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We have directly demonstrated that megakaryocytes are a major site of synthesis and storage of transforming growth factor-β1 (TGF/β1) by combined immunohistochemical, immunocytochemical, and in situ hybridization methods. The presence of TGF/β1 messenger RNA (mRNA) in mature megakaryocytes in adult rat spleen and bone marrow (BM) was established by in situ hybridization. Localization of TGF/β1 protein to intact α-granules of megakaryocytes, its putative storage site, was accomplished in glycol-methacrylate embedded porcine BM with an immunoperoxidase technique and light microscopy. The TGF/β1 was sequestered in intracytoplasmic granules in a pattern virtually identical to that of another α-granule marker protein, fibrinogen. This observation strongly suggests packaging of TGF/β1 into this organelle within megakaryocytes. That TGF/β1 mRNA was localized to megakaryocytes suggests that the TGF/β1 found in the α-granules in platelets originates with megakaryocyte synthesis. The α-granule localization of TGF/β1, as well as fibrinogen, was also demonstrated in isolated platelets at the ultrastructural level by electronmicroscopy (EM) and postembedding colloidal-gold immunocytochemistry, thus directly demonstrating that α-granules are the final storage site for TGF/β1 in mature platelets.

To date, three closely-related TGFβ genes have been found in the mouse; TGFβ1, TGFβ2 and TGFβ3. Previous experiments have indicated that TGFβ1 and TGFβ2 may play important roles during mouse embryo-genesis. The present study now reports the distribution of transcripts of TGFβ3 in comparison to the other two genes and reveals overlapping but distinct patterns of RNA expression. TGFβ3 RNA is expressed in a diverse array of tissues including perichondrium, bone, intervertebral discs, mesenteries, pleura, heart, lung, palate, and amnion, as well as in central nervous system (CNS) structures such as the meninges, choroid plexus and the olfactory bulbs. Furthermore, in several organ systems, TGFβ3 transcripts are expressed during periods of active morphogenesis suggesting that the protein may be an important factor for the growth and differentiation of many embryonic tissues.

Bone morphogenetic protein-2A (BMP-2A) is a member of the transforming growth factor β (TGF β) gene family that has been implicated in cartilage and bone formation. Here we use in situ hybridization to show that BMP-2A RNA is expressed in a variety of embryonic epithelial and mesenchymal tissues outside of the developing skeletal system, including cell populations known to play important roles in morphogenesis. Thus, high levels of transcripts are found in developing limb buds (ventral ectoderm and apical ectodermal ridge), heart (myocardium of the atrioventricular canal), whisker follicles (ectodermal placodes, hair matrix and precortex cells), tooth buds (epithelial buds, dental papilla and odontoblasts), and craniofacial mesenchyme, as well as a number of other sites. The expression patterns of BMP-2A are different from those of TGF β-1, -2 and -3, and this is illustrated in detail in the developing whisker follicles. These results suggest that BMP-2A plays multiple roles in morphogenesis and pattern formation in the vertebrate embryo.

We have studied the temporal and spatial expression of transforming growth factor β2 (TGF/β2) RNA in mouse embryos from 10·5 days post coitum (p.c.) to 3 days post partum (p.p.) by in situ hybridization analysis. TGFβ2 RNA is expressed in a variety of tissues including bone, cartilage, tendon, gut, blood vessels, skin and fetal placenta, and is in general found in the mesenchymal component of these tissues. The expression of TGF/12 RNA changes during development in a manner consistent with a role for the gene product in mediating mesenchymal-epithelial interactions.