🧭 Four basic tissues
- Epithelium – avascular, minimal ECM, covers/lines, secretion
- Connective tissue – abundant ECM, vascular (most), support/defense
- Muscle – contraction (skeletal, cardiac, smooth)
- Nervous – signal conduction (neurons + glia)
🔬 key Epithelium: no blood vessels; CT: defined by ECM & ground substance.
🧶 ECM · fibers
- Collagen I – bone, tendon, ligament, dermis (tensile)
- Collagen II – hyaline & elastic cartilage
- Collagen III (reticular) – lymphoid organs, liver, marrow
- Collagen IV – basement membrane (non‑fibrillar)
- Elastic – elastin + fibrillin; large arteries, lung
⚠️ Scurvy (Vit C def) → impaired hydroxylation of collagen. Osteogenesis imperfecta → type I collagen defect (blue sclerae, brittle bones).
🧫 CT cells
- Fibroblast – produces collagen, GAGs (main resident)
- Macrophage – phagocytosis, antigen presentation
- Mast cell – histamine/heparin; IgE‑mediated degranulation
- Plasma cell – antibody secretion; clock‑face nucleus
- Adipocyte – white (storage) / brown (thermogenesis, UCP‑1)
mast cell tissue‑resident; basophil = circulating counterpart.
📦 CT proper subtypes
- Loose (areolar) – beneath epithelia, around vessels
- Dense regular – tendons, ligaments (parallel collagen)
- Dense irregular – dermis, capsules (multidirectional)
- Reticular – lymph nodes, spleen, marrow (type III)
- Adipose white – unilocular, energy storage
- Adipose brown – multilocular, UCP‑1, thermogenesis
- Mesenchyme – embryonic, gives rise to all CT
- Mucous (Wharton jelly) – umbilical cord
🦴 Cartilage
- Hyaline – type II collagen; articular, trachea, nose (glassy)
- Elastic – type II + elastic fibers; ear, epiglottis
- Fibrocartilage – type I + II; IV discs, pubic symphysis
- Perichondrium – absent in articular & fibrocartilage
- Growth: appositional (perichondrium) + interstitial (chondrocyte division)
⚠️ Achondroplasia = FGFR3 gain‑of‑function → impaired endochondral growth (most common dwarfism).
🏗️ Bone
- Osteoblast – synthesizes osteoid (type I collagen); becomes osteocyte
- Osteoclast – multinucleated, resorption (H⁺ ATPase, acid hydrolases)
- Compact – osteons (Haversian systems), Volkmann canals
- Spongy – trabeculae, marrow spaces, higher surface area
- Intramembranous – skull, clavicle (no cartilage model)
- Endochondral – long bones, growth plate (cartilage model)
osteoporosis osteoclast > osteoblast (postmenopausal) · osteopetrosis defective osteoclast (marble bone)
🩸 Blood & hematopoiesis
- RBC – anucleate, biconcave, 120d lifespan
- Neutrophil – multilobed, acute bacterial infection
- Eosinophil – bilobed, bright granules; allergy, parasite
- Basophil – histamine/heparin; type I hypersensitivity
- Lymphocyte – B/T/NK, adaptive immunity
- Monocyte – kidney‑shaped nucleus → macrophage
- Platelet – megakaryocyte fragment, hemostasis
hematopoiesis yolk sac (3‑8w) → liver/spleen (fetal) → bone marrow (postnatal). Extramedullary in marrow failure (myelofibrosis, thalassemia).
💪 Muscle tissue
- Skeletal – striated, multinucleated, voluntary, satellite cells (limited repair)
- Cardiac – striated, single central nucleus, intercalated discs, involuntary
- Smooth – non‑striated, single nucleus, involuntary, good regeneration
sarcomere A band (myosin, constant), I band (actin, shortens), H zone (myosin only, shortens), Z line (α‑actinin), M line (myosin anchor)
⚠️ Duchenne – dystrophin (X‑linked), pseudohypertrophy calves. Myasthenia gravis – anti‑AChR Ab, fatigable weakness.
🧠 Nervous tissue
neuron
- Dendrites – receive input
- Axon – conducts AP; hillock = trigger zone
- Nissl bodies = RER (protein synthesis)
glia
- Astrocyte – BBB, GFAP, K⁺ buffering
- Oligodendrocyte – CNS myelin (multiple axons)
- Schwann cell – PNS myelin (single axon)
- Microglia – CNS macrophage
- Ependymal – CSF production
MS CNS demyelination (oligodendrocyte) · GBS PNS demyelination (Schwann cell)
🧴 Glands
- Exocrine – duct to surface (salivary, pancreas acinar)
- Endocrine – ductless → hormone to blood (thyroid, adrenal)
- Merocrine – exocytosis (salivary, eccrine)
- Apocrine – apical pinch‑off (mammary, axillary)
- Holocrine – entire cell disintegrates (sebaceous)
⚠️ Clinical pearls · USMLE
- Scurvy – Vit C → collagen hydroxylation
- OI – type I collagen (blue sclerae)
- Achondroplasia – FGFR3 (endochondral)
- Osteoporosis – osteoclast > osteoblast
- Osteopetrosis – defective osteoclast
- Duchenne – dystrophin X‑linked
- MS – CNS oligodendrocyte
- GBS – PNS Schwann cell
- Paget – mosaic cement lines
- Ehlers‑Danlos – collagen cross‑linking defect
🔬 framework Collagen synthesis defect · matrix mineralization defect · cell activity defect (osteoblast/osteoclast/chondrocyte)