CD44 expression in the developing and growing rat intervertebral disc.
CD44 has been identified at the time of extracellular matrix formation and expansion in several sites of the developing embryo (Wheatley et al. [1993] Development 119:295-306). The nucleus pulposus, consisting of a hydrated extracellular matrix tissue at birth, not previously closely analyzed, was examined for expression of CD44 in the developing and aging rat intervertebral disc. CD44 was identified solely on notochordal cells from the first onset of intervertebral disc formation (day 15 embryo) through the loss of notochordal cells from the nucleus pulposus (12-24 months of age). No CD44 expression was found in the notochordal cells prior to disc formation or in any cells other than the notochordal cells in the annulus fibrosus or nucleus pulposus of the intervertebral disc. Using reverse transcriptase-polymerase chain reaction methodology, the single 365 amino acid CD44 standard, CD44s, open reading frame was amplified from notochordal cells isolated from the nucleus pulposus. Western blot analysis of a cultured nucleus pulposus notochordal cells total protein extract identified a single CD44 species devoid of chondroitin sulfate with a mass of approximately 85 kDa, characteristic of CD44s. Cell surface detection for CD44 was co-localized with hyaluronan and proteoglycans at first appearance of disc formation in the nucleus pulposus.