Embryology Learning Resources
Duke University Medical School
Schedule (w/in Gross Anatomy) UNSW Embryology (awesome site!)

Embryogenesis: fertilization, implantation, gastrulation, somitogenesis

Suggested readings from Langman's Medical Embryology (11th. ed.):
Ch. 3 pp. 36-43
Ch. 4 pp. 47-54
Ch. 5 pp. 55-66
Ch. 6 pp. 67-84
Ch. 7 pp. 104-106

Click here to launch the Simbryo Early Embryogenesis animation (and some really trippy music -you'll understand once the window opens...)

  1. Understand the general processes that occur in the early embryo AND the approximate time in development at which they occur such as:
    1. Cleavage
    2. Compaction
    3. Implantation
    4. Gastrulation
    5. Body axis and left-right patterning
    6. Neurulation
    7. Somitogenesis
    8. Embryonic folding and body wall closure
    9. Placentation and formation of embryonic membranes

  2. Understand the etiology of birth defects that would arise due to perturbation(s) in the processes listed above, such as:
    1. Situs inversus
    2. Caudal agenesis (“sirenomelia”), (and VATER/VACTERL sequence)
    3. Sacrococcygeal tumor
    4. Neural tube defects
    5. Body wall closure defects (e.g. ectopia cordis)

  3. Understand the spatial relationship and functional role of extramembryonic structures such as:
    1. Umbilical cord
    2. Yolk sac
    3. Chorion/placenta
    4. Amnion

  4. Understand the formation and fate of the three embryonic germ layers (ectoderm, mesoderm, and endoderm) AND be able to list the general derivatives of each layer (Note: the neural crest is sometimes considered a "4th germ layer" but recall that it is derived from the neural plate and is therefore ultimately derived from ECTODERM.)

  5. Be able to list the derivatives of each subdivision of mesoderm:
    1. axial (notochordal)
    2. paraxial mesoderm:
      1. sclerotome
      2. dermatome
      3. primaxial myotome
      4. abaxial myotome
    3. intermediate mesoderm
    4. lateral plate mesoderm
      1. splanchnic (visceral) mesoderm
      2. somatic mesoderm


Practice Questions


Multiple choice/multiple correct. Any, all, or none of the answers to each question may be correct. Please identify all the correct answers.

1. Implantation of human embryos typically occurs:

  1. about 1 day after fertilization
  2. about one week (i.e. 7 days) after fertilization
  3. about two weeks (i.e. 14 days) after fertilization
  4. after gastrulation
  5. at the same time as neural tube closure
  6. NONE of the above



2. Which of the following is derived from ENDODERM?

  1. endoneurial fibroblasts and Schwann cells of peripheral nerves
  2. endothelial lining of blood vessels
  3. epithelial lining of the respiratory tract
  4. cells lining the amniotic membrane
  5. cytotrophoblast placental cells
  6. NONE of the above



3. During the development of the placenta:

  1. cytotrophoblast cells divide and their progeny fuse to form syncytiotrophoblast cells.
  2. syncytiotrophoblast cells invade the uterine wall.
  3. fetal blood vessels attach directly to materal vessels such that there is mixing of fetal and maternal blood.
  4. the placenta develops from the chorionic plate.
  5. most of the placenta is derived from embryonic intermediate mesoderm.
  6. NONE of the above



Questions 4-7 refer to the image below. For each of these questions, choose the SINGLE BEST answer.



4. The approximate age of this embryo is:

  1. 1 week
  2. 2 weeks
  3. about 4 weeks
  4. about 12 weeks



5. The label indicating the amniotic cavity is:

  1. A
  2. B
  3. C
  4. D



6. The label indicating a space lined by endoderm is:

  1. A
  2. B
  3. C
  4. D



7. The label indicating a space containing fluid that is produced by the kidneys in the developing fetus is

  1. A
  2. B
  3. C
  4. D


Questions 8 and 9 refer to the following case. For each of these questions, choose the SINGLE BEST answer.

A 19-year-old pregnant woman at 29 weeks gestation underwent routine prenatal sonography. A large fluid filled, intrauterine structure was found and she subsequently was referred to a tertiary care facility for further evaluation and management. She denied nausea, vomiting, acute abdominal pain, or recent trauma, and her pregnancy had been progressing well. An MRI was obtained to further delineate the lesion found on sonography.

MRI showed large cystic mass arising from the sacrococcygeal region of the fetus (Fig. 1). There was no evidence of polyhydramnios. The patient was counseled and monitored until she presented in labor eight weeks later, with contractions every ten minutes, nausea, vomiting, and apparently decreased fetal activity. She was taken urgently to the operating room where a cesarean section was performed and the fetus was delivered without complication. At the time of delivery, the female neonate was noted to have a large, cystic sacrococcygeal mass (Fig. 2) measuring 19.1 cm by 9.7 cm by 13.2 cm. The mass had no solid components, and there was no involvement of the neural tube.

The neonate was subsequently taken to surgery where the lesion was resected and coccygectomy was performed. Post-operatively, the mother and child both did well and were discharged home in stable condition.

Figure 1

Figure 1. MRI (T2 weighted images) of the maternal pelvis depicting a large cystic mass arising from the sacrococcygeal region of the fetus (the head of the fetus is down).

Figure 2

Figure 2. Gross image of the sacrococcygeal mass following cesarean section delivery of the neonate.


8.    The MOST LIKELY cause for this congenital defect is:

  1. abnormal migration and proliferation of lumbosacral neural crest.
  2. abnormal persistence of primitive streak tissue after completion of gastrulation.
  3. premature regression of the primitive streak during gastrulation.
  4. abnormal closure of the posterior (caudal) neuropore.
  5. abnormal left-right patterning due to ciliary dysfunction.



9. Histologic examination of the mass would reveal derivatives of which germ layer(s)?

  1. endoderm
  2. mesoderm
  3. ectoderm
  4. ALL of the above




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Updated 10/09/11 - Velkey