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MCAT Embryology, Germ Layers and Stem Cell Potency Practice Questions

By StudyStore · Published 2026-10-20 · 9 original questions · about 14 min

Nine free MCAT-style embryology practice questions on fertilization, cleavage, implantation, gastrulation, germ layer derivatives, neural crest, induction and stem cell potency, with full explanations.

Quick concept primer

Early development follows a fixed order: fertilization, cleavage, morula, blastocyst, implantation, gastrulation, then neurulation and organ formation. At fertilization, acrosomal enzymes let the sperm cross the zona pellucida, and a rise in calcium in the egg triggers the cortical reaction that blocks further sperm and lets the secondary oocyte finish meiosis II. Cleavage divisions make more, smaller cells without growth, so the embryo stays about the same size. The blastocyst has an outer trophoblast, which forms the fetal part of the placenta and secretes hCG to keep the corpus luteum making progesterone, and an inner cell mass, which forms the embryo.

Gastrulation creates three germ layers. Ectoderm gives epidermis, the nervous system and, through the neural crest, melanocytes, peripheral ganglia and the adrenal medulla. Mesoderm gives muscle, bone, blood, heart, kidneys, gonads and the adrenal cortex. Endoderm gives the epithelial lining of the gut and airways plus the liver and pancreas. Cells become determined before they look different, and induction by neighboring tissue, such as the notochord acting on ectoderm, steers that choice. Potency narrows from totipotent zygote to pluripotent inner cell mass to multipotent adult stem cells.

Traps: the adrenal medulla is neural crest while the adrenal cortex is mesoderm, and determination comes before differentiation.

9 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

Which sequence lists events of early human development in the correct order?

  1. Fertilization, gastrulation, cleavage, implantation, neurulation
  2. Cleavage, fertilization, blastocyst formation, neurulation, gastrulation
  3. Fertilization, implantation, cleavage, blastocyst formation, gastrulation
  4. Fertilization, cleavage, blastocyst formation, implantation, gastrulation, neurulation
Show answer and explanation

Answer: D. Fertilization, cleavage, blastocyst formation, implantation, gastrulation, neurulation

The zygote forms at fertilization and then undergoes cleavage divisions to a morula and a blastocyst, which implants in the uterine wall about a week after fertilization. Gastrulation then creates the three germ layers, and neurulation follows as the notochord induces the neural plate. Each other option places cleavage, implantation or gastrulation out of order.

Question 2

Each of the following is derived mainly from mesoderm EXCEPT:

  1. The kidneys
  2. Skeletal muscle
  3. The epithelial lining of the airways
  4. The adrenal cortex
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Answer: C. The epithelial lining of the airways

The airways bud from the embryonic gut tube, so their epithelial lining comes from endoderm, like the lining of the digestive tract. The kidneys, skeletal muscle and the adrenal cortex all develop from mesoderm. The adrenal medulla, by contrast, comes from neural crest cells, which are ectodermal.

Question 3

A mutation prevents neural crest cells from migrating away from the neural tube. Which tissue is most likely to be affected?

  1. Pigment-producing cells of the skin
  2. Motor neurons of the spinal cord
  3. Hepatocytes of the liver
  4. Skeletal muscle of the limbs
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Answer: A. Pigment-producing cells of the skin

Neural crest cells leave the edges of the folding neural tube and migrate widely to form melanocytes, sensory and autonomic ganglia, Schwann cells and the adrenal medulla, so a migration defect would leave patches of skin without pigment. Spinal motor neurons develop inside the neural tube itself, hepatocytes come from endoderm, and limb muscle comes from mesoderm.

Question 4

A spherical zygote 100 micrometers in diameter undergoes 5 rounds of equal cleavage divisions with no growth. If each blastomere is roughly spherical, what is the approximate diameter of each blastomere?

  1. 3 micrometers
  2. 20 micrometers
  3. 31 micrometers
  4. 50 micrometers
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Answer: C. 31 micrometers

Five divisions give 2^5 = 32 cells sharing the original volume, so each cell has 1/32 of the zygote volume. Volume scales with the cube of diameter, so each diameter is 100 divided by the cube root of 32, which is about 3.17, giving about 31 micrometers. Dividing the diameter by 32 gives 3 micrometers, a common error that treats diameter like volume.

Question 5

An egg is treated so that its intracellular calcium cannot rise when a sperm fuses with it. Which event is most directly prevented?

  1. Binding of the sperm to the zona pellucida
  2. Release of acrosomal enzymes by the sperm
  3. Implantation of the blastocyst in the endometrium
  4. The cortical reaction that blocks entry of additional sperm
Show answer and explanation

Answer: D. The cortical reaction that blocks entry of additional sperm

Sperm fusion triggers a wave of calcium release in the egg, which causes cortical granules to release enzymes that harden the zona pellucida and prevent polyspermy, and also lets the oocyte complete meiosis II. Sperm binding and the acrosome reaction occur in the sperm before fusion, and implantation happens days later.

Question 6

During the first weeks of pregnancy, what keeps progesterone levels high enough to maintain the endometrium?

  1. hCG secreted by the trophoblast keeps the corpus luteum producing progesterone
  2. An LH surge from the anterior pituitary every few days stimulates the ovary
  3. The inner cell mass secretes progesterone directly
  4. FSH from the anterior pituitary stimulates new follicles to make progesterone
Show answer and explanation

Answer: A. hCG secreted by the trophoblast keeps the corpus luteum producing progesterone

Without pregnancy, the corpus luteum degenerates after about two weeks and progesterone falls. The trophoblast of the implanted embryo secretes hCG, which acts like LH to keep the corpus luteum alive until the placenta takes over progesterone production. High progesterone suppresses pituitary LH and FSH, and the inner cell mass forms the embryo, not a hormone source.

Question 7

How do cells of the inner cell mass differ in developmental potential from the zygote?

  1. They are unipotent and can form only one cell type
  2. They are pluripotent and can form all embryonic germ layers but not trophoblast-derived placental tissue
  3. They are multipotent and can form only cells of a single germ layer
  4. They are totipotent and can form both the embryo and the placenta
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Answer: B. They are pluripotent and can form all embryonic germ layers but not trophoblast-derived placental tissue

The zygote and very early blastomeres are totipotent: they can give rise to the whole embryo and to extraembryonic tissue such as the trophoblast. Inner cell mass cells, the source of embryonic stem cells, are pluripotent: they can make derivatives of all three germ layers but no longer form trophoblast. Multipotent and unipotent describe more restricted adult stem and progenitor cells.

Question 8

Notochord tissue from one amphibian embryo is transplanted beneath belly ectoderm of a second embryo, where the ectoderm would normally form skin. A second neural plate forms above the graft. Which process does this demonstrate?

  1. Induction
  2. Apoptosis
  3. Determination by maternal cytoplasmic determinants
  4. Senescence
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Answer: A. Induction

Induction is a change in the fate of one tissue caused by signals from a neighboring tissue. The notochord releases signals that cause overlying ectoderm to become neural plate, so grafted notochord turns belly ectoderm into neural tissue. Apoptosis is programmed cell death, cytoplasmic determinants act within a cell from the egg, and senescence is aging-related loss of division.

Question 9

Presumptive skin ectoderm was transplanted into the region that normally forms brain in host embryos at two stages.

Stage of donor and hostFate of transplanted ectoderm
Early gastrulaForms brain tissue, matching its new location
Late gastrulaForms skin tissue, matching its original location

Which conclusion is best supported?

  1. Ectoderm can never become neural tissue
  2. The fate of these ectoderm cells becomes determined between the early and late gastrula stages
  3. The cells are fully differentiated skin cells at the early gastrula stage
  4. Brain tissue forms only from mesoderm
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Answer: B. The fate of these ectoderm cells becomes determined between the early and late gastrula stages

At the early gastrula stage the transplanted cells adopt the fate of their new surroundings, so they are not yet committed. By the late gastrula they follow their original fate despite the new location, so they have become determined. The early result shows ectoderm can become neural tissue, and the cells clearly were not differentiated skin early on.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.