Development of the chick columella: immunohistochemical studies with anti-collagen monoclonal antibodies

Can you hear me now? Understanding vertebrate middle ear development

The middle ear is a composite organ formed from all three germ layers and the neural crest. It provides the link between the outside world and the inner ear, where sound is transduced and routed to the brain for processing. Extensive classical and modern studies have described the complex morphology and origin of the middle ear. Non-mammalian vertebrates have a single ossicle, the columella. Mammals have three functionally equivalent ossicles, designated the malleus, incus and stapes. In this review, I focus on the role of genes known to function in the middle ear. Genetic studies are beginning to unravel the induction and patterning of the multiple middle ear elements including the tympanum, skeletal elements, the air-filled cavity, and the insertion point into the inner ear oval window. Future studies that elucidate the integrated spatio-temporal signaling mechanisms required to pattern the middle ear organ system are needed. The longer-term translational benefits of understanding normal and abnormal ear development will have a direct impact on human health outcomes.

Localization of type X collagen in canine growth plate and adult canine articular cartilage

Type X collagen was extracted from ends of canine growth plates by pepsin digestion after 4 M guanidine hydrochloride extraction, purified by stepwise salt precipitation (2.0 M NaCl in 0.5 M acetic acid), and chromatographed on a Bio-Gel A1.5 M column in 1.0 M CaCl2. Without reduction on sodium dodecyl sulfate (SDS) polyacrylamide gels, the preparation yielded a single, high-molecular-weight (mol wt) band; after reduction, a single band of relative mol wt 5.0 x 10(4) was found. Polyclonal sera were raised against the purified collagen and used in the immunolocalization of canine type X collagen. As expected, indirect immunoperoxidase (IP) or indirect immunofluorescent staining with the polyclonal sera demonstrated that most of the immunoreactivity was localized in the zone of provisional calcification of the growth plate and in cartilage remnants in the metaphyseal region of the physis. A progressive decrease in staining toward the diaphysis of the fetal canine long bone was apparent as the trabecular structures were remodeled to bone. Unexpectedly, type X collagen was also detected in the zone of calcified, mature articular cartilage. It was concentrated in the pericellular matrix of the chondrocytes, appeared at or just above the tidemark, and was expressed immediately before mineralization. Identification of type X collagen in both the canine growth plate and the zone of calcified articular cartilage suggests that cells in the deep layer of cartilage and in the zone of calcified cartilage in the adult animal retain some characteristics of a growth plate and may be involved in regulation of mineralization at this critical interface. The expression of growth plate-like properties would allow the deep chondrocytes of mature articular cartilage to play a role in remodeling of the joint with age and in the pathogenesis of osteoarthritis.

Intrinsic and extrinsic controls of the hypertrophic program of chondrocytes in the avian columella

Immunohistochemical studies of the chick columella have shown that the extracellular matrix of this ossicular cartilage template is composed largely of type II collagen. As development proceeds, synthesis of type X collagen, a hypertrophic cartilage-specific molecule, is initiated by endochondral chondrocytes within the zone of cartilage cell hypertrophy. Subsequently, these cells and their surrounding extracellular matrix are removed, resulting in marrow cavity formation. We have examined which of these processes are programmed within the columella chondrocytes themselves, and which require involvement of exogenous factors. Prehypertrophic columella from 12-day chick embryos were grown either in organ culture on Nuclepore filters or as explants on the chorioallantoic membrane of host embryos. Chondrocytes from the same source were grown in monolayer cell cultures. In both organ culture and cell culture, chondrocytes developed to the stage at which some of them entered the hypertrophic program and initiated the production of type X collagen as determined by immunofluorescence histochemistry with a monoclonal antibody specific for that collagen type. The organ cultures, however, did not progress to the next stage, in which detectable removal of the type X collagen-containing matrix occurs. When identical columella were grown on the chorioallantoic membrane of host chicks, the type X collagen-containing matrix which formed was rapidly removed, resulting in the formation of a marrow cavity. Thus, progression of endochondral chondrocytes to the deposition of type X collagen-containing matrix seems to be programmed within the cells themselves. Subsequent removal of this matrix requires the involvement of exogenous factors.