Transforming growth factor-beta 2 and TGF-beta 3 regulate fetal rat cranial suture morphogenesis by regulating rates of cell proliferation and apoptosis

Cranial vault sutures are the major intramembranous bone growth sites during rapid expansion of the neurocranium. To function as bone growth sites, sutures need to remain patent, while allowing rapid bone formation at the edges of the bone fronts. Premature osseous obliteration of sutures (craniosynostosis) by fusion of bone fronts across the suture site prevents further bone formation at this site, often leading to severe facial dysmorphology. Although several growth factor receptor and transcription factor mutations have been implicated in craniosynostosis, the underlying mechanisms leading to sutural obliteration remain unclear. Previous studies have shown that dura secreted soluble factors responsible for maintaining suture patency and that suture fusion observed in the absence of dura was preceded by elevated levels of DNA synthesis and collagen production in the suture region. The use of neutralizing antibodies in a fetal calvarial culture model further demonstrated that removal of transforming growth factor (TGF) -beta 3 activity induced premature sutural obliteration, whereas removal of TGF-beta 2 activity prevented sutural obliteration. Data presented here demonstrate that suture obliteration induced by removal of TGF-beta 3 activity was preceded by elevated levels of DNA synthesis, similar to that seen upon removal of the dura. Addition of exogenous TGF-beta 3 to calvaria cultured without dura both prevented suture obliteration and reduced DNA synthesis to levels comparable to those seen with intact dura. Addition of exogenous TGF-beta 2 to calvarial cultures induced sutural fusion accompanied by elevated levels of cell proliferation. However, sutures rescued from obliteration by removal of TGF-beta 2 activity did not have decreased levels of cell proliferation, but rather appeared to be due to inhibited differentiation. In all calvaria in which sutures remained patent in culture, numbers of apoptotic cells were high within the suture, whereas in sutures destined to fuse, numbers of apoptotic cells were low. Results indicate that one of the critical regulators of suture patency is cell number. Alterations in cell number can trigger premature differentiation of cells, resulting in sutural obliteration. Furthermore, a complex interplay between closely related molecules is required to maintain cranial vault sutures in an unossified state, while allowing new bone to be formed at the edges of the bone fronts.

Hyaluronan is essential for the expansion of the cranial base growth plates

Exquisite control of chondrocyte function in the zone of hypertrophy results in expansive growth of cartilaginous growth plates, and is a prerequisite for normal skeletal lengthening. We hypothesize that hyaluronan-mediated hydrostatic pressure causes lacunae expansion in the zone of hypertrophy; an important mechanism in cartilaginous growth plate and associated skeletal expansion. The role of hyaluronan and CD44 in this mechanism was studied using organ culture of the bipolar cranial base synchondroses. Hyaluronan was present in the hypertrophic zones, pericellular to the hypertrophic chondrocytes, while no hyaluronan was detected in the resting, proliferating and maturing zones. This localization of hyaluronan was associated with increased lacunae size, suggesting that chondrocytes deposit and retain pericellular hyaluronan as they mature. In comparison, Toluidine Blue staining was associated with the territorial matrix. Hyaluronidase, the hyaluronan-degrading enzyme, and CD44, the receptor for hyaluronan which also participates in the uptake and degradation of hyaluronan, were co-localized within the zone of ossification. This pattern of expression suggests that cells in the early zone of ossification internalize and degrade hyaluronan through a CD44-mediated mechanism. Treatment of the cultured segments with either Streptomyces hyaluronidase or hyaluronan hexasaccharides inhibited lacunae expansion. These observations demonstrate that hyaluronan-mediated mechanisms play an important role in controlling normal skeletal lengthening.

A pilot study of short- and long-term sequelae to rigid fixation across metacarpal physes in a baboon model

The use of rigid fixation for fractures of the extremities has become commonplace. The short- and long-term effects of rigid fixation on the growing hand, however, have not been studied thoroughly. In this project, the use of rigid fixation across metacarpal growth plates (physes) in growing primate hands was examined. The hypothesis to be tested was that long-term placement of rigid fixation devices across physes during stabilization of mid-shaft osteotomies will cause the physes to close prematurely. Fixation devices with screws placed in the epiphysis and left in place for 4 months or 1 year resulted in open physes, in support of the null hypothesis. However, in physes plated for 1 year, biochemical changes associated with increased bone differentiation were apparent. Findings suggest that rigid fixation across physes for as long as 1 year can be used appropriately in growing individuals when necessary. However, until additional investigation establishes whether the open physes are still capable of producing bone-lengthening hypertrophic chondrocytes, caution should be used in long-term placement of rigid fixation devices.

Cranial suture obliteration is induced by removal of transforming growth factor (TGF)-beta 3 activity and prevented by removal of TGF-beta 2 activity from fetal rat calvaria in vitro

Cranial suture morphogenesis requires soluble, heparin-binding factors secreted by the dura mater to resist premature osseous obliteration. Elevated levels of transforming growth factor (TGF)-beta 1, TGF-beta 2, and TGF-beta 3 have previously been noted in cranial sutures undergoing normal and premature sutural obliteration. To examine the role of TGF-beta s in regulating cranial suture morphogenesis, an established in vitro, serum-free, calvarial culture system was used. In this system, fetal rat coronal sutures undergo apparently normal suture morphogenesis in the presence of dura mater, but undergo osseous obliteration in the absence of dura mater. Neutralizing polyclonal antibodies to TGF-beta 1, TGF-beta 2, or TGF-beta 3 were added to cultures of fetal day 19 rat calvaria, which were harvested at 3, 4, or 5 days, processed for histology, sectioned, and examined. Coronal sutures from calvaria cultured in the presence of dura mater resisted obliteration, either alone or in the presence of TGF-beta 1 or TGF-beta 2 neutralizing antibodies. However, sutures from calvaria cultured in the presence of TGF-beta 3 neutralizing antibodies became obliterated. Conversely, sutures from calvaria cultured in the absence of dura mater became obliterated by bone, either alone or in the presence of neutralizing antibodies to TGF-beta 1 or TGF-beta 3. However, those sutures cultured in the presence of neutralizing antibodies to TGF-beta 2 were rescued from osseous obliteration.

Dura mater maintains rat cranial sutures in vitro by regulating suture cell proliferation and collagen production

Craniosynostosis, the premature osseous obliteration of cranial vault sutures, can result from mutations in genes encoding components of growth factor signaling systems or the extracellular matrix (ECM). Little is known of the capacity of osteoprogenitor cells of the cranial sutures to divide or to synthesize ECM in situ. Osteoblasts derived from patients with prematurely fused sutures were reported to express alkaline phosphatase and osteocalcin at elevated levels, while proliferating at a rate comparable to control cells [DePollack et al., JBMR, 1996]; however, the suture osteoprogenitors, the population most likely to show proliferative abnormalities, were not present in the fused sutures used for this study. A model in which rat coronal sutures and associated bones develop normally in vitro, but in which sutures can be induced to fuse in the absence of dura mater, was used to examine cell proliferation and total protein synthesis in unfused sutures cultured in the presence of dura mater or in sutures induced to fuse in the absence of dura mater. Significantly increased cell proliferation was seen in suture cells prior to sutural obliteration, which returned to control levels as sutural fusion proceeded. Collagen synthesis in fusing sutures was elevated compared to non-fusing sutures and comparable to that seen in bone. Results indicated that in the absence of intercellular signals provided by the dura mater, suture cell proliferation increased initially, followed by increased synthesis of collagenous ECM within the suture and subsequent osseous obliteration of the suture. Thus factors originating in the dura mater affected suture cell proliferation and ECM production and were required for the maintenance of suture patency.

Elevated levels of transforming growth factors beta 2 and beta 3 in lambdoid sutures from children with persistent plagiocephaly

OBJECTIVE

To analyze the pertinent history and physical findings specific to the subset of patients with a progressive posterior skull deformity, requiring surgery to correct their deformity.

PATIENTS

Since the Academy of Pediatrics issued its recommendation on supine positioning of infants to prevent sudden infant death syndrome (SIDS) in 1992, 73 children have presented to the University of Virginia Craniofacial Anomalies Clinic with posterior-skull deformities. The majority were successfully managed with conservative therapy, but in six patients, the deformity was severe and persistent, requiring surgical correction. All six children were older (7.5-12 mo), presenting with more severe morphologic appearances and a higher incidence of associated neurodevelopmental delay. Three had family backgrounds of isolated craniosynostosis.

METHODS

Characteristics of these patients were examined to determine why they may have differed from those that responded to conservative management. Immunohistochemical staining of their lambdoid sutures was performed.

RESULTS

Significantly increased staining for TGF-beta 2 and TGF-beta 3, potent stimulators of bone cell growth and differentiation, was seen in all 'affected' sutures from the flattened side of the skull, compared to unaffected sutures from the protruding side of the skull-a pattern similar to that seen during normal bony obliteration of calvarial sutures.

CONCLUSION

The majority of patients with posterior plagiocephaly associated with positioning responded to conservative management, while a small subset of patients with persistent posterior skull deformation required surgical intervention. A genetic basis for the latter patients' persistent plagiocephaly, rather than positioning, cannot be ruled out. Genetics, prolonged external pressure against the sutures, or a combination of these factors may lead to permanently raised levels of growth factors in 'affected' sutures.

TGF-beta 1, TGF-beta 2, and TGF-beta 3 exhibit distinct patterns of expression during cranial suture formation and obliteration in vivo and in vitro

Cranial sutures function as bone growth centers while themselves remaining unossified. Rat frontonasal sutures become obliterated by neonatal day 21 (N21), while coronal sutures do not fuse over the life of the animal. Coronal sutures induced to undergo osseous obliteration in vitro after removal of the dura mater were found to require soluble, heparin-binding factors present in dura mater to resist osseous obliteration. Transforming growth factor beta 1 (TGF-beta 1), beta 2, and beta 3, heparin-binding factors known to regulate bone cell proliferation and differentiation, were considered likely candidates. The presence and distribution of these factors in calvarial tissues both in vivo and in vitro were established by immunohistochemical analysis, while reverse transcription followed by polymerase chain reaction (RT/PCR) was employed to determine the presence of transcripts for these factors in mRNA isolated from microdissected dura mater. Results indicated that the presence of TGF-beta 1 and TGF-beta 2 were associated with developing coronal and frontonasal sutures, and that the continued presence of these factors was associated with osseous obliteration of the frontonasal suture. However, increased TGF-beta 3 immunoreactivity was associated with the coronal suture remaining unossified. RT/PCR demonstrated the presence of transcripts for TGF-beta 1, beta 2, and beta 3 in dural tissues isolated from rat calvaria. These data support the notion of a role for TGF-beta s in regulating cranial suture morphogenesis and establish the in vitro model as a valid system for examining mechanisms by which growth factors regulate both suture morphogenesis and bone growth at the suture site.

Cranial sutures require tissue interactions with dura mater to resist osseous obliteration in vitro

A chemically defined serum-free medium, which supports the development of bones and fibrous tissues of rat calvaria from nonmineralized mesenchymal precursor tissues, was employed to investigate tissue interactions between the dura matter and overlying tissues. Fetal calvarial rudiments from stages prior to bone and suture morphogenesis (fetal days 19 and 20) and neonatal calvarial rudiments with formed sutures (day 1) were cultured with and without associated dura mater. Removal of calvaria for in vitro culture allowed the examination of suture morphogenesis in the absence of tensional forces exerted on the sutures via fiber tracts in the dura mater originating in the cranial base. Ossification of frontal and parietal bones proceeded in a fashion comparable to development in vivo, but the cranial (coronal) sutures--primary sites for subsequent skull growth--were obliterated by osseous tissue union in the absence of dura mater. Bony fusion did not occur when rudiments were cocultured with dura mater on the opposite sides of 0.45 microns polycarbonate transwell filters, suggesting that the influence of dura mater on sutural obliteration was mediated by soluble factors rather than cell-cell or cell-matrix interactions. These results indicate that cell signaling mechanisms rather than biomechanical tensional forces are required for morphogenesis of the calvaria.

Repair of critical size rat calvarial defects using extracellular matrix protein gels

In this study the authors examined the capacity of gels of reconstituted basement membrane, laminin, and type I collagen to mediate repair of critical size defects in rat calvaria. Although autografts are widely used to repair bone defects caused by trauma or surgical treatment of congenital malformations, neoplasms, and infections, an adequate quantity of graft is not always available. Allogenic bone is readily available, but its use is associated with an increased incidence of nonunion, fatigue fracture, and rejection. Biologically active, purified components of basement membranes, which have been shown to promote osteogenic differentiation and angiogenesis in vitro and type I collagen (the major constituent of bone extracellular matrix) can be formed into native isotonic space-filling gels. In this study critical size calvarial defects were created in retired male Sprague-Dawley rats. Thirty-six animals were divided into seven groups. Group 1 (control) received no treatment for the defects. Group 2 animals were implanted with methylcellulose. Groups 3, 4, 5, and 6 were implanted with gels of type I collagen, reconstituted basement membrane, or laminin, respectively. The last group of three animals (Group 7) was implanted with 100 micrograms of type I collagen gels (identical to Group 3) and sacrificed at 20 weeks following a single CT scan to determine if complete healing could be obtained with this method given sufficient time. Except for rats in the type I collagen group that was evaluated by multiple computerized tomography (CT) scans biweekly from 2 to 12 weeks, bone repair was evaluated using CT at 12 weeks. Healing was quantified using three-dimensional reconstruction of CT. Following the final CT scan in each experimental group, animals were sacrificed, and a sample of tissues was evaluated by conventional histology. Animals treated with type I collagen gels showed 87.5% repair of the area of the defects at 12 weeks and 92.5% repair by 20 weeks. Increasing the gel volume 1.5 x accelerated complete repair to 3 months. Murine-reconstituted basement membrane and laminin gels induced 55.5% and 46.3% repair, respectively, at 3 months. In untreated control animals 7% repair of the area of the defects showed at 3 months. Histological analysis confirmed new bone formation in partial and completely healed defects. Bioengineered native collagen gels may have wide applicability for bone repair as an alternative bone graft material alone, in combination with autograft or marrow aspirate, or as a delivery system for osteogenic growth factors.

In the absence of periosteum, transplanted fetal and neonatal rat coronal sutures resist osseous obliteration

Normal craniofacial development depends on expansion of the cranial vault by growth at the sutures. Inappropriate development of the sutures leads to global disruption of patterns of craniofacial growth. Tissue interactions between dura mater and suture matrix play a critical role in the phenotypic maintenance of cranial sutures. However, the function of the periosteum in this process remains under-reported and controversial. To examine the contribution of periosteum in maintaining the patency of coronal sutures, fetal and neonatal rat coronal sutures were transplanted to surgically created defects in adult rat host parietal bones. These sutures were examined for their ability to persist in the host milieu in the presence and absence of both donor and host periosteum. This study established that removal of both host and transplant periosteum, unlike removal of dura mater, did not lead to obliteration of either fetal or neonatal sutures. Thus, periosteum and dura mater are nonequivalent tissues with respect to influence on suture patency.

Subterranean mole-rats naturally have an impoverished calciol status, yet synthesize calciol metabolites and calbindins

Mole-rats (Family Bathyergidae) have no obvious source of calciol. They live in an environment devoid of sunlight and consume a herbivorous diet. Calciol status, metabolism and expression were examined in six species of Bathyergids. Serum levels of calcidiol in all species were < 5 micrograms/l and those of calcitriol were low (18.0 +/- 11.0 (SD) ng/l, N = 57) when compared to other rodents. Within 72 h of injecting animals with tritium-labelled calciol, most of the labelled prohormone had been metabolized to more polar metabolites. Three times more tritium-labelled calcitriol (19.3 +/- 2.9%) was present than (24R)-hydroxycalcidiol (6.2 +/- 10%). The natural absence of detectable circulating concentrations of calcidiol and the threefold greater amount of calcitriol to (24R)-hydroxycalcidiol produced indicate that calciol naturally is in short supply. Calciol-dependent calbindins were absent in the duodenum. Calbindin-D28k was present in the Purkinje cells of the cerebellum and in some collecting ducts and proximal and distal convoluted tubules of the kidney. Calbindin-D9k also was present but was localized uniquely in the juxtaglomerular cells of the five southern African species. These data confirm that Bathyergid mole-rats naturally have an impoverished calciol status. Despite the presence of calbindins in renal tissues, the functional importance of this hormone in calbindin synthesis and other normal mole-rat physiology is not known.

Tissue interactions with underlying dura mater inhibit osseous obliteration of developing cranial sutures

Cranial sutures play a critical role in calvarial morphogenesis, serving as growth centers during skull development. Both biomechanical tensile forces originating in the cranial base and biochemical factors present in dura mater have been postulated as determinants of suture morphogenesis and patency. A rat transplant model free of the putative biomechanical influence of the dura and cranial base was used to investigate the role of the dura mater in both the initial morphogenesis and maintenance of sutures during skull growth. Day 19 fetal presumptive (F19) and day 1 neonatal differentiated (N1) coronal sutures, including associated frontal and parietal bones, were transplanted with or without underlying dura mater to the center of adult parietal bones. After 1, 2, and 3 weeks, transplanted tissues were examined histologically and histomorphometrically to determine whether sutures formed and whether they were obliterated by ossification in the absence of dura mater. Both F19 and N1 sutures remained patent for 2 weeks either in the presence or the absence of transplant dura mater. However, at 3 weeks, in the absence of transplant dura mater, sutures were obliterated by bone, while in the presence of dura mater sutures resisted ossification, demonstrating an essential requirement for interactions with the transplant dura mater in maintaining functional sutures. Both F19 and N1 transplants showed comparable bone growth (cross-sectional surface area), regardless of the presence of transplant dura mater. These experiments suggest that tissue interactions of a biochemical nature, rather than biomechanical forces generated through the cranial base, are required to maintain the suture as a non-ossified growth center. Furthermore, while the presence of dura mater was essential for maintenance of suture patency, fetal dura mater was not required for initial suture formation.

Ontogeny of calbindin-D28K and calbindin-D9K in the mouse kidney, duodenum, cerebellum and placenta

The appearance of the calcium-binding proteins (CaBP-D28K and CaBP-D9K) in embryonic mice tissues was determined using a sensitive immunohistochemical assay. CaBP-D28K first appears in myenteric nerve plexuses of the duodenum on day E15, in duodenal villus cells on day E16, in Purkinje cells of the cerebellum on day E19, in cells of the mesonephric duct on day E11 and in the metanephric duct on day E12. CaBP-D9K first appears in enterocytes of the duodenum on day E18, in trophoblastic giant cells (TGC) of the placenta on day E10, and in the metanephric duct on day E15. A differential time of appearance and colocalization of the two CaBPs is demonstrated in the embryonic mouse kidney, suggesting either that vitamin D does not control both CaBPs in the foetus or that the vitamin D control is unequal. The early appearance and location of CaBP-D9K in TGCs may suggest that these cells play an important role in transplacental transfer of calcium.

Noncytoplasmic and filamentous appearance of calbindin-D28k and tubulin in double, indirect immunofluorescent staining of embryonic chick tissue

Double indirect immunofluorescent labeling of embryonic chick tissue was undertaken for the vitamin D-induced calcium binding protein, calbindin-D28k, and microtubules. Immunoreactivities for both calbindin-D28k and tubulin were found to exhibit a filamentous staining pattern in mesonephros, metanephros, intestine, and brain. In the intestine, staining was absent at 19 days, while immunolabeling of tubulin became evident at 20 days, and both antigens were present in 21-day tissue. In intestinal epithelium, as well as in 10-day metanephros, it was strikingly evident that cells either stained for both antigens or were negative for both calbindin-D28k and tubulin. In 11-12-day metanephros, an increased number of cells with both immunoreactivities were found. In 15-17-day brain, tubulin was evident within all cells but stained most intensely in Purkinje cells which were also positive for calbindin-D28k. Mesonephros from 4-5-day embryos revealed immunolabeling of both tubulin and the calcium binding protein. A statistical analysis of the various cell types revealed that the vast majority contained either both antigens or neither of the immunoreactivities. Of the more than 600 cells scored, none were found to be positive for calbindin-D28k, while at the same time negative for tubulin. It is concluded that calbindin-D28k exhibits a noncytoplasmic distribution in all tissues tested and that the filamentous appearance may reflect localization of the antigen in tubulo-vesicular organelles associated with cytoskeleton.

Estrogen inhibits calbindin-D28k expression in mouse uterus

The cellular localization and hormonal controls of calbindin-D9k expression in the rodent reproductive tract have suggested new functions for this protein. The present studies were undertaken to extend the earlier studies of calbindin-D9k to the related protein, calbindin-D28k. Immunohistochemical studies revealed that calbindin-D28k was absent from female rat reproductive tissues, but was abundantly expressed in immature mouse uterus and oviduct. Immunoreactivity was restricted to the endometrial and glandular epithelium of the uterus and the oviductal epithelium. Neither 1,25-dihydroxyvitamin D- nor strontium-containing diets (to blunt 1,25-dihydroxyvitamin D production) affected expression of calbindin-D28k. Uterine, but not oviductal, calbindin-D28k decreased markedly at sexual maturity; this pattern persisted in pregnant mice and was reproduced in immature mice by the administration of estradiol (3 micrograms/day for 3 days). RNA extraction and Northern analyses demonstrated that estrogen markedly decreased calbindin-D28k mRNA abundance in the uterus, but not in the oviduct. These findings suggest that estrogen affects mammalian calbindin-D28k expression and represent a rare example of estrogen-induced down-regulation of gene expression.

Immunohistochemical localization of calbindins (28K and 9K) in the tissues of the baboon Papio ursinus

An indirect immunoperoxidase procedure was used to detect the presence of calbindin-D28K and calbindin-D9K in the cerebellum, kidney, and duodenum of the baboon Papio ursinus. Antibodies to chick calbinding-D28K and to both rat and mouse calbindin-D9K were used. The cerebellum and kidney were shown to contain calbindin-D28K; the doudenum contained calbindin-D9K. In the cerebellum, positive staining was found in the Purkinje cells only; in the kidney, positive staining was found in the distal convoluted tubules, connecting tubules, and collecting tubules, extending deep into the medullary regions of the kidney. Staining in the duodenum was confined to the enterocytes of the villi, with no stain present in the crypt regions or goblet cells. Thus the baboon, a primate, contains the larger of the calbindins in both the cerebellum and kidney as does the human and monkey, but its distribution in the kidney is more generalized than that found in humans. The molecular weight of calbindin-D9K was found to be similar to that found in other animals. However, the calbindin-D28K from the baboon tissues appears to be slightly smaller than the protein found in other animals and may therefore be of similar size to the human calbindin-D28K (Mr 26,000).

Appearance during chick embryogenesis of vitamin D-dependent calcium-binding protein (calbindin-D28K)

The appearance of the vitamin D-dependent calcium-binding protein (calbindin-D28K) in embryonic chick tissues was determined using a sensitive immunohistochemical assay to elucidate the possible role of calbindin-D28K during embryogenesis. This protein first occurs in renal, cerebellar and intestinal cells during the final stages of maturation and/or differentiation. It is during this period of their development that these tissues become functional. On the basis of our observations, we suggest that the onset of cell function may be coupled with the appearance of calbindin-D28K.

Induction of supernumerary tracheal buds and the stimulation of DNA synthesis in the embryonic chick lung and trachea by epidermal growth factor

Epidermal growth factor (EGF) has been found to stimulate DNA synthesis in both the trachea and bronchial tree of 5-day-old chick embryo lung rudiments in organ culture. After 20 h culture in the presence of 10 ng/ml EGF, the incorporation of tritiated thymidine into DNA is stimulated two- to three-fold following a 2 h labelling period, as revealed by scintillation counting. Autoradiographic data indicate that this stimulation is most marked in the epithelial tissue component of both the trachea and bronchial tree. Supernumerary "lung" buds have been induced in the normally unbranched tracheal epithelium by agarose pellets containing EGF, such buds having been previously induced only by grafting a variety of mesenchymal tissues alongside the tracheal epithelium. Since EGF has been shown to be a potent stimulator of tracheal DNA synthetic activity it is suggested that the induction of supernumerary buds by the EGF-agarose pellets is achieved through a localized stimulation of cell proliferation in the tracheal epithelium. These data would further suggest that the induction of supernumerary tracheal buds by various mesenchymal tissues is similarly due to a localized increase in mitotic activity resulting from the action of some mitotic stimulator substance(s) emanating from the inducing mesenchymal tissue. This conclusion may be extended to include normal bud formation which occurs during branching morphogenesis in several developing organ systems.

Induction of supernumerary tracheal buds and the stimulation of DNA synthesis in the embryonic chick lung and trachea by epidermal growth factor

Epidermal growth factor (EGF) has been found to stimulale DNA synthesis in both the trachea and bronchial tree of 5-day-old chick embryo lung rudiments in organ culture. After 20 h culture in the presence of 10 ng/ml EGF, the incorporation of tritiated thymidine into DNA is stimulated two- to three-fold following a 2 h labelling period, as revealed by scintillation counting. Autoradiographic data indicate that this stimulation is most marked in the epithelial tissue component of both the trachea and bronchial tree. Supernumerary ‘lung’ buds have been induced in the normally unbranched tracheal epithelium by agarose pellets containing EGF, such buds having been previously induced only by grafting a variety of mesenchymal tissues alongside the tracheal epithelium. Since EGF has been shown to be a potent stimulator of tracheal DNA synthetic activity it is suggested that the induction of supernumerary buds by the EGF-agarose pellets is achieved through a localized stimulation of cell proliferation in the tracheal epithelium. These data would further suggest that the induction of supernumerary tracheal buds by various mesenchymal tissues is similarly due to a localized increase in mitotic activity resulting from the action of some mitotic stimulator substance(s) emanating from the inducing mesenchymal tissue. This conclusion may be extended to include normal bud formation which occurs during branching morphogenesis in everal developing organ systems.