USMLE Review: Embryology. General Embryology Timeline: 1. Week 1: implantation, hCG secretion 2....

USMLE Review: Embryology

General Embryology

Timeline:

1. Week 1: implantation, hCG secretion

2. Week 2: bilaminar disk (epiblast / hypoblast)

3. Week 3: gastrulation, notochord, neural plate

4. Weeks 3 – 8 (embryonic period): organs form, neural tube

5. Week 4: heart beats, limb buds form, neural tube closes

6. Weeks 8 – 36 (fetal period): looks human, movement, sexual differentiation, continued neural development.

7. Respiratory system develops late (RDS – hyaline membrane)

General Embryology

Timeline:

General Embryology

Teratogens:

1. alcohol (FAS)

2. smoking

3. thalidomide

4. diethylstilbesterol (DES)

5. folic acid / folate

General Embryology

Placental Deveopment:

1. fetus – chorion (cytotrophoblast / synctiotrophoblast)

2. maternal – decidua (lacunae with maternal blood)

Umbilical cord:

1. one umbilical vein

2. two umbilical arteries

Cardiovascular Embryology

Heart:

1. Single tube to four chamber

2. Atrial and ventricular development

3. Atrial and ventricular septation

4. Outflow tracts: truncus arteriosus

Great vessels:

1. Aortic arches – 3rd, 4th and 6th

2. Cardinal veins

Third arch

Fourth arch

Sixth arch

Cardiovascular Embryology

Erythropoieses:

1. Yolk sac: 3 – 8 weeks

2. Liver: 6 – 30 weeks

3. Spleen: 9 – 28 weeks

4. Bone marrow: 28 weeks – adult

5. Therefore, liver and spleen can produce blood in disease states

Cardiovascular Embryology

Atrial septal defect:

1. Most common congenital anomaly

2. Left to right shunt (↑ pulmonary flow)

3. Maybe asymptomatic until adulthood

LA

RA

Cardiovascular Embryology

Atrioventricular septal defect:

1. About ½ of all congenital heart anomalies in infancy

2. Partial: atrial defect along with left atrioventricular valve insufficiency

3. Complete: atrial and ventricular wall defect

Cardiovascular Embryology

Tetralogy of Fallot:

1. Ventricular septal defect

2. Pulmonary stenosis

3. Overriding aorta

4. Right ventricular hypertrophy

1

2

3

4

Cardiovascular Embryology

Fetal Circulation:

1. Umbilical vein

2. Ductus venosus

3. Inferior vena cava, right atrium

4. Foramen ovale to left atrium, left ventricle to aorta

5. Right ventricle, pulmonary trunk

6. Ductus arteriosus

7. Aorta

8. Umbilical arteries

1

2

3

4

5

6

7

8

Cardiovascular Embryology

Adult Circulation:

1. Ligamentum teres hepatis (umbilical vein - closed)

2. Ligamentum venosum (ductus venosus - closed)

3. Inferior vena cava, right atrium

4. Right ventricle, pulmonary trunk, lungs (ligamentum arteriosum: ductus arteriosus - closed)

5. pulmonary veins to left atrium, left ventricle to aorta

6. Medial umblical ligaments (umbilical arteries – closed)

1

2

3

4

4

4

5

5

5

6

Neural Embryology

Development:

1. Neural tube and neural crest cells

2. Spinal cord

a. sensory: alar plate (posterior / dorsal)

b. motor: basal plate (anterior / ventral)

Neural Embryology

Development:

3. Rhombencephalon (mylencephalon / metencephalon)

a. medulla

b. pons / cerebellum

c. 4th ventricle

4. Mesencephalon: midbrain (cerebral aqueduct)

5. Diencephalon: hypothalamus, thalamus, epithalamus (3rd ventricle)

6. Telencephalon: cerebral hemispheres (lateral ventricles)

Neural Embryology

Development:

Myelencephalon

Metencephalon

Mesencephalon

Diencephalon

Telencephalon

Neural Embryology

Neural tube defects:

Neural Embryology

Anencephaly:

Neural Embryology

Chiari malformation:

Branchial Arch and Facial Embryology

Branchial (Pharyngeal) clefts, arches and pouches:

Branchial Arch and Facial Embryology

Development:

1. Branchial clefts

a. 1st = external auditory meatus

b. no others develop – cysts could remain

Branchial Arch and Facial Embryology

Development:

2. Branchial arches

a. 1st = trigeminal nerve (V); mandible, malleus, incus

b. 2nd = facial nerve (VII); stapes, styloid process, lesser horn of hyoid

c. 3rd = glossopharyngeal nerve (IX); greater horn of hyoid

d. 4th – 6th = vagus nerve (X); laryngeal cartilages

Branchial Arch and Facial Embryology

Development:

3. Branchial pouches

a. 1st = middle ear, auditory tube

b. 2nd = palatine tonsil

c. 3rd = thymus, inferior parathyroids

d. 4th = superior parathyroids, ultimobranchial body (parafollicular cells)

Branchial Arch and Facial Embryology

Development:

(1st arch)

(2nd arch)

(3rd arch)

(4th & 6th

arches)

Branchial Arch and Facial Embryology

Cranial nerve association:

Branchial Arch and Facial Embryology

Bones and Cartilage:

Branchial Arch and Facial Embryology

Pharyngeal clefts:

Branchial Arch and Facial Embryology

Cervical cysts and fistulas:

Branchial Arch and Facial Embryology

Thyroid and thymus migration:

Branchial Arch and Facial Embryology

Thyroid migration:

Branchial Arch and Facial Embryology

Thyroglossal duct and cysts:

Branchial Arch and Facial Embryology

Tongue development:

Branchial Arch and Facial Embryology

Cleft lips and palate:

Branchial Arch and Facial Embryology

Cleft lip (1:1000) and palate (1:2500):

Incomplete cleft lip Bilateral cleft lip Cleft lip/palate

Cleft palate Facial cleft Midline cleft lip

Gastrointestinal Embryology

Bowel development:

1. Foregut: esophagus, stomach (celiac artery)

2. Midgut: duodenum, small intestine, 2/3rds of large intestine (superior mesenteric artery)

3. Hindgut: 1/3 large intestine, sigmoid colon, rectum (inferior mesenteric artery)

GI organs:

1. Pancreas (celiac & superior mesenteric artery)

2. Liver (celiac artery)

Gastrointestinal Embryology

Tracheoesophageal fistula:

Gastrointestinal Embryology

Annular pancreas:

1. Ventral pancreas

2. Duodenal stenosis

Gastrointestinal Embryology

Midgut bowel rotation:

1. 180 degrees counterclockwise

2. Around superior mesenteric artery

Gastrointestinal Embryology

Abnormal bowel rotation:

1. A = incomplete bowel rotation

2. B= incorrect clockwise rotation

Gastrointestinal Embryology

Omphalocele:

1. Failure of intestine to return to abdomen

Gastrointestinal Embryology

Bowel atresia and stenosis:

Gastrointestinal Embryology

Vitelline duct:

Gastrointestinal Embryology

Hindgut malformations:

Rectovaginal fistulaUrorectal fistula

Imperforate anusRectal atresia

Urinary Embryology

Kidney development:

1. pronephros: up to 4th week, degenerates

2. mesonephros: interim kidney, contributes to male reproductive

Urinary Embryology

Kidney development:

3. metenephros: permanent kidney

a. ureteric bud – outgrowth from mesonephros; forms collecting system of kidney (collecting ducts through ureter)

b. metanephric blastema – forms glomerulus through distal convoluted tubule

Urinary Embryology

Kidney development:

4. urogenital sinus: bladder, urethra

Urinary Embryology

Kidney ascent difficulties:

Pelvic kidney Horseshoe kidney

Urinary Embryology

Double ureters: A, B, D & E

Ectopic ureter: C

Urinary Embryology

Urachal fistula, cycst or sinus:

Urinary Embryology

Bladder exstrophy with or without epispadias:

Reproductive Embryology

Male and female development:

1. Paramesonephric duct is the default (female)

2. Mesonephric duct must be stimulated to remain (male)

a. SRY gene – testis determining factor

b. Mullerian inhibitory factor – suppresses paramesonephric

c. Androgens stimulate mesonephric

Reproductive Embryology

Male development (mesonephric ducts):

16 weeks 9 months

Reproductive Embryology

Female development (paramesonephric ducts):

8 weeks 9 months

Reproductive Embryology

Adult structures:

Mesonephric ducts Paramesonephric ducts

Reproductive Embryology

Adult homologous structures:

Male Female

Labioscrotal swelling

Genital tubercle

Musculoskeletal Embryology

Development:

1. Somites develop for each spinal nerve level

2. Sclerotome, myotome and dermatome from each somite

3. 4th week: Limb buds form

Musculoskeletal Embryology

Development:

1. Somites develop for each spinal nerve level

2. Sclerotome, myotome and dermatome from each somite

3. 4th week: Limb buds form

5 weeks 6 weeks

7 weeks

Musculoskeletal Embryology

Development:

4. Myotomes subdivide to

a. epimere (dorsal ramus)

b. hypomere (ventral ramus)

Musculoskeletal Embryology

Development:

5. Dermatomes

Musculoskeletal Embryology

Syndactyly:

Musculoskeletal Embryology

Amelia:

Musculoskeletal Embryology

Meromelia:

thalidomide

Musculoskeletal Embryology

Cranial sutures:

Musculoskeletal Embryology

Acrocephaly:

premature closure of coronal suture

Embryology

Good Luck on the USMLE!!

Feel free to contact me with any questions:

tchampney@med.miami.edu