From bone lining cell to osteocyte--an SEM study.

We describe the SEM appearance of the rat endosteal bone lining cell ( BLC ) population, and the sequence of morphological changes of these cells as they self-incorporate into unmineralized bone matrix (osteoid), establish intercellular connections, and construct lacunae. The osteoblast/nascent osteocyte series was progressively unsheathed by gentle digestion of the osteoid with 0.25% collagenase. The osteoblasts which leave the polygonally packed BLC compartment rapidly develop numerous complexly branched processes that contact the processes elaborated by previous generations of maturing and mature osteocytes. As osteoblasts mature and approach the mineralization front, they appear to lose processes. The mature cells begin to form osteocyte lacunae by depositing an asymmetric perimeter of woven collagen fibrils, such that as the cells roof-over, the lacunae appear as pocketlike constructions. The collagen fibrils on the perilacunar matrix are oriented in a tangential or circular pattern, while those in the more distal matrix are arranged in a parallel pattern. With the completion of a lacuna, its wall appears to mineralize quickly, for lacunae could be recognized only when they are forming.

Craniofacial, caudal, and visceral anomalies associated with mutant sirenomelic mice.

Craniofacial anomalies were correlated with mutant murine sirenomelia. Ninety-eight newborn sirens from heterozygous matings were examined and analyzed. In the 96 sirens that had intact craniofacial structures, micrognathia was seen in 39% of the sirens, microstomia in 34%, macroglossia in 26%, and cleft palate in 21%. Even when not cleft, the siren palates were narrower and more highly arched than those of nonsiren littermates. The frequency of abnormal craniofacial development was greater in those sirens that were more severely affected caudally. Even though some earlier studies had indicated a preponderance of males, 46 of the 95 sirens with intact pelvic viscera were females. Fifty-three percent of the sirens were monopodal, 35% were apodal, and 11% were dipodal. A penile-like projection on the genital tubercle occurred on 15 apodal sirens and four monopodal sirens; all but three of these sirens were males. Bladder agenesis was seen in 100% of the sirens, anal atresia in 80%, and bilateral renal agenesis in 43%. No siren was found with bilaterally normal kidneys. The srn gene responsible for sirenomelia might either directly affect the embryo at both the caudal and cranial regions or indirectly affect the embryo by producing lateral mechanical compression at both these sites. The srn gene was earlier characterized as autosomal-recessive; our data confirm this. Sirenomelia was found in only 11% of the newborns from crosses of carrier mice in the colony. Analyses of uterine contents at days 12-14 suggest that the srn gene is fully penetrant, but often lethal, during the fetal period.

A cellular investment of bone marrow.

We describe a method for clearly separating the cell layers at the bone-marrow interface, which reveals that the myeloid tissue is invested by an epithelial-like layer of specialized squamous cells we call the marrow sac. The scanning electron microscope showed that the sac was fenestrated and that some of its cells pass as perivascular elements with the marrow capillaries that penetrate the bony cortex. The transmission electron microscope (TEM) showed that the cells comprising the marrow sac are less than 0.1 micrometer thick, overlap at their margins without specialized cell junctions, and are more electron dense than the reticular or fibroblastic cells of the marrow stroma. The fenestrations in the sac were intercellular and were usually occupied by cells having an ultrastructure compatible with an osteoprogenitor cell (OPC) lineage. The observation of a close proximity between the cells of the marrow sac and the osteogenic cells that line the endosteal surfaces of bone suggest that the sac cells, along with the OPCs of the superficial marrow stroma, should be included in any morphological or functional definition of an endosteum.