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Developmental

History and Basic Concepts

Textbook: Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith J. (2007) Principles of . 3th ed. London: Oxford university press.

Gilbert SF. (2003) Development Biology. 7th ed. Sunderland: Sinaure Associates Inc. 1 2

Development is a fundamental part of biology

Developmental Biology deals with complex mechanisms and many layers of “biological information” superimposed one upon another. Recent advances in , and has and will continue to further our understanding of development unlike any time in the past. Embryogenesis ( formation) determines the overall . (Fertilized embryo body plan ) OiOrganogenesis ( formati on) d et ermi nes sub sec tions o f th e body (examples: limb, eye). Many , proteins, pathways and cell behaviours are common to both processes.

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1 Model in Fig. 1.3 Fig. 1.2 Lizard Although people are mostly interested in human development (for South African claw- egocentric reasons), many aspects of development are Fig. 1.1 Drosophila toed conserved in distantly related species. The major model organisms used to study the principles of development are... – nematode () –fruit fly ( ) – sea urchins – South African claw-toed FROG ( laevis ) – chick – mouse Released its tail in defense – () 5 Useful model for 6

Important words to grow by Experimental Approaches A/P axis: anterior ~ ; posterior ~ tail. D/V axis: dorsal ~ upper or back; ventral ~ lower or front. P/D axis: proximal ~ near; distal ~ far. Lateral: to the side. Anatomical/descriptive studies haploid ~ 1 set (of chromosomes) . Experimental approaches diploid ~ 2 sets (of chromosomes). Genetic studies

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2 Anatomical/Descriptive Studies Anatomical/Descriptive Studies

Aristotle () • 1820s –4th century BC – Pander – Reproductive strategies – Describes 3 germ layers – Cell division patterns – First indication of induction – Placental functions – Epigenesis further substantiated 1600s –“ex ovo omnia” (Everything from an egg ) – First studies • descriptions of chick development 1700s – Epigenesis vs . Preformation

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Anatomical/Descriptive Studies 3) The embryo of a given species departs from the stages of Ernst von Baer Principles (~1820s) lower rather than through them; therefore, the early embryo of higher animals is never like the lower adult but 1) General features of a large group (i.e. only its early embryo ) appear earlier in development than specialized features of a smaller group. All look similar after and then develop specialized features typical of class, order, and species. All vertebrates have the same early Visceral cleft of embryonic birds or developmental structures - gill arches, , mammals do not resemble the gill and primitive kidneys. slits of adult fish . Rather they resemble the visceral clefts of the 2) Less general characters are developed from the more general until embryonic fish and other embryonic the most specialized appear. vertebrates. These are later specialized to form the gills in fish All vertebrates initially have and eustachian tubes in mammals thesametype of skin. OlOnly Duck later does the skin specialize to form scales, features, hair, and claws. Human embryos never pass through is the Chick a stage equivalent to the adult fish or same in vertebrates and bird. Rather they initially share then becomes wing, leg, characteristics only with the embryo arm, webbed foot…. More General Less General11 12

3 von Baer’s Principles Epigenesis vs. preformation

Generalized features of broad groups of animals (phylum, class) A scientific approach to explaining development stated 5th century BC. form before more specialized features of specific groups (family, addressed the development problem: two possibilities. genus) Epigenesis and preformation – Early -Brain,,p spinal cord, , 17th century, develop ment === Preformation – Late - Limbs, hair. Character development: Generalized Specific Preformation theory suggests that all structures exist from the very Embryo of a species resembles embryo of other groups, not adult beginning, they just get larger. forms. Early embryo of one group is never exactly like that of another, it 17th century Italian embryologist , supported that only shares characteristics. the embryo was already present form the very beginning (Fig. 1.4) One preformation theory, the theory of the homunculus, suggested that a little human embryo was hidden in the head of every . [Theory has fallen out of favor. X]

13 14 Phylum Class Order Family Genus Species

Fig. 1.4 Marcello Malpighi Late 1700s Wolff (1759) Observations of chick embryo Preformation Structures entirely different in embryos vs : epigenesis

Top: chick early embryo, and at 2 days’ incubation. It suggested that ever ythin g was already present from the very begin

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4 1672 Marcello Malpighi – First Microscopic Account Of Development Epigenesis vs. preformation

Preformation Preformation theory suggests that all structures exist from the very beginning, they just get larger.

17th cen tury Ita lian em bryo log is t Marce llo,sugges te d tha t the embryo was already present form the very beginning (Fig. 1.4) One preformation theory, the theory of the homunculus, suggested that a little human embryo was hidden in the head of every sperm. [Theory has fallen out of favor. X]

17 Fig. 1.5 18 Began the great debate of epigenesis and preformation

Epigenesis Vs. Preformation Epigenesis vs. preformation

Epigenesis (~upon formation) is a theory of development that states that Epigenesis (~upon formation) is a theory of development that states that new structures arise by progressing through a number of different stages. new structures arise by progressing through a number of different stages. Originally proposed by Aristotle in 4th Century BC. Preformation theory suggests that all structures exist from the very Since an embryo grows to be an adult and that adult produces another beginning, they just get larger. Prominent theory of the time. (Malpighi – embryo and so on indefinitely , according to the theory of preformation, the very first embryo must have included within itself supported by his observation that the un- incubated egg already had a tiny copies of all the future embryos. great deal of structure) The embryo was already formed, therefore it only had to grow One preformation theory suggested that a little human embryo was hidden in the head of every sperm (although not widely excepted). Since an embryo grows to be an adult and that adult produces another embryo and so on indefinitely, according to the theory of preformation, the very first embryo must have included within itself tiny copies of all 19 the future embryos. Difficult to explain interracial effects, etc. 20

5 -August Weisman (1883) Experimental Approaches – theory: Autonomous specification

What causes cell differentiation: cyyptoplasm or nucleus? – Defect experiment:

Defect experiments Isolation experiments Recombination experiments Transplantation experiments

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Actually both autonomous and conditional specification are Hans Dreisch (1892): Isolation experiment seen:

AitAgainst on ly Autonomous specification

Autonomous specification ???

Conditional specification Yes 23 24

6 Fate Maps transplant experiments: – Embryonic induction – Spemann (1924): Embryonic organizer

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Box 1A Basic stages of Xenopus laevis development Mature egg

Animal pole: pigmented upper surface 12 times Vegetal pole: lower region Yolk cell Animal half become GlGerm layers formation anterior (head) stage Laid clutch of : fluid filled cavity Germ Layers -, - muscle, cartilage, internal organ (, blood kidney) - gut, , 27 28

7 Cytoplasm rearrangement , first Gastrulation 2-cell 8-cell

Early blastula late blastula

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Archenteron

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8 Early Xenopus development: fertilization

The unfertilized egg is a sin gle lar ge cell. The animal pole, the upper part of the egg, has a pigmented surface. The vegetal pole, lower region, contains the yolk. After fertilization, the male nucleus (from sperm) and female nucleus (from egg) fuse to form one nucleus. After fertilization, cleavage begins without growth (mitotic division only).

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Xenopus Xenopus gastrulation & neuralution

After ~12 cycles of division a layer of small cells is formed surrounding a fluid-filled cavity (the blastocoel) that sits on top of the large yolk cells. Gastrulation is an extensive rearrangement of embryonic cells. Mesoderm Three germ layers are mesoderm, endoderm and ectoderm and endoderm move to the inside of the embryo to give the basic body The mesoderm is located at the "equator" and becomes muscle, cartilage, plan. bone, heart, blood, kidney For the most part, the inside of the frog is now inside and the "outside" The endoderm is above the mesoderm and below the ectoderm and except for the skin is outside. becomes gut, lungs and liver Notochord is a rod-like structure that runs from the head to the tail and The ectoderm sits above the endoderm and becomes the epidermis and lies beneath the nervous system. nervous system are segmented blocks of mesoderm which form on either side of In the blastula, these layers are all on the surface and they interact! the notochord. They become muscles, spinal column and dermis (skin). Neuralution occurs when ectoderm above the notochord folds to form (becomes spinal cord & brain).

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9 changed the conception of and 19th century German heredity Germ cells------egg and sperm, are not influenced by the body that bears them. Develop between 1820-1880. All living organism consist of cells. One Somatic cells (body cells)---form , can not transmitted to the germ cell. cell develop to mutilcellular interaction and form. Fig. 1.6 Orggp,anisms are composed of cells, the basic unit of . Both animals and are multicellular composites that arise from a single cell, therefore development must be epigenetic and not preformational since a single cell (; the fertilized egg) results in many different types of cells. Only the germ cells (egg and sperm) pass characteristics on to the offspring. SoSomaticmatic cecellslls aarere notnot ddirectlyirectly involvedinvolved in passpassinging oonn ttraitsraits to tthehe nextnext generation and characteristics acquired during an animal's life are not passed onto the offspring. Remember that “a hen is only an egg's way of making another egg.”

Only in Germ cell and transmit to offspring 37 Somatic cell 38

Meiosis and fertilization

Meiosis is the reduction division that allows diploid precursor cells to generate haploid germ cells. At fertilization, a diploid is reformed by joining two haploid germ cells. The diploid zygote contains equal numbers of chromosomes from each of two parents. Observations of eggs revealed that after fertilization the egg contains 2 nuclei which fuse to form a single nucleus The nucleus must then contain the "physical basis of heredity."

Somatic cell ---- , one cell two cells, chromosome no change Germ cell---Meiosis, one cell four cells, chromosome n = n / 2

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10 Cell Theory (Late 19th Century by August Weismann) Mosaic and regulative methods of development

Organisms are composed of cells, the basic unit of life. How cells become different from one another emerged. Both animals and plants are multicellular composites that arise from a Mosaic development depends upon specific determinants in the one- single cell, therefore, development must be epigenetic and not celled zygote that are not divided equally between the daughter cells preformational since a single cell (the fertilized egg) results in many (asymmetric division). Fig.1.7 Weismann’s theory of nuclear determination. different types of cells. Roux (1880's) destroyed one cell of a two-celled embryo (with a hot pin) Only the germ cells (egg and sperm) pass characteristics on to the to result in ~1/2 frog embryo. Based on a mosaic mechanism. To offspring. (First distinction between somatic and germ cells) check Weismann’s theory. Somatic cells are not directly involved in passing on traits to the next Regulative development depends upon interactions between 'parts' of generation and characteristics acquired during an animal's life are the developing embryo and can result in causing different tissues to form (even if parts of the original embryo are removed). not passed onto the offspring. Driesch destroyed one cell of a sea urchin embryo at the two cell stage “a hen isonlly an egg'sway of making another egg.” (Samuel Btl)Butler) and a normal appearing but smaller sea urchin resulted.

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Mosaic and regulative methods of development Mosaic and regulative methods of development How cells become different from one another emerged.

Mosaic development depends upon specific determinants in the Fig.1.7 Weismann’s theory of nuclear determination one-celled zygote that are not divided equally between the daughter cells (asymmetric division). Fig.1.7 Weismann’s theory of nuclear determination. Roux (1880's) destroyed one cell of a two-celled embryo (with a hot pin) to result in ~1/2 frog embryo. Based on a mosaic mechanism. To check Weismann’s theory. Fig 1.8 Driesch destroyed one cell of a sea urchin embryo at the two cell stage and a normal appearing but smaller sea urchin larva resulted. Regulative development depends upon interactions between 'parts' of the developing embryo and can result in causing different The factors in the nucleus that were distributed asymmetrically to tissues to form (even if parts of the original embryo are daughter cells during cleavage removed).

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11 Mosaic and regulative methods of development Mosaic and regulative methods of development How cells become different from one another emerged.

Fig.1.8 Roux’s experiment to check Weismann’s theory Mosaic development depends upon specific determinants in the one-celled zygote that are not divided equally between the daughter cells (asymmetric division). Fig.1.7 Weismann’s theory of nuclear determination . Roux (1880's) destroyed one cell of a two-celled embryo (with a hot pin) to result in ~1/2 frog embryo. Based on a mosaic mechanism. To check Weismann’s theory. Damage Driesch destroyed one cell of a sea urchin embryo at the two cell area stage and a normal appearing but smaller sea urchin larva resulted. Regulative development depends upon interactions between 'parts' of the dev eloping embry o and can resu lt in cau sing different No develop tissues to form (even if parts of the original embryo are removed). Development of the frog is based on a mosaic mechanism

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Mosaic and regulative methods of development Regulative Development Drastically different results from the predictions of Weismann or Roux Fig.1.9 Driesch’s experiment on sea urchin embryo, which Figure 3.15. Driesch's first demonstrated the phenomenon of regulation demonstration of regulative development. (A) An intact 4-cell seaurchinembryo generates a normal pluteus larva. (B) When one removes the 4-cell embryo from its fertilization envelope and isolates each of the four cells, each cell can form a smaller, but normal, pluteus larva. (All larvae are drawn to the same scale.) NtNote that the four larvae diderive din this way are not identical, despite their ability to generate all the small necessary cell types. Such variations are also seen in adult sea urchins formed in this way. Mosaic development ??????????????? 47 48

12 Mosaic versus regulative methods of development Regulatory development: induction

Mosaic development depends upon specific determinants in Induction is a type of regulatory development via cell-cell the one-celled zygote that are not divided between the interaction or communication. daughter cells (asymmetric division). Lack cell types if This is a process where one directs the development of regions of embryo are removed (Autonomous specification). another tissue. A classical experiment: Spemann & Mangold (1924) - graft of the Regulative development depends upon interactions between blastopore lip of one newt onto another. 'parts' of the developing embryo and can result in causing Note: The blastopore is the opening formed in early gastrulation different tissues to form (even if parts of the original embryo are removed) (Conditional specification). through which cells migrate inside. The Spemann Organizer can induce the formation of an ectopic axis (twinned embryo)

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Fig.1.9 The importance of induction and cell-cell interaction in development was regulative proved dramatically in 1924 Spemann development; and his assistant Hilde Mangold. cell fate determined late organizer

more mosaic than regulative Organizer: controlling the organization of a completer embryonic body mosaic development; 1935 Nobel Prize for and cell fate determined early

The same embryo grow the same organism? 51 52

13 Five processes of development One fertilized egg split into two different 1. Cleavage Division: No increase in total cell mass (every cell size ↓) , copy . Fig. 1.12 Genetic vs. Embryology

GtGenotype vs. ph htenotype

Environment factor

Genotype Phenotype regulated Fig. 1.10

Phenotype: visible appearance, internal structure, ….

53 54 Xenopus eggs after four cell divisions

Five processes of development Five processes of development

2. : A/P and D/V axes: Coordinate system Fig. 1.13 3. : take 3D form, migrates far. 1 egg→250 types Fig. 1.14

2. Different ggyerm layers Box 1B

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14 Five processes of development Five processes of development

4. Cell Differentiation: cells 1. Cleavage Division: No increase in total cell mass become structurally and functionally different. (every cell size ↓) , copy gene. Fig. 1.12 5. Growth: cell multiplication 2. Pattern Formation: A/P and D/V axes: Coordinate system (proliferation), increase in cell Fig. 1 .13 size, deposit extracellular material (bone, shell) growth 2. Different germ layers Box 1B can be morphogenetic. Fig. 1.15 3. Morphogenesis: take 3D form, neural crest migrates far. 1 egg→250 types Fig. 1.14 4. Cell Differentiation: cells become structurally and functionally different. 5. Growth: cell multiplication, increase in cell size, deposit extracellular material (bone, shell) growth can be morphogenetic. Fig. 1.15 57 58

Five cell behaviours provide the link between gene action and Genes control cell behavior by controlling which protein are made by development a cell

• results in cell behavior and development. Gene protein regulation • Gene activity gives cell identity. 1. Cell-cell communication 2. Cell shape changes 3. Cell movement (adhesion molecular) Fig. 1.16 4. Differential gene activity controls development 5. Cell (apoptosis)

The behavior modulated by 1. gene—protein 2. cell-cell interaction 3. positional information 59 Fig. 1.17 Gene expression and protein synthesis 60

15 Cell fate Differential gene expression cell FATE specification, determination

Regulative mosaic development “Cell fate” is what cells should become (not differentiation). Specified cells keep their fate even when isolated. This is tested by transplantation (some cells change their fate). Early embryonic cells are not narrowly determined, latter ones are. DNA mRNA Protein transcription & processing nuclear export, translation & modification Differential gene expression controls cell differentiation. Common house-keeping genes do not cause cells to differ. Developmentally specific transcription factors direct differential Next step gene expression. 分化 Differentiated differentiation Start from asymmetrical division

Fig. 1.18 The distinction between cell fate, determination , and specification61 62 (Idealized)

The timing plays an important role of Differential gene activity controls Inductive interaction development How the gene expression in one cell affected another?

Inductive interaction is the process by which one group of cells change the fate of another ggproup of cells. The information to cause induction passes Fig 1.19 from cell to cell in the form of: Determination 1. secreted diffusible molecule (signal of the eye transduction) region with 2. surface molecule (competent) time in 3. gap junction (channel) amphibian development Competence : the state of being able to respond to inductive signals due to the presence of receptor or transcription factors.

63 Fig. 1.20 An inducing signal can be transmitted from 64 one cell to another

16 Positional information directs pattern formation The on pattern formation Fig. 1.22 Positional information directs pattern formation by giving positional values to cells. Fig. 1.21 This biological information must first be specified and then the value must be interpreted . (Nearest more directly) varies in concentration and directs different fates at Fig. 1.21 different concentrations. Fig. 1.22 Source Sink [high] [low] Threshold concentrations: Different fates, different levels (ranges) Morphogen: A chemical whose of morphogen. concentration varies, and which is involved in pattern formation

Cell fate dependent on: 1. asymmetrical division 2. Timing Pattern formation: 3. Inductive information Concentration and threshold effect 4. Positional information 65 66

Development is progressive

1. Lineage dependent fate: Cytoplasmic localization Different order produced and asymmetric cell division control the fate of different pattern resulting cells. Daughter cells become different and give different lineages. Fig. 1.25

One cell two different cell four………

It can produce many cell types

asymmetrical division

Fig. 1.23 Positional information could be used to generate an enormous variety of pattern 67 68 Fig 1.26 Stem asymmetric cell division produced differentiate and renew

17 Development is progressive Development is progressive 2.Generative program rather than a descriptive program: Development 3. Lateral Inhibition: Many structures are regularly spaced. Fig. 1.24 depends upon a progressive series of instructions.

Descripppgtive program: DNA contain a full descrippgtion of the organisms to which it will giver rise; It is a “blueprint” for the organism. DNA Generative program: Descriptive program combine with cytoplasmic effect.

An embryo needs each action to be built upon the previous action and that on the one before . Via inhibitory molecule acts the nearest cell, prevent development Development instructions are not a "blue print" but are a structural list of actions Cells that form a structure stop neighbouring cells from doing the same (feathers, compound eye faucets).

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Summary Development is progressive 1. All the information for embryonic development is contained with the 4. The reliability of development is achieved by a variety of means fertilized egg (diploid zygote). 2. Cytoplasmic constitute plays important role of the embryonic Redundancy : There are two or more ways of carrying out a development. particular process Negative feedback 3. Gene encode protein regulated embryonic development. Many feedback system (positive or negative ) regulated the 4. Major process involved in development are cell division, pattern development formation, morphogenesis, celldifferentiation, cell migration, cell death and grow. 5. The complexity of embryonic development is due to the complexity of cells themselves 5. Development is progressive. 6. Cell in early embryo usually have much greater potential for i. Different gene and different timing development. ii. More complex independent external signal 7. Many gene are involved in controlling the complex interaction, and 71 72 reliability is achieved in a variety was

18 Transgenic mice

Udder

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Transgenic mice Northern blotting (for m-RNA detection)

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19 Reverse transcriptase-polyermerase chain reaction (RT-PCR) Microarray-technique

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In situ hybridization

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20 1. Specific term: A/P, D/V 2. Epigenesis vs. preformation 3. Mosaic vs. regulative development 4. five steps of development 5. Cell fate

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