Yolk

About: Yolk is a research topic. Over the lifetime, 9055 publications have been published within this topic receiving 197383 citations.

Yolk protein uptake in the oocyte of the mosquito aedes aegypti. l.

Abstract: Yolk proteins are thought to enter certain eggs by a process akin to micropinocytosis but the detailed mechanism has not been previously depicted. In this study the formation of protein yolk was investigated in the mosquito Aedes aegypti L. Ovaries were fixed in phosphate-buffered osmium tetroxide, for electron microscopy, before and at intervals after a meal of blood. The deposition of protein yolk in the oocyte was correlated with a 15-fold increase in 140 mµ pit-like depressions on the oocyte surface. These pits form by invagination of the oocyte cell membrane. They have a 20 mµ bristle coat on their convex cytoplasmic side. They also show a layer of protein on their concave extracellular side which we propose accumulates by selective adsorption from the extraoocyte space. The pits, by pinching off from the cell membrane become bristle-coated vesicles which carry the adsorbed protein into the oocyte. These vesicles lose the coat and then fuse to form small crystalline yolk droplets, which subsequently coalesce to form the large proteid yolk bodies of the mature oocyte. Preliminary radioautographs, and certain morphological features of the fat body, ovary, and midgut, suggest that the midgut is the principal site of yolk protein synthesis in the mosquito.

Ontogeny of the haemopoietic system: yolk sac origin of in vivo and in vitro colony forming cells in the developing mouse embryo.

Abstract: Summary. The mouse yolk sac has been shown to contain in-vivo colony forming cells capable of producing granulocytic, megakaryocytic and erythroid spleen colonies; in-vitro colony forming cells producing granulocytic and mononuclear-macrophage colonies in agar; and cells capable of repopuiating the lymphoid and myeloid tissue of lethally irradiated hosts. Similar haemopoietic precursor cells were also demonstrated in the blood at the time of initiation of the circulation and in the early embryonic liver. Organ cultures of 7 day embryos with intact yolk sacs, and embryos or yolk sacs after separation have shown the autonomous nature of the development of haemopoiesis in the yolk sac and the dependence of intra-embryonic haemopoiesis, particularly in embryonic liver, on colonization by yolk sac haemopoietic cells. Both in-vivo and in-vitro colony forming cells have been involved in the first migration stream, between yolk sac and embryonic liver, and evidence has been presented for the role of local environmental factors in controlling the differentiation of these cell types. These results support the view that development of haemopoietic organs in both embryo and adult is dependent on colonization by circulating cells and that these circulating stem cells originate initially in the yolk sac. This indicates that the yolk sac is the only site of genuine de novo formation of haemopoietic stem cells.

Yolk is a source of maternal testosterone for developing birds

Abstract: The sex steroid hormones that affect development in birds have been thought to be produced exclusively by the embryo or neonate. I used radioimmunoassay to measure the amounts of androstenedione, 5 alpha-dihydrotestosterone, testosterone, 17 beta-estradiol, and corticosterone in the yolk of freshly laid canary (Serinus canaria) and zebra finch (Poephila guttata) eggs. Testosterone was found in both canary and zebra finch eggs, but its contents were much higher in the former than in the latter. The testosterone content of canary eggs in a same clutch increased with the order of laying, regardless of the genetic sex of the offspring that hatched from these eggs. Yolk testosterone was also present in the eggs of female canaries that were kept without a male, indicating that it is of maternal origin. The social rank of juvenile canaries was positively correlated with the concentration of yolk testosterone in the eggs from which they hatched, suggesting that the development of aggressive behavior of offspring might be subject to modification by maternal testosterone. These findings indicate that female songbirds can bestow upon their eggs a dose of hormone that modifies the behavior of offspring. Variable doses of these hormones might explain some of the individual variation in offspring behavior.

Intraspecific variation in egg size and egg composition in birds: effects on offspring fitness

Abstract: 1. There is little unequivocal evidence to date in support of a positive relationship between egg size and offspring fitness in birds. Although 40 studies (of 34 species) have considered the effect of variation in egg size on chick growth and/or survival up to fledgling only 12 studies have controlled for other characters potentially correlated both with egg size and offspring fitness. Of these only two have reported a significant residual effect of egg size on chick growth (in the roseate tern and European blackbird) and three a residual effect on chick survival (all in seabirds: common tern, lesser black-backed gull and kittiwake). 2. More consistent evidence exists, though from fewer studies, for a positive relationship between egg size and offspring fitness early in the chick-rearing period; chick growth and chick survival being dependent on egg size in 8 of 10 studies and 4 of 5 studies respectively. It is suggested that the most important effect of variation in egg size might be in determining the probability of offspring survival in the first few days after hatching. 3. Egg size explains on average 66% of the variation in chick mass at hatching (n = 35 studies) but only 30% of the variation in chick body size (n = 18). When effects of hatching body size are controlled for chick mass remains significantly correlated with egg size, though the reverse is not true. This supports the hypothesis that large eggs give rise to heavier chicks at hatching, i.e., chicks with more nutrient (yolk) reserves, rather than structurally larger chicks. 4. Egg composition increased isometrically with increasing egg size in about half the studies so far reported (n equals approximately 20). However, in seabirds, and some passerines, larger eggs contain disproportionately more albumen, whilst in some waterfowl percentage yolk content increases with increasing egg size. Changes in albumen content largely reflect variation in the water content of eggs, but changes in yolk content involve variation in lipid content, and therefore in egg 'quality.' The adaptive significance of variation in egg composition is considered; females may adjust egg composition facultatively to maximise the benefits to their offspring of increased reproductive investment. 5. Considerations for future research are discussed with particular emphasis on experimental studies and the application of new techniques.