B2.3_Cell specialization

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B2.3 Cell Specialization

What are the roles of stem cells in multicellular organisms? How are differentiated cells adapted to their specialized functions

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B2.3.1— Production of unspecialized cells following fertilization and their development into specialized cells by differentiation.

The zygote then divides by mitosis (D2.1.7) to form an embryo composed of genetically identical cells.

The cells in the zygote and embryo are unspecialized stem cells (B2.3.2).But as the cells of the embryo continue to divide, they begin to specialize in structure and function

B2.3.1— Production of unspecialized cells following fertilization and their development into specialized cells by differentiation.

B2.3.1— Production of unspecialized cells following fertilization and their development into specialized cells by differentiation.

B2.3.1— Production of unspecialized cells following fertilization and their development into specialized cells by differentiation.

But remember!

• All cells of a multicellular organism share an identical genome – each cell contains the entire set of genetic instructions for that organism
• The activation of different instructions (genes) within a given cell by chemical signals will cause it to differentiate into different cell types

B2.3.1— Production of unspecialized cells following fertilization and their development into specialized cells by differentiation.

Differenciation is driven by the release of gene regulating chemicals (transcription factors) called morphogensThe impact of the morphogen will be determined by its relative concentration:

• Cells closer to the morphogen source receive higher concentrations of morphogen, resulting in the activation of more genes
• Cells further away from the morphogen source receive lower concentrations of morphogen, resulting in the expression of fewer genes

Morphogen gradients control the differential expression of genes within an early-stage embryo

B2.3.2— Properties of stem cells.

B2.3.4— Differences between totipotent, pluripotent and multipotent stem cells.

B2.3.4— Differences between totipotent, pluripotent and multipotent stem cells.

Stem cell research requires the destruction of an embryo in order to extract the pluripotent stem cells. These cells can be used to advance medical treatments for many diseases. Different groups would argue that life begins at varying stages of embryonic development.

B2.3.3— Location and function of stem cell niches in adult humans.

A stem cell niche is a microenvironment within the organism in which the stem cells live in preparation for future proliferation and differentiation

Interactions via signalling molecules with other cells or with the extracellular matrix of the niche can activate, or prevent genes from transcribing. As a result, the stem cell can:

• remain dormant (inactive)
• divide into more of the same kind of stem cell
• become differentiated into another kind of cell.

B2.3.3— Location and function of stem cell niches in adult humans.

Bone Marrow

• Haemopoietic stem cells are located within the bone marrow and give rise to the different types of blood cells (e.g. erythrocytes, leucocytes and thrombocytes)
• Bone marrow transplants are commonly employed to replace the haemopoietic stem cell niche following chemotherapy for leukemia (blood cell cancer)

B2.3.3— Location and function of stem cell niches in adult humans.

Hair follicles

• The hair follicles contain a range of epidermal stem cells that are involved in cyclic bouts of hair growth, skin innervation, vascularisation and wound repair
• These stem cells could potentially be harvested and used to regenerate skin tissue in burns victims (or stimulated to promote hair regrowth in bald individuals)

​​B2.3.5— Cell size as an aspect of specialization.

The size of cells can vary significantly in multicellular organisms in order to optimise the specific function of a cell

Neurons need to transmit signals throughout the body and can be over 1m in length (but with a width of only ~10 µm)

Striated muscle fibres consist of fused muscle cells – they can have a width of 20–100 µm and a length of up to 12 cm

A human ovum (female egg) is one of the largest cells with a diameter of 120 µm, while the male sperm is extremely small (~5 µm)

white blood cells are larger than red blood cells and range from about 10 to 20 µm. They also vary in structure and function.

Red blood cells need to squeeze through narrow capillaries

B2.3.6- Surface area-to-volume ratios and constraints on cell size.

• The rate of metabolism of a cell is a function of its mass / volume (larger cells need more energy to sustain essential functions)
• The rate of material exchange is a function of its surface area (large membrane surface equates to more material movement)

As a cell grows, volume increases faster than surface area, leading to a decreased SA:Vol ratio

• If metabolic rate exceeds the rate of exchange of vital materials and wastes (low SA:Vol ratio), the cell will eventually die

• Hence growing cells tend to divide in order to maintain a high SA:Vol ratio suitable for survival

B2.3.6- Surface area-to-volume ratios and constraints on cell size.

Activity: Kognity, section B2.3.5-6

B2.3.7— Adaptations to increase surface area-to-volume ratios of cells.

Cells that are specialized for exchange of materials and have adaptations to increase the SA:V ratio. For example:

• Small size (e.g. prokariotes)
• Compartmentalisation (eukariotes)
• Flattended shape (pneumocytes type 1 in alveoli, capillary endothelial cells)
• Villi (e.g. enterocytes in the digestive epithelium)
• Invaginations (e.g. mitochondria and chloroplasts)

B2.3.8- Adaptations of type I and type II pneumocytes in alveoli.

In the alveolar epithelium, two different cell adaptations that fulfil very different roles, are required for the overall function of the tissue:

Lamellar bodies (secretory vessicles)

B2.3.9- Adaptations of cardiac muscle cells and striated muscle fibers.

Skeletal muscle fibers:

• Up to 12 cm
• Cilindrical and unbranched -> unidirectional contraction

• Multinucleated (syncytium) -> why?

Both cardiomyocites and skeletal muscle fibers:

• Contain many mitochondria
• Myofibrils of actin and myosin with sarcomeres (striated appearance)
• Myofibrils shorten during contraction

Cardiomyocites:

• 150 µm
• Single central nucleus -> why?
• Branched
• Intercalated discs with gap junctions
• Synchronised contraction

B2.3.10- Adaptations of sperm and egg cells.

Both nuclei are haploid

Sperm cell: Smaller and motile Rich in mitochondria No storage of nutrients (provided by seminal fluid) Acrosome with digestive enzymes

Egg: Larger Cytoplasm rich in polysaccharides, proteins and lipids External layers prevent multiple fertilization

Linking questions

• What are the advantages of small size and large size in biological systems?

• How do cells become differentiated?