11.3.3 Development of the Embryo

• Fertilization and Cleavage:

After fertilization, the zygote (fertilized egg) begins to divide through cleavage, a process where the zygote divides into smaller cells. The developing mass of cells moves down the oviduct and reaches the uterus, where it is called the blastocyst.

• Implantation:

The blastocyst embeds itself into the inner wall of the uterus in a process called implantation. This step is crucial for the embryo’s further development. After implantation, the embryo begins to develop into a fully formed human body.

• During pregnancy, menstruation does not occur, and the embryo (later called fetus) develops in the uterus for approximately 38-40 weeks.

Placenta: Structure and Function

• Placenta Formation:

The placenta is an organ that connects the fetus to the mother’s tissue. After 4-5 days of implantation, some cells from the developing embryo, along with the endoderm from the uterus, form the placenta. This organ is fully formed by 12 weeks after fertilization.

• The placenta acts as a temporary organ that separates the fetus from the mother but allows the exchange of materials between the two.

• Placenta’s Role:

• Nutrition: The placenta transfers nutrients like carbohydrates (glucose), proteins (amino acids), minerals, and water from the mother’s blood to the developing embryo.

• Gaseous Exchange: It functions like a lung, where oxygen from the mother’s blood diffuses into the fetus’s blood, while carbon dioxide from the fetus is expelled into the mother’s blood.

• Excretion: The placenta acts like a kidney, eliminating metabolic waste products from the fetus’s blood into the mother’s bloodstream.

• Hormone Secretion: The placenta secretes hormones that help protect the embryo and support its development.

• Umbilical Cord:

The fetus is connected to the placenta by the umbilical cord, a tube that facilitates the exchange of nutrients, gases, and waste products between the mother and fetus.

Fetal Membranes

• Embryonic Membranes:

The fetus is surrounded by a series of protective membranes that help with nutrition, gaseous exchange, and the elimination of waste. These membranes also protect the fetus from external harm and regulate important developmental functions.

Embryonic and Fetal Development Timeline

1. 36 Hours After Fertilization:

The fertilized egg begins to divide into two cells (cleavage).

2. 72 Hours After Fertilization:

The fertilized egg forms a 16-cell structure, a stage known as the morula. The cells continue to divide as they travel to the uterus.

3. 4 Weeks After Fertilization:

The embryo floats in a fluid-filled sac. The heart starts beating, and the brain begins to develop.

4. 5 Weeks After Fertilization:

The embryo is growing and developing. Limbs begin to form, such as buds for arms and legs.

5. 8 Weeks After Fertilization:

The embryo is now called a fetus. The organs are still small but are present.

6. 28 Weeks After Fertilization:

The fetus is developing rapidly. The head moves downward in preparation for birth.

7. 38 Weeks After Fertilization:

By this stage, the fetus is fully developed and ready for birth.

Labor and Childbirth

• Labor:

As pregnancy reaches its final stages, the anterior pituitary and placenta release hormones that trigger regular uterine contractions. These contractions, known as labor pain, become more intense as childbirth nears.

• Childbirth Process:

During childbirth, the embryonic membrane bursts, releasing the amniotic fluid. The baby is then delivered through the birth canal, completing the birth process.