WEEK 1-2: Vertebrate Embryology and Animal Developmental Biology Notes

Vertebrate Embryology/Animal Developmental Biology

• The study of embryos.

Embryo

• Early stage when the developing animal does not yet resemble the adult of the species.

Vertebrate Embryology

• The study of structural changes in embryonic development.

Animal Developmental Biology

• Study of embryonic developmental processes of integrated complex phenomena.

Development

• From fertilization to birth, hatching, and metamorphosis.

Types of Development:

1. Ontogenic

• Processes involved in the transformation of a fertilized single oocyte into a new organism.

  • Major accomplishments:

  • Growth: Generation of cell number.

  • Differentiation: Cellular diversity within generation.

  • Morphogenesis: Cellular order within generation.

Modes of Ontogenetic Development

• Mosaic development:

  • Fate of a cell depends upon specific cytoplasmic determinants in the zygote.

  • If a part of the embryo is removed, certain cell types would be lacking in later stages of development.

  • If a blastomere is isolated, it cannot develop.

• Regulative development:

  • Fate of a cell depends upon interactions with neighboring cells, not on what piece of cytoplasm it has.

  • When a blastomere is isolated early in cleavage, it can form a new complete individual.

2. Phylogenetic

• Transformation of the forms of life (evolutionary development).

Key Processes of Development

• Gametogenesis

  • Oogenesis

  • Spermatogenesis

• Cleavage division

• Pattern formation (body axes formation)

• Morphogenesis

• Cell differentiation

• Growth

Problem of Differentiation

• How does the same genetic information result in different cell types?

• How can the fertilized egg generate different cell types?

Problem of Morphogenesis

• How do cells form ordered structures?

• How are cells positioned in the right place at the right time?

• How do form and pattern emerge from the simple beginnings of a fertilized egg?

• Morphogenesis investigates how regulation of cell fates contributes to the form and structure of organisms and their component parts.

Problem of Growth (Cell Division)

• How are cell division and growth tightly regulated?

Problem of Reproduction

• How are reproductive cells set apart during embryonic development?

  • Only the germ cells pass characteristics on to the offspring.

Problem of Evolution

• How do changes in development create new body forms, and what changes are possible?

• Why is the distinction between analogous and homologous structures important?

• The question of environmental integration:

  • How is the organism’s phenotype influenced by the environment?

Concept of Guidelines

• Guidelines: Directive influences on development.

  • Preformed guidelines: Present right at the start of ontogeny; maternal genes/maternal effect genes.

  • Progressively formed guidelines: Appear gradually in every step of ontogeny; zygotic genes.

Preformed Guidelines

• Maternal genes/maternal effect genes: Distribution of key maternal factors in the oocytes (maternal mRNA).

Maternal Effect Genes/Factors in Amphibian and Fish Oocyte

• Balbiani body (at the vegetal pole): Accumulation of mitochondria and cytoplasmic granules (germ granules) containing silenced mRNAs.

• Maternal mRNAs:

  • Organized in the cytoplasmic granules together with several regulatory proteins responsible for their post-transcriptional processing and thus translational regulation.

  • At egg activation and fertilization.

• Balbiani body:

  • A vehicle for transporting and localizing maternal factors to the vegetal cortex during oogenesis by means of microtubule network and motor proteins.

Vegetal Pole

• The end with the highest concentration of yolk.

Progressively Formed Guidelines

• Guidelines that appear gradually in every step of ontogeny.

• Cleavage:

  • Pattern is under mother’s control.

• Blastula:

  • Participants of father’s genes, new sets of genes, new proteins synthesized.

• Gastrula:

  • New sets of genes, new proteins synthesized.

• Anterior-posterior axis:

  • Coupled to gastrulation.

  • Developmental potential & inducing properties of cells in the dorsal lip of blastopore (DLB) change with time.

  • Early cells in the DLB → anterior mesoderm → neural tissue.

  • Latter cells of the DLB → posterior → induce posterior neural structures.

• Wnt signal activity:

  • High in posterior; low in anterior.

Genes Defining Embryo Territories

• Gap genes: Define broad territories of the embryo; enable the expression of the pair-rule genes.

• Pair-rule genes: Divides the embryo into regions about two segments wide.

• Segment polarity genes: Divide the embryo into segment-sized units along the anterior-posterior axis.

• Homeotic genes: Define the identities of each of the segments in the spatial domain.

Fate

• The range of cell types that a particular embryonic cell can give rise to.

• Dependent on:

  • Cell asymmetries: Inequality in cell.

  • Unequal cytoplasmic determinants.

  • Inductive information: Signaling molecules.

  • Morphogens: Chemical substances that will help establish the body of the embryo.

Maturation Factors

• Oct4: Required for maturation of ICM (inner cell mass).

• Cdx2: Required for the maturation of TE (trophoectoderm).

Potency

• The ability of a cell to follow a developmental pathway.

• Embryonic stem cells: Unspecialized; can undergo unlimited self-renewal.

  • Totipotent: Total potential of any cell.

  • Pluripotent cells: Can differentiate into any body tissue; it cannot support full development of the entire organism/embryo.

  • Multipotent: Differentiates into different cell types within a given lineage.

  • Unipotent: Fully specialized; can generate its own specific type.

• Determination:

  • The gradual commitment to a certain cell fate; geared to follow a certain developmental pathway.

Embryonic Induction

• Evocative influence of cells.

• Chordamesoderm: the capacity to induce.

• Ectoderm: competence to respond.

Concept of Regulation of Developmental Processes

• Regulation of developmental processes:

  • It is the precise control of gene expressions, cell-to-cell signaling pathways, and environmental interactions, to ensure the proper formation & function of tissues and organs.

• Dysregulation of these processes can lead to:

  • Developmental abnormalities

  • Congenital disorders

  • Diseases

Gene Regulation

• Control of gene expression; specific genes turned on or off at precise times.

Cell Signaling Pathways

• Pathways that play a crucial role in cell-cell communication, influencing decisions regarding cell proliferation, differentiation, and survival.

Epigenetic Modifications

• Can influence gene expression without altering the DNA sequence, an additional layer of regulatory control over developmental processes.

  • DNA methylation

  • Histone modification

Key Signaling Pathways in Animal Development

• BMP/TGF signaling pathway: Signals through SMAD proteins.

• FGF signaling pathway: Activates the MAPK and Akt pathways.

• Wnt signaling pathway: Promotes pluripotency.

• Notch signaling pathway: Controls cell fate during animal development.

• Hedgehog signaling pathway: Plays a key role in determining cell fate during embryonic development.

Concept of Inevitability

• Apoptosis: Programmed cell death in mammalian tissues.

  • PCD is an important process during embryonic development.

  • Serves to remove the superfluous cells and tissues.

  • Has a crucial role in a variety of morphogenetic events.

  • Can shape an organ by the simple elimination of cells that are no longer required, without inducing tissue remodeling.

• PNS Development:

  • Neurons are overproduced; survival depends on competition for limited amounts of survival-promoting factors produced in target tissues.

  • PCD controls cell number by deleting cells which fail to partner.

  • Quantitative matching of neurons with their targets.

• Sculpting:

  • Hollowing out to create lumen or cavities; formation of tubes and vesicles.

• Biological clock: The somite segmentation clock.

• Somites: Repeated structures in embryogenesis; they are sequentially generated.

Oscillatory Genes

• Oscillate every 2-hr cycle.

Notch Signaling

• Somite segmentation clock.

• Higher notch activity: Longer clock period.

• Lower notch activity: Shorter clock period.

Cell Intrinsic Timers

• Intracellular developmental programs that change precursor cells over time.

Intrinsic Timing Mechanism

• Helps determine when cells stop dividing and differentiate.

• Signaling factor (PDGF - platelet-derived growth factor): Serves as a timer component and measures elapsed time.

• Effector (TH - thyroid hormone): Stops cell division; initiates differentiation at the appropriate time.

• p27/Kip1: A cell cycle inhibitor that accumulates in the precursor cells as they proliferate.

Homeobox Genes

• The master developmental control genes; act at the top of genetic hierarchies, regulating aspects of morphogenesis and cell differentiation in animals.

Concept of Differentiation Early History

• Aristotle: 1st systematic study of embryos; recorded different stages in the development of the chick embryo; recognized that there are multiple ways that organisms reproduce.

• William Harvey & Graaf: Described the ovarian follicle and Graafian follicle.

• Hamm & Leuwenhoek: Observed human sperm; laid down the theory of preformation.

• Splanzani: States that both male and female sex products are necessary for the initiation of development.

• Wolff: States that embryological development occurs through progressive growth and development; laid down the epigenetic concept; theory of epigenesis.

• Lazzaro Spallanzani: Successfully performed the first artificial insemination (using frog eggs).

• Caspar Wolff: First person to demonstrate morphogenesis; saw the development of structure out of structureless materials.

• Karl Ernst Baer: Made significant strides in descriptive embryology searching for the vital force; the first person to note the many similarities between the embryos of vertebrates, particularly amniotes.

• Karl Ernst Von Baer: Father of embryology.

Von Baer’s Law

• The more general features that are common to all members of a group of animals develop earlier than the more special features which distinguish the various members of the group.

• Ernst Heckel: Laid down the concept that ontogeny recapitulates phylogeny.

Law of Biogenesis

• Ontogeny is a shortened/modified recapitulation of phylogeny.

• Christian Panderm: Existence of germ layers.

• Heinrich Rathke: Affinity between embryos of higher and lower vertebrates (pharyngeal pouches).

• Schleiden & Schwann: Laid the foundation of Modern Embryology & Histology.

• Wilhelm Roux: Founder of experimental embryology; he began in the 1880s an experimental program using frog eggs.

• August Weismann: The germplasm theory; self-reproducing determinants as guiding force for morphogenesis.

• Oscar and Richard Hertwig: Was the first man to observe sexual reproduction.

• Hans Spemann and Hilde Mangold: Introduced the organizer effect (dorsal lip of blastopore) and concept of embryonic induction.

• Edward & Sleptoe: Introduced IVF.

Two Early Views How Animals Developed from an Egg

1. Theory of Preformation:

   • All parts of the future embryo were imagined to be already in the egg but these were transparent, folded, small and cannot be seen

2. Theory of epigenesis:

   • the egg does not contain a preformed embryo but only the material of which the embryo is formed

Approaches in the Study of Embryology

1. Descriptive embryology:

   • involves detailed study of the structure and arrangements of minute internal organs

   • Concerned with explanations of structural features

   • Investigates when and how a process is carried out

2. Comparative embryology:

   • establishes relationships between developmental stages

3. Experimental Embryology:

   • fins out why a process is carried out at a specific time in a specific manner

4. Ascertains which activate or regulate the developmental process

5. Chemical embryology:

   • involves biochemical investigations of the embryo; ushered in molecular biology

6. Tertology:

   • study of embryonic malformations

7. Reproductive embryology:

   • techniques in fertilization, implantation of embryos; concepts of conception and contraception

8. Developmental biology:

   • broader approach from embryonic development to postnatal development