Section 2: Fertilization and clevage

What is fertilization?

Fertilization is when a sperm cell fuses with an egg cell to form a *zygote*. It usually happens in the ampulla of the uterine tube about a day after ovulation. This union restores the full chromosome number (46) and marks the start of a new organism.

Where does fertilization occur?

Fertilization most often takes place in the *ampulla* of the uterine tube, the widest and most accessible part. This location gives sperm time to mature before they meet the egg. 📍 Mnemonic: "Ampulla = Arrival point."

What must happen before fertilization?

Sperm must undergo *capacitation*, a process that removes surface proteins so the sperm can penetrate the egg's outer layers. This takes a few hours in the female reproductive tract. ⚡ Mnemonic: "Cap off to get in."

What is the acrosomal reaction?

The acrosomal reaction happens when the sperm releases enzymes from its acrosome cap to digest the egg's protective coat (zona pellucida). This allows the sperm to reach the oocyte membrane.

What prevents multiple sperm from entering the egg?

Once one sperm fuses, the egg triggers a *cortical reaction* that hardens the zona pellucida. This blocks other sperm from entering and prevents polyspermy. 🚫 Mnemonic: "First come, first served."

What happens to the oocyte during fertilization?

The secondary oocyte finishes meiosis II after sperm entry, forming the mature ovum and a second polar body. The sperm and egg nuclei then merge to form a diploid zygote.

When does the zygote form?

The *zygote* forms immediately after the sperm and egg nuclei fuse. It contains a complete set of 46 chromosomes — half from each parent.

What is cleavage?

Cleavage is a rapid series of mitotic divisions of the zygote without growth between divisions. It increases the number of cells (blastomeres) while the embryo stays the same size. ✂️

Why doesn't the embryo grow during cleavage?

Because it's still surrounded by the *zona pellucida*, which keeps its size constant. Only the number of cells increases, not the overall mass.

What is a morula?

Around *day 3, the embryo becomes a solid ball of 16-32 cells called the morula*. It looks like a mulberry and marks the first compact structure of the embryo. 🍇

When does the blastocyst form?

By *day 5, fluid fills the morula, forming a cavity and two cell types: the trophoblast (outer) and inner cell mass (inner). This new hollow sphere is called a blastocyst*. Mnemonic: "5 days → Blastocyst alive."

What are the parts of the blastocyst?

The *trophoblast becomes the placenta, and the inner cell mass (embryoblast) forms the embryo itself. The cavity in the middle is the blastocoel*. 🌱

What happens to the zona pellucida before implantation?

The blastocyst "hatches" by shedding the zona pellucida around day 6. This allows it to attach to the uterine wall for implantation. Mnemonic: "Hatch before attach."

How does the blastocyst get nutrition before implantation?

Before connecting to maternal blood, it absorbs nutrients from secretions of the uterine glands. These secretions keep it alive for the first few days. 🍽️

What is the main function of the trophoblast?

The trophoblast helps the blastocyst attach and invade the uterine lining. It later forms the fetal part of the placenta.

What is the main function of the inner cell mass?

The inner cell mass (embryoblast) gives rise to all tissues of the embryo and some extraembryonic membranes. It's the true beginning of the baby itself. 👶

What determines if twins are identical or fraternal?

If one zygote splits → *monozygotic twins (identical) if two separate eggs are fertilized → dizygotic twins* (fraternal). Timing of division affects how many placentas and sacs form. 👯

When does the embryo enter the uterus?

The embryo reaches the uterus about *3-4 days after fertilization* as a morula. It floats freely for about 2 more days before implantation. 🕊️

How long after ovulation does implantation occur?

Implantation begins around *day 6-7* after fertilization. This is when the blastocyst attaches to the uterine wall. 📅

Why is fertilization important biologically?

It restores the full chromosome number, determines the baby's sex (XX or XY), and starts embryonic development. It's the point when life officially begins.

What causes polyspermy?

Polyspermy happens when more than one sperm enters the egg, leading to an abnormal zygote. The cortical reaction normally prevents this. ⚠️

What is the zona pellucida?

The zona pellucida is a thick glycoprotein layer around the egg that protects it and controls sperm binding. After fertilization, it hardens to stop other sperm from entering.

What is the perivitelline space?

It's the small gap between the oocyte membrane and the zona pellucida. After the sperm enters, this space fills with cortical granules that block polyspermy.

What is the significance of the male and female pronuclei?

The sperm and egg nuclei are called pronuclei until they fuse. Their fusion creates a single nucleus with both parents' genetic material. ❤️

When does cleavage end?

Cleavage ends when the blastocyst forms and begins implantation. After this, cell specialization starts, leading to the bilaminar disc stage. 📅

What causes genetic variation during fertilization?

Each sperm and egg carry unique DNA because of meiosis and crossing-over. Fertilization randomly combines them, creating genetic diversity. Mnemonic: "Mix, meet, make unique."