25.3
Fécondation
Pendant la fécondation, un ovocyte et un spermatozoïde fusionnent pour créer une nouvelle structure diploïde. Chez la femme, le processus se produit u…
La fécondation chez les humains correspond à la fusionentre un ovule et un spermatozoïde,produisant potentiellement un nouvel individu. Avant la fécondation, un ovule haploïde recouvertd'une enveloppe glycoprotéique interne,appelée la zone pellucide,et d'une matrice externe de cellules du cumulusquitte un ovaire et migre vers une trompe de Fallope. En parallèle, des spermatozoïdes haploïdesdoivent être présents dans les voies génitaleset, dû en partie aux contractions utérines,sont également conduits dans les trompes de Fallope.
Une fois là-bas, ils entrent dans un processus de maturationappelé la capacitation,ce qui implique que des protéines de fécondation d'ovulesoient acheminées à la tête du spermatozoïde. Les mouvements de la queue du spermatozoïdecommencent également à s'intensifier à ce moment-là. Cela les propulse vers l'ovule,qu'ils localisent grâce à des signaux chimiques,comme la progestérone, sécrétée par les cellules du cumulus.
À leur arrivée, les spermatozoïdes effectuentla réaction acrosomique durant laquelleune vésicule contenant des enzymes digestivesdans la tête du spermatozoïde, l'acrosome, rejetteson contenu dans l'espace extracellulaire,lui permettant d'entrer dans la zone pellucide. Une fois que le premier spermatozoïde a pénétré totalementl'enveloppe glycoprotéique,sa membrane et celle de l'ovule fusionnent. Cela modifie l'ovule et la zone pellucide se durcit,ce qui empêchent les autres spermatozoïdes d'entrer.
La fécondation se termine lorsque le spermatozoïdelibère son contenu dans l'ovule. La structure diploïde qui en résulte, contenant maintenantle matériel génétique des deux parents,créera finalement un nouvel individu.
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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.