51. Temporal expression of N-cadherin is extremely important during early development of the mammalian embryos. Accordingly, which one of the following statements about N-cadherin is true?
(1) Injection of N-cadherin antibodies just prior to condensation of mesenchymal cells will aid cartilage formation.
(2) Presence of N-cadherin just prior to condensation will facilitate nodule formation and development of the limb skeleton.
(3) The border between the nervous system and skin will form properly only if epidermal cells are experimentally made to express N-cadherin.
(4) Expression of N-cadherin is redundant during separation of neural and epidermal precursor cells.

The correct statement about N-cadherin during early mammalian embryo development is:

(2) Presence of N-cadherin just prior to condensation will facilitate nodule formation and development of the limb skeleton.

Explanation:

N-cadherin is a calcium-dependent cell adhesion molecule that plays a critical role in tissue morphogenesis during early development. Its temporal expression is essential, particularly just before the condensation of mesenchymal cells that leads to nodule formation, a precursor to cartilage and limb skeleton development. N-cadherin mediates the cell-cell adhesion necessary for these processes, helping mesenchymal cells aggregate properly.

Injecting N-cadherin antibodies before condensation would disrupt this adhesive function and impede cartilage formation, contradicting option (1).

The proper formation of the boundary between the nervous system and skin relies on differential cadherin expression, typically with neural cells expressing N-cadherin and epidermal cells expressing E-cadherin, so forcing epidermal cells to express N-cadherin experimentally would likely disrupt this boundary rather than form it properly, opposing option (3).

Expression of N-cadherin is not redundant in separating neural and epidermal precursors, as cadherin switching (from E-cadherin in epidermal to N-cadherin in neural cells) is a key developmental event, ruling out option (4).

Hence, N-cadherin’s presence prior to condensation is directly linked to facilitating mesenchymal aggregation leading to limb skeleton development.