasm at the time of GVB, but some processes of cytoplasmic maturation related to successful preimplantation development probably still occur without coordination with nuclear maturation. Thus, continued differentiation of GV-stage oocytes is necessary after the acquisition of competence to undergo nuclear maturation, to allow for the deposition of the maternal factors required for the development of preimplantation embryos beyond the 2-cell stage.