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# ⭐ **First & Second Week of Embryonic Development**

*(Fertilization → Implantation → Bilaminar Disc)*

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## **WEEK 1 — Key Events (Day 0–6)**

### **1️⃣ Fertilization (Day 0)**

* Occurs in **ampulla of fallopian tube**.
* Sperm penetrates corona radiata → zona pellucida → oocyte plasma membrane.
* Oocyte completes **meiosis II** → forms **zygote (diploid 46,XX / XY)**.
* **Zona reaction** prevents polyspermy.

### **2️⃣ Cleavage (Day 1–3)**

* Rapid mitotic divisions without growth.
* 2-cell → 4-cell → 8-cell stage.
* At **8–16 cells**, forms **morula**.

### **3️⃣ Morula enters uterus (Day 3–4)**

* Surrounded by zona pellucida (prevents ectopic implantation).

### **4️⃣ Blastocyst formation (Day 4–5)**

Blastocyst consists of:

* **Trophoblast → placenta**
* **Inner cell mass (embryoblast) → embryo**
* **Blastocoel** (fluid cavity)

### **5️⃣ Implantation begins (Day 6)**

* Blastocyst attaches to **posterior superior uterine wall**.
* Trophoblast differentiates into:

* **Cytotrophoblast**
* **Syncytiotrophoblast** → invasive, produces **hCG**.

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## **WEEK 2 — Key Events (Day 7–14)**

**Mnemonic – “Two’s in Week 2”**

* **2 layers** (epiblast, hypoblast)
* **2 cavities** (amniotic cavity, yolk sac)
* **2 trophoblast layers** (cyto + syncytio)
* **2 membranes** (chorion, amnion)

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### **1️⃣ Completion of Implantation (Day 7–10)**

* Syncytiotrophoblast erodes endometrium → embeds blastocyst fully.
* **Lacunar spaces** form → maternal blood enters → **uteroplacental circulation starts**.
* Trophoblast secretes **hCG**, detectable in maternal blood by **day 8–10**.

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### **2️⃣ Bilaminar Germ Disc Formation**

Inner cell mass → **two layers**:

* **Epiblast** (forms all 3 germ layers later)
* **Hypoblast** (extraembryonic endoderm for yolk sac)

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### **3️⃣ Amniotic Cavity Formation**

* Appears above epiblast.
* **Amnioblasts** form **amnion membrane**.

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### **4️⃣ Yolk Sac Formation**

* Hypoblast cells line blastocoel → **primary yolk sac**.
* Later replaced by **secondary yolk sac**.

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### **5️⃣ Extraembryonic Mesoderm Development**

* Forms between trophoblast and yolk sac.
* Cavities appear → merge → form **extraembryonic coelom** (→ chorionic cavity).

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### **6️⃣ Chorion Formation**

Extraembryonic somatic mesoderm + cytotrophoblast + syncytiotrophoblast = **chorion**.
Forms **chorionic villi** → future fetal placenta.

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### **7️⃣ Prechordal Plate Formation (End of Week 2)**

* Thickened hypoblast area → marks **future head region**.

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# ⭐ SUMMARY TABLE

| Week | Key Events |
| ---------- | ----------------------------------------------------------------------------------------- |
| **Week 1** | Fertilization → Cleavage → Morula → Blastocyst → Start of implantation |
| **Week 2** | Bilaminar disc → Amniotic cavity → Yolk sac → Chorion → Full implantation → hCG secretion |

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# ⭐ High-Yield Exam Points (NEET PG / INI CET)

* hCG detectable in maternal blood by **Day 8–10**.
* Syncytiotrophoblast is **invasive** and secretes **hCG**.
* Implantation normally occurs on **posterior superior uterine wall**.
* Embryoblast → **epiblast + hypoblast in week 2**.
* Amniotic cavity from **epiblast**; yolk sac from **hypoblast**.
* Chorion has **3 components**: extraembryonic mesoderm + cytotrophoblast + syncytiotrophoblast.

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