Osteoblasts in the diaphysis of the tibia during endochondral ossification in young rats are attached to one another by nexus, by "adhaerens" junctions, and by simple appositions. "Adhaerens" junctions and nexus also occur between preosteoblasts and osteoblasts. Furthermore, to osteoblasts exhibit a network of microfilament bundles in the cell periphery overlying the osteoid. From this network bundles extend into the cell processes which protrude into the unmineralized matrix. The mean diameter of individual microfilaments is 5.9 +/- 0.06 nm. A possible role of nexus and microfilaments in controlled bone growth and differentiation is discussed.

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Some characteristics of early premelanosomes (PM) suggest that primarily a continuous cisternal complex of the endoplasmic reticulum (ER) is transformed simultaneously to PM. These characteristics are: (i) the form and size, which are similar to ER cisternae; (ii) the localization in groups in the ER; )iii) the same stage of maturation within a group; (iv) the continuities between early PM, and (v) the lack of continuities between ER and PM. Comparative measurements reveal that the limiting membrane of PM, with a total thickness of 7.6 +/- 0.19 nm and a center-to-center distance of 5.2 +/- 0.06 nm, is significantly thicker than the ER membrane (6.3 +/- 0.15 nm and 4.3 +/- 0.04 nm, respectively) and the melanosome limiting membrane (6.5 +/- 0.22 nm and 4.4 +/- 0.05 nm, respectively). Therefore, during the formation of melanosomes, the limiting membrane must be transformed from a thin (ER) to a thick (PM) and again to a thin (melanosome)stage.