SEARCH · PubMed Health

Results for “segmentation clock”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18Linked to original sources

PubMed1999

Concordant upregulation of type II-TGF-beta-receptor, the cyclin-dependent kinases inhibitor P27Kip1 and cyclin E in human atherosclerotic tissue: implications for lesion cellularity.

Atherosclerosis is a 'response-to-injury' process associated with chronic inflammation, tissue repair and a considerable cell turnover. These growth-related processes are controlled by the 'cell cycle clock' which is composed of cyclin-dependent kinases (Cdks), their activating subunits, the cyclins, and by inhibitors of Cdks (Ckis). P27 is a Cki which associates with cyclin A-Cdk2, cyclin D-Cdk4 and with cyclin E (CE)-Cdk2 complexes thereby abrogating their catalytic activity leading to potent inhibition of late G1 to S-phase transition. Furthermore, TGF-beta1 mRNA and immunoreactivity are locally increased in atherosclerotic lesions. Since TGF-beta1 growth suppressive function in the late G1 phase may be mediated by p27, blocking the catalytic activity of CE-Cdk2 complexes, via the stimulation of TGF-beta-RI and TGF-beta-RII, we investigated the topographical association between TGF-beta-RI, TGF-beta-RII, P27Kip1 and CE by immunohistochemistry in coronary artery segments without atherosclerosis and carotid atheromatous plaques of 11 patients undergoing carotid endarterectomy. P27-immunoreactivity was present in 11/11 atherosclerotic (92.7 +/- 3.3% of the cells) and 5/5 control (80.9 +/- 3.7% of the cells; P < 0.002 versus control) specimens and localized to nuclei of macrophages (CD68-positive), vascular smooth muscle cells (alpha-actin positive), T-lymphocytes (CD3-positive) as well as to the nuclei of endothelial cells. In the atherosclerotic tissue, TGF-beta-RI and TGF-beta-RII-immunoreactivity was present in 11/11 specimens and localized to inflammatory cells and to cells with VSMC-like-morphology. TGF-beta-RI-immunoreactivity was present in 87.4 +/- 5.3% (controls 75.3 +/- 7.48%; n.s.) and TGF-beta-RII-immunoreactivity was present in 83.7 +/- 6.8% (controls 39.5 +/- 7.3%; P < 0.002) of the cells. Double immunolabeling, and investigation of serial sections revealed co-expression of TGF-beta-RI and TGF-beta-RII in virtually all cells positive for P27. In the atherosclerotic specimens, CE-immunoreactivity was present in all specimens in macrophages (CD68-positive), vascular smooth muscle cells (alpha-actin positive) and in endothelial cells in 12.58 +/- 13.58% of the nuclei whereas in the controls CE staining was restricted to 0.19 +/- 0.43% of the cells (P < 0.001). Importantly, as shown by immunofluorescent double-labeling, we found cells expressing P27 that were simultaneously positive for CE. In summary, the present study provides evidence that TGF-beta1 present in human atherosclerotic tissue may mediate its growth suppressive activity also by p27, blocking the activity of CE-Cdk2 complexes. Quantitative analysis revealed that TGF-beta-RII, p27 and CE are concordantly upregulated in the atherosclerotic tissue with chronic inflammation, supporting the view that TGF-beta1, p27 and CE may play an important role in the processes associated with chronic inflammation and cell turnover in advanced human atherosclerotic plaques. Taken together, these results provide a possible link between the chronic inflammation associated with advanced atherosclerosis, the effects of extracellular growth factors and cell cycle control.

Arteriosclerosis↗

PubMed2006

Dynamic changes in the direction of the theta rhythmic drive between supramammillary nucleus and the septohippocampal system.

Neurons in the supramammillary nucleus (SUM) of urethane-anesthetized rats fire rhythmically in synchrony with hippocampal theta rhythm. As these neurons project to the septum and hippocampus, it is generally assumed that their role is to mediate ascending activation, leading to the hippocampal theta rhythm. However, the connections between SUM and the septohippocampal system are reciprocal; there is strong evidence that theta remains in the hippocampus after SUM lesions and in the SUM after lesioning the medial septum. The present study examines the dynamics of coupling between rhythmic discharge in the SUM and hippocampal field potential oscillations, using the directionality information carried by the two signals. Using directed transfer function analysis, we demonstrate that during sensory-elicited theta rhythm and also during short episodes of theta acceleration of spontaneous oscillations, the spike train of a subpopulation of SUM neurons contains information predicting future variations in rhythmic field potentials in the hippocampus. In contrast, during slow spontaneous theta rhythm, it is the SUM spike signal that can be predicted from the preceding segment of the electrical signal recorded in the hippocampus. These findings indicate that, in the anesthetized rat, SUM neurons effectively drive theta oscillations in the hippocampus during epochs of sensory-elicited theta rhythm and short episodes of theta acceleration, whereas spontaneous slow theta in the SUM is controlled by descending input from the septohippocampal system. Thus, in certain states, rhythmically firing SUM neurons function to accelerate the septal theta oscillator, and in others, they are entrained by a superordinate oscillatory network.

Action Potentials↗

PubMed2002

Response of a pacemaker neuron model to stochastic pulse trains.

We investigated the response of a pacemaker neuron model to trains of inhibitory stochastic impulsive perturbations. The model captures the essential aspect of the dynamics of pacemaker neurons. Especially, the model reproduces linearization by stochastic pulse trains, that is, the disappearance of the paradoxical segments in which the output firing rate of pacemaker neurons increases with inhibition rate, as the coefficient of variation of the input pulse train increases. To study the response of the model to stochastic pulse trains, we use a Markov operator governing the phase transition. We show how linearization occurs based on the spectral analysis of the Markov operator. Moreover, using Lyapunov exponents, we show that variable inputs evoke reliable firing, even in situations where periodic stimulation with the same mean rate does not.

Action Potentials↗

PubMed2000

Rod-type transducin alpha-subunit mediates a phototransduction pathway in the chicken pineal gland.

The chicken pineal gland is a photosensitive neuroendocrine organ producing melatonin in circadian clock-regulated and light-sensitive manners. To understand the relationship between the photoreceptive molecule pinopsin and the light-dependent melatonin suppression that is sensitive to pertussis toxin treatment, we have searched for pertussis toxin-sensitive G protein alpha-subunits expressed in the chicken pineal gland. Here we report the cDNA cloning of the pineal transducin alpha-subunit (Gtalpha), which is highly homologous to human retinal rod cell-specific Gt(1)alpha. Concurrent cDNA cloning of chicken retinal Gt(1)alpha and Gt(2)alpha (rod and cone cell-specific alpha-subunits of transducin, respectively) revealed that the chicken pineal Gtalpha is identical to the retinal Gt(1)alpha. Double-immunostaining analysis of the chicken pineal sections localized Gt(1)alpha-immunoreactivity in the rudimentary outer segments of both follicular and parafollicular pinealocytes that were immunopositive to anti-pinopsin antibody. To examine whether pineal Gt(1)alpha is involved in the pineal phototransduction pathway, trypsin protection assay was applied for detecting the conversion of GDP-bound Gt(1)alpha into the guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS)-bound form in the pineal membrane homogenate. It was clearly demonstrated that the pineal Gt(1)alpha is activated in a light-dependent manner in the presence of GTPgammaS. These data together suggest strongly that pineal Gt(1)alpha mediates the phototransduction pathway triggered by pinopsin in the chicken pinealocytes.

Amino Acid Sequence↗

PubMed1997

Analysis of frequency (frq) clock gene homologs: evidence for a helix-turn-helix transcription factor.

The frq gene plays a key role in the organization of the Neurospora crassa circadian clock. Our previous analysis of a C-terminal fragment of the putative FRQ protein suggested it is a nuclear transcription factor but did not identify a known DNA-binding domain. If the hypothesis is correct that FRQ is a transcription factor, sequences consistent with this function should be conserved in distantly related species. To investigate, we have cloned frq homologs from other filamentous fungi including Chromocrea spinulosa, and Leptosphaeria australiensis. Alignment of the Leptosphaeria and Chromocrea proteins with the published (complete) sequences for Neurospora crassa and Sordaria fimicola shows that they are respectively about 47%, and 43%, identical to both Neurospora and Sordaria. The alignment identifies several short regions of high conservation punctuated by regions showing near total divergence. Sequences consistent with FRQ being a transcription factor are generally conserved. Most importantly, we show that a highly conserved segment of the protein has strongly predicted helix-turn-helix (HTH) structure as supported by three independent methods. Further, this segment shows the defining sequence characteristics of known HTH DNA-binding domains. Amino acids at positions altered in frq mutant alleles are conserved in all species examined. Transformation of the Neurospora frq9 (conditionally arrhythmic) mutant with the Chromocrea homolog rescued the pigmentation and conidiation defects of the mutant but not the circadian defect; the Leptosphaeria homolog failed to rescue any defect. Together, these data provide the first testable hypotheses concerning several specific aspects of FRQ structure and function.

Amino Acid Sequence↗

PubMed2002

[A case of breast reconstruction with a rectus abdominis musculocutaneus flap following re-lumpectomy for local recurrence of breast cancer after breast conserving therapy].

A 42-year-old woman underwent breast conserving operation with irradiation for Stage I (T1 N0 M0) breast cancer at the 12 o'clock axis of her right breast. At six years after surgery, a solitary local recurrent tumor (1.2 cm in diameter) was found at the 2 o'clock axis of her right breast. No metastases were found at other sites. By the desire of the patient to conserve her breast, we performed re-lumpectomy with surgical margin (2 cm) and reconstructed with a rectus abdominis musculocutaneus flap to fill the large defect of breast tissue. One year and 5 months after surgery, the patient was doing well without recurrence, and was satisfied with the cosmetic effect.

Breast Neoplasms↗

PubMed2006

Long ciliary processes with no ciliary sulcus and appositional angle closure assessed by ultrasound biomicroscopy.

PURPOSE: Nonpupil block mechanisms and appositional angle closure after laser iridotomy (LI) have been reported as common findings in Asians. We evaluated the presence of these findings in a cohort of Brazilian patients using ultrasound biomicroscopy (UBM). METHODS: This observational case-control study included 22 open angle eyes and 31 eyes with occludable angles on gonioscopy (defined by 2 examiners). UBM radial scans through a typical ciliary process were obtained in both light and dark conditions, at 6 and 12-o'clock positions. Long ciliary processes with no ciliary sulcus were determined on the basis of a reference line drawn perpendicular to the iris plane passing through a point located 750 mum from scleral spur. Trabecular ciliary processes distance was measured on 6-o'clock UBM images. RESULTS: After LI, 52% of occludable angle eyes had appositional angle closure in both 6 and 12-o'clock UBM images. We also observed this finding in 14% and 23% of the control eyes (in 6 and 12-o'clock UBM images, respectively). A long ciliary process with no ciliary sulcus was observed in 61% of occludable angle eyes, and also in 32% of control eyes (6-o'clock UBM images). Control eyes had longer trabecular ciliary processes distance than occludable angle eyes (P<0.001). CONCLUSIONS: The UBM finding of long ciliary processes associated with the absence of ciliary sulcus is not necessarily related to an anterior positioning of the ciliary processes. Whether UBM appositional angle closure after LI is associated with further angle closure process and/or poor intra-ocular pressure control remains to be evaluated.

Aged↗

PubMed2006

Fractal dimension in health and heart failure.

BACKGROUND: Non-linear analysis of heart rate variability (HRV) can give additional information about autonomic control of the heart rate. This study applied the fractal dimension (FD) in a congestive heart failure (CHF) population. METHODS: FD and HRV were evaluated in a healthy population (n=21) and an end-stage heart failure population (n=21) using 1-h segments during the day and night from Holter recordings. RESULTS: CHF patients presented a loss of circadian variation in both FD and conventional time- and frequency-domain HRV indices. FD was higher in the CHF population both during the day and night. In the CHF population the correlation between FD and high-frequency power of HRV was lost. CONCLUSION: Day-night variations of heart rate fluctuations are lost in heart failure. Changes in FD reflecting physiological and pathophysiological changes were observed.

Algorithms↗

PubMed1999

Chronopharmacology and its impact on antihypertensive treatment.

Circadian rhythms have been documented throughout the plant and animal kingdom at every level of eukariotic organization. Circadian rhythms are endogenous in nature, driven by oscillators or clocks, and persist under free-running (e.g. constant darkness) conditions. The genes expressing the biological clock have been identified in various species. The important feature of endogeneous biological rhythms is their anticipatory character. Rhythmicity inherent to all living systems, allows them to adapt more easily and to better survive under changing environmental conditions during the 24 hours of a day as well as during changing seasons. Having this in mind it is easy to conceive that not only must the right amount of the right substance be at the right place, but also this must occur at the right time. Also in man nearly all functions of the body including those influencing pharmacokinetic parameters such as drug absorption and distribution, drug metabolism and renal elimination display significant daily variations. Also the onset and symptoms of diseases such as coronary infarction, angina pectoris, stroke, ventricular tachycardia are circadian phase dependent. Myocardial infarction and angina attacks as well as silent ischemias (ST-segment depression) in stable angina pectoris have an early morning peak between 8-12 h. In contrast, ECG abnormalities and angina attacks in variant angina mainly occur at night. Blood pressure and heart rate in normotensives and essential (primary) hypertensive patients display highest values during daytime followed by a nightly drop and an early morning rise. In about 70% of forms of secondary hypertension (e.g. renal disease, hyperthyroidisms, hormonal diseases, gestational hypertension), however, this rhythmic pattern is abolished or even reversed exhibiting nightly peaks in blood pressure. This form of hypertension is accompanied by increased end organ damages. Thus, different subtypes of a disease (angina pectoris, hypertension) can display different circadian patterns in symptoms. These observations are a challenge for basic and clinical research to get a better understanding on the underlying mechanisms of regulation. Moreover, they call for a circadian time-specified drug treatment. From above it is evident that pharmacokinetics may also not be constant within a day. Chronopharmacokinetics have been shown for several cardiovascular active drugs (propranolol, nifedipine, verapamil, enalapril, isosorbide-5-mononitrate, digoxin, etc.). Far more drugs were shown to display significant daily variations in their effects (chronopharmacodynamics, chronotoxicology) even after chronic application or constant infusion. In conclusion, there is clear evidence that the dose/concentration-response relationship of drugs can be significantly dependent on the time of day. Thus, circadian time has to be taken into account as an important variable influencing a drug's pharmacokinetics and/or its effects or side effects.

Animals↗

PubMed2000

Voiding dysfunction in the orthotopic neobladder.

The orthotopic neobladder has become the preferred method of handling the urinary tract after removal of the bladder in men and women. However, an improved quality of life compared to urinary diversion fails to be realized when voiding dysfunction arises. These difficulties with urination range from retention to incontinence. Voiding dysfunction following neobladder construction that persists beyond 6-12 months warrants fluoro-urodynamic evaluation to determine the cause and plan therapy. Although colonic, ileocolonic, gastric, and ileal neobladders have all been deemed acceptable, the S or W-configured, spheroidal shaped neobladders created from ileum are the most popular. Voiding pressures and micturition patterns depend on the type, length, and configuration of bowel segment harvested. These variables also determine the risk of voiding dysfunction, along with the choice of surgical technique, and the age and sex of the patient. Urinary retention is more common in women, especially after urethral nerve sparing. Obstruction is often due to inferior displacement of the bladder neck, which can angulate the urethra. Daytime stress incontinence arises from reduced urethral outlet resistance accentuated by low neobladder capacity, reduced compliance, or elevated neobladder pressures. Night-time incontinence develops as a consequence of absent sensation that permits excessive nocturnal volumes to overcome the impaired continence mechanisms of the bladder outlet. This situation is exacerbated by the loss of physiological storage reflexes. Therapy for retention rests primarily with intermittent self-catheterization. Stress urinary incontinence can be treated with periurethral bulking agents or an artificial urethral sphincter. Nocturnal enuresis is often effectively managed by the use of an alarm clock to awaken the patient several times per night. Knowledge of the pathogenesis and identification of risk factors implies that prevention through proper design of the neobladder, meticulous surgical technique, and patient selection is paramount.

Humans↗

PubMed1989

[Automatic computer analysis of fetal cardiac rhythm].

In utero cardiotocographic recording is currently one of the best means of diagnosis of fetal distress. The purpose of automatic tracing of the fetal heart rate is to prevent inter- and intra-individual variability in order to better evaluate the condition of the fetus. This study was carried out with the assistance of a cardiotocograph HP 8040A connected to a microcomputer equipped with a clock card. The heart signal may then be continuously recorded and stored on floppy disks, which are then reread and processed by microcomputer. The data are divided into time intervals (3, 5 or 10 seconds), selected by the physician. At each stage, mean, median and sample variation are calculated. Losses of signal are read. In a second stage, the use of a digital, linear and predictive filter (Kalman filter) applied to heart rates, allows baseline extraction as well as the detection of variations about this baseline (accelerations, decelerations). With the use of this filter on sample variations, it is possible to detect various variability periods, via two different approaches: classification of variability values in three groups: 0-5 bpm, 5-10 bpm, greater than 10 bpm, detection of increases or decreases of the variability as compared to the mean variability during the recording. Comparison of automatic analysis and visual analysis by segmentation is satisfactory in terms of variability. Detection of accelerations and decelerations is more problematic, for two reasons: the notion of baseline, the accuracy of digital detection: 15 bpm, 15 sec. All these problems are discussed.

Female↗

PubMed2001

Wavelet entropy: a new tool for analysis of short duration brain electrical signals.

Since traditional electrical brain signal analysis is mostly qualitative, the development of new quantitative methods is crucial for restricting the subjectivity in the study of brain signals. These methods are particularly fruitful when they are strongly correlated with intuitive physical concepts that allow a better understanding of brain dynamics. Here, new method based on orthogonal discrete wavelet transform (ODWT) is applied. It takes as a basic element the ODWT of the EEG signal, and defines the relative wavelet energy, the wavelet entropy (WE) and the relative wavelet entropy (RWE). The relative wavelet energy provides information about the relative energy associated with different frequency bands present in the EEG and their corresponding degree of importance. The WE carries information about the degree of order/disorder associated with a multi-frequency signal response, and the RWE measures the degree of similarity between different segments of the signal. In addition, the time evolution of the WE is calculated to give information about the dynamics in the EEG records. Within this framework, the major objective of the present work was to characterize in a quantitative way functional dynamics of order/disorder microstates in short duration EEG signals. For that aim, spontaneous EEG signals under different physiological conditions were analyzed. Further, specific quantifiers were derived to characterize how stimulus affects electrical events in terms of frequency synchronization (tuning) in the event related potentials.

Adult↗

PubMed2005

Electrophysiological effects of 4-aminopyridine on fictive locomotor activity of the rat spinal cord in vitro.

Recently the K+ channel blocker 4-aminopyridine (4-AP) has been suggested to be useful to improve motor deficits due to spinal cord lesions. There is, however, little basic research support for this action of 4-AP. In this study we have used as a model the neonatal mammalian spinal cord in vitro that generates a rhythmic activity termed fictive locomotion (induced by bath-application of NMDA + 5-HT) with phasic electrical discharges alternating between flexor and extensor motor pools and between left and right motoneurons within the same segment. When 4-AP was added in the presence of sub-threshold concentrations of NMDA + 5-HT, there was facilitation of fictive locomotion which appeared with alternating patterns on all recorded ventral roots (VR). Furthermore, in the presence of 4-AP, weak dorsal root (DR) stimuli, previously insufficient to activate locomotor patterns, generated alternating discharges from various VRs. The present data show that 4-AP could strongly facilitate the locomotor program initiated by neurochemicals or electrical stimuli, indicating that the spinal locomotor network is a very sensitive target for the action of 4-AP.

4-Aminopyridine↗

PubMed2005

Maintenance of multiple working memory items by temporal segmentation.

Experimental work based on single cell recordings supports the hypothesis that working memory representations are retained by sustained neuronal firing. While this hypothesis can account for the maintenance of a single memory item, it remains unclear how multiple working memory items are represented. This account will discuss the possible physiological mechanism responsible for the maintenance of multiple working memory items including mechanisms based on sustained firing and synaptic encoding. The focus will be on temporal segmentation by phase encoding, namely the idea that several working memory items are activated sequentially at different points in time. It has been proposed that a mechanism of nested gamma (30-80 Hz) and theta (4-8 Hz) oscillations is responsible for controlling the reactivation of the memory list. This mechanism has been shown to be compatible with multiple behavioral findings on working memory such as the data from the Sternberg experiment. The theta/gamma mechanism has also received support from a large set of electrophysiological findings, however, more experimental work is required to further substantiate or falsify the model.

Action Potentials↗

PubMed1997

A novel zinc finger-containing RNA-binding protein conserved from fruitflies to humans.

The Drosophila lark gene encodes an essential RNA-binding protein of the RNA recognition motif (RRM) class that is required during embryonic development. Genetic analysis demonstrates that it also functions as a molecular element of a circadian clock output pathway, mediating the temporal regulation of adult emergence in the fruitfly. We now report the molecular characterization of a human gene with significant similarity to lark. Based on fluorescence in situ hybridization and radiation hybrid mapping, the human gene has been localized to chromosome region 11q13; it is closely linked to several identified genes including the locus of Bardet-Biedl syndrome type 1. The lark-homologous human gene expresses a single 1.8-kb size class of mRNA in most or all tissues including brain. Additional database searches have identified a mouse counterpart that is virtually identical to the human protein. Similar to lark protein, both mammalian proteins contain two copies of the RRM-type consensus RNA-binding motif. Unlike most RRM family members, however, the Drosophila and mammalian proteins also contain a retroviral-type (RT) zinc finger that is situated 43 residues C-terminal to the second RRM element. Within a 184-residue segment spanning the RRM elements and the RT zinc finger, the human and mouse proteins are 61% similar to the Drosophila lark sequence. These common sequence features and comparisons among a large collection of RRM proteins suggest that the human and mouse proteins represent homologues of Drosophila lark.

Adult↗

PubMed2001

Generation of a novel functional neuronal circuit in Hoxa1 mutant mice.

Early organization of the vertebrate brainstem is characterized by cellular segmentation into compartments, the rhombomeres, which follow a metameric pattern of neuronal development. Expression of the homeobox genes of the Hox family precedes rhombomere formation, and analysis of mouse Hox mutations revealed that they play an important role in the establishment of rhombomere-specific neuronal patterns. However, segmentation is a transient feature, and a dramatic reconfiguration of neurons and synapses takes place during fetal and postnatal stages. Thus, it is not clear whether the early rhombomeric pattern of Hox expression has any influence on the establishment of the neuronal circuitry of the mature brainstem. The Hoxa1 gene is the earliest Hox gene expressed in the developing hindbrain. Moreover, it is rapidly downregulated. Previous analysis of mouse Hoxa1(-/-) mutants has focused on early alterations of hindbrain segmentation and patterning. Here, we show that ectopic neuronal groups in the hindbrain of Hoxa1(-/-) mice establish a supernumerary neuronal circuit that escapes apoptosis and becomes functional postnatally. This system develops from mutant rhombomere 3 (r3)-r4 levels, includes an ectopic group of progenitors with r2 identity, and integrates the rhythm-generating network controlling respiration at birth. This is the first demonstration that changes in Hox expression patterns allow the selection of novel neuronal circuits regulating vital adaptive behaviors. The implications for the evolution of brainstem neural networks are discussed.

Animals↗

PubMed2001

Phase-coupled oscillator models can predict hippocampal inhibitory synaptic connections.

What factors are responsible for propagating electrical activity in the hippocampus? Using an intact, isolated hippocampus preparation, it is possible to observe spontaneous delta (< or = 4 Hz) waves of rhythmic field potentials. These rhythmic potentials are inhibitory in nature, mediated by GABAergic inhibitory potentials originating from a population of principal neurons. They start in the ventro-temporal region and move longitudinally towards the dorso-septal region with a phase lag of approximately 10% between the extracellular recordings. We use the mathematical framework of phase-coupled oscillators (PCO) to gain some insight into the underlying network system. A chain of 15 nearest-neighbour bidirectionally coupled PCOs is used where each oscillator refers to a segment of the CA1 region of the hippocampus that can generate these slow field potentials. We find that ventro-dorsal delta waves exist if there is a dominance in coupling strength in one direction. Without a one-way coupling dominance, ventro-dorsal waves can still exist, but then the coupling strengths need to be much larger. The relationship between entrained and intrinsic frequencies and the variation of propagation speeds along the longitudinal axis can be used to determine which case applies. Currently available experimental data supports one of the cases, predicting that there is a stronger ventral to dorsal inhibitory effect.

Action Potentials↗

PubMed2000

Segmentation: a view from the border.

Segmentation, or metamerism, consists of the subdivision of the body into discrete units that subsequently acquire regional specializations. In vertebrates, the most obvious manifestation of this phenomenon is seen during the formation of the mesodermal somites and their derivatives. This review surveys three different models for how somites form, and how they relate to recent molecular data suggesting the involvement of transcription factors and cell surface molecules. A new model (the "Morse code" model) is proposed to convey positional information to somitogenic cells. Finally, the molecular events of boundary formation (during the initial epithelialization of somites) and boundary maintenance (between adjacent somite halves as well as in resegmentation) are discussed.

Animals↗