During osteogenesis, osteoblasts lay down osteoid and transform into osteocytes embedded in mineralized bone matrix. Despite the fact that osteocytes are the most abundant cellular component of bone, little is known about the process of osteoblast-to-osteocyte transformation. What is known is that osteoblasts undergo a number of changes during this transformation, yet retain their connections to preosteoblasts and osteocytes. This review explores the osteoblast-to-osteocyte transformation during intramembranous ossification from both morphological and molecular perspectives. We investigate how these data support five schemes that describe how an osteoblast could become entrapped in the bone matrix (in mammals) and suggest one of the five scenarios that best fits as a model. Those osteoblasts on the bone surface that are destined for burial and destined to become osteocytes slow down matrix production compared to neighbouring osteoblasts, which continue to produce bone matrix. That is, cells that continue to produce matrix actively bury cells producing less or no new bone matrix (passive burial). We summarize which morphological and molecular changes could be used as characters (or markers) to follow the transformation process. Developmental Dynamics 235:176 -190, 2006.

The neural crest (NC) is a major contributor to the vertebrate craniofacial skeleton, detailed in model organisms through embryological and genetic approaches, most notably in chick and mouse. Despite many similarities between these rather distant species, there are also distinct differences in the contribution of the NC, particularly to the calvariae of the skull. Lack of information about other vertebrate groups precludes an understanding of the evolutionary significance of these differences. Study of zebrafish craniofacial development has contributed substantially to understanding of cartilage and bone formation in teleosts, but there is currently little information on NC contribution to the zebrafish skeleton. Here, we employ a two–transgene system based on Cre recombinase to genetically label NC in the zebrafish. We demonstrate NC contribution to cells in the cranial ganglia and peripheral nervous system known to be NC–derived, as well as to a subset of myocardial cells. The indelible labeling also enables us to determine NC contribution to late–forming bones, including the calvariae. We confirm suspected NC origin of cartilage and bones of the viscerocranium, including cartilages such as the hyosymplectic and its replacement bones (hymandibula and symplectic) and membranous bones such as the opercle. The cleithrum develops at the border of NC and mesoderm, and as an ancestral component of the pectoral girdle was predicted to be a hybrid bone composed of both NC and mesoderm tissues. However, we find no evidence of a NC contribution to the cleithrum. Similarly, in the vault of the skull, the parietal bones and the caudal portion of the frontal bones show no evidence of NC contribution. We also determine a NC origin for caudal fin lepidotrichia; the presumption is that these are derived from trunk NC, demonstrating that these cells have the ability to form bone during normal vertebrate development.

Background: A sample of 1448 students in grades 7 and 9 was drawn from public schools in Atlantic Canada to explore students' knowledge of science and mathematics requirements for science, technology, engineering, and mathematics (STEM) careers. Also explored were their mathematics self-efficacy (MSE), their future career interests, their preferences for particular career activities, and their likelihood to pursue a STEM career.

Scleral ossicles are dermal bones that are present in the eye of many vertebrates. Despite this, little is understood about their development. This study investigates the cellular dynamics during and after induction, and attempts to identify inducing factors. Both cell death and proliferation were found to play limited roles in mesenchymal condensation formation, but are involved in development of the inducing epithelium overlying the presumptive ossicle. Real-time reverse transcriptase polymerase chain reaction of candidate genes identified significant increases in sonic hedgehog (SHH) expression. In situ hybridization confirmed that SHH is exclusively expressed in the conjunctival (scleral) papillae and not in the mesenchyme. Direct localized inhibition of Hedgehog signaling, by means of cyclopamine, supports the finding that SHH may play a role in scleral ossicle induction. In addition, a nonfluctuating asymmetry with respect to the number of ossicles per eye was found. This study provides significant insight into understanding the development of the neural crest derived dermal bones. Developmental Dynamics 237: 3240 -3251, 2008.

Scleral ossicles are bones within the sclera of the eye. A total of 547 teleost species (744 specimens) from 36 orders and 163 families were investigated with respect to scleral ossicle presence/absence and number. This is the first extensive investigation into the distribution of scleral ossicles in living teleosts. Derived orders were found to have the most variable scleral ossicle numbers (zero, one, or two per eye), while more basal groups tend to have no ossicles. Whereas more data on the activity level of individual families and on family-level interrelationships is needed, significant findings were nevertheless made. Ninety-four percent of the families investigated have a consistent ossicle number, indicating that family level is a reliable predictor of scleral ossicle presence/absence. In a subgroup analysis of 28 families, additional trends were observed with regard to activity level, namely that 80% of the families that are described as sluggish have no scleral ossicles while 100% of those that are very active have two ossicles per eye. In addition, fish that inhabit deep sea environments are the most likely ones to lack scleral ossicles. The analysis also supports the hypothesis that scleral ossicle number declined from the basal condition of four elements per eye seen in Cheirolepis to no ossicles in the eye, early in teleost evolution. At least 24 evolutionary steps are needed to account for the scleral ossicle distribution seen in teleost orders today. This study describes the variation of scleral ossicles in the most diverse group of vertebrates, Teleostei, and provides the first step in understanding the evolvability of these elements in bony fishes.