Developmental Biology Terms Starting With Y
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Yolk Platelet
/ YOHK PLAYT-let / · From Old English 'geolca' meaning yellow part of egg, and French 'platelette' meaning small plate or flat piece.
Yolk platelet is a membrane-bound organelle found in oocytes and early embryos of oviparous and ovoviviparous species that stores concentrated proteins and lipids to nourish the developing embryo before external feeding begins.
Yolk platelets form during oogenesis when the oocyte endocytoses vitellogenins, yolk precursor proteins synthesized in the liver or fat body and transported through the bloodstream. These structures contain crystalline reserves of phosphoproteins, lipoproteins, and neutral lipids that can constitute up to 95 percent of egg volume in birds and reptiles. In Xenopus laevis eggs, yolk platelets measure 1 to 5 micrometers in diameter and contain a crystalline core of phosvitin and lipovitellin surrounded by lipid droplets within a membrane envelope.
During embryonic development, yolk platelets are progressively catabolized through autophagy and lysosomal digestion, releasing amino acids, fatty acids, and phosphate to fuel biosynthesis and energy metabolism. Their distribution within the egg profoundly influences cleavage patterns, with heavily yolked vegetal regions undergoing slower and incomplete cell division compared to the rapidly dividing animal pole.
Sturgeon eggs, harvested as caviar, are among the most yolk-rich vertebrate eggs outside of birds and reptiles, with vitellogenin-derived proteins accounting for more than 30 percent of egg dry mass. The beluga sturgeon requires 20 or more years to reach reproductive maturity, meaning the yolk platelets in a single clutch represent decades of maternal investment in nutrient accumulation.
Yolk platelets are simply stored fat. They contain highly organized crystalline protein structures, primarily vitellogenin-derived phosphoproteins called phosvitins and lipovitellins, along with lipid components, making them structured nutritional organelles rather than simple lipid droplets.
In Xenopus laevis eggs, yolk platelets concentrate in the vegetal hemisphere, creating a visible color gradient from the dark animal pole to the pale vegetal pole, and this asymmetry directly causes the unequal cleavage pattern that produces large vegetal blastomeres and small animal blastomeres by the 32-cell stage. Sea urchin eggs contain relatively sparse yolk platelets and instead rely heavily on dissolved nutrients and maternal mRNA, allowing nearly equal cleavage divisions that produce blastomeres of roughly uniform size across the entire embryo.
Yolk Sac
/ YOHK SAK / · Old English geolca; Latin saccus, bag
Yolk sac is an extraembryonic membrane attached to the embryo that supplies nutrients, produces the first blood cells, and generates primordial germ cells during early development.
In mammals, the yolk sac forms even though it contains almost no nutritive yolk, and its most significant early job is hematopoiesis. Between weeks 2 and 8 of human development, this structure generates the first red blood cells and white blood cells before the liver and eventually the bone marrow take over that function. Primordial germ cells also originate in the yolk sac wall and migrate along the hindgut mesentery to colonize the developing gonads, making this membrane indispensable for reproductive system formation.
In egg-laying vertebrates such as the chicken (Gallus gallus domesticus), the yolk sac absorbs lipid-rich yolk through a network of blood vessels and delivers it directly to the embryo via the vitelline circulation.
In zebrafish (Danio rerio), the yolk sac syncytial layer actively transports fatty acids and amino acids into the embryo for roughly the first five days of life, and mutations disrupting this transport cause severe growth defects even though the yolk itself remains physically intact.
The yolk sac only stores and supplies yolk as nutrition. In mammals it produces the first blood cells and generates the primordial germ cells that will eventually become eggs or sperm.
In human development, the yolk sac is visible by ultrasound from about week 5 of gestation and reaches peak size of 3 to 5 millimeters in diameter by week 10 before regressing as placental function takes over. Despite containing virtually no nutritive yolk, the human yolk sac synthesizes alpha-fetoprotein from week 4 onward at concentrations detectable in maternal serum, providing the biochemical basis for the maternal AFP screening test used to detect open neural tube defects and Down syndrome in the first and second trimesters. Primordial germ cells originate in the yolk sac wall at approximately week 3 and migrate along the hindgut mesentery to colonize the developing gonads by week 5, traveling distances of several millimeters guided by the chemoattractant SDF-1 secreted by the genital ridge.
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