## Hepatic Sinusoid Ultrastructure — Key Distinguishing Features **Key Point:** Hepatic sinusoids are uniquely permeable due to their fenestrated endothelium *without* a basement membrane. This architecture allows free exchange of plasma proteins, metabolites, and blood cells between the sinusoid and the space of Disse. ### Normal Hepatic Histology — Sinusoidal Characteristics | Feature | Details | |---------|----------| | **Endothelium** | Fenestrated (pores 50–200 nm diameter) | | **Basement membrane** | ABSENT — permits direct contact with hepatocytes | | **Space of Disse** | Subendothelial space containing plasma ultrafiltrate | | **Kupffer cells** | Resident macrophages within sinusoid lumen | | **Stellate cells (Ito cells)** | Located in space of Disse; store vitamin A; produce collagen in fibrosis | | **Permeability** | High — allows rapid exchange of nutrients, hormones, drugs | **High-Yield:** The sinusoid is the most permeable capillary bed in the body. Tight junctions and continuous endothelium are features of the blood–brain barrier and kidney glomerulus, NOT the liver. **Clinical Pearl:** In cirrhosis, hepatic stellate cells activate and proliferate, producing excessive collagen and reducing sinusoidal permeability (capillarization). This impairs nutrient exchange and contributes to portal hypertension. **Warning:** Do not confuse hepatic sinusoids with systemic capillaries. The absence of a basement membrane is a *feature*, not a defect — it enables the liver's metabolic and filtration functions. ### Why Option D is Wrong Option D describes a **continuous, tight-junctioned endothelium** — this is the opposite of what hepatic sinusoids possess. Tight junctions and continuous endothelium are found in: - Blood–brain barrier (cerebral capillaries) - Kidney glomerular filtration barrier - Intestinal epithelium None of these apply to the liver.
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