reticular fiber

STRUCTURE/BIOCHEMISTRY: Reticular fibers are composed of type III collagen (COL3A1), a fibrillar collagen with thinner fibrils (about 20-45 nm) than type I. They are not bundled into thick fibers and form fine, branching, mesh-like networks. Their carbohydrate content is high, so they are PAS-positive. They are argyrophilic (stain black with silver impregnation, e.g., Gomori/Reticulin stain) but stain poorly with H&E. Fibroblasts and reticular cells synthesize them. In the liver, hepatic stellate (Ito) cells also make them.

LOCATION/FUNCTION: They form the stroma, a supportive framework, of hematopoietic and lymphoid organs (bone marrow, spleen, lymph nodes), the liver (space of Disse), and adipose tissue. They also occur in smooth muscle, endoneurium, the basement membrane (reticular lamina, which lies beneath the basal lamina), kidney, and the early stages of wound healing/granulation tissue. They provide a scaffold for cells such as lymphocytes and macrophages, and they allow organs to expand and change shape.

COLLAGEN SYNTHESIS (high yield): (1) Preprocollagen is synthesized in the rough ER. (2) Hydroxylation of proline and lysine residues requires vitamin C (deficiency causes scurvy). (3) Glycosylation of hydroxylysine and triple-helix formation produce procollagen. (4) Procollagen is secreted, and procollagen peptidases cleave the terminal propeptides to make tropocollagen. (5) Lysyl oxidase, which requires copper, cross-links tropocollagen into collagen fibrils. (Menkes disease involves decreased copper and lysyl oxidase function.)

CLINICAL CORRELATES: - Vascular (type IV) Ehlers-Danlos syndrome: COL3A1 mutation (type III collagen) causes weak blood vessels, hollow organs (bowel, uterus) prone to rupture, aneurysm and arterial dissection, and easy bruising. Skin and joint hypermobility are less prominent than in classical EDS. - Mnemonic: "Be So Totally Cool, Read Books" / "Collagen types: Bone (I), Cartilage (II), Reticulin (III), Basement membrane (IV)." Another mnemonic: type III is "thr-EEE", the collagen of "granulation tissue" (the early stage of wound repair), later replaced by type I. - Wound healing: Type III collagen is deposited first in granulation tissue (about days 3-14) and is replaced by stronger type I collagen during remodeling (needs matrix metalloproteinases, which require zinc). - Reticulin stain in pathology: It assesses bone marrow fibrosis (e.g., primary myelofibrosis), liver architecture (loss or collapse of the reticulin framework in hepatocellular carcinoma or in cirrhosis/necrosis), and some tumors (e.g., it highlights nests in carcinomas versus individual cells in sarcomas/lymphomas). - Keloids: Excess collagen (type III initially, then disorganized type I) with a TGF-β contribution.

KEY COMPARISONS: Type I collagen (most common, bone, skin, tendon, late wound repair; defect causes osteogenesis imperfecta). Type II (cartilage, vitreous, nucleus pulposus). Type III (reticulin: skin, blood vessels, uterus, fetal tissue, granulation tissue). Type IV (basement membrane/basal lamina; defect causes Alport syndrome; target in Goodpasture syndrome). Elastic fibers (elastin, fibrillin-1; defect causes Marfan syndrome) are separate from reticular fibers.