Step 1 Content | Embryology

System

Normal Development

Abnormal Development

General Embryology

- Gametogenesis (spermatogenesis, oogenesis) - Fertilization, cleavage, implantation - Germ layer formation (ectoderm, mesoderm, endoderm) - Neurulation and folding - Embryonic and fetal circulation - Placental and umbilical development

- Teratogenesis (environmental, genetic, infectious) - Chromosomal abnormalities (aneuploidy, translocations) - Congenital malformations (multifactorial inheritance)

Hematopoietic & Lymphoreticular

- Hematopoiesis (yolk sac → liver → bone marrow) - Development of lymphoid organs (thymus, spleen, lymph nodes)

- Congenital immunodeficiencies (DiGeorge syndrome – thymic aplasia) - Hemoglobinopathies (sickle cell, thalassemia)

Central & Peripheral Nervous

- Neural tube formation and closure - Brain vesicle development (forebrain, midbrain, hindbrain) - Spinal cord and neural crest derivatives - Myelination and neuronal migration

- Neural tube defects (anencephaly, spina bifida, encephalocele) - Hydrocephalus, microcephaly, holoprosencephaly - Chiari and Dandy-Walker malformations - Congenital cranial nerve defects

Skin & Connective Tissue

- Epidermis and dermis differentiation - Hair, nails, and sweat gland formation - Development of connective tissues (cartilage, tendons, ligaments)

- Albinism, ichthyosis, epidermolysis bullosa - Ehlers-Danlos syndrome, Marfan syndrome

Musculoskeletal

- Limb bud formation, segmentation - Endochondral and intramembranous ossification - Muscle differentiation and migration

- Limb abnormalities (polydactyly, syndactyly, amelia, phocomelia) - Achondroplasia, osteogenesis imperfecta, congenital scoliosis

Respiratory

- Lung bud formation (week 4) - Bronchial branching morphogenesis - Alveolar development and surfactant production

- Tracheoesophageal fistula - Pulmonary hypoplasia (e.g., Potter sequence) - Congenital diaphragmatic hernia

Cardiovascular

- Heart tube formation and looping - Aortic arch and great vessel development - Septation of atria and ventricles - Fetal circulation changes at birth

- Congenital heart defects (ASD, VSD, Tetralogy of Fallot, transposition of great arteries) - Patent ductus arteriosus, coarctation of the aorta

Gastrointestinal

- Gut tube formation and rotation - Liver, pancreas, and biliary system development - Enteric nervous system formation

- Malrotation, volvulus, intestinal atresia - Meckel’s diverticulum, omphalocele, gastroschisis - Hirschsprung disease (aganglionosis) - Annular pancreas, biliary atresia

Renal/Urinary

- Kidney development (pronephros → mesonephros → metanephros) - Ureteric bud and nephron formation - Bladder and urethra development

- Renal agenesis, polycystic kidney disease - Horseshoe kidney, vesicoureteral reflux - Urachal abnormalities (patent urachus)

Reproductive

- Gonadal differentiation (testis, ovary) - Müllerian (paramesonephric) and Wolffian (mesonephric) duct development - External genitalia formation

- Disorders of sexual development (androgen insensitivity, Turner syndrome) - Cryptorchidism, hypospadias, epispadias - Müllerian agenesis, uterine malformations

Endocrine

- Development of hypothalamus, pituitary, thyroid, adrenal glands - Pancreatic islet cell formation

- Congenital adrenal hyperplasia - Thyroid dysgenesis, ectopic thyroid - Diabetes-related genetic syndromes