Muscle Diseases & Erythropoiesis
Dermatomyositis: The Skin Fighter 🧴
The hallmark: Symmetrical proximal muscle weakness + pathognomonic skin findings.
Signature skin findings:
Fascia around GROUPS of muscle = perimysium. CD4+ T cells do the damage outside the fibers.
Muscle pathology: Inflammation with CD4-positive T cellsHelper T cells - coordinate immune response infiltrating the perimysium (fascia around muscle fiber bundles).
Lab findings:
- ↑ Creatine phosphokinase (CPK)
- ↑ Aldolase
- Abnormal EMG (positive sharp waves, fibrillations)
- Anti-Jo-1 antibodies (present in ~20-30%)
- Positive ANA
⚠️ BOARD TRAP: Must screen for occult visceral malignancy (malignancy-associated myositis in ~25% of cases). Order CT abdomen/pelvis AND CXR. Dermatomyositis has paraneoplastic potential!
Polymyositis: The Muscle Fighter 💪
The hallmark: Symmetrical proximal muscle weakness (especially shoulders/hip flexors) WITHOUT skin findings.
Muscle pathology: Muscle fibers show necrosis, degeneration, regeneration with inflammatory cell infiltrate. Inflammation with CD8-positive T cellsCytotoxic T cells - kill infected/abnormal cells directly infiltrating the endomysium (fascia around INDIVIDUAL muscle fibers).
Fascia around INDIVIDUAL fibers = endomysium. CD8+ T cells kill from the inside.
Lab findings:
- ↑ CPK
- ↑ Lactate dehydrogenase (LDH) ← Polymyositis-specific!
- ↑ Aldolase
- ↑ AST and ALT (liver enzymes elevated from muscle damage)
- Anti-Jo-1 antibodies (present in some cases)
- Positive ANA
- Abnormal EMG
Key difference: Polymyositis does NOT require cancer screening (unlike dermatomyositis).
Quick Reference: Derm vs Poly
| Feature | Dermatomyositis | Polymyositis |
|---|---|---|
| Skin findings | YES (Gottron's, heliotrope, shawl) | NO |
| Fascia type | Perimysium (groups) | Endomysium (individual) |
| T cell type | CD4+ | CD8+ |
| LDH elevation | Sometimes | YES — reliable marker! |
| Cancer screening | REQUIRED | Not routine |
Erythropoiesis: Building Blood Red Cells 🫀
Red blood cells are made in different locations depending on age. Move the slider to explore where erythropoiesis happens at different ages:
4 months gestation:
RBC production happens in the yolk sacTemporary structure that produces blood before liver takes over. Still very early!
6 months gestation:
Liver and spleen become the primary sites. Flat bones (ribs, pelvis, spine) also join in. The body is preparing for independent living!
8 months gestation:
Long bones (femur, humerus, tibia) activate. By this point, we have 5 sites producing RBCs: liver, spleen, AND three types of bone (flat, long, pelvis).
1 year of age (after birth):
Liver and spleen mostly shut down. Bone marrow takes over as the sole factory. This is where adults spend their whole life producing RBCs.
Adult facts: Average adult has 5 liters of blood. Each RBC lives ~120 days. When one dies, bone marrow makes a new one.
Adult: If bone marrow is destroyed...
WARNING: Spleen and liver reactivate in a desperate attempt to replace bone marrow function. This causes MASSIVE splenomegaly and hepatomegaly (can grow to 5x normal size!). Only diseases that wipe out the ENTIRE bone marrow trigger this.
Polycythemia note: Newborns exposed to birth canal hypoxia undergo erythropoiesis in utero → polycythemia (too many RBCs). This resolves as they breathe after birth.
Why this matters for medicine: Understanding where blood cells are made helps you diagnose bone marrow diseases, anemia, and recognize when the body is compensating for marrow failure.
Organizing Metabolic Pathways: The Framework 🗂️
Every metabolic pathway has three essential features. Learn these and you've got the foundation:
- Product: What does this pathway make?
- Rate-limiting enzyme: Which step controls the speed?
- Starting substrate: What goes in first?
• Synthesis (anabolic): Happens in the CYTOPLASM 🏗️
• Breakdown (catabolic): Happens in the MITOCHONDRIA ⚙️
• EXCEPTION: Glycolysis is catabolic BUT lives in cytoplasm (makes sense—centrally located to feed other pathways immediately)
Five pathways that break the rules: These pathways operate in BOTH compartments:
- Heme synthesis
- Gluconeogenesis
- Urea cycle
- Fatty acid synthesis
- Pyrimidine synthesis
These five are your "hard mode" pathways—master them and you'll crush boards. They appear everywhere because they're metabolically central.
Test Your Knowledge 🎯
Show 6 random questions from a question bank. Each question shuffles its answers. Wrong answers teach, not just tell you "no."