卵黄がないのに卵黄嚢

人間の発生の途中で、卵黄嚢というものが形成されます。卵黄嚢の本来の意味は、卵黄が入った袋であるはずですが、人間の卵細胞には卵黄がありません。ニワトリの卵には卵黄（卵の黄身の部分）がありますし、カエルの卵にも卵黄タンパク質がたっぷり入っています。魚の卵にも卵黄がたくさんあるので卵黄が少ない上側の部分で細胞が分裂します。

なぜ人間には卵黄がないのでしょうか。卵黄は胎児が育つための栄養の貯蔵なわけですが、人間は胎盤を通じて母親から栄養を供給されるので、卵黄を持っている必要がないというわけです。

Human eggs do not contain yolk proteins in the same way that bird or reptile eggs do. Here’s an explanation of the key differences:

Mammalian vs. Non-Mammalian Eggs

Non-Mammalian Eggs

Birds, reptiles, and many other non-mammalian species produce eggs with distinct yolks. These yolks are rich in proteins, lipids, vitamins, and minerals that provide nutrition to the developing embryo

 In chicken eggs, for example, the yolk contains about 43% of the total egg protein and all of the egg’s fat

Human/Mammalian Eggs

Human eggs, like those of other mammals, have a fundamentally different structure:

  1. No Distinct Yolk: Human eggs do not have a separate yolk compartment
  2. Different Nutrient Source: Instead of relying on a yolk for nutrition, human embryos receive nutrients directly from the mother’s body through the placenta after implantation
  3. Size Difference: Human eggs are much smaller than bird or reptile eggs, as they don’t need to store large amounts of nutrients.

Protein Content in Human Eggs

While human eggs don’t have yolk proteins per se, they do contain proteins that are crucial for early development:

  • Maternal Proteins: Human eggs contain various maternal proteins essential for fertilization and early embryonic development.
  • Cytoplasmic Components: The egg cytoplasm contains organelles and molecules necessary for initial cell divisions and embryo formation.

Evolutionary Perspective

The absence of yolk in human and other mammalian eggs is an evolutionary adaptation. Mammals have developed a more direct method of nourishing their offspring through placental connections, eliminating the need for a large, nutrient-rich yolk

In summary, while human eggs contain proteins vital for early development, they lack the specific yolk proteins found in the eggs of birds, reptiles, and other non-mammalian species.

(Perplexity.ai)

 

参考

  1. https://www.try-it.jp/chapters-15192/sections-15193/lessons-15235/point-3/