Problem 13 - Biology

Question

On about the seventh day of development, the human embryo (a) implants in the wall of the uterus (b) has a fully developed placenta for obtaining nutrients and oxygen (c) releases human chorionic gonadotropin (d) both a and c (e) a, b, and c.

Step-by-Step Solution

Answer

The correct answer is (d) both a and c.

Step 1: Understand Implantation Timing

The human embryo implants in the wall of the uterus around the seventh day of its development. This process is called implantation, and it allows the embryo to start the process of receiving nutrients from the mother. Thus, option (a) is correct.

Step 2: Evaluate Placenta Development

On the seventh day, the placenta is not yet fully developed. The placenta begins to form after implantation but becomes fully functional several weeks later. Therefore, option (b) is incorrect.

Step 3: Identify Hormone Release

Around the time of implantation, the embryo starts releasing human chorionic gonadotropin (hCG), which is a hormone crucial for maintaining the pregnancy. Therefore, option (c) is also correct.

Step 4: Determine Combined Options

Since both options (a) and (c) are correct, the answer is option (d). The embryo implants in the uterus and releases hCG around the seventh day.

Key Concepts

Implantation

Implantation is a critical milestone in human embryonic development that occurs approximately seven days after fertilization. At this stage, the fertilized egg, now called a blastocyst, travels down the fallopian tube and reaches the uterine lining, known as the endometrium.
When the blastocyst adheres to the endometrial epithelium, it begins to embed itself within the thickened uterine wall. This process is essential as it allows the embryo to establish a physical and nutritional connection with the mother's body.

• Implantation enables exchange of nutrients and waste between the mother and the embryo.
• It triggers the start of embryonic development beyond the single-cell stage.
• The success of implantation sets the foundation for a viable pregnancy.

Implantation initiates a cascade of events that eventually lead to the development of the placenta and other structures critical for supporting fetal growth.

Placenta Development

Following successful implantation, the development of the placenta begins. The placenta is a vital organ that forms to connect the developing baby to the mother’s uterine wall.
Initially, tiny hair-like projections called villi extend into the endometrium, forming the early structure of the placenta. This facilitates the exchange of gases, nutrients, and waste products between the mother and the fetus. However, on the seventh day post-fertilization, the placenta is not fully mature. It continues to develop throughout the first trimester and becomes fully functional after several weeks.

• The maturation of the placenta is crucial for adequate nutrient supply.
• A fully developed placenta acts like a lifeline, ensuring the fetus receives essential oxygen and nutrients.
• It also produces hormones required for maintaining pregnancy, including progesterone and estrogen.

In summary, while the implantation process marks the beginning of placenta formation, its complete development is not immediate and progresses along with pregnancy.

Human Chorionic Gonadotropin (hCG)

Human chorionic gonadotropin (hCG) plays a vital role in early pregnancy. Released by the cells of the newly implanted embryo, hCG is the hormone responsible for signaling the mother's body to maintain the uterine lining.
This hormone does several important things:

• It supports the corpus luteum, allowing continuous production of progesterone, crucial for maintaining the uterine lining.
• hCG levels rise rapidly after implantation and can be detected in maternal blood and urine.
• The presence of hCG is what home pregnancy tests typically detect.

Moreover, hCG is vital for ensuring that the growing embryo receives steady nutritional support until the placenta is capable of taking over these hormone production roles in the subsequent weeks. In essence, hCG acts as a biochemical messenger that sustains early pregnancy and supports the developing embryo’s needs.