25.3
受精
受精とは、卵子と精子の細胞が融合して新しい2倍体を作ることです。人間の場合、卵巣から放出された卵子が卵管に入ると受精します。このプロセスにはいくつかの重要なステップがあります。1）性器に存在する精子が卵子を見つけます。2）卵子を見つけたら、精子は酵素を放出して、卵子の保護膜である透明帯を突き破ります…
人間の受精は卵子と精子の細胞を接合し、新しい個体を生み出すことができるものです。受精前に、透明帯と呼ばれる 内側タンパク質コートに囲まれた一倍体卵、および支持卵丘細胞の外側マトリックスが 卵巣を出て卵管に移動します。同時に、半数体の精子は生殖管に 存在しており，部分的に子宮収縮のために、卵管へと追いやられます。一旦そこに着くと、受精能獲得と呼ばれる成熟過程に入ります。卵子融合タンパク質が精子頭部に輸送されます。精子も尾や鞭毛の動きの増加を 示し始めています。卵丘複合体から滲出したプロゲステロンのように、化学信号を感知することによって 細胞が卵に向かって進みます。到着すると、精子は先体反応を受け、その間に、精子の先端にある 消化酵素を含む小胞である先体が、その内容物を細胞外空間に追い出し、細胞が透明帯を貫通できるようにします。最初の精子がプロテインコートを完全に貫通すると、卵細胞膜と融合します。これにより卵が変化し、透明帯が硬化し、他の細胞の侵入が阻止されます。受精は、精子がその内容物を卵の中に 放出することで終わります。得られた二倍体構造は、両親の遺伝物質を含んでおり、最終的には新しい個体を生み出します。
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Q1: What happens to sperm cells before they can fertilize an egg?
Sperm undergo capacitation, a maturation process where egg-fusing proteins transport to the sperm head and flagellum movements increase. Sperm then locate the egg by sensing chemical signals like progesterone released by cumulus cells. Finally, they undergo the acrosome reaction, expelling digestive enzymes that allow them to burrow through the zona pellucida surrounding the egg.
Q2: How does the zona pellucida prevent multiple sperm from entering the egg?
After the first sperm penetrates the zona pellucida and fuses with the egg, calcium signaling triggers cortical granules beneath the egg's membrane to release enzymes. These enzymes cleave sperm-binding proteins on the zona pellucida's surface, hardening it and blocking additional sperm entry. This polyspermy block ensures only one sperm fertilizes the egg.
Q3: What role does calcium signaling play in egg activation after fertilization?
Sperm-egg fusion initiates calcium ion waves pulsing through the egg cytoplasm. These Ca2+ oscillations activate kinases in the egg that restart meiosis, which was arrested when the egg was released from the ovary. Calcium signaling essentially awakens the egg, allowing it to resume division and prepare for embryonic development.
Q4: What happens to the egg's chromosomes during meiosis completion after fertilization?
After fertilization triggers meiosis resumption, the egg divides into a large mature ovum containing most cytoplasm and a smaller polar body that dissolves. The mature egg's nucleus, called the egg pronucleus, contains one set of unduplicated chromosomes. This haploid nucleus will soon fuse with the sperm pronucleus to form a diploid embryo.
Q5: How do the sperm and egg pronuclei combine to form a diploid nucleus?
When the sperm nucleus enters the egg, its membrane dissolves and chromosomes unpack. A new nuclear membrane forms around the loosened sperm material, creating the sperm pronucleus. The sperm's centrosome orients between both pronuclei, forming microtubules that draw them together. Their membranes dissolve, chromosomes mingle, and mitosis begins, combining both parents' genetic material.
Q6: What structures guide sperm toward the egg in the fallopian tube?
Sperm are driven into the fallopian tubes partly by uterine contractions. Once there, they sense chemical signals like progesterone released by the cumulus cell complex surrounding the egg. These chemical cues guide sperm toward the egg, enabling them to locate and approach it for fertilization.
Q7: What happens after the sperm and egg membranes fuse during fertilization?
Once sperm and egg membranes fuse, the sperm releases its contents—including its nucleus and centrosome—into the egg's cytoplasm. The zona pellucida hardens, blocking other sperm from entering. The diploid structure now contains genetic material from both parents and will undergo embryogenesis cleavage and blastulation to eventually generate a new individual.