Surviving cells contain less RER, and their Golgi complexes contain many smooth vesicles and lysosomal-like structures. The development of a ruffled border, against the surface of the mineralized enamel, signifies the start of the rapid removal of water and protein from the enamel. At the completion of transition, the ameloblasts are shortened to half their previous height (Figs 3-8a and 3-8b). They are now referred to as maturation ameloblasts.
Formation of the Papillary Layer
During the final phase of secretion, and progressing through the transition stage, the OEE, SR, and SI are transformed into the papillary layer, an epithelium believed to be specialized for transport.11 This conversion is preceded by a reduction in the size of the intercellular spaces of the SR and by a loss of glycosaminoglycans. The redifferentiated cells of the OEE, SR, and SI arrange themselves into numerous epithelial folds, or papillae, located between the ameloblasts and a well-developed capillary bed (see Figs 3-8a and 3-8b).
The former OEE, SR, and SI cells are no longer distinguishable as separate cell types, and are now referred to as papillary cells. Papillary cells contain numerous mitochondria, large numbers of pinocytotic vesicles, and extensive gap junctions.108–111 Numerous microvilli increase the papillary cell surface area several fold.
The cytoplasmic features of the papillary cells, along with their association with a rich bed of fenestrated capillaries, suggest that at this stage the enamel organ has become specialized to perform transport functions related to enamel maturation.112 The fact that papillary cells form extensive gap junctions with adjacent maturation ameloblasts leads to the conclusion that these two types of cells are acting in concert during maturation.11
Figs 3-8a and 3-8b Papillary layer (PL) cells situated between the capillaries and the maturation ameloblasts (MA). (Hematoxylin-eosin stain. Original magnification × 600.) (a) Cross section depicting the MA through their long axis, and the alternating arrangement of papillae and indenting blood vessels. (E) Endothelial cells. (b) Tangential section through the maturation enamel organ, illustrating the close contact between papillary cells and capillaries filled with red blood cells (rbc).
Papillary cells have been shown to endocytose exogenous tracer material and to transport it to lysosomal bodies.111 This has led to the speculation that the papillary layer participates directly in the removal and degradation of enamel matrix breakdown products that gain access to the intercellular spaces of the papillary layer. However, there is no direct evidence that enamel matrix degradation products diffuse into the intercellular spaces of the papillary layer.
An alternative hypothesis suggests that sodium-potassium-ATPase activity in papillary cells generates an intercellular osmotic gradient across the enamel, drawing water and small matrix polypeptides toward the maturation ameloblasts.113 The polypeptide matrix fragments would undergo endocytosis and additional degradation in secondary lysosomes of maturation ameloblasts (Figs 3-9 to 3-11).
Fig 3-9 Maturation ameloblast phenotypes. Ruffle-ended and smooth-ended maturation ameloblasts cycle back and forth during the maturation phase. Cycling (C) of the two phenotypes involves extensive remodeling of the distal cytoplasm and junctional complexes at both ends of the cells. The Golgi complexes (GA) and the lysosomal (Ly) apparatus are well developed in both cell configurations. Zonula adherens (ZA) and zonula occludens (ZO) shift from a distal position in the ruffle-ended ameloblasts to a proximal position in the smooth-ended ameloblasts. Mitochondria (M) are located primarily in the distal cytoplasm. Endosomes (E) containing enamel matrix (EM) are present in highest amount in the ruffle-ended ameloblasts.
Fig 3-10a Distal part of a ruffle-ended maturation ameloblast. (Em) Enamel matrix; (Ncl) nucleolus; (Nuc) nucleus. (Original magnification × 9,000.)
Fig 3-10b Golgi complex containing numerous Golgi cisternae (Gc) in a maturation ameloblast. (Original magnification × 16,000.)
Fig 3-10c Ruffled border (RB) and endocytosis vesicles (Ev) at higher magnification. (Original magnification × 20,000.)
Fig 3-11 Proposed pathway of enamel protein (EMP) reabsorption and digestion by ruffle-ended ameloblasts. The intercellular space (ICS) is sealed by a zonula occludens (ZO). Enamel proteins are endocytosed from the labyrinthine spaces of the ruffled border into endosomes (E) that fuse with larger secondary lysosomes (SL). Lysosomal enzymes are transported to the SLs via primary lysosomal vesicles (L) originating in the area of the Golgi apparatus. H+-adenosine triphosphatase, expressed at high levels, is responsible for secretion of H+ into the enamel. High levels of alkaline phosphatase are correlated to calcium transport at the ruffled border. (CV) Coated vesicle.
Structure of Maturation-Stage Ameloblasts
During the maturation stage, water and enamel matrix degradation products are removed from the enamel, and mineralization continues until the final enamel achieves a composition (by weight) of 95% mineral and only 4% water and 1% organic matrix.26,114 Biochemical analysis of enamel indicates that there is a rapid loss of matrix during the initial phase of maturation. Prior to this stage, the enamel is soft and porous, and the crystallites have yet to grow to their final thickness.115 During the final stages of the maturation process, water is lost as mineral continues to be added to the growing crystallites. Ever-smaller quantities of matrix proteins are released and removed by the maturation ameloblasts until the enamel reaches its mature state prior to eruption.
Maturation ameloblasts (and perhaps the secretory ameloblasts) contribute proteolytic enzymes that are involved in an extracellular enzymatic cleavage of matrix proteins into small peptides prior to removal by endocytosis.84,116,117 One such enzyme is enamelysin, a matrix metalloproteinase (MMP-20) that degrades amelogenin.59 A serine proteinase (ameloprotease) capable of degrading the entire amelogenin molecule has been isolated from pig enamel matrix.69,118 Membrane-type matrix metalloproteinase (MT-MMP) is also expressed in ameloblasts.119 It has been suggested that MT-MMP might function as an activator of extracellular MMPs close to the cell surface during enamel maturation.
Enamel maturation is more time consuming than the preceding secretory stage. Maturation ameloblasts remain in contact with the enamel surface for approximately two thirds more time than do the secretory ameloblasts. Failure of enamel maturation leads to the eruption of enamel that is relatively soft, porous, and easily discolored by food and/or blood and serum.
On completion of the transition phase, maturation ameloblasts develop a ruffled border, a zone of cytoplasmic folds and invaginations along the distal end of the cell in contact