B2.3 Cell Specialisation

Fertilization and Early Development:
- Following fertilization, specialized cells arise from unspecialized cells through differentiation.
- The zygote undergoes rapid division to become an embryo, followed by a fetus, then an infant.
Gene Expression:
- Early-stage embryos exhibit gene expression that leads to different cell types based on signal gradients.
- Various cells develop due to the expression of certain genes, controlled by signaling molecules called morphogens that specify regions in the embryo.
Examples and Implications:
- Bicoid: A morphogen in fruit flies influencing anterior-posterior axis; defects can cause developmental issues.
- Cancer Cells: Show rapid reproduction and improper differentiation, leading to tumor formation.

Stem Cells Overview:
- Stem cells have unlimited division capacity and can differentiate into various cell types.
- Plants: Contain stem cells in meristematic regions for growth and regeneration.

Stem cell niches provide the environment for self-renewal and differentiation.
Examples of Niches:
- Bone Marrow: Contains hematopoietic stem cells (multipotent).
- Hair Follicles: Involves epithelial stem cells contributing to hair regeneration and skin maintenance.
Self-Renewal Mechanism: Upon cell division, both stem cells and differentiated cells can be produced.

Totipotent:
- Can develop into any tissue, found in early embryo stages.
Pluripotent:
- Can form nearly all cell types, cannot form a full organism.
Multipotent:
- Limited to certain subsets of cells, e.g., blood cells from bone marrow.
Unipotent:
- Can produce only one cell type, involved in repair and maintenance (e.g., skin cells).

Size Variation: Includes male and female gametes, various blood cells, neurons, and muscle fibers.
Adaptations:
- Sperm: Small and motile for genetic material transport.
- Eggs: Larger and round, non-motile.
- Red Blood Cells: Biconcave shape enhances oxygen transport; no nucleus increases internal space.
- White Blood Cells: Larger, involved in immune defense with specialized functions.
- Neurons: Long axons facilitate impulse transmission over distances.

Mathematical Relationship: Volume increases faster than surface area, limiting cell size for efficient function.
Importance of SA:V Ratio:
- Efficient material exchange depends on a high surface area relative to volume.
- Cells maximize exchange through adaptations (e.g., folding, projections).
Impact: Larger organisms have more numerous small cells rather than fewer large ones, enhancing function.

Erythrocytes: Biconcave shape maximizes surface area for oxygen absorption.
Proximal Convoluted Tubule Cells: Microvilli increase reabsorption surface area, supported by extensive mitochondria for active transport.

Type I Pneumocytes:
- Thin and flat for minimal diffusion distance, covering 95% of alveolar surface.
Type II Pneumocytes:
- Produce surfactant to reduce surface tension; more cytoplasmic space for secretion.

Striated Muscle Fibres:
- Long and multinucleated for voluntary movement; contraction is based on the arrangement of myofibrils.
Cardiac Muscle Cells:
- Branching structure, single nuclei, interconnected to enable coordinated contractions across the heart.