Learning Objectives
- Identify the microscopic features of a normal arteriole and a normal venule.
- Recognize the structure of a small peripheral nerve in cross-section.
- Understand the organization of a neurovascular bundle at the microvascular level.
- Distinguish between vessel wall thickness in resistance vessels.
Overview of Microvasculature
At high magnification, the vascular supply and nerves to tissues are often grouped into a neurovascular bundle. This ensures that even small sections of tissue receive simultaneous perfusion and innervation.
Characteristic Features: Arterioles vs. Venules
- Normal Arteriole: These are the primary resistance vessels. They have a thick wall relative to their lumen, consisting of one or two layers of smooth muscle.
- Normal Venule: These have a thin wall and a larger, often irregularly shaped lumen. They serve as the primary site for white blood cell emigration during inflammation.
- Small Peripheral Nerve: Seen in cross-section as a collection of axons surrounded by Schwann cells and a protective perineurium.
Microscopic Organization
- Endothelium: A single layer of squamous cells lining both the arteriole and venule.
- Precapillary Sphincters: Arterioles regulate blood flow into capillary beds via smooth muscle contraction.
Clinical Relevance: Microvascular Pathology
- Hypertension: Chronic high blood pressure often manifests as hyaline arteriolosclerosis, where the wall of the arteriole thickens and becomes pink and “glassy,” narrowing the lumen.
- Diabetic Microangiopathy: Diabetes can cause thickening of the basement membrane in small vessels, leading to ischemic damage in organs like the kidneys and retina.
- Peripheral Neuropathy: Damage to the small peripheral nerve within these bundles can lead to loss of sensation or paresthesia, often seen in metabolic or toxic clinical conditions.