development & embryology. sea urchins are models for the study of the early development of...

Development & Embryology

• Sea urchins are models for the study of the early development of deuterostomes. More later.– Sea urchin eggs are fertilized externally.– Sea urchin eggs are surrounded by a jelly coat.

Fertilization activates the egg and bring together the nuclei of sperm and egg

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 47.2


• The Acrosomal Reaction.– Acrosomal reaction: when exposed to the

jelly coat the sperm’s acrosome discharges it contents by exocytosis.• Hydrolytic enzymes enable the acrosomal process

to penetrate the egg’s jelly coat.

• The tip of the acrosomal process adheres to the vitelline layer just external to the egg’s plasma membrane.


– The sperm and egg plasma membranes fuse and a single sperm nucleus enters the egg’s cytoplasm.• Na+ channels in the egg’s plasma membrane open.

– Na+ flows into the egg and the membrane depolarizes: fast block to polyspermy.


FAST BLOCK TO POLYSPERMY

• The Cortical Reaction.– Fusion of egg and sperm plasma membranes

triggers a signal-transduction pathway.• Ca2+ from the eggs ER is released into the cytosol

and propagates as a wave across the fertilized egg IP3 and DAG are produced.– IP3 opens ligand-gated channels in the ER and the Ca2+

released stimulates the opening of other channels.


– High concentrations of Ca2+ cause cortical granules to fuse with the plasma membrane and release their contents into the perivitelline space.• The vitelline layer separates from the plasma

membrane.• An osmotic gradient draws water into the perivitelline

space, swelling it and pushing it away from the plasma membrane.

• The vitelline layer hardens into the fertilization envelope: a component of the slow block to polyspermy.

• The plasma membrane returns to normal and the fast block to polyspermy no longer functions.


Cortical Granule Fusion Following Egg Fertilization Video

Watch as cortical granules fuse with the egg cell plasma membrane immediately following fertilization by a sperm cell.


These images were made using a lipophilic dye that binds to the membranes of the granules, allowing us to see individual fusion events.


Notice how these fusion events, which are triggered by calcium, sweep across the egg surface immediately following sperm entry at the upper left.

Credit: Courtesy of Mark Terasaki

Calcium Release Following Egg Fertilization Video

• See the changes in free calcium in an egg cell immediately following fertilization by a sperm cell.


Simultaneous images of the fertilization event were taken using phase contrast microscopy to show the cell (left) and Calcium Green fluorescence microscopy to reveal areas of free calcium (right).


Notice how a wave of free calcium moves quickly across the cell, starting at the point of sperm entry in the upper right.

• Credit: Courtesy of Mark Terasaki

• Activation of the Egg,– High concentrations of Ca2+ in the egg

stimulates an increase in the rates of cellular respiration and protein synthesis.

– In sea urchins, DAG activates a protein that transports H+ out of the egg.• The reduced pH may be indirectly responsible for

the egg’s metabolic responses to fertilization.


– In the meantime, back at the sperm nucleus...• The sperm nucleus swells and merges with the egg

nucleus diploid nucleus of the zygote.– DNA synthesis begins and the first cell division occurs.


• Fertilization in Mammals.• Capacitation, a function of the female reproductive

system, enhances sperm function.– A capacitated

sperm migratesthrough a layerof follicle cellsbefore it reachesthe zona pellucida.

– Binding ofthe sperm cellinduces anacrosomalreaction similarto that seen in thesea urchin.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 47.5

• Enzymes from the acrosome enable the sperm cell to penetrate the zona pellucida and fuse with the egg’s plasma membrane.– The entire sperm enters the egg.– The egg membrane depolarizes: functions as a fast block to

polyspermy.– A cortical reaction occurs.

• Enzymes from cortical granules catalyze alterations to the zona pellucida: functions as a slow block to polyspermy.


– The envelopes of both the egg and sperm nuclei disperse.• The chromosomes from the two gametes share a

common spindle apparatus during the first mitotic division of the zygote.


Fertilization

• The nucleus of the egg and sperm unite to form a zygote

• Within an hour, the single celled diploid zygote begins to divide making many smaller cells called blastomeres– There is no time nor energy for growth of the

cells between divisions—it just makes more and more cells that are smaller and smaller!

– Soon a solid ball of cells is formed called a morula

Calcium Wave Propagation in Fish Eggs Video

See how waves of calcium move across a fertilized egg, beginning at the point of sperm entry. These eggs were injected with aequorin, a protein that emits light when bound to calcium ions.

Calcium Wave Propagation in Fish Eggs Video

Notice, at the end of the video, how similar waves of calcium can be induced in an unfertilized egg by calcium ionophore treatment.

Credit: Courtesy of Lionel Jaffe

Cleavage, no NOT that kind!

• The initial period of cell division is called cleavage.

• The embryo’s size is NOT increasing, just the number of cells

• The two ends of the zygote are referred to as the animal and vegetal poles [not a typo—it is vegetal not vegetable!] The very first division in a frog

embryo. You can clearly see the first cleavage furrow

The animal pole

• Generally, the blastomeres of the animal pole will go on to form the external tissues of the body

The thick, clear membrane you see outside the forming blastula protects against polyspermy, thus polyploidy, which is usually fatal.

Cleavage of a Fertilized Egg

Watch as a fertilized frog egg undergoes multiple rounds of mitosis and cytokinesis. Each round of cell division parcels the original cytoplasm into smaller and smaller cells. Notice how the cell divisions appear to be synchronized.

Credit: Courtesy of Dr. Huw Williams and Professor Jim Smith

The vegetal pole

• Generally, the cells of the vegetal pole will form the internal tissues

Here’s something you’d never guess!

• The initial dorsal-ventral orientation of the embryo is determined at fertilization by the location where the sperm nucleus enters the egg

• This point corresponds to the future belly [ventral] side

Formation of the blastula

• Tight junctions join the cells on the outside of the blastomere

• Like a belt of protein that encircle a cell and weld it firmly to its neighbors

• Connect adjacent cells in a sheet; prevents leaks

• Found in the walls of organs—lining of gut as well as cells on the outside of a blastula The tight junctions pictured are

found much later in the lining of the gut of a mammal

Formation of the blastula

• At about the 16-cell stage the cells in the interior of the mass begin to pump Na+ from their cytoplasm into the spaces between cells– Osmotic gradient established– Water is drawn into the CENTER of the cell

mass, enlarging the intercellular spaces– The spaces coalesce to form a single large

cavity within the cell mass

A blastocoel forms within the morula blastula

The resulting HOLLOW ball of cells is called the blastula

Increased Na+ causes water to fill intercellular

spaces creating the hollow center.

• In birds the yolk is so plentiful that it restricts cleavage to the animal pole: meroblastic cleavage.

• In animals with less yolk there is complete division of the egg: holoblastic cleavage.

Gastrulation

• Some of the cells then push inward forming a gastrula that is invaginated [inward pinching]

• These cells crawl over each other using lamellipodia—cellular extensions

Lamellipodia in Cell Migration Video

Watch how a cell is able to move using projections known as lamellipodia. These images of a migrating fish epidermal cell were taken using differential interference contrast microscopy. Notice how the lamellipodium forms a thin sheet at the forward edge of the cell, attaches to the substrate, and then pulls the cell forward.

Credit: Courtesy of Mark S. Cooper

Gastrulation

• This process creates the main axis of the vertebrate body

• Converts the blastula into a bilaterally symmetrical embryo with a central gut

• From this point on the embryo has 3 germ layers

Gastrulation

Primitive gut—endoderm—future stomach, lungs, liver and most of the other internal

organs

Cells that remain on the exterior—derivatives include: skin on

the outside and nervous system on the inside

Cells that break away from the invaginating cells and invade the space between the gut and

the exterior wall become mesoderm.Eventually form the notochord, bones, blood

vessels, connective tissues and muscles

Neurulation

• The process that forms the neural tube which gives rise to the brain and spinal cord– A broad zone of ectoderm begins to thicken on the

dorsal surface of the embryo– Triggered by the presence of the notochord beneath it

Neurulation• The thickening is produced by elongation of

certain ectodermal cells– The cells assume a wedge shape by contracting

bundles of actin filaments at one end causing the tissue at tone end to roll up into a tube

– The tube pinches off from the rest of the ectoderm forming the neural tube

Cell Migration

• A variety of cells migrate to form distant tissues following specific paths through the embryo– Neural crest cells

pinch off from the neural tube and form a number of structures making up the body’s sense organs

Cell Migration

• Somites are made from cells that migrate from central blocks of muscle and form the skeletal muscles of the body

• Precursors to blood cells and gametes also move

Somites

These darkened joint-like internal ridges are

somites. It lets researchers age an embryo prior to limb

bud formation.

Xenopus Neurulation(time lapse) Video

This video shows the development of an embryo of Xenopus, the African clawed frog. This animal is a favorite of embryologists, as many of the developmental processes seen in a frog are also observed in other vertebrates. This time-lapse video starts with the end of gastrulation and progresses to the beginning of organogenesis—particularly the beginnings of the nervous system.

Credit: Ray Keller and John Shih, University of California, Berkeley, sponsored by the Society for Developmental Biology

Xenopus Neurulation (time lapse) Video

Late in gastrulation, cleavage has produced numerous cells too small to be seen individually. The cells roll over the lip of the blastopore and into the interior of the gastrula. Inside, lighter-colored yolk-laden cells that originated at the vegetal pole form the endoderm of the rudimentary digestive tract. Cells from the animal pole spread over the embryo and form ectoderm. In between, a layer of mesoderm is taking shape. Credit: Ray Keller and John Shih, University of

California, Berkeley, sponsored by the Society for Developmental Biology


Late in gastrulation, cleavage has produced numerous cells too small to be seen individually. The cells roll over the lip of the blastopore and into the interior of the gastrula. Inside, lighter-colored yolk-laden cells that originated at the vegetal pole form the endoderm of the rudimentary digestive tract. Cells from the animal pole spread over the embryo and form ectoderm. In between, a layer of mesoderm is taking shape. Credit: Ray Keller and John Shih, University of

California, Berkeley, sponsored by the Society for Developmental Biology


At about 9 seconds, you are seeing the back of the embryo, with what will become the head at the top. The blastopore later becomes the anus. Under the dorsal surface, from head to tail, a rod of mesoderm called the notochord is forming; later it will be replaced by the vertebral column. Above the notochord, the ectoderm thickens, forming a neural plate bounded by two neural folds. This is the beginning of neurulation, the formation of the nervous system.

Credit: Ray Keller and John Shih, University of California, Berkeley, sponsored by the Society for Developmental Biology

Adult Xenopus

Organogenesis and Growth

• The embryo is a few mm long by now

• It has 105 cells• Over the course of

development it will enlarge 100 times and have a million times as many cells

Most Multicellular Organisms Develop according to Molecular

Mechanisms

• Six major mechanisms– Cell Movement and Induction– Determination– Pattern formation– Expression of Homeotic Genes– Programmed Cell Death– Aging [that part after the birth!]

Cell movement and Induction

• One way cells move is by pulling themselves along using cell adhesion molecules such as cadherin proteins

• There are at least a dozen different types of cadherins identified thus far


• Each type of cadherin attaches to others of its own type at its terminal segments forming a 2-cadherin link between the cytoskeletons of adjacent cells

• If cells expressing two different cadherins are mixed, they quickly sort themselves out aggregating into 2 separate masses


• Determinants—developmental signals• Regulative development—Each cell

receives an equivalent set of determinants. Body form is determined by cell-cell interactions. MAMMALS

• Mosaic development—initial cells created by cleavage divisions contain different determinants. INSECTS


• Separate the cells of an early blastula and allow them to develop independently– Animal pole blastomeres develop ectoderm– Vegetal pole blastomeres develop endoderm– NEITHER develop mesoderm!

• Place animal and vegetal cells next to each other, some of the animal pole cells will develop as mesoderm! What gives?


• The interaction between cells triggers the switch in the developmental path of the cells

• Induction—when a cell switches from one path of development to another as a result of interaction with an adjacent cell

How?

• Inducing cells secrete proteins that act as intercellular signals

• Morphogens--These molecules cause abrupt changes in the patterns of gene transcription

Organizer cell clumps set up a gradient of the morphogen

• Organizers—groups of cells that produce diffusible signal molecules that convey positional information to other cells

The Zone of Polarizing Activity in the Chick Limb Bud

• The ZPA is an organizer– Secretes Sonic hedgehog,

a protein growth factor [morphogen].

– Required for pattern formation of the limb along the anterior-posterior axis.

It sets up a gradient, cells close to it get a high concentration of the morphogen while those farther away get a lesser concentration of the morphogen

• Apical ectodermal ridge (AER).– Secretes fibroblast growth factor (FGF)

proteins.– Required for limb growth and patterning along

the proximal-distal axis.– Required for

pattern formationalong thedorsal-ventralaxis.


Fig. 47.23a

The Zone of Polarizing Activity in the Chick Limb Bud


Fig. 47.23a

My undergraduate research!

• I was a bit freaked out when I found this example in a book!

• It was what I did the summer of 1982 [I know. I’m old!] with a team of other students

• We transplanted cells, carefully mapping their locations and watched what happened! Mirror images of the ZPA results in

mirror limb images. Rather sadistic, don’t you think?

Xenopus again

• The morphogen activin in low concentrations causes animal pole cells to develop into epidermis

• Slightly higher levels induce muscle

• Higher still notochord

Determination

• Mammalian eggs are symmetrical in both contents and shape

• All of the early blastoderm cells are equivalent up to the 8-cell stage– Separate the 8 cells from an 8-cell stage

embryo 8 identical twins– Join 2 separate ones from 2 DIFFERENT 8-

cell-stage embryos one organism forms

Is that weird, or what?

• Totipotent—potentially capable of expressing all of the genes of their genome

• All 8 cells are totipotent, once they become 16-cells, they have begun determination

• Chimera—contains cells from different genetic lines—that’s what we call our single organism created from joining the 2 cells from the 2 different 8-cell stage embryos!

Determination

• The process begins by cell-cell interactions

• Determination—the commitment of a particular cell to specialized developmental path

• Differentiation—cell specialization that occurs at the end of the developmental path

• Cells are first determined to a tissue type, then differentiated into those tissues

The Mechanism of Determination

• Gene regulatory proteins are the tools used by cells to initiate developmental changes

• When genes encoding these proteins are activated they often reinforce their own activation [kind of like one baby in the nursery cries and it sets off a chain reaction!]

The Mechanism of Determination

• Cells may not undergo differentiation until some time later

• Other factors interact with the regulatory protein and cause it to activate still other genes

• Need I say, “puberty”?

Can Determination Be Reversed?

1984—Steen Willadsen, Danish, working in TEXAS, cloned a sheep

His success was due to picking an embryo in VERY early stages of development

January 1996—Scotland—Keith Campbell and Ian Wilmut developed a technique to allow older cells to accomplish cloning

July 5, 1996—Dolly was born!

More about Dolly

• The ewe that donated the mammary cell was 6 years old

• That means the cells had been differentiated for a VERY long time!

• Previously, only embryonic cells had been successful—the key was getting the mammary cell nucleus [clone] and the egg nucleus of the host cell to be in the same stage of cell division

http://members.fortunecity.com/ggord4/parton/page1.htm

More about Dolly

• To prepare the mammary cells, they were starved in steps over a 5-day period

• This arrested cell division & caused them to pause at the beginning of the cell cycle

• Eggs from a ewe were enucleated• Both cells were surgically combined in

January of 1996 by inserting the mammary cells inside the covering around the egg cell

More about Dolly• A brief electrical shock was administered• Caused the plasma membranes of the two cells

to become leaky so that the nucleus of the mammary cell passed into the egg cell—a very cool trick!

• The shock also kick-started the cell cycle, causing cell division

• After 6 days, in 30 of 277 tries, the dividing embryo reached the blastula stage and 29 of these were transplanted into surrogate mother sheep

• One gave birth to Dolly on July 5, 1996

Pattern Formation

• Drosophila – the fruit fly has the best understood pattern formation mechanism

• During oogenesis, mRNA molecules are deposited in one end of the egg by nurse cells

• This marks the embryo’s front end.

Expression of Homeotic Genes

• After pattern formation has established the number of body segments in Drosophila

• A series of homeotic genes act as master switches to determine the forms these segments will assume

• Homeotic genes—code for proteins that function as transcription factors

Programmed Cell Death

• Webbed fingers and toes• Human tails • Tadpole tails• Necrosis—cell death caused by injury—

cells typically swell and burst, releasing their contents into the extracellular fluid

• Apoptosis—cells shrivel and shrink and their remains are taken up by surrounding cells

Gene Control of Apoptosis• A “death program” is

activated• All animal cells possess

such programs• Nematodes—the same

131 cells always die during development in a predictable and reproducible pattern of apoptosis

• Three genes govern the process and constitute the death program itself

How?

• In humans the bax gene encodes the cell death program

• An oncogene called bcl-2 represses it• The bax protein seems to induce

apoptosis by binding to the permeability pore of the cell’s mitochondria, increasing its permeability and in doing so, trigger cell death

How?

• The bcl-2 prevents damage from free radicals

• Antioxidants are almost as effective as bcl-2 in blocking apoptosis

Aging—my least favorite!

• 4 different theories• Accumulated

Mutation Hypothesis• Telomere Depletion

Hypothesis• Wear-and-Tear

Hypothesis• Gene Clock

Hypothesis

Accumulated Mutation Hypothesis

• The oldest theory [tee hee !]• Cells accumulate mutations as they age,

leading to eventual lethal damage• Somatic mutations do indeed accumulate

– Tend to accumulate the modified base 8-hyrdroxyguanine; an –OH group is added to G

– Little evidence these mutations CAUSE aging– No acceleration in aging occurred among

survivors of Hiroshima and Nagasaki

Telomere Depletion Hypothesis

• In a seminal exp. In 1961, Leonard Hayflick demonstrated that fibroblast cells will divide only a certain number of times

• About 50 for that type of cell• Cell division stops, the cell cycle blocked

just before DNA replication• Freeze a cell sample after 20 doublings,

thaw it, and it resumes growth for 30 more doublings then stops!


• An explanation of the “Hayflick limit” was suggested in 1986 by Howard Cooke

• He glimpsed an extra length of DNA at the ends of chromosomes


• Telomeric regions—repeats of TTAGGG• Shorter in older somatic tissue• Cooke speculated that a 100 base-pair

portion of the telomere cap was lost by a chromosome during each cycle of DNA replication

• After 50 replications, perhaps the protective telomeric cap would be used up


• Senescence—the state a cell enters when it is no longer able to proliferate

• Proof: In 1998 researchers transferred into human primary cell cultures a gene that leads to the expression of telomerase

• Telomerase—builds TTAGGG telomeric caps• New caps were added to the chromosomes of

the cells• These cells did NOT senesce at the Hayflick

limit and continued to divide for more than 20 additional generations

Wear-and Tear Hypothesis

• The idea that cells wear out.• Just a statistical limit—nothing designed

into the cells• Over time, disruption, wear and damage

eventually erode a cell’s ability to function properly

• Free-radicals provide evidence; they are molecules with ONE unpaired electron


• Glycation--Free radicals cause glucose to become linked to proteins

• Two of the most common proteins involved are collagen and elastin—key components of the connective tissues in our joints

• Glycated collagen and elastin are NOT replaced and individual molecules may be as old as the individual!


• Glycation of collagen, elastin and a plethora of other proteins within a cell produces a complex mixture of glucose-linked proteins, advanced glycosylation end products or AGEs for short. Cute, huh?

• They can cross-link; reducing flexibility in the joints—a symptom of aging

Gene Clock Hypothesis

• Very little doubt that at least some aspects of aging are under the direct control of genes

• How else do you account for Cher, Goldie Hawn, Dick Clark and Bo Derek?


• Hutchinson-Gilford syndrome—mutations in the “aging” genes

• Growth, sexual maturation and skeletal development are retarded

• Atherosclerosis and strokes usually lead to death by age 12

• Only some 20 cases have been reported• Recessive


• Werner’s Syndrome• Not as rare—10 people per million• Werner found this in a family in 1904• Premature aging, appears in adolescence,

producing death before age 50 of heart attack or a form of rare connective tissue cancer

• Gene identified in 1996 on the short arm of Chromosome 8


• Affects a helicase enzyme involved in the repair of DNA

• The mutant helicase enzyme may fail to activate critical tumor suppressor genes

• Work on Nematodes has been done that mutates a set of genes thought to govern an intrinsic clock

• The life span has been increased 5 fold!• It spends more time in each phase of its life

cycle

Embryology and Evolution

• Two kinds of embryos as we travel from simple to complex down the evolutionary chain

• Protostome— “first mouth”; these organisms have embryos in which the blastula indents to form a two-layer thick ball with a blastopore opening to the outside. In mollusks, annelids and arthropods, the mouth develops from or near the blastopore

• Deuterostome—”second mouth”; the blastopore becomes the anus

Deuterostomes evolvedfrom protostomes more than 630

million years ago.

• Both are coelomates—have a body cavity

• Deuterostomes differ in many other aspects of embryo growth, including the plane in which the cells divide.

Differences

• Deuterostomes differ in many other aspects of embryogrowth, including the plane in which the cells divide.

• most importantly, the cells that make up an embryonicprotostome each contain a different portion of theregulatory signals present in the egg, so no one cell of the embryo (or adult) can develop into a complete organism.

• In marked contrast, any of the cells of a deuterostome can develop into a complete organism.

Amniote embryos develop in a fluid-filled sac within a shell or

uterus

• The amniote embryo is the solution to reproduction in a dry environment.– Shelled eggs of reptiles and birds.– Uterus of placental mammals.

• Avian Development.• Cleavage is meroblastic, or incomplete.• Cell division is restricted to a small cap of

cytoplasm at the animal pole.• Produces a blastodisc, which becomes arranged

into the epiblast andhypoblast thatbound theblastocoel, theavian versionof a blastula.


Fig, 47.12 (1)

• During gastrulation some cells of the epiblast migrate (arrows) towards the interior of the embryo through the primitive streak.

• Some of these cells move laterally to form the mesoderm, while others move downward to form the endoderm.


Fig, 47.12 (2)

• In early organogenesis the archenteron is formed as lateral folds pinch the embryo away from the yolk.

• The yolk stalk (formed mostly by hypoblast cells) will keep the embryo attached to the yolk.

• The notochord, neural tube, and somites form as they do in frogs.

• The three germlayers and hypoblastcells contribute tothe extraembryonicmembrane system.


Fig, 47.12 (3)

• The four extraembryonic membranes are the yolk sac, amnion, chorion, and allantois.– Cells of the yolk sac digest yolk providing

nutrients to the embryo.– The amnion encloses the embryo in a fluid-

filled amniotic sac which protects the embryo from drying out.

– The chorion cushions the embryo against mechanical shocks.

– The allantois functions as a disposal sac for uric acid.



Fig. 47.14

• Mammalian Development.– Recall:

• The egg and zygote do not exhibit any obvious polarity.

• Holoblastic cleavage occurs in the zygote.

– Gastrulation and organogenesis follows a pattern similar to that seen in birds and reptiles.


– Relatively slow cleavage produces equal sized blastomeres.• Compaction occurs at the eight-cell stage.

– The result is cells that tightly adhere to one another.


–Step 1: about 7 days after fertilization.• The blastocyst reaches the uterus.• The inner cell mass is surrounded by the

trophoblast.

Fig. 47.15 (1)


• Step 2: The trophoblast secretes enzymes that facilitate implantation of the blastocyst.– The trophoblast thickens, projecting into the

surrounding endometrium; the inner cell mass forms the epiblast and hypoblast.

– The embryo will develop almostentirely from the epiblast.


• Step 3: Extraembryonic membranes develop.– The trophoblast gives rise to the chorion, which

continues to expand into the endometrium and the epiblast begins to formthe amnion.

– Mesodermal cells are derived from the epiblast.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 47.15 (2) and (3)

· Step 4:· Gastrulation: inward movement of epiblast cells

through a primitive streak form mesoderm and endoderm.

Fig. 47.15 (4)


· Once again, the embryonic membranes – homologous with those of shelled eggs.· Chorion: completely surrounds the embryo

and other embryonic membranes.· Amnion: encloses the embryo in a fluid-filled

amniotic cavity.· Yolk sac: found below the developing embryo.

· Develops from the hypoblast.· Site of early formation of blood cells which later migrate to

the embryo.· Allantois: develops as an outpocketing of the

embryo’s rudimentary gut.· Incorporated into the umbilical cord, where it

forms blood vessels.· Organogenesis begins with the formation of

the neural tube, notochord, and somites.


Purple Sea Urchin Embryonic Development

Female (top) exuding eggs after being injected with KCl

Male (bottom) exuding sperm after being injected with KCl

Sea Urchin Embryonic Development1) The unfertilized egg is about

100 micrometers (µm) in diameter, similar to that of humans, and is surrounded by an extracellular layer called the vitelline layer. Upon fertilization by the first sperm, the vitelline layer becomes raised off the surface of the egg and hardens, forming the protective structure known as the fertilization envelope. All cleavages up to the blastula stage occur within this envelope.

Sea Urchin Embryonic Development

Purple Sea Urchin Egg with Fertilization Membrane

Sea Urchin Embryonic Development (time lapse) Video


2) During first cleavage, the nuclear envelope breaks down, and the duplicated chromosomes separate into two complete sets, followed by cytokinesis. In the two new cells, or blastomeres, you can clearly see the two new nuclei.

Sea Urchin Embryonic Development3) Second cleavage,

progressing from 2 to 4 cells, is seen here. Cleavages will proceed synchronously, approximately every 30 minutes, passing through the morula stage (16-64 cells) when the cells are loosely attached to each other, up to the blastula stage (more than 128 cells).

Sea Urchin Embryonic Development4) The blastula stage is

seen at the end of this clip. This stage is made up of a hollow ball of 1000 or so cells, arranged in a single-layered epithelium. The cells are tightly packed together, maintaining a space in the center called the blastocoel cavity.

Sea Urchin Embryonic Development5) At the beginning of gastrulation, a number of

cells in the flattened "vegetal pole," shown next at the bottom of the embryo, move as individual cells into the blastocoel cavity. In this cavity the cells migrate around, fuse with each other in a ring, and begin secreting elements of the calcium carbonate skeleton of the embryo. Because these cells are the first to move as individual cells in the embryo, they are called the primary mesenchyme cells (PMCs). The remaining cells in the vegetal pole fill in the gaps, restoring a complete epithelial sheet.

Sea Urchin Embryonic Development6) While the PMCs are migrating around,

archenteron formation, or formation of the embryonic digestive tract, begins. The first stage involves the pushing in of the vegetal pole to form a short, wide, blind-ended tube.

Sea Urchin Embryonic Development• 7) This tube then narrows and

elongates by a process that includes extensive cell rearrangement. Following this elongation, a subset of cells (secondary mesenchyme cells) at the tip of the archenteron will extend processes that contact a specific site on the inside of the ectodermal wall and tow the archenteron toward that spot. The wall of the ectoderm will bend inward and fuse with the tip of the archenteron to form the mouth.

Sea Urchin Embryonic Development• The wall of the ectoderm will bend inward and

fuse with the tip of the archenteron to form the mouth. The digestive tract will differentiate into an esophagus, a stomach, and an intestine, and the embryo will begin to feed. Four to 8 or 12 arms will extend, supported by internal skeletal elements. This feeding larva will float around in the plankton, eating algal cells, for 5 or 6 weeks, then will metamorphose into the adult form of the sea urchin.

• Credit: Rachel Fink, editor, “A Dozen Eggs,” Society for Developmental Biology

development & embryology. sea urchins are models for the study of the early development of...

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