Chapter 2: Connective Tissue & Blood
Connective tissue is defined less by its cells than by what surrounds them — the extracellular matrix. Blood counts as a connective tissue too: its "matrix" is plasma, and its cells float free instead of being embedded in fibers.
The resident cells
- Fibroblasts are the workhorses — they synthesize collagen, elastin, and the ground substance that makes up the matrix. They're the most abundant cell in loose and dense connective tissue.
- Macrophages phagocytose debris and pathogens and present antigen to T cells, linking innate and adaptive immunity.
- Mast cells are packed with metachromatic granules containing histamine and heparin; degranulation drives allergic and inflammatory reactions.
- Plasma cells are terminally differentiated B cells with an eccentric "clock-face" nucleus, built to mass-produce antibody.
Cartilage: three flavors
| Type | Matrix | Found | Notes |
|---|---|---|---|
| Hyaline | Glassy, type II collagen | Articular surfaces, trachea, fetal skeleton | Has a perichondrium (except at joints) |
| Elastic | Elastic fibers + collagen | External ear, epiglottis | Flexible, recoils to shape |
| Fibrocartilage | Dense type I collagen | Intervertebral discs, menisci | No perichondrium; heals poorly |
Bone in three stages
Osteoblasts line the bone surface and secrete unmineralized osteoid. Once matrix mineralizes around them, they become trapped as osteocytes, sitting in lacunae and communicating through thin canaliculi. Multinucleated osteoclasts, derived from the monocyte lineage, resorb bone from the opposite side of the same remodeling process — the two cell types are in constant balance.
Blood as connective tissue
- Erythrocytes: anucleate, biconcave discs (~7.5 µm) built to maximize surface area for gas exchange.
- Platelets: anucleate cytoplasmic fragments of megakaryocytes, central to clot formation.
- Granulocytes (neutrophil, eosinophil, basophil) and agranulocytes (lymphocyte, monocyte) make up the white cell line — covered in detail on a peripheral smear, where granule color and nuclear shape are the two fastest identifying features.
Key teaching point
When a slide question asks you to identify a connective tissue cell, work through the same two cues every time: nuclear shape and granule color. It gets you to neutrophil vs. eosinophil vs. basophil, and to fibroblast vs. macrophage vs. mast cell, faster than trying to memorize whole-field gestalt.