CELL SPECIALISATION (B2.3)

How are stem cells formed?

• At fertilisation, the diploid single cell called a zygote begins dividing. After it becomes a solid ball of 16-32 cells, it is called a morula.

• Cell continues to divide and after 5-6 days, the morula will start to differentiate into a hollow ball of cells called a blastocyst

  • This contains outer layer of cells called the trophoblast, and the inner cell mass

  • Trophoblast will eventually develop into the placenta, whereas the ICM will become the embryo.

What happens despite cells having identical DNA sequences?

Cells differentiating into various types with specific functions

What is differentiation?

Involves the selective expression of gens, leading to the formation of specialised cell types.

What are 2 different cell types that differentiate?

• Embryonic cells

  • Start as identical but develop into diverse cell types during embryogenesis

• Morphogens

  • Secreted by certain cells in the early embryo, play a crucial role in establishing body patterns

How do morphogens create concentration gradients as they diffuse away from their source?

• Cells sense their positions in the concentration through surface receptors

• The positions in the gradients determines the fate of cells, guiding their development into specific cell types

• This mechanism enables the organisation of cells and the determination of body patterns with minimal genetic input

What is the differentiated cell produced from totipotent stem cells? ( egg cell of morula)

• Can differentiate into any type of cell including placental cell

• Can give rise to a complete organism

• Found in morula stage

• Contributes to the formation of the embryo, placenta and embryonic sac tissues

What is the differentiated cell produced from pluripotent stem cell? (embryonic stem cells of the blastocyst)

• Can differentiate into all body cells, but cannot give rise to a whole organism

• Present in the inner cell mass of the blastocyst

• Differentiate into cells of 3 germ layers; endoderm, mesoderm and ectoderm

What is the differentiated cell produced from multipotent stem cells? (umbilical cord stem cells)

• Can differentiate into a few closely related types of body cell

What is the differentiated cell produced from unipotent stem cells?

• Can only differentiate into their associated cell type

• EG. liver stem cells can only make liver cells

What is a stem cell niche?

Refers to the microenvironment where stem cells reside within an organism, influencing their behaviour and differentiation.

What do signalling molecules do?

Activate or inhibit gene transcription, regulating stem cells behaviour.

What do stem cells do?

• Communicate with neighbouring cells within the niche

• Interact with extracellular matrix surrounding them

• Some remain dorment , while others proliferate to maintain the stem cell pool

• The need to be maintained for the continuous supply of functional cells throughout an organisms life

Examples of stem cell niches

• Blood: Resides in the bone marrow, where they interact with supportive cells to regulate blood cell production

• Skin: Stem cells in the hair follicles in the ‘bulge’ region, regulate hair grwoth trough cycles of degeneration, grwoth, and rest

Human cell sizes

• 50-100 trillion cells

• Cells range from about 7.5( red blood cells) to 150 micrometers (human egg cell)

Egg cell vs sperm cell

• Egg cell volume is 10 million times larger than sperm cell volume

• Egg cell is one of the largest cells, and sperm is one of the smallest

What are the reasons for size differential in sperm vs egg?

• Egg cell large size is specialised for containing necessary nutrients for early embryo development

• Sperm is small due to its lack for need for nutrient content

What are some specialised human cells?

• Some nerve cells; those in sciatic nerve can be incredibly long, with axons exceeding 1 meter

• Red blood cells; smallest human cells, measuring approximately 7.5 micrometers in diameter

• White blood cells; larger than red blood cells, and range from about 10 - 20 micrometers and possess various shapes of nuclei

• Striated muscle fibres; large, multi nucleated cells which can be extremely long

How do cell regulate growth?

Through cell surface receptors and growth factors in the environment

Discuss the idea of SA : V

• As cell grows larger, SA:V ratio decreases

• Larger SA:V ratio facilitates nutrient intake and waste removal