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 μ m sections of aldehyde-fixed, Epon-embedded material. `Microvessels' (diameter less than 2 μ m)) had a surface area density S_V (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, Q_A, were 211, 395, 157 and 43 mm^-2 respectively. The length density of microvessels J_V = 2 × Q_A. The microvessel density was significantly greater in synaptic neuropil (NP) than neuron cell body (CB) zones. Total vessel volume density V_V was 3.49, 4.73, 1.88 and 0.28%, in good agreement with previous estimates using intravascular tracers. Mean microvessel diameter {d} was in the range 4.1-6.5 μ m (mode 3.9-4.9 μ m). The radius of the Krogh cylinder, R, was 28, 20, 32 and 61 μ m. 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 S_V, Q_A, J_V and {d} in Sepia brain are similar to those of the rat, while Sepia muscle vascularity is less than in the rat.