Paraffin sections of air respiratory organ of catfish measured 24.6 (A,B), 26.3 (C,F), 20.9 (D,E) cm in length, stained with safranin O "A, C-F" and H&E "B". The black arrows refer to penetration of mesenchymal cells into lacunae. The red arrows refer to the perichondral originating undifferentiated cells. The arrow heads refer to the opened lacunae. The double arrow refers to vascular inasion. m: mesenchymal cells. Scale bars represent 20 μm.
