Flashcards in Lecture 5: Vertebrate Ontogeny Deck (21)
How are eggs classified based on the amount of yolk present?
- Microlecithal eggs: Contain a small amount of yolk. Usually mammalian eggs.
- Mesolecithal eggs: Contain a moderate amount of yolk (70% of the egg volume). Usually amphibian eggs.
- Macrolecithal Eggs: Contain a large amount of yolk (99% of egg volume). Usually bird and reptile eggs.
How are eggs classified based on their Distribution of yolk?
Isolecithal Yolk - Contain a small amount of yolk (usually also microlecithal) that is evenly distributed throughout the egg. Again, mostly mamallian eggs.
Telolecithal Yolk: The other two sizes of the eggs that is usually concentrated towards one side of the egg.
The egg with all the yolk in it is called the vegetal pole. The yolk-free side is called the animal pole.
What characteristics does the size of the yolk give about the egg's function?
- Microlecithal eggs have a small amount of nutrient availability, and have to rely on maternal blood for the rest of it's nurturing. In doing so, the young are born as functional "miniature adults"
- Mesolecithal eggs provide enough yolk to feed the larva post-birth, so the larva is born a lot earlier, and feeds on the egg for a while before it undergoes metamorphosis.
- Macrolecithal eggs have enough yolk to produce an adult-like form, care might be needed after they are born, but they appear to be mini-adult formed once they've hatched.
The fertilized egg starts by cleaving. Describe the first two cleavage plans?
- The first cleavage plane goes passes through the animal-vegetal axis of the egg, producing two daughter-cells, or blastomeres. This "unzipping"-like cleavage plans slows down when it reaches the thicker yolk. The second cleavage begins before the first cleavage ends. Gotta move fast when you're makin' life.
- The second cleavage plane runs in the same direction, but perpendicular (+shaped) to the first cleavage. Now there's four daughter cells. Proving once again that math doesn't get much wrong.
Describe the process after the second cleavage, up until the formation of the internal cavity.
- The third cleavage attempts to be perpendicular to the other two, but now that's getting harder, so it fails. Now we've got 8 daughter cells. Four of them are small and yolk-less (micromeres) and four of them are larger and yolky (macromeres).
- This whole cleave-happy thing keeps going until there's 64+ daughter cells. Micromeres continue to gravitate towards the animal pole, and macromeres continue to gravitate towards the vegetal pole.
- The cells begin to separate to form an "internal cavity". Due to the thickness of the yolk, the cavity (blastocoel) forms on the animal pole.
Describe what happens following blastulation
- The blastula becomes even more proliferated until it forms the gastrula. This is a hollow-ball shape with blastospores in the center, and the outer layer composed of future-neural tube cells. At this point, the cavity is also known as the archenteron, and is not the original blastocoel.
- The surface layer begins to fold into it's neural tube phase, undergoing neuralation and becoming the CNS.
Review and make sure you understand the diagram, Figure 4, on Slide 17
Pretty simple. The more colorful one in Lecture 1 is probably more relevant.
How are the first three cleavages in a bird embryo different than in a fish embryo?
- The cleavages don't go through the yolk; they're too big.
But at the point where there are 8 daughter cells, all of the are in contact with the yolk.
- The third cleavages is really "two cleavages", one on either side of the (+) sign. Still not even though.
Describe the 4th cleavage of the avian embryo.
The fourth cleavage is peripheral around the others to make a circle. (I think the outside of this circle sets the stage for the outer layer). Meanwhile the yolk undergoes a process called dissolution, which creates a subgerminal space.
Describe how the epiblast and hypoblast are formed in the avian embryo
- The cells in contact with the yolk develop a membrane between the two called the blastodisc. The blastodisc cleaves to produce multiple layers of blastomeres.
- The cell mass as a whole becomes the blastoderm, which is divided into the small, yolk-free cells, and the larger, yolk granuled cells.
- Important: The small cells go on the Outside, That's the epiblast. The smaller yolk cells go on the inside. That's the hypoblast.
Describe the formation of the blastocoel/gastrocoel in the avian embryo
- The epiblast and hypoblast separate, producing a cavity in between these two layers. This is called delamination. The initiation site of delamination marks the future Posterior end of the embryo.
- When completed, the hypoblast sinks back into the center, widening the cavity. This cavity becomes the blastocoel/gastrocoel.
Describe the formation of the trophoblast and inner-cell mass in mammals.
- The egg is holoblastic and the yolk is small so the cleavage will pass through the entire egg. So this is more comparable to an amphibian egg. After the 4th cleavage, we start to see the first unequal cells (1 big-ass central one, and 11 small ones surrounding it)
- A day or two later, this has become 8 large inner cells, and 99 outer cells surrounding it.
- The Inner Cells are the Inner Cell Mass, the Outer Cells are called the trophoblast.
Describe what happens to the trophoblast
- Trophoblast connects to the uterus and divides into two tissue types, cytotrophoblast, and syncitiotrophoblast.
- Cytotrophoblast is on the side of the blastocyst that's more internal than syncitiotrophoblast. This forms the "chorion," which contributes to the placenta.
- Syncitiotrophoblast - The outermost portion of the trophoblast. Separated by a membrane from the rest of the embryo. Acts as a "burrower" to dig it's way through the uterus. The Syncitiotrophoblast and the cytotrophoblast combine to form the complete placenta.
Describe the inner cell mass
Essentially forms the rest of the embryo. Including the amniotic membrane. The inner cell mass forms two different cavities. The first one is the yolk sack, which is useless in mammals. This continuously shrinks as the other (amniotic) cavity gets bigger.
What are some advantages of an embryo developing in an amniotic environment?
Carbon dioxide and oxygen diffuse readily between the embryo and the environment.
- Nitrogenous waste products (usually ammonia) can be readily excreted.
- Problem of desiccation is reduced.
What are some disadvantages of embryos developing in a terrestrial environment?
Keeping cell membranes moist, a must if diffusion is to occur, becomes difficult.
- Copious amounts of water are not available to dilute the highly toxic ammonia waste product.
- Food for the developing embryo may not be readily available in a terrestrial environment.
List the 4 extraembryonic membranes and their germ layers.
- Amnion: Composed of ectoderm and mesoderm
- Chorion: Composed of ectoderm and mesoderm
- Yolk Sac - Composed of endoderm and mesoderm
- Allantois - Composed of endoderm and mesoderm
- and honorary mention: Chorioallantoic membrane, which is a fusion of allantois and the chorion.
Describe the chorion
- Surrounds the embryo as well as the other membranes
- Not directly attached to the embryo
- Lies just below the shell of eggs of reptiles and birds
- In reptiles and birds, it fuses with the allantois to form the chorioallantoic membrane, which is important for gas exchange.
- It mammals, it is involved in the formation of the placenta.
Describe the amnion
- Surrounds the embryo proper except where the yolk sac and allantoic sac are attached
- Creates a fluid-filled environment important in protecting the embryo from desiccation.
Describe the yolk sac
Continuous with gut endoderm of the embryo
- In reptiles, birds, and egg-laying mammals, the yolk sac contains a massive amount of yolk necessary for nutrition for the developing embryos.
- In other mammals, the yolk sac eventually disappears but serves as the preliminary site for the formation of blood cells and germ cells.