Download View PDF

Fertilization:
1. Key ideas:
a) Sperm (n) and egg (n) nuclei fuse to form 2n zygote.
b) Sperm contact with egg results in initiating metabolic processes in egg
and thus embryonic development.
c) Study sea urchins, as are deutrostomes, like vertebrates.
2. Process:
a) Acrosomal reaction (sperm). Figs. 47.3, 47.6
b) Cortical reaction (egg). Fig. 47.4
a) acrosome releases hydrolytic enzymes, breakdown of jelly coat of egg,
binding of specific receptor on vitelline layer, fusion of membranes,
ion channels open.
b) Na+ ions move in, depolarization of egg membrane results ini)
Fast block
ii)
Ca2+ released from ER moves as a wave across fertilized egg.
Brings about fusion of cortical granules with plasma membrane
of egg, contents released into perivitelline space, degrade
proteins holding v. membrane in place and osmotic gradient
created by mucopolysaccharides so water moves in, leads to
swelling forming fertilization envelope = slow block. Also,
increase in metabolism via translation of mRNAs. Increased
cellular respiration and protein synthesis (activated egg).
3. Differences in Mammals cf Sea urchins:
i)
Molecules in female reprod. Tract increase sperm motility.
ii)
Sperm move through follicle cells.
iii)
Cortical reaction results in hardening of zona pellucida.
iv)
Whole sperm taken in.
v)
Nuclei fuse AFTER first 2n division.
vi)
Process slower than in sea urchins.
vii)
Fertilization in oviduct.
viii) Eggs small, little food reserve.
ix)
No obvious polarity regarding cytoplasmic contents and cleavage.
Cleavage:
Frog:
-
-
Chicken:
-
Mammals:
-
S and M only, no G1 or G2 (NO GROWTH).
Division of cytoplasm into blastomeres.
Zygote to morula (5-7 divisions) to blastula (blastocoel and
single layer on outside (MULTIPLE in frog blastula).
Polarity in all animals EXCEPT mammals.
Yolk concentrated at vegetal pole.
Animal pole becomes head end (anterior).
Opposite side to fertilization point give rise to dorsal
structures (vegetal cortex interacts with animal cortex via
cytoplasmic determinants.
First cleavage goes through gray crescent, determines L-R axis.
Holoblastic cleavage.
Gray crescent determines dorsal side.
First two divisions form A-V poles.
3rd cleavage is horizontal, yolk pushes cytokinesis to A pole so
blastocoel in A-hemisphere. Fig. 47.9.
LOTS of yolk. Fig. 47.10.
Yolk is cell with nutrients with small dish of cytoplasm at Apole.
Meroblastic cleavage.
Forms blastoderm (equivt. Blastula)
Hypoblast (less/lower) and epiblast (top/above).
SINGLE layer of cells cf in frog.
Holoblastic, blastomeres equal in size.
Slow, 1st div. by 36hrs.
Blastocyst (equivt. Blastula)
ICM equivt. Epiblast and hypoblast.
Trophoblast cells on outside involved in implantation.
Once implanted, gastrulation starts.
Gastrulation:
-
Rearrangement of cells by migration.
Form embryo with 3 germ layers and primitive gut.
Changes in cell shape, motility, extracellular adhesion to other
cells and molecules of ECM.
Cells from outside move inside embryo.
New positions result in new interactions.
Sea Urchin Fig. 47.11
Frog Fig. 47.12
Chick Fig. 47.13
- Mesenchyma cells give
rise to blastocoel.
- Invaginations at
blastopore.
- Endodoerm cells give
rise to archenteron.
Pulled by mesenchyme
cells’ filapodia.
-Invag. from dorsal lip on
dorsal side.
- Cells of blastopore
involute, spread
horizontally and meet on
both sides.
-SAME time, cells of
animal pole move over
OUTER surface.
- Some of epiblast cells
move to midline of
blastoderm, detach, move
in and form thickening
(primitive streak) along
A-P axis.
- Equivt. Blastopore lip.
-Displace hypoblast cells,
form endoderm.
- Others move laterally
into blastocoel, form
mesoderm.
- Those on surface form
ectoderm.
- Hypoblast helps DIRECT
formation of primitive
streak and normal
development.
-Later hypoblast forms
yolk sac and stalk.
- Chorion for gas
exchange.
-Yolk sac surrounds yolk.
-Allantois forms disposal
sac.
- Amnion surrounds
embryo and contains
amniotic fluid.
Mammals:
- EEMS form as in chick. Yolk sac – blood cells, Allantois – umbilical cord.
Organogenesis:
- 3 germ layers give rise to rudimentary organs.
- Localized changes in tissue and cell shape.
- Evidenced by folds, splits and clustering of cells.
- Carried out by cells signaling to each other.
Frog Fig. 47.14
-Mesoderm forms notocord.
-Signals from notochord to ectoderm
above forms neural plate.
-Neural plate folds to form neural tube.
- A-P axis, forms CNS.
- Neural tube pinches off and neural
crest cells formed, migrate, give rise to
nerves, teeth, parts of skull.
-Mesoderm forms somites, give rise to
muscles, migrate.
Mesoderm next to somites split into two
layers, form lining of body cavity
(coelom).
NB: pseudo-, a- and true coelomates.
Chick Fig. 47.15
- Similar formation of notochord and
neural tube as in frog.
- By 2-3 days basic structures set.
Morphogenesis: (Mechanisms)
- Shape of embryo.
- Result of movement of cells (ANIMALS ONLY, NOT PLANTS).
- Changes in cell shape and cell position.
Changes in Cell Shape:
- Involves reorganization of cytoskeleton. Fig. 47.19. Neural tube
formation.
- Microtubules are parallel to D-V axis to lengthen cells.
- Dorsal end of EACH cell has actin filaments orientated
crosswise.
- These contract forming wedge shape.
- Forces ectoderm to bend inwards.
- Similar processes in invaginations and evaginations of a cell.
Changes in cell position;
- Cytoskeleton and cell migration.
- Cells use cytoskeleton to “crawl” by extending and retracting cell
protrusions (equivt. Amoeboid). BUT here are FLAT sheets
(lamellapodia) or spikes (filapodia). Ex. gastrulation. Movement of
cells deep into embryo via filapodia at leading edge of migrating
tissue.
Blastopore lip cells drag cells behind them. Whole sheet gives rise to
Endododerm and mesoderm. Fig. 47.12.
- Sheets can become narrower and longer (converges and extends).
Convergent extension. Fig. 47.20. Exs. Archenteron elongation,
involution in frog.
- Also, INDIVIDUAL cell migrations ex. somites, neural crest cells.
- Signaling pathways in cell guiding via ECM glycoproteins (fibronectin).
- Ex. Involution in frog – fibronectin fibers line roof of blastocoel and
cells at free edge of mesoderm migrate along fibronectin. Fig. 47.21.
- Supported by no migration when Abs to fibronectin injected.
- Other substances inhibit migration in other directions so the nonmigrating cells along pathway can direct movement of other cells.
- CAMs on surface of cells bind CAMS on other cells, vary in amount
and/or chemical identity. Ex. Cadherins, need Ca2+ to function in
formation of frog blastula ( 47.22), tight junctions froming at 8-cell
stage in mammalian embryos.
Review:
How is fate of a cell determined?
a) Cytoplasmic determinants – frog, fruit fly.
b) Cell location – ICM and trophoblasts.
c) Inductive signals – cell to cell contact, signal pathways.
a) Fate maps – Fig. 47.23.
Fig. 47.24. Gray crescent cytoplasmic determinants dictate dorsal structures.
Fate of LATE gastrula cells fixed. Ectoderm expt. Pg. 1005.
c) - Ex. Vulva formation in C. elegans.
- Ex. Dorsal lip of blastopore organizes neural tube, notocord and other organs.
BMP-4 (bone morphogenic protein-4), active in cells on ventral side of gastrula.
Organizer Inactivates BMP-4 on dorsal side by proteins that bind BMP-4, so
dorsal structures formed.
- Ex. Pattern formation (spatial organization of organs etc. in 3D), via positional
information.
Earlier signals determine fore and hind limb locations.
Limb bud. Fig. 47.26.
AER (ectoderm) produces FGF causes outgrowth of limb from body and D-V axis.
ZPA (mesoderm) under ectoderm, for A-P axis. Cells NEAREST ZPA give rise to
posterior structures (cf your pinkie location to thumb). Fig. 47.27.