The place on the surface where meiosis occurred, and the two polar bodies were produced is called the animal pole, & usually drawn at the top.
The exact opposite point on the surface is called the vegetal pole.
(like if the north pole were the animal pole then the south pole would be the vegetal pole)
Sea urchins :members of the phylum Echinodermata.
(like also starfish)
Small egg cells, about 100 micrometer spheres;
But notice that and average adult cell would be only about 10 microns in diameter, so the egg cells have about 1000 times the volume
Transparent, not too much yolk;
And the yolk is almost evenly distributed;
by coincidence VERY similar to egg cells of mammals like human or mouse.
{Don't tell anybody, but a few species of echinoderms have larger yolky eggs; but don't bother to remember this, because hardly anybody knows}
Just after fertilization, a special protective layer forms to protect the egg from being reached by any more sperm. This is called the fertilization membrane. If a second sperm gets in, the egg will not develop normally, so all species have special mechanisms to protect eggs from "polyspermy"
A special property of sea urchin eggs is that at the fourth mitosis, the 4 cells closest to the vegetal pole cleave very unequally, each dividing into one tiny cell ("micromere") and a relatively larger cell ("macromere")
Micromeres have a special fate, and lots of research has been done on them.
As more cell divisions occur, the cells form a hollow sphere, with the cells packed tightly together as an epithelial sheet.
The space inside is called the "blastocoel"
At this state of development, the embryo is called a "blastula".
It is an "unsolved problem" what mechanism causes this hollow space to form and expand. Many people think the cells push each other apart each time they divide, and some evidence that seems to favor this idea is that if you measure the blastocoel volume, it expands in jumps (the graph of volume versus time looks like stair-steps). Other people (me, for example) think water is pumped into the blastocoel, inflating it.
If you put sea urchin blastulas in salt solutions that are just like sea water, except no calcium ions, then the cells separate. Then, if you put these separated cells back in regular sea water, the cells will reaggregate into little solid balls of cells, which re-form blastocoels! Descendants of the micromeres aggregate at one end of each of these!! In other words, they can re-form their correct anatomy! This is a good puzzle how!
These cells are called the primary mesenchyme:
later they form the skeleton.
Other cells near the vegetal pole also become mesenchymal and move into the blastocoel; these are called the secondary mesenchyme,
and later will become muscle cells.
At the vegetal pole, the cell sheet actively folds inward.
Such infolding is called invagination,
& embryology includes hundreds of other examples in which parts of epithelial cell sheets undergo invagination.
One theory about the cause is active 'contraction' of muscle-like proteins (called "cytoplasmic actin" and myosin) concentrated at the concave surfaces of those cells. Another theory is based on swelling of an extracellular gel layer. Yet another explanation depends on active rearrangements of cell-cell adhesions. Personally, I would bet on the acto-myosin.
This invagination forms an inside tube, called the archenteron, most of which will later form the digestive tube of the larva (endoderm).
Eventually the invaginating archenteron reaches all the way up through the blastocoel to near the animal pole. A smaller infolding forms up there:
called the stomodeum, and the cells of the tip of the archenteron rearrange to form an opening, which becomes the mouth of the larva.
Two sideways outfoldings from the sides of the archenteron separate from it as hollow sacks, that become the body cavity. the "coelom"
Long stiff arms extend outward from the embryo,
each supported by a rod of skeleton.
The embryo has now become a pluteus (larva)
When plutei were first discovered in plankton, they were thought to be a separate kind of animal, and pluteus was their genus name! (They swim by cilia, or really flagella)
{To me they look like Klingon Battle-Cruisers in Star-Trek;
but pluteus was a Latin word for a special kind of wheeled shield, mentioned by Caesar's De Bello Gallico!}
Frogs and salamanders are both members of the class of vertebrates called amphibians.
Their egg cells are much larger, millimeters in diameter,, and have black pigment granules concentrated in their surface cytoplasm, in the half closest to the animal pole. The other side is heavier, so the vegetal pole rotates downward. The black top, yellow bottom has the effect of camouflage, so that swimming animals can't see them as well.
{notice how many fresh water turtles and fish are dark on the top and yellow or white on the top, resulting in this same kind of camouflage.}
Cleavage is holoblastic (despite so much yolk)
But horizontal cleavage furrows form nearer the animal pole, resulting in the cells near the vegetal pole being 5 or 10 times bigger than cells nearer the animal pole, but the difference in size is NOT because of more divisions at the animal end.
(as is sometimes taught in some sections of Biol 52)
A small blastocoel forms below the animal pole.
Gastrulation is by invagination along one side,
not at the vegetal pole (in contrast to urchins).
The blastopore forms about at the latitude of Australia
About half the blastula surface rolls inward in gastrulation; becoming mesoderm & endoderm.
additional notes on gastrulation from a past year's lecture
The mesoderm cells rearrange to form:
The notochord: a solid rod of vacuolated cells
tightly wrapped by spiral layers of collagen fibers
Two columns of "paraxial mesoderm"
one on each side of the notochord.
These columns subdivide into rows of somites,
which are block-like segments.
These form first near the front, then a pair at a time from front to rear. (Not head paraxial mesoderm)
Each somite later subdivides into 4 parts:
dermatome, myotome, anterior sclerotome,
& posterior sclerotome.
Mesoderm just lateral to the paraxial mesoderm forms rods (one on each side)
called intermediate mesoderm.
To either side of these strips of intermediate mesoderm
forms sheets called lateral plate mesoderm.
Each sheet of lateral plate mesoderm splits, so as to form a hollow space inside itself.
This space will be the coelomic cavity.
The cells that are left behind at the surface of an embryo in gastrulation become the ectoderm
The part of the ectoderm above notochord & somites
rolls up to form the neural tube
(sort of like invagination, except as a tube)
The edges of the neural tube fuse together,
and the cells there become mesenchymal
and crawl down inside.
These are called the neural crest.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The rest of the ectoderm is called somatic ectoderm
The brain, spinal cord and motor nerves
develop from neural tube ectoderm
The sensory nerves, autonomic nerves, pigment cells, and facial skeleton
(the rest of the skeleton develops from mesodermal)
develop from neural crest ectoderm
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"I am not making this up, you know" Anna Russell
"And so Brunhilde is the first woman that Siegfried has ever met who is not his aunt"
The endoderm forms the lining of the digestive tract - esophagus, stomach, intestine
The lungs, the liver, the pancreas, thyroid gland
gill slits, and maybe some of the salivary glands
form by invaginations of parts of the endoderm.
The notochord supports swimming by tadpoles and young fish, is needed for normal elongation,
but later degenerates in higher vertebrates.
Some people think it forms intervertebral discs
but they are wrong (but don't argue with them).
The dermatome forms dermis of the skin.
All voluntary muscles come from myotome.
Each pair of anterior sclerotomes forms the posterior half of one of your vertebrae (bones)
Guess what posterior sclerotomes form!
(yes! they form anterior halves of vertebrae!!)
Intermediate mesoderm forms kidneys (3 pairs!)
and male sex ducts.
Lateral plate mesoderm forms
the lining of the coelom,
the heart, the female sex ducts,
and the mesodermal parts of limb buds,
except NOT the muscles (which are myotome)
Next,
let's vote what is the weirdest fact so far!
and
What happens in animal development?
I) The parents' bodies form sperm cells and egg cells.
(& this is more complicated than you think)
II) A sperm cell fertilizes an egg cell.
(there are mechanisms to block more than one sperm)
III) This fertilized egg cell divides into hundreds
or thousands of cells. (trillions in humans, eventually)
IV) These cells "turn on" different subsets of genes,
becoming dozens or hundreds of differentiated cell types. (becoming liver cells, or nerve cells, or skin cells, or muscle cells, or pigment cells etc. 250 in humans/ a dozen in Hydra)
V) As embryonic cells begin their differentiation, they also begin to crawl around, and otherwise actively rearrange. The first set of rearrangements are called gastrulation; then comes neurulation, followed by other active cell rearrangements.
(*) "Growth" is seldom if ever the driving force that causes these rearrangements; Historically, however, people's first hypothesis has almost always been growth, in the sense either of enlargement or of mitotic divisions. For example, gastrulation in fish embryos; neural tube closure in amphibian, mammal and bird neurulation.