25.3
Fertilization
During fertilization, an egg and sperm cell fuse to create a new diploid structure. In humans, the process occurs once the egg has been released from…
Fertilization in humans joins egg and sperm cells, potentially yielding a new individual.
Prior to fertilization, a haploid egg, surrounded by an inner protein coat termed the zona pellucida, and an outer matrix of supportive cumulus cells, exits an ovary and migrates into a fallopian tube.
Concurrently, haploid sperm must be present in the genital tract, and partly due to uterine contractions, are similarly driven into the fallopian tubes. Once there, they enter a maturation process called capacitation, which involves egg-fusing proteins being transported to the sperm head.
Here, sperm also begin to demonstrate increased tail or flagellum movements. This propels the cells towards the egg, which they locate by sensing chemical signals, like progesterone, exuded by the cumulus egg complex.
Upon arriving, sperm undergo the acrosome reaction, during which a vesicle harboring digestive enzymes in a sperm's tip, the acrosome, expels its contents into the extracellular space, enabling the cells to burrow through the zona pellucida.
Once the first sperm fully penetrates the protein coat, its, and the egg cell's membrane coalesce. This alters the egg, and the zona pellucida hardens, which blocks other cells from entering.
Fertilization ends with the sperm releasing its contents into the egg. The resulting diploid structure, now containing both parents' genetic material, will eventually generate a novel individual.
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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.