USMLE Step 1 Embrology and
Reproduction
For the following important embryological gene, describe where they are made in the
developing fetus, what their role is and what happens when they are absent:
Sonic Hedgehog Gene
WNT-7 gene
FGF-gene
Hox genes (homeobox) ------- ✔ CORRECT ANSWER ✓✓Sonic Hedgehog Gene:
- produced at the base of limbs in zone of polarizing activity
- involved in patterning along AP axis and CNS development
- mutation can cause holoprosencephaly
WNT-7 gene
- produced at apical ectodermal ridge (thickened ectoderm at distal end of each developing
limb)
- necessary for proper organization along dorsal-ventral axis
FGF-gene:
- produced at apical ectodermal ridge
- stimulates miosis of underlying mesoderm providing for lengthening of limbs
,Hox genes (homeobox)
- involved in segmental organization of the embryo in a craniocaudal direction
- codes for transcription factors
- HOX mutations --> appendages in the wrong location
At the following time points after fertilization of the egg, describe the key findings in the
developing embryo:
Within 1 week
Within 2 weeks
Within 3 weeks
Weeks 3-8 (embryonic period)
Week 4
Week 6
Week 8
Week 10 ------- ✔ CORRECT ANSWER ✓✓Within 1 week:
- hCG secretion begins around the time of implantation of the blastocyst
- blastocyst STICKS at day 6!
Within 2 weeks:
- bilaminar disc forms (epiblast, hypoblast)
- 2 weeks = 2 layers!
Within 3 weeks:
- gastrulation forms the trilaminar embryonic disc
,- cells from the epiblast invaginate --> forming the primitive streak --> endoderm, mesoderm,
ectoderm.
- the notochord arises from the midline mesoderm; overlying ectoderm becomes the neural
plate
- NOTE: 3 weeks = 3 layers
Weeks 3-8 (embryonic period):
- neural tube formed by neuroectoderm and closes by week 4
- organogenesis
- the fetus is EXTREMELY susceptible to teratogens
Week 4:
- heart begins to beat
- upper and lower limb buds begin to form
- 4 weeks = 4 limbs and 4 heart chambers
Week 6:
- fetal cardiac activity visible by transvaginal ultrasound
Week 8:
- fetal movements start
- remember GAIT at EIGHT weeks!
Week 10
- genitalia have male and female characteristics
, What is derived from ectoderm? ------- ✔ CORRECT ANSWER ✓✓Ectoderm = external/outer
layer
Three major categories:
1. Surface ectoderm
- skin, parotid, sweat, mammary glands
2. Neuroectoderm
- gives rise to neural tube --> CNS
3. Neural crest
- PNS + non-neural structures nearby!
- melanocytes, chromaffin cells of adrenal medulla, parafollicular cells of the thyroid (C cells),
pia and arachoid, bones of the skull, odontoblasts, endocardial cushions
What is derived from the mesoderm?
What are the findings of mesodermal defects? ------- ✔ CORRECT ANSWER ✓✓Remember:
*middle or MEAT layer*
So includes:
- muscle, bone, connective tissue, serous lining of the body, spleen, lymphatics, blood, wall of
gut tube, upper vagina, kidneys, adrenal cortex, dermis, testes, ovaries.
*What are the findings of mesodermal defects?*
- remember VACTERL
Reproduction
For the following important embryological gene, describe where they are made in the
developing fetus, what their role is and what happens when they are absent:
Sonic Hedgehog Gene
WNT-7 gene
FGF-gene
Hox genes (homeobox) ------- ✔ CORRECT ANSWER ✓✓Sonic Hedgehog Gene:
- produced at the base of limbs in zone of polarizing activity
- involved in patterning along AP axis and CNS development
- mutation can cause holoprosencephaly
WNT-7 gene
- produced at apical ectodermal ridge (thickened ectoderm at distal end of each developing
limb)
- necessary for proper organization along dorsal-ventral axis
FGF-gene:
- produced at apical ectodermal ridge
- stimulates miosis of underlying mesoderm providing for lengthening of limbs
,Hox genes (homeobox)
- involved in segmental organization of the embryo in a craniocaudal direction
- codes for transcription factors
- HOX mutations --> appendages in the wrong location
At the following time points after fertilization of the egg, describe the key findings in the
developing embryo:
Within 1 week
Within 2 weeks
Within 3 weeks
Weeks 3-8 (embryonic period)
Week 4
Week 6
Week 8
Week 10 ------- ✔ CORRECT ANSWER ✓✓Within 1 week:
- hCG secretion begins around the time of implantation of the blastocyst
- blastocyst STICKS at day 6!
Within 2 weeks:
- bilaminar disc forms (epiblast, hypoblast)
- 2 weeks = 2 layers!
Within 3 weeks:
- gastrulation forms the trilaminar embryonic disc
,- cells from the epiblast invaginate --> forming the primitive streak --> endoderm, mesoderm,
ectoderm.
- the notochord arises from the midline mesoderm; overlying ectoderm becomes the neural
plate
- NOTE: 3 weeks = 3 layers
Weeks 3-8 (embryonic period):
- neural tube formed by neuroectoderm and closes by week 4
- organogenesis
- the fetus is EXTREMELY susceptible to teratogens
Week 4:
- heart begins to beat
- upper and lower limb buds begin to form
- 4 weeks = 4 limbs and 4 heart chambers
Week 6:
- fetal cardiac activity visible by transvaginal ultrasound
Week 8:
- fetal movements start
- remember GAIT at EIGHT weeks!
Week 10
- genitalia have male and female characteristics
, What is derived from ectoderm? ------- ✔ CORRECT ANSWER ✓✓Ectoderm = external/outer
layer
Three major categories:
1. Surface ectoderm
- skin, parotid, sweat, mammary glands
2. Neuroectoderm
- gives rise to neural tube --> CNS
3. Neural crest
- PNS + non-neural structures nearby!
- melanocytes, chromaffin cells of adrenal medulla, parafollicular cells of the thyroid (C cells),
pia and arachoid, bones of the skull, odontoblasts, endocardial cushions
What is derived from the mesoderm?
What are the findings of mesodermal defects? ------- ✔ CORRECT ANSWER ✓✓Remember:
*middle or MEAT layer*
So includes:
- muscle, bone, connective tissue, serous lining of the body, spleen, lymphatics, blood, wall of
gut tube, upper vagina, kidneys, adrenal cortex, dermis, testes, ovaries.
*What are the findings of mesodermal defects?*
- remember VACTERL
