sclerotome

**Embryology (paraxial mesoderm lineage):** Paraxial mesoderm segments into somites (somitomeres cranially) starting ~week 3. Each somite differentiates into: (1) **Sclerotome** (ventromedial) → vertebrae, ribs, annulus fibrosus of intervertebral discs; (2) **Myotome** (dorsomedial/central) → skeletal muscle of the back (epaxial → deep back muscles via dorsal primary rami) and body wall/limbs (hypaxial → ventral primary rami); (3) **Dermatome** (dorsolateral) → dermis of back; (4) syndetome (tendons) is a sclerotome-derived compartment in some descriptions.

**Molecular signaling:** Notochord and ventral neural tube (floor plate) secrete **Sonic hedgehog (SHH)** (with noggin), which induces sclerotome formation and represses dorsal fates. This upregulates **PAX1 and PAX9** (and NKX3.2/SOX9) driving chondrogenesis and vertebral formation. Dorsal neural tube (Wnt) induces dermomyotome. Segmentation itself depends on the segmentation clock (Notch, Wnt, FGF) and **HOX genes** specify regional vertebral identity.

**Resegmentation (key concept):** Each sclerotome splits into a cranial and a caudal half. The caudal half of one sclerotome fuses with the cranial half of the next to form a single vertebra. Thus, each vertebral body is formed from two adjacent sclerotomes, so myotomes (and spinal nerves) bridge across the intervertebral disc, allowing movement of the spine. Spinal nerves exit at the intervertebral foramina, between vertebrae, and intersegmental arteries pass across the vertebral bodies.

**Notochord remnant:** Nucleus pulposus of the intervertebral disc derives from the **notochord**, whereas the annulus fibrosus derives from the **sclerotome**. (High-yield distinction.)

**Clinical correlations:** - **Herniated disc:** the nucleus pulposus (notochord) herniates through the annulus fibrosus (sclerotome), typically posterolaterally at L4-L5/L5-S1 due to the posterior longitudinal ligament. - **Chordoma:** tumor from notochord remnants (clivus/sacrococcygeal). - **Spina bifida:** failure of the vertebral arches (sclerotome-derived) to fuse; occulta (bony defect only), meningocele, myelomeningocele. Associated with low folate; elevated AFP. - **Neural tube defects/VACTERL, Klippel-Feil syndrome:** failure of cervical vertebral segmentation (fused cervical vertebrae, short neck, low hairline). - **Congenital scoliosis:** hemivertebrae/block vertebrae from defective sclerotome formation/segmentation. - **Spondylocostal dysostosis:** defects in segmentation clock genes (e.g., DLL3, MESP2) causing vertebral and rib anomalies. - **Holoprosencephaly/SHH defects:** impaired SHH signaling disrupts midline and axial skeletal development. - **Sclerotome vs. "sclerotome" in pain:** In clinical medicine, a *sclerotome* may also refer to the area of bone/deep tissue innervated by a single spinal segment (deep pain referral), distinct from a dermatome (skin) and myotome (muscle).

**Mnemonic:** **S**clerotome = **S**keleton (vertebrae, ribs); **M**yotome = **M**uscle; **D**ermatome = **D**ermis.