Bioveterinary Sciences &
Biological Sciences Notes
Royal Veterinary College, D300
Contents
DEVELOPMENT ............................................................................................................................ 1
The Developing Embryo ............................................................................................................ 1
Introduction to Developmental Biology ......................................................................................... 1
The Beginnings of Life ..................................................................................................................... 4
Gastrulation .................................................................................................................................... 6
Somitogenesis ................................................................................................................................. 9
Limb Development ........................................................................................................................ 15
Neurulation ................................................................................................................................... 19
The Neural Crest ........................................................................................................................... 25
Genes and Development .............................................................................................................. 27
Drosophila Development .............................................................................................................. 35
Gills and Lungs Development ....................................................................................................... 40
Heart and Foetal Circulation Development .................................................................................. 47
Regeneration................................................................................................................................. 57
Teratogenesis ................................................................................................................................ 63
Stem Cells and Stem Cell Therapy ................................................................................................ 67
Ethics and Philosophy of Science ............................................................................................. 72
Use of Animals in Research........................................................................................................... 72
Introduction to Ethics, Ethical Theories, and the History of Animal Welfare Ethics .................... 76
Ethical Issues, Law, and Research ................................................................................................. 80
Philosophy of Science ................................................................................................................... 84
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Introduction to Developmental Biology
Describe the main anatomical models used in developmental biology.
Discuss the advantages and disadvantages of each model.
Understand that developmental biology research informs other areas of
medical and biological research.
Developmental biology is the study of the transient stages between egg and birth, i.e. how does the
egg produce an adult and vice versa:
The generation of cells – cell division and growth.
The generation of different cells – cell differentiation (differentiated cells have taken on a
final character).
o Totipotent cells can produce any type of cell
o Pluripotent cells can produce several types of cells
The generation of tissues/organs/organism – morphogenesis (the creation of structure or
form). Changes in cell shape, behaviour and organisation drive morphogenesis, e.g.
gastrulation.
o Mesenchyme – single or loosely linked cells of irregular shape
o Epithelial – cells tightly attached to each other or a common membrane, with
regular e.g. cuboidal or columnar shape.
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Ways of studying developmental biology
Anatomical embryology – the study of how anatomy changes within and between embryos.
Genetic embryology – how genes control development.
Experimental embryology – the mechanics of development.
All three are interdependent on each other. They all require experimental animal models or model
organisms. Many biological processes are the same in all animals – particularly during development.
o Experimental and anatomical embryology is easier in larger embryo that develop
externally.
o Genetic embryology is easier in small animals with short generation times.
Model organisms
Easy to breed all year round
Easy to maintain in a lab in large numbers
Fast development
Have some similarity to human/veterinary patients
Fruit fly
Drosophila Zebrafish Frog Chick Mouse
melanogaster Danio rerio Xenopus laevis Gallus gallus Mus musculus
Small, cheap and easy Mammalian,
Short life span.
to keep. therefore closes to
Develop external to human.
Easy to handle Large eggs.
Transparent – good mother.
large numbers.
for anatomical Same number of
Develop external to
development. Large embryo. genes, in same
Easily mutated and mother.
order.
easy to make
Good for genetics as Easily
transgenic. Easily manipulated
easy to mutate, make experimentally Good for genetics:
for experimental
transgenics. manipulated. transgenics –
Genome embryology.
Close to humans knockouts.
sequenced.
Sequenced genome. than Xenopus.
Sequenced genome.
Small, short
Ideal genetic
Can also perform Sequenced genome. breeding cycle.
model.
experimental biology
experiments Sequenced genome.
Several mammalian Genetics difficult as Poor for
Not vertebrate. Not good for
organs are not they have four experimental
genetics.
present. copies of each gene. studies.
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No single model is sufficient. Most studies use a combination of different models. This tells us more
about development in general but also species differences. These are key to understanding changes
in the body plan over time (evolution).
Developmental studies inform the following areas:
- Birth defects research - Stem cells
- The role of genes in disease - Regeneration
- Cancer - Evolution
This permits development of preventative regimes and treatments for pathological conditions:
Cancer
o Control of the cell cycle and proliferation
Stem cells
o Characterisation of potency and identification of stem cells
Regeneration
o Re-iteration of developmental process
Evolution
o Control of changes in the body plan over time
Overview of Embryonic Development
Body Planes and Axis
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The Beginnings of Life
Describe the interaction between sperm and oocyte.
Describe how cell division and cleavage occurs in the fertilised egg during the
formation of the blastocyst.
Correlate this to how the egg/zygote/blastocyst moves through the oviduct.
Describe changes in the organisation of the blastula following implantation.
Formation of germ cells
Primordial germ cells are diploid. They undergo meiosis to form germ cells – sperm and egg (these are
haploid, with an N number of 1). The egg goes through 1st meiosis at ovulation.
Zygotic Stage
Entry of sperm into oocyte leads to the formation of zygote. Sperm entry causes the surface of the
oocyte (zona pellucida) to become impenetrable. Then, the diploid (2N) status is attained; 1N
chromosomes from the sperm and 1N chromosomes from the oocyte. This initiates cell division.
Cleavage Stage
Cell division is initiated. Each cell is called blastomere. There’s a solid mass of cells, each in contact
with Zona Pellucida. Cell division is asynchronous in mammals, which means that cell can be different
sizes and can divide at a different speed.
Morula Stage
This is the 16 cell stage. During this stage, organisational changes occur. Cells divide but do not grow,
therefore the size of the embryo does not increase.
Blastocyst Stage
Continued cell division causes some cells to become located on the inside of the ball of cells. These
are the inner cell mass (ICM). This is the first “differentiation” event. Cells on the outside pump fluid
into the embryo to form a blastocyst cavity. Whilst the cells on the inside of blastocyst clump forming
a condensed inner cell mass, the cells on the outside are known as trophoblasts. The blastocyst then
breaks out of zona pellucida and starts to grow.
Normal and ectopic pregnancies
The embryo develops as it moves along the oviduct. Implantation should occur outside the oviduct, in
the uterus. Ectopic pregnancies are due to the implantation outside of the uterus, >90% in the oviduct.
This is mostly caused by infection and scarring, which impeded movement of the egg into the uterus.
As an ectopic pregnancy develops, it can rupture the oviduct. The embryo usually dies and a massive
haemorrhaging can occur. A life threating to mother surgery can be required.
Implantation
Hatched blastocyst implants into the uterine wall as it needs to obtain nutrition and excrete waste.
Trophoblast cells in contact with a uterus are induced to divide and start to invade the endometrium.
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Invading trophoblasts lose cell membranes and become multi-nuclear (syncytiotrophoblasts).
Trophoblasts that retain membranes are called cytotrophoblasts.
Differentiation of Inner Cell Mass (ICM)
The embryo becomes buried in the endometrium. The inner cell mass forms two layers:
Epiblast – columnar cells adjacent to syncytiotrophoblasts.
Hypoblast – cuboidal cells facing the blastocyst cavity (yolk sac).
Only the epiblast cells will become the embryo. All other cells carry out a supportive role for the
embryo and will be discarded later.
Loss of Totipotency
The blastocyst stage is the best stage for obtaining embryonic stem cells as they are easy to remove
and they have a lot of potentials to differentiate.
The Fate of Epiblast and Hypoblast
Summary
Fertilisation starts cell division but also protects the
embryo from abnormal genetic development and
prevents ectopic pregnancy.
Cell division is not initially linked to growth.
Blastocyst hatches and establishes contact with
mother to obtain nutrients.
The outside of blastocyst is involved in communication.
Inside (ICM) forms two layers, epiblast and hypoblast.
Epiblast gives rise to all
embryonic tissues.
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Gastrulation
Define gastrulation as the mechanism that generates 3 definitive germ layers.
Describe the elongation of the primitive streak and formation of the node.
Explain how cells ingress along the primitive streak and describe the tissue that
they form.
Gastrulation is a process where a single layer gives
rise to three distinct layers.
Vertebrates have three germs layers:
Ectoderm – outer layer
Mesoderm – middle layer
Endoderm – inner layer
Blastula embryo consists of two layers: epiblast and
hypoblast (each 1 cell thick).
Epiblast gives rise to all three germ layers.
Hypoblast makes no contribution to germ
layers.
Two new layers develop from epiblast: mesoderm and
endoderm. Cells remaining in the epiblast become ectoderm:
Cells in the middle of the epiblast converge to the middle of the disk. They thicken and become
compressed. They form the primitive streak. This begins at the posterior end (tail) and extends to the
anterior end (head). They don’t pile up on top – instead they ingress as individual cells – not attached
to each other. Cells continually keep moving from the edge towards the midline and ingressing.
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The Endoderm
The first cells to ingress (posterior cells) drop down to contact the hypoblast and become endoderm
of the body. These form a sheet of cells.
o Prospective endodermal cells ingress individually. Epithelium to Mesenchyme Transition.
o Endodermal cells then intercalate with hypoblast reverting to epithelial state.
o Continued endodermal intercalation pushes hypoblast cells to margins. Endoderm spreads
laterally and anteriorly as a sheet.
o Epithelial cells regularly arranged, adherent and on basement membrane.
o Cell shape and adhesion change.
o Basal lamina is broken down and cells elongate (bottle shape).
o Cells break the intercellular connection and migrate. This gives rise to the mesenchyme.
o The mesenchymal cell is more motile and can migrate upon the hypoblast.
The Mesoderm
The next cells to ingress through the streak (more anterior) sit between the epiblast and the endoderm
and become mesoderm of the body.
Summary
Epiblast cells form the ectoderm.
Cells converge at the posterior midline, extend
anteriorly and form a primitive streak.
Cells change from epithelia to mesenchyme.
Cells migrate through streak, undergo mesenchymal
to epithelial transition, forming the endoderm.
Next population of cells goes through the streak,
remaining mesenchymal, forming the mesoderm.
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The Node
Primitive streak elongation stops about 75% length of the axis. Cells condense at the anterior end of
the steak to form the node – a knot of dividing stem cells. Epiblast cells that ingress through the node
migrate anteriorly to form part of the head.
The first cells to ingress through the node migrate anteriorly to form head endoderm. The next group
of cells to ingress through the node migrate anteriorly to form all head mesoderm.
Then, the node starts regressing posteriorly, leaving behind cells. These are mesoderm cells that form
two structures – the notochord and somites.
Streak and node gastrulation are called primary gastrulation. This accounts for tissues of head and
body to hindlimb level. The node runs of out stem cells at hindlimb. Mouse, chicks, humans etc. have
more posterior tissue. This posterior tissue comes from……
The tailbud is a knot of stem cells in the posterior of the streak. It moves posteriorly, leaving cells
behind. However, it only forms mesoderm cells of the posterior notochord and somites, in never forms
the endoderm. This is called secondary gastrulation.
Summary
Epiblast cells ingress through the primitive
streak to form endoderm and mesoderm of the
body.
Cells that stay in the epiblast become ectoderm.
Epiblast cells ingressing through the node form
head endoderm and mesoderm.
As the node regresses, it leaves behind cells
forming notochord and somites.
Tail notochord and tail somites form by
gastrulation through the tailbud.
33-hour chick embryo
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Somitogenesis
Describe the adult vertebrate tissues that form from somites.
Understand that somites form intermediate tissues and these then develop into
adult tissues.
Describe the experimental evidence that supports our understanding of somite
formation, maturation and anterior-posterior patterning.
Explain intrinsic and extrinsic influences on somite formation, maturation and
anterior-posterior patterning.
Somites are dorsal paired segments of mesoderm occurring along the
notochord in vertebrate embryos. They develop into muscle and bone
in the adult animal. Somites develop from the paraxial mesoderm (the
mesoderm located at either side of the developing neural tube).
Cell division
Cell division in the presomitic mesoderm provides more tissue to differentiate into somites.
The length of the presomitic mesoderm stays constant. The number of somites generated
and the frequency they are generated depends on the organism. Temperature changes can
affect the frequency of generation but the normal number of somites are ultimately
formed.
Mesenchymal to epithelial transition
In a somitomere, the cells are mesenchymal. In a somite, the cells on the outside become
more organised and they become epithelial, with some mesenchymal cells in the centre.
Presomitic mesoderm is predetermined to generate the right number of somites. Adjacent
tissues do not influence somitogenesis. Moreover, the number of cells in the presomitic
mesoderm is not important, so if you remove a section of presomitic mesoderm somites
will still form at a normal rate – only smaller in size.
