Lecture Thirteen - Animal development I Flashcards
Describe how an organisms development is determined, and define cell differentiation and morphogenesis.
An organisms development is determined by the genome of the zygote and by differences that arise between early embryonic cells.
Important events regulating development occur during fertilization and each of three successive stages that begin to build the animal’s body (cleavage, gastrulation and organogesesis).
Cell differentiation is the specialisation of cells in their strucutre and function.
Morphogenesis is the process by which an animal takes shape.
Describe fertilization.
The main function of fertillisation is to bring the haploid nuclei of sperm and egg together to form a diploid zygote.
When the sperm makes contact with the egg’s surface it initiates metabolic reactions within the egg that triggers onset of embryonic development.
There are two important reactions:
- Achosomal reaction.
- Cortical reaction.
Describe the process of fertilisation in sea erchins.
1) Contact: The sperm cell contacts the egg’s jelly coat.
2) Acrocomal reaction: Hydrolytic enzymes released from the acrosome make a hole in the jelly coat, while growing actin filaments from the acrosomal process. This structure proturdes from the sperm head and penetrates the jelly coat, binding to receptors in the egg cell membrane that exend throuhh the vitelline layer.
3) Contact and fusion of sperm and egg membrane: A hole is made in the vitelline layer, allowing contact and fusion of the gamete plasma membranes (sperm and egg membranes). The membrane becomes depolarized, resulting in the fast block to polyspermy (fast block = acromsomal reaction).
Polyspermy = egg being fertillised by more than one sperm.
4) Entry of sperm nucleus into the egg.
5) Cortical reaction: Fusion of the gamete membranes triggers an increase of Ca2+ in the egg’s cytosol, causing cortical granules in the egg to fuse with the plasma membrane and discharge their contents. This leads to swelling of the perivitelline space, hardening of the vitelline layer, and clipping of sperm-binding receptors. The resulting fertillisation envelope is the slow block to polyspermy.
Illustrate the cortical reaction.
Describe how fertillisation in mammals works.
In mammals, cortical reaction modifies the zona pellucuda as a slow block to polyspermy, instead of the vitaline layer, as in sea erchins.
1) Sperm migrates though the coat of follicle cells and binds to receptor molecules in the zona pellucida of the egg. (Receptor molecules are not shown in the diagram).
2) This binding induces the acrosomal reaction, in which the sperm releases hydrolytic enzymes into the zona pellucida.
3) Breakdown of the zona pellucida by these enzymes allows the sperm to reach the plasma membrane of the egg. Membrane proteins of the sperm bind to receptors on the ehh membrane, and the two membranes fuse.
4) The nucleus and other components of the sperm cell enter the egg.
5) Enzymes released during the cortical reaction harden the zona pellucida, which now functions as a block to polyspermy.
Describe the cleavage process.
Fertillisation is followed by cleavage, a period of rapid cell division without growth.
Cleavage partitions that cytoplasm of one large cell into smaller cells called blastomeres.
Continued cleavage produces a solid ball of cells known as a morula.
A fluid filled cavity forms within the morula known as a blastocoel.
This creates a hollow ball call the blastula.
In some mammals, the development of the embryo is temporarily suspended at the blastula stage in a process known as embryonic diapause, e.g. in kangaroos.
Describe the process of axis formation.
The eggs and zygote of many animals (exept mammals) have definite polarity.
The polarity is defined by the uneven distribution of substances (e.g. yolk) in the cytoplasm.
Yolk is more concentrated at teh vegetal pole and least concentrated at the animal pole.
Development of body axes in frogs is influenced by polarity of the egg.
1) The polarity of the egg determines the anterior-posterior axis before fertilisation.
2) At fertilisation, the pigmented corte slides over the underlying cytoplasm toward the point of sperm entry. This rotation (red arrow in the diagram) exposes a region of lighter-coloured cytoplasm, the gray crescent, which is a marker of the dorsal side.
3) The first cleavage division bisects the gray crescent. Once the anterior-posterior and dorsal-ventral axes are defined, so is the left-right axis.
What implications does the uneven distribution of yolk have on how an organism develops?
Uneven distribution of yolk in frog zygotes means that cleavage at animal pole is more rapid resulting in smaller blastomeres.
Because of this unequal cell division, the blastocoel is located in the animal hemisphere.
In animals whose eggs contain less yolk (e.g. sea urchin), the blastomeres are more likely to be of similar size and the blastocoel is centrally located.
Some animal eggs have plentiful yolk and this can have pronounced effecs on cleavage.
Describe the difference between holoblastic and meroblastic cleavage.
Holoblastic cleavage refers to complete division of eggs and is seen in animals whos eggs contain little or moderate amounts of yolk.
In animals with plentiful yolk (e.g. birds) cleavage is restricture to a small disk of yolk-free cytoplasm located at the animal pole.
Since cleavage cannot penetrate the tolk, it remains uncleaved.
This incomplete division of a yolk-rich egg is known as meroblastic cleavage.
Describe the process of gastrulation.
Gastrulation is the morphogenetic process whereby cells of the blastula are rearranged to form a three layered embryo (gastrula) with a primitive gut.
The three layers (which are known as embryonic germ layers) are:
1) Ectoderm - forms the outer layer of gastrula.
2) Endoderm - lines the embryonic digestive tract.
3) Mesoderm - partly fills the space between the ectoderm and endoderm.
Eventually these layers develop into all the tissues and organs of the adult animal.
Describe gastrulation in sea urchins.
Gastrulation in sea urchins:
- The blastula consists of a single layer of ciliated cells surrounding the blastocoel. Gastrulation begins with the migration of mesenchyme cells from the vegetal pole into the blastocoel.
- The vegetal plate invaginates (buckles inward). Mesenchyme cells migrate throughout the blastocoel.
- Endoderm cells form the archenteron (future digestive tube). New mesenchyme cells at the tip of the tube begin to send out thin extensions (filopodia) toward the ectoderm cells of the blastocoel.
- Contraction of these filopodia then drags the archenteron acrossthe blastocoel.
- Fusion of the archenteron with the blastocoel wall completes formation of the digestive tube with a mouth and an anus. The gastrula has three germ layers and is covered with cilia, which function in swimming and feeding.
Describe gastrulation in frogs.
1) Gastrulation begins as cells move inwards to form the dorsal lip of the blastopore in a process called invagination. Additional cells then roll inward over the dorsal lip (involution) and move into the interior, where they will form endoderm and mesoderm. Meanwhile, cells of the animal pole, the future ectoderm, change shape and begin spreading over the outer surface.
2) Continued involution of cells expends the endoderm and mesoderm and the archentreon begins to form, as a result, the blastocoel becomes smaller.
3) Late in gastrulation, the endoderm lined archenteron has completelu replaced the blastocoel and the three germ layers are in place. The circular blastopour surrounds a plus of yolk filled cells.
Describe the process of gastrulation in birds.
Recall that cleavage in birds results in blastoderm consisting ofupper and lower layers (epiblast and hypoblast) lying ontop of the egg yolk.
During gastulation, some cells of the epiblast migrate into the interior of the embryo through a groove called the ‘primitive streak.’
Some of these cells will form endoderm, others will form mesoderm.
Those left behind on surface will become ectoderm.
