Lecture 5 Flashcards
Describe yolk distribution and developmental strategies
Yolk is synthesized in liver and delivered to ovary via bloodstream (transferred in soluble form, becomes insoluble in ovary) Microlecithal eggs - very small amount of yolk (mammals, young need external source of nutrients) Mesolecithal eggs - moderate amount (70% of egg volume, amphibians, provides enough energy to produce free-swimming larval form that can hatch and feed while continuing to develop) Macrolecithal eggs - large amount (99% volume, bird and reptilian eggs, enough energy to produce adult-like form though birds still need parental care, reptiles feed on their own) Isolecithal eggs - small amount of yolk evenly distributed throughout egg (mammals) Telolecithal eggs - Moderate to large amount of yolk that is concentrated towards one pole (vegetal pole) yolk free pole (animal pole), amphibial, reptilians and avian eggs
Describe major developmental steps of the frog embryo: cleavage –> gastrulation
(Look at slide for figures) - Cleavage is mitotic activity not accompanied by growth or differentiation and results in a multicellular mass - The 1st cleavage planes passes through animal-vegetal axis of the egg and produces two identical daughter cells (blastomeres). Cleavage plane slows down when it encounters yolk in the vegetal pole - The 2nd cleavage will begin before the first cleavage has been completed, results in four daughter cells equal in size and amount of yolk - The 3rd cleavage plane passes perpendicular to first two and is pushed above the equator of the embryo into the animal pole - 3rd cleavage will result in 8 cells, 4 cells will be smaller and relatively yolk-free (micromeres) 4 cells will be larger and densely packed with yolk (macromeres) - Cleavage continues but is slowed down in the vegetal pole which results in a solid ball of cells (32-64 cells) consisting of more and smaller cells at the animal pole and fewer/larger yolk-filled cells at the vegetal pole - Cells begin to separate to form internal cavity (blastocoel), because of the interference of yolk, the blastocoel is eccentrically displaced towards the animal pole - Following blastulation, cell movements and folding of cell layers results in rearranged hollow ball stage referred to as the gastrula in a process called gastrulation - The gastrula superficially resembles blastula, but it consists of several layers of cells, and the cavity (gastrocoel or archenteron) is not the original blastocoel, there is an opening called the blastopore but large mass of yolk-filled cells form a yolk plug that occludes the blastopore - Following gastrulation, surface of layer cells covering gastrocoel will undergo a folding process called neurulation resulting in the neural tube (will form entire CNS)
Relate frog gastrula to the vertebrate body plan
(Look at slide for figures) - After neurulation, the embryonic body plan resembles the generalized vertebrate body plan discussed in lecture 1
Describe the basic steps in the early cleavage and development of the avian embryo and compare with amphibian embryo
(Look at slide for figures) - Instead of cleavage occurring through a ball, this pattern occurs within a flattened disk (blastodisc), this type of cleavage is referred to as meroblastic/discoidal cleavage - The 1st cleavage is represented by a furrow in the cytoplasm of the cytoplasmic disc at the top of the animal pole, the cleavage plane doesn’t extend down into yolk itself but will spread outward and deeper as yolk is converted into cytoplasm - The 2nd cleavage furrow is perpendicular to the 1st - After the 2nd cleavage furrow, there are 4 equal sized blastomeres whose cytoplasm is in contact with yolk underneath and continuous cytoplasm on the periphery - The 3rd cleavage furrow lies parallel on either side of the first producing eight connected blastomeres - The 4th cleavage is a circular furrow that forms the periphery of the first 3, creating 16 blastomeres - The yolk underneath the central blastomeres undergoes dissolution creating a subgerminal space - At about the 16-32 cell stage, cleavage furrows begin to appear parallel to the surface of the blastodisc creating multiple layers of blastomeres - At about the several hundred cell stage, the cell mass is referred to as the blastoderm and consists of two cell populations; small yolk free cells and larger cells with yolk granules - The cells separate out forming an upper layer of small cells (epiblast) and a deeper layer of large cells (hypoblast) - The epiblast separates from the hypoblast forming a small cavity between the two layers (separation is called delamination and begins at one side of the disc and precedes to the opposite side, initiation site marks future posterior) - When delamination is complete, hypoblast sinks into the subgerminal space, creating a space between the epi and hypoblasts
Describe basic steps in the early cleavage and development of mammalian embryo and compare with avian
(Look at slide for figures) - The mammalian egg is isolecithal - The cleavage planes will pass entirely through the egg (holoblastic) - The first several cleavages are similar to that of the amphibian egg: equal and perpendicular to each other. - After about the fourth cleavage (3-day human embryo) cleavages become unequal: The 3 day human embryo consists of one large central cell and eleven small cells surrounding it - At about 4.5 days of cleavage, the human embryo consists of approximately 8 larger inner cells and approximately 99 smaller outer cells (inner cells = inner cell mass, outer cells = tropoblast layer • Trophoblast: • At the time of implantation into the uterine wall the trophoblast will attach to the lining of the uterus (endometrium) and will form two kinds of tissues: a cytotrophoblast and a syncytiotrophoblast. • The cytotrophoblast consists of a layer of cells on the inside of the blastocyst separated from each other by cell membranes. • The cytotrophoblast will give rise to the chorion which, in mammals, contributes to the placenta. • The syncytiotrophoblast forms on the side of the blastocyst that has initiated the implantation process. It consists of a syncytium, composed of cells whose cytoplasm and cell membranes have fused into a common mass
Distinguish between the inner mass cells and trophoblast in terms of their developmental potential
• Trophoblast: • The syncytiotrophoblast will erode its way into the uterine endometrium and will form cavities (trophoblast lacunae) that will become filled with maternal blood. This blood will provide nutrients and oxygen to the developing embryo. • The cytotrophoblast and syncytiotrophoblast will combine to form the placenta. • Inner cell mass: • Cells of the inner cell mass will form the embryo proper as well as additional membranes that surround and support the embryo. • These membranes include the amnion, yolk sac, and allantoic stalk.
List advantages for embryos developing in an aquatic 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.
List disadvantages for 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 four extra embryonic membranes
(Look at slide for figure) • 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)
Describe function of 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 function of 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 function of 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.
Describe function of Allantois
• Projects from gut endoderm of embryo just caudal to yolk sac • In reptiles and birds the allantois stores nitrogenous waste in the form of uric acid and fuses to the chorion to form the chorioallantoic membrane necessary for gas exchange. • In mammals, only the proximal part of the allantois is retained; it forms the umbilical cord connecting the embryo to the placenta (chorion).
Identify the Respective Letter to the Animal type
A= Microlecithal isolecithal egg of the echinoderm
B= Mesolecithal,telolecithal egg of the frog
C=Macrolecithal telolecithal of the chick
D=Centrolecithal egg of the insect
What animal is this type of Cleavage occuring in?
Frogs
What Animal TYPE is this cleavage pattern occuring in?
Mammals
Identify the Arrows
*Ignore “trophoblastic lacunae” on this picture*
Identify the parts
Identify these structures.
*They are the same
Identify the structures the arrows are pointing to
What animal is this cleavage occuring in?
Birds
