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# ⭐ **Third Week of Embryonic Development (Day 14–21)**

*A complete, high-yield clinical + conceptual reference*

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## ✅ **1. Overview**

The **3rd week** marks the beginning of **gastrulation**, formation of **three germ layers**, **notochord**, **neural plate**, and the start of **body axis formation**.
It is one of the **most critical periods** because major congenital anomalies arise from errors in this week.

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## ✅ **2. Key Events of Week 3**

### **A. Gastrulation (Starts Day 15–16)**

Formation of **ectoderm, mesoderm, endoderm** from epiblast.

**Steps:**

1. Formation of **primitive streak** at caudal end
2. **Invagination** of epiblast cells
3. Replacement of hypoblast → **endoderm**
4. New middle layer → **intra-embryonic mesoderm**
5. Remaining epiblast → **ectoderm**

**Clinical:**

* **Caudal regression syndrome** → abnormal gastrulation
* **Sacrococcygeal teratoma** → persistent primitive streak
* **Sirenomelia** → defective mesoderm migration

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### **B. Notochord Formation (Day 16–17)**

* Primitive node cells migrate cranially → **notochordal process**
* Notochord defines **body axis**, induces formation of **neural plate**
* Basis of **nucleus pulposus** of intervertebral discs

**Clinical:**

* **Chordomas** arise from remnants of notochord
* Abnormal induction → neural tube defects

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### **C. Neural Plate & Neurulation Begins (Day 18–21)**

Notochord releases signals → ectoderm thickens → **neural plate**
Neural folds appear → neural groove
Start of **neural tube formation**

**Cranial neuropore closes ~Day 25**
**Caudal neuropore closes ~Day 27**

**Clinical:**

* Failure of closure → **Anencephaly**, **Spina bifida**
* Prevented by **folic acid supplementation**

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### **D. Mesoderm Differentiation**

| Mesoderm Region | Derivatives |
| ----------------- | -------------------------------------------------------------------------- |
| **Paraxial** | Somites → bone, muscle, dermis |
| **Intermediate** | Kidney, ureters, gonads |
| **Lateral plate** | Splanchnic + somatic layers → CVS, gut wall, limb buds, body cavity lining |

**Clinical:**

* Renal agenesis = intermediate mesoderm defect
* Limb defects = lateral plate mesoderm issues

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### **E. Blood Vessel & Heart Development (Day 18–21)**

* **Angiogenesis** begins in splanchnic mesoderm
* Formation of **blood islands**
* **Heart begins beating by Day 21**
* First functional organ in embryo

**Clinical:**

* Early heart anomalies originate now: VSD, ASD, trunk defects

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## ✅ **3. Two Important Membranes Form**

* **Buccopharyngeal membrane** (cranial): ectoderm + endoderm → no mesoderm
* **Cloacal membrane** (caudal): ectoderm + endoderm → no mesoderm

Errors → anal agenesis, oropharyngeal defects

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## ✅ **4. Summary Timeline (SUPER HIGH-YIELD)**

| Day | Event |
| ------------- | ------------------------------------------- |
| **Day 14–15** | Primitive streak formation |
| **Day 16** | Gastrulation continues, notochordal process |
| **Day 17** | Notochord forms |
| **Day 18** | Neural plate induction |
| **Day 19–20** | Neural folds start |
| **Day 21** | Heart starts beating |

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## 🧠 **5. NEET PG High-Yield Points (One-liners)**

* **Gastrulation** = formation of **3 germ layers**
* **Primitive streak** = first sign of gastrulation
* **Notochord** induces neural tube formation
* **Somites appear** → segmentation
* **Allantois & blood vessels** appear in week 3
* **Heart starts beating on day 21**
* Neural tube defects prevented by folic acid

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