Oocyte intracellular lipids are mainly stored in lipid droplets (LD) providing energy for proper growth and development. Lipids are also important signalling molecules involved in the regulatory mechanisms of maturation and hence in oocyte competence acquisition. Recent studies show that LD are highly dynamic organelles. They change their shape, volume, and location within the ooplasm as well as their interaction with other organelles during the maturation process. The droplets high lipid content has been correlated with impaired oocyte developmental competence and low cryosurvival. Yet the underlying mechanisms are not fully understood. In particular, the lipid-rich pig oocyte might be an excellent model to understand the role of lipids and fatty acid metabolism during the mammalian oocyte maturation and their implications on subsequent monospermic fertilization and preimplantation embryo development. The possibility of using chemical molecules to modulate the lipid content of oocytes and embryos to improve cryopreservation as well as its biological effects during development is here described. Furthermore, these principles of lipid content modulation may be applied not only to germ cells and embryo cryopreservation in livestock production but also to biomedical fundamental research.
Oocyte quality is one of the key limiting factors in female fertility [
The cumulus-oocyte complex (COC) composed of the female gamete and the surrounding cumulus cells (CC) is a complete functional and dynamic unit playing a pivotal role in oocyte metabolism during maturation. The bidirectional exchanges of nutrients and regulatory molecules between oocyte and contiguous CC are crucial for oocyte competence acquisition, CC expansion, and early embryonic development [
During
Oocyte competence to complete nuclear maturation is acquired at least in two steps: firstly, oocytes are able to resume meiosis, undergo germinal vesicle breakdown (GVBD), and progress to metaphase I; secondly, oocytes are competent to advance beyond metaphase I, enter anaphase, and proceed to MII [
Several mediator factors are involved in the maturation of an oocyte [
The oocyte cytoplasmic maturation is a complex process comprising many organelles and compounds [
In spite of several efforts, oocyte cytoplasmic maturation remains a key limiting step for ART. The reasons why fully grown oocytes are not capable of becoming viable embryos are still elusive, but incomplete cytoplasmic maturation and/or asynchrony between nuclear and cytoplasmic maturation are certainly among those critically responsible.
The use of morphological characteristics and metabolites involved in COC maturation can provide valuable information for the preselection of high-quality oocytes to maximize embryonic developmental outcomes [
Morphological appearance of immature pig oocytes.
Conversely, other morphological characteristics can be used to predict oocyte quality. The huge number of LD, as well as their distribution pattern or their interaction with other organelles and cytoplasmic pigments, is responsible for the dark colour tone that characterizes the oocyte of some mammalian species, namely, the pig, cattle, horse, or even of the minke whale [
Besides morphological characterization, metabolic markers are also eligible criteria to evaluate oocyte quality and estimate its fertilization ability [
The pig oocyte is known as one of the most lipid rich oocytes in domestic animals [
Regarding LD composition, a core of neutral lipids is enveloped by a phospholipid monolayer containing a wide variety of proteins, embedded in both, the phospholipid monolayer and within the core [
A model for lipid droplets (LD) biogenesis ((a), (b)) during pig oocyte (c) maturation: (a),
Immature pig oocyte with lipid droplets (LD) highlighted in white colour (LD areas were measured using Image J software) and scale bar 50
Lipid droplets can be found in association with other organelles linked to cellular metabolism such as mitochondria, ER, endosomes, peroxisomes, and cytoskeleton [
As referred, during human and pig oocyte aging, a dark colour tone became more pronounced and an irregular ooplasm was presented [
Plasmalogens represent another important class of phospholipids in oocyte that has been identified as membranes constituents, both cytoplasmic and inner cell membranes, participating in the regulation of their dynamics [
Analysis of FA composition of cattle and pig oocytes during maturation showed that 16:0, followed by oleic (
Infertility is a huge concern throughout the world both in animals and humans. Although the economic importance of the reproductive efficiency in livestock is recognised, herd fertility has declined over the past 30 years. Reduced oocyte and embryo quality were identified as major problems of this substandard fertility [
Similar reports exist concerning the women fertility decline due to age and obesity [
Deregulation of neutral lipids storage in LD has been linked to a variety of metabolic diseases [
Lipid modulators are substances that are capable of reducing and/or modifying intracellular lipid content of cells. These substances have been successfully applied in ART, namely, in oocyte maturation and embryo production [
Hypothetical model for the effects of forskolin and
The diterpenoid forskolin is a chemical stimulator of lipolysis through the activation of adenyl cyclase, whose effects in pig have also been demonstrated in both oocytes and embryos [
Alternatively, PES is another lipid modulator that regulates embryo metabolic pathways. PES increases glucose metabolism through PPP during embryo culture [
Considerable progress has been made in improving and simplifying oocyte and embryo cryopreservation procedures to be routinely used in transfer programs. In general, cryopreservation by slow freezing is a process where extracellular water crystallizes, resulting in an osmotic gradient that draws water from the intracellular compartment until intracellular vitrification occurs [
The plasma membrane of oocytes and embryos is the first cellular structure whose integrity is affected by thermotropic phase transition. During cooling, irreversible damage occurs shortly after exposure to low, but not freezing, temperatures just below 15°C [
Differences in LD colour tone of fresh immature and vitrified-warmed pig oocytes were identified: gray in fresh and slightly dark in vitrified oocytes [
As referred, the high lipid content that has been related to an increased sensitivity to chilling injury during cryopreservation is particularly important in pigs, but also in cattle [
The lipid content of the pig oocyte, as well as the asynchrony between nuclear and cytoplasmic maturation, renders it a good model in the field of oocyte biology research. Knowledge of the pathways and key molecules regulating these processes may highlight therapeutic possibilities to prevent the excessive accumulation or to modulate lipid composition of cytoplasmic droplets. Furthermore, lipid modulators may also be applied in germ cells and embryo cryopreservation to improve livestock production. Finally these molecules might provide tools to overcome lipid metabolic disorders related to infertility.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The “Fundação para a Ciência e a Tecnologia” is greatly acknowledged for funding a Grant for E. G. Prates (SFRH/BD/42359/2007). The authors acknowledge Ann Barreiro for her kind assistance in revising this paper.