The primitive streak appears on the dorsal surface of the bilaminar disc during the second week of human development. Which of the following is the correct sequence of appearance and migration of the primitive streak?

Explanation

## Primitive Streak Development Timeline **Key Point:** The primitive streak is a transient midline structure that appears during the **second week** of development (around day 15) and is the site of **gastrulation**—the formation of the three primary germ layers. ### Temporal and Spatial Sequence **High-Yield:** The primitive streak follows a precise spatiotemporal pattern: 1. **Appearance (Day 15):** First appears at the **caudal end** of the bilaminar disc (the posterior/caudal epiblast) 2. **Elongation (Days 15–21):** Elongates **cranially** (toward the head region), reaching maximum length by approximately day 21 3. **Regression (Weeks 3–4):** Regresses **from cranial to caudal**, disappearing completely by the end of week 4 ### Why This Pattern Matters ```mermaid flowchart TD A["Day 15: Primitive streak appears at caudal end of epiblast"]:::outcome B["Days 15-21: Streak elongates cranially"]:::action C["Week 3: Cranial regression begins"]:::action D["Week 4: Complete regression, caudal end disappears last"]:::outcome A --> B B --> C C --> D ``` **Mnemonic:** The primitive streak **starts caudal, grows cranial, and retreats cranial-to-caudal** — think of it as "arriving from the tail, leaving from the head." **Clinical Pearl:** Persistent primitive streak remnants (at the caudal end, which regresses last) can lead to: - **Sacrococcygeal teratomas** (most common extragonadal germ cell tumor in neonates/children) - **Chordomas** (from notochord remnants) ### Textbook Reference Per **Langman's Medical Embryology** (14th ed.) and **Moore & Persaud's The Developing Human**: the primitive streak first becomes visible at the caudal end of the embryonic disc around day 15, elongates cranially to establish the embryonic axis, and then regresses in a cranial-to-caudal direction, normally disappearing by the end of the 4th week. Failure of complete regression underlies sacrococcygeal teratoma formation. ### Fate of Primitive Streak Cells As the streak elongates and regresses, epiblast cells continuously ingress to form: - **Axial mesoderm** (notochord, prechordal plate) - **Paraxial mesoderm** (somites) - **Intermediate mesoderm** - **Lateral plate mesoderm** - **Definitive endoderm**