Find study materials for any course. Check these out:
Browse by school
Make your own
To login with Google, please enable popups
To login with Google, please enable popups
Don’t have an account?
To signup with Google, please enable popups
To signup with Google, please enable popups
Sign up withor
= the formation of gametes.
Formation of gametes take place in the gonads
germ line cells, whereas all other tissues in the body are derived from
Oocytes accumulate components, in the form of mRNA and proteins for:
- Translational machinery
- DNA replication and cell division
fusion of both pronuclei during fertilization
Contains glycoproteins essential for species
specificity & sperm bindingZona pellucida (mammals) extra coating
made ofExtracellular matrix
(mammals): layer of cells that nurture the
Beneath the cell membrane
Gel-like cytoplasm - may help sperm entry into the cell
Fertilization: 4 major events
-Sperm and egg make contact and must
recognize each other as the same species-ONE (and only one) sperm enters egg
-Fusion of the genetic material
-Activation of egg to begin development
cyclic GMP (cGMP)
is set in motion triggering
faster sperm swimming.
Within an ATTRACTANT GRADIENT,
- sperm shows RAPID and SUDDEN CHANGES in
theirnormal swimming trajectories due to
-TRANSIENT INCREASES in the sperm’s flagellum [Ca2+]i
conc.(or Ca2+ bursts) when the sperm detects
-LOCAL ATTRACTANT CONCENTRATIONS of RESACT in SU
etc. dynamic changes in the flagella by CHEMOTAXIS
2. Docking (recognision and contact with jelly layer)
3. acrosomal exocytosis
5. penetration of egg coat
a protien that binds to the sperm durring the acrosomal process and allows for the fusion of cellular membranes.,
1. sperm squeeze through cells left over from the follicule
2. sperm digest the jelly coat via acrosomal enzymes
3. proteins on sperm head bind to receptors on egg
4. plasma membranes fuse
5. sperm nucleus moves into the cytoplasm of egg
6. cortical reaction results in the formation of the fertilization envelope
7. Nuclei fuse in egg
This induces fusion of the acrosomal
membrane with sperm cell membrane
Differences in recognition of sperm and egg in mammals
ZP1, ZP2, ZP3. ZP3 is responcible for binding to the sperm bindin (the sperm binds to carbohydrate moieties on ZP3) and initiating the acrosomal reaction. This is mediated by an influx of calcium. in addition ZP2 must be bound to for the sperm to cross the zona.
oocyte cumulus complexes are ovulated from ovaries (#1),
picked-up by the outer surface
of the infundibulum (#2),
(unlabeled arrow) by ciliary
beating then into the ampulla
for fertilization (#3).
Fertilized eggs and embryos
are transported through the
isthmus to the uterine cavity
where they then can implant in
the uterine wall
Capacitated spermatozoa are guided from the storage site to the egg by a combination of chemotaxis, thermotaxis and, perhaps, oviductal contractions
lysosomal enzyme that is found in the acrosome and enables penetration of the egg.
neuramidase + acrosin found in acrosome in mammals.
= removal of adherant seminal plasma proteins
- reorganization of plasma membrane lipids and proteins
-possible influx of calcium ions
- increase in cAMP and decrease in intracellular pH
-takes several hours in the uterus and sperm must be removed from seminal fluid
ACE ( testicular angiotensin converting enzyme) in egg fertilization.
proteins by its GPIas activity allowing binding to the zona pellucida and fertilization
ZP3 allows species-specific sperm binding
ZP2 mediates subsequent sperm binding
ZP1 cross-links ZP2 and ZP3.
-zona block: biochemical changes that eliminates sperm binding
-cortical reaction: cortical granual exocytosis
CELLULAR EVENTS AFTER
Completion of meiosis (if necessary)
Activation of protein synthesis
Activation of DNA synthesis
First mitosis and
beginning of cleavage stage
COMMON FEATURES of cleavage
1. MODIFIED CELL CYCLE: no G1 or G2 phase
2. LACK OF OVERALL GROWTH: blastula same size as fertilized zygote
STEPS: morula=> blastocoel/bastodisk
The process by which this cavity forms involves two steps: compaction and cavitation.
MANY DIFFERENT TYPES OF CLEAVAGE
1) FIXED or NON-FIXED cleavage plan
2) AMOUNT AND POSITION OF YOLK
3)plan of successive divisions
PLAN OF SUCCESSIVE DIVISIONS
discoidal- birds and reptiles
Compaction results from changes in cell- cell adhesion and cell-cell junction. Cells change shape, flatten and"touch" each other over a larger area.
Cell-cell adhesion is mediated by Cell Adhesion Molecules (CAMs). E-cadherin is the CAM found in embryos and is responsible for compaction.
Cell adhesion molecules: CAM and cadherin - CAMs: membrane proteins capable of homo- and heterotypic binding. - Cadherin: subtype of CAMs where adhesion is Ca-dependent - Binding is fast, selective and easily reversible (like Velcro).
E-cadherin becomes adhesive. Several molecules cluster forming large plaques. Cell-cell contact is increased. Actin filaments attach to the periphery of the E-cadherin plaques. There is no actin filaments within the plaque itself.
E-cadherin plaque formation
and reorganization of actin network lead to cell polarization. Cells become epithelial in nature. F-actin is found towards the apical side and E-cadherin plaques are on the basolateral side.
- Na+ is transported inside the cells from the outside through Na+ channel and Na+/H+ pump. These are located on the apical side of the outside cells. Na+ is secreted out of the cell, into the intercellular space though Na+/K+ pump. This movement of Na+ drives the movement of water from outside the embryo to within the intercellular space. All the intercellular spaces coalesce, leading to the formation of a fluid-filled cavity: theblastocoel
There is basically no yolk present in sea urchin oocyte, therefore the cleavage is holoblastic. It is a fixed radial cleavage plan.
Unequal cytokinesis leads to daughter cells of different sizes. Micromeres (small cells) form at the very "bottom" vegetal row with macromeres (large cells) on top of them.
Blastula is composed of one cell layer surrounding a cavity, the blastocoel.
cells epithilial in nature
DROSOPHILA cleavage (part 1)
Average amount of yolk is centrally located, leaving a narrow, more fluid, subcortical region. This leads to superficial meroblastic cleavage.
1) Early stage: Chromosome duplication, separation and segregation into two daughter nuclei follow the regular mitotic pathway. However, during early development mitosis is NOT followed by cytokinesis (cell division). CONSEQUENCE: Many nuclei accumulate within an undivided cell: syncytiumor syncytial blastoderm
2) Mid stage: Most nuclei move in synchrony to the cell surface.
3) Late stage: Synchronized cytokinesis leads to plasma membrane formation and cell separation (cellularization).
This leads to the formation of a cellular blastoderm with central yolk and NO blastocoel. One cell layer of epithelial nature. Embryo is called a blastoderm
Lots of yolk. Small region of fluid cytoplasm containing nucleus is found on top of the yolk. 1) First stage: Cleavage furrows never completely penetrate the yolk region (meroblastic cleavage). Cytokinesis is mostly limited to the original fluid cytoplasmic region. There is no fixed cleavage plan. This organization leads to a discoidal meroblastic cleavage.
Subgerminal space is formed between cell mass and yolk.
bird cleavage (part 2)
Cells at one end of the disc (Koller's sickle region) start moving and migrating underneath the disc within the subgerminal space. These cells form the hypoblast layer. Cells also delaminate from the epiblast into the subgerminal space. The delaminated cells become incorporated with the migrating cells of the hypoblast layer. Formation of two layers of cells, the hypoblast and epiblast
Moderate amount of yolk, asymmetrically distributed with larger amount towards the vegetal pole. Cytokinesis is slowed down by the large yolk amount but still occurs, this leads to a holoblastic cleavage.
The cleavage plan is fixed, following a radial pattern.
Fast cytokinesis occurs where there is a minimal yolk amount. Cytokinesis is slowed down by larger yolk amount. Therefore, cell division is more rapid at the animal pole compared to vegetal pole. This means that within a specific length of time thre are more divisions at the animal pole. Therefore, animal pole cells are more numerous and consequently smaller than vegetal pole cells. Cells have different size because yolk amount affects the speed of cytokinesis.
The mammalian cleavage is rotational holoblastic. There is no fixed plan of cleavage.
Cell cycle is longer than in other species but still, early on, there is no G1 or G2
Compaction occurs between the 8 and 16 cell-stage. At the 8-cell stage, compaction starts and polarization occurs.
At the 16 cell stage, tight junctions assemble at the apical side.
Compaction,polarization and cavitation processes lead to a blastocyst composed of two cell groups: inner cell mass (ICM) and trophoblast, with a blastocoel.
- Cells with side facing the outside becomes trophectoderm/trophoblast (polarized) - Cells with no side facing outside becomes ICM (non-polarized).
The inner cell mass will subdivide into epiblast and hypoblast. The embryo proper will develop from part of the epiblast. The hypoblast will lead to part of the extra-embryonic membranes together with the trophoblast.
radial cleavage occurs such that there is one polorized cell (trophectoderm) and one non polarized (no microvilli) (ICM)
tangental cleavage occurs such that there are two polar cells
Microvilli are located on the apical side
Number of cells in each group varies and cell can be reallocated from one group to the other.
WORD LIST (cleavage)
morula, blastula/blastocyst/blastoderm, blastocoel, compaction, cell polarization, gap and tight junction, epithelial cavitation, CAM and cadherin, cytokinesis, meroblastic, holoblastic, syncytium, hypoblast
Onset of morphogenic movements which lead to spatial rearrangement of the blastula/blastoderm cells.
Some cells move from the outside to the inside of the blastula/blastocyst
either singly or as a sheet (epithelium).
Endoderm: inner layer (epithilial)
Ectoderm: outer layer (epithilial)
mesoderm: layer in between (usually mesenchyme)
CELLULAR EVENTS NECESSARY FOR CELL MOVEMENTS
1. change in cell shape: cytoskeleton
2. change in cell adhesion to its neighbors: Cadherin and ECM
4. Cell division
Ingression: single cell movement
Invagination: cells move as an epithilium
involution: cells move as an epithilium
Epiboly: cell streaming
Delamination: layer splitting
Requires changes in cell shape and adhesiveness: epithilial to mesenchymal transition (EMT)
Bending of the epithelium is driven by swelling of the inner lamina as well as change in cell shape due to actin and myocin. Outer lamina keeps dimentions and thus causes a bending to occur.
Elongation of a “tube”, process is driven by cell intercalation
the thinning and spreading of cells
Driven by both cell intercalation and change in cell shape
- Formation of the blastopore lip on the dorsal side first, at the lower limit of the gray crescent. Cells move in by involution. Formation of a new
cavity, the archenteron or primitive gut.
- Blastopore lip extend laterally and finally ventrally. Cells keep moving in by involution together with extensive epiboly movements on the
- Yolk-filled vegetal cells are pulled in passively and ultimately form the yolk plug.
- Cells bordering the new cavity become endoderm, cells staying on the outside form ectoderm, cells in between are mesoderm.
ROLE OF THE ECM DURING GASTRULATION AND CELL
Mesodermal cells do not migrate over the "turned inside-out" surface.
Migrating cells need a component present on the inner surface of cells
bordering the blastocoel, probably ECM fibers
IS FIBRONECTIN PART OF THE REQUIRED COMPONENT?
-Gastrulation involves the formation of a primitive streak which is functionally equivalent to the blastopore lip in Xenopus.
-Formation of the primitive streak occurs first at the future posterior end of the embryo (Kohler's sickle cells). The streak then moves forward
by convergent extension and the accompanying movements of gastrulation. The tip of the primitive streak is called the Hensen's node.
TYPES OF MOVEMENTS INVOLVED DURING
1) EPIBOLY: cells move on the surface towards the primitive ridges.2) INVOLUTION: cells move over the primitive ridges, into the groove, down to the botton of the primitive pit. Once there..
3) INGRESSION: at the bottom of the pit, cells move into the blastocoelic cavity by ingression. Once in the blastocoele, cells can either stay as mesenchymal cells within the blastocoelic cavity and become mesoderm or invade hypoblast which becomes endoderm.
1) The primitive streak elongates up to its most anterior point. Then,
2) The primitive streak regresses back towards the posterior end.In the wake of the retraction, the next stage of organogenesis starts, including neurulation (formation of the nervous system).
This is why anterior portion is more developed than posterior in birds and higher level organisms.
Movements and stages similar to birdsDerivatives of ICM and trophoblastHuman blastocyst and implantation
Sign up for free and study better.
Get started today!