lec164

Cell Division

One cell creates approximately 10^14 cells.

Cell Types - number in humans

Humans have over 200 distinct cell types.

DNA Uniformity

All cells contain the same DNA.

Tissue Formation

Different cell types form various tissues and organs.

Cell Growth Limitation

Cells know when to stop dividing.

Tissue Maintenance

Replacement of dead cells in tissues.

Morphogenesis

Cells organized into tissues and organs.

Hippocrates

Recognized the process of development.

Aristotle

Studied how embryos are formed.

Preformation

Organism is preformed in egg or sperm.

Epigenesis

New structures arise progressively during development.

Harvey's Theory

All animals originate from eggs.

Homunculus

Miniature human concept in sperm.

Cell Theory

Cells are basic units of life.

Von Baer's Laws

General features appear before specialized features.

Embryonic Development

Early embryos resemble earlier stages of lower animals.

Fixed Body Plan

Final body plan established at birth in vertebrates.

Modular Growth

Plants grow by adding organs continuously.

Morphological Plasticity

Plants can adjust growth based on environment.

Pattern Formation

Generation of order within a structure.

Cell Differentiation

Specialization of cell function.

Growth

Increase in size of organisms.

ideas in preformation

entire organism is contained in miniature in the egg/sperm - homunculus - implies all beings existed from the beginning of life

problems with preformation

fitted religious trends and doesn't account for mixed inheritance in hybrids

models for studying development - vertebrates

frogs, chicks, zebrafish

models for studying development - invertebrates

drosophila, nematode worm, sea urchin

models for studying development - plants

arabidopsis thaliana

Cleavage

Cell division without growth in organism.

Fertilised egg

Zygote that undergoes cell division.

Differentiation

Process where cells become specialized.

Body axes

Orientation established during early development.

Gastrulation

Formation of gut and germ layers.

Blastula

Ball of cells formed during cleavage.

Blastocoel

Fluid-filled cavity in the blastula.

Gastrula

Stage after blastula, forms gut structure.

Archenteron

Primitive gut formed during gastrulation.

Blastopore

Opening of the archenteron in gastrula.

Blastomeres

Smaller cells resulting from cleavage.

Holoblastic cleavage

Complete division of egg with little yolk.

Meroblastic cleavage

Incomplete division due to yolk presence.

Radial cleavage

Equal cell division in sea urchins.

Unequal cleavage

Asymmetric division due to yolk distribution.

Discoidal cleavage

Cell division limited to a small region.

Superficial cleavage

Nuclei migrate to form a single cell layer.

Rotational cleavage

Perpendicular division of cells in mammals.

Morula

Solid ball of cells before blastocyst stage.

Trophectoderm

Outer cells contributing to placenta.

Mid blastula transition (MBT)

Shift to embryonic control of cell cycle.

steps within cleavage

cleavage of zygote leads to eight cell stage, further cleavage forms blastula

organisms with holoblastic cleavage

echinoderms, annelids, molluscs, mammals, amphibians - species whose eggs have small/moderate amounts of yolk

organisms with meroblastic cleavage

fish, reptiles, birds, insects - species with yolk-rich eggs

sea urchin cleavage

radial - simple planes of cleavage divide cells equally

amphibians cleavage

unequal cleavage due to yolk distribution - majority of animal body derived from divisions towards animal (top) pole

frog cleavage

first 2 cleavage furrows form 4 equal size blastomeres, 3rd cleavage is asymmetric - forms unequal blastomeres due to yolk in vegetal hemisphere (bottom)

fish/bird cleavage

discoidal - creates cellular region above dense yolk

chick egg cleavage

limited to a small layer of cells on top of the yolk

insect cleavage

superficial - nuclei migrate to outer surface and become enclosed in membranes, form a single layer of cells over embryo surface

mammals cleavage

rotational, during 2nd cleavage the 2 cells are divided in perpendicular planes

human embryo - egg to implantation steps

egg is fertilised, first cleavage forms 2 cell stage, forms morula, becomes blastocyst, early stage implantation

morula - inner 8 cells

inner cell mass - becomes proper embryo

morula - outer 20 cells

trophoectoderm - embryonic contribution to placenta

embryo cleavage cell cycle

no gap phases, rapid cell divisions, initially controlled by factors stored in the egg

MBT

mid blastula transition

what happens after MBT

2 G phases added, new embryonic mRNA transcription, cell division becomes asynchronous

pattern formation

generation of order within a structure and establishment of main body axes

3 axes of vertebrate body

dorsal-ventral, anterior-posterior, left-right

dorsal-ventral is defined by ..

point of sperm entry

anterior-posterior is defined by ..

polarity of egg before fertilisation

right-left defined by ..

first cleavage

dorsalising factors from egg define ..

dorsal-ventral axis and site of future blastopore

morphogens

signalling molecules - gradient of morphogens determines pattern of development

maternal effect genes

embryo phenotype depends on mothers genes

morphogenesis

change in form, generation of main structure of body, gastrulation and organogenesis

gastrulation

movement of cells from blastula surface to embryo interior - forms germ layers

organogenesis

formation of organs

gastrulation results in formation of. .

a body cavity, independent cell layers

archenteron

primitive gut

germ layer

region of cells that give rise to specific tissues/organs

ectoderm

outer germ layer, forms epidermis, brain and CNS

mesoderm

middle germ layer, forms blood, heart, kidneys, gonads, bones, muscles, connective tissues

endoderm

inner germ layer, forms lungs, inside of digestive tract and associated organs

2 kinds of cell movements in sea urchins

migration of cells to form mesoderm - invagination at vegetal pole creates archenteron and blastopore

diploblastic organisms eg. cnidaria

2 germ layers - ecto/endoderm - tissues but no organs, radial symmetry

triploblastic organisms eg. vertebrates

3 germ layers - tissues and organs, bilateral symmetry

stages in drosophila life cycle

fertilisation, cleavage, gastrulation, hatching, metamorphosis

cleavage in drosophila

superficial - cleavage surrounds large central yolk

how many cleavage cycles

13 - no cell division, very rapid

cleavage produces

a syncytium of around 6000 nuclei

cellularisation

occurs during cycle 14 - produces cellular blastoderm - single layer of cells surrounding yolk

cleavage is driven by ..

maternal mRNAs and proteins derived from the egg

what happens after cellularisation

major switch to zygotic gene expression which drives differentiation, all subsequent tissues are derived from the single layer of cells around embryo

endoderm forms ..

gut

mesoderm forms ..

muscle, heart, blood

ectoderm forms ..

nervous system, cuticle

how is muscle formed

tube of mesoderm forms via invagination of the ventral furrow - will form muscle

how is midgut formed

endoderm forms by invaginations at anterior and posterior ends - will join to form midgut

main body regions are specified by ..

movements of ectodermal/mesodermal cells in the germ band - positions future body parts, followed by segmentation

maternal genes establish ..

body plan and polarity

maternal effect genes

phenotype depends on gene in the mother

how does bicoid work

transcribed after fertilisation, forms a gradient across syncytium - it is a morphogen and activated expression of hunchback