Changes in the types of collagen synthesized during chondrogenesis of the mouse otic capsule

Inner ridge cells may be the main source of tectorial membrane type II collagen: evidence from quantitative mRNA in situ hybridization

In a previous study we showed that COL2A1 mRNA is expressed in both ectodermally and mesodermally derived structures of second trimester human fetal cochlea, whereas type II collagen is present in mesodermally derived structures and in tectorial and basilar membranes. Because the tectorial membrane is acellular and therefore does not make its own proteins, the source of type II collagen and proteoglycans in this membrane has been of interest. We have attempted to address this issue, at least in part, by performing quantitative cRNA mRNA in situ hybridization on second trimester human fetal cochlear sections using a COL2A1 probe. By counting the number of silver grains cell in the interdental cells, inner sulcus cells and inner ridge Kolliker organ cells and by an analysis of variance of these quantitative data. inner ridge cells were found to have significantly higher levels of COL2A1 mRNA than interdental and inner sulcus cells (p < 0.0001). On the basis of significantly higher COL2A1 mRNA levels in inner ridge cells and their higher numbers than interdental and inner sulcus cells we postulate that type II collagen for human fetal tectorial membrane is derived mostly from inner ridge Kolliker organ cells. The lower COL2A1 mRNA in interdental cells appears to provide type II collagen for the spiral limbus and the tectorial membrane. The inner sulcus cells, hair cells. Deiter's and Hensen's cells also appear to contribute lesser amounts of type II collagen to the tectorial membrane. In analogy to these findings it is possible that other tectorial membrane proteins, including proteoglycans and other collagens, are also largely derived from these cells during human fetal development.

Fine structure of extracellular matrix and basal laminae in two types of abnormal collagen production: L-proline analog-treated otocyst cultures and disproportionate micromelia (Dmm/Dmm) mutants

L-Azetidine-2-carboxylic acid (LACA), a naturally occurring vegetable imino acid, can be incorporated into mammalian proteins in place of proline, thereby eliciting an inhibitory effect on collagen secretion. Exposure of explants of the embryonic mouse inner ear to LACA reduces the number of collagen fibrils in the otic capsule, gives rise to a dose-dependent derangement of the basal lamina, and ultimately results in dysmorphogenesis and retarded differentiation of the inner ear. Disproportionate micromelia (Dmm) is an incomplete dominant form of dwarfism characterized by a reduced quantity of type II collagen in the cartilaginous extracellular matrix (ECM). Abnormal morphogenesis in homozygotic Dmm mice resembles the abnormal morphogenesis observed in LACA-exposed otic explants, resulting in malformed inner ears with a bulky cartilaginous capsule and a lack or reduction of defined perilymphatic spaces (Van De Water and Galinovic-Schwartz, 1987). In this study, we examined by ultrastructural analysis LACA-exposed otic explants and inner ears of Dmm/Dmm mouse embryos for abnormalities in the collagenous constituents of the basal laminae and capsular ECM. We demonstrate, in comparison to normal embryonic mouse inner ears, a reduction in collagen fibrils and irregular cytodifferentiation of chondrocytes in the ECM of LACA-exposed and Dmm/Dmm inner ears as well as in the basal laminae of LACA-exposed specimens. In addition, we provide evidence of dysmorphogenesis of the otic capsule and perilymphatic spaces in LACA-exposed explants. Moreover, while previous studies demonstrated the anomalous development of sensory structures in otocyst explants following LACA exposure, in this study we provide evidence of the normal morphogenesis of otic epithelial-derived sensory structures in homozygotic Dmm/Dmm mouse embryos.

Expression and localization of COL2A1 mRNA and type II collagen in human fetal cochlea

The expression and localization of COL2A1 mRNA and protein was examined in human fetal cochlea to study the role of this gene in hearing and to begin to understand the pathogenesis of mutations in COL2A1 in hearing disorders. Northern blot analysis revealed COL2A1 expression in fetal membranous cochlea to be markedly greater than that in fetal skin, kidney, cartilage, eye and brain. In situ hybridization revealed COL2A1 expression in marrow cells, osteoblasts, fibroblasts and some osteocytes, in addition to chondrocytes in otic capsule. In the membranous cochlea, connective tissue elements (spiral ligament, spiral limbus and modiolar connective tissue), neuronal cells, secretory cells (stria vascularis) and organ of Corti cells (sensory hair cells) were found to express COL2A1. Immunohistochemistry was performed to assess distribution of type II collagen and correlation with COL2A1 mRNA in these morphologically and functionally diverse cell populations. In otic capsule, only cartilage was found to stain positively, and in membranous cochlea, only connective tissue structures including spiral ligament, spiral limbus, tectorial and basilar membranes, modiolar and spiral lamina cartilage contained type II collagen. Nonconnective tissue cells, marrow cells and osteoblasts did not contain immunohistochemically identifiable protein. Absence of type II collagen in a subset of cochlear cells may reflect potentially either inability to detect low levels of protein in these cells or posttranscriptional regulation.

The membranous skeleton: the role of cell condensations in vertebrate skeletogenesis

Elements of the vertebrate skeleton are initiated as cell condensations, collectively termed the 'membranous skeleton' whether cartilages or bones by Grüneberg (1963). Condensations, which were identified as the basic cellular units in a recent model of morphological change in development and evolution (Atchley and Hall 1991) are reviewed in this paper. Condensations are initiated either by increased mitotic activity or by aggregation of cells towards a centre. Prechondrogenic (limb bud) and preosteogenic (scleral ossicle) condensations are discussed and contrasted. Both types of skeletogenic condensations arise following epithelial-mesenchymal interactions; condensations are identified as the first cellular product of such tissue interactions. Molecular characteristics of condensations are discussed, including peanut agglutinin lectin, which is used to visualize prechondrogenic condensations, and hyaluronan, hyaladherins, heparan sulphate proteoglycan, chondroitin sulphate proteoglycan, versican, tenascin, syndecan, N-CAM, alkaline phosphatase, retinoic acid and homeo-box-containing genes. The importance for the initiation of chondrogenesis or osteogenesis of upper and lower limits to condensation size and the numbers of cells in a condensation are discussed, as illustrated by in vitro studies and by mutant embryos, including Talpid3 in the chick and Brachypod, Congenital hydrocephalus and Phocomelia in the mouse. Evidence that genes specific to the skeletal type are selectively activated at condensation is discussed, as is a recent model involving TGF-beta and fibronectin in condensation formation. Condensations emerge as a pivotal stage in initiation of the vertebrate skeleton in embryonic development and in the modification of skeletal morphology during evolution.

Immunohistochemical localization of fibronectin-like protein in the inner ear of the developing gerbil and rat

Immunohistochemistry was used to demonstrate the distribution of fibronectin-like protein within the developing inner ear of two species of altricial rodents: gerbils and rats. While there were temporal differences between the two species, the developmental sequence of immunostaining was virtually identical. Most notably, in rats from embryonic day 18 through day 1 postpartum, and in gerbils from birth through day 4 postpartum, intense, discrete fibronectin-like immunoreactivity was observed in the cochlea immediately beneath the inner and outer hair cells, sites of active auditory nerve fiber growth and nerve-hair cell synaptogenesis at these ages. The results suggest that fibronectin is appropriately positioned spatially and temporally to play a significant role in promoting, guiding and/or maintaining neural innervation within the developing organ of Corti. The temporo-spatial pattern of immunostaining in Schwann cells and auditory (VIIIth cranial) nerve neurons implies that fibronectin also plays a significant role in the early formation of myelin. In non-neural elements of the cochlea, fibronectin is a major structural component within the basilar membrane at all of the developmental stages investigated.

Ornithine decarboxylase activity during development of the mouse inner ear in vivo and in vitro

Ornithine decarboxylase activity was determined during the development of the peripheral auditory system in the murine otocyst with the goal of understanding the role of this enzyme in the morphological and functional maturation of the inner ear. At gestational days 11 and 12 enzyme activity was more than 10-fold higher than adult levels. A sharp decline occurred between day 12 and 13 after which activity rose to a peak around day 15. Activity then dropped continuously until near-adult levels were reached at birth. A lower specific activity of ODC but a similar time-course was seen in otocysts explanted at gestational day 13 and subsequently cultured for 6 days. For two stages of development, enzyme activity and binding of 3H-alpha-difluoromethylornithine were compared. The four-fold difference in enzymatic activity on gestational days 15 and 17 was paralleled by a similar difference in binding. Ornithine decarboxylase activity during inner ear development therefore seems primarily regulated at the level of protein synthesis. Ornithine decarboxylase activity correlates with major inductive events in the morphogenesis of the cartilagenous otic capsule that serves as a template for the formation of the bony labyrinth. The pattern of activity may reflect the changes in the head mesenchyme that is recruited by the otocyst to aggregate and form its protective otic capsule.

Type II collagen distribution in the ear of the guinea pig fetus

By using monoclonal antibodies to type II collagen and immunohistochemical techniques, we studied the distribution of type II collagen in the developing guinea pig ear. Type II collagen appearance and disappearance corresponded to cartilage development and resorption. Type II collagen was identified in Meckel's and Reichert's cartilages, the cartilage plate of the auricle and external acoustic meatus, the ossicles, eustachian tube cartilage, and the otic capsule. Type II collagen also appeared with the development of structures in noncartilaginous parts including the tympanic membrane, tympanic annulus, basilar membrane, spiral limbus, spiral ligament, and osseous spiral lamina, Rosenthal's canal, the maculae of the utricle and saccule, and the semicircular canal membrane, crista ampullaris, and endolymphatic duct. Type II collagen is distributed widely in the ear after the early stages of development. Thus, type II collagen should be considered an important structural component of the ear.

Transient expression of a chondroitin sulfate-related epitope during cartilage histomorphogenesis in the axial skeleton of fetal rats

A monoclonal antibody (MC21C), raised in mouse in response to a mixture of bone proteins, was found to exhibit a unique reactivity toward native chondroitin sulfate chains. Indirect immunofluorescence and immunoperoxidase assays were performed on tissue sections at different stages of fetal rat development, in order to investigate the distribution of the MC21C epitope during cartilage morphogenesis and differentiation. This extracellular marker was present in the sclerotome and its distribution subsequently followed the segmentation pattern of the precartilaginous vertebral column. In addition, changes in the MC21C-immunostaining pattern strongly correlated with the initial growth of the vertebrae. In the axial skeleton (spinal column, basis cranii), the immunostaining by MC21C was maximum in precartilaginous condensations and then rapidly disappeared during the process of chondrification. Also, the perinotochordal matrix was intensely immunostained.

Hyaluronate production by the inner ear during otic capsule and perilymphatic space formation

Otosclerosis has been hypothesized to result from a disorder of the extracellular matrix of the cartilaginous rests present in the adult temporal bone. Matrix relationship to bone formation and remodeling, as well as the fact that the pathogenesis of otosclerosis is expressed by the action of both of these processes, strongly suggests that more knowledge is needed about the process of otic capsule development. In pursuit of this goal, otic complexes were explanted from mouse embryos that ranged in age from 10.5 to 16 days old and were then exposed to 3H-glucosamine (50 microCi/ml) for 6 hours in vitro. Total labeled glycosaminoglycans (GAGs) and labeled hyaluronate content of each age group of otic explants were measured, and the results were compared to a developmental series of the otic regions of whole embryos stained with either toluidine blue or alcian blue. Increases in the synthesis of the total GAGs were observed on embryonic day 11 and for a prolonged period extending from gestation day 13.5 through day 16. The first increase of GAGs occurred at the initiation of metachromasia and positive staining by toluidine blue of the region of aggregated periotic mesenchyme cells that form the otic capsule. The second increase in GAGs was correlated with chondrification of the capsule. Hyaluronate production revealed a different pattern. Synthesis of hyaluronate exhibits peaks at 10.5, 12.5, and for an extended period of from 13.5 to 14.5 days of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)

Collagen type II in the otic extracellular matrix. Effect on inner ear development

Immunocytochemistry was used to demonstrate type II collagen distribution during normal development of the mouse inner ear and in two malformed inner ears. Patterns of inner ear abnormalities and type II collagen distribution were compared between the malformed labyrinth of a mouse mutation (disproportionate micromelia, Dmm) and otic explants exposed to the teratogenic action of an L-proline analog, L-azetidine-2-carboxylic acid (LACA). The results suggest that type II collagen is an important constituent of the developing inner ear's extracellular matrix. Disruptions of the spatial and temporal pattern of collagen type II can adversely affect morphogenesis of the inner ear. A common mechanism of action is postulated for the causation of both the genetic and teratogen-induced inner ear malformations (i.e. disruption of the secretion of collagens to the otic extracellular matrix).