QUESTION 4 Describe the events that lead to pregnancy from the time a zygote is formed, up to the formation of the foetus including the development of the placenta and umbilical cord. Content: Synthesis:

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To describe the events that lead to pregnancy from the formation of a zygote to the development of a fetus, including the formation of the placenta and umbilical cord, we can break down the process into several key stages:

### 1. Formation of the Zygote
- **Fertilization**: The process begins when a sperm cell successfully penetrates an egg cell (oocyte) in the fallopian tube. This fusion of genetic material from both parents results in the formation of a zygote, which is a single-cell organism with a complete set of chromosomes (46 in humans).

### 2. Cleavage and Morula Formation
- **Cell Division**: The zygote undergoes a series of rapid mitotic divisions known as cleavage. This process does not increase the overall size of the embryo but divides the zygote into smaller cells called blastomeres.
- **Morula Stage**: After several divisions, the embryo becomes a solid ball of cells known as a morula (approximately 16-32 cells). This stage occurs about 3-4 days post-fertilization.

### 3. Blastocyst Formation
- **Blastocyst Development**: The morula continues to divide and develops into a blastocyst, which consists of an outer layer of cells (trophoblast) and an inner cell mass (embryoblast). The blastocyst forms a fluid-filled cavity called the blastocoel.
- **Implantation**: Around 5-6 days after fertilization, the blastocyst travels down the fallopian tube and reaches the uterus. It then implants itself into the uterine lining (endometrium), where it will receive nutrients and support.

### 4. Development of the Placenta
- **Trophoblast Differentiation**: The trophoblast cells begin to differentiate and invade the uterine lining, forming the placenta. The placenta is a vital organ that facilitates nutrient and gas exchange between the mother and the developing embryo.
- **Chorion Formation**: The outer layer of the trophoblast forms the chorion, which contributes to the formation of the placenta. The chorion develops finger-like projections called chorionic villi that extend into the uterine tissue, increasing the surface area for exchange.

### 5. Formation of the Umbilical Cord
- **Connecting the Fetus to the Placenta**: As the embryo develops, a structure called the umbilical cord forms. The umbilical cord contains blood vessels (two arteries and one vein) that connect the fetus to the placenta, allowing for the transport of oxygen, nutrients, and waste products.
- **Development of the Amniotic Sac**: The amniotic sac, filled with amniotic fluid, surrounds the developing embryo, providing cushioning and a stable environment.

### 6. Embryonic Development
- **Gastrulation**: Following implantation, the inner cell mass differentiates into three germ layers (ectoderm, mesoderm, and endoderm) during a process called gastrulation. These layers will give rise to all the organs and tissues of the body.
- **Organogenesis**: The organs begin to form from these germ layers, and by the end of the eighth week, the embryo is referred to as a fetus. Major systems such as the nervous system, circulatory system, and digestive system start to develop.

### 7. Fetal Development
- **Growth and Maturation**: The fetus continues to grow and develop over the next several months, with significant changes occurring in size, organ function, and overall complexity. By the end of the pregnancy (approximately 40 weeks), the fetus is fully developed and ready for birth.

### Summary
In summary, the journey from zygote to fetus involves a series of complex and coordinated events, including fertilization, cleavage, implantation, placental development, umbilical cord formation, and the differentiation of embryonic tissues into functional organs. Each stage is crucial for the successful development of a healthy fetus.
Pregnancy begins with the formation of a zygote when a sperm fertilizes an egg. The zygote divides into a morula and then a blastocyst, which implants in the uterus. The trophoblast cells develop into the placenta, a vital organ that connects the fetus to the mother. A structure called the umbilical cord forms, allowing the fetus to receive nutrients and oxygen from the placenta. As the embryo grows into a fetus, organs and systems develop from the three germ layers formed during gastrulation. By the end of pregnancy, the fetus is fully developed and ready for birth.

QUESTION 4 Describe the events that lead to pregnancy from the time a zygote is formed, up to the formation of the foetus including the development of the placenta and umbilical cord. Content: Synthesis:

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