Unit 18: Development, Stem Cells, and Model Organisms
How one fertilized cell commits to hundreds of fates, why some fates can be reversed, and how a whole animal can be rebuilt from a single adult nucleus.
6 topics · 21 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Cell Differentiation and the Stability of Cell Fate
- How it worksHow does a cascade of transcription factors lock in a specific gene expression pattern as a cell differentiates?
- Common mistakeWhy doesn't cell differentiation happen by cells losing the genes they no longer need?
- Going furtherWhat happens to a differentiated cell's fate when a master regulatory transcription factor gets forcibly expressed in it?
Your turn1 of 2
A skin cell and a neuron in the same body have identical DNA sequences, yet only the neuron expresses genes for building synapses and only the skin cell expresses genes for building keratin. A student assumes each cell type must have physically lost the genes it doesn't use. Why doesn't cell differentiation happen by cells losing the genes they no longer need?Embryonic Stem Cells and Pluripotency
- How it worksHow do transcription factors like Oct4, Sox2, and Nanog maintain an embryonic stem cell's pluripotent state?
- Common mistakeWhy isn't a pluripotent stem cell the same thing as a totipotent one that can also build placental tissue?
- Going furtherWhat happens to an embryonic stem cell's pluripotency the moment those core transcription factors are switched off?
Your turn1 of 2
An embryonic stem cell can give rise to every cell type found in the adult body. A student assumes this must mean an embryonic stem cell can also build the placenta and other extra-embryonic tissues needed to support a developing embryo. Why isn't a pluripotent stem cell the same thing as a totipotent one that can also build placental tissue?Induced Pluripotent Stem Cells
- How it worksHow does forcing just four transcription factors into a mature cell reprogram it back into a pluripotent state?
- How it worksWalk me through how an induced pluripotent stem cell's chromatin gets reset to an embryonic-like pattern during reprogramming.
- Common mistakeWhy aren't induced pluripotent stem cells identical in every way to embryonic stem cells?
- Ties togetherHow does iPSC reprogramming reverse the same chromatin-locking mechanisms that keep a differentiated cell's fate stable?
Your turn1 of 2
Induced pluripotent stem cells (iPSCs) are reprogrammed from mature adult cells and can differentiate into a wide range of cell types, much like embryonic stem cells can. A student assumes iPSCs must therefore be functionally identical in every respect to true embryonic stem cells. Why aren't induced pluripotent stem cells identical in every way to embryonic stem cells?Morphogen Gradients and Positional Information
- How it worksHow does a morphogen released from one source form a concentration gradient that cells interpret differently at different distances?
- How it worksWalk me through how different threshold concentrations of the same morphogen switch on different genes in nearby cells.
- Common mistakeWhy isn't a morphogen gradient simply a signal that turns one gene on or off in an all-or-nothing way?
- Going furtherWhat happens to a developing embryo's body plan when a morphogen's source is experimentally moved to the opposite end?
Your turn1 of 2
Cells at different distances from a morphogen's source in a developing embryo end up expressing different sets of genes and adopting different fates. A student assumes a morphogen must work as a simple all-or-nothing switch, either fully turning one gene on or fully leaving it off in every cell it reaches. Why isn't a morphogen gradient simply a signal that turns one gene on or off in an all-or-nothing way?Stem Cell Niches and Adult Tissue Renewal
- How it worksHow does a stem cell niche's local signals keep adult stem cells dividing while pushing most daughter cells to differentiate?
- Common mistakeWhy isn't every adult tissue equipped with the same regenerative stem cell population?
- Going furtherWhat happens to tissue renewal when a stem cell niche's signaling is disrupted by injury or aging?
Your turn1 of 2
The lining of the gut is replaced every few days by actively dividing adult stem cells, while most neurons in the adult brain persist essentially for a lifetime with very limited stem cell renewal. A student assumes every adult tissue must have the same kind of active regenerative stem cell population. Why isn't every adult tissue equipped with the same regenerative stem cell population?Cloning and Nuclear Reprogramming
- How it worksHow does somatic cell nuclear transfer place an adult cell's nucleus into an egg that's had its own nucleus removed?
- How it worksWalk me through how factors in the egg's cytoplasm reprogram that adult nucleus back to an embryonic state.
- Common mistakeWhy doesn't a cloned animal like Dolly the sheep have DNA identical to the animal that donated the egg?
- Going furtherWhat happens to a cloned embryo's epigenetic marks compared to a normally fertilized embryo's?
Your turn1 of 2
Dolly the sheep was cloned by transferring an adult cell's nucleus into an egg cell that had its own nucleus removed, with that egg coming from a different sheep than the one that donated the adult nucleus. A student assumes Dolly's DNA must be entirely identical to the sheep that donated the egg, since the egg provided the environment Dolly developed in. Why doesn't a cloned animal like Dolly the sheep have DNA identical to the animal that donated the egg?