Chapter 2 How Development Works

CHAPTER 2 How development works

I.

GAMETOGENESIS A. Spermatogenesis B. Oogenesis

II. FERTILIZATION, F ERTILIZATION, CLEAVAGE, CLEAVAGE, BLASTULATION, GASTRULA GASTRUL ATION III. FIVE MAIN PROCESSES OF DEVELOPMENTAL BIOLOGY A. Regional specification - from a population population of similar similar cells, the formation formation of a set of territories of cells each committed committed to become a different different structure or type ( ex. Head, trunk, tail) B. Cell differentiation - format formation ion of specialized specialized cells cells C. Morphogenesis - cell and and tissue movem movements ents D. Growth -increase in size E. Timing - coordina coordinated ted in time time

I.

GAMETOGENESIS A. Spermatogenesis B. Oogenesis

II. FERTILIZATION, F ERTILIZATION, CLEAVAGE, CLEAVAGE, BLASTULATION, GASTRULA GASTRUL ATION III. FIVE MAIN PROCESSES OF DEVELOPMENTAL BIOLOGY A. Regional specification - from a population population of similar similar cells, the formation formation of a set of territories of cells each committed committed to become a different different structure or type ( ex. Head, trunk, tail) B. Cell differentiation - format formation ion of specialized specialized cells cells C. Morphogenesis - cell and and tissue movem movements ents D. Growth -increase in size E. Timing - coordina coordinated ted in time time

II. FERTILIZATION, CLEAVAGE, BLASTULATION, GASTRULATION

Generalized sequence of early development

II. FERTILIZATION- from the time of sperm entry to cleavage

Common features of fertilization 1. Block to Polyspermy - rapid change in egg structure that excludes the fusion of any sperm 2. Activation of the Inositol triphosphate signal transduction pathway

Rapid increase in extracellular calcium

Causes exocytosis of cortical granules

Fertilization membrane

Increase rate of protein synthesis

Trigger cytoplasmic rearrangements that position determinants

Start the second meiotic division

ZYGOTE- fertilized egg; stage after the fusion of the male and female pronucleus

II. CLEAVAGE and BLASTULATION – early cell division

-there is no growth phase between successive divisions - Blastomeres – product of cleavage -

The embryo’s own genome remains inactive during part or all of the cleavage phase Protein synthesis is directed by mRNA transcribed during oogenesis

Maternal effects – properties of the cleavage stage embryo depends entirely on the genotype of the mother TYPES OF CLEAVAGE 1. Meroblastic - part of the zygote cleaves, remainder ( e.g. yolk) does not 2. Holoblastic - whole zygote becomes subdivided into blastomeres 3. According to arrangement of blastomeres: a. radial, bilateral, rotational, spiral superficial

Microlecithal / Oligolecithal - little yolk - echinoderms, coelenterates, amphioxus, mammals Medialecithal / Mesolecithal - moderate amount of yolk - annelids, mollusks, lampreys, lungfishes, amphibians Megalecithal/Macrolecithal - eggs have large quantities of yolk - arthropods, hag fishes, bony fishes, reptiles, birds, monotremes Isolecithal- refers to an even distribution of yolk Telolecithal- most of its yolk concentrated on one pole - birds (highly), amphibians ( telolecithal)

III. GASTRULATION- phase of morphogenic movements in early development that brings about the formation of the three germ layers GASTRULA- three layered structure

Different processes during gastrulation

Invagination - infolding of a cell sheet to form an internal protrusion or pocket - initiated from a localized apical constriction - constriction causes the cell sheet to buckle -> constricted region of cells forms a protrusion into the interior

Involution - internalization of a cell sheet by movement led by a free edge - initiated from a localized apical constriction - migration of cells around the edge of the constricted surface - will involve the formation of a free edge in the involuting tissue

Epiboly – active spreading and increase in area of a cell sheet

surface

sections

Migration of cells around the edge of the constricted surface

Constricted region of cells forms a protrusion in the anterior

Sheets of cells surround and enclose another population

GASTRULATION- convergent extension

Intercalation of cells leading to axial elongation ( convergent extension) - Individual cells intercalate in between each other - cause a constriction of the sheet in the direction of intercalation - Elongation of the sheet at right angles to the intercalation

GASTRULATION- Ingression

Ingression – produces mesenchymal cells at the onset of gastrulation - During the epithelial-mesenchymal transition (EMT), the primary mesenchymal cells (PMCs) detach from the epithelium and become internalized msenchymal cells that can migrate freely.

Cells lie scattered within an extracellular matrix

Lose its adhesion to hyaline, loses adhesion and cadherin. Ingressing cells squeeze through the matrix

REGIONAL SPECIFICATION and DETERMINANT

A. Regional specification - from a population of similar cells, the formation of a set of territories of cells each committed to become a different structure or type ( ex. Head, trunk, tail)

Signaling center Determinant – important for future regional specification of the embryo - substance/s located in part of an egg or blastomere - guarantees the assumption of a particular state of commitment by the cells that inherit it during cleavage ( animation 4) - can be mRNA o protein - can be transcription factors


Determinants are of considerable importance for the very earliest stages of the embryonic development. Often responsible for the establishment of the first two or three distinctly specified regions in the embryo. Sometimes RNA or proteins. Becomes expressed, localized and controls the fate of the first two blastomeres.

REGIONAL SPECIFICATION and DETERMINANT Generation of bilateral symmetry with two determinants

Two gradients partition the embryo into territories along two axes The resulting embryo has territories arranged symmetrically around a medial plane.

Localization of a determinant by a symmetry breaking process

Localization of a determinant. Segregates some substances to one region of the zygote and other substances to other regions.


PIE-1 - a bifunctional protein that regulates maternal and zygotic expression - inhibit mRNA transcription - promotes germ cell fate : 1. inhibition of transcription, which blocks zygotic programs that drive somatic development 2. promotes primordial cell development by activating the protein expression from nos-2 and possibly other maternal RNAs

REGIONAL SPECIFICATION and DETERMINANT Germ cell during embryonic development 1. Multiplication 2. Migration to the gonads 3. Become fully integrated in the gonads 4. In mid-development- key determination of sex determination is made 5. Gonad is determined to become ovary or testes

INDUCING FACTOR

PROPERTIES OF A MORPHOGEN GRADIENT Normal development of an animal with head and three segments

Graft of the posterior source to the anterior causes formation of a U-shaped gradient and produces a double-posterior animal.

Insertion of an impermeable barrier causes formation of a large gap in pattern Morphogen- a type of inducing factor to which competent cells can make at least two different responses at different threshold concentrations. Responding cell will form a different committed territories

Absence develops head

High develops tail

HOMEOTIC MUTANTS Homeotic genes /selector genes – controls the commitment of different body parts - a gene whose expression distinguishes two body parts. If mutated then one body part will be converted into the other. - Expression is controlled by cytoplasmic determinant or inducing factors

Normal genotype and phenotype

Loss-of-function mutation of gene 2 causes second body segment to resemble the first.

Gain-of-function mutation of gene 2 causes first body segment to resemble the second.

DEVELOPMENTAL CONTROL GENES – all encode transcription factors

BISTABLE SWITCH – positive feedback regulation - Molecular mechanism which has two stable steady states that can be interconverted by some external signal

The figure depicts a temporal sequence. In step 2, the gene is upregulated by a regulator In step 3, it is also upregulated by its product In step 4, it remains “on” because of the product even though the regulator is gone

“on” by a regulator ( either a cytoplasmic determinant or a transduction pathway activated by an inducing factor)

AXES AND SYMMETRY

Principal axes of an animal

Anatomical planes of an early embryo

Axes of a fertilized egg after it has acquired a dorsoventral symmetry

Ventral view of an animal showing deviation from bilateral symmetry

TYPES OF CELL MOVEMENTS -

Many morphogenic movements depend on the movement of individual cells Long range ex. Migration of neural crest cells or germ cells

Short range ex. Adhesion or shape changes

1. Large flat Lamellipodia 2. Multiple Thin filipodia 3. Microfilament bundles Flat process/ extension rich in microfilaments

Apical constriction reduce the apical surface area - Increase the length of cells Preliminary to invagination

CLASSIFICATION OF MORPHOGENIC MOVEMENTS

Localized apical constriction Constricted region of cells forms a protrusion in the anterior

Migration of cells around the edge of the constricted surface

-processes that generate multilayered structure from a single epithelium - Found in gastrulation, neurulation, and in the formation of glands, sense organs and appendages

Sheets of cells surround and enclose another population Expands to cover the whole embryo


Cells form aggregates ex. Somites and skeletal elements formation - Arises partly by: 1. increased cell division 2. reduction of matrix secretion 3. increased cell-cell adhesion

generation of hollow ball or tube of cells - May occur either by cell rearrangement or by apoptosis of cells in the interior ex. Formation of lumen


Also called delamination -cells leave epithelium and move off as individuals -reduction in cell-cell adhesion in cells Ex. Chick epiblast to form hypoblast formation of neural crest from the dorsal neural tube

-formation of coelomic lining of epithelium or of kidney tubules


Individual cells intercalate in between each other -

Causing constriction in the direction of intercalation And elongation of the sheet at right angles to the intercalation - Cells all need to be polarized in the same direction

Acquisition of a polarity by cells in an epithelium in the direction of the plane of epithelium - Found in most epidermal structures


-characteristic of organogenesis rather than early development -

Epithelial bud grows into a mesenchymal mass

-

The number of growing points progressively increases to generate a branched structure

-

Ex. Tracheal system, lung or kidney

CELL ADHESION Adhesion of early embryo cells is dominated by CADHERINS Cadherin-based adhesion is homophilic- cells carrying E-cadherin will stick more strongly than to cells bearing N-cadherin

If cells with different adhesion systems are mixed -> will sort out into separate zones, eventually forming dumbbell-like configuration or even separate

with same adhesion systems : If cell type A is more adhesive than B, then B will eventually surround A

CELL ADHESION

Experiment demonstrating Cell sorting by differential adhesion Red and green cells are colored with vital dyes 2.4 greater N-cadherin transfected in green cells Cell types are Mixed together in an aggregate

More adhesive green cells in the middle

TYPES OF CELL DIVISION

CONCEPT OF GROWTH

True growth – increase in size In later development, growth does not require an increase in cell number

Involve an increase in cell size Increase in the amount of the extracellular matrix

* No real growth in free-living embryos Xenopus, zebrafish or sea urchin - Only caused by uptake of water -

Mammals, birds, reptiles with external nutrient supply can grow extensively during development

Link for viewing animations online:

http://bcs.wiley.com/hebcs/Books?action=resource&bcsId=7612&itemId=0470923512&resourceId=29584

Chapter 2 How Development Works

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