1
Q
Development is determined by
A
zygote’s genome.
2
Q
Molecules in the egg called
A
cytoplasmic determinants
3
Q
Cell differentiation
A
the specialization of cells in structure and function.
4
Q
Morphogenesis
A
the process by which an animal takes shape.
5
Q
The first stage of development
A
gametogenesis (formation of gametes)
6
Q
Formation of sperm
A
spermatogenesis.
7
Q
Formation of eggs
A
oogenesis.
8
Q
Production of primary oocytes
A
is complete before birth.
9
Q
Production of mature gametes
A
ceases at 50
10
Q
fertilization
A
process is the fusion of the sperm and egg to form a diploid zygote.
11
Q
fertilization in sea urchins
A
The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development.
12
Q
fertilization in sea urchins
A
- Acrosomal reaction is stimulated by contact with egg jelly (stimulatory molecules is fuses sulfate).
- Fuses sulfate binds to sperm and activates the Ca2+ transport channel
- Elevated Ca2+ triggers fusion of acrosome and cell membranes.4.Proteolytic enzymes digest a path through jelly coat to egg surface.5.Ca2+ influx also stimulates actin polymerization to form the acrosomal process
- Acrosomal process adheres to vitelline envelope via binding proteins found in egg membrane.
- Sperm Acrosomal process membrane fuses with egg membrane.
- The sperm’s nucleus, and centriole enters the egg after fusion occurs.
13
Q
egg activation in sea urchins
A
- Ca2+ in the cytosol increases the rate of cellular respiration and protein synthesis by the egg cell.
- Collection of events are referred to as egg activation.
- Proteins and mRNAs needed for activation are already present in the egg.
- The sperm nucleus fuses with the egg nucleus, and cell division begins.
14
Q
blocking polyspermry in sea urchins
A
- Ca2+ also triggers the cortical reaction.
- Cortical granules release their content via exocytosis.
- Hyalin and other enzymesseparate the vitelline envelope from the egg.
- Hyalin-sugar-rich molecule that attracts water via osmosis into the space between egg and vitelline envelope.
- Polyspermy-fusion of additional sperm to the fertilized egg.
15
Q
fertilization in mammals
A
- Process is similar to sea urchins but much slower (12-36 hours).
- Sperm must travelthrough a layer of follicle cells.
- Egg outer layer is not a jelly coatbut rather a zona pellucida.
- Fertilization is internal.
16
Q
formation of the fertilisation envelope in mammals
A
- Cortical granules release enzymes that harden the zona pellucida.
- Enzymes also strip the sperm receptors found in the egg membrane.
- Hyalinattracts water via osmosis.
**In sea urchins, sodium (Na+) influx changes membrane potential of the egg.
17
Q
cleavage
A
- Fertilization is followed by cleavage, a period of rapid cell division without growth.
- Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres.
- The blastula is a ball of cells with a fluid-filled cavity.
- a fluid-filled cavity is called a blastocoel
18
Q
Cleavage of egg with moderate to little yolk
A
- In species whose eggs have little or moderate amounts of yolk, such as sea urchins and mammals, cleavage occurs throughout the whole egg.
- This pattern is termed Holoblastic cleavage-complete division of the egg.
- Results in cells of equal size surrounding the blastocoel.
19
Q
Cleavage in eggs with large yolk
A
- The yolk in these species slows down cell division and result in:
1. Meroblastic cleavage-incomplete division of the egg.
2. Occurs in species with yolk-rich eggs, such as reptiles and birds
20
Q
Meroblastic cleavage
A
incomplete division of the egg
21
Q
Cleavage patterns in frogs
A
- The egg yolk affects cleavage patterns.
- The eggs and zygotes of many animals, except mammals, have a definite polarity -defined by distribution of yolk:
- Vegetal pole has more yolk.
- Animal polehas less yolk. - The polarity causes unequal cell divisions.
- That results in the blastocoel forming entirely in the animal hemisphere
22
Q
Establishment of body axes in frogs
A
- The three body axes are established by the egg’s polarityand by a cortical rotation following binding of the sperm
- The animal –vegetal asymmetry dictates where the anterior –posterior axis will form.
- Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles.
- Once the anterior-posteriorand dorsal-ventral axis are established, the right and left axis is fixed.
23
Q
establishment of the body axes in chicks
A
- Gravity plays a part in establishing the anterior-posterior axis.
- pH levels between the two sides of the blastodermestablish the dorsal-ventral axis.
24
Q
Gastrulation
A
- rearranges the cells of a blastula into a three-layered embryo called a gastrula.
- A Gastrulahas a primitive gut.
- The three layers produced by gastrulation are called embryonic germ layers.
25
embryonic germ layers
1. The ectodermforms the outer layer.
2. The endodermlines the digestive tract.
3. The mesodermpartly fills the space between the endoderm and ectoderm.
26
Gastrulation in sea Urchins
1. The blastula consists of a single layer of cells surrounding the blastocoel.
2. Mesenchyme cells migrate from the vegetal pole into the blastocoel.
3. The vegetal plate forms from the remaining cells on the vegetal pole and buckles inward.
4. The endoderm cells form an invagination.
5. The newly formed cavity is called the archenteron
6. This opens through the blastopore, which will become the anus.
27
Gastrulation in frogs
1. The frog blastula is many cell layers thick.
2. Cells of the dorsal lip originate in the gray crescent.
3. Cells below the dorsal lip invaginate to create the archenteron.
4. Cells continue to move from the embryo surface into the embryo by involution until the blastocoel disappears.
5. These cells become the endodermand mesoderm
6. The blastoporeencircles a yolk plug when gastrulation is complete.
7. The surface of the embryo is now the ectoderm, the innermost layer is endoderm, and the middle layer is mesoderm
28
Gastrulation in chicks
1. Cleavage forms a small cap called a blastoderm instead of a blastula.
2. Blastodermforms on top of the yolk mass.
3. The upper layer of the blastodermis called the epiblast.
4. The lower layer is the hypoblast.
5. The epiblastmoves toward the midline of the blastodermand then into the embryo toward the yolk.
6. Cell movements result in a pile-up of cells and in the formation of the primitive streak.
7. primitive streak-The thickened midline in the blastoderm.
8. The migration of cells gives rise to the endoderm,mesoderm, and ectoderm.
9. The epiblast cells continues to migrate and sort-a process called convergent extension.
10. This lengthens and narrowsthe primitive streak.
11. The hyploblastcells are displaced to form the sac that surrounds the yolkand the stalk that connect the yolk to the embryo
12. The embryo develops from the epiblastand the hypoblastform the extraembryonic membrane.
29
Gastrulation in humans
1. Human eggs have very little yolk.
2. Ablastocyst is the human equivalent of the blastula.
3. The inner cell mass is a cluster of cells at one end of the blastocyst.
4. The trophoblastis the outer epithelial layer of the blastocyst and does not contribute to the embryo
5. Instead, the trophoblast initiates implantation.
6. Gastrulation is preceded by implantation.
7. During implantation, the trophoblast secretes enzymes that break the uterine lining allowing invagination of the blastocyst.
8. The cells of the inner cell mass form a flat disc of cells. 9. The inner layer is the epiblastand the outer layer is thehypoblast.
10. The trophoblastalso forms finger-like projections and a structure called Chorion.
11. Cells of the epiblast migrate to form a primitive streak.
12. Some of those cells will gives rise to the endoderm,mesoderm, andectoderm.
13. Four extraembryonic membranes form from the epiblast.
14. At the end of gastrulation, four distinct membranes surround the embryo.
15. The cells of the trophoblast, epiblastand the endometrial tissues will form the placenta
30
how long does it take for the extra-embryonic membranes to develop in the placenta
2 weeks
31
Developmental Adaptations of Amniotes
1. Embryos of birds, other reptiles, and mammals develop in a fluid-filled sac in a shell or the uterus.
2. Organisms with these adaptations are called amniotes.
3. Reproduction outside of aqueous environments required development of:
- Shelled egg (e.g. birds, other reptiles).
- Uterus (e.g. marsupial and mammals).
32
The four extra-embryonic membranes that form around the embryo are
1. The chorionfunctions in gas exchange
2. The amnion encloses the amniotic fluid
3. The yolk sac encloses the yolk.
4. The allantoisdisposes of waste products and contributes to gas exchange
33
Organogenesis
1. During organogenesis, various regions of the germ layers develop into rudimentary organs.
2. Organogenesis-formation of organs in their proper location.
3. Adoption of particular developmental fates may cause cells to change shape or even migrate to a new location.
4. A cell’s fate can be established by inheritance of cytoplasmic determinants or by interaction with neighboring cells.
34
Neurulation in frogs
1. The first step of organogenesis in vertebrates is neurulation and somitogenesis.
2. Neurulation begins with the development of a dorsal nerve rod called the notochord.
3. The notochord forms from dorsal mesoderm.
4. The notochordis a rod of cartilage that supports the body of the developing embryo.
5. Signaling molecules secreted by the mesodermcause the ectoderm to form a neural plate.
6. Neural plate formation is an example of induction.
7. The neural platecells change shape and curve inward, forming the neural tube.
8. The neural tube will become the central nervous system (brain and spinal cord).
9. A set of cells develop along the border of the neural tube and are called the Neural crest.
10. These cells migrate to form various parts of the embryo (nerves, parts of teeth, skull bones).
35
Somitogenesis in frogs
1. While the neural tube is forming the rest of the body is being established.
2. A group of mesoderm cells next to the notochord separate into blocks called somites.
3. Somites form the segmented structure of the vertebrate body.
4. These somite cells end up forming the vertebrae, ribs and associated muscles.
36
Spina bifida
1. Spina bifida is a birth defect resulting from faults during neurulation.
2. Can lead to nerve damage and paralysis.
37
Cell migration in neurulation
1. Organogenesis relies in cell migration (e.g. cells of the neural crest).
2. The microtubulesand microfilamentsare essential to cell migration.
3. These are found in the cytoskeleton and they:
- Bringing about changes in cell shape.
- Enabling a cell to migrate to a new location
4. Somite cells and neural crest cells migrate to various location by “crawling” using the cytoskeletal fibers.
5. Cell adhesion molecules (glycoproteins) play a role by promoting cell-cell interactions.
6. The extracellular matrix also guide cell through their migration.
38
Changes in cell shape during neurulation
1. Microtubulesorientate themselves from dorsal to ventral side in a sheet of ectodermal cells.
2. At the epical end of the cell is a bundle of actin filaments which contract to give the cells a wedge shape.
3. This process bends the ectodermal layer inward developing an invagination
39
Changes in cell shape -Convergent Extensions
1. Convergent extension-rearrangement that causes a sheet of cells to become narrower and longer.
- E.g. formation of the primitive streak.
2. The cells elongate with their ends pointing at the direction they will travel.
3. Cell wedge together to form fewer columns.
40
Morphogenesis -Larval and Maturity stages
1. Once rudimental organs are formed cells continue to dived and become specialised in structure and function.
2. Differences in cell types is the result of cell differentiation.
3. Results from the expression of different genes.
41
Cell differentiation
1. During cell differentiation a cell or a group of cells become committed to a particular developmental fate. 2. The cytoplasmic determinants that the cells inherit are important in cell fate specification.
3. Biologists use fate mapping to show structures arising from each region of the embryo.
42
Fate mapping
1. reveals where cells in the blastula end up in the gastrula.
2. Biologist use a dyeto mark cells during cleavage and track their developmental fate.
43
Fate mapping in C. elegans
1. Complexes of RNA and proteins regulate germ cell’s fate.
2. These cytoplasmic determinants are called P granules.
3. P granules are present in both the larvae and adult gonads.
4. cytoplasmic determinants are important in cell fate determination.
44
Role of cilia in cell fate determination
1. Ciliais important in specifying cell fate in human embryos.
2. Humans have two types of cilia;
- Monocilia–present on the surface of all cells.
- Motile cilia -restricted to cells that propel fluids e.g. epithelial cells of the airway.
3. Monocilia act as antennaeon the cell surface.
4. Motile cilia generates a leftward fluid flow breaking symmetry between left and right sides.
5. Without this, individuals develop situs inversus.
45
situs inversus
the reversal of the normal left –right asymmetry of the organs in the chest and abdomen
46
Programmed cell death
1. Fate mapping also revealed 131 cells die during the development of C. elegans.
2. This programmed cell death is called Apoptosis.
3. Apoptosis is regulated by a single gene that is similar between organisms.
4. Structures that no longer offer an advantage are targeted for cell death.
5. The webbing between fingers of most mammals is eliminated via apoptosis.
6. In some cases a structure functions in early stages and is eliminated during later development.
7. The tail of a tadpole undergoes apoptosis during metamorphosis.
8. Extra neurons are eliminated by apoptosis as the embryo ages.
47
Restricting cell developmental potential
1. Work by Hans Spemann showed that the first two cells of a frog embryo are totipotent.
2. Totipotent-can develop into all the different types of cells for that species.
3. In mammals the first eight cells are totipotent.
- Identical twins result from totipotent cells if they separate before the trophoblastand inner cell mess is differentiated.
4. Cells are generally fixed by late gastrulation.
48
Totipotent
can develop into all the different types of cells for that species.
49
Spermann’sorganizers
1. Spemann also discovered that cells of the gastrula can induce pattern formation -arrangement of tissues and organs in the three dimensional space.
2. Cells that can induce pattern formation are called organizers
3. Biologists later discovered that spemannorganizers contained a protein called bone morphogenicprotein 4 (BMP-4) -a growth hormone that get inactivatedin the dorsal side.
4. Inactivation of BMP-4 allows these cells to form dorsal structures.
5. Spemann discovered that cells of the dorsal lip triggered gastrulationof the surrounding tissue.
6. Inductive signals play a major role in pattern formation.
7. The molecular cues (morphogens) that control pattern formation are called positional information.
- This information tells a cell where it is with respect to the body axes.
- It determines how the cell and its descendants respond to future molecular signals.
50
Formation of chick limb
1. The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds.
2. Two regions have profound effects on pattern formation of chick limb.
- Apical ectodermal ridge (AER) -thickened ectoderm at the bud’s tip.
- Zone of polarizing activity (ZPA) -mesodermal tissue under the ectoderm where the posterior side of the bud is attached to the body.
3. The positional information indicating location along three axes (Proximal-distal, Anterior-posterior, Dorsal-ventral axis)
4. AER secrets a signal protein calledFibroblast growth factor (FGF).
- Removing AER blocks limb outgrowth.
5. ZPA secretes a signal protein called Sonic hedgehog. - Tissue transplantation experiments support the hypothesis that the ZPA produces an inductive signal that conveys positional information indicating “posterior”
APES1001 Block 2
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