Biomedical subjects

D Summerbell

Publications and source records attributed to D Summerbell.

50 records · Page 3Linked to original sources

PubMed1977

Regulation of the deficiencies along the proximal distal axis of the chick wing-bud: a quantitative analysis.

The wing-bud shows zero regulation following removal of a whole slice of the proximal-distal axis from stage 22 or later. Prior to this it is possible that it may show some regulation. Some of the apparent regulation may be explained by the way in which the limb grows. The bud never shows perfect size regulation or morphallaxis. The proximal-distal axis of the bud between shoulder and wrist expands uniformly from stage 20.

Animals↗

PubMed1977

The development of functional innervation in the chick wing-bud following truncations and deletions of the proximal-distal axis.

Selections of the proximal-distal axis were removed so as to produce wings lacking entire skeletal levels. The innervation of the resulting limb from the appropriate spinal nerves was tested by electrophysiological recording, gross dissection and serial sections. Ther nerves form repeatable and ordered patterns in the modified limbs which can be readily related to the normal pattern. In general the segmental nerves only innervate their appropriate territory irrespective of its location; the route followed by the nerves is characteristic for the skeletal level they traverse. If a target is missing then so is the normal nerve branch to that region.

Action Potentials↗

PubMed1976

A descriptive study of the rate of elongation and differentiation of the skeleton of the developing chick wing.

Differentiation of the wing skeleton is clearly visible in whole mounts from stage 24. It proceeds in proximo-distal and postero-anterior sequence. It is possible to map the part of the limb giving rise to each skeletal element as it appears and subsequently. This enables one to estimate: (a) a rate of elongation curve for each segment level, (b) the intrinsic rate of change of elongation of the cartilage, (c) a definitive extrapolation back to classical presumptive fate maps, (d) the normal range of variation in the proportion of the limb occupied by the three main segmental levels (stylopod, autopod, zeugopod).

Animals↗

PubMed1975

Positional signalling and specification of digits in chick limb morphogenesis.

The interpretation of positional information can provide the basis for pattern formation in limb morphogenesis. The gradient in positional information along the antero-posterior axis, which is specified with respect to a localised boundary region, can be modified by grafting this region to successive positions along the axis. The pattern of digits obtained is consistent with a model based on diffusion of a labile morphogen and is thus similar to models proposed for the development of pattern in invertebrates.

Animals↗

PubMed1975

Morphogenesis of the vertebrate limb.

Pattern formation in both the developing and the regenerating vertebrate limb can be understood in terms of the assignment of positional values to cells. The positional value may then control the selection of a course of cytodifferentiation as, say, muscle or cartilage. It is proposed that the proximodistal coordinate of positional value is extablished by a mechanism based on autonomous change with time in a 'progress zone' at the tip of the limb bud. The anteroposterior coordinate seems to be specified by a positional signal from an orgainzing region. Only cells in the progress zone respond to this signal. The change of character in the progress zone may be linked to cell division. It seems that each skeletal rudiment may initially be the same length, corresponding to the amount of tissue that leaves the progress zone during one cell cycle. The lengths on the skeletal elements are already specified shortly after exit from the progress zone, and are capable of very little regulation. The positional value may determine the pregramme of later growth. The relevance of the progress zone model to epimorphic regulation, and amphibian limb regeneration in particular, is discussed.

Animals↗

PubMed1975

Time, place and positional value in the chick limb-bud.

Quantitative experimental evidence is presented for the progress zone theory of limb development. The theory, here formulated mathematically, states that the parts of the limb are specified in proximo-distal succession by an autonomous timing mechanism operating in a 'progress zone' of undifferentiated growing mesenchyme under the influence of the apical ectodermal ridge. By the exchange of distal tips between young and old wing-buds, it is shown that there are no long-range morphogenetic signals from proximal to distal tissue. The width of the progress zone is calculated, and it is found autoradiographically that practically all its cells are dividing.

Animals↗

PubMed1974