Fertilization and Early Embryo Development

The process of fertilization in humans involves several intricate steps, from the meeting of sperm and egg to the formation of a zygote and its subsequent development into an embryo. This page covers the key stages of fertilization, early embryo development, and the formation and functions of the placenta.

Fertilization Process

The journey begins when spermatozoa swim to the fallopian tube to meet the secondary oocyte. As they travel, sperm undergo capacitation, preparing them for fertilization.

Vocabulary: Capacitation is the process that allows sperm to penetrate and fertilize an egg.

When capacitated sperm reach the oocyte, the acrosome reaction begins:

1. Proteases break down the corona radiata (outer layer of granulosa cells).
2. Sperm reach the zona pellucida (a glycoprotein layer surrounding the oocyte).
3. An increase in Ca²+ concentration triggers the sperm head to release acrosin.
4. Acrosin breaks down the zona pellucida, allowing sperm to reach the cell membrane.

Highlight: The first sperm to penetrate the zona pellucida fuses its membrane with the oocyte's, incorporating its genetic material.

Immediately after membrane fusion, the cortical reaction occurs to prevent polyspermy:

1. The oocyte's smooth endoplasmic reticulum releases Ca²+.
2. This triggers cortical granules to fuse with the membrane.
3. The contents alter the zona pellucida, making it impermeable to other sperm.

Zygote Formation and Early Development

After fertilization:

1. Male and female pronuclei combine when the cell is ready for its first mitosis.
2. The resulting cell, called a zygote, begins to divide.
3. Over 3 days, cell division continues in the fallopian tube, forming a 16-cell morula.
4. By day 7, a blastocyst forms, consisting of an inner cell mass (future fetus) and outer trophoblast cells.

Definition: A blastocyst is a structure formed in early embryonic development, consisting of an inner cell mass and a fluid-filled cavity called the blastocele.

Implantation and Placenta Formation

The blastocyst embeds in the uterus using trophoblastic villi, which grow into the endometrial layer to form the placenta. This process occurs between days 6-10, after which the woman is considered pregnant.

Placenta Functions

The placenta is a temporary organ formed from both maternal and fetal tissues. Its key functions include:

1. Acting as a physical barrier between maternal and fetal blood systems.
2. Protecting the fetus from the mother's immune system.
3. Facilitating material exchange between maternal and fetal blood.
4. Secreting hormones.
5. Protecting fetal capillaries from blood pressure changes.

Highlight: The placenta's structure, with its chorionic villi, increased surface area, and counter-current flow, allows for efficient exchange of materials between mother and fetus.

Hormones in Pregnancy

Several hormones play crucial roles during pregnancy:

1. Testosterone
2. Estrogen (inhibits FSH and LH release)
3. Progesterone (prevents hormone cycle)
4. Gonadotropin-releasing hormone (GnRH)
5. Follicle-stimulating hormone (FSH)
6. Luteinizing hormone (LH)

These hormones work together to maintain pregnancy and regulate fetal development.