Fig. 2. Immunoperoxidase staining with QCPN antibody of chick-quail chimeras sacrificed at E19-20. (A,B) Detection of quail nuclei in the osteogenic perichondrium at leading edge of periosteal ossification. (A) Alkaline phosphatase staining, ALP; (B) QCPN immunostaining. Note the marked reactivity for ALP of multiple cell layers in the richly vascular osteogenic perichondrium, but not in the adjacent fibroblastic layer. QCPN immunoreactive cells reside within the ALP-positive layer and cluster around thin-walled blood vessels (bv). Arrows in B indicate quail nuclei. (C,D) More advanced stage of bone deposition, marked by a recognizable bony collar (arrows). C, ALP staining; D, QCPN immunostaining. Note the presence of quail nuclei within the boundaries of the ALP-positive osteogenic layer of the perichondrium/periosteum. (E,F) Quail chondrocytes are observed in superficial (sub-perichondral) regions of hyaline cartilage (E, QCPN immunostaining; F, QCPN immunostaining/Alcian Blue), and in chondrocytes surrounding the marrow cavity and vascular canals (G). Quail nuclei also occur frequently in hematopoietic cells (hc) in the bone marrow (H,I). Adjacent HE stained (H) and QCPN immunostained (I) sections depicting the hematopoietic tissue in the marrow cavity of a long bone. (J) QCPN-immunoreactive osteocyte (arrowhead) residing in a typical osteocytic lacuna in well-formed bone.