Biomedical subjects

T Sachs

Publications and source records attributed to T Sachs.

18 recordsLinked to original sources

PubMed2001

Plant morphogenesis: long-distance coordination and local patterning.

The overall morphology of a plant is largely determined by developmental decisions taken within or near the terminally positioned apical meristems of shoots and roots. The spatial separation of these developmental centers emphasizes the need for long-distance signaling. The same signaling events may simultaneously coordinate differentiation within meristems and in the connecting vascular tissues. Recent genetic and molecular analyses not only confirm the proposed role of auxin as a coordinating signal across the plant, but also implicate auxin as a patterning signal in embryo and meristem organization.

Morphogenesis↗

PubMed2000

Integrating cellular and organismic aspects of vascular differentiation.

Vascular differentiation can be studied at two levels, and they should complement one another: as an aspect of integrated plant development and as cellular processes. The differentiation of organized strands that connect between organs is induced by polar auxin flow, towards the roots. Anatomy, therefore, can be a complementary method of observing polarity and its changes. As expected for a self-correcting and essential system, vascular patterning mutations are relatively rare and have pleiotropic effects, including modifications of responses to auxin and its transport. Tissue polarity both expresses and depends on auxin transport, a feedback that could account for the determined nature of polarity as well as the gradual canalization of differentiation to vascular strands. This predicts that the molecules responsible for polarity will be localized gradually as differentiation proceeds. Further, a modified location of these molecules can be expected to precede anatomical expressions of a new, regenerated, polarity. Tracheary differentiation is probably the best studied example of cell differentiation. Within the plant, however, this differentiation is coupled to oriented cell growth either along or at right angles to the axis of auxin flow, depending on tissue competence. Differentiation is also coupled to the differentiation of the other components of the vascular system. There are, presumably, early joint stages to these differentiation processes, but what they are remains an intriguing problem.

Cell Differentiation↗

PubMed1991

Development of immature stomata: evidence for epigenetic selection of a spacing pattern.

In Sansevieria trifasciata as many as half the potential stomata remain immature. The development of all stomatal structures started at the same time and the early stages of the development of immature stomata had no special characteristics. Statistical analysis showed that the mature stomata were more evenly spaced than all potential stomata, both mature and immature. Furthermore, the distribution of mature stomata per unit area was more predictable or orderly than comparable structures of a random model that developed in the same way. These facts indicate that a nonrandom loss of many stomata by "immaturity" is a major determinant, acting during rather than preceding development, of the distribution of the mature, functional stomata. Thus in Sansevieria there is a selection of an epidermal pattern from an excess of cells that undergo the early stages of stomatal development.

Cell Differentiation↗

PubMed1991

An effective self-retaining retractor for anorectal procedures.

A self-retaining retractor technique is described for anorectal surgery eliminating many of the problems inherent in the commonly used instruments. It is particularly useful in endorectal pull-through procedures in which the exact level of anastomosis within the anorectal canal is of great importance.

Anal Canal↗

PubMed1991

Squamous carcinoma of the mid-esophagus. A survival study.

A survival study for squamous carcinomas of the mid-esophagus treated by Southern California Permanente Medical Group in the interval of 1954 to 1988 was undertaken. Radiation therapy and surgery were equally efficacious in terms of 5-year survival for patients without distant disease and performance status sufficient to tolerate treatment (11% and 16%, respectively). There was no survival benefit for patients treated with palliative surgery. Less invasive endoscopic means along with chemotherapy and radiation for palliation are recommended except for special circumstances. Optimal treatment combinations remain to be discovered.

Carcinoma, Squamous Cell↗

PubMed1990

Adenocarcinoma arising in Barrett's esophagus.

A review of nine adenocarcinomas of the esophagus arising in Barrett's epithelium was undertaken. We found the disease among white males disproportionately. Risk factors and incidence rate remain to be clarified. Only one patient was in a surveillance program and only he had carcinoma discovered "early." He still survives while only one of the eight whose diagnosis followed investigation of symptoms remains alive.

Adenocarcinoma↗

PubMed1988

Epigenetic selection: an alternative mechanism of pattern formation.

It is commonly assumed that patterned development is specified by pre-patterns or programs, actual development being a necessary consequence of previous conditions. However, the variability of normal development and its regenerative capacities are evidence for additional patterning processes. Predictable mature structures could result from continued "epigenetic selection" of the most appropriate developmental events. This selection would occur from an excess of possibilities that are genetically equivalent. The final balanced state would be specified by the genes, and the developmental system could gravitate towards this state without a detailed program. Selection could result from competition between cells and tissues for limiting developmental signals. The success or continuation of events that could start randomly would depend on feedback relationships with complementary developmental events. Specialized processes could be gradually localized if differentiation itself consumed limiting signals and if this consumption increased as differentiation proceeded. Spatial patterns could be formed if the movement of signals was gradually facilitated along the axes where it were initiated by diffusion. For example, induced facilitated transport could be the basis of an advantage of multicellular centers over scattered cells that have specialized in the same way. If epigenetic selection has a developmental role it requires a revision of common views concerning the cellular traits and the gene functions necessary for patterned development. An example of these traits is that cells should be expected to respond to changes in signal availability, not necessarily to signal concentration at any given time.

Animals↗

PubMed1986

Cellular interactions in tissue and organ development.

Developmental patterns result from combinations of interactions and intracellular programmes. The purpose here is to define the roles of interactions wherever possible and to consider their major parameters, such as the timing of their action, their specificity and the distances over which they occur. The approach is one of a board survey, attempting to outline major interaction systems on the basis of information from different sources. The evidence concerning interactions that control development must come primarily from development itself. Both disturbed, as during regeneration and grafts, and normal development are relevant. Growing apices interact over relatively long distances. They reduce the development of similar apices and induce the development of axial tissues that connect them to the plant. Young shoot tissues also induce the development of root apices and vice versa. These various effects can be understood on the basis of a hormonal feedback involving auxins and cytokinins. Vascular differentiation, furthermore, is a cellular expression of these interactions. It occurs along the flow of auxin from the young tissues of the shoot towards the root apices. This flow is canalized by a positive feedback between cell polarization and the polar flow that both controls and results from this polarization. Structural relations between pholem and xylem, limitations of regeneration and the formation of rays all indicate the existence of additional, radial interactions. The fate of individual cells in development is varied and often follows no recognizable rules. This indicates interactions that operate on the size of a tissue or organ rather than on its precise cellular development. On the other hand, the continuity of plasmodesmata, wall thickenings and cytoplasmic strands demonstrate local interactions between neighbouring cells. It is concluded that, though the possibilities of developmental interactions may be bewildering, the list, when known, may not be all that long. The interactions specify orientation and quantity rather than the precise fate of cells. Their effects are gradual and they involve feedback loops. Growth factors, even known growth factors, play surprisingly large roles in cellular interactions. Such controls whose effect is gradual and general could provide a basis for developmental plasticity.

Cell Communication↗

PubMed1981