Chapter 2. Specification and Patterning
Developmental biologists study forces within the embryo that cause cells to take on thwir fate as development unforlds. Differentiation- the process producing overt change in appearance or biochemistry in cells.
How does the fate of a cell become determined? Is it an all or none process? Internal cues in zygitre, egg and early embryo that have cellular interactions and induction- which makes the cells differentiate.
As it is committed to a fate, there is restriction of potential fate
Specification - labile, will differentiate in neutral environment. They have information about what it can become.
Determination - determined cells will differentiate autonomously even if placed in another region of the embryo. Fate is set and is now irreversible.
Determined cells are thought to be irreveraibly committed
From these levels and types of commitment, different developmental patterns were observed in embryos of different species.
Autonomous specification (leads to mosaic developmental pattern)- predetermination, cells from early embryos (blastomeres) will differentiate to their normal fate when isolated
This occurs in the correct time frame and with the appropriate cellular structures formed. When the embryo loses these cells, it lacks the structures they would normally have formed.
Mosaic developmental patterns seen mostly in invertebrates.
Macho development- localized in egg, maternally derived. Has muscle specific actin
Syncytial specification- one cell, many nuclei, due to incomplete cell division. The DNA replicates without destroying the cyoplasm.
AUTONOMOUS VS SYNCYTIAL
Prospective potency = prospective fate.
In insects: early developmental patterns due to unequal distribution of cytoplasmic determinants.
Bicoid- two tail, caudal
Nanos gradients make body regions such as head, tail, thorax, abdomen.
IF SOMETHING DOESNT HAVE A BICOID- THEY COULD HAVE TWO TAILS
Conditional specification leads to regulative development- cells from early embryos have a flexible fate
Isolation experiment: possibility for twinning- bastomeres can form an entire embryo and can take information from the environment
Drieschs demonstration of a regulative developmental pattern in vertebrates and some invertebrates. Pluck blastomeres apart- make more complete embryos. Conditional specification.
Prospective potency > prospective fate. Can indicate conditional specification. Recombination experiment: separate cells so they can mix and reassemble. Reaggregation, Recombination and regression experiment. Transplantation experiment. Defect experiment. Physical pressure to slter nuclei localization. Transfer of cytoplasmic content.
Cellular interactions are important guides- cells direct and restrict each other.
Autonomous specification. Predominant in vertebrates. Invariant cleavages produce the same lineages, blastomere fates are generally invariant.
WHAT TYPE OF EXPERIMENTS CAN BE DONE TO DETERMINE THE PATTERN AND TIMING OF COMMITMENT FOR A SPECIES?
FIND IT, LOSE IT, MOVE IT
STRENGTHS OF EVIDENCE:
CORRELATIVE
LOSS OF FUNCTION
GAIN OF FUNCTION
ACTIN AND MYOCIN ARE HOUSEKEEPING GENES
FIND IT: FINDING SOMETHING
LOSE IT: REMOVE IT, SEE WHAT HAPPENS. IS IT REQUIRED?
MOVE IT: STICK IT IN DURING A DIFFERENT TIME IN DEVELOPMENT (NEW ENVIRONMENT).
What are sources and how do these cells recieve signals for induction into specific pathways of differentiation to form tissues and organs.
Current methods for cell analysis of cell identities and fate mapping.
Chapter 4: Cell- Cell communication and morphogenesis
Early development- whether autonomous or regulative specification.
Cleavege to become multicellular
Regulative specification
Remodeling and activating zygotic genome
Gastrulation: rearrangement of cells in the blastocyst
Patterns of cellular movement
Cleavage and gastrulation patterns
Cell adhesion
Axis specification
How do cells interact for recognition, inductions and development of form? How do cells recognize and react to each other?
Reaggregation experiments show that similar cell types adhere to each other
Types of interactions and components
Juxtacrine signaling: hemophilic binding, heteophilic binding, ECM
Paracrine Signaling: ligands and receptors
Ligands: Proteins that are secreted from a cell and designed to communicate a response in another cell are generally referred to as signaling proteins
Receptors: the proteins within a membrane that function to bind either other membrane-associated proteins or signaling proteins
Heterophilic binding: adhesion molecules bind to other molecules of the same type
Hemophilic binding: when adhesion molecules bind to different types
Differentiation: determined fate
Specification: still changeable
Paracrine factors: affect adjacent cells, long distance
Juxtacrine factors: cells must be close to interact
Mesenchymal cells are undifferentiated germ cells.
Organisms express stage and tissue specific CAMS that arise in cell sorting, ordering and movements dueing morphogenesis.
Cell contact and affinity- germ layers cell types and selective affinity evidenced with dissociation and reggregation assays.
Quantity and quality of CAMs produces different levels of surface tension and reaggregation, and migration patterns.
Cadherens: Calcium dependent adhesion molecules
Cell surface proteins hold cells together and interact with ECM and cytoplasm. CAMS.
P- cadherin: Mediates cell-cell adhesion by binding to other P-cadherin molecules on neighboring cells.
E- cadherin: a transmembrane protein that plays a crucial role in cell-cell adhesion in epithelial tissues
E-cadherin is widely expressed in most epithelial tissues, while P-cadherin is more restricted to specific basal or myoepithelial cell layers, particularly in the breast tissue, making it considered a marker for these specialized cells; functionally, loss of E-cadherin is often associated with cancer cell invasion, whereas P-cadherin can sometimes play a role in tumor progression depending on the context
B catenin- structural signaling, links to cytoskeleton
Integrins- another class of surface adhesion proteins critical for development. FORM DIMERS
Extracellular- binds to RGD, which is an amino acid sequence that integrins bind to.
Epithelial mesenchymal transitions – critical for numerous types of morphogenesis. Epithelial cells are cells (from any germ layer) that are found in sheets or tubes, while mesenchymal cells (often mesoderm or neural crest derived) are loosely packed, disconnected cells
Epithelial cells are able to find each other and look for similar binding sites.
Extracellular matrices are critical during development
Proteins: collagen, lamnin and proteoglycans
Proteoglycans: huge. Many polysaccharides with peptide chains
Glycoproteins: proteins with sugar groups: fibronectin.
Induction: the process of one cell type influencing or changing the behavior of another nearby cell
Requires an inducing cell and a responding cell
Competence: the ability of the responding cell to respond to inductive signals.
Eye formation in Xenopus: the optic vesicle (part of the neural plate) induces the overlying ectoderm (presumtive epidermis) to form the tissue of the lens
Eye formation is due to multiple inductive events. Reciprocal inductions indicate that signaling between cell types continues throughout the differentiation process
Mutations in what types of proteins can cause a failure in these inductive interactions
Epithelial and mesenchymal interactions: inductive interactions often occur between two general cell types and are common across closely related species.
Epidermal structures of the chick: ther dermis and epidermis induction in the chick shows regional specificity of ther dermal mesenchyme. Depending on the trgion of the origin of the inducting cells, the competent epithelial epidermal cells form different types of structures.
Paracrine signaling
Local signaling: morphogens
Concentration gradients and variable responses TGF-B family and activin as an example
What types of molecules and signaling pathways are involved in induction during development
FGF and TRK signaling pathways
Hedgehog pathway
Cell surfaces with receptors in paracrine signaling.
Depriving cells of calcium
Adjacent cells cant find each other
Integrins could not bind
Calcium binding sites wouldnt work
Calcium free media when cells need to not adhere
EDTA buffer chelating binds to things with 2 charges (calcium, lead, magnesium and heavy metals)
Application: biomaterials for wound healing, 3D tissue printing, cell interaction based on adhesive surfaces
Density and specificity of proteins and glycoproteins and proteoglycans can set up pattern for migration of cells along the ECM.
Fibronectin
Ecm is good for movements in gastrulation
Timing and adjacency for exposure to signals is important
Axes of bilaterally symmetrical animals
Where and when does the information for axis formation arise? Belly vs back
Case study: 2 heads are better than one
Proscholdt- how does embryonic axis organize
Used DBL cells- DORSAL BLASTOPORE LIP
Noggin injected into embryo moves signal
Chordin and noggin are secreted from the DBL, but do not directly induce dorsal fates.
Morphogens can change the fate of tissue- induce it to differentiate via regulatory mechanisms
Dorsal vs ventral PH
Ventral vs dorsal- gravity
Competence: ability to respond to inducing signals. Not passive
All germ layers are involved in producing
Competence of ectoderm
Mutant: change in dna sequence: genome level is called a knockout
Pax6- important in ectoderm- lens formation and competence. Transcription factors
Ingression: sheet to individual
Integrins: bind to RGD, connects to basal lamina
In order to move away, turn off e- cadherin to degrade in order for it to migrate away and become mesenchymal
Mesenchyme has regional specificity. Can bind to epithelium to differentiate
induction:
Competence is needed as well as an inductive signal
Multiple and reciprocal inductions
Often epithelial and mesenchymal interactions
Key for cellular inyteractions has gastrulation and organogenesis
Cyclops: 1 eye. Sonic hedgehog gene.
Chapter 14 and 7
Overarching question: when does human personhood begin? What are key processes that mark development in mammals
Specific stages and processes with homo sapiens, similar to other species, but some differences.
Fertilization
Bastocyst
Implantation
Gastrulation
Organogenesis
Fetal growth. Fetus is 9-10 weeks post fertilization
Birth
How are the structures of the ganetes optimized for their function
Spermatid structure and contents STRUCTURE. FUNCTION. FATE
Tail- motility. Disintegrates
Nucleus- haploid which contains the fathers half of genetic information needed for zygote and development. Sex chromosomes (X or Y). Sperm determines sex.
Enzymes for degradation of ECM (Zona pelluda) found around egg
Motochindria: ATP for motility
Cell membrane with specific proteins
Acrosomal vessible- gives reaction for fertilization.
Centiole- organizes motility of tail and becomes centromere, separation of chromosome in mitosis.
Ovum structure and contents
Haploid. Mothers half of genetic information, sex chromosome
Cytoplasm for maternal transcripts
ECM for the jelly coat. Importabnt that it sticks around until fertilization. Prevents polyspermy and early implantation
Cortical granules- enzymes that prevent polyspermy
The ECM, zona pellusa, is important because it has
The location where sperm meets egg
Helps maintain protective coating
What are potential problems with oocytes or spermatids that may prevent fertilization or development – thus lead to infertility?
Age. females stop producing eggs (menopause). Males never stop producing but quality diminishes.
Dna damage- spermatosis in men
Disruption of signal molecules- endocrine disruptors in specific pathways
Endometriosis- female issue with uterine lining
Immune disorders
Hormone imbalances- stress and cortisol
PCOS- where female makes more testosterone
Untreated STD- ectopic pregnancies
Surgeries
How does IVF overcome some types of infertility (or not)
Motility issues such as immotile sperm
ICSI- IVF
Ivf is very expensive though- around 20 k
Infertile: being unable to become preganant for a full year
What mechanisms may be different in internal vs. external fertilization?
Contact and recognition
Prevention of polyspermy
Fusion of pronuclei
Activation of egg metabolism
Internal vs expernal fertilization
External
Cues to replicate:
Temp
pH
Time of year
Light
Chemically released cues
Migration patterns
Once one spawns, the others do too. (sea urchins)
Number of gametes are higher
Chemoattractants from egg ECM laters, sperm binding and passage through ECM, eventually fusion of egg and sperm membranes
Prevention of polyspermy- 1+ sperm fertilizes an egg- triploid. Extra chromosomes.
What happens if polyspermy does occur?
Triploid
4 centromeres
Double mitosis
Uneven chromosomes
Death of embryo
After fertilization- what happens to the successful sperm embryo
Degrades
Mitochondira degrades
Centroile remains and forms centrosomes
Sex chromosomes remain
Nucleus maintains and remodeled
Flagellum degrades
Membrane fuses with egg membrane
Nuclear membrane breaks down
During spermatogenesis- dna is packaged with protamines for higher levels of condensation than in somatic cells
Ovum, now zygote, has cytoplasm restructures in the chromatin
Dna condenses
Oocyte derived histones bind and recondense the dna
polar body is product of mitosis II
Trophectoderm vs inter cell mass (ICM)
Trophectoderm becomes
ICM becomes part of placenta
Hatching and implantation vs epiblast layer
Enzymes help degrade ECM
L selectins make sperm and egg interact. Its a ligand.