Yolk uptake

Acetylcholine is synthesised in nerve terminals from its precursor choline, which is not formed in the CNS but transported there in free form in the blood. It is found in many foods such as egg yolk, liver and vegetables although it is also produced in the liver and its brain concentration rises after meals. Choline is taken up into the cytoplasm by a high-affinity (Am = 1-5 pM), saturable, uptake which is Na+ and ATP dependent and while it does not appear to occur during the depolarisation produced by high concentrations of potassium it is increased by neuronal activity and is specific to cholinergic nerves. A separate low-affinity uptake, or diffusion (Am = 50 pM), which is linearly related to choline concentration and not saturable, is of less interest since it is not specific to cholinergic neurons. 

As mentioned in Sect. 2.2.3, the biodistribution of HPMA copolymers depends on many factors. Molecular weight influences the uptake in the isolated tissue of yolk sac [266] as well as the elimination in vivo [124, 125,267,268]. Nonspecific increase in the rate of polymer uptake can be achieved by incorporation of positively charged or hydrophobic comonomers into the HPMA copolymer structure, such as methacryloyloxyethyltrimethylammonium chloride [22], N-methacryloyltyrosinamide [21], or N-[2-(4-hydroxyphenyl)ethyl]acrylamide [267]. The incorporation of hydrophobic moieties may influence the solution properties of the HPMA copolymer conjugates [132,134,269]. The interaction with the cellular surface may depend on the association number and the stability of the micelles. 

Roth, T.F., Porter, K.R. Yolk protein uptake in the oocyte of the mosquito Aedes aegypti. J Cell Biol 20 (1964) 313-330. 

It has been proposed that a low molecular weight phosphoprotein of phosvitin nature (47) which binds iron with high affinity may be involved in the cytosolic transport of iron in liver cells (48, 49), but no energized uptake of iron has been demonstrated such as in isolated rat liver mitochondria using a Fe(III)-phosvitin from avian egg yolk (mol wt 40,000) as the substrate (50). The biochemical significance of the cytosolic iron binding phosphoprotein is therefore still uncertain. 

Reader, J. P, N. C. Everall, M. D. J. Sayer, and R. Morris. 1989. The effects of eight trace metals in acid soft water on survival, mineral uptake and skeletal calcium deposition in yolk-sac fry of brown trout, Salmo trutta L. J. Fish. Biol. 35 187-198. 

Calbindin-D9k has been identified in both the placenta and yolk sac of rats and mice, and increases in the later stages of gestation when there is considerable fetal uptake of calcium for mineralization of the skeleton. In both birds and mammals, calcitriol is also required for ovulation (Halloran, 1989). 

Matsubara, T. and K. Sawano. Proteolytic cleavage of vitellogenin and yolk proteins during vitellogenin uptake and oocyte maturation in barfin flounder ( Verasper moseri). J. Exp. Zool. 272 34-45, 1995. 

In adult females of most insect species, JHs are involved in the regulation of reproduction and oocyte maturation. Juvenile hormone-dependent processes include the synthesis of vitellogenin, the precursor of the yolk protein, by the fat body and the release of vitellogenin and its subsequent uptake and incorporation into the oocyte [44,45]. 

Remon, J.P., Duncan, R. and Schacht, E. (1984) Polymer-dmg combinations pinocytic uptake of modified polysaccharides containing procaimide moieties by rat visceral yolk sacs cultured in vitro. J. Controlled Rel. 1 47-56. 

Within the developing embryo, lipoprotein lipase activity is high in both heart and adipose tissue at EI4 but is absent from liver and brain. A big increase in activity occurs in adipose tissue between EI2 and EI6 and this coincides with the period of lipid uptake from the yolk and deposition in the adipocytes. More than 90% of the energy required by the developing embryo is obtained from oxidation of fatty acids present in yolk triglycerides. A further increase in lipoprotein lipase activity also occurs on hatching (Speake, Noble McCartney, 1993). 

Properties, hormonal regulation of hepatic synthesis and ovarian uptake, and conversion to yolk platelets. Am. Zool., 14 1159. 

Immediately internal to this, the visceral yolk sac endoderm is rich in transcripts of cellular retinol binding protein (CRBP I) (6). The mechanism of transfer involves receptor-mediated uptake of retinol from maternal RBP-retinol by the visceral yolk sac endoderm, where it binds to CRBP I and interacts with the enzymes mediating RA synthesis (7). The assumption that retinol is retained specifically in CRBP I-expressing embryonic tissues has been verified by using 14C-labeled retinyl acetate (delivered intravenously to the pregnant dam) as a source of retinol (8). 

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