25.3
Fertilización
Durante la fertilización, un óvulo y un espermatozoide se fusionan para crear una nueva estructura diploide. En los humanos, el proceso ocurre una vez…
- [Instructor] La fertilización en humanos
une las células del óvulo y del esperma, potencialmente
generando un nuevo individuo.
Antes de la fertilización, un huevo haploide,
rodeado por una capa interna de proteínas
denominada zona pelúcida, y una matriz externa
de células de apoyo del cumulus,
sale de un ovario y migra a una trompa de Falopio.
Al mismo tiempo, esperma haploide debe estar presente
en el tracto genital, y en parte debido
a las contracciones uterinas, son también impulsados
a las trompas de Falopio.
Una vez allí, entran en un proceso de maduración
llamado capacitación, que involucra a proteínas que fusionan
el huevo siendo transportadas a la cabeza del esperma.
Aquí, los espermatozoides también comienzan a demostrar
mayor movimiento de la cola o flagelo.
Esto impulsa las células hacia el huevo,
al que localizan mediante la detección de señales químicas,
como la progesterona, exudada por el cumulus del huevo.
Al llegar, los espermatozoides sufren la reacción acrosoma,
durante la cual una vesícula que alberga enzimas digestivas
en la punta del espermatozoide, el acrosoma, expulsa su
contenido en el espacio extracelular, permitiendo que las
células excaven a través de la zona pelúcida.
Una vez que el primer espermatozoide penetra completamente
la capa proteica, su membrana y la de la célula del huevo
se unen. Esto altera el huevo, y la zona pelúcida se
endurece, lo que bloquea la entrada de otras células.
La fertilización termina con el esperma liberando su
contenido dentro del huevo.
La estructura diploide resultante, que ahora contiene
el material genético de ambos padres,
eventualmente generará un nuevo individuo.
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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.