No abstract

No abstract

No abstract

No abstract

No abstract

It has been suggested that positional information along the proximo-distal axis of the limb-bud is specified by time spent in the progress zone. Mesenchyme cells have been killed by X-irradiation, reducing the rate cells leave the zone. The time spent there by some cells is thus increased. When limbs, stage 18/19, stage 21, or tips of stage 24, are treated with increasing doses of X-irradiation, from 1000 rads to 2500 rads proximal structures are progressively lost, whereas distal ones— the digits—are relatively unaffected. There was no evidence for intercalation of missing parts. These effects are due to killing or damage of mesenchyme cells: the ectoderm is not affected at these doses. The results are consistent with a quantitative analysis based on the progress zone model, in which viable cells repopulate the progress zone and gradually restore it to normal as non-dividing cells are diluted out. It is suggested that any treatment causing damage to the mesenchyme at early stages will give similar results. The mesenchyme cells appear to be surprisingly resistant to radiation damage. The form of the limb-bud is not altered by damaging the mesenchyme. Differences in the development of structures at similar proximo-distal levels, following irradiation, is considered in terms of the requirement of a threshold number of cells.

The mammary gland is a system of hollow interconnecting tubes which develops from an invasive branching cord of epithelial cells. This ultrastructural study of the developing mammary gland focuses on how the lumen forms and establishes the polarized epithelial lining of the gland. The earliest signs of lumen formation are many small cavities and crevices lined with microvilli which appear at scattered sites throughout the branching cords and neck of the gland. It is suggested that these initial small lumina form quite simply by separation of cells whose opposing faces are non-adhesive. The continuous central lumen of the gland develops by fusion and enlargement of the many small lumina. The cells adjacent to the developing lumen will form the polarized epithelial lining of the gland. Excess, more basal, epithelial cells degenerate. The lumen begins to appear when the branching pattern is almost complete. Thus, during morphogenesis, invasion by the mammary gland epithelium involves penetration of the mesenchyme by a solid cord of cells. We suggest that this cellular organization may be a fundamental characteristic of invasive epithelia and that a crucial step in the development of malignant epithelial tumours is a change in cell organization from a polarizedcell sheet to a solid cord of cells which can invade.

The role of gap junctional communication during patterning of the chick limb has been investigated. Affinity-purified antibodies raised against rat liver gap junctional proteins were used to block communication between limb mesenchyme cells. Co-injection of the antibodies and Lucifer yellow into mesenchyme cultures demonstrated that communication was inhibited almost immediately. When antibodies were loaded into mesenchyme tissue by DMSO permeabilization, [3H]nucleotide transfer was prevented for at least 16 h. Polarizing region tissue from the posterior limb bud margin causes digit duplications when grafted to the anterior margin. Quail polarizing region cells were loaded with gap junction antibody and grafted into chick wing buds. The antibody had no effect on growth or survival of the grafted cells. As very few polarizing region cells are required to initiate duplications, the number of polarizing region cells in the grafts was reduced by diluting 1:9 with anterior mesenchyme tissue. When either polarizing region or anterior mesenchyme tissue in the graft was loaded separately with antibody, there was little effect on respecification of the digit pattern. However, loading both tissues in the graft caused a significant decrease in duplications. This indicates that a major role of gap junctions in limb patterning may be to enable polarizing region cells to communicate directly with adjacent anterior mesenchyme. A role for gap junctional communication between anterior mesenchyme cells cannot be excluded. The results are discussed in relation to the role of retinoic acid as a putative morphogen.