Developmental Biology

An Understanding of Everything

Stages & Events of Chordate Embryogenesis

Developmental ProcessEmbryonic Stage

Fetus

NeurulationNeurulation

Zygote

Morula

Blastula

Gastrula

Neurula

Pharyngula

Gametogenesis

• Process of producing gametes Spermatogenesis Oogenesis

• Meiotic cell division

• Packaging of material into oocytes

• Removal of cytoplasm from sperm

Accumulation of Vitellogenin during Oogenesis in Xenopus

Localization of Developmental Regulatory Factors

• Dsh, Xcat-2, Xlsirt, Vg-1 mRNAs localized to vegetal pole of vertebrate eggs

Xlsirt mRNA

Bicoid Gradient in Drosophila Egg & Embryo

Fertilization - Sperm and Egg Fusion

Fertilization Induces a Rearrangement of Cytoplasmic, Localized Factors

• Initial localization of material in eggs is radially symmetrical

• Fertilization creates a point of asymmetry and causes rotational reorganization of cytoskeleton to generate bilateral symmetry

Gray crescent

Reorganization of Cytoplasmic Maternal Factors Set Up Signaling Cascades

Figure 20.12 Molecular Mechanisms of the Primary Embryonic Organizer

Cleavage Distributes Maternal Components to Blastomeres

Figure 19.7 Asymmetry in the Early Embryo (Part 1)

Figure 19.7 Asymmetry in the Early Embryo (Part 2)

Autonomous Development of Separated Tunicate Blastomeres

Figure 19.8 The Principle of Cytoplasmic Segregation

Figure 20.10 Hans Spemann’s Early Experiments

Figure 20.11 The Dorsal Lip Induces Embryonic Organization

Fate Map of a Frog Blastula

Figure 20.9 Gastrulation in the Frog Embryo (Part 1)

Figure 20.9 Gastrulation in the Frog Embryo (Part 2)

Figure 20.9 Gastrulation in the Frog Embryo (Part 3)

Figure 20.15 Neurulation in the Frog Embryo (Part 1)

Figure 20.15 Neurulation in the Frog Embryo (Part 2)

Figure 20.13 Gastrulation in Amniotes (Part 1)

Figure 20.13 Gastrulation in Amniotes (Part 2)

Figure 20.16 The Development of Body Segmentation

Mouse embryo

Drosophila Homeotic and Vertebrate Hox Genes Control A-P Patterning

Hox Genes Pattern A-P Axis

Concepts in Developmental Biology

• Polarity Established by localization of maternal gene products Established by inductive signaling events

• Morphogenesis Cellular movements and embryonic structure formation Regulated by cell-signaling & cell adhesion mechanisms

• Differentiation Specialization of cells to a particular fate

• Growth Increase in cell number Increase in cell size

Cell Specification

• Differentiation The process and the processes associated with a cell

becoming specialized Occurs in multiple steps

Cell Specification

• Autonomous All differentiation information is

contained within the cell

• Conditional Differentiation information supplied

through interactions with other cells

• Commitment Specification Determination

• Terminal differentiation

Cell Specification - Steps

Cell Specification - Commitment

• Specification A cell is said to be specified when: Cells differentiate autonomously when removed from normal

environment (embryo) and placed in a neutral environment (culture medium)

Placing cells into a non-neutral environment (a different place in the embryo) causes the cells to follow the fate of other cells the new location rather than their original fate

• Determination A cell is said to be determined when: Cells differentiate autonomously even when placed in

a non-neutral environment When moved to a different location within the embryo,

the transplanted cells differentiate according to their original fate

Cell Specification - Committment

• When a cell can no longer change or be changed into anything other than the cell type it is

• Can be associated with permanent changes in DNA DNA Methylation is a prominent factor B-cells (plasma cells) rearrange the immunoglobulin (Ig) genes

so that they can now only form a single type of Ig

Cell Specification - Terminal Differentiation

Spemann’s Specification Experiments

Presumptive neural plate ectoderm in the early gastrula was uncommitted. Later gastrula neuroectoderm was committed to a neural fate.

Inductive signals trigger conditional specification, determination and differentiation

Dorsal Lip Transplantation

Reversal of Terminal Differentiation

• Embryonic Stem cells Totipotent or pluripotent cells

• Dedifferentiated stem cells Pluripotent Derived from previously differentiated cells

• Cloning proves nuclear equivalence

Figure 19.3 Cloning a Plant (Part 1)

Cloning by Nuclear Transplantation

• Nuclear transplant experiments have shown that somatic cells contain the entire genome.

• Nucleus of an unfertilized egg is replaced with the nucleus of a somatic cell

• These experiments led to two important conclusions:

No information is lost in the early stages of embryonic development (a principle known as genomic equivalence).

The cytoplasmic environment around a nucleus can modify its fate.

The First Cloning Experiment – Nuclear Transplantation in Xenopus laevis

Cloning of the frog Xenopus laevis by nuclear transplantation of albino gut cell nuclei into enucleated, wt oocytes. All progeny are albino & female

oocytetadpole

nucleus

First Mammalian Clone

Dolly & Bonnie

Figure 19.10 Induction during Vulval Development in C. elegans (Part 1)

Figure 19.10 Induction during Vulval Development in C. elegans (Part 2)

A Gene Cascade Controls Pattern Formation in the Drosophila Embryo

Maternal effect genes

Gap genes

Pair rule genes

Homeotic genesSegment polarity genes

Bicoid and Nanos Protein Gradients Provide Positional Information (Part 1)

Figure 19.14 Bicoid and Nanos Protein Gradients Provide Positional Information (Part 2)

Figure 19.16 A Homeotic Mutation in Drosophila

Antennapedia

Figure 19.12 Organ Identity Genes in Arabidopsis Flowers (Part 1)

Figure 19.12 Organ Identity Genes in Arabidopsis Flowers (Part 2)

Figure 19.13 A Nonflowering Mutant

Northern Analysis: Gel

Electrophoresis

Formaldehyde gelsMethyl-mercury-OH gels

Northern Analysis: Probing

Spatial expression information

RNA LocalizationDevelopmental Stages

temporal expression information