Histology of Blood Vessels, Blood, and Red Bone Marrow

Classic classroom specimen: Aorta (also pulmonary trunk and immediate branches)
Functional overview
- Serve as high-pressure conduits directly leaving the heart.
- Abundant elastic fibers smooth out the pulsatile output of the ventricles ("wind-kessel" effect) → converts intermittent systole/diastole into a more continuous blood flow.
- Clinical tie-in: Loss of elasticity (e.g., in arteriosclerosis) increases systolic pressure and cardiac afterload.
Histological layers
- Tunica interna (intima)
- Simple squamous endothelium lining the lumen.
- Subendothelial loose connective tissue (CT) with occasional smooth muscle cells.
- Internal elastic membrane is poorly defined because it merges with the elastic lamellae of the media.
- Tunica media – diagnostic hallmark
- Dominated by concentric elastic lamellae (≈30–50 layers in the adult aorta).
- Intermixed circularly oriented smooth muscle cells (SMCs).
- Scattered fibroblasts & ground substance (rich in proteoglycans for resiliency).
- No fibroblasts/elastic fibers are produced after early childhood → pathologic tears (dissection) occur here.
- Tunica externa (adventitia)
- Predominantly dense irregular white fibrous CT (collagen type I) + areolar CT.
- Houses vasa vasorum (small vessels), lymphatics, and autonomic nervi vasorum.
- Anchors the artery to surrounding structures.
Mnemonic: "I MEA-n elastic" = Intima, Media (Elastic), Adventitia.

Intermediate calibre (most "named" arteries: radial, femoral, coronary, etc.).
Purpose: Distribute blood to specific organs; tone of SMCs finely controls regional flow.
Key histological traits
- Tunica interna
- Endothelium + thin subendothelial CT.
- Internal elastic lamina (IEL) is prominent & wavy → dark scalloped line in LM; useful for identification.
- Tunica media
- Smooth muscle cells predominate (up to $40$ layers) arranged concentrically.
- Only few elastic fibers interwoven → far less than in elastic arteries.
- External elastic lamina (EEL)
- Separates media from adventitia; visible in medium-sized arteries, fades in small ones.
- Tunica externa
- White fibrous/areolar CT with collagen, elastin, fibroblasts.
Functional correlation: Muscular arteries adjust lumen diameter → major component of systemic vascular resistance (SVR).
Pharmacologic note: α₁-adrenergic agonists cause SMC contraction → raise blood pressure; Ca²⁺ channel blockers relax them.

Store ≈ $70\%$ of blood volume at rest → "blood reservoir".
Lower pressure; walls thinner, lumen larger, shape often irregular or collapsed in sections.
Layer composition
- Tunica interna
- Endothelium + thin subendothelial layer; may form valves in medium veins (esp. limbs) to prevent retrograde flow.
- Tunica media
- Much thinner than in arteries; only a few layers of circular SMCs; collagen & minimal elastin.
- Tunica externa
- Thickest coat in veins; dense collagen bundles + elastic fibers; merges gradually with surrounding CT (no distinct outer boundary).
Histologic ID tips
- Compare paired artery & vein in same section: vein shows larger lumen, thinner wall, collapsed outline, less prominent elastic laminae.
Clinical relevance: Loss of valve competence → varicose veins; sluggish flow predisposes to deep vein thrombosis (DVT).

Definition: Small arteries, capillaries, and veins supplying the outer layers of large arteries and veins (where diffusion from the lumen is insufficient).
Location: Predominantly in tunica externa and outer media of elastic arteries & large veins.
Importance
- Necessary for metabolic support of thick vascular walls.
- Atherosclerotic plaques often originate near vasa vasorum entry sites.
Analogy: Like service roads feeding a highway’s maintenance depots.

Subdivided by presence/absence of specific granules.

Neutrophils
- Multi-lobed nucleus ("PMNs").
- Pale lilac cytoplasmic granules containing lysozyme, defensins.
- Function: Acute bacterial defense; phagocytosis & respiratory burst.
Eosinophils
- Bi-lobed nucleus; large orange-red granules with major basic protein.
- Attack parasites; modulate allergic responses.
Basophils
- S-shaped nucleus obscured by dark blue granules rich in histamine & heparin.
- Analogous to mast cells; mediate hypersensitivity + vasodilation.

Lymphocytes
- Round, dense nucleus occupying most of cell; scant cytoplasm.
- Subtypes: $T$ cells (cell-mediated immunity), $B$ cells (differentiate into antibody-secreting plasma cells), NK cells.
Monocytes
- Largest WBC; kidney-shaped nucleus, gray cytoplasm with "ground glass" appearance.
- Differentiate into macrophages, osteoclasts, dendritic cells.

Cell fragments derived from megakaryocytes.
Key roles: Hemostasis – platelet plug formation, secretion of clotting & growth factors.
Appear as small purple granules on smear.

Spongy network of reticular CT within trabecular bone (e.g., vertebrae, ribs, pelvis).
Major cellular residents
- Megakaryocytes
- Giant polyploid cells $50–100\,\mu m$ with multilobed nucleus.
- Peripheral cytoplasm buds off to form platelets via proplatelet extensions penetrating sinusoid endothelium.
- Developing erythroblastic islands (central macrophage surrounded by erythroblasts).
- Myeloid series (myeloblast → promyelocyte → myelocyte → metamyelocyte → band cell → mature granulocyte).
- Lymphoid progenitors en route to thymus or peripheral tissues.
Sinusoidal capillaries allow mature cells to enter circulation.
Regulatory factors: $EPO$ for RBCs, $TPO$ for platelets, $G\text{-}CSF$ for neutrophils, etc.

Elastic artery: Thick media with many pink, wavy elastic lamellae; indistinct IEL; wide lumen.
Muscular artery: Prominent scalloped IEL; thick SMC media, visible EEL; round lumen.
Vein: Thin media, thick adventitia; collapsed or irregular lumen; absent/indistinct elastic laminae.

Pulse pressure increases with diminished aortic compliance (aging/elastin fragmentation).
Ankle–brachial index (ABI) screens for peripheral artery disease (PAD) in muscular arteries.
Central venous catheterization targets large veins; knowledge of thin walls prevents inadvertent arterial puncture.
Bone marrow biopsy (posterior iliac crest) relies on identifying megakaryocytes & normal hematopoietic ratios.

Use paired artery/vein slides to refine ID skills – contrast thickness and laminae.
Trace IEL in muscular arteries at low power; then zoom for cellular detail.
For blood smears: perform a differential count; correlate morphology with clinical cases.
Red bone marrow: locate sinusoids first (empty lumina), then scan for giant megakaryocytes (low frequency but striking).