Generation development biology

1. BS1070 Biodiversity and Behaviour
Developmental Biology
Will Norton
Adrian room 333
whjn1@le.ac.uk

2. Overview of the lecture
• Comparative embryology
• Animal body plans
• Animal development
• Organogenesis

3. What is Developmental Biology?
Essentially, Developmental Biology seeks to answer the
following question:
“How does a single cell – the fertilised egg – give rise to
a multicellular organism, in which a multiplicity of
different cell types are organised into tissues and
organs to make up a three dimensional body?”
From Wolpert, Principles of Development 4th Edition. Oxford University Press.

4. What happens during development?
• Animal embryos begin life as a single celled zygote
• This zygote generates a complex adult organism:

5. How does this happen?
• Three basic processes:
• Cell division.
– The zygote divides to produce two daughter cells.
– The two daughter cells divide to produce four cells and so on
(e.g. our bodies are made of ~37 trillion cells).
• Cell differentiation.
– Early embryonic cells have no unique identity. As
development proceeds, cells begin to acquire unique
structures and functions. This process is called
“differentiation”
• Morphogenesis.
– Unlike adults, early developing embryos typically comprise a
spherical ball of cells. Morphogenesis is the process by which
the embryo changes shape to establish the body plan.

6. Overview of development
Cell division, differentiation and morphogenesis overlap
in time
Zygote
(fertilized egg)
Eight cells Blastula
(cross section)
Gastrula
(cross section)
Adult animal
(sea star)
Cell
movement
Gut
Cell division
Morphogenesis
Observable cell differentiation
Seed
leaves
Shoot
apical
meristem
Root
apical
meristem
Plant
Embryo
inside seed
Two cells
Zygote
(fertilized egg)
Figure 21.4
We will first briefly look at how animal development helps us to
understand evolutionary relationships between species
Animal development. Most
animals go through some
variation of the blastula and
gastrula stages. The blastula
is a sphere of cells surrounding
a fluid-filled cavity. The gastrula
forms when a region of the
blastula folds inward, creating
a tube—a rudimentary gut.
Blastocoel

7. Comparative Embryology
• Adult organisms may look very different from species to species
• However, closely related organisms progress through embryonic stages
where they look very similar to one another, before diverging in form as
they continue to grow

8. Comparative Embryology
• Adult organisms may look very different from species to species
• However, closely related organisms progress through embryonic stages
where they look very similar to one another, before diverging in form as
they continue to grow
Example: “gill pouches”
in developing vertebrates

9. • Even though adult forms may look very different from one another,
developmental similarities (and differences) can help us understand the
evolutionary relationships between different species
• This idea is based on Darwin’s principle of “descent with modification”:
traits passed on from one generation to the next can sometimes change,
which can alter development and thus adult form. This can result in
adaptation, or even the generation of a new species
• Features that emerge during development (such as body plan, tissue
types, body symmetry and modes of development) help us to understand
phylogenetic (i.e. evolutionary) relationships between animals
Comparative Embryology

10. Phylogenetic trees
Closer animals are on tree – more closely related. Shorter branches =
more closely related
Can be based upon morphological features or gene / protein sequences
Evolutionary relationships can be visualised using phylogenetic trees

11. How do we classify animals?

12. Dichotomous branch points in the phylogenetic tree
• 1) Tissue type
– Parazoa (sponges) – no true tissues or organs: just collections of cells
– Eumetazoa (all other animals) – true tissues and organs
• 2) Body symmetry (for eumetazoa)
– Radiata (radially symmetrical) (two embryonic tissues types)
– Bilateria (bilaterally symmetrical) (three embryonic tissue types)
• 3) Body cavity (coelom) (for bilateria)
– Acoelomate (no body cavity)
– Pseudocoelomate (a ‘false’ body cavity)
– Coelomate (a true body cavity)
• 4) Modes of development (for bilateria)
– Protostome (some are coelomate, others acoelomate or pseudocoelomate)
– Deuterostome (these are all coelomate)

13. Classification based on body symmetry

14. Classification based on body symmetry
• A major dichotomous branch point in the phylogenetic tree is
“body symmetry”
• In biology, symmetry represents a roughly balanced distribution
of body morphology
• There are three ways to classify symmetry in animals:
– Asymmetry
– Radial symmetry
– Bilateral symmetry

15. Asymmetry: Parazoa
• Asymmetry: most sponges (parazoa) are asymmetric
• No matter how you cut a sponge, the resulting halves will
look different.

16. • Eumetazoa can be radially or bilateral symmetric
• Radial symmetry: found in the “radiata”, animals that
have a top and a bottom, but no front-back/left-right.
• Sea anemone: cutting from top to bottom along several
planes produces two halves that are (roughly) the same.
Symmetry: Eumetazoa

17. • Bilateral symmetry: found in the “bilateria”, animals that
have a front and back and a top and bottom
• Animals with bilateral symmetry produce two equal
halves only when cut down the central axis of the animal.
• Subdivides animals into right and left halves.
Symmetry: Eumetazoa

18. Symmetry: Eumetazoa
• Bilaterians have:
– A dorsal (top) side and a ventral (bottom) side
– A right and left side
– Anterior (head) and posterior (tail) ends
– Cephalization, the development of a head

19. One reason why animal body plans vary is because types of embryonic
tissue varies (tissues are collections of specialized cells isolated from other
tissues by membranous layers)
Tissues of adult animals arise from two or three basic tissue types that form
in the embryo. These are called “germ layers”
The germ layers are:
• 1) Endoderm (endo=inner, derm=skin)
• 2) Mesoderm (meso=middle)
• 3) Ectoderm (ecto=outer)
• The embryos of diploblasts contain endoderm and ectoderm
• The embryos of triploblasts contain endoderm, mesoderm and
ectoderm

20. The Germ Layers Give Rise to Different Tissue
Types in the Adult Organism
Generally the:
• Endoderm produces the gut and its associated organs (all animals
are heterotrophic so they need a gut!)
• Mesoderm produces muscles and other organs (such as
circulatory system)
• Ectoderm produces the outer surface of the animal (for example
skin) and in some animals it also produces the nervous system.

21. Radiata are diploblasts
Their embryos contain ectoderm and endoderm
These include the cnidarians (e.g. jellyfish) and ctenophores (comb jellies)
Bilateria are triploblasts
Their embryos contain ectoderm, endoderm and mesoderm
Includes
• Flatworms
• Arthropods (insects, crustaceans)
• Vertebrates
Eumetazoan body plans differ in part due to
differences in embryonic tissue types
Diploblastic embryo Triploblastic embryo
• The evolution of the mesoderm led to the emergence of larger animals with more
complex body plans

22. A major dichotomous branch in the bilateria is determined by the presence or
absence of the coelom (body cavity).
• The coelom is a fluid-filled cavity surrounded by mesoderm.
• Radiata are acoelomates (lacking a coelom). This is because the radiata are diploblasts
(and thus lack mesodermal tissue).
• The situation is more complex for the bilateria. Some possess a coelom, some don’t and
some possess a “pseudocoelom”.
• Body cavities arising from the coelom can play several different roles:
– Hydrostatic skeleton
– Protection of internal organs against mechanical shock
– Enable internal organs (such as digestive system) to move independently of body
wall
NB: the coelom is different from the digestive tract. For example in humans the coelom
forms the pleural, pericardial and peritoneal cavities.
A closer look at body cavities

23. Bilaterian body plans: body cavity
• Acoelomates such as flatworms lack a body cavity
between the digestive tract and outer body wall
Figure 32.8
Body covering
(from ectoderm) Tissue-filled
region (from
mesoderm)
Digestive tract
(from endoderm)

24. Bilaterian body plans: body cavity
• Pseudo-coelomates such as nematodes have a body
cavity only partially lined by tissue derived from
mesoderm
Figure 32.8
Pseudocoelom
Muscle layer
(from
mesoderm)
Body covering
(from ectoderm)
Digestive tract
(from endoderm)

25. Bilaterian body plans: body cavity
• Coelomates such as annelids and vertebrates have a
true coelom: a body cavity completely lined by tissue
derived from mesoderm
Coelom
Body covering
(from ectoderm)
Digestive tract
(from endoderm)
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
Figure 32.8

26. Protostomes and Deuterostomes
The bilateria can also be classified by differences in
archenteron (early digestive tube) development
Modes of archenteron development differ according to the
developmental origins of the mouth:
Protostomes (proto = first, stoma = mouth)
Deuterostomes (deutero = second, stoma =
mouth)
• In protostome development: the mouth forms first and
the anus forms second
• In deuterostome development: the anus forms first and
the mouth forms second

27. Origins of the embryonic mouth
• During development “blastopore”
formation marks the beginning of
archenteron (primitive gut)
development.
• The blastopore can produce either
the mouth or the anus.
Figure 32.9
Anus
Anus
Mouth
Mouth
Protostome: mouth develops
from blastopore
Deuterostome: anus develops
from blastopore
Digestive tube
The blastopore of a frog embryo

28. • Protostomes and deuterostomes also differ in two
other key ways. These are:
– Pattern of early cell division
– How the coelom forms during development

29. Differences in cell division
• After fertilisation, the initial rounds of mitotic cell
division without growth are called “cleavage”.
• The basic pattern of cell cleavage differs in
protostomes and deuterostomes.
• Cleavage in protostomes is “spiral”
– Here the planes of cell division occur at a diagonal angle
relative to the vertical axis of the embryo and is
determinate.
• Cleavage in deuterostomes is “radial”
– Here the planes of cell division occur at an angle that is
perpendicular or parallel to the vertical axis of the
embryo.
Diagonal
8 cell stage
Perpendicular
Parallel

30. Differences in coelom formation
• The coelom forms during a developmental period called “gastrulation”.
Archenteron
(primitive gut)
Blastopore
Mesoderm
Coelom
Mesoderm
Schizocoelous: solid masses of
mesoderm split and form coelom
Enterocoelous: folds of
archenteron form coelom
Coelom
Protostome
(molluscs, annelids, arthropods)
Deuterostome
(echinoderms, chordates)
Figure 32.9

31. 3. Gastrulation
• Gastrulation is a highly complex process during which the ball-
shaped blastula is reorganised to form a complex multi-layered
structure
• Gastrulation results in the formation of the two (diploblasts) or
three (triploblasts) germ layers: the endoderm, ectoderm and
mesoderm
• The details of gastrulation vary widely between species, but in all
cases it involves:
– Migratory movements of cells
– Changes in cell shape and size
– Changes in the way cells interact with each other

32. Frog gastrulation
African claw-toed frog

33. Frog gastrulation
Blastula Gastrula
• Ectoderm
• Endoderm
• Mesoderm

34. Frog gastrulation
• In frogs, gastrulation begins at the future dorsal region, which lies at equator between
animal and vegetal hemispheres.
• The first cells to move are mesodermal cells which invaginate: they change shape and push
inwards to produce a small crease, the blastopore.
• The top surface of the crease is called the “dorsal blastopore lip”
• The blastopore lip is the site where the mesoderm and endoderm move from the outside
to the inside of the embryo.
Dorsal blastopore lip

35. • Mesoderm and endoderm cells then migrate over the blastopore lip and
through the crease, into the interior of the embryo in a process called
“involution”
• At same time, ectoderm cells in the animal pole change shape and begin to
spread out over the outer surface of the embryo
SURFACE VIEW CROSS SECTION
Animal pole
Dorsal lip
of blastopore
Dorsal lip
of blastopore
Vegetal pole
• Endoderm
• Mesoderm
• Ectoderm
Expanding
ectoderm

36. • While this is happening the blastopore lip slowly expands and more cells
push inwards, and the blastopore becomes ring-shaped.
• Mesoderm and endoderm cells involute around all parts of this ring
• Meanwhile, the ectoderm continues to expand over the surface of the
embryo pole. It pushes the ring of involuting cells vegetally as it does so.
• As endoderm involutes and expands, the archenteron begins to form and the
blastocoel shrinks
Expanding
ectoderm
Archenteron
Involuting
mesoderm/endoderm
• Ectoderm
• Endoderm
• Mesoderm
Blastocoel
shrinking

37. • By the end of gastrulation
• The ectoderm has covered the entire surface of the embryo
• Involution has internalised all of the mesoderm and endoderm
• The archenteron is expanded and the blastocoel reduced in size
Close to the end of gastrulation
Yolk plug is fully encircled
by ectoderm at end of
gastrulation
• Ectoderm
• Endoderm
• Mesoderm

38. Recap: frog gastrulation
• Invagination at equatorial region to form
blastopore
• Involution of mesoderm and endoderm through
the blastopore
• Spreading out of ectoderm from animal pole to
enclose embryo

39. Zebrafish gastrulation
Starts with a single cell at the animal pole: meroblastic cleavage

40. 4. Organogenesis
• Cellular specialisation in different regions of the 3 germ
layers and interactions between the tissues lead to
organ formation
– Folding of tissue layers
– Splitting of tissue blocks
– Clustering of cells
??
•Wolpert “Principles of Development” p.150

41. 4. Organogenesis
• Elements of the nervous system
are among the first organs to
develop in chordates
Dorsal mesoderm specialises into
the notochord
• Dorsal ectoderm thickens above
the notochord to form the neural
plate
– A key mechanism is signalling
between the notochord and the
ectodermal cells
Neural folds
1 mm
Neural fold
Neural plate
Notochord
Ectoderm
Mesoderm
Endoderm
Archenteron
Figure 47.14

42. 4. Organogenesis
• The neural plate bends down
• The lateral margins push
inwards
• The neural plate pinches off to
form the neural tube, which
becomes the brain and spinal
cord
• A subset of the ectodermal
cells become ‘neural crest’
cells that migrate throughout
the animal
Neural
fold
Neural plate
Neural crest
Outer layer
of ectoderm
Neural crest
Neural tube Figure 47.14

43. 4. Organogenesis
• This process is called
neurulation
• The neural crest cells form:
– Peripheral nerves
• (e.g. sensory neurones)
– Skull bones
– Teeth
Neural
fold
Neural plate
Neural crest
Outer layer
of ectoderm
Neural crest
Neural tube Figure 47.14

44. 4. Organogenesis
• The mesoderm lateral to the
notochord is induced to
form blocks (somites) that
will become trunk muscles
• Its layers also separate to
form the coelom
• The development of these
blocks (and indeed most
structures) progresses from
anterior to posterior
Eye Somites Tail bud
1 mm
Neural tube
Notochord
Neural
crest
Somite
Archenteron
(digestive cavity)
Coelom
Figure 47.14

45. 4. Organogenesis
• The steps described so far lay down the basic body plan
of the organism
• Further regional cell specialisation and interactions
between tissues, controlled by many cell-signalling
pathways lead to the development of individual adult
organs

46. What you need to know…
• How major groups of animals are classified by features
of their development
• Differences between protostome and deuterostome
development
• Origin and eventual roles of the 3 germ primary tissue
layers (more in the next lecture)
• Basic principles of frog gastrulation
Chapter 32, Biology by Campbell and Reece