Characterization of yolk DNA from Xenopus laevis oocytes ovulated in vitro

Open questions on the functional biology of the yolk granules during embryo development

Biogenesis and consumption of the yolk are well-conserved aspects of the reproductive biology in oviparous species. Most egg-laying animals accumulate yolk proteins within the oocytes thus creating the source of nutrients and energy that will feed embryo development. Yolk accumulation drives the generation of a highly specialized oocyte cytoplasm with maternal mRNAs, ribosomes, mitochondria, and, mainly, a set of organelles collectively referred to as yolk granules (Ygs). Following fertilization, the Ygs are involved in regulated mechanisms of yolk degradation to fuel the anabolic metabolism of the growing embryo. Thus, yolk accumulation and degradation are essential processes that allow successful development in many species. Nevertheless, the molecular machinery and mechanisms dedicated to the programmed yolk mobilization throughout development are still enigmatic and remain mostly unexplored. Moreover, while the Ygs functional biology as a nutritional source for the embryo has been acknowledged, several reports have suggested that Ygs cargoes and functions go far beyond yolk storage. Evidence of the role of Ygs in gene expression, microbiota harboring, and paracrine signaling has been proposed. In this study, we summarize the current knowledge of the Ygs functional biology pointing to open questions and where further investigation is needed.

Yolk platelets in artemia embryos: are they really storage sites of immature mitochondria?

We have used semi-quantitative polymerase chain reaction (PCR) technology to determine the mitochondrial DNA (mtDNA) content of yolk platelets isolated from embryos of the brine shrimp, Artemia franciscana, and ultrastructural analysis of yolk platelet formation to determine whether these organelles contain mitochondria as reported previously. Using six different isolation and purification protocols, we found one yolk platelet preparation to be devoid of mtDNA, while four yolk platelet preparations contained mtDNA ranging from 16.4 to 85 pg/10(6) yolk platelets. One preparation contained 600 pg mtDNA per 10(6) yolk platelets. Based on our PCR analyses, the mtDNA component of Artemia yolk platelets represented 0.16-4.5% of the total DNA isolated from the platelets. We calculated that Artemia yolk platelets contain, on average, approximately 1.78 molecules of mtDNA/platelet. Direct analysis of mtDNA in "free" mitochondria isolated from yolk platelet-free preparations of Artemia embryos and newly hatched larvae yielded 0.76-0.80 ng/animal. Based on these values, the mtDNA content of yolk platelets was approximately 0.2% of total mtDNA in Artemia embryos. Microscopic analysis of yolk platelet formation during oogenesis in Artemia failed to show the inclusion of mitochondria during the assemblage of yolk platelets. The "mitochondria-like" structures that appear in yolk platelets during their utilization lack the well defined inner and outer membranes characteristic of mitochondria making it unlikely that the yolk platelet inclusions are mitochondria. Our results from PCR technology and ultrastructure analysis demonstrate that mtDNA in yolk platelets of Artemia franciscana embryos is a minor component of the total mtDNA in the embryo, and they fail to support the notion that yolk platelets in Artemia are a major source of immature mitochondria for development.

Presence, isolation and characterization of yolk DNA from chicken eggs

Yolk DNA was detected with immunocytochemistry in fertilized chicken eggs. Yolk spheres were purifield with Ficoll-400 density gradient centrifugation, followed by extraction of DNA therein. Yolk DNA is sensitive to various restriction endonucleases. but shows no obvious bands, demonstrating its sequence heterogeneity. There is only a little, if any, methylated CpG in yolk DNA. judgmg from MspI and HpII digestion. Southern blot indicated that the sequence of yolk DNA accounts for a small portion of that of genomic DNA. Its uniqueness suggests that yolk DNA may play a crucial role in early development of chickens.

Absence of DNA in yolk platelets of Bufo arenarum oocytes

The absence of DNA in the yolk platelets of Bufo arenarum oocytes was verified. Oocytes from the coelomic cavity were used in order to avoid possible contamination by the ovarian tissue. Four methods for DNA analysis were employed, and the possibility of interference was carefully checked. The presence of three possible contaminants, which produced positive reactions from the DNA analysis, was detected. These were the glycosaminoglycan from the superficial layer of the platelets, sucrose, and polyvinylpyrrolidone from the buffer commonly used during the isolation of platelets. The present evidence points to the necessity of using at least two different methods for DNA determination with due regard for their sensitivity and specificity before any firm conclusion can be reached on the presence or absence of DNA in nonnuclear and mitochondrial parts of the oocyte.

The origin of yolk-DNA in Xenopus laevis

Xenopus laevis serum and plasma was found to contain an average of 25 microgram DNA/ml. Isolated X. laevis oocytes incubated in medium containing 25 microgram DNA/ml labeled with either 125I, 32P or 14C and from three different sources (bovine, E. coli and X. laevis), incorporated the label at an average rate of 0.11 ng.mm-2.hr-1. Sucrose gradient fractionation of oocytes revealed that 40-75% of the acid-precipitable label incorporated was associated with the yolk platelets. Additional incubations of oocytes in unlabeled medium demonstrated that the DNA incorporated into the yolk platelets was undergoing turnover; only 20% of the yolk-associated DNA was still present after a one-week incubation. Our data suggest that yolk-DNA arises by the adventitious uptake of DNA present in the maternal serum by vitellogenic oocytes.

Utilization of yolk platelets and lipid bodies during the myogenesis of Xenopus laevis (Daudin)

The premyoblast and differentiating mononuclear myoblasts of the metameric striated muscles of Xenopus laevis were analysed for vitellolysis and lipid body utilization. In the course of myoblast differentiation the stainability of platelets stained with safranin and fast green shows essential variations. The platelets appear to lose their affinity for safranin and subsequently begin to stain with fast green. The yolk platelets were found to contain basic proteins, non-histone proteins, and phospholipids. The lipid bodies appear in the myoblast cytoplasm at the onset of vitellolysis and they disappear after yolk reserves have been utilized. After the deutoplasmatic material has been used the myoblast nuclei begin to divide and this leads to the formation of polykaryocytes.