D2.1.12 Cell proliferation for growth, cell replacement and tissue repair (HL) Notes

Cell Proliferation for Growth, Cell Replacement, and Tissue Repair

1. Cells proliferate for three key reasons:
   1. Growth: Developing from a single fertilized egg to a multicellular organism.
   2. Cell replacement: Replacing worn-out or damaged cells during routine maintenance.
   3. Tissue repair: Healing injuries by regenerating lost or damaged tissue.

For Growth: Early-Stage Animal Embryos

1. Growth begins with a single zygote (fertilized egg).
2. The zygote undergoes rapid mitotic divisions, forming a blastula, a hollow ball of cells.
3. These divisions create the foundation for all tissues and organs.
4. Cell proliferation is the mechanism that transforms one cell into billions.

For Growth: Plant Meristems

1. Plants grow through meristems, regions of undifferentiated cells capable of continuous division.
2. Apical meristems at root and shoot tips drive growth in length (taller plants, deeper roots).
3. Lateral meristems (e.g., cambium) drive growth in thickness.

For Cell Replacement: Skin as a Model

1. The epidermis (outer skin layer) constantly loses cells due to friction and wear.
2. Basal cells in the lower epidermis divide to replace lost cells.
3. As new cells move upward, they produce keratin, becoming flattened and waterproof before dying and shedding.
4. In routine replacement, cell division balances cell death, maintaining constant cell numbers.

For Tissue Repair: Wound Healing in Skin

1. When skin is injured, stem cells in the basal layer divide rapidly to replace damaged cells.
2. The rate of division exceeds cell death, allowing the wound to close.
3. If the dermis (deeper layer) is damaged, stem cells repair the tissue, though this takes longer.
4. In severe cases where stem cells are lost, skin grafts may be needed.