Thrombospondin‐4 is expressed by early osteogenic tissues in the chick embryo

Abstract

The thrombospondins are a family of related glycoproteins found in the embryonic extracellular matrix. To date, five members of this family have been identified. Thrombospondin‐1 and thrombospondin‐2 have similar primary structure, but are expressed in different tissues at different times during development. Thrombospondins‐3, ‐4, and cartilage oligomeric protein belong to a second thrombospondin subgroup in which the carboxyl‐half of each molecule is most similar to thrombospondin‐1 and ‐2. Here, we report the cloning and sequencing of a novel probe to avian thrombospondin‐4. We have used this probe to determine the origins of thrombospondin‐4 in the chick embryo by in situ hybridization. Thrombospondin‐4 transcripts first appear in the mesenchyme surrounding bone anlage coinciding with the initial stages of osteogenesis. The expression in osteogenic tissues is transient: thrombospondin‐4 mRNAs are not seen in the osteoblasts of bone collars in developing long bones. This pattern is distinct from avian thrombospondin‐2, which is expressed in perichondrium and embryonic fibrous connective tissues. Our observations indicate that connective tissues are the principal site of thrombospondin‐4 expression in the chick. The diverse origins of different thrombospondin gene family members imply distinctive roles for these proteins related to the growth and differentiation of cartilage, tendons, and bone. ©1995 Wiley‐Liss, Inc.