Introduction to the main cellular components of development Flashcards
Origins of developmental biology
- Aristotle was amongst the first to think about development nearly 2 and a half millennia ago
- Thought that the organism was in the head of the sperm whether that be a human or a dog etc.
A frog’s life
The stages through through fertilisation to hatching is called what?
embryo genesis
What is development controlled through?
Development is controlled through a number of different regions of the embryo called organizers
What are you no longer considered an embryo?
What continues to still happen?
Beyond hatching development still progresses
Once out of egg you are no longer an embryo, as embryo is all in egg. But development still continues outside of the egg
Comparative embryology
2-day chick
- Marcello Malpighi, 1672, dorsal, looking down
- Lillie, 1908, ventral looking up with a dissecting scope
- 3d image, superimposing 2 images
What are the two main patterns of cleavage?
- Holoblastic (complete) cleavage
- Meroblastic (incomplete) cleavage
Tell me about the types holoblastic (complete) cleavage?
Tell me about the types of meroblastic (incomplete) cleavage
What are the types of cell movement during gastrulation?
What are these?
Why is rearranging cells crucial?
- Rearranging cells is crucial to make the right structures, cells give rise to tissues, give rise to organs
- Early patterns of development are crucial for getting cells, not only into the correct germ layer, but into the right place to set up the body axes
Naming the parts and layers
- Once in the right place, other signals kick in, forming the right cell types/tissues/organs in the right place.
- The evolution of pharyngeal arch structures in the vertebrate head
The cells that form the gills supports in fish form the middle ear bones of mammals, and jaw structures in amphibians, birds and ‘reptiles.
German embryologists
- The pattern known as von Baerian divergence, as illustrated by the embryos of four vertebrates, fish, hen, cow and human, shown at three different stages – early, middle and late. Note the pattern of early similarity giving way to later differences. (Redrawn from A Theory of the Evolution of Development, John Wiley & Sons, Ltd., 1988.)
Keeping track of moving cells
A huge part of understanding development is understanding what cells are doing. The exciting stuff is happening inside the body of the embryo, so how do we follow that?
Direct observation of living embryos
Conklin’s work published in 1905 is amazing. I was still using it to help understand tunicate development in 2000-2004!
Dye marking
Evolutionary embryology
Summary
- What development and embryology is.
- The origins of developmental biology.
- The frog’s life cycle.
- Patterns of cleavage, early development, and homology.
- The importance of embryology/dev biology across the discipline.
How does a fertilised egg (zygote) become an animal?
Embryonic development is a timed controlled process whereby a single celled, unspecialized zygote divides and selectively activates expression of genes to produce a complex organism composed of many cell types.
1. Proliferation and growth
2. Differentiation
3. Morphogenesis
Most cells in a multicellular organism have the same genome or DNA. Genes must be turned on and turned off differentially during development.
Whats proliferation?
The zygote undergoes successive cell divisions to produce the billions of cells that comprise the adult organism.
Whats cell growth?
Growth of the developing embryo results from increase in both the number and size of cells.
Both balanced by programmed cell death (“Apoptosis”). Essential during morphogenesis
Tell me how a cell differentiates and what the stages of differentiation it goes through
Once the zygote is formed, it begins mitotic divisions to produce more cells, which are totipotent.
As the cells divide, their developmental potential decreases and their cell fate becomes determined.
What are the two steps to differentiation (as its a multi-step process)?
Differentiation: it is a multi-step process that generates specialized cell types
- A cell ceases to divide
- And develops specialized structural elements and distinct functional properties
Differentiation of cells is proceded by commitment. Tell me about this?
Commitment of a cell to a certain fate: The cell might not look different from its nearest or more distant neighbors and show no visible signs of differentiation, but its developmental fate has become restricted
What are the two stages to commitment of a cell?
- specification
- determination
Tell me about the specification stage of commitment of a cell?
Specification where a cell is capable of differentiating autonomously (i.e. by itself) when placed in a developmentally neutral environment (culture dish)
Tell me about the determination stage of cell commitment?
Determination where a cell is capable of differentiating autonomously even when placed in a non-neutral environment or moved to another region of the embryo
This diagram shows commitment of a cell, explain what its showing
- A cell becomes specificed –> determination –> differentiated
- once the cell is differentiated it’s irreversible
- reversibility decreases as the cell becomes more specialised
What are the three types of specification?
- Autonomous (mosaic development) specification
- Conditional (regulative) specification
- Syncytial specification
Whats autonomous specification?
- Cells develop only according to early fate
- Characteristic of most invertebrates
Whats conditional specification?
- Cell fate depends on context
- Characteristic of vertebrates (and some invertebrates)
Whats syncytial specification?
- Cell fate depends on exposure to cytoplasmic determination in a syncytium
- Characteristic of most insects
Tell me why autonomous specification also gives rise to mosaic development?
- Characteristic of most invertebrates.
- The cell “knows” very early what it is to become without interacting with other cells.
- Cell fate is determined by the specific cytoplasmic morphogenic determinants (proteins and RNA) apportioned to each cell as the fertilized egg divides.
- If cleavage patterns are invariant, then cell fates will be invariant. Blastomere fates are generally invariant: cleavage is important, need to segregate the cytoplasmic determinate in a way that will give rise to all the bits of the embryo
- Gives rise to mosaic development. Cells cannot change fate if a blastomere is lost
Autonomous specification of the tunicate (sea squirt)
Are blastomeres (a cell formed by cleavage of a fertilized ovum) commited at a early or late stages in mosaic development?
very early stage
If a blastomere is split, what does it form and contain?
- If split, each dissociated blastomere pair forms original structures
- Each blastomere contains positional information in the form of specific proteins and genes
The macho gene regulates muscle development in the tunicate
Tell me how conditional specification has capacity for regulative development?
- Characteristic of all vertebrates and few invertebrates.
- Specification by interactions between cells. Relative positions are important.
- Variable cleavages produce no invariant fate assignments to cells.
- Massive cell rearrangements and migrations precede or accompany specification.
- Capacity for “regulative” development allows cells to acquire different functions
Cell date depends on interactions between what?
Cell fate depends on interactions with neighbouring cells.
- Cell-to-cell contacts
- Secreted signals (paracrine factors)
- Physical properties of the microenvironment (mechanical factors)
What is meant by regulation in embryonic cells?
Embryonic cells can change fates to compensate for missing parts = Regulation
Conditional specification produces what?
regulative development
Conditional specification in frog transplantation experiments
Whats syncytium?
Syncytium – nuclear division without cell division; results in cytoplasm with many nuclei. This embryo is called syncytial blastoderm
Tell me how syncytial specification can cause both autonomous and conditional specification after cellularisation
- Characteristic of most insect classes.
- Begins before fertilization. Maternal messages are key.
- Specification of body regions by interactions between cytoplasmic regions prior to cellularization of the blastoderm.
- Variable cleavage produces no rigid cell fates for particular nuclei.
- After cellularization, both autonomous and conditional specification are seen.
Whats superficial cleavage?
Superficial cleavage: nuclear division without cell division. Cells form later from invaginating membrane of egg
Tell me about the importance of morphogen gradients
Maternal messages:
- Bicoid – anterior
- Nanos – posterior
Bicoid and Nanos proteins are morphogens
Each morphogen establishes a gradient throughout the embryo (like a diffusion gradient)
Bicoid: Nanos ratio determines anterior-posterior identity
Cells identity depends on their position in multiple gradients
Model of anterior-posterior pattern generation in drosophila
Questions about morphogenesis to think about…
- How are tissues formed from populations of cells?
- How are organs constructed from tissues?
- How do organs form in particular locations and how do migrating cells reach their destinations?
- How do organ and their cells grow, and how is their growth coordinated throughout development?
- How do organs achieve polarity?
A signal elicits a cellular response which has effects on what and influences what
Cell communication
Summary lecture 2
- Development is a multistep process: cell proliferation & growth; differentiation and morphogenesis
- The zygote is totipotent. Gives rise to the myriad of cell type of our body through the process of differentiation
- Differentiation is a multistep process preceded by commitment of cells to various fates.
- Commitment can be reversible (specification) or irreversible (determination)
- There are three strategies of specification: autonomous; conditional and syncytial
- During development, cells interact and communicate
