Review Questions for Third Exam


Posted March 22, 2016


You should know all ectodermal, mesodermal and endodermal organs.

What subdivision of the ectoderm / mesoderm / endoderm each organ and tissue develops from, and be able to sketch and describe whatever folding or fusions of cell sheets occur in the formation of each.

Neural crest, neural tube, somatic ectoderm, epidermis, hair, feathers, scales of reptiles {epidermis; but fish scales are mesodermal!} pigment cells, Schwann cells, [In the facial area, neural crest cells form all tissues that develop from mesoderm in other parts of the body, including the skeleton] Lobes of the brain, lens of the eyes, optic cup, choroid fissure, pigmented retina, neural retina, ganglion cells, optic nerve, optic chiasm, optic tectum, retino-tectal projection, neural projections, ephrin protein, spinal cord, spinal nerves, sensory nerves, post-ganglionic autonomic nerves {from the neural crest; what else!}, placodes, olfactory nerve, nostril, palatal shelves, lens of eye, neuromast cells, semicircular canals, otoliths, cochlea, lateral line, ameloblasts, odontoblasts, enamel, dentine, teeth, pharyngeal pouches, gill slits, eustachian tube, tonsils, parathyroid glands, thymus organ, athymic mice, "nude mice", Rathke' pocket, salivary glands, thyroid gland, lungs, alveoli, pancreas, liver, cloaca, bladder, rectum, cartilage, bone, notochord, paraxial mesoderm, somites, dermatome, myotome, sclerotome, intermediate mesoderm, pronephros, pronephric duct = Wolffian duct = Vas deferens = mesonephric duct, mesonephros, metanephros, ureter, lateral plate mesoderm, coelomic cavity, heart, vena cava, ductus arteriosus, inter-ventricular septum, septal defects, "blue babies", situs inversus, genital ridges, ovary, oviduct, testis, sperm duct, uterus

primordial germ cells, (are regarded as not being part of any of the 3 germ layers)

Extraembryonic membranes: (in teleosts) enveloping layer, yolk syncytial layer
(in reptiles birds and mammals) chorion, amnion, "bag of waters", yolk sac, allantois, extraembryonic coelom.

Chorion and amnion : epidermal ectoderm and lateral plate mesoderm. yolk sac and allantois : endoderm and lateral plate mesoderm

Three alternative causes of identical twinning:

If both twins are enclosed in the same amnion, then there were 2 primitive streaks in the same inner cell mass. (and they will also be inside the same chorion)

If both twins are together inside the same chorion, but each separate in their own two amnions, then they developed from two separate inner cell masses, inside the same trophoblast.

If twins are both in their own separate chorion, then…

They developed from separate trophoblasts, and might not be identical twins.

Incidentally a very great immunological discovery was accidentally made by using skin grafts to (try to!) distinguish between cattle twins that were genetically identical versus which were "fraternal" twins.

Researchers expected that only non-identical twins would "reject" organ grafts from each other.
But immune tolerance VERY unexpectedly developed in non-identical twin cattle.
Their immune systems didn't attack each other! Can you figure out, or guess, why not?
(Note: this doesn't occur in human twins, for some unimportant reason.)




Which organs are formed by infolding placodes of somatic ectoderm? Which are formed from pharyngeal pouches in the neck? Name as many differentiated cells as you can that develop from neural crest cells.

What are the three subdivisions of each somite? And what cell types differentiate from each of the three?

What are some differences between the epidermis and the dermis? Which do scales develop from?

What are some other names for the Wolffian duct? From what do the sperm ducts develop?

What do the oviducts develop from? (Guess why many animals develop two uteruses) By what experimental disturbance could you cause two uteruses to develop, in a species that normally forms only one)?

How can you cause an embryo to develop two hearts?

List as many examples as you can in which the boundary between two different tissues or differentiated cell types develops at the location of a sharp fold in an epithelial sheet of cells.

How might the developing Wolffian duct tips be guided toward the cloaca?Maybe by chemotaxis? Maybe by haptotaxis (i.e. an adhesion gradient)? Can you devise experiments to distinguish between alternative guidance mechanisms?

* If a diffusible chemical caused localized increases in either the adhesiveness or the contractile strength of parts of crawling tissue cells (i.e. some component cells of our body, or of an embryo) then would concentration gradients of that chemical cause cells to orient their locomotion up concentration gradients of this diffusible chemical? Hint: yes. Would this be chemotaxis? or haptotaxis? or both? Which kind of chemotaxis?

Stomodeum is to palate as cloaca is to the tissue that separates the bladder from what?
from the rectum

Cleft palate is to the stomodeum as septal defects are to what?
the heart

Which organs develop from invaginations in the stomodeal ectodermal epithelium?

What is a placode? Where do they develop? Sketch a cross section of a placode in the process of forming.

How does the inner bone-like dentine layers of teeth form? Neural crest cells aggregate on the inner surface of sheets of ameloblasts.

Why are alligators able to continue replacing their teeth all during their life? Because a strip of invaginated stomodeal epithelium continues to exist and bud off vesicles of cells that differentiate into ameloblasts.

What embryological difference causes birds not to form teeth? Inductive signals are no longer sent from their neural crest cells to induce stomodeal ectoderm to differentiate into ameloblasts.

What is the evidence for that explanation?

Kollar & Fisher grafted mouse neural crest cells to locations next to stomodeal ectoderm cells from chicken embryos, and teeth developed.

What were some other possible genetic changes that could have resulted in evolutionary loss of teeth? Loss or mutation of receptors of the inductive signals that stimulate either ameloblast or odontoblast development. Mutations in genes needed for synthesis of either enamel or dentine. Several other possibilities, that you ought to be able to invent.

Imagine the possibility that some of the grafted mouse neural cells changed their differentiated state and became ameloblasts: Explain why the Kollar and Fisher experiment would then have led to erroneous conclusions.

What if some mouse stomodeal epithelial cells had accidentally been included among the grafted mouse neural crest cells; how could that have resulted in erroneous results?

Suggest how to test for the possibility of such errors of interpretation.
For example, what if fossil teeth from prehistoric birds had enamel which differed in some clear way from mammal enamel?
What if a green fluorescent marker had been inserted into mouse genes for making enamel? Then, how could you use this to reduce the possibility of misinterpreting the Kollar and Fisher experiment?

The lenses of the eyes develop from what, in what part of which germ layer?


"Chemotaxis" is the name given to any case in which cells or neural axons are guided toward targets by gradients of concentrations of any chemical.

Are there more than one possible mechanism that can produce chemotaxis?
Describe three. *Maybe you can invent a fourth. * Would you use this same word ("chemotaxis") to describe any means of accumulating cells at locations where some chemical's concentration is higher than elsewhere?

Which kind of chemotaxis is used by bacteria? By white blood cells? By Dictyostelium amoebae? By ciliate protozoa? By nerve growth cones of the ganglion cells, as they form the optic nerve by crawling out of the retina, under the brain, up the side of the midbrain, and connecting to the optic tectum to create a neural projection.

Do nerve fibers from the eye innervate the brain (optic tectum)?
Or do nerve fibers from the brain innervate the eye? (actually, some do.)

How are ephrin proteins and ephrin receptor proteins believed to participate in the formation of the neural projection? *In what sense is it "repulsion" for a chemical to stimulate detachment of cells? *Would less strongly contractile cells be less strongly "repelled" by stimulation of detachment? Would larger cells be repelled at longer distances? Or at least seem to be.

Ephrin proteins have been found to have locally increased concentrations in the boundary areas where somitomeres are splitting apart into separate somites: Explain why this helps explain segmentation of vertebrate bodies.

Steve Roth demonstrated that more cells from dorsal halves of chicken retinas will stick to ventral halves of chicken mid-brains ("optic tectums" [the real plural of tectum is tecta . Second declension neuter. Which you don't need to know]
This greater accumulation was interpreted as evidence that each part of the retina is more adhesive to the part of the tectum that it innervates. * Would that evidence have convinced you? *Would it have occurred to you that the real cause would be greater stimulation of detachment in the areas not innervated?

* Symmetry? Does less detachment equal more adhesion? … Less adhesion = more detachment? More pressure = less pulling; etc. Experimentally, can one distinguish whether any given phenomenon is caused by more of one variable, as opposed to less of its opposite? (Not an easy question; and not just "semantic".)

The next time you learn that a given phenomenon is caused by maximization of "X" will you try to invent a symmetrical reverse-explanation based on minimization of whatever is the reverse of X? (If X plus Y = constant. then maximization of X equals minimization of Y? Are these the same? *Or are they just difficult to tell apart? Can you think of some examples of this kind of similarity of double-opposites, either from embryology, biology, or any other field.

This question is not without interest; would you not disagree?







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