Intro To Development

Question 1

Oviparity

Answer 1

• eggs are laid, with little or no embryonic development in the mother
• internal or external fertilisation
• invertebrates, fish, amphibians, reptiles, birds, monotreme (egg laying) mammals

Question 2

Oviviparity

Answer 2

• eggs are retained in the mother’s body until ready to hatch
• no placental connection to mother
• internal fertilisation, born live
• invertebrates, fish, amphibians, reptiles

Question 3

Viviparity

Answer 3

• eggs are retained in the mother’s body and connected to the mother via a placenta
• internal fertilisation and young are born live
• most mammals, rare in invertebrates, fish, reptiles

Question 4

Preformation

Answer 4

Organs and tissues are preformed and correctly positioned in the fertilised egg. They simply enlarge during embryonic development. (Historically an argument of some scientists but has been disproved)

Question 5

Epigenesis

Answer 5

Organs and tissues are formed gradually, the complexity of the embryo increasing with time (Aristotle)

Question 6

Homunculus (little man) idea of spermists:

Answer 6

Preformationist idea in late 17th C - fully formed human could be seen in the head of each sperm. Each sperm male has sperm of its own? -> reductio ad absurdum

• mother make no contribution to next generation apart from suitable environment for embryo to develop? but mother’s characteristics are inherited.

Question 7

First person to use microscopes to study developing (chick) embryos?

Answer 7

Marcello Malpighi

Question 8

How was the argument finally in favour of epigenesis?

Answer 8

Science is constantly evolving, technology improves and understanding changes.

Development of better microscopes and staining techniques - chick embryos increase in complexity as development progresses, they don’t have tiny beaks that just enlarge

Question 9

What stage are embryonic stem cells from?

Answer 9

The blastula (fluid filled sphere)

Question 10

How is the multilayered structure of the larva created and what are the layers called?

Answer 10

A coordinated series of cell movements called gastrulation: forming three germ layers that subsequently form the tissues and organs of the adult:

Ectoderm, mesoderm and endoderm

Question 11

What is lineage restriction?

Answer 11

When a differentiated cell line ceases to give rise to all tissue types and can only give rise to a certain sub- set of cell types

Question 12

Who discovered the germ layers?

Answer 12

Christian Pander when studying chick embryos

Question 13

What do triploblastic and diploblastic mean?

Answer 13

Triplobastic: having three primary germ layers

Diploplastic: possess two germ layers - lack mesoderm (simpler organisms: jelly fish, corals, hydra

Question 14

Differentiation

Answer 14

The process by which cells acquire their functional characteristics by specialising into a certain cell type

Question 15

The phylotypic stage

Answer 15

The time point in the development of an animal when it most closely resembles other species

Question 16

What are pharyngeal arches and who discovered them?

Answer 16

Rathke discovered them (branchial, visceral and gill arches)

They are found at the phylotypic stage of all vertebrate embryos

They become the gills of fish, but the jaws, ears, neck of mammals

Question 17

How did Darwin use the pharyngeal arches as an argument for evolution?

Answer 17

Evolution tinkers with the pharyngeal arches so these same structures can give rise to a jaw, or gills depending on the different species. Mammalian embryos don’t have gills at any stage of development

Question 18

Who proposed mosaic development?

Answer 18

August Weismann - eggs contain a mosaic of different determinants that are distributed asymmetrically during cleavage divisions and direct the future development of cells that inherit them (preformation at a molecular level)

Question 19

Who found the different coloured cytoplasms?

Answer 19

Edwin Conklin - studied ascidian eggs and saw some species had different colours of cytoplasm - following development of the cytoplasms, he saw what each part of the egg subsequently became

Question 20

What did Conklin conclude yellow cytoplasm contained?

Answer 20

A muscle determinant as yellow cytoplasm was always found in muscle cells of the larva

Question 21

What did Wilhelm Roux do?

Answer 21

Killed a single cell of the 2-cell frog embryo using a hot needle. Remaining cell formed only half an embryo -> frog egg was a mosaic of self differentiating parts, where the surviving cell had only received a subset of all the determinants in the egg

Question 22

Who determines that a cells fate depends on its position in the embryo?

Answer 22

Hans Driesch dissociated cleavage stage sea urchin embryos and allowed them to develop separately

Instead of forming partial larvae but instead they formed small complete larvae -> not a mosaic of localised determinants but display regulative development

Question 23

What is regulative development?

Answer 23

Cells split into two - embryo doesn’t continue developing as everything is normal and realised that there is no cell next to it. -> cells communicate with each other to check that development is happening correctly

Question 24

Who separated 2 cell newt embryos into individual cells and to show what?

Answer 24

Hans Spemann -> each cell formed a half-sized larva that was otherwise normal. -> amphibian embryos can also regulate. Regulation not observed by Roux as he did not remove the dead cell.

Question 25

What did Hans Spemann and Hilde Mangold do?

Answer 25

Transplanted the prospective notochord of a newt gastrula into opposite side of similarly staged host. (Newt cells with different levels of pigmentation to follow transplanted cells) -> conjoined twin was formed, transplant formed the notochord and parts of the nervous system

Most of the nervous system formed was by host cells that would’ve otherwise formed epidermis - the fate of the host cells were changed from epidermal to neural

Question 26

What is neural induction?

Answer 26

During gastrulation the dorsal mesoderm migrates anteriorly beneath the dorsal ectoderm, releasing signals that induce the nervous system. In the absence of the signals, dorsal ectoderm will form epidermis.

Question 27

Embryonic induction

Answer 27

The process whereby a cell or tissue signals to another cell or tissue in the embryo, thereby affecting the development of the responding cell or tissue

Question 28

Embryonic competence

Answer 28

The ability of a cell or tissue to respond to an inducing signal (embryonic tissues only remain competent for a limited period of time)

Question 29

Cell fate

Answer 29

Not normally decided in a single step but over a number of steps that restrict its potency - the range of cell types it will form

The cell follows a series of decisions (eg ectoderm or mesoderm) before differentiating into final cell type

Question 30

What are the earliest decisions of cell fate vs later ones?

Answer 30

Earliest ones involve cytoplasmic determinants, but later decisions are made by cell to cell interactions (e.g .induction)