DFF lecture 20/21: animal development

what are the 2 basic/extreme reproductive strategies? Describe them

*1. r-selected*- many offspring with little investment per offspring (lots of eggs and sperm made, but little resources are provided to them and not a high success rate, often just thrown into the water) *2. K-selected*- few offspring with a lot of investment per offspring (more yolk + resources, parental care)

describe frog gastrulation

- begins when cells in gray crescent region move inward - cells of animal pole spread out (becoming ectoderm), and they push surface cells below them toward and across the dorsal lip. These cells involute into the interior of the embryo, where they form the endoderm and mesoderm - involution creates the archenteron and destroys the blastocoel. The blastopore lip forms a circle, with cells moving to the interior all around the blastopore. The yolk plug is visible through the blastopore

describe the emergence of pattern in the egg. What is established prior to fertilization? What is established after?

- initially, the anterior/posterior axis is already established in the egg prior to fertilization. - in some, yolk is concentrated in posterior/vegetal region - other axes (dorsal/ventral, right/left) appear during or after fertilization

describe bird gastrulation

- posterior epiblast cells change shape and thicken, forming the primitive streak - cells migrate, converging at the primitive streak and causing it to elongate - the primitive streak narrows and lengthens... - this forms the primitive groove: the chick blastopore. Cells migrate inward through the primitive groove and Hensen's node (primitive node, where it stops) - Cells generated in Hensen's node and passing into the gastrula migrate anteriorly and form head structures and the notochord

describe sea urchin gastrulation

- the vegetal pole of the blastula flattens - some cells invaginate to form archenteron, while other cells involute, becoming the mesenchyme (mesoderm) - thin extensions of mesenchyme cells attach to the overlying ectoderm - the archenteron elongates, assisted by the contraction of mesenchyme cells - the mouth forms where the archenteron meets the ectoderm

gastrulation

a series of cell and tissue movements in which the blastula-stage embryo folds inward, producing a three-layered embryo, the gastrula.

what types of cells do epiboly during gastrulation?

almost always ectoderm cells

what types of cells do passive envelopment during gastrulation?

always yolk-filled endoderm cells

induction

an interaction between embryonic cells in which one cell influences the fate and the expression of the genes of another cell

animal pole

anterior region of egg where fertilization takes place (the receptors for sperm are along here) contains the least yolk

what question does fate mapping answer?

are the cells from a certain region destined to become a certain part of the later embryo?

when does the blastocyst implant into the uterine wall?

beginning of 2nd week, BEFORE gastrulation at this point, there are only epiblast and hypoblast layers (in embryoblast) and then a trophoblast, and a blastocoel with fluid

invagination and involution occurs in the ____________

blastopore

the dorsal lip becomes the ____________

blastopore

The dorsal mesoderm cells used in the Mangold-Spemann transplantation experiment are ______________ to become notochord a determined but not yet fated b fated but not yet determined c both fated and determined d neither fated or determined e Not enough data to know about fate or determination of these cells

c

invagination

cells attached in a layer move inward by changing their shapes, resulting in the change in shape of the layer like pushing finger into balloon, balloon cells don't detach from each other cells move in together, attached as one, changing shape of tissue

how do determination and fate differ in regards to transcription factors?

in determination, there are certain transcription factors that will be active no matter what, while in fate that's not true (involves lots of timing!!) sl . 36

how is cleavage different from typical mitosis?

in normal mitosis, you have long periods of G1, G2, transcription, etc. whereas in cleavage it's mostly just a series of replicating DNA (S phase) and mitosis, and repeat. And in cleavage, the cells aren't focused on getting bigger, they're actually getting smaller

meroblastic (incomplete) cleavage

incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds the yolk is really hydrophobic so the motility and microtubule proteins don't work well in this environment and you don't get cleavage in the entire embryo- so blastomeres only form from the animal pole, and the cells kinda float on a big thing of yolk

involution

individual cells move inward by actively crawling through, beneath, and around other cells may end up in same place, but INDIVIDUAL not maintaining desmosomes and other attachments

in mammal gastrulation, the blastodisc forms from ________ ________ cells

inner mass this is reptilian-like gastrulation (aka similar to birds, like we learned)

in what major way is sea urchin gastrulation different from other animals?

invagination is assisted by the pulling of mesenchymal cells attached to ectoderm (kinda stringy-looking things)

what is unique about mammalian cleavage?

it has many planes of division

what would happen if an embryo lacked a gray crescent?

it wouldn't develop a nervous system

frog fate map

know what region becomes what

epiboly

many stacked layers of cells reorganize their attachments resulting in a longer but fewer layered tissue that stretches over the surface of the embryo on outer surface

what types of cells typically do invagination during gastrulation?

mesoderm and endoderm cells

what types of cells typically do involution during gastrulation?

mesoderm cells, some endoderm

at the late blastula stage, the ___________ is determined while the ____________ is not

mesoderm; ectoderm

what does the neural plate form?

neural tube

What type of movement do the cells that form the neural tube undergo? What about the neural crest cells?

neural tube: invagination crest: involution

what 3 structures does the mesoderm subdivide into following gastrulation?

notochord, somites and lateral

what is the evolutionary explanation of the mammalian pattern of development?

o Echinoderm pattern could be perhaps more practical. o Historic relic of the evolutionary origin of mammals. Development is a complex and important phenotype that tends to be highly conserved through evolution.

the notochord is the ____________

organizer

vegetal pole

posterior region of egg where the yolk is concentrated

in birds, a __________ ______ is formed while in frogs, a _________ _______ is formed

primitive streak; dorsal lip

Define each as holoblastic or meroblastic: - sea urchin - frog - human - bird

sea urchin: holoblastic frog: holoblastic human: holoblastic bird: meroblastic consider how much yolk there is!

neuralizing signal

signal sent out by the organizer that binds to bone morphogenic protein 4 (BMP4), inactivating it. BMP4 encourages the formation of ventral epidermal structures (ectoderm) and prevents neural development. This signal released from the organizer therefore inhibits an inhibitor of neural formation so that the nervous system can develop

in animals with a lot of yolk, smaller cells would be located at the _________ pole and larger cells would be located at the _____________ cell

small: animal big: vegetal

what is used to determine fate vs determination?

transplantation experiments

The pigmented cortical cytoplasm tilts toward the direction where the sperm entered, and that tilts the animal/vegetal poles. How much tilt depends on where the sperm entered. Closer to equator = more tilt occurs, and you end up with a gray crescent

uhh learn this

at what point could the blastomeres in a mammal fall apart, resulting in twins? Why?

up until about the 16 cell stage/morula- because the cells are very loosely connected at this point so they could fall apart. However, at the 16 cell stage, the molecules tend to compact, they form cell adhesion molecules and can't break apart anymore (this is called a morula)

Each somite divides into 3 layers. What do they become?

upper: skin middle: muscle lower: cartilage of vertebrae and ribsf

determination

when a cell commits to be a certain type of cell no matter what, even in big trauma (basically, a toe cell will form into a toe cell even if it's by the heart) when a cell is determined, even if you move it within the embryo, it will become the thing it was fated to become or die trying

do microtubules aid in cortical rotation?

yes

Are cellular descendants of the grey crescent necessary to induce ectodermal cells to form a neural tube in frog? What experiment's result demonstrates the answer?

yes Mangold-Spemann organizer experiment (where they isolated the zygote, with the gray crescent only on one side)

Are the presence of dorsal lip cells of the organizer sufficient to induce overlying ectoderm cells to form a neural tube in frogs? What experiment's result demonstrates the answer?

yes Mangold-Spermann organizer experiment (when they took a chunk of mesoderm and placed it on the other side and it made another notochord)

if trauma occurred to a mesodermal cell at the late blastula stage, could its fate be changed to benefit the organism?

yes- determination doesn't occur until gastrulation BUT it would have to stay a mesoderm cell, it couldn't switch to ectoderm or endoderm because those are already determined wait...this might be wrong

passive envelopment

yolk-filled cells that don't actively move but are covered by other cells undergoing epiboly. Look like they move inward but in reality they stay in the same place while other cells cover over them don't move but appear to move

describe the process of neurulation

Above the notochord, part of the ectoderm flattens and becomes the neural plate. This forms a neural groove and then bends to create a neural tube (invagination). Cells along the edges are called neural crest cells and move independently (involution) and form the PNS (nerves, spinal ganglia, other tissue)

describe a transplantation experiment on amphibia, and the conclusions

Late blastula ectodermal cells were moved from the region of future epidermis to the region of future neural tissue and vice versa. conclusion: ectodermal cells decide during gastrulation whether to later become epidermal or neural cells. Spatial information is important in the decision making process. These cells are not yet determined at the late blastula stage

when does the archentron form in sea urchins and frogs? What about in birds?

SE/dogs: during gastrulation birds: after gastrulation

archenteron

The endoderm-lined cavity, formed during gastrulation, that develops into the digestive tract of an animal. primitive gut

describe the process of a single fertilized egg becoming a blastula

The fertilized egg undergoes cleavage, which is usually frequent and synchronous at first (except in humans and some mammals). Blastomeres (daughter cells) increase in number, but the total size stays about the same. These cells are generally pretty loosely connected. Once there are about 8-16 cells, this is called a morula. At this point, the cells tend to be more compacted. Finally, the blastomeres will bunch in one region and will be surrounded by fluid, making a blastocyst/blastula with a blastocoel

the initial steps of mammalian gastrulation is more similar to _________, while the later steps are more similar to ___________-

frog and sea urchin; reptiles initially it's more similar to frog and sea urchin because of a small amount of yolk and the distribution of it, causing all to have holoblastic cleavage. Later it's more similar to reptiles/birds because we share a more recent common ancestor. Also, mammals share the primitive streak with birds.

how is bird gastrulation different from other animals?

gastrulation occurs posterior to anterior, different from other animals in which gastrulation is simultaneous in different regions.

what is the future site of the dorsal lip and organizer? What determines this region?

grey crescent this is formed by the fertilization of the egg by the sperm- the region of the egg in the animal pole rotates toward the site of sperm entry. This establishes the dorsal/ventral axis and produces the grey crescent

the archenteron forms the _________

gut

What are the 2 types of cleavage?

holoblastic and meroblastic

do holoblastic and meroblastic cleavage result in same or different size cells? Why?

holoblastic: different meroblastic: same because in holoblastic, the cells can still develop in yolk region but cleavage is more difficult there (so the cells are larger) in meroblastic, there's so much yolk that it inhibits growth of cells in vegetal region and thus they only grow in animal pole and are the same size

what is the major difference between r-selected and K-selected reproductive strategies? What is an exception?

how much yolk is present yolk has a hydrophobicity on how fertilized eggs will develop exception: placental mammals- there isn't lots of yolk but high investment instead based on implantation of small fertilized egg into uterus, which provides environment, develop placenta to feed embryo directly from mother

describe the Mangold- Spemann Organizer Experiment and what it showed us

A chunk of the dorsal lip in the blastula (prior to gastrulation) was placed on the other side of the cell. 2 blastopores formed, with 2 notochords and 2 separate neural tubes, resulting in twins attached at the belly with separate nervous systems Conclusion: dorsal mesoderm is already determined in blastula stage, and it signals the ectoderm around it what to become

is the yolk hydrophilic or hydrophobic?

hydrophobic

what did fate mapping prove?

in an undisturbed embryo, a region of the later embryo consistently develops from a particular region of the earlier embryo. Thus, this technique is successful in determining the fate of cells key: undisturbed embryo

what are the 3 germ layers formed by gastrulation? What do they become?

1. Ectoderm (outer)- forms from epiblast layer and will become epidermis/skin and nervous system 2. Mesoderm (middle)- forms from rapidly dividing cells in gastrulation, will become bones, connective tissue and muscles 3. Endoderm (inner)- forms from hypoblast layer, will become linings of gut, linings of most blood vessels and urethra, as well as secretory portions of endocrine and exocrine glands

what 2 signals does the organizer send out?

1. neuralizing signal (inactivates BMP4 so that the nervous system develops) 2. a signal which instructs the lateral adjacent mesoderm to form the somites

cleavage results in cells that are greater in number but actually get smaller. Why is this?

1. not much time for growth 2. fertilization envelope prevents the cell from obtaining energy and mass from external sources

how many layers do organs typically have?

3! endo, meso, ecto

describe the process of fate mapping

Cells of an early embryo are marked with a dye which does not spread or fade (now done with intracellular markers; originally external). Then, you determine which structures have dye in later stages.

why is the inactivation of BMP4 necessary?

If BMP4 weren't inhibited, all of the ectoderm would become skin/epidermis, but the inactivation of BMP4 prevents this in areas where it should not form, like the neural plate (encourages nervous system to develop, and not skin)

which side of the developing frog embryo has a higher concentration of B-catenin?

dorsal

the gray crescent will later become the _________ _____________ and _______________

dorsal mesoderm and organizer

at what point do ectodermal cells decide whether to become skin or CNS cells?

during gastrulation

the animal pole becomes the __________, while the vegetal pole becomes the ____________

ectoderm; endoderm

passive envelopment often occurs because of ___________

epiboly

T or F: the purpose of gastrulation is to form the archentron

false! the purpose is to form the 3 germ layers

fate vs determination

fate is when a cell will develop into a given type only in an undisturbed embryo, while determination is when the cell will develop into a given type no matter what, even if it's placed in a different area of the body

if something is determined, it must also be ________

fated

the neural crest cells migrate between layers of what?

somites the neural crest cells become nerves and other tissue in PNS

cleavage

special mitosis that begins immediately after fertilization

the entry point of the sperm determines what?

the dorsal/ventral part of the embryo where the sperm enters is ventral dorsal = gray crescent

holoblastic (complete) cleavage

the entire egg cleaves (complete division of egg) common when there isn't a lot of yolk, like in sea urchins and frogs

embryonic induction

the process of a cell becoming determined to a particular fate

organizer

the region of the dorsal lip that induces (causes formation of) the neural plate

describe how the emergence of pattern occurs in a frog

there are two proteins distributed throughout the cell. One inhibits the other. In the vegetal pole there is another protein that inhibits the inhibitor. Thus, in the animal pole the inhibitor is dominant, while in the vegetal pole that first protein is dominant. specifically... B-catenin is distributed throughout the cytoplasm of the egg, as well as GSK-3 proteins that degrade the B-catenin. Vesicles in the vegetal pole (dorsal side) move on microtubule tracks to the side opposite of sperm entry The vesicles release a GSK-inhibiting protein, so now there is lots of B-catenin on dorsal side (bc no GSK to kill it) and on the ventral side there's lots of GSK but not B-catenin (bc GSK kills B-catenin). It's a gradient.

Where do somites develop? What do they become?

they develop alongside the neural tube during its formation, and become skin, muscles, cartilage, vertebrae and ribs

why are structural changes in cells so important?

they lead to functional specialization (different anatomy leads to different physiology)