USMLE Step 1 Review/Embryology

From Wikibooks, the open-content textbooks collection

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.

Contents 1 General principles 1.1 Embryogenesis 1.2 Germ layers 2 Clinical relevance 2.1 Pancreas divisum 2.2 Malrotation 2.3 Meckel's diverticulum 2.4 Cryptochidism 2.5 Branchial cleft cysts 2.6 Persistent thyroglossal duct 2.7 Germ cell tumors

[edit] General principles

[edit] 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

[edit] Germ layers

Epiblast → endoderm, mesoderm, ectoderm

[edit] Clinical relevance

[edit] Pancreas divisum

[edit] 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.

[edit] 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.

[edit] Cryptochidism

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.

[edit] 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.

[edit] 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.

[edit] Germ cell tumors

Primordial germ cells originate in yolk sac, then migrate along central axis to the gonads. Migration can go awry in any way, leading often to midline germ cell tumors (eg, teratoma).