These are extensions of the lacunae and are filled with extracellular fluid.

Options

A. Volkmann's canals
B. Haversian canals
C. Osteons
D. Canaliculi

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the structures that are extensions of lacunae and filled with extracellular fluid.

2. **Identifying Lacunae**: Lacunae are small spaces found in bone tissue, specifically between the lamellae (the concentric rings of the bone matrix). They house mature bone cells known as osteocytes.

3. **Exploring the Options**:
- **Wolkmann's Canal**: This option is incorrect. Wolkmann's canals are channels in bone tissue that carry blood vessels and are not extensions of lacunae.
- **Haversian Canal**: This option is also incorrect. Haversian canals are central canals in the osteon that contain blood vessels and nerves, not extensions of lacunae.
- **Osteon**: This is the basic structural unit of bone, which includes lamellae, lacunae, canaliculi, and Haversian canals. Therefore, this option is incorrect as well.
- **Canaliculi**: This option is correct. Canaliculi are indeed extensions of lacunae and form a network of minute canals that facilitate the transport of nutrients and waste between osteocytes.

4. **Conclusion**: The correct answer is canaliculi, as they are the extensions of lacunae filled with extracellular fluid.

**Final Answer**: Canaliculi are the extensions of lacunae and are filled with extracellular fluid.

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