## Hypoblast (Primitive Endoderm) Derivatives **Key Point:** The **hypoblast** (also called **primitive endoderm**) is the layer beneath the epiblast. During gastrulation, hypoblast cells do NOT ingress through the primitive streak; instead, they **migrate cranially and laterally** to form extraembryonic structures. ### Primary Hypoblast Derivatives 1. **Yolk Sac Wall** - Hypoblast cells migrate around the yolk sac - Forms the visceral endoderm lining the yolk sac - Provides nutritive and hematopoietic support early in development 2. **Allantois** - Outgrowth of the yolk sac wall near the primitive streak - Extends into the connecting stalk - Forms the **umbilical vessels** (allantois becomes the median umbilical ligament postnatally) - Contributes to placental development ### Germ Layer Origin Summary | Layer | Origin | Ingression Route | Key Derivatives | |---|---|---|---| | **Epiblast** | Bilaminar disc | Primitive streak | All three germ layers, amnion | | **Hypoblast** | Bilaminar disc | NO ingression | Yolk sac, allantois, extraembryonic mesoderm | | **Mesoderm** | Epiblast → ingresses | Primitive streak | Somites, connective tissue, cardiovascular | | **Endoderm** | Epiblast → ingresses | Primitive streak | GI tract, respiratory, endocrine glands | | **Ectoderm** | Epiblast → remains | Surface epiblast | Nervous system, epidermis, neural crest | **High-Yield:** The hypoblast does **NOT** form any **intraembryonic** structures. It is purely **extraembryonic**. All three germ layers of the embryo proper come from the **epiblast**. **Mnemonic:** **HYPO = Extra** — Hypoblast gives rise to extraembryonic structures (yolk sac, allantois, chorion). **Clinical Pearl:** Abnormal allantois development can lead to **urachus anomalies** (patent urachus, urachal cysts, urachal sinuses) because the allantois is connected to the developing bladder. 
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