Dev Bio Exam 3

Metazoans

Clade of life composed of animas

• all metazoans, and thus all animals gastrulate

• Lewis Wolpert was on to something

Early development patterns let give us some clues into the relationships between the branches of the tree of life

• The number of germ layers created during gastrulation

• Diploblastic - 2 germ layers: ectoderm and endoderm

  • ex: cnidarians

• Triploblastic - 3 layers: ectoderm, mesoderm, and endoderm

  • ex: arthropods and chordates

Early development patterns let give us some clues into the relationships between the branches of the tree of life

• Triploblast can be further subdivided by which opening the gut develops first

  • Protostomes - Greek “mouth first”, mouth formed first at opening to the gut made during gastrulation

    • ex: arthropods

  • Deuterostomes - Greek “mouth second”, anus formed first at opening to the gut made during gastrulation

    • ex: chordates and echinoderms

Early development patterns give us some clues into the relationships between the branches of the tree of life

• Deuterostomes are further divided by whether they produce a notochord during development

• Notochord: characteristic of chordates, along with pharyngeal arches

  • transient mesodermal rod along dorsal most portion of embryo, induces formation of neural tube (future central nervous system)

The basic principles of early development are:

Cleavage: rapid, exponential cell divisions seen in morula and blastula generation

Gastrulation: cells produced by cleavage more to acquire new neighbors and generate germ layers

Axis specification: the major and local axes of the body are established to allow for patterns of differentiation

The process of cleavages can be divided into two basic concepts

Cleavage patterns:

• the amount of yolk distribution determines where cleavage can occur, effect symmetry and angles

• cytoplasmic factors influence angle of mitotic spindle (speed)

Mitotic rate:

• maternally deposited mitosis promoting factors (MPFs) determine rates and phase of cell division

(goldilocks rule; if too fast lethal if too slow doesn’t develop correctly)

Cleavage patterns fall under two broad categories

Holoblastic: “whole”

• complete cleavage of egg and yolk

• typical of eggs with sparse, evenly distributed yolk

Meroblastic: “part”

• only a portion of cytoplasm is cleaved

• typical of eggs with large volumes of dense yolk

Holoblastic cleavage has several subdivisions based on cleavage symmetry and angle

Isolecithal: (most)

• modes of dividing with spare, distributed yolk in cells

Mesolicithal: (some)

• divisions with slight to moderate preference for vegetal yolk distribution

Holoblastic cleavage has several subdivisions based on cleavage symmetry and angle

Radial - untwisted

Spiral - each level twisted 90 degrees

Meroblastic cleavage is very asymmetric and distinguished by cell division completion

Typical in animals with large yolk deposits maintained at length through development

• Telolecithal (dense yolk throughout most of the cell)

1. Bilateral cleavage; cephalopod mollusks

2. Discoidal cleavage; fish, reptiles, and birds

• Centrolecithal (yolk in center of egg)

  • superficial cleavage; most insects

Mitotic divisions are controlled by cytoplasmic determinates

• Mitosis-promoting factor (MPF): is a major factor responsible for the resumption of meiotic cell division post-fertilization in frog eggs

• also regulates the cell cycle of early blastomeres

MPF controls mitotic divisions of early blastomeres by cyclically degrading the cyclin B subunit

MPF consist of two subunits

• Cyclin B (large subunit)

• Cyclin-dependent kinase (CDK, CDC in diagram)

Maternal deposition of Cyclin B mRNA allows for easy synthesis in early divisions

• results in rapid, synchronous, 2-phase cell cycle (S phase, M phase)

Generation of zygote-produced MPFs eventually transitions cell cycle to multi-part interphase

Pre-MBT: M and S

Post-MBT: M, G1, S, G2

Degradation of maternal mRNA leads to expansion of interphase with addition of G-phases

• cell divisions desynchronize, zygote genome begins to generate cyclins

• called the mid-blastula transition (MBT), or material-to-zygotic transition (MZT)

Once the blastula is formed, gastrulation allows for the generation of germ layers through cell movements

• Gastrulation leads to full realization of metazoan gut, a fact not lost on early biologist

• Ernst Haeckel, a friend and proponent of Charles Darwin, proposed the terms “gastrula” and “gastrulation” in the late 1800s

Gastrulation consists of several types of cell movement

Invagination

Ingression

Epiboly

Involution

Delamination

Convergent extension

Invagination

Movement of a continuous layer of cells inward, infolding of a sheet (epithelium) of cells, much like the indentation of a soft rubber ball when it is poked

(e.g. sea urchin endoderm)

Involution

allows a leading edge to “crawl” under and along an outer layer of cells, inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells

(e.g. amphibian mesoderm)

Ingression

involves the complete delamination of cells from a layer and independent migration inward, migration of individual cells become mesenchymal (i.e. separate from one another) and migrate independently (e.g. sea urchin mesoderm)

Delamination

allows for the splitting of one layer of cells into two separated layers, splitting of one cellular sheet into two more or less parallel sheets. while on a cellular basis it resembles ingression, the results is the formation of new (additional) epithelial sheet of cells (e.g. hypoblast formation in birds and mammals)

Epiboly

allows for the concerted movement of a layer of cells to “engulf” a deeper layer of cells, movement of epithelial sheets (usually ectodermal cells), spreading as a unit (rather than individually) to enclose deeper layers of the embryo. Can occur by cells dividing, by cells changing into their shape, or by several layers of cells radially intercalating into fewer layers (arrows); often, all three mechanisms are used (e.g. ectoderm formation in sea urchins, tunicates, amphibians, and zebrafish)

Convergent extensions

allows cells to push the anterior-posterior axis outwards (circle or ovoid or rod), movement of more lateral cells of all germ layers toward the midline. This leads to a convergence of those cells causing their intercalation along the medial to lateral axis. As these cells converge en masse, the embryo extends along the anterior-to-posterior axis (e.g. ectoderm and mesoderm of amphibians, zebrafish, birds, and mammals).

The basic principles of early development are cleavage, gastrulation, and axis specification

Cleavage:

• rapid, exponential cell divisions seen in morula and blastula generation

Gastrulation:

• cells produced by cleavage move to acquire new neighbors and generate germ layers

Axis specification

• the major and local axes of the body are established to allow for patterns of differentiation

Mitotic divisions are controlled by cytoplasmic determinates

• Mitosis-promoting factor (MPF) is a major factor responsible for the resumption of meiotic cell division post-fertilization in eggs

  • frog eggs are a great model for this principle

• Also regulates the mitotic cell cycle of early blastomeres

MPF controls mitotic divisions of early blastomeres by cyclically degrading the cyclin B subunit

MPF consist of two subunits

• Cyclin B (large subunit)

• Cyclin-dependent kinase (CDK, CDC in diagram)

Maternal deposition of Cyclin B mRNA allows for easy synthesis in early divisions

• results in rapid, synchronous, 2-phase cell cycle (S phase, M phase)

Generation of zygote-produced MPFs eventually transitions cell cycle to multi-part interphase

Degradation of maternal mRNA leads to expansion of interphase with addition of G-phases

• cell divisions desynchronize, zygote genome begins to generate cyclins

• called the mid-blastula transition (MBT), or maternal-to-zygotic transition (MZT)

  • (mom’s mRNA could mask effects of a cyclin B knockout for a while)

Gastrulation

movement of cells to create new neighbors and generate germ layers, allows for primitive gut to begin to form

Neurulation

process of folding the neural plate and closing the neural tube

Invagination depend on maintaining tight cellular connections and apical constriction

Apical constriction: asymmetric change in cell shape that results in the apical (side away from basal lamina, or closest to lumen or exterior) surface “constricting” and narrowing compared to the base

Tight junctions between cells in the layer create “buckling” effect that starts invagination

Evidence for evolution of the cellular mechanisms for invagination found in close animal relatives: choanoflagellates

Have collar closure when actin-myosin ring is not constricted and that’s how they feed. When the actin-myosin ring is contracted the collar opens and makes a motile form which has the apical constriction

What end of the sperm cell is the apical side?

The flagella

Sea urchins undergo ingression followed by invagination to migrate mesoderm and endoderm respectively

• ingression is driven by epithelia-to-mesenchymal transitions (EMT), allowing cells to delaminate and migrate

  • in sea urchins, mesoderm ingresses followed by invagination of endoderm

Birds and mammals reverse this process in the formation of the primitive streak

• Discoidal embryo has radial symmetry broken by formation of “virtual” blastopore-the primitive streak- at future and posterior position

• Invagination (apical constriction) moves upward towards anterior, with ingression forming mesoderm and endoderm

Ingression and Invagination get the party started but Involution and Epiboly keep it going all night

• Both involution and epiboly continue to power the gastrulation process

  • involution further folds and pushes the cells from the blastopore inwards

• Epiboly drives this process by pushing the epiblast (outer layer of cells of blastula) cells to spread through cell division, shape change, and intercalation

  • Intercalation - the merging movement of cells from two or more adjacent populations into fewer (or one) layer

Involution involves expansion of epiblast so that the folding started by invagination and ingression can continue to push that tissue up into the embryo

The ingression of the tissue at the blastopore leads to the ability to have involution progress

Epiboly uses radial intercalation to drive the epiblast over the rest of the embryo and contributes much of the force maintaining involution

Cells at leading end of involuting mesendoderm (precursor to both germ layers) craw along the basal lamina of the epiblast

Convergent extension uses the force of these other cell movements in concert to elongate