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Neovascularization into the epiphyseal growth plate : a morphological study of the metaphyseal vessels Hunter, William Lawrence


The metaphyseal vessels which invade the calcified cartilage of the epiphyseal growth plate were examined by a variety of techniques to determine their morphology, replicative dynamics and growth patterns as they relate to endochondral ossification. For descriptive purposes four different regions of these vessels were characterized: 1) Sprout Tips - the terminal ends of the capillary sprouts which actually impinge upon the hypertrophic chondrocytes of the growth plate. 2) Region of Extended Calcified Cartilage - those vessels deeper within the metaphysis which are surrounded by an extracellular matrix predominantly composed of extended septa of calcified cartilage. 3) Region of Bone Deposition - further still from the epiphyseal cartilage the microvessels are contained within a network of active bone deposition laid down upon a calcified cartilage framework. 4) Region of Primary Vessels - at a distance of 350-500 μm from the hypertrophic chondrocytes are dilated vessels with one or two layers of smooth muscle in their walls, that supply and drain the metaphyseal capillary plexus. Electron microscopic examination following perfusion fixation at physiological conditions of pressure and flow rate with either 2.3% glutaraldehyde alone or mixed with 2% tannic acid, demonstrated several ultrastructural features. The sprout tips of the metaphyseal capillaries are continuous blind-ended vessels lined by an attenuated fenestrated endothelium with no underlying basement membrane. Progressing deeper into the metaphysis, an abluminal basement membrane-like material is found in regions adjacent to the endothelial cell nucleus but not beneath more attenuated portions of the cell. Upon reaching the region of bone deposition, a more complete basement membrane is present and covers an increasingly larger percentage of the abluminal cell surface; although it is never entirely continuous. Dividing endothelial cells are most frequently found in the region of bone deposition at an average of 175-200 μm behind the apicies of the growing sprout tips. Serial sections revealed that dividing endothelial cells retain junctional attachments to neighbouring cells of the capillary wall throughout mitosis. The cells also form microvillar adhesion sites between daughter cells (and adjacent cells) prior to the completion of cytokinesis, such that they are junctionally linked before dissolution of the cell bridge. Unlike many angiogenic vessels, in the metaphyseal capillaries endothelial cell division occurs at a location where circulatory flow has already been established, and it must produce the cells necessary for continued growth while maintaining an intact vascular wall. Throughout the metaphyseal sprouts the endothelial cells display many features associated with growing vessels including luminal microvilli, abluminal cellular projections, and abundant cytoplasmic organelles. Dividing pericytes are characteristically found distributed evenly between the regions of extended calcified cartilage and bone deposition (50-350 μm from the sprout tip). The capillaries and post-capillary venules which act as the parent vessels from which the metaphyseal capillaries are derived are thought to be located within the region of bone deposition near their union with the larger primary vessels (250-350 μm from the sprout apecies). The metaphyseal capillary sprouts represent a continuous unidirectional angiogenic vascular network which grows via a constant radiating elongation from a growth centre that remains a fixed distance behind the growth front.

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