Regulators of growth plate maturation

Estrogen is known to play an important role in longitudinal bone growth and growth plate maturation, but the mechanism by which estrogens exert their effect is not fully understood. In this thesis this role is further explored. Chapter 1 contains a general introduction to longitudinal bone growth and the regulation of the growth plate in respect to relevant topics further studied in this thesis. Estrogen can act through a genomic or a nongenomic pathway. Both pathways are explored in rats at the onset of maturation in chapter 2. Estrogen stimulates VEGF expression in uterus and bone, which is an important growth factor for chondrocyte differentiation and chondrocytes survival in the growth plate. In chapter 3 the effect of estrogen on VEGF expression in the growth plate was studied in the rat and human growth plate. Another effect of estrogen is that it accelerates growth plate senescence. Senescence is one of the postulated intrinsic mechanisms by which the growth plate matures and finally fuses. In chapter 4 we investigated senescence in relation to proliferation, by investigating a cell cycle inhibitor p27Kip1. In animal models, catch-up growth is suggested to be caused by delayed growth plate senescence. In chapter 5 this hypothesis was further tested in humans. With puberty estrogen levels increase, the growth plate matures and at the end growth ceases with epiphyseal fusion through mechanisms not yet completely understood. In order to further explore growth plate maturation we subjected two growth plate tissues of the same patient, but with one year and one pubertal Tanner stage in between, to microarray analyses. Gene expression patterns and transcription factor binding sides in relation to pubertal maturation were studied in a longitudinal study within this single patient in chapter 6. In addition, we collected extra prepubertal and pubertal growth plate tissues and studied these samples with microarray techniques as well in chapter 7. In chapter 8 the process of epiphyseal fusion and apoptosis was studied in human growth plates. Animal models are frequently used but not fully representative for the human growth plate. Therefore we investigated a promising human in vitro model with multipotent mesenchymal stem cells (MSCs) that can differentiate into chondrocytes. MSCs can be isolated from various tissues. In chapter 9 we investigated the chondrogenic potential of MSCs from different origins and in chapter 10 we compared this model with the epiphyseal growth plate by analyzing gene expression patterns and pathways with micro-array analyses. Chapter 11 contains general conclusions and a discussion regarding the results