Fate of the mammalian cranial neural crest during tooth and mandibular morphogenesis

Neural crest cells are multipotential stem cells that contribute extensively to vertebrate development and give rise to various cell and tissue types. Determination of the fate of mammalian neural crest has been inhibited by the lack of appropriate markers. Here, we make use of a two-component genetic system for indelibly marking the progeny of the cranial neural crest during tooth and mandible development. In the first mouse line, Cre recombinase is expressed under the control of the Wnt1 promoter as a transgene. Significantly, Wnt1 transgene expression is limited to the migrating neural crest cells that are derived from the dorsal CNS. The second mouse line, the ROSA26 conditional reporter (R26R), serves as a substrate for the Cre-mediated recombination. Using this two-component genetic system, we have systematically followed the migration and differentiation of the cranial neural crest (CNC) cells from E9.5 to 6 weeks after birth. Our results demonstrate, for the first time, that CNC cells contribute to the formation of condensed dental mesenchyme, dental papilla, odontoblasts, dentine matrix, pulp, cementum, periodontal ligaments, chondrocytes in Meckel's cartilage, mandible, the articulating disc of temporomandibular joint and branchial arch nerve ganglia. More importantly, there is a dynamic distribution of CNC- and non-CNC-derived cells during tooth and mandibular morphogenesis. These results are a first step towards a comprehensive understanding of neural crest cell migration and differentiation during mammalian craniofacial development. Furthermore, this transgenic model also provides a new tool for cell lineage analysis and genetic manipulation of neural-crest-derived components in normal and abnormal embryogenesis.

Suppression of polyglutamine-mediated neurodegeneration in Drosophila by the molecular chaperone HSP70

At least eight inherited human neurodegenerative diseases are caused by expansion of a polyglutamine domain within the respective proteins. This confers dominant toxicity on the proteins, leading to dysfunction and loss of neurons. Expanded polyglutamine proteins form aggregates, including nuclear inclusions (NI), within neurons, possibly due to misfolding of the proteins. NI are ubiquitinated and sequester molecular chaperone proteins and proteasome components, suggesting that disease pathogenesis includes activation of cellular stress pathways to help refold, disaggregate or degrade the mutant disease proteins. Overexpression of specific chaperone proteins reduces polyglutamine aggregation in transfected cells, but whether this alters toxicity is unknown. Using a Drosophila melanogaster model of polyglutamine disease, we show that directed expression of the molecular chaperone HSP70 suppresses polyglutamine-induced neurodegeneration in vivo. Suppression by HSP70 occurred without a visible effect on NI formation, indicating that polyglutamine toxicity can be dissociated from formation of large aggregates. Our studies indicate that HSP70 or related molecular chaperones may provide a means of treating these and other neurodegenerative diseases associated with abnormal protein conformation and toxicity.

CREB-binding protein sequestration by expanded polyglutamine

Spinal and bulbar muscular atrophy (SBMA) is one of eight inherited neurodegenerative diseases known to be caused by CAG repeat expansion. The expansion results in an expanded polyglutamine tract, which likely confers a novel, toxic function to the affected protein. Cell culture and transgenic mouse studies have implicated the nucleus as a site for pathogenesis, suggesting that a critical nuclear factor or process is disrupted by the polyglutamine expansion. In this report we present evidence that CREB-binding protein (CBP), a transcriptional co-activator that orchestrates nuclear response to a variety of cell signaling cascades, is incorporated into nuclear inclusions formed by polyglutamine-containing proteins in cultured cells, transgenic mice and tissue from patients with SBMA. We also show CBP incorporation into nuclear inclusions formed in a cell culture model of another polyglutamine disease, spinocerebellar ataxia type 3. We present evidence that soluble levels of CBP are reduced in cells expressing expanded polyglutamine despite increased levels of CBP mRNA. Finally, we demonstrate that over-expression of CBP rescues cells from polyglutamine-mediated toxicity in neuronal cell culture. These data support a CBP-sequestration model of polyglutamine expansion disease.

Evidence for proteasome involvement in polyglutamine disease: localization to nuclear inclusions in SCA3/MJD and suppression of polyglutamine aggregation in vitro

Spinocerebellar ataxia type 3, also known as Machado-Joseph disease (SCA3/MJD), is one of at least eight inherited neurodegenerative diseases caused by expansion of a polyglutamine tract in the disease protein. Here we present two lines of evidence implicating the ubiquitin-proteasome pathway in SCA3/MJD pathogenesis. First, studies of both human disease tissue and in vitro models showed redistribution of the 26S proteasome complex into polyglutamine aggregates. In neurons from SCA3/MJD brain, the proteasome localized to intranuclear inclusions containing the mutant protein, ataxin-3. In transfected cells, the proteasome redistributed into inclusions formed by three expanded polyglutamine proteins: a pathologic ataxin-3 fragment, full-length mutant ataxin-3 and an unrelated GFP-polyglutamine fusion protein. Inclusion formation by the full-length mutant ataxin-3 required nuclear localization of the protein and occurred within specific subnuclear structures recently implicated in the regulation of cell death, promyelocytic leukemia antigen oncogenic domains. In a second set of experiments, inhibitors of the proteasome caused a repeat length-dependent increase in aggregate formation, implying that the proteasome plays a direct role in suppressing polyglutamine aggregation in disease. These results support a central role for protein misfolding in the pathogenesis of SCA3/MJD and suggest that modulating proteasome activity is a potential approach to altering the progression of this and other polyglutamine diseases.

Uranium stabilization of c28: a tetravalent fullerene

Laser vaporization experiments with graphite in a supersonic cluster beam apparatus indicate that the smallest fullerene to form in substantial abundance is C(28). Although ab initio quantum chemical calculations predict that this cluster will favor a tetrahedral cage structure, it is electronically open shell. Further calculations reveal that C(28) in this structure should behave as a sort of hollow superatom with an effective valence of 4. This tetravalence should be exhibited toward chemical bonding both on the outside and on the inside of the cage. Thus, stable closed-shell derivatives of C(28) with large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps should be attainable either by reacting at the four tetrahedral vertices on the outside of the C(28) cage to make, for example, C(28)H(4), or by trapping a tetravalent atom inside the cage to make endothedral fullerenes such as Ti@C(28). An example of this second, inside route to C(28) stabilization is reported here: the laser and carbon-arc production of U@C(28).

The second BRCT domain of BRCA1 proteins interacts with p53 and stimulates transcription from the p21WAF1/CIP1 promoter

Inherited mutations in the breast and ovarian cancer susceptibility gene BRCA1 are associated with high risk for developing breast and ovarian cancers. Several studies link BRCA1 to transcriptional regulation, DNA repair, apoptosis and growth/tumor suppression. BRCA1 associates with p53 and stimulates transcription in both p53 dependent and p53-independent manners. BRCA1 splice variants BRCA1a (p110) and BRCA1b (p100) associates with CBP/p300 co-activators. Here we show that BRCA1a and BRCA1b proteins stimulate p53-dependent transcription from the p21WAF1/CIP1 promoter. In addition, the C-terminal second BRCA1 (BRCT) domain is sufficient for p53 mediated transactivation of the p21 promoter. Previous studies emphasized the importance of the BRCT domain, which shows homology with p53 binding protein (53BP1), in transcriptional activation, growth inhibition and tumor suppression. Our findings demonstrate an additional function for this domain in protein-protein interaction and co-activation of p53. We also found that BRCA1a and BRCA1b proteins interact with p53 in vitro and in vivo. The p53 interaction domain of BRCA1a/1b maps, in vitro, to the second BRCT domain (aa 1760-1863). The BRCT domain binds to the central domain of p53 which is required for sequence specific DNA binding. These results demonstrate for the first time the presence of a second p53 interaction domain in BRCA1 proteins and suggests that BRCA1a and BRCA1b proteins, like BRCA1, function as p53 co-activators. This BRCT domain also binds in vitro to CBP. These results suggest that one of the mechanisms by which BRCA1 proteins function is through recruitment of CBP/p300 associated HAT/FAT activity for acetylation of p53 to specific promoters resulting in transcriptional activation.

Heterozygous loss of Six5 in mice is sufficient to cause ocular cataracts

Myotonic dystrophy (DM) is an autosomal dominant disorder characterized by skeletal muscle wasting, myotonia, cardiac arrhythmia, hyperinsulinaemia, mental retardation and ocular cataracts. The genetic defect in DM is a CTG repeat expansion located in the 3' untranslated region of DMPK and 5' of a homeodomain-encoding gene, SIX5 (formerly DMAHP; refs 2-5). There are three mechanisms by which CTG expansion can result in DM. First, repeat expansion may alter the processing or transport of the mutant DMPK mRNA and consequently reduce DMPK levels. Second, CTG expansion may establish a region of heterochromatin 3' of the repeat sequence and decrease SIX5 transcription. Third, toxic effects of the repeat expansion may be intrinsic to the repeated elements at the level of DNA or RNA (refs 10,11). Previous studies have demonstrated that a dose-dependent loss of Dm15 (the mouse DMPK homologue) in mice produces a partial DM phenotype characterized by decreased development of skeletal muscle force and cardiac conduction disorders. To test the role of Six5 loss in DM, we have analysed a strain of mice in which Six5 was deleted. Our results demonstrate that the rate and severity of cataract formation is inversely related to Six5 dosage and is temporally progressive. Six5+/- and Six5-/- mice show increased steady-state levels of the Na+/K+-ATPase alpha-1 subunit and decreased Dm15 mRNA levels. Thus, altered ion homeostasis within the lens may contribute to cataract formation. As ocular cataracts are a characteristic feature of DM, these results demonstrate that decreased SIX5 transcription is important in the aetiology of DM. Our data support the hypothesis that DM is a contiguous gene syndrome associated with the partial loss of both DMPK and SIX5.

Gingivae contain neural-crest- and mesoderm-derived mesenchymal stem cells

Gingivae represent a unique soft tissue that serves as a biological barrier to cover the oral cavity side of the maxilla and mandible. Recently, the gingivae were identified as containing mesenchymal stem cells (GMSCs). However, it is unknown whether the GMSCs are derived from cranial neural crest cells (CNCC) or the mesoderm. In this study, we show that around 90% of GMSCs are derived from CNCC and 10% from the mesoderm. In comparison with mesoderm MSCs (M-GMSCs), CNCC-derived GMSCs (N-GMSCs) show an elevated capacity to differentiate into neural cells and chondrocytes and induce activated T-cell apoptosis in vitro. When transplanted into mice with dextran sulfate sodium (DSS)-induced colitis, N-GMSCs showed superior effects in ameliorating inflammatory-related disease phenotype in comparison with the M-GMSC treatment group. Mechanistically, the increased immunomodulatory effect of N-GMSCs is associated with up-regulated expression of FAS ligand (FASL), a transmembrane protein that plays an important role in MSC-based immunomodulation. In summary, our study indicates that the gingivae contain both neural-crest- and mesoderm-derived MSCs with distinctive stem cell properties.

Carbon ARC Generation of C60

Generation of C60 at a rate of more than 10 grams per day has been accomplished by operation of a carbon arc in an atmosphere of helium. Optimum yield of 15% was found to occur near 100–200 torr, but yields greater than 3% were found throughout the range between 50 and 760 torr. A model is proposed to explain the observed behavior based on competition between annealing of graphitic sheets to curve so that they minimize dangling bonds, and further rapid growth of these sheets in the gas phase to form giant fullerenes. In agreement with predictions of this model, laser vaporization of graphite targets was found to produce macroscopic quantities of C60 only when performed in an oven above 1000 C.

Specific transforming growth factor-beta subtypes regulate embryonic mouse Meckel's cartilage and tooth development

Members of the transforming growth factor-beta (TGF-beta) superfamily have emerged as critical regulators for cell growth and differentiation. Whereas the different TGF-beta subtypes are equipotent in the majority of biological assays using cell lines cultured in vitro, there are indications that in more complex systems involving epithelial-mesenchymal interactions, the TGF-beta subtypes differ in their biological activities. To test the hypothesis that TGF-beta subtypes specifically regulate either Meckel's cartilage or tooth morphogenesis, we designed experiments to compare loss of function effects of TGF-beta 1, TGF-beta 2, and TGF-beta 3 subtypes using a serumless, chemically defined medium to culture embryonic mouse E10 (42-44 somite pairs) mandibular explants. The major effect of loss of function resulting from abrogation of TGF-beta 1 using antisense treatment resulted in a 20% increase (P < 0.05) in chondrocyte number, a decrease in extracellular matrix, and dysmorphology of the rostral region of Meckel's cartilage. Exogenous TGF-beta 1 provided indistinguishable recovery to the normal phenotype. TGF-beta 2 antisense treatment produced a threefold enlargement (P < 0.05) of tooth organs and advanced their development to the cap stage. TGF-beta 2 provided recovery to the normal phenotype (e.g., reduced tooth size and development to the bud stage), whereas TGF-beta 1 or TGF-beta 3 polypeptides had no effect. TGF-beta 3 antisense treatment resulted in a reduction of approximately 15% in the length of Meckel's cartilage. We interpret these results to suggest that TGF-beta 1 functions to regulate the number of chondrogenic cells, the amount of extracellular matrix, and the rate of developmental assembly of the rostral to posterior segments in forming Meckel's cartilage. TGF-beta 2 appears to regulate tooth size and stage of development without affecting cartilage. TGF-beta 3 appears to regulate Meckel's cartilage size without altering tooth size or shape. The results are discussed in terms of the regulatory functions of the TGF-beta subtypes during embryonic craniofacial morphogenesis.

The mechanism of TGF-β signaling during palate development

Cleft palate, a malformation of the secondary palate development, is one of the most common human congenital birth defects. Palate formation is a complex process resulting in the separation of the oral and nasal cavities that involves multiple events, including palatal growth, elevation, and fusion. Recent findings show that transforming growth factor beta (TGF-β) signaling plays crucial roles in regulating palate development in both the palatal epithelium and mesenchyme. Here, we highlight recent advances in our understanding of TGF-β signaling during palate development.

The role of protein composition in specifying nuclear inclusion formation in polyglutamine disease

Intracellular inclusions are a unifying feature of polyglutamine (polyQ) neurodegenerative diseases, yet each polyQ disease displays a unique pattern of neuronal degeneration. This implies that the protein context of expanded polyQ plays an important role in establishing selective neurotoxicity. Here, in studies of the spinocerebellar ataxia type 3 disease protein ataxin-3, we demonstrate that the protein sequence surrounding polyQ specifies the constituents of nuclear inclusions (NI) formed by the disease protein. The nuclear proteins cAMP response element-binding protein-binding protein (CBP) and Mastermind-like-1 strongly colocalize only to NI formed by full-length ataxin-3, whereas the splicing factor SC35 colocalizes only to NI formed by a polyQ-containing, carboxyl-terminal fragment of ataxin-3. These differences in NI formation reflect specific protein interactions normally undertaken by ataxin-3, as both normal and mutant full-length ataxin-3 co-immunoprecipitate with CBP and sediment on density gradients as macromolecular complexes. Moreover, normal ataxin-3 represses cAMP response element-binding protein-mediated transcription, indicating a functional consequence of ataxin-3 interactions with CBP. Finally, we show that mutant ataxin-3 forms insoluble intranuclear complexes, or microaggregates, before NI can be detected, implying a precursor-product relationship. These results suggest that protein context-dependent recruitment of nuclear proteins to intranuclear microaggregates, and subsequently to NI, may contribute to selective neurotoxicity in polyQ diseases.

A homozygous microdeletion in helix 7 of the luteinizing hormone receptor associated with familial testicular and ovarian resistance is due to both decreased cell surface expression and impaired effector activation by the cell surface receptor

In this report, the genomic DNA was examined from two siblings with gonadal LH resistance. A 46,XY pseudohermaphrodite presented with female external genitalia and his 46,XX sister exhibited menstrual irregularities (oligoamenorrhea) and infertility. Exons 1-11 of the LH receptor (LHR) gene were amplified by the PCR using different sets of intronic primers and were directly sequenced. Sequencing revealed that both individuals carried a deletion of nucleotides 1822-1827, resulting in the deletion of Leu-608 and Val-609 within the seventh transmembrane helix. This mutation was introduced into a recombinant human (h) LHR cDNA. Transfections of 293 cells with hLHR(wt) vs. hLHR(deltaL608,V609) revealed that very little of the mutant receptor was expressed at the cell surface. This was due to both a decrease in the total amount of receptor expressed as well as to an increased intracellular retention of the mutant receptor. In spite of the decreased cell surface expression of the mutant, sufficient amounts were present to allow for assessment of its functions. Equilibrium binding assays showed that the cell surface hLHR(deltaL608,V609) binds hCG with an affinity comparable to that of the wild-type receptor. However, the cells expressing the hLHR(deltaL608,V609) exhibit only a 1.5- to 2.4-fold stimulation of cAMP production in response to hCG. In contrast, cells expressing comparably low levels of hLHR(wt) responded to hCG with 11- to 30-fold increases of cAMP levels. Therefore, the testicular and ovarian unresponsiveness to LH in these patients appears to be due to a mutation of the hLHR gene in which Leu-608 and Val-609 are deleted. As a consequence, the majority of the mutant receptor is retained intracellularly. The small percentage of mutant receptor that is expressed at the cell surface binds hormone normally but is unable to activate Gs.

TrlR, a defective TraR-like protein of Agrobacterium tumefaciens, blocks TraR function in vitro by forming inactive TrlR:TraR dimers

Octopine-type Ti plasmids of Agrobacterium tumefaciens require the quorum-sensing proteins TraR and TraI and the diffusible pheromone 3-oxooctanoyl homoserine lactone (AAI) to regulate genes required for conjugal transfer. TraR activity is inhibited by a protein called TrlR, which closely resembles amino acids 1-181 of TraR but is truncated as a result of a shift in the reading frame at codon 182. This frameshift does not affect synthesis of the amino-terminal domain, which is thought to bind autoinducer and mediate protein dimerization, but abolishes translation of the carboxyl-terminal, DNA-binding domain. In this study, we show that TrlR, like TraR, requires AAI for solubility when overexpressed in Escherichia coli. TrlR bound one molecule of AAI per protein monomer, supporting the prediction that the amino-terminal domain of TraR contains the AAI binding site. Purified TrlR blocked TraR for both specific DNA binding and transcription of a tra promoter, supporting previous studies performed with whole cells. When TrlR and a TraR fusion protein were co-expressed in E. coli, these proteins readily formed heterodimeric complexes that were inactive in DNA-binding activity. These data support the hypotheses that (i) the amino-terminal half of TraR binds AAI and mediates protein dimerization; (ii) both DNA-binding domains in a TraR dimer are required for stable DNA binding; and (iii) TrlR blocks TraR by direct protein-protein interactions.

Regulation of cell growth by oxidized LDL

The first reports of the influences of oxidized LDL (oxLDL) on cell function pertained to negative effects on cell growth-growth arrest, injury, and toxicity. Since these studies, it has become apparent that sublethal levels of oxLDL cause some, but not all, cells to proliferate. This review highlights the growth-promoting effects of oxLDL rather than its inhibitory or injurious effects. Smooth muscle cells (SMCs) and monocyte-macrophages proliferate after exposure to oxLDL; endothelial cells do not. Scavenger receptors are involved in the proliferative effects on monocyte-macrophages, whereas the effects of oxLDL on SMCs appear to be receptor independent. Lysophosphatidylcholine (lysoPC), and structurally related lipids are among the growth-promoting constituents of oxLDL. OxLDL exerts at least a part of its effects by inducing expression or causing the release of growth factors. OxLDL (or lysoPC) can cause the release of basic fibroblast growth factor (bFGF) from SMCs; oxLDL (or lysoPC) can induce heparin binding EGF-like growth factor (HB-EGF) synthesis and release from macrophages. An imposing array of changes in cytokine and growth factor expression and/or release can be imposed by oxLDL on a wide variety of cell types. These effects and the studies probing the cell signaling events leading to them are described.

TGF-beta receptor II in epithelia versus mesenchyme plays distinct roles in the developing lung

Transforming growth factor (TGF)-beta signalling plays important roles in regulating lung development. However, the specific regulatory functions of TGF-beta signalling in developing lung epithelial versus mesenchymal cells are still unknown. By immunostaining, the expression pattern of the TGF-beta type II receptor (TbetaRII) was first determined in the developing mouse lung. The functions of TbetaRII in developing lung were then determined by conditionally knocking out TbetaRII in the lung epithelium of floxed-TbetaRII/surfactant protein C-reverse tetracycline transactivator/TetO-Cre mice versus mesenchyme of floxed-TbetaRII/Dermo1-Cre mice. TbetaRII was expressed only in distal airway epithelium at early gestation (embryonic day (E)11.5), but in both airway epithelium and mesenchyme from mid-gestation (E14.5) to post-natal day 14. Abrogation of TbetaRII in mouse lung epithelium resulted in retardation of post-natal lung alveolarisation, with markedly decreased type I alveolar epithelial cells, while no abnormality in prenatal lung development was observed. In contrast, blockade of TbetaRII in mesoderm-derived tissues, including lung mesenchyme, resulted in mildly abnormal lung branching and reduced cell proliferation after mid-gestation, accompanied by multiple defects in other organs, including diaphragmatic hernia. The primary lung branching defect was verified in embryonic lung explant culture. The novel findings of the present study suggest that transforming growth factor-beta type II receptor-mediated transforming growth factor-beta signalling plays distinct roles in lung epithelium versus mesenchyme to differentially control specific stages of lung development.

Molecular and cellular regulatory mechanisms of tongue myogenesis

The tongue exerts crucial functions in our daily life. However, we know very little about the regulatory mechanisms of mammalian tongue development. In this review, we summarize recent findings of the molecular and cellular mechanisms that control tissue-tissue interactions during tongue morphogenesis. Specifically, cranial neural crest cells (CNCC) lead the initiation of tongue bud formation and contribute to the interstitial connective tissue, which ultimately compartmentalizes tongue muscles and serves as their attachments. Occipital somite-derived cells migrate into the tongue primordium and give rise to muscle cells in the tongue. The intimate relationship between CNCC- and mesoderm-derived cells, as well as growth and transcription factors that have been shown to be crucial for tongue myogenesis, clearly indicate that tissue-tissue interactions play an important role in regulating tongue morphogenesis.

TGF-beta signaling and its functional significance in regulating the fate of cranial neural crest cells

Members of the transforming growth factor-beta (TGF-beta) superfamily regulate cell proliferation, differentiation, and apoptosis, and control the development and maintenance of most tissues. TGF-beta signal is transmitted through the phosphorylation of Smad proteins by TGF-beta receptor serine/threonine kinase. During craniofacial development, TGF-beta may regulate the fate specification of cranial neural crest cells. These cells are multipotent progenitors and capable of producing diverse cell types upon differentiation. Here we summarize evidence that TGF-beta ligands and their signaling intermediates have significant roles in patterning and specification of cranial neural crest cells. The biological function of TGF-beta is carried out through the regulation of transcriptional factors during embryogenesis.

Overexpression of Smad2 reveals its concerted action with Smad4 in regulating TGF-beta-mediated epidermal homeostasis

Members of the transforming growth factor-beta (TGF-beta) superfamily are critical regulators for epithelial growth and can alter the differentiation of keratinocytes. Transduction of TGF-beta signaling depends on the phosphorylation and activation of Smad proteins by heteromeric complexes of ligand-specific type I and II receptors. To understand the function of TGF-beta and activin-specific Smad, we generated transgenic mice that overexpress Smad2 in epidermis under the control of keratin 14 promoter. Overexpression of Smad2 increases endogenous Smad4 and TGF-beta 1 expression while heterozygous loss of Smad2 reduces their expression levels, suggesting a concerted action of Smad2 and -4 in regulating TGF-beta signaling during skin development. These transgenic mice have delayed hair growth, underdeveloped ears, and shorter tails. In their skin, there is severe thickening of the epidermis with disorganized epidermal architecture, indistinguishable basement membrane, and dermal fibrosis. These abnormal phenotypes are due to increased proliferation of the basal epidermal cells and abnormalities in the program of keratinocyte differentiation. The ectodermally derived enamel structure is also abnormal. Collectively, our study presents the first in vivo evidence that, by providing an auto-feedback in TGF-beta signaling, Smad2 plays a pivotal role in regulating TGF-beta-mediated epidermal homeostasis.

Parenchymal cell proliferation and mechanisms for maintenance of granular duct and acinar cell populations in adult male mouse submandibular gland

To evaluate proliferation as a factor in maintenance of parenchymal cell populations in adult male mouse submandibular glands, a variety of surveys were conducted following a pulse with 3H-thymidine. Striated granular duct (SGD) cells had the highest labeling index, followed by intercalated duct (ID) cells, then acinar (AC) cells, and granular duct (GD) cells had the lowest. These cell types showed from 30% to 60% completion of mitosis by 24 hr, with SGD, AC, and GD showing a likely second wave of mitosis sometime between 2 and 7 days after the pulse. About 40% of the pulse-labeled cells still remained as single cells at 42 days after the pulse. Repeat divisions in daughter cells of the primary labeled cells were very rare. A shift in the pattern of labeled cells at the ID-GD junction indicates that ID and SGD cells in this compartment are differentiating to GD cells. Further comparison of the magnitude of this conversion with the amount of noncompartmental GD cell proliferation provided a basis for calculating that approximately 70% of GD cell population maintenance occurs by self-proliferation, and the remaining 30% is contributed by differentiation from ID and SGD cells. A similar survey at the ID-acinus junction showed no evidence of conversion of ID cells to AC cells indicating that most, if not all, proliferative activity leading to AC cell population maintenance occurs by self-proliferation. Finally, based in part on structural changes at the ID-GD junction during the survey period, a pattern of cell conversion described as "in situ differentiation" is proposed. When this pattern is carried to fruition, this explains several structural features of the secretory complex typical to the male pattern submandibular gland. The proposed mechanism is supported by a three-dimensionally reconstructed sequence of likely intermediate structures.