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Cellular Mechanisms
of Development
Chapter 19
Biology Dual Enrollment
Mrs. Mansfield
1
Process of development
• Process of systematic gene-directed
changes throughout an organism’s life
cycle
• 4 subprocesses:
– Cell division
– Differentiation
• Nuclear reprogramming
– Pattern formation
– Morphogenesis
2
Cell Division
• Very first process that must occur during
embryogenesis
• After fertilization, the diploid zygote
undergoes a period of rapid mitotic
divisions
• In animals, controlled by cyclins and
cyclin-dependent kinases (Cdks)
– Exert control over checkpoints in the cycle of
mitosis
3
Cell division
• Cleavage
– In animal embryos, period of rapid cell
division following fertilization
– Blastomeres – Enormous mass of the zygote
is subdivided into a larger and larger number
of smaller and smaller cells
– Not accompanied by any increase in the
overall size of the embryo
– G1 and G2 phases of cell cycle short or
eliminated
4
Cell cycle
• In contrast to the cell cycle of adult
somatic cells:
• Dividing cells of early frog embryos lack G1
and G2 stages
• Cleavage stage nuclei rapidly cycle between
DNA synthesis and mitosis
• Large stores of cyclin mRNA present in the
unfertilized egg
5
• Caenorhabditis elegans
– One of the most completely described models
of development
– Adult worm consists of 959 somatic cells
– Transparent, so cell division can be followed
– Researchers have mapped out the lineage of
all cells derived from the fertilized egg
– Fate of each cell is the same in every C.
elegans individual
6
• Plant growth
– Animals can move while plants can’t
– Plants compensate by allowing development
to accommodate local circumstances
– Body plan is not fixed – assembled
throughout life span from modules
• Leaves, roots, branch nodes, flowers
– Each module is rigidly controlled but utilization
is adjustable to environmental conditions
– Body is built outward from stem cells in
meristems
7
Cell differentiation
• Human body contains 210 major types of
differentiated cells
• Cell determination – molecular decision to
become a particular type of cell
• Cells become determined prior to
differentiation
• Standard test for determination is to move
cell
8
Test for determination
• Determination has a time course
• Depends on a series of intrinsic or
extrinsic events
– A cell in the brain region of amphibian early
gastrula stage has not yet been determined
– If transplanted elsewhere in embryo, it
develops according to the site of transplant
9
• Determination often takes place in stages
– Cell first becomes partially committed
• Acquires positional labels that reflect its location in
the embryo
• For example, transplanted leg tissue may be
determined as leg tissue, but not a specific part of
the leg
10
• Molecular basis of determination
– Cells initiate developmental changes by using
transcription factors to change patterns of
gene expression
– Once the initial “switch” is thrown, the cell is
fully committed to its future developmental
path
– Cells become committed via
• Differential inheritance of cytoplasmic determinants
• Cell–cell interactions
11
• Differential inheritance of cytoplasmic
determinants
– Tunicates are marine invertebrates
– Swimming tadpolelike larval stage
– Muscles that move the tail develop on either
side of the notochord
– Colored pigment granules asymmetrically
localize to tail muscle cell progenitors
12
– Experimentally shifting colored pigment
granules causes other cells to become
muscle cells
– Female parent provides egg with mRNA
encoded by macho-1 gene
• Gene product has been shown to be a
transcription factor that can activate the expression
of several muscle-specific genes
13
• Induction
– Change in cell fate due to interaction with an adjacent
cell
– Demonstrate the importance of cell–cell interactions
in development by separating the cells of an early
frog embryo and allowing them to develop
independently
– Ectoderm develops from cells of the animal-pole
– Endoderm develops from cells of the vegetal-pole
– No mesoderm develops unless animal-pole cells and
vegetal-pole cells are placed next to each other
14
• Another example of inductive cell interactions is the
formation of the notochord and mesenchyme in tunicate
embryos
• Muscle, notochord, and mesenchyme all arise from
mesodermal cells
15
• Prospective mesodermal cells receive signals from the
underlying endodermal precursor cells that lead to the
formation of notochord and mesenchyme
• Combination of fibroblast growth factor (FGF) signal and
macho-1 muscle determinant leads to 4 different cell
types
16
• Stem cells
– Cells that are capable of continued division,
but can also give rise to differentiated cells
– Degree of determination
• Totipotent – cell that can give rise to any tissue in
an organism (embryo and extraembryonic
membranes)
• Pluripotent – give rise to all cells in the adult
organism’s body
• Multipotent – give rise to limited number of cells
• Unipotent – give rise to only a single cell type
17
• Embryonic stem cells (ES cells)
– Form of pluripotent stem cells
– Made from mammalian blastocysts
– ES cells isolated from inner cell mass and
grown in culture
– In mice, have been shown to develop into any
type of cell in the tissues of the adult
• Cannot develop into extraembryonic membranes
– 1998 – first human ES cells
• Great promise and controversy
18
• ES cells offer a way to study the
differentiation process in culture
– ES cells have been used to recapitulate in
culture the early events in mouse
development
– Offers the promise of understanding the
molecular cues that are involved in the
stepwise determination of different cell types
• Human ES cells could be used to make
hematopoeitic stem cells or
cardiomyocytes for use as treatments
19
Nuclear reprogramming
• Reversal of determination allowed cloning
– Experiments carried out in the 1950s showed
that single cells from fully differentiated tissue
of an adult plant could develop into entire,
mature plants
• Cells of an early cleavage stage
mammalian embryo are also totipotent
– Natural twinning
– Producing multiples artificially for commercial
agricultural lines of cattle
20
• Early experiments showed nuclei could be
transplanted between cells
– Cells do not appear to undergo any truly
irreversible changes, such as loss of genes
– More differentiated the cell type, the less
successful the nucleus in directing
development when transplanted
• Nuclear reprogramming – nucleus from a
differentiated cell undergoes epigenetic
changes that must be reversed to allow
the nucleus to direct development
21
• Early amphibian work showed that adult
nuclei have remarkable developmental
potential, but cannot be reprogrammed to
be totipotent
• Nuclear transfer in mammals did not result
in reproducible production of cloned
animals
– Did lead to discovery of imprinting
22
• 1984 – sheep was cloned using the
nucleus from a cell of an early embryo
• 1996 – Dolly, the first clone generated
from a fully differentiated animal cell
– Used somatic cell nuclear transfer (SCNT)
– Dolly matured into fertile adult
– Established beyond all dispute that
determination in animals is reversible
23
• Reproductive cloning
– Uses SCNT to create animal genetically
identical to another
– Efficiency is quite low and other problems
• Only 3–5% of adult nuclei transferred to donor
eggs result in live births
– Due to lack of genomic imprinting
• Normal mammalian development depends on
precise genomic imprinting
• Organization of chromatin in adult and embryo
very different
24
• Much work has been put into trying to find ways
to reprogram adult cells to become pluripotent
cells without the use of embryos
• Different lines of inquiry showed that
reprogramming of somatic nuclei was possible
• 2006 – genes for 4 different transcription factors
introduced into fibroblast cells in culture
– Named induced pluripotent stem cells (iPS cells)
– Appear to be similar to ES cells in terms of
developmental potential, as well as gene expression
pattern
25
Reprogramming adult cells
• Cells of adult organisms can be
reprogrammed to pluripotent cells
– Nuclei from somatic cells transplanted into
oocytes as during cloning
– Somatic cells fused to ES cells
– Germ cells after prolonged culture can
reprogram
– Somatic cells in culture can be reprogrammed
using factors.
26
• Therapeutic cloning
– Produce patient-specific lines of embryonic
stem cells
– Artificial embryo created using same process
as Dolly (SCNT)
– Its cells are used as embryonic stem cells for
transfer to injured tissue
– Body readily accepts these cells with no
immune rejection
– May be obsolete with development of iPS
cells
27
Pattern formation
• All multicellular organisms seem to use
positional information to determine the
basic pattern of body compartments and
overall body
• Positional information then leads to
intrinsic changes in gene activity
– Cells ultimately adopt a fate appropriate for
their location
28
• Pattern formation can be considered the
process of taking a radially symmetrical
cell and imposing two perpendicular axes
to define the basic body plan
– Anterior–posterior (A/P, head-to-tail) axis
– Dorsal–ventral (D/V, back-to-front) axis
• Polarity – acquisition of axial differences in
developing structure
29
• Fruit fly Drosophila melanogaster
– 2 bodies during development
•
•
•
•
Larva – tubular eating machine
Adult – flying sex machine
Metamorphosis – passage from one body to next
Embryogenesis – process of going from fertilized
egg to larva
30
• Development begins before fertilization with
construction of egg
• Specialized nurse cells that help the egg grow move
some of their own maternally encoded mRNAs into the
maturing oocyte
• Action of maternal, rather than zygotic, genes
determines the initial course of Drosophila development
31
• Syncytial blastoderm – 12 rounds of nuclear division
without cytokinesis
• Nuclei space themselves out
• Membranes grow forming cellular blastoderm
• Embryonic folding and primary tissue development
soon follow
• Within a day of fertilization, embryogenesis creates a
segmented, tubular body
32
• Anterior/Posterior axis in Drosophila
– Based on opposing gradients of two different
proteins produced by nurse cell mRNAs:
Bicoid (anterior) and Nanos (posterior)
– Morphogens – proteins whose concentration
gradients can specify different cell fates
– 2 other maternal messages, hunchback and
caudal, are evenly distributed across the egg
• Bicoid inhibits translation of caudal mRNA
• Nanos inhibits translation of hunchback mRNA
33
• Dorsal–ventral axis in Drosophila
– Established by actions of the dorsal gene product
– Maternal transcripts of the dorsal gene are put into
the oocyte – not asymmetrically distributed
– Gurken
• First, the oocyte nucleus synthesizes gurken mRNA
• Gurken mRNA accumulates in a crescent between the
nucleus and the membrane
• This will be the future dorsal side of the embryo
• No Gurken signal is released from the other side of the
oocyte, and the follicle cells on that side of the oocyte adopt
a ventral fate
– Dorsal
• Signaling molecule activated on ventral surface that causes
transport of Dorsal into ventral nuclei
34
• Note that many Drosophila genes are named for
the mutant phenotype that results from a loss of
function in that gene
– A lack of dorsal function produces dorsalized
embryos with no ventral structures
• Unifying factor controlling the establishment of
both A/P and D/V polarity in Drosophila is that
bicoid, nanos, gurken, and dorsal are all
maternally expressed genes
35
• Pattern formation in Drosophila along the
A/P axis
– Determination of structures is accomplished
by the sequential activation of three classes of
segmentation genes
– Create body plan of three fused head
segments, three thoracic segments, and eight
abdominal segments
36
• Within 3 hr after fertilization, a highly
orchestrated cascade of segmentation gene
activity transforms the broad gradients of the
early embryo into a periodic, segmented
structure with A/P and D/V polarity
–
–
–
–
Bicoid
Hunchback and other gap genes
Pair-rule genes
Segment polarity genes
37
• Drosophila mutants with particular segments
that seem to have changed identity
• Mutations in homeotic genes lead to the
appearance of perfectly normal body parts in
inappropriate places
38
• Homeotic gene complexes of Drosophila
– Bithorax complex
• Several homeotic genes map together to 3rd
chromosome
• Control the development of body parts in the rear
half of the thorax and all of the abdomen
• Order of genes corresponds to order of segments
– Antennapedia complex
• Governs the anterior end of the fly
• Order also corresponds to order of segments
39
• Homeodomain
– All of the homeotic gene complexes contained a
conserved sequence of 180 nucleotides coding for a
60-amino-acid, DNA-binding domain
– Homeobox DNA that encodes homeodomain
• Hox gene
– Homeobox-containing gene that specifies the identity
of a body part
– Function as transcription factors that bind DNA using
their homeobox domain
– Ultimate targets of Hox gene function must be genes
that control cell behaviors associated with organ
morphogenesis
– Found in organisms as primitive as cnidarians
40
• Evolutionary split between plant and
animal cell lineages occurred about 1.6
BYA
– Before the appearance of multicellular
organisms with defined body plans
– Multicellularity evolved independently in
plants and animals
– Genetic control of pattern formation in plants
is fundamentally different from that of animals
• Meristems add modules throughout lifetime
41
• Plants have MADS-box genes
– Homeotic gene family
• Have homeobox-containing genes
• Do not possess complexes of Hox genes similar to the
regional identity ones in animals
– Family of transcriptional regulators found in most
eukaryotic organisms
• Plants have many; animals only a few
– In flowering plants, control transition from vegetative
to flowering growth, root development, floral organ
identity
42
Morphogenesis
• Generation of ordered form and structure
• Product of changes in cell structure and cell
behavior
• Animals regulate
–
–
–
–
–
The number, timing, and orientation of cell divisions
Cell growth and expansion
Changes in cell shape
Cell migration (not used in plants)
Cell death
43
• Orientation of the mitotic spindle determines the
plane of cell division in eukaryotic cells
– Plane determined by spindle placement
– Great diversity of cleavage patterns in animal
embryos is determined by differences in spindle
placement
• In animals, cell differentiation is often
accompanied by profound changes in cell size
and shape
– Neurons, muscle cells
44
• Apoptosis
– Programmed cell death a part of development
• Human embryos begin with webbed fingers
– Necrosis – cells that die due to injury
– In C. elegans, due to 3 genes
– Mechanism of apoptosis appears to have
been highly conserved during the course of
animal evolution
• C. elegans genes similar to human genes
45
• Cell migration important during many
stages of animal development
– Involves adhesion and loss of adhesion
• Cell-to-cell interactions are often mediated through
cadherins
– Also involves cell-to-substrate interaction
• Cell-to-substrate interactions often involve integrinto-extracellular-matrix (ECM) interactions
– Extracellular matrix controls extent or route of
migration
46
• Cadherins
– Large gene family with several subfamilies
– All are transmembrane proteins that share a common
motif
– Cadherin domain
• 110-amino-acid domain in the extracellular portion of the
protein that mediates Ca2+-dependent binding between like
cadherins (homophilic binding)
– Cells with the same cadherins adhere specifically to
one another, while not adhering to other cells with
different cadherins
47
• Integrins
– Attached to actin filaments of the cytoskeleton and
protrude out from the cell surface in pairs, like two
hands
– “Hands” grasp a specific component of the matrix
– Actions
•
•
•
•
Provides anchor
Can initiate changes in cell
Alter growth of cytoskeleton
Activate gene expression
– Gastrulation depends on fibronectin–integrin
interactions
48