Soft tissue

• The circulatory system facilitates the continuous movement of the body fluid including the blood and the lymph
• 2 main elements:
  • Blood vascular system
    • Arterial system
    • Microcirculation: capillaries
    • Venous system
  • Lymph vascular system
• Vasa vasorum: small blood vessels responsible for the supply of the walls of the big blood vessels
• Normal vessels contain endothelial cells facing lumen and pericytes, smooth muscle cells and glomus cells towards outside of vessel

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• Normal histology:
  • Structural transition is gradual between the different types of vessels
  • General structure of the arterial and venous system (thickness depends on size and type of vessel)
    • Tunica adventitia (TA):
      • Longitudinally arranged collagen
      • Layer is larger in veins than arteries
    • Tunica media (TM):
      • Concentrically arranged smooth muscle
      • Diameter is altered by autonomic control
      • Smooth muscle may secrete collagen, elastic fibers, elastic lamellae and proteoglycans
      • Larger diameter in arteries than veins of similar size
    • Tunica intima (TI):
      • Inner endothelium
      • Variable outer subendothelial connective tissue (SCT) composed of scattered fibroblasts and myointimal cells producing collagen and elastin fibers
        • With age myoepithelial cell can accumulate lipids causing tunica intima thickening
• Arterial system:
  • Artery types:
    • Elastic artery
    • Muscular artery
    • Arteriole (lumen < 0.3 mm)

	Tunica intima	Tunica media	Tunica adventitia
Elastic artery	Thick	Broad layers of fenestrated elastic sheets (40 - 50) + collagen layers + smooth muscle	Vasa vasorum are present Vasa vasorum also penetrates the outer layer of tunica media
Muscular artery	Thick	Internal elastic lamina (IEL) Thick layer of circumferential smooth muscle layer External elastic lamina (EEL)	No vasa vasorum
Arteriole (large; lumen < 0.3 mm)	Thin, with almost no subendothelial connective tissue	Thin internal elastic lamina 3 - 6 layers of smooth muscle No external elastic lamina

• Microcirculation:
  • Direction of blood flow:
    • Arterioles → capillaries → postcapillary venules → small muscular venules
    • AV shunt: direct connection between the arterioles and venules
  • Arteriole (small):
    • Lumen < 0.1 mm)
    • TI with almost no subendothelial connective tissue
    • No internal elastic lamina
    • < 2 layers of smooth muscle
    • No external elastic lamina
  • Capillaries and sinusoids:
    • 3 - 40 micrometers
    • Single endothelial layer with nuclei bulging into the capillary lumen
    • No muscular layer or elastic lamina
    • 3 types:
      • Continuous capillaries: uninterrupted endothelial layer (brain, muscle, lung, connective tissue)
      • Fenestrated capillaries: capillaries with multiple large pores (kidney, endocrine organ)
      • Sinusoid with discontinuous endothelium: disrupted Interface between the tissue and capillary lumen (spleen sinusoid, liver sinusoids)
    • Pericytes:
      • Present but difficult to identify on H&E
      • Contractile cell that regulates microvascular blood flow (SMA+) (Histol Histopathol 1991;6:269, Microvasc Res 2009;77:235)
  • Postcapillary venules - small muscular venules:
    • Series of postcapillary draining vessels with increasing luminal diameter and wall thickness
• Venous system:
  • Low pressure draining system returning the blood to the heart
  • Blood flow is maintained by:
    • Negative pressure created in the chest cavity / right atrium during inspiration
    • Compression of veins during contraction of skeletal muscle
    • Venous valves
  • Elastic and muscular components of the veins are less prominent
• Lymphovascular system:
  • Low pressure draining system returning the lymph to the venous system
  • Lymph flow is maintained by
    • Negative pressure created in the chest cavity during inspiration
    • Compression of vessels during contraction of skeletal muscle
    • Lymphovascular valves
  • Lymphoid capillary:
    • Similar size to the regular capillary
    • No pericytes
    • No or minimal basement membrane
    • Greater permeability compared to regular capillary
  • No lymphatic vessels in CNS, cartilage, bone, teeth, thymus, placenta
  • Lymph nodes:
    • Site of antigen presentation, antibody secretion, activated lymphoid cells entry into lymph
• Glomus body:
  • Encapsulated glomeriform arteriovenous anastomosis (glomus body) richly innervated by autonomic nervous system fibers
  • Located in dermis
  • High number in finger and toes
  • Involved in the thermal regulation
  • Glomus cells:
    • Specialized modified smooth muscle cell with indistinct borders, pale cytoplasm, uniform nuclei; surround arterial segment of glomeriform arteriovenous anastomosis (glomus body)
    • SMA+, MSA+, type IV collagen (outlines basement membrane), desmin variable / +, S100-, CK-
• Normal aging related changes:
  • Normal vascular structure changes during life, resulting with progressive vascular stiffness
  • Elastic arteries:
    • Increase in diameter
    • Fibrous intimal thickening
    • Fragmentation of elastic laminae
    • Thickening of vasa vasorum walls
  • Muscular arteries:
    • Dilatation and tortuosity
    • Fibrous intimal thickening
    • Fragmentation and calcification internal of elastic laminae
    • Fibrosis and hyalinization of tunica media
  • Arterioles:
    • Fibrous intimal thickening
    • Hyalinization of tunica media
  • Capillaries:
    • Basement membrane thickening
  • Venules and veins:
    • Fibrous intimal thickening
    • Hypertrophy of the muscle bundles

Contributed by Norbert Sule, M.D., Ph.D.

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Which of the following is part of the arteriolar wall structure?

1. External elastic lamina (EEL)
2. Fenestrated elastic sheets
3. Internal elastic lamina (IEL)
4. Muscularis propria

C. Internal elastic lamina (IEL). The arteriolar wall is composed of thin tunica intima with endothelial layers and subendothelial connective tissue, tunica media with thin internal elastic lamina (answer C) and smooth muscle and tunica adventitia. External elastic lamina is present only in muscular type arteries and the fenestrated elastic sheets are present in the elastic type arteries. Muscular wall of a luminal visceral organ is called muscularis propria. The muscular wall of a blood vessel is called tunica media.

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Reference: Histology-blood vessels

The above section shows a portion of the blood vessels stained with elastin stain. Based on the image, this blood vessel represents

1. Arteriole
2. Elastic type artery
3. Large muscular vein
4. Muscular type artery
5. Sinusoid

D. Muscular type artery. The depicted portion of this blood vessel shows 2 (internal and external) elastic lamina. Arterioles and large muscular vein have only internal elastic lamina. Elastic type artery has multiple layers of elastic membranes in the tunica media. Muscularis type artery as shown has 2 distinct internal and external elastic laminae. Sinusoid is a type of capillary and has only a single layer of endothelium without tunica media / adventitia or elastic lamina.

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Reference: Histology-blood vessels