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What are the developmental mechanisms?
A. Cellular Divisions
Both the mitotic and meiotic types of cell divisions are involved in the reproductive chain of events. Specifically, the meiotic division (meiosis) ensures the development of haploid gametes, while the mitotic division (mitosis) is associated with multiplication of diploid cells as seen with primordial germ cell proliferation, cleavage and the series of post-fertilization cells proliferation and differentiation.
B. Cellular Induction
The influence of one cell group (inducer) over a neighboring cell group (induced) during embryogenesis is known as induction. Embryonic induction is considered to play an important role in human development and experts believe that the first major induction phenomenon occurs during the final stages of gastrulation. It is associated with the release of inductive signals from one group of cells to influence the development of another group of cells.
Scientists have identified three main ways in which signals can be passed between cells i.e. by diffusible signal, directly contact, or through gap junctions. In the first mechanism, a diffusible signal is sent through the extracellular space, and is received by a cell-surface receptor, which further transmits the signals by way of second messengers. In the second mechanism, cells directly contact each other through transmembrane proteins located on their surfaces; while in the third mechanism, the cytoplasm of two cells is connected through gap junctions, allowing the signal to pass directly from one cell to another cell.
Various eye structures (lens, optic cup, and so on), internal ear structures, as well as several tissues (for example, vertebral cartilage) emerge from cells which were acted upon by inducer tissues. In addition, limbs, kidney, nasal structures, salivary glands, pancreas, teeth, feathers, and hair are organs which require inductive stimuli. It is not known whether a single common mechanism underlies each of those inductions.
Many scientists believe that the inductive interaction mediated by cell-cell contacts is dependent on the developmental information that resides at the cell surface. Perhaps the surface of the responding tissue recognizes the signal molecules present on the surface of the inducing tissue. In other instances, a secreted protein might move among various cells or tissues and exert its effects on competent cells.
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