loose connective tissue

**Definition & Structure** - Connective tissue is divided into embryonic (mesenchyme, mucous), connective tissue proper (loose and dense), and specialized types (cartilage, bone, blood, adipose). - Loose connective tissue (LCT) has more ground substance and cells than fibers, which gives it a loosely woven, flexible, highly vascular structure. - Subtypes: **areolar** (the prototype), **adipose**, and **reticular**.

**Components** - *Fibers*: collagen type I (tensile strength, the most abundant), thin elastic fibers (fibrillin and elastin; recoil), and reticular fibers (collagen type III; supportive stroma). - *Ground substance*: glycosaminoglycans (hyaluronic acid, chondroitin sulfate, heparan sulfate), proteoglycans, and adhesive glycoproteins (fibronectin, laminin). It is hydrated and permits diffusion of nutrients and waste. - *Resident cells*: - **Fibroblasts**: synthesize fibers and ground substance, and are key to wound healing. - **Macrophages**: derived from monocytes, they phagocytose and present antigen. - **Mast cells**: contain heparin and histamine, and mediate type I hypersensitivity through IgE (FcεRI). - **Plasma cells**: secrete antibodies. - **Adipocytes** and **pericytes**. - *Transient (wandering) cells*: neutrophils, eosinophils, lymphocytes, and monocytes migrate in during inflammation.

**Locations** - Lamina propria beneath epithelia (GI, respiratory, and urinary tracts) - Papillary dermis of the skin - Superficial fascia (hypodermis) - Around blood vessels, nerves, and glands (adventitia) - Mesenteries and omentum - Stroma of glands and organs (the reticular type forms the framework of lymph nodes, spleen, liver, and bone marrow)

**Functions** - Support and binding of epithelia to deeper structures - Nutrient and gas exchange (via capillaries) with avascular epithelia - Defense and immune surveillance - Cushioning, plus fluid and water storage (edema accumulates here) - Wound repair (granulation tissue forms from LCT)

**Clinical Correlations (High-Yield)** - **Edema** collects in loose connective tissue because it can expand readily (e.g., periorbital edema in nephrotic syndrome, which is seen early because the periorbital tissue is especially loose). - **Collagen synthesis defects**: - *Ehlers-Danlos syndrome*: defective collagen (types I, III, or V), causing hyperextensible skin and joint hypermobility. - *Scurvy*: vitamin C deficiency impairs hydroxylation of proline and lysine, causing perifollicular hemorrhage and poor wound healing. - *Osteogenesis imperfecta*: type I collagen defect. - *Marfan syndrome*: fibrillin-1 defect (elastic fibers). - **Type I hypersensitivity**: mast cell degranulation in LCT leads to urticaria, angioedema, and anaphylaxis. - **Cellulitis**: infection spreading through the loose subcutaneous tissue planes. **Necrotizing fasciitis** spreads along fascial planes. - **Wound healing**: fibroblasts proliferate and deposit collagen type III (early), which is then remodeled to type I. - **Contrast with dense connective tissue**: dense regular (tendons, ligaments, aponeuroses) and dense irregular (reticular dermis, organ capsules) have more fibers and fewer cells and ground substance.

**Histology Pearls** - On H&E, there are sparse pink collagen fibers in a loose, haphazard arrangement, with many scattered nuclei and open spaces. - Elastic fibers need special stains (Verhoeff-van Gieson, orcein). Reticular fibers need silver stain.