MCAT Biology Review
Chapter 3: Embryogenesis and Development
1. Which of the following developmental stages has the greatest nuclear-to-cytoplasmic ratio?
1. Eight-cell embryo
2. Which of the following associations of a primary germ layer and an adult organ is correct?
1. Endoderm—cardiac muscle
2. Endoderm—lens of the eye
3. From which of the following layers does the notochord form?
4. The influence of a specific group of cells on the differentiation of another group of cells is called:
5. Which of the following is likely to be found in maternal blood during pregnancy?
1. Immunoglobulins produced by the fetus
2. Fetal hemoglobin released from fetal red blood cells
3. Progesterone produced by placental cells
4. Carbon dioxide exhaled from fetal lungs
6. A cell releases a substance that diffuses through the environment, resulting in differentiation of a nearby cell. This is an example of what type of cell–cell communication?
7. A cancer cell is removed from a patient and cultured. The cells in this culture seem to be able to divide indefinitely with no cellular senescence. Which peptide is likely activated in these cells that accounts for this characteristic?
1. Epidermal growth factor
2. Sonic hedgehog
3. Transforming growth factor beta
8. A tumor is removed from a patient, and when investigated by pathologists, appears to contain tissue resembling placenta, hair, thyroid tissue, and cardiac muscle. What was the likely potency of the cells from which this tumor originated?
9. Which of the following is FALSE with regard to adult stem cells?
1. They retain inherent pluripotency if harvested from selected organs.
2. They are less controversial than embryonic stem cells.
3. They require treatment with various transcription factors.
4. There is a reduced risk of rejection if patient’s own stem cells are used.
10.A child is born with webbed toes. This pathology is most likely accounted for by a failure of:
1. cell differentiation.
2. cell determination.
11.Following a myocardial infarction, the heart often heals by the creation of a scar by fibroblasts. This is an example of:
1. complete regeneration.
2. incomplete regeneration.
12.Neurofibromatosis type I, or von Recklinghausen’s disease, is a disorder that causes formation of tumors in multiple nervous system structures as well as the skin. While all cells carry the same mutation on chromosome 17, selective transcription of the genome appears to cause the most significant tumorogenesis in which of the following primary germ tissue layers?
13.Which of the following shows the correct order of early developmental milestones during embryogenesis?
1. Blastula → gastrula → morula
2. Morula → gastrula → blastula
3. Morula → blastula → gastrula
4. Gastrula → blastula → morula
14.A pregnant women is accidentally given a single dose of a teratogenic drug late in the third trimester. The baby is born three days later. Which of the following is the most likely outcome?
1. Complete failure of organ development and death of the fetus
2. Partial failure of organ development with survival of the fetus
3. Serious disfigurement of the fetus
4. Respiratory distress at birth, but no long-term effects
15.Which of the following statements regarding fetal circulation is FALSE?
1. In the umbilical cord, there are more arteries than veins.
2. The foramen ovale is the only shunt that connects two chambers of the heart.
3. Blood flow in the ductus arteriosus is from the aorta to the pulmonary artery.
4. The ductus venosus is the only shunt that bypasses the liver.
Answers and Explanations
The question is asking us to determine the developmental stage with the greatest nuclear-to-cytoplasmic material ratio. During the series of rapid mitotic divisions known as cleavage, the number of cells increases dramatically, but the overall volume of the embryo does not change. In fact, the volume of the embryo does not significantly increase until after the blastula has already implanted. As such, a high ratio of nuclear to cytoplasmic material will be found at the stage with the greatest amount of cells during early development. From the given choices, the stage with the greatest number of cells is the blastula. This is further supported by the fact that the cells are displaced from the center by the blastocoel.
To answer this question, it could be useful to review quickly the embryonic layers. The ectoderm gives rise to the integument (the epidermis, hair, nails, and the epithelia of the nose, mouth, and anal canal), the lens of the eye, and the nervous system (including the adrenal medulla). The endoderm gives rise to the epithelial linings of the digestive and respiratory tracts and parts of the liver, pancreas, thyroid, and bladder. Finally, from the mesoderm arise the musculoskeletal system, the circulatory system, the excretory system, the gonads, and the adrenal cortex. Therefore, the only incorrect association can be found in choice (C) because the fingernails are derived from ectoderm.
A rod of mesodermal cells called the notochord develops along the longitudinal axis just under the dorsal layer of ectoderm. Through inductive effects from the notochord, the overlying ectoderm starts bending inward and forms a groove on the dorsal surface of the embryo. The dorsal ectoderm will eventually pinch off and develop into the spinal cord and brain. While the neural tube forms from ectoderm, the notochord itself is mesodermal.
The influence of a specific group of cells on the differentiation of another group of cells is termed induction. For example, the eyes are formed through a constant back-and-forth of reciprocal development between the brain and the ectoderm. Competence refers to the ability of a cell to respond to a given inducer, but not the influence of the group of organizing cells, eliminating choice (A). Senescence is a term for biological aging, eliminating choice (B). Determination may be the result of induction, but this term does not refer to the general concept of the effect of one group of cells on the differentiation on another group of cells, eliminating choice (C).
During pregnancy, the placenta produces estrogen and progesterone to maintain the endometrium. These hormones are necessary for proper gestation of the fetus and should be measurable in maternal blood because they act on maternal organs. Prior to birth, the fetus is immunologically naïve and does not yet produce immunoglobulins, eliminating choice (A). It is worth noting, though, that maternal immunoglobulins cross the placenta to enter fetal blood. Fetal hemoglobin is a large protein and, thus, cannot easily cross the placenta. Further, red blood cells are much too large to cross the barrier themselves, eliminating choice (B). Carbon dioxide from fetal metabolism can be found in maternal blood, but the lungs are nonfunctional prior to birth as the fetus is suspended in amniotic fluid. Carbon dioxide is transferred across the placenta directly from the fetal bloodstream, eliminating choice (D).
The question stem states that a cell releases a substance that diffuses through the environment and causes differentiation of a nearby cell. Because the cell is acting on another nearby cell, and the molecule spreads by diffusion, this is an example of paracrine signaling. Autocrine signaling, choice (A), occurs when a molecule secreted by a cell results in an effect on the same cell. Juxtacrine signaling, choice (B), occurs between adjacent cells, but the signal does not spread by diffusion. In endocrine signaling, choice (D), a molecule is secreted that travels via the bloodstream to a distant target. Choice (C) is therefore the correct answer.
Cells that are able to divide indefinitely with no senescence are not exhibiting normal cell behavior. Normally, somatic cells divide a limited number of times until the telomeres become too short to be effective protectors of genomic material. When this occurs, the cells stop dividing. However, in this case, the cells have continued to divide indefinitely. It is likely that the enzyme telomerase has been activated, which allows for synthesis of telomeres to counteract shortening during mitosis.
This question stem describes a tumor composed of multiple types of embryonic tissues as well as tissues derived from the three germ cell layers. Because this tumor contains placental tissue, the tumor must have originated from a cell that was able to produce those types of tissues. Cells capable of producing placental tissue as well as ectoderm, mesoderm, and endoderm are, by definition, totipotent.
Embryonic stem cells are controversial because they require destruction of an embryo to harvest, eliminating choice (B). Adult stem cells are significantly less controversial, but require treatment with various transcription factors in order to induce an increased level of potency, eliminating choice (C). Rejection is a concern when foreign cells are introduced into an individual; using one’s own stem cells should remove this risk, eliminating choice (D). Adult stem cells are not naturally pluripotent, unless pluripotency has been induced by strategic use of transcription factors. Therefore, choice (A) is the correct answer.
During development, programmed cell death occurs in multiple locations in order to ensure development of the correct adult structures. One of the places in which this occurs is between fingers and toes. If apoptosis does not occur correctly in these locations, then an infant may be born with webbed fingers or toes. Failure of determination or differentiation would likely result in an absence of digits, eliminating choices (A) and (B). Failure of neurulation would lead to an absence of a nervous system and would not be compatible with life, eliminating choice (D).
After an injury, healing occurs by some sort of regenerative process. In humans, some tissues, such as the liver, are capable of regenerating tissue with much the same function and structure as the previous tissue. However, the heart is not capable of this sort of regeneration, often forming a fibrous scar in an area of injury. This is an example of incomplete regeneration, in which newly formed tissues are not identical in structure or function to the tissues that have been injured or lost.
Here, the mutation affects the skin and the nervous system, both of which are derived from ectoderm. The other germ tissue layers do not lead to skin or nervous system formation, eliminating choices (B) and (C). The notochord is not actually a primary germ tissue layer, and thus cannot be an answer to the question, eliminating choice (D).
After the first cell divisions occur, the embryo consists of a solid ball of cells known as a morula. Then a hollow center forms, creating the blastula. Finally, as the cells begin to differentiate into the three germ layers, the embryo is considered a gastrula. Choice (C) is therefore the correct answer.
The question stem states that the pregnant woman was given the drug three days before the baby was born. It is important to remember that organogenesis occurs during the first trimester. The last structure to become fully functional is the lungs. Because the organs were already largely formed prior to the administration of the teratogenic drug, it is likely that there was no major effect on the development of any organ as a result of exposure to the teratogen, eliminating choices (A), (B), and (C). However, because lung tissues are so sensitive and because they mature so late, it is likely that the infant may have some respiratory distress at birth.
Blood flow in the ductus arteriosus is from the pulmonary artery to the aorta. The direction of flow is determined by the pressure differential between the right side of the heart (and pulmonary circulation) vs. the left side of the heart (and systemic circulation). Unlike in adults, the right side of the heart is at a higher pressure during prenatal life than the left side, so blood will shunt from the pulmonary circulation to the systemic circulation through both the foramen ovale and ductus arteriosus.