Chondrocytes interact with their neighbours through their cartilaginous extracellular matrix (ECM). Chondrocyte__matrix interactions compensate the lack of cell__cell contact and are modulated by... Show moreChondrocytes interact with their neighbours through their cartilaginous extracellular matrix (ECM). Chondrocyte__matrix interactions compensate the lack of cell__cell contact and are modulated by proteoglycans and other molecules. The epiphyseal growth plate is a highly organized tissue responsible for long bone elongation. The growth plate is regulated by gradients of morphogens that are established by proteoglycans. Morphogens diffuse across the ECM, creating short- and long-range signalling that lead to the formation of a polarized tissue. Mutations affecting genes that modulate cell__matrix interactions are linked to several human disorders. Homozygous mutations of EXT1/EXT2 result in reduced synthesis and shortened heparan sulphate chains on both cell surface and matrix proteoglycans. This disrupts the diffusion gradients of morphogens and signal transduction in the epiphyseal growth plate, contributing to loss of cell polarity and osteochondroma formation. Osteochondromas are cartilage-capped bony projections arising from the metaphyses of endochondral bones adjacent to the growth plate. The osteochondroma cap is formed by cells with homozygous mutation of EXT1/EXT2 and committed stem cells/wild type chondrocytes. Osteochondroma serves as a niche (a permissive environment), which facilitates the committed stem cells/wild-type chondrocytes to acquire secondary genetic changes to form a secondary peripheral chondrosarcoma. In such a scenario, the microenvironment is the site of the initiating processes that ultimately lead to cancer. Show less
In view of the complex roles of the canonical Wnt signaling during skeletal devel-opment and disease, it is important to accurately distinguish the specific roles of this signaling cascade at... Show moreIn view of the complex roles of the canonical Wnt signaling during skeletal devel-opment and disease, it is important to accurately distinguish the specific roles of this signaling cascade at specific time windows during embryogenesis as well as postnatally in the maintenance of the skeleton. Moreover, a proper understanding of these multi-faceted roles will ultimately aid us in identifying new therapeutic targets for the treat-ment of growth disorders, osteoporosis and osteoarthritis. Most of the animal models that furnish our knowledge of the effects of canonical Wnt signaling during skeletal development and maintenance use the forced expression of a stabilized and thereby oncogenic _-catenin. The roles of intracellular _-catenin regulators and thereby of wild type _-catenin levels during skeletogenesis, bone mass accrual or AC maintenance are largely unknown. The research described in this thesis aimed at describing the role of two major intracellular regulators of _-catenin, namely Apc and Gsk3_ in regulation of SPC differentiation, bone mass accrual and cartilage maintenance. Show less
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... Show moreEstrogen 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. Show less
Longitudinal growth is the key characteristic that distinguishes children from adults. Growth is regulated in the growth plates, which are layers of cartilage located at the ends of the long bones.... Show moreLongitudinal growth is the key characteristic that distinguishes children from adults. Growth is regulated in the growth plates, which are layers of cartilage located at the ends of the long bones. The cartilage cells are called chondrocytes and go through a coordinated program of proliferation, maturation, hypertrophic differentiation, apoptosis and replacement by bone. This process is called endochondral bone formation. A complex network of hormones (endocrine regulators) is involved in this process. The mechanism of actions of these hormones is not completely understood. A possible mechanism of action is the interaction of systemic hormones with locally produced growth factors (paracrine regulators), like the Parathyroid Hormone (PTH) related Peptide (PTHrP) and its receptor, the type 1 PTH/PTHrP receptor (PTHR1). PTHrP and PTHR1 are among the key regulators in the process of endochondral bone formation. The working mechanism of PTHrP signalling, alone or in combination with other growth factors or systemic hormones, in endochondral bone formation is not completely understood. In this thesis, we have further addressed the actions of PTHrP in the complex network of endocrine and paracrine regulation of endochondral bone formation. Show less
