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BIO 185: Topics in Biology – Fall 2003
Developmental Biology – Outline 5
V. Selected Topics: The final section of this course will cover several distinct topics.
We’ll finish our discussion of the germinal layers and their products by discussing the
development of the endoderm. We will then finish embryological mechanics all together
with a section on development of the tetrapod limb. We will follow this with discussions of
two important stage in maturation development: sexual development and ageing. Finally,
we will discuss the burgeoning field of Evolutionary Developmental Biology. As you have
probably gathered, there are many more topics in the life-long development of the
organism that we could discuss. Hopefully, your curiosity has been piqued and you’ll follow
the inevitable discussion of these topics in the news as our human culture struggles with the
wealth of information that Developmental Biology holds in store!
A. The Endoderm: This last of the three germinal layers starts out at the “bottom” after
gastrulation and winds up on the inside of the organism. This is due to the inward folding events
that we discussed briefly in heart development above. The primary tissues formed by the
endoderm are: 1. the gastrointestinal tract and its organ derivatives, the liver, gallbladder and
pancreas; 2. The airways of the bronchial tubes and lungs; and 3. The extraembryonic
membranes that provide the developing embryo with nutrient, waste and gas exchange. The
endoderm also plays a key role in the induction of the mesodermal layer, as well as many of its
derivative tissues, such as the notochord, heart and blood vessels.
1. Formation of the Primary Endodermal Tube – the Primitive Gut. (p. 511)
a. Development is anterior to posterior (like everything else).
1. Budding from this tube gives the GI tract organs and airways.
2. The first distinctions are the separation of foregut and hindgut.
3. The tube starts out covered and closed by ectoderm – the stomodeum.
a. This ectoderm is still in contact with neural ectoderm.
b. The stomodeum forms Rathke’s pouch and glandular pituitary.
c. Nearby neural cells become infundibulum and neural pituitary.
b. The most anterior structure is the pharynx (remember the pharyngeal arches?).
1. We produce four pairs of pharyngeal pouches with the arches between.
a. The first pair becomes auditory canals and eustachian tubes.
b. The second pair gives the walls of the tonsils.
c. The third gives the thymus and one pair of parathyroid glands.
d. The fourth pair gives the other pair of parathyroid glands.
1. The floor beneath buds off into respiratory tract.
2. We also form a small tissue structure under the second pair of arches.
a. Forms the thyroid gland in combo with mesodermal cells.
2. The Gastrointestinal Tract and its Derivatives. (p. 511)
a. Constrictions in the tube form esophagus, stomach, small and large intestine.
1. Different regions are induced by different splanchnic mesoderm.
2. Mesodermal mesenchyme recruited to form smooth muscle layer.
3. The back end also starts covered by ectoderm – cloacal membrane.
b. The derivative organs.
1. The liver forms much like the kidney – buds induced by mesenchyme.
a. The key mesenchyme is heart-forming region.
b. Mesenchyme also induces branching and differentiation.
c. The gallbladder forms as early drainage duct but remains.
d. Interstingly, the liver induces proepicardial formation in turn.
2. Two distinct endodermal buds fuse to form pancreas.
a. Notochord is key inducer, heart cells seem antogonistic.
3. The Respiratory Tract. (p. 515)
a. Bud extends from pharyngeal floor as laryngotracheal groove.
1. The tube bifurcates into paired bronchi and lungs.
2. Mesodermal mesenchyme implicated here as well.
a. Also recruits smooth muscle layer.
b. Alveolar development is last to develop in terrestrial animals.
1. Surfactant is finally secreted as late as 34 weeks in humans.
4. The Extraembryonic Membranes of Terrestrial Vertebrates. (p. 517)
a. The embryo must avoid dessication – forms the amnion.
1. Secretes amniotic fluid.
b. The embryo must exchange gasses from an enclosed place (egg or uterus).
1. Forms the chorion.
a. The membrane inside the shell in reptiles and birds
b. The placenta in humans.
c. The embryo must remove waste – forms the allantois.
1. A membraneous sack that holds waste until hatch or birth.
2. Ours is vestigial, pigs use it tremendously.
d. The embryos gotta eat – forms the yolk sac.
1. surrounds the entire yolk, connects to the midgut and blood.
B. Development of the Tetrapod Limb: The limb is a pretty amazing thing. It always
forms as two mirror image pairs directly opposite each other on the sides of the trunk. It
changes from the shoulder or hip to the fingers or toes (proximal-distal axis). It changes
from the thumb or big toe in “front” to the pinky finger or little toe in the “back”
(anterior-posterior axis). It also changes from the knuckles to the palm or bottom of your
foot (dorsal-ventral axis). How does such a complex pattern form so regularly (look
around, it’s pretty remarkably consistent)? This process is a microcosm of embryonic
Pattern Formation – development of coordinate structure in four dimensions of space and time.
1. Specification of the Limb Bud – Getting It All Started.
a. Specification along the animals’ anterior-posterior axis.
1. Forelimb buds always form at anterior-most Hox d-6 in midline.
2. Mesoderm in limb-forming region then recruits somitic myoblasts.
3. Retinoic acid from Henson’s Node is critical.
a. Remove tadpole’s tail, give RA – get several legs!
b. The limb field has complete potential for all limb components.
1. Transplantation gives ectopic limbs.
2. Separation gives multiple limbs.
c. Accumulation of lateral plate and paraxial mesoderm beneath the ectoderm.
1. Bones will come from lateral plate, muscle from paraxial.
2. Bud is a sub-ectodermal bulge of mesoderm
3. FGF10 from lateral plate gets it all rolling.
a. Beads soaked in it will give ectopic limb buds.
2. Generation of the Proximal-Distal Axis
a. Generation of the apical ectodermal field – organizing power!
1. Mesodermal FGF10 causes adjacent ectoderm to form ridge.
2. Ectoderm in turn stimulates mesodermal proliferation
3. The mesoderm has competence for limb cell types, ectoderm signals.
b. The Progress Zone carries the proximal-distal information.
1. Progress Zone is end-cap mesoderm and ectodermal ridge.
2. Has the information for humerous, radius-ulna, digits.
a. Transplantaion of older PZ gives older cell types
b. Not sure how!
c. hox-gene related, of course.
1. Hox d-9 and d-10 exzpressing cells form humorous
2. Hox d9 – d13 form posterior radius-ulna, Hox d-9 only in anterior
3. Hox a and d9-d12 split up the digit formation
3. Generation of the Anterior-Posterior Axis
a. Generation of the Zone of Polarizing Activity – more organizing power!
1. The ZPA arises very early in limb bud stage.
a. From a small patch of mesoderm in posterior.
b. Cells expressing Hox b-8 and dHAND only.
2. Sonic hedgehog is its “power” molecule.
b. Shh and digit formation.
1. Shh release causes gradients of BMP2 and BMP7.
2. Gradients cause interdigital mesoderm to specify anterior digit.
a. Then they apoptose.
4. Generation of the Dorsal-Ventral Axis
a. Controlled by the different ectodermal tissues close to the mesoderm
1. Wnt’s from ectoderm cause mesoderm to form pads or knuckles.
2. Mesoderm appears to cause ectodermal changes.
3. Hmm.... chicken or the egg?
C. Sexual Development: This is an area of Developmental Biology that often surprises
people. We tend to be familiar with the chromosomal determination of sex (at least in
mammals) and of the hormonal influences that drive the development of secondary sexual
characteristics during puberty. These are the beginning and the end of the process.
Chromosomal determination starts at the point of conception and the development of
secondary sexual characteristics occurs well after birth. During the embryonic period some
rather odd things are known to happen…
1. Chromosomal Determination of Sex.
a. Mammals
1. Females have a matched pair of sex chromosomes: XX
a. Haploid sex cells (eggs) have an single X
2. Males have an unmatched pair of sex chromosomes: XY
a. Haploid sex cells (sperm) have an X or a Y
3. Offspring pick up one from each parent.
b. Birds
1. Females have an unmatched pair: ZW
2. Males have a matched pair: ZZ
c. Bees
1. Females are fertilized diploids.
2. Males are unfertilized haploids.
d. Flies
1. Determined by the numeric ratio of X and somatic chromosomes
2. Development of the Gonads.
a. Develop from the intermediate mesoderm near the developing kidney.
b. Early development is uniform in males and females: “indifferent stage”.
1. Selected mesenchyme condenses into sex cords.
a. Development is by mutual induction with mesenchyme.
b. Forms the Mullerian duct by interconnection of cords.
c. There is no default stage in humans.
1. Need two X’s to develop functional ovaries.
a. With one X ovarian follicles develop but can’t be maintained.
2. Need two Y’s to develop testes.
a. Y carries a gene SRY that provides testes determination.
d. Gonad development prepares for the final differentiation of germ cells.
1. Granulosa and thecal cells are specified in females.
2. Leydig cells in males.
3. Development of the Primordial Germ Cells.
a. Germ line cells are determined in the epiblast during cleavage phase!
1. In the posterior portion near the start site of the primitive streak.
2. Express two genes used as markers: fragilis and stella.
b. They have to walk to the gonads – in humans it can take weeks.
1. Migrate through the hind gut in a little row.
2. Reach the gonads determined, differentiate there based on hormones.
4. Development of Secondary Sexual Characteristics.
a. External genitalia, internal duct-work, hair, breasts, larynx (you know the list).
b. All of this is dependent on the gonadal hormonal milieu.
1. Female characteristics dependent on ovarian estrogen expression.
a. Mullerian duct becomes uterus, oviducts, upper end of vagina.
b. In puberty all the rest of the effects kick in.
1. Male dependent on testosterone and Mullerian Inhibiting Substance.
a. Mullerian duct degenerates.
b. Penis, seminiferous tubules and ducts develop.
1. Vas deferens, epididymous are mesonephric remnants
c. In puberty all the rest of the effects kick in.
D. Ageing: The cold, cruel truth – time and entropy always win. There comes a point in
every life when catalysis outpaces synthesis. Metabolically speaking, we spend our lives
making big molecules out of little molecules, and then trying to keep it all together. We
have complex synthetic pathways and intricate repair mechanisms but normal wear and
tear eventually leads to accumulation of damaged molecules. The evolutionary viewpoint
has long been that organisms keep themselves together until they have passed on their
genes. Recent advances have begun to shed light on some molecular mechanisms that
control ageing. The big question is, “Do we lose the race or do we just quit trying?” As with
everything else – only time will tell….
1. Senescence is the Physiological Deterioration that Precedes Death.
a. A relatively new phenomenon.
1. Only seen regularly in the last century.
b. The senescence phenotype varies greatly between species.
1. Ours, we know quite well.
a. Neuronal decline: Mental faculties and sensory organs.
b. Skeletomuscular decline: Osteoporosis, muscle mass, joint flexibility.
c. Epidermal decline: Gray hair, wrinkling and sagging skin.
d. Organ diseases.
2. Maximum Life Span and Life Expectancy.
a. The oldest known human life span is ~120 years.
1. Tortoises and lake trout are champs at ~150 years.
2. Laboratory mice ~4.5 years (the wild mouse rarely lasts the first year!).
3. Fruit flies ~3 months.
b. Life expectancy is not characteristic of species, but instead of populations.
1. In the U.S.: males ~72 years, females ~80 years.
a. It’s only been above 40 years since the mid-1800’s!
b. It remains below 40 years many places today.
3. Causes of Ageing
a. General wear-and-tear and genetic instability.
1. The first and longest lived theory.
2. Over the years small traumas to the tissues, cells and DNA build up.
a. Point mutations increase and protein function declines.
1. Protein synthesis enzymes.
2. DNA synthesis enzymes.
b. DNA repair enzymes deserve special attention.
1. Species with efficient repair systems live longer.
2. Mutations are associated with premature ageing.
b. Oxidative Damage Hypothesis.
1. Damaging molecules are the result of our own metabolism.
2. Reactive oxygen species (ROS): superoxide anion, hydroxyl radicals, H2O2.
a. 2-3% of O2 molecules in mitochondria are inadequately reduced.
3. Enzymes superoxide dismutase and peroxidase break them down.
a. Overexpression gives longer life in flies and worms.
b. The fly methuselah gene gives longer life and also destroys radicals.
4. This is harder to get a fix on in mammals.
a. Have greater redundancy, gene expression changes are less obvious
c. Mitochondrial Damage Hypothesis.
1. Mutation rate is higher due to fewer repair mechanisms.
a. Key spot for metabolic activity and free radical production.
2. Recent reports suggest there are “hot spots” for age-related mutation.
a. Mutations allow these mitochondria to reproduce faster.
b. Out-compete undamaged mitochondria.
d. Mutations Causing Pre-Mature Ageing.
1. Hutchinson-Gilford progeria syndrome and mouse klotho gene.
a. Have the entire senescent phenotype.
b. Don’t know what the genes do yet.
e. Telomere Length and Ageing.
1. So far, no correlation between length and life span.
2. No correlation between length and a person’s age.
4. Genetically Programmed Ageing in Cells.
a. Many links to the insulin signaling pathway (it decreases ROS synthesis!).
1. Caloric restriction in mammals.
a. Nearly all mammals, including humans, show longer life.
b. Evidence links it directly to lower insulin levels (of course).
2. Mice with loss-of-function mutations in the insulin signaling pathway.
a. Live longer than littermates.
3. Drosophila with loss-of-function mutations in the insulin receptor gene.
a. Weak decrease in function give 85% longer life!
4. C. elegans and diapause.
a. Worms have a natural decrease with short food supplies.
b. Insulin decreases and worms live longer – suspended animation.
5. Dogs with low levels of insulin-like growth factor 1 (IGF-1).
a. Related hormone system.
b. Breeds with low levels live longer than other breeds.
6. Mice lacking one copy of the IGF-1 receptor gene.
a. 25% longer life.
b. The gonadal system has also been implicated.
1. Longer life after removal of the germ-line cells in C. elegans.
2. Cells appear to release an inhibitor to a life lengthening steroid.
E. Evolutionary Developmental Biology: “Evo-Devo” is the combined study of
developmental molecular biology/genetics with evolutionary, or population, genetics. The
first is part of the ontogenic approach – the study of developmental mechanics of
individuals; the second is the phylogenic approach – the study of the genetics of taxonomy.
The goal is to use the information from both disciplines to explain the Earth’s diversity and
relationships between species. The basic idea is that speciation is driven by heritable
alterations in development (eg. big changes have to change the development program). The
difference between a human arm and a whale fin is not the basic limb bone and muscle
pattern but an adaptive change in the organization and expression of the pattern.
Developmental approaches can thus provide tremendous insight into the process by
pinpointing the timing of changes in gene sequences or expression.
1. Evolutionary Theory: Modern evolution is based on two ideas that preceded Darwin. The first was
that the human hand and the fin of the whale are individual constructs, built by the Creator completely
separately, which allow them to adapt to their “Conditions of Existence”: The second was that similarities
between the human hand and whale fin were modifications of a single plan and the plan could be
discerned by comparing those similarities, or homologies. This theory was thought of as “Unity of Type”.
a. Charles Darwin’s Synthesis: Descent with Modification.
1. Incorporated “Unity of Type” in the concept of common ancestry.
a. Used the ideas of von Baer and Rathke to show that these similarities develop
first in the embryo.
2. Incorporated “Conditions of Existence” in the concept of natural selection.
a. Used the ideas of von Baer and Rathke to show that divergent characteristics
develop much later in the embryo.
3. Darwin’s theory is the theory of evolutionary gradualism.
b. The Modern Synthesis: Fusion of the work of Darwin and Mendel.
1. Random mutations in genes were selected by the environment.
a. The most reproductively capable dominate the gene pool.
b. Assumed genes like globin genes for oxygen carrying capacity.
c. Mutations that produce individual variation ultimately give species variation.
1. Requires reproductive isolation
2. The differences between phyla were major differences between genomes.
c. The New Developmental Synthesis:
1. A few flaws in population genetics “Modern Synthesis”.
a. Assumed gradualism.
1. Punctuated Equilibrium (Eldridge, Gould and Stanley).
a. Species are characterized by morphologic stability
b. Provided evidence for rapid evolutionary change.
1. Change occurs with cataclysm
a. At the population level – asteroids and stuff.
b. Assumed accumulated mutation in structural genes.
1. Punctuated Equilibrium provides evidence for rapid change.
a. Implies mutation in key developmental regulatory genes.
1. At the species level – one or two genes.
2. Humans and chimps >99% identical at the
nucleic acid level.
c. Assumed genetic disparity in taxons.
1. So not true!
2. We share many genes in common with bacteria, yeast, flies.
a. Pax6 gene in eye formation in all visual systems.
b. Tinman/Nkx2.5 gene in hearts of flies and humans.
2. The Evolutionary Development Synthesis.
a. Mutation in key regulatory genes causes fundamental shift in development.
1. Hox genes are the classic – anterior/posterior pattern in all animals
a. Changes in hox gene number.
b. Changes in hox gene patterns within a body region.
c. Changes in hox gene expression between body segments.
d. Changes in the proteins encoded by hox genes.