# USMLE Step 1 Review/Embryology

Embryology is the study of embryonic and fetal development, an understanding of which is required for the USMLE Step 1. Though embryology is a vast area of study, the USMLE requires only a very basic understanding; these can be summarized in a few basic principles about development and augmented with some details about embryology relevant to particular disease conditions.

## General principles
### Embryogenesis

Sperm + oocyte → zygote → proliferation → blastomere → blastocyst → inner cell mass (embryoblast) + trophoblast differentiation
    Inner cell mass (embryoblast) →
       Epiblast → embryo, amnion
       Hypoblast → yolk sac
    Trophoblast → syncytiotrophoblast + cytotrophoblast → placenta

### Germ layers

Epiblast → endoderm, mesoderm, ectoderm

## Clinical relevance
### Malrotation

Intestinal structures outgrow abdominal cavity → forced outside abdominal cavity → return at ~10 weeks → rotate into final position. Failure of proper rotation results in malrotation.

### Meckel's diverticulum

True diverticulum of distal ileum, formed by remnant of vitelline (omphalomesenteric) duct. Gut is formed when trilaminar disk folds in on itself, coopting yolk sac to make gut tube; omphalomesenteric duct is communication between midgut and yolk sac, which normally obliterates. Failure of obliteration → Meckel's diverticulum. Rule of 2s: 2% of population, 2% symptomatic, 2 inches long, 2 feet from ileocecal valve, 2 types of tissue (pancreatic/gastric). Associated with abdominal pain, lower GI bleeding. 99m-Technetium (Meckel's) scan identifies ectopic gastric tissue. May lead to volvulus, intussusception, obstruction.

### Cryptorchidism

Failure of testicular descent by 1 year → cryptorchidism. Testes derive from embryonic kidney (retroperitoneal) → descend via gubernaculum → internal inguinal ring → scrotum. Cryptorchid testis can be found anywhere along this path.

### Branchial cleft cysts

Branchial structures include clefts, arches, and pouches. Clefts are ectodermal, arches mesodermal, and pouches endodermal. Most have an associated nerve, skeletal structures, and muscles worth remembering. Clefts II-IV are meant to obliterate; when this does not occur, a branchial cleft cyst or sinus may occur, which is a lateral structure of the neck.

### Persistent thyroglossal duct

Base of tongue → foramen cecum → thyroid → produces thyroglossal duct as it migrates to neck → duct normally obliterates, leaving pyramidal lobe as remnant. Failure of duct obliteration can lead to persistent thyroglossal duct or thyroglossal duct cyst, the most common cause of a midline neck mass.

### Germ cell tumors

Primordial germ cells originate in yolk sac, then migrate along the embryo's midline longitudinal axis to the gonadal ridge. Migration can go awry, and may lead to germ cell tumors (eg, teratomas), which are frequently midline.

### Kartagener's syndrome

Kartagener's syndrome, also called primary ciliary dyskinesia, is associated with situs inversus (organs on the wrong side of the body), infertility, and recurrent lung infections due to bronchiectasis, among other things -- all resulting from a loss of cilia. The situs inversus is most interesting in the discussion of embryology. During embryogenesis, the sonic hedgehog (Shh) protein is partly responsible for determining which side of the body an organ should develop on (eg, the liver, heart, spleen). Normally, cilia sweeps Shh to one side of the developing embryo to help signal this sidedness. In primary ciliary dyskinesia, the dynein arms of cilia are dysfunctional and thus cilia malfunction. Thus the normal sweeping motion is impossible and the Shh sidedness signal is lost, resulting in inversion of these organs' normal position.