37. Embryonic development – regulative and mosaic. Morphogens and embryonic induction.

Embryonic development - regulative and mosaic. Morphogens and embryonic induction.

• Embryonic development, also known as embryogenesis, is the complex process through which a fertilized egg transforms into a fully developed organism.
• This process occurs inside protective structures, such as the egg envelopes in oviparous species or the maternal placenta in mammals.
• Development after birth or hatching can follow two routes:
• Indirect development involves a larval stage and metamorphosis, seen in organisms like amphibians.
• Direct development skips the larval stage entirely and is seen in reptiles, birds, and mammals.

Cleavage Types Based on Yolk Content

• Holoblastic cleavage occurs in eggs with little or moderate yolk:
• The entire egg divides.
• Found in mammals (rotational), amphibians (radial), and echinoderms.
• Meroblastic cleavage occurs in eggs with abundant yolk:
• Only a portion of the egg divides.
• Seen in birds (discoidal) and insects (superficial).

Mosaic vs Regulative Development

Mosaic Development

• Also known as determinate development.
• Each blastomere (cell from cleavage) has a pre-set fate from the beginning.
• Loss or damage to a blastomere results in the absence of the corresponding body part or structure.
• This pattern relies on the uneven distribution of specific molecules (morphogens) within the egg cytoplasm.
• A classic example is the nematode Caenorhabditis elegans, where the fate of each cell is fixed, and the adult organism always has the same number of cells.

Regulative Development

• Also known as indeterminate development.
• All early blastomeres retain the ability to form a complete embryo.
• If a cell is lost or damaged, others can compensate and produce a normal organism.
• There are no pre-set gradients of morphogens in the egg.
• Determination of cell fate happens later through interactions between cells and signals from neighboring tissues.
• This is the developmental strategy used by mammals, where the embryo adjusts and compensates during early stages to ensure normal development.

Morphogens: Definition and Function

• Morphogens are signaling molecules that guide cells to take on specific identities during development.
• Their primary role is to direct the spatial organization of different cell types and tissues.

Key Characteristics of Morphogens

1. 1. Morphogens are often produced in specific locations and spread out to create concentration gradients.
2. 2. Cells respond to different concentrations of a morphogen by activating different sets of genes.
3. 3. In mosaic development, morphogens are inherited directly from the oocyte and segregated into specific regions of the egg.
4. 4. In regulative development, morphogens are produced and secreted by cells of the embryo itself.

Gradient Formation Mechanisms