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Microvessel surface area, density and dimensions in brain and muscle of the cephalopod Sepia officinalis

The microvasculature of brain and muscle in the cuttlefish Sepia was studied with stereological techniques to provide information about the surface area for exchange at the blood-tissue interface which was necessary for a parallel study of the permeability of the blood-brain barrier in Sepia. Microvessel density, length, dimensions and volume fraction, and the radius of the ‘Krogh cylinder’ of tissue supplied by each microvessel were also estimated. Vertical lobe (VL) and optic lobe (OL) of brain, outer collar valve muscle (VM) and tentacle muscle (TM) were analysed in 1 μm sections of aldehyde-fixed, Epon-embedded material. ‘Microvessels’ (diameter less than 20 μm) had a surface area density Sv (in the order VL, OL, VM, TM) of 134, 176, 67.9 and 13.8 cm2 cm-3 respectively. The numbers of microvessels per unit area tissue, QA, were 211, 395, 157 and 43 mm-2 respectively. The length density of microvessels JV = 2 x QA. The microvessel density was significantly greater in synaptic neuropil (NP) than neuron cell body (CB) zones. Total vessel volume density VV was 3.49, 4.73, 1.88 and 0.28%, in good agreement with previous estimates using intravascular tracers. Mean microvessel diameter d̄ was in the range 4.1-6.5 μm (mode 3.9-4.9 μm). The radius of the Krogh cylinder, R, was 28, 20, 32 and 61 μm. Calculations with the Krogh-Erlang equation show that brain and valve muscle are unlikely to be hypoxic under physiological conditions, while tentacle muscle may be. The vascular parameters correlate well with the known biochemistry of cephalopod tissues. This study represents a detailed analysis of the microvasculature in a complex invertebrate and permits useful comparisons with vertebrate tissues. Values for microvascular SV, QA, JV and d̄ in Sepia brain are similar to those of the rat, while Sepia muscle vascularity is less than in the rat.