developmental biology Flashcards
List the processes that contribute to development, e.g. growth, differentiation.
cell division - Cleavage divisions can set up asymmetries by segregating determinants.
emergence of pattern - Pattern formation is the process by which a spatial and temporal pattern of cellular activities is organised within the embryo so that a well-ordered structure develops.
An early step is allocation to different germ layers.
morphogenesis - (creation of structure and form)
* differential proliferation
* change in cell shape and size
* cell movement
* cell fusion
* cell death
Gastrulation moves the germ layers relative to one another
growth - There is little growth during early development, while the basic body plan is being established.
Differential growth rates can result in a change in body proportions.
progressive cell commitment - Early embryonic cells are pluripotent; over time their potential is gradually restricted.
Differentiation - is the process of cells becoming structurally and functionally specialised, reflecting activation and maintenance of a particular pattern of gene expression.
cleavage
Cleavage produces a cluster of blastomeres
* Divisions occur in the absence of growth
* Spherical blastomeres form a loose clump
The morula forms 3 days after fertilisation, and undergoes compaction
E-cadherin becomes restricted to regions of intercellular contact
increased cell-cell adhesion maximises contact between blastomeres, forming a compact ball of cells held together by tight junctions
Subsequent tangential cleavages produce one polarised and one nonpolarised daughter cell
(microvilli on left side of parent cell)
radial cleavage=horizontal resulting in 2 polarized daughter cells(microvilli split evenly between 2 cells)
tangential cleavage=vertical resulting in 1 polarized(all microvilli on this cell) and one nonpolarized cell(no microvili)
The outer cells have distinct apical and basal surfaces. The nonpolarised cells form the inner cell mass. The inner cells
communicate extensively through gap junctions.
apical - the exterior surface; microvilli
basal or basolateral - internal surfaces; E-cadherin
The outer and inner cells give rise to two distinct lineages: trophectoderm and inner cell mass (ICM)
this segregation is the first step towards differentiation
A fluid-filled cavity develops, and the embryo is now called a blastocyst
- The inner cell mass will give rise to the embryo proper (and some extra-embryonic structures)
- The zona pellucida prevents implantation in the oviduct
- Flattened epithelial cells of the trophectoderm will form extra-embryonic tissues
- Fluid-filled blastocyst cavity(orblastocoel)
Active transport of sodium ions leads to fluid accumulation in the blastocoel
Tight junctions between outer cells act as a permeability barrier.
Na+ is actively transported into the blastocoel.
As the ion concentration in the blastocoel increases, water flows in by osmosis.
The resulting hydrostatic pressure inflates the blastocoel.
Enzymes digest through the zona pellucida and the embryo hatches(day5.5-6)
- Zonapellucida - outer layer that the embyo hatches from
- Trophoblast(derived from trophectoderm) - outer layer of cells on the embyo(hatched)
- Hypoblast - part of inner cell mass
- Blastocyst cavity - fluid filled cavity
- Epiblast - part of inner cell mass
day 6 - endometrium(progestational stage)
Implantation requires interactions between trophoblast integrins and laminin & fibronectin
extracellular matrix proteins expressed by the epithelial cells of the uterine mucosa.
The inner cell mass is now called the embryoblast.
The embryoblast forms a flat disc of two layers, epiblast and hypoblast, with different fates
Epiblast (blue)
* columnar cells adjacent to newly formed amniotic cavity
* will form the embryo proper
Hypoblast (yellow)
* small cuboidal cells adjacent to the blastocyst cavity
* will form extra-embryonic structures that will connect to the mother’s circulation
the primitive streak forms after 2 weeks
The primitive streak forms on the surface of the epiblast in the region that will become the posterior of the embryo.
This is the first sign of the anteroposterior axis.
During gastrulation, epiblast cells migrate towards the primitive streak and invaginate (move inwards), displacing the hypoblast.
During gastrulation, epiblast cells migrate through the primitive streak to create the three germ layers
The first cells to invaginate form endoderm.
The next cells to invaginate form mesoderm.
The remaining epiblast cells form ectoderm.
The germ layers are morphologically distinct and have different fates. After gastrulation, the flat embryo rapidly folds into a three-dimensional body.
During neurulation the ectoderm folds along its central axis to form the neural tube
The neural folds elevate and fuse.
Neural crest cells form near the site of fusion and migrate away.
The epidermis fuses above the neural tube.
the paraxial mesoderm segments into somites
somites generate trunk and limb muscles, dermis and vertebrae
the endoderm is internalised and gives rise to the epithelial lining of the gastrointestinal tract.
endoderm also provides the stomach, liver and pancreas and the epithelial lining of the respiratory tract
During the embryonic period (weeks 3-8) the germ layers give rise to tissues and organ systems (organogenesis)
Stem cell populations establish each of the organ primordia.
Organogenesis is sensitive to genetic and environmental influences.
This is the period when most structural birth defects are induced.
development of most animals proceed through common stages:
- fertilisation
- cleavage to form the blastula
- gastrulation to reorganise the structure of the embryo and generate the germ layers
- neurulation
- organogenesis
Development is progressive: a simple embryo with a few cell types in a basic pattern is gradually refined to generate a complex organism with many cell types showing highly detailed organisation. This process is known as epigenesis.
Explain the concept of genomic equivalence, and how it can be demonstrated by somatic cell nuclear transfer (SCNT).
Genomic equivalence: Somatic Cell Nuclear Transfer shows that (almost) all somatic cells have a complete copy of the genome
A differentiated cell’s nucleus can direct development of a new individual, proving it had not lost any of its genetic potential as it became specialised.
