morphogenesis Flashcards
what is involved in the development of cells?
what does morphogenesis mean?
what is morphogenesis like?
what are examples of simple morphogenesis?
what are some examples of refined differentiation?
Morphogenesis
Development involves not only differentiation of cells but also their morphogenesis into specific multicellular arrangements such as tissues and organs.
Literally, morphogenesis means the generation of embryonic shape or form.
Morphogenesis is a dynamic rather than static process, as tissues can change in shape repeatedly throughout development.
Examples of simple morphogenesis include change in shape of early embryonic tissues such as nueral tube formation and separation of different tissue layers in the embryo (endoderm, mesoderm, ectoderm). Examples of refined differentiation include the eye and retina tissue where cells are exquisitely layered and are considered by some to reflect the pinnacle of histogenesis
what are the major questions that should be kept in mind about morphogenesis?
what is usually discussed about in morphogenesis?
how are lymph nodes good examples of morphogenesis?
Major questions that form the framework of discussions of morphogenesis include:
1) How are tissues formed from cells? How do neural retina cells stick to other cells and not become integrated into the pigmented retina or iris cells next to them?
2) How are organs constructed from other tissues?
3) How do migrating cells reach their destinations and how do organs form in particular locations? Eyes develop in head and nowhere else. What stops eye development everywhere else?
4) How do organs grow and how is their growth coordinated throughout development?
5) How do organs achieve polarity?
Some cell behaviors associated with morphogenesis include cell shape change, cell division, cell motility, cell growth, cell death, and changes in the composition of cell membrane or secreted products.
Usually in morphogenesis we are discussing groups of cells in a tissue, so these cell behaviors are generally patterned in space and time across the cells in the tissue.
Recently, we looked at follicles in the lymph node; these follicles are a nice example of morphogenesis driven by some of these cell behaviors.
Under stimulation, lymph node follicles grow and their centers become paler.
This is due to cell division (in the center) and rearrangement (movement) of cells (dividing lymphoblasts in the center and lymphocytes on the outside).
what was Just most well known for?
no objectives that says i need to know this
Experimental Embryology and the concept of Differential Cell Affinity
Ernest Everett Just was an accomplished African-American embryologist active in the early part of the 20th century.
E.E. Just was born in South Carolina and grew up on James Island, which (at that time) was very rural, and rich with diversity of nature.
Just studied at Dartmouth and University of Chicago and was later hired to teach at Howard University. Originally he was hired to teach English but because there was a need for Biology teachers, Just soon moved into the Biology Department.
E.E. Just also spent numerous (up to 25) summers at Woods Hole Marine Biology Laboratory in Cape Cod where he had access to diverse types of embryos, and could interact with the most renowned embryologists of the time.
E.E. Just is most well known in biology in general for his important work demonstrating the fast block to polyspermy (the mechanism by which oocytes can be fertilized by one and only one sperm).
what was one of the most important concepts that EE Just contributed to?
explain how EE just performed these experiments.
what was a viewpoint that EE just supported that was not fully accepted at the time?
In terms of morphogenesis, E.E. Just also performed experiments that paved the way for one of the most important concepts in morphogenesis- differential cell adhesion.
E.E. Just performed these experiments on Astaria (Starfish) embryos. In this set of experiments, E.E. Just obtained cleavage stage embryos with different numbers of cells (with 2, 4, 8, 16 or 32 cells) and physically separated the individual blastomeres.
He then placed the blastomeres back in physical proximity and assessed whether the blastomeres would re-adhere.
His results demonstrated that only at early stages blastomeres would re-adhere.
At later stages the blastomeres could not re-adhere and remained separated from one another.
From this elegant work, E.E. Just concluded that there were stage specific variations or regulation in cell-cell adherence during embryonic development.
This was the first time that the concept of differences in adhesion in embryos was promoted.
A final note: Much of E.E. Just’s work purported an “organic” viewpoint where the whole (the embryo) was greater than the sum of the parts (cells).
This type of “emergent properties” viewpoint did not go over well with many of the other embryologists at Woods Hole who were strict reductionists, and believed, for example, that genes could explain all of development.
no question just read. no part of the objectives says this is relevant to know for exam.
Johannes Holtfreter was a German born experimental embryologist who overlapped with E.E. Just but was active more in the second half of the 20th century. Holtfreter and Just met when E.E. Just traveled to Germany in the 1930s where his ‘organic’ ideas were better received than they had been at Woods Hole. Holtfreter is an icon and character of embryology.
He was a traveler and an artist who spent much time when he was younger unemployed and traveling and drawing.
Later when he had his own laboratory, Holtfreter would take off a month every summer simply to travel and draw.
During these summers, he regularly spent time in Bali and other similar far-flung, seemingly exotic, locations. Holtfreter was able to enter Canada when the Nazis came to power in Germany in the 1930s and he spent time in a Refugee Camp in Canada.
Later Holtfreter obtained a faculty position at University of Rochester where he spent the rest of his career.
what did Townes and Holtfreter write about that is most well known?
what is described in these papers?
what was found in their studies?
how else did Townes and Holtfreter also repeat these experiements?
how did these scientists interpreted their findings?
Holtfreter made numerous contributions to the field of embryology, publishing many beautiful papers with gorgeous hand drawn illustrations.
He worked exclusively on amphibian embryos. Holtfreter’s papers published with Townes (his student) on Differential Cell Affinity are probably his most well known, and the most relevant for our discussion of Morphogenesis.
In the experiments described in these papers, Townes and Holtfreter took different tissues from amphibian embryos (e.g. neural and ectoderm) and dissociated the cells from those tissues.
They then combined the cells together and the cells would spontaneously reaggregate.
Amazingly, the cells from the different tissues would sort out.
For example, neural cells would adhere to other neural cells, and ectoderm cells would adhere together but form a separate tissue layer, distinct from the neural cell layer.
Townes and Holtfreter repeated these experiments with other tissue layers, including mesoderm, endoderm and ectoderm and obtained similarly striking results.
They interpreted these experiments as indicating that differential cells affinity is what separates different tissue layers and contributes to organized morphogenesis.
This concept of differential cell affinity remains the fundamental principle of morphogenesis.
what are the moecules that mediate differential cell affinity in morphogenesis?
what are cadherins?
what does each cadherin have?
what do the EC domains have for Cadherins?
what is the interaction site for Cadherins like?
what is required for cadherin adhesion?
intracellulary what do Cadherins bind to?
what allows Cadherins to bind to actin cytoskeleton?what have recent experiments shown?
Molecular Basis of Differential Cell Affinity
Molecules that mediate differential cell affinity in morphogenesis include Cadherins – remember our old friends from Epithelial Histology.
Cadherins are a family of homophilic cell-cell adhesion molecules.
Each Cadherin is a transmembrane protein containing 5 extracellular (EC) domains and a short intracellular segment.
The EC domains contain a binding site for Cadherins (that mediates homophilic binding).
Note that this Cadherin interaction site is specific so only the same types of Cadherins can bind to one another.
The EC domains of Cadherins also contain a Ca2+ binding site.
Ca2+ is required for Cadherin mediated adhesion. (Because of this, if you culture an embryo in Ca2+ free medium it will dissociate.)
Intracelluarly, Cadherins bind to adaptor proteins called catenins (including beta-catenin and alpha- catenin).
alpha- catenin links Cadherins to the actin cytoskeleton, although recent experiments have shown that this is an indirect coupling with some complex dynamics.
how do cadherin subtypes differ?
what do all classic cadherins have in common?
what can cadherins bind to?
what are cadherins a good candidate for?
what are the different types of cadherins?
what is the most striking example of differential cadherins expression?
what could this difference mediate?
what will disruption of N-cadherin cause?
what regulates cadherins?
what are the signaling pathways involved?
what is hevaly studied in Cadherins interaction , and why?
There are many different types of Cadherin molecules. Cadherin subtypes differ in their EC domains that mediate homophilic binding.
All classic Cadherins have the same intracellular segment that links to catenins and actin.
Because of the differences in EC binding domains, like Cadherins can only bind to like Cadherins.
This specificity of binding makes Cadherins good candidates to mediate differential, specific, tissue affinity.
Different types of Cadherins include E-cadherin that is expressed only in epithelial tissues, P-Cadherin (placental Cadherin) that is expressed both in the placental cells of the mammalian embryo that contact the uterine wall and the uterine wall cells, N-cadherin that is highly expressed in the neural ectoderm and in differentiating neurons, and C-cadherin that is expressed in blastomeres of, and mediates gastrulation movements in Xenopus embryos.
The most striking example of differential Cadherin expression in different tissues is in the ectoderm (E-cadherin) and prospective neural tube (N-Cadherin). Thus, the differential expression of E- and N-Cadherins could mediate the sorting out or separation of the neural tube from the ectoderm.
Indeed, disruption of N-cadherin function in the neural tube results in neural tissue invading ectoderm tissue.
Many signaling pathways regulate Cadherins or their intracellular partners the catenins.
These signaling pathways include those defined by Wnts, Fibroblast Growth Factors (FGFs) and Platelet Derived Growth Factors (PDGFs).
In particular the interactions between Wnt pathways and Cadherins are heavily studied.
This is because beta-catenin is a downstream component of both pathways.
Recall that earlier in the semester, I presented my research studying novel interactions between Wnt and Cadherin effectors, and their effects on optic axon pathfinding behaviors.
what is convergent extension?
how does this process work?
what happens during convergent extension?
what does convergent extension look like in tissues?
what does convergent extension look like in vertebrate embryos?
what does convergent extension look like in the mesoderm?
where does convergent extension look like in an embryo?
how does convergent extension occur in the neural tissue?
what would be a good analogy to explain this?
Convergent extension is the narrowing and lengthening of a tissue.
It is a morphogenetic process that is important for establishing the body axis in all vertebrates.
We will study convergent extension as a nice, concrete and specific example of morphogenesis.
During convergent extension, a tissue narrows (converges) along one axis and lengthens (extends) along a perpendicular axis.
In all the tissues we will discuss today, convergent extension involves convergence (narrowing) along the medio-lateral axis of the tissue/embryo, and extension (lengthening) along the antero-posterior axis of the tissue/embryo.
In vertebrate embryos, convergent extension takes place in (at least) two tissue layers, the mesoderm and the neural tissue layers (also called the neural ectoderm).
In the mesoderm, convergent extension is coupled with and is thought to be responsible for gastrulation movements that establish the future GI tract and anus of the embryo.
Convergent extension occurs in the mesoderm tissue while it is involuting inside the embryo.
In the neural tissue, convergent extension occurs while the neural tissue is rolling up into a tube and is thus involved in establishing the prospective spinal cord. Convergent extension in the neural tissue is thought by some to generate the driving force responsible for neural tube formation.
By analogy, if you extend a rubber sheet it will roll up along the long axis of the sheet.
what is the work of Ray keller involve?
Ray Keller is an embryologist who has been active since the 1970s.
His work includes beautiful and powerful descriptions of morphogenetic movements in Xenopus embryos. Like Holtfreter, Ray Keller has worked predominantly on amphibian embryos.
Also, like Holtfreter, Ray Keller is a character and an icon of embryology.
Keller grew up on a farm in SE Missouri, and was drafted in, and fought in the Korean War.
He then returned to the states and began a career in biology.
He has a passion for motorcycles and considered that as an alternative career to biology.
Currently, Ray Keller owns 15 motorcycles (Harley Davidson and other types).
In his PhD thesis Ray Keller did a masterful mapping of the movements of different tissue layers in the frog embryo.
Ray Keller did a post doc with JP Trinkaus at Yale University, who was a pioneer in the study of cell motility, especiaqlly in fish embryos.
Ray Keller was a tenured professor at UC Berkeley from the mid 1980s-1996.
He then moved with his wife to the University of Virginia where he still runs a lab though he is semi-retired.
His work involves detailed microsurgical manipulations of embryonic tissues, combined with molecular perturbations and biomechanical measurements.
what did kellers lab first show?
what is the Keller Sandwich?
what did john shih and ray keller describe ? how was this done?what were Shih and Keller able to show?
what did the mesodernal cells extend?
what led the mesodernal cells to mediolaterally intercalate?
what resulted in convergent extension of the mesodernal tissue?
what did one study show what is required for mesodermal convergent extension?
what have other labs been able to show?
what was lance davidson and ray keller able to show?
Work from Ray Keller’s lab first showed that you can recapitulate the convergent extension of the neural and mesodermal tissues that occurs in the embryo in explants (tissues that are extirpated from the embryo). Specifically, they invented the “Keller Sandwich” which includes two neural/mesodermal tissues explanted back to back.
This Keller sandwich is popular because it is easy to make, and can be used easily in assays for molecular regulation of convergent extension.
John Shih and Ray Keller described the cellular mechanism of convergent extension in the mesoderm tissue in the early 1990s.
To do this, they had to develop a new type of explant called the mesodermal shaved explant.
This explant consisted of a single layer of mesodermal deep cells that would undergo convergent extension autonomously.
Using high-resolution timelapse recordings and detailed cellular kinematic analysis, Shih and Keller showed that mesodermal cells underwent a sequence of cell behaviors that led to convergent extension of the tissue.
Specifically, the mesodermal cells extended large protrusions both medially and laterally.
The mesodermal cells applied these protrusions onto one another, exerted tension and then pulled past one another.
This led to the mesodermal cells mediolaterally intercalating, resulting in fewer cells along the mediolateral axis of the tissue and more cells along the anteroposterior axis of the tissue.
This is what resulted in convergent extension of the mesodermal tissue.
More recent studies have defined the molecular basis of mesodermal convergent extension.
One study from Paul Skoglund in the Keller lab showed that myosin II is required for mesodermal convergent extension, by regulation of mediolateral protrusions on meosdermal cells, and for formation of actin foci (dense spots) within the mediolaterally protruding mesodermal cells.
Other labs have shown that actin-microtubule regulators such as a RhoGEF are also required for mesodermal convergent extension.
Finally, Lance Davidson, a former postdoc with Ray Keller showed that Wnt pathways can also regulate the actin foci, or contractions, that are observed in mesodermal cells that have mediolateral protrusions
what has been less studied than mesodermal convergent extension?
what did Dr. Elul do define cellular mechanism of neural convergent extension?
what were the differences between Dr. Eluls experiment and mesodenal convergent extension?
Neural convergent extension has been less studied that mesodermal convergent extension.
This was the subject of my PhD work in the later 1990s and it was challenging!
To define the cellular mechanism of neural convergent extension, I had to develop a single layered neural deep cell explant, analogous to the mesodermal shaved explant made by Shih and Keller.
Once I developed this explant, I could define the basic cellular mechanisms of neural convergent extension.
I concluded first that the neural deep cells could converge and extend autonomously, and then, using timelapse recordings and detailed kinematic analyses, that the neural deep cells used mediolateral intercalation and also had medially and laterally directed protrusions.
This sounds similar to the mesodermal convergent extension but there are several significant differences between the two mechanisms of convergent extension. Specifically, neural deep cells were not as elongated as mesodermal deep cells, only had episodic medial and lateral protrusions (rather than continuous as in the mesoderm), and underwent a promiscuous mediolateral intercalation in which they exchanged neighbors with cells that originally were far away from them.
Thus, neural deep cells seemed to undergo a more inefficient process of convergence extension than the mesoderm.
After defining the basic cellular mechanism of neural convergent extension in deep neural explants, I then compared this mechanism to that in deep neural explants with underlying mesoderm.
These deep neural over mesodermal explants had a tissue configuration more similar to that in the embryo (in which the neural tissue overlies the mesoderm) and thus retained the full inductive interactions that occurred in the embryo.
The question I addressed then was how does neural convergent extension change in the presence of prolonged inductive interactions with the mesodermal tissue.
I discovered that the neural deep cells now showed regionally defined cell motility.
Specifically, lateral neural deep cells now showed medially directed protrusions and medially located neural deep cells showed randomly directed cell motility.
In addition, the mediolateral intercalation of the lateral neural deep cells was now conservative.
Together, these two studies defined neural convergent extension mechanisms in the presence and absence of signals from the mesoderm.
A few studies have defined the molecular basis of neural convergent extension but many questions remain.
Myosin IIB is required, and the Wnt pathway is involved but how these molecules interact, nor how they differentially regulate mesodermal and neural convergent extension cell motility behaviors is not known.
In addition, a recent study published this year from Margo in Ann Sutherland’s laboratory at the University of Virginia defined the cellular mechanism of neural convergent extension in mouse embryos.
This required development of techniques to culture embryos in order to perform confocal timelapse imaging of mouse neural ectoderm.
What are the similarities and differences of mouse and frog convergent extension?
That is the subject of a proposal I am working on, to create bioinformatics infrastructure to quantitatively and visually compare neural convergent extension in diverse species, including frog, mouse, zebrafish and chick embryos.
