The Muse: the embryo
Developmental Biology
Major Questions
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Growth: regulation of size
Morphogenesis: generation of ordered form
Differentiation: generation of cellular diversity
Reproduction: why and how is the germ line so special
Evolution: how do changes in development create new body
forms
• Environmental integration: how does the environment influence
development
Approaches
• Anatomical : Comparative and Evolutionary Embryology,
Teratology, Modeling
• Experimental
• Genetic
The Anatomical approach
Anatomical approach : descriptive, based on the observation of
morphological changes
Discovery of primary germ layers, inductive interactions amongst
them.
Comparative embryology
• Aristotle in 350 B.C noticed that:
1) Animals are born in different ways:
- oviparity: from eggs
- viviparity: live birth
- ovoviviparity: producing a egg that hatches inside the
body
2) Two major cell division patterns by which embryos are
formed:
- holoblastic pattern of cleavage: the entire egg is divided
into smaller cells
- meroblastic pattern of cleavage: only part of the egg is
destined to become the embryo.
Epigenesis and preformation
• In 1672, Marcello Malpighi published the first microscopic
account of chick development.
• Debate: epigenesis X preformation
- Epigenesis: organs of the embryo are formed from scratch
- Preformation: organs of the embryo are already present in
miniature form
• In 1767, Kaspar Wolff proclaimed that he believed in the truth
of epigenesis after working with chick embryos and seeing the
heart and blood vessels to develop.
• The end of preformationism did not come until 1820s with
Christian Pander, Karl Ernst von Baer, and Heinrich Rathke.
Christian Pander- 1820s
• Pander studied the chicken embryo and discovered the germ
layers:
- Ectoderm: generates the outer layer of the embryo. It
produces the surface area (epidermis) of the skin abd forms of
the brain and nervous system.
- Mesoderm: generates the blood, heart, kidney, gonads, bones,
muscles and connective tissue.
- Endoderm: innermost layer of the embryo and produces the
epithelium of the digestive tubes and its associated organs
(including the lungs).
• Triploblastic- animals with 3 germ layers
• Diploblastic- animals with 2 germ layers
1.4 The notochord in the chick embryo
Karl Ernst von Baer discovered
the notochord and the mammalian
egg.
Comparative / Evolutionary Embryology
Von Baer rules: vertebrate embryos are very similar, sharing various
structures. As they develop, they diverge. (pages 9 and 10)
Darwin: community of embryonic structure reveals community of embryonic
descent; comparison of embryonic forms help in establishing
evolutionary relationships
1.6 Fate maps the early gastrula stage
• Cell lineage: following individual cells to see what they become.
• Techniques to follow normal development: FATE MAPPING (vital dye
marking, radioactive or fluorescent labeling, genetic marking).
1.7 Fate map of the tunicate embryo – Edwin Conklin, 1905
1.8 Vital dye staining of amphibian embryos
1.9 Fate mapping using a fluorescent dye- zebrafish (Part 1)
1.9 Fate mapping using a fluorescent dye- zebrafish (Part 2)
1.10 Genetic markers as cell lineage tracers
Chimeric embryos: graft of
quail cells inside of a
chicken embryo. Also called
“mosaic” embryos.
The Cellular Basis of Morphogenesis
• Cells are constantly changing during embryogenesis
• Morphogenesis is brought about through a limited repertoire of
variations in cellular processes within these two types of
arrangements:
- Direction and number of cell divisions
- Cell shape changes
- Cell movement
- Cell growth
- Cell death
- Changes in the composition of the cell membrane or
secreted products
• Cell migration: one of the most important contributions of fate
maps has been their demonstration of extensive cell migration
during development.
1.11 Neural crest cell migration
(A) Mary Rawles (1940) showed that melanocytes
of the chicken originate in the neural crest (NC).
(B)(C) Weston (1963)
demonstrated that
migrating NC cells
gave rise to
melanocytes, and
peripheral neurons.
Medical Embryology
• Between 2 and 5% of live births show observable anatomical
abnormalities.
• In the lack of experimental data…birth defects help us understand
normal human development.
• Some birth defects are produced by mutant genes or chromosomal
abnormalities, while others are produced by environmental factors.
• Genetic defects: malformations – often appear as syndromes
Use of animal models to identify genetic, cellular and molecular basis
of human syndromes.
Ex: kit mutation leading to piebaldism
• Environmental defects: disruptions
Responsible agents: teratogens – chemicals, viruses, radiation
Ex: thalidomide leadind to phocomelia
1.15 Developmental anomalies caused by genetic mutation
• Piebaldism- dominant mutation on gene Kit. Includes anemia,
sterility, unpigmented regions of the skin and hair, deafness,
and absence of nerves that cause peristalsis in the gut.
1.16 Developmental anomalies - environmental agent (Part 1)
• Disruptions: abnormalities caused by exogenous agents (certain
chemicals or viruses, radiation, or hyperthermia).
• The agents responsible for these disruptions are called teratogens.
1960s: increase of a
previous rare syndrome
of congenital
abnormalities.
Phocomelia caused by
the drug thalidomide
which were prescribed
to pregnant women as a
mild sedative.
1.16 Developmental anomalies - environmental agent (Part 2)
Mathematical Modeling
• Base development on formal mathematical and physical
principles.
• Mathematical modeling is strong in pattern formation and
growth.
• Growth
- Isometric: all components grow at the same rate, uniform
(mollusk shells)
- Allometric: components grow at different rates (human
body)
• Patterning
- Turing: reaction-diffusion model (tooth development)
1.17 Equiangular spiral growth patterns
1.18 Allometry in humans
1.20 Reaction-diffusion (Turing model)
1.21 Photograph of the snail Oliva porphyria (L), and a computer model
1.22 Mammalian tooth cusp pattern modeled by reaction-diffusion equations
1.22 Mammalian tooth cusp pattern modeled by reaction-diffusion equations