Test: Embryology - 2 - MCAT MCQ

The acrosome is an organelle located at the tip of the sperm head and contains enzymes necessary for fertilization. It plays a crucial role in the penetration of the zona pellucida, a glycoprotein layer surrounding the egg. If a single point mutation modifies the proteins inside the acrosome, it can result in an inability to dissolve the zona pellucida's glycocalyx, which is necessary for successful fertilization. This would prevent the sperm from reaching and penetrating the egg, leading to infertility or reduced fertility. Therefore, option A is the correct answer.

If the acrosomal reaction fails, it can indeed lead to higher trisomy risk and sterility, respectively. The acrosomal reaction is essential for the sperm to penetrate the zona pellucida and reach the egg for fertilization. If this process is impaired or disrupted, it can result in difficulties in sperm-egg interaction and fertilization.

Without a successful acrosomal reaction, the sperm may not be able to penetrate the zona pellucida effectively, leading to a higher risk of polyspermy and subsequent abnormalities in chromosome number (trisomy). Additionally, if the acrosomal reaction is consistently unsuccessful, it can contribute to infertility as fertilization cannot occur.

Therefore, option D, higher trisomy risk and sterility, respectively, is the correct answer.

Placenta accreta is a condition where the placenta attaches abnormally to the myometrium, the muscular layer of the uterus. The attachment of the placenta occurs during the early stages of embryonic development.

In early embryonic development, after fertilization, the zygote undergoes cleavage, resulting in a solid ball of cells called the morula. Subsequently, the morula undergoes further cell division and forms a fluid-filled cavity called the blastocyst. The blastocyst consists of two main cell populations: the trophoblast and the inner cell mass (ICM).

The trophoblast is responsible for implantation and the subsequent formation of the placenta. It is the outer layer of cells in the blastocyst that interacts with the maternal tissues. The trophoblast differentiates into two distinct layers: the outer syncytiotrophoblast and the inner cytotrophoblast.

During blastulation, which is the process of blastocyst formation, the trophoblast cells play a crucial role in attaching the blastocyst to the uterine wall. The trophoblast cells invade and attach to the endometrium of the uterus, facilitating implantation.

Therefore, the earliest embryonic stage at which placenta accreta could occur is during blastulation, specifically during the attachment of trophoblast cells to the uterine wall.

The correct answer is option A: Blastulation, trophoblast.

Malformation of the peripheral nervous system is linked to specific groups of cells formed during neurulation. These cells are known as neural crest cells, and they are derived from the ectoderm, which is one of the three embryonic germ layers.

During neurulation, which is the process of neural tube formation, the neural plate folds and forms the neural tube, which gives rise to the central nervous system. However, at the edges of the neural plate, some cells undergo a transformation and detach from the neural tube. These cells are the neural crest cells.

The neural crest cells migrate extensively throughout the developing embryo and give rise to a wide range of cell types, including neurons and glial cells of the peripheral nervous system, as well as other cell types in various tissues such as the craniofacial structures, melanocytes, and adrenal medulla.

Therefore, the specific group of cells linked to the malformation of the peripheral nervous system during neurulation is the neural crest cells, and they are derived from the ectoderm.

The correct answer is option C: Neural crest cells, ectoderm.

Embryological cells that will form the spine will form first, followed by the lungs, and then the muscles.

In this case, the order of formation is:

• Embryological cells that will form the spine (derived from mesoderm)
• Embryological cells that will form the lungs (derived from endoderm)
• Embryological cells that will form the muscles (also derived from mesoderm)

During the first week of gestation, the fertilized egg undergoes several divisions and forms a solid ball of cells called a morula. By the end of the first week, the morula develops into a blastocyst, which is a hollow ball of cells with an inner cell mass and an outer layer called the trophoblast.

Fertilization typically occurs in the fallopian tubes within 24 hours after ovulation. After fertilization, the zygote undergoes cleavage divisions during the first week, eventually forming the blastocyst. The blastocyst then travels through the fallopian tube and reaches the uterus by the end of Week 1 or the beginning of Week 2, where it implants into the uterine lining.

Therefore, the correct answer is B. Week 2 of gestation.

cells called the blastocyst. This early stage of development involves rapid cell division without significant growth in overall size. The blastocyst contains an inner cell mass, which will give rise to the embryo, and an outer layer of cells called the trophoblast.

It is during the subsequent stages of development, such as implantation and organogenesis, that we start to observe a significant increase in the size of the embryo. During implantation, the blastocyst attaches to the uterine wall and begins to invade the surrounding tissues. As development progresses, the embryo undergoes organogenesis, during which the major organs and organ systems start to form, leading to further growth in size and complexity.

Neurulation is the process during embryonic development where the neural tube, which will give rise to the central nervous system, is formed. It is a critical stage in establishing the bilateral symmetry of mammals. During neurulation, the ectoderm in the dorsal midline region of the embryo folds inward to form the neural plate. The neural plate then undergoes further folding and fusion to create the neural tube.

The neural tube divides the embryo into two halves, establishing bilateral symmetry. The anterior (head) and posterior (tail) ends of the neural tube will give rise to the brain and spinal cord, respectively. The formation of the neural tube and the establishment of bilateral symmetry are important milestones in early embryonic development.

Therefore, the correct answer is A. Neurulation.

The mesoderm is one of the three primary germ layers formed during early embryonic development. It gives rise to various tissues and organs in the adult body, including bones and muscles.

The mesoderm differentiates into mesenchyme, which further develops into different types of connective tissues, including bone tissue. Bones are formed through a process called ossification, where mesenchymal cells differentiate into osteoblasts, which produce the extracellular matrix of bone.

Additionally, the mesoderm also gives rise to muscle tissue. Myoblasts, derived from the mesoderm, differentiate into skeletal muscle, smooth muscle, and cardiac muscle cells.

Therefore, if a radioactive marker is applied to the mesoderm of a mammalian embryo, the tissues or organs that would have this marker in the adult are bones and muscles.

1. Sperm bypasses corona radiata: The corona radiata is the layer of follicular cells surrounding the oocyte. The sperm needs to penetrate through this layer to reach the oocyte.
2. Sperm binding with vitelline membrane: Once the sperm reaches the oocyte, it binds to the receptor proteins on the vitelline membrane, which is the outer layer of the oocyte.
3. Cortical granules release enzymes: After the sperm binding, cortical granules in the oocyte release their contents. These enzymes modify the zona pellucida, the glycoprotein layer surrounding the oocyte, preventing polyspermy.
4. Digestion of zona pellucida by acrosomal enzymes: The acrosome of the sperm contains enzymes that help in the penetration of the zona pellucida. These enzymes digest the zona pellucida, allowing the sperm to reach the oocyte's plasma membrane.

Therefore, the correct order is 2, 1, 3, 4.