Biochemical Study of Lipid, Protein and Carbohydrate in Ovary of Teleost: A Review

Nutritional research has shown that optimal levels of dietary proteins notably improve ovarian development, fecundity and reproductive performance in teleost fishes .Nutritional research has shown that optimal levels of dietary proteins notably improve ovarian development, fecundity and reproductive performance in teleost fishes ^[3]

Lipids represent one of the most crucial biochemical constituents in fish reproduction, acting as the primary source of metabolic energy and as essential precursors for structural and hormonal compounds during oogenesis. They play a fundamental role in yolk synthesis, membrane formation and steroid hormone production, all of which are indispensable for vitellogenesis and oocyte maturation ^[4].

Hormonal control, with estrogen induced vitellogenesis as the most striking example, has been recognized as the master regulator in orchestrating these physiological and biochemical changes ^[5]. The hormonal biochemical cross talk also determines the reproductive cycle, with estrogen and progesterone fluctuations directly modulating lipid depots, vitellogenin uptake and protein synthesis in the ovarian follicles ^[6] .The biochemical quality of eggs is highly impacted by broodstock diet and enrichment with long chain polyunsaturated fatty acids (LC-PUFAs) has been demonstrated to improve embryonic development, increase larval viability and raise total reproductive performance ^[7].

PUFA plays a crucial role in fertility and early development. PUFA functions as cell membrane stabilizer and modulator in maintaining membrane fluidity and protecting cells from apoptosis. Given their role in the formation and function of the cell membranes, PUFAs have been associated with various areas of reproductive biology, exerting an influence on the reproduction process at the levels of hypothalamus pituitary ovary axis, ovarian follicular microenvironment and embryo development. supplying the larvae with the necessary energy to survive the initial days of life. It is the precise chemical composition of this fat that decides whether or not the next generation lives on. Research by ^[8] illustrates this connection clearly they found that supplementing the diet of Blue gourami (Trichopodus trichopterus) with arachidonic acid not only boosted reproductive performance but also directly altered the fatty acid profiles of the ovaries and offspring. This highlights just how much a fish's reproductive potential relies on its dietary lipid intake.

The diet of fish has an impact on the amount of lipid that is deposited in muscle and in the ovary, which in turn, is related to the reproductive capacity. For instance, the different amounts of lipid in the diet of salmonids, will differently alter the expression of the genes linked to lipid metabolism that in turn will influence the reproductive performance ^[9]. Furthermore, oocytes also contain energy reserves that are influenced by macromolecular components like proteins and nucleic acids, which are essential for embryonic development.

^[10]The developmental changes demonstrated in winter flounder larvae , such as the increase of RNA, DNA and protein content, thus proving maternal provisioning of these items to be important in the early life stages. The fish nutrition part of the hypothesis is the least important. Besides, fish reproductive success is primarily governed by the substances secreted by the endocrine system and the ones that stimulate the growth of the fish. These internal signals are what keep the entire process on track, ensuring that growth and maturation happen exactly as they should instance,^[11] found that insulin like growth factor I (IGF-I) is a key driver of cell proliferation and steroidogenesis in the ovarian follicles of Coho salmon (Oncorhynchus kisutch), illustrating how hormonal pathways directly control oocyte maturation. These hormonal cues work in tandem with the energy stored during vitellogenesis. A clear example of this reliance on accumulated reserves is seen in the European eel (Anguilla anguilla) because these eels often stop feeding during their spawning migration, their entire gonadal development depends strictly on the lipid stores they built up beforehand ^[12].

Nutrition of breeders also has effects at later life stages, as observed in a study with Nile tilapia (Oreochromis niloticus) that showed a direct effect of maternal diet on performance at the larval and juvenile stage, particularly in relation to growth and feed efficiency. Broodstock nutrition also determines egg quality and larval survival. ^[13] Found that Atlantic cod (Gadus morhua) fed optimized diets produced eggs of higher quality and viable larvae. Specifically, the lipid content of early life stages has an impact on growth and survival as shown in Atlantic mackerel (Scomber scombrus) larvae where different lipid contents influenced larval development.^[14] Collectively these findings emphasize that oocyte development and fecundity in teleost fishes are tightly regulated by the interplay between diet, energy reserves and endocrine signals^[15].

LIPID

Lipids in fish are not just a form of stored energy but also the primary energy source necessary for the initiation of the reproductive process. Lipids are the metabolic fuel that provides the energy for the final stages of egg development, egg maturation and ovulation. They also facilitate final maturation of eggs and sperm, including the process of packaging maternal RNA and proteins in the egg that are needed for the first few days of the embryo's life. Without all this, the embryo can't even develop ^[16]. This process depends on a sophisticated transport system where the liver synthesizes lipoproteins. These carriers deliver a payload of phospholipids, triglycerides and cholesterol to the ovaries during vitellogenesis, fueling the rapid growth of the follicles^[17] .

However, this metabolic system is highly sensitive to external changes. Research indicates that the lipid profile of the ovaries reacts quickly to both diet and environmental shifts. Even slight tweaks to the fatty acid composition in broodstock feed can trigger significant changes in the ovary's lipid makeup. These shifts eventually trickle down to affect the fish’s overall fecundity, the health of the developing embryo and the survival rates of the hatchlings ^[18]. The fatty acid composition of neutral and polar lipids in Salmo salar eggs and reported marked variations depending on developmental stage. Confirmed that adequate lipid metabolism is vital for maintaining the energy supply required during fertilization and embryonic growth ^[19].

PROTEIN

Proteins are the major constituents of yolk, which give an essential foundation for embryonic development ^[20] had reported that dietary protein is necessary for the growth and reproduction and also during vitellogenesis. ^[21] Pointed out that protein yolk deposition is increasing gradationally with the maturation of the oocyte which further indicates that there is an increase in protein synthesis in the ovary. Vitellogenin a yolk precursor protein is synthesized in the liver and is regulated by estrogen it is then transported to the ovary when the oocyte grows. ^[22] had reported that vitellogenin synthesis is regulated by the level of circulating estrogen and even estrogen also transports the biochemical concentration of the oocyte which is in the developing stage ^[23].further they studied that ovarian protein concentration is plays main role in embryo quality and early larval survival.

CARBOHYDRATE AND GLYCOGEN

Although lipids serve as the primary energy source, carbohydrates also contribute to ovarian energy metabolism ^[24] developed a standard method for glycogen estimation that has been widely applied in studies of ovarian carbohydrate reserves.^[25] Explained that glucose metabolism supports energy homeostasis during reproductive phases, while ^[26] reported that ovarian glycogen mobilization supplies energy for intense vitellogenin activity and follicular growth. Histochemical studies by ^[27] revealed differential accumulation of carbohydrate granules during various oocyte maturation stages, indicating shifting metabolic priorities during ovarian development.

NUTRITIONAL INFLUENCE ON OVARY

Dietary composition plays a decisive role in determining ovarian biochemical reserves.^[28] Showed that higher dietary protein intake promotes ovarian growth and vitellogenic development in Oncorhynchus mykiss.^[29] Further demonstrated that high-quality dietary protein improves ovarian protein content, fecundity and hatching success in freshwater teleosts.

Nutritional programming is a dynamic process wherein a stimulus or series of stimuli occurring during a sensitive period in an organisms developmental processes contributes to persistent alterations in the organisms metabolism and physiology. It can also have long term consequences for offspring health with effects on subsequent generations. The inclusion of balanced PUFA levels in broodstock feed therefore contributes significantly to improved reproductive efficiency and overall egg quality ^[16].

HORMONAL AND ENDOCRINE REGULATION

Studied the mobilization and utilization of biochemical reserves during oogenesis is regulated by endocrine factor^[30]. Revealed the role of the insulin-like growth factor (IGF) system in the coordination of growth and reproduction^[31]. reported that the IGF signaling can enhance lipid mobilization, protein synthesis and vitellogenin uptake.Steroid hormones such as estrogen and progesterone also influence ovarian lipid and protein synthesis.^[32] Demonstrated that fluctuations in hormone levels correspond with shifts in ovarian biochemical composition during different reproductive phases.

ENVIRONMENTAL AND STRESS INDUCED ALTERATIONS IN OVARIAN

The biochemical pathways in the ovary react strongly to its environment. For example, even everyday temperature changes can make a fish redistribute its energy storage and deviate lipids from reproductive storage^[33]. Under heat stress, the effects are more pronounced^[34] described that lipid metabolism and endocrine signaling are disturbed and the production of the yolk precursor vitellogenin is severely affected.

Chemical interference complicates that balance even further. For example ^[35], his discovery that copper exposure and in Nile tilapia (Oreochromis niloticus) to the ”biochemical bottleneck” of synthesis his lipid and protein and resulted in a measurable decrease in egg quality. In addition to chemical pollutants, the total cost of stress on the environment or human losses is sufficient to disproportionately impact the HPG axis. This has been effectively demonstrated by the work of ^[36] in Rainbow Trout (Oncorhynchus mykiss), indicating that hormonal shifts induced by stress reduce reproductive productivity. Overall, these works show that maintaining a stress stable environment is not just a preference, but a biological necessity for successful broodstock management.

EGG QUALITY, LARVAL SURVIVAL AND OVARIAN BIOCHEMISTRY

The biochemical composition of oocytes determines egg quality and larval viability.^[37] Highlighted that maternal provisioning of lipids, proteins and carbohydrates directly influences larval survival^.[10]Showed that RNA–DNA ratios serve as reliable biochemical indicators of protein synthesis and larval growth potential.^[38]Reviewed global egg-quality research and emphasized that maternal biochemical reserves are key determinants of fertilization success and embryo viability.

Table 1: Review of carbohydrate, lipid and protein in different fish species