Connective Tissue (CT) = supporting tissue with cells + EXTENSIVE matrix (fibres + ground substance).

Components:
• Cells: fibroblasts (build matrix), macrophages, mast cells, plasma cells, adipocytes, leukocytes.
• Fibres: collagen (white, strength), elastic (yellow, recoil), reticular (fine, support).
• Ground substance: GAGs + proteoglycans + glycoproteins.

Classification:
• CT proper:
– Loose (areolar, adipose, reticular).
– Dense (regular = tendon/ligament; irregular = dermis).
• Specialized CT:
– Cartilage (hyaline, elastic, fibrocartilage).
– Bone (compact + spongy).
– Blood (covered separately).

CONNECTIVE TISSUE — THE GREAT SUPPORTER

The most diverse of the four basic tissues. Origin: mesoderm (mesenchyme). All CTs share three components in different proportions: cells + fibres + ground substance.

I. CT CELLS:
• Fibroblasts — most numerous; spindle-shaped; make collagen + elastin + ground substance. Quiescent form = 'fibrocyte'.
• Macrophages — heterogeneous resident phagocytes; recruited monocytes contribute to many populations, while several tissue-resident populations have distinct embryonic origins; include Kupffer and alveolar macrophages; microglia, osteoclasts and Langerhans cells are related but distinct tissue-resident populations. Antigen presentation.
• Mast Cells — packed with histamine + heparin granules; central to allergic reactions.
• Plasma Cells — antibody factories; clock-face nucleus.
• Adipocytes (Fat cells) — lipid storage, signaling, insulation. White vs brown adipose.
• Leukocytes — neutrophils, lymphocytes, eosinophils, basophils, monocytes (transient migrants from blood).
• Mesenchymal stromal/stem cells — multipotent; in many adult CTs.

II. CT FIBRES:
• Collagen fibres — most abundant protein in body. White, parallel, immense tensile strength. ~28 types of collagen (Type I in tendons/skin/bone; Type II in cartilage; Type III = reticular; Type IV in basement membrane).
• Elastic fibres — made of elastin; yellow; recoil after stretch. In ligamentum nuchae, vocal folds, skin, large arteries.
• Reticular fibres — fine type III collagen; form mesh in lymphoid organs (lymph node, spleen, bone marrow), liver, endocrine. Stained by silver (argyrophilic).

III. GROUND SUBSTANCE:
• Gel-like matrix between cells & fibres.
• Glycosaminoglycans (GAGs) — long polysaccharide chains (hyaluronic acid, chondroitin sulfate, keratan sulfate, heparan sulfate).
• Proteoglycans = core protein + many GAGs.
• Glycoproteins — fibronectin, laminin (binds cells to matrix).
• Holds water; resists compression.

IV. CLASSIFICATION:

A. CONNECTIVE TISSUE PROPER:
1. Loose (Areolar) CT: All cells + fibres + abundant ground substance loosely arranged. The 'packing material' under epithelia, around vessels. Very vascular; site of inflammation.
2. Adipose Tissue: Specialized for lipid storage.
– White adipose — large unilocular fat cells; energy store; insulation; mechanical cushion. Most adult fat.
– Brown adipose — multilocular fat cells with many mitochondria; thermogenesis (UCP-1 protein); abundant in newborns + hibernating animals.
3. Reticular CT: Network of reticular fibres + reticular cells; stroma of lymph nodes, spleen, bone marrow, liver.
4. Mucous CT: Embryonic tissue; persists in umbilical cord ('Wharton's jelly').
5. Dense Regular CT: Parallel collagen bundles. Tendons + ligaments + aponeuroses. Few fibroblasts.
6. Dense Irregular CT: Collagen in random directions. Dermis of skin, capsules of organs (kidney, spleen, liver), submucosa.
7. Elastic CT: Predominantly elastic fibres. Ligamentum nuchae, ligamenta flava, vocal cords, walls of large arteries.

B. SPECIALIZED CONNECTIVE TISSUES:
Cartilage:
• Cells = chondroblasts (active) + chondrocytes (mature, in lacunae).
• Avascular; nutrition by diffusion; perichondrium (covering CT) supplies it.
• 3 types:
– Hyaline cartilage — homogeneous matrix; articular surfaces, costal cartilages, growth plates, trachea, larynx (most cartilages).
– Elastic cartilage — many elastic fibres; pinna, epiglottis, auditory tube.
– Fibrocartilage — type I collagen bundles + chondrocytes; intervertebral discs, menisci, pubic symphysis.

Bone (Osseous tissue):
• Cells = osteoblasts (form), osteocytes (maintain), osteoclasts (resorb).
• Mineralized matrix of collagen + hydroxyapatite.
• Compact bone — Haversian systems (osteons): central canal + concentric lamellae + lacunae + canaliculi.
• Spongy bone — trabeculae aligned with stress lines; marrow in spaces.