Development Biology

1
Development Biology
INTRODUCTION
Ms. Lucky Juneja Lecturer, School of
Biotechnology, DAVV
2
Aristotle The first embryologist Development
slow process of progressive change Development
of a multicellular organism begins with a single
cell the fertilized egg, or zygote, which divides
mitotically to produce all the cells of the
body Objectives of development . generates
cellular diversity . order within each
generation . it ensures the continuity of life
from one generation to the next.
3
FEW TERMS Differentiation A single cell, the
fertilized egg, gives rise to hundreds of
different cell type. This generation of
cellular diversity is called differentiation.
Morphogenesis differentiated cells are not
randomly distributed. Rather, they are organized
into intricate tissues and organs. This creation
of ordered form is called morphogenesis. Homologo
us structures are those organs whose underlying
similarity arises from their being derived from a
common ancestral structure. For example, the wing
of a bird and the forelimb of a human are
homologous. Analogous structures are those whose
similarity comes from their performing a similar
function, rather than their arising from a common
ancestor. Therefore, for example, the wing of a
butterfly and the wing of a bird are
analogous. Abnormalities due to exogenous agents
(certain chemicals or viruses, radiation, or
hyperthermia) are called disruptions. The agents
responsible for these disruptions are called
teratogens (Greek, "monster-formers"), and the
study of how environmental agents disrupt normal
development is called teratology.
4
QUESTIONS ABOUT DEVELOPMENT The question of
differentiation Since each cell of the body
(exceptions) contains the same set of genes, we
need to understand how this same set of genetic
instructions can produce different types of
cells? The question of morphogenesis How can
the cells form such ordered structures? The
question of growth How do our cells know when to
stop dividing? How is cell division so tightly
regulated? The question of reproduction How
are sperm and egg cells set apart to form the
next generation, and what are the instructions in
the nucleus and cytoplasm that allow them to
function this way? The question of evolution
How do changes in development create new body
forms? The question of environmental
integration How is the development of an
organism integrated into the larger context of
its habitat?
5
Circle of Life The Stages of Animal Development
1. Immediately following fertilization,
cleavage occurs. Cleavage is a series of
extremely rapid mitotic divisions wherein the
enormous volume of zygote cytoplasm is divided
into numerous smaller cells. These cells are
called blastomeres, and by the end of cleavage,
they generally form a sphere known as a blastula.
2. After the rate of mitotic division has
slowed down, the blastomeres undergo dramatic
movements wherein they change their positions
relative to one another. This series of extensive
cell rearrangements is called gastrulation, and
the embryo is said to be in the gastrula stage.
As a result of gastrulation, the embryo contains
three germ layers the ectoderm, the endoderm,
and the mesoderm. 3. The cells interact with
one another and rearrange themselves to produce
tissues and organs. This process is called
organogenesis.
6
4.Germ cell in many species a specialized
portion of egg cytoplasm gives rise to cells
that are the precursors of the gametes (the sperm
and egg). The gametes and their precursor cells
are collectively called germ cells, and they are
set aside for reproductive function. All the
other cells of the body are called somatic cells.
5. In many species, the organism that hatches
from the egg or is born into the world is not
sexually mature. Indeed, in most animals, the
young organism is a larva that may look
significantly different from the adult. Larvae
often constitute the stage of life that is used
for feeding or dispersal. In many species, the
larval stage is the one that lasts the longest,
and the adult is a brief stage solely for
reproduction.
7

8
Fertilization is the initiating step in
development. The zygote, with its new genetic
potential and its new arrangement of cytoplasm,
now begins the production of a multicellular
organism. Between these events of fertilization
and the events of organ formation are two
critical stages cleavage and gastrulation. Clea
vage, a series of mitotic divisions whereby the
enormous volume of egg cytoplasm is divided into
numerous smaller, nucleated cells. These
cleavage-stage cells are called blastomeres. In
most species (mammals being the chief exception),
the rate of cell division and the placement of
the blastomeres with respect to one another is
completely under the control of the proteins and
mRNAs stored in the oocyte by the mother. First
the egg is divided in half, then quarters, then
eighths, and so forth. This division of egg
cytoplasm without increasing its volume is
accomplished by abolishing the growth period
between cell divisions (that is, the G1 and G2
phases of the cell cycle). Meanwhile, the
cleavage of nuclei occurs at a rapid rate.
9
One consequence of this rapid cell division is
that the ratio of cytoplasmic to nuclear volume
gets increasingly smaller as cleavage progresses.
This decrease in the cytoplasmic to nuclear
volume ratio is crucial in timing the activation
of certain genes. For example, in the frog
Xenopus laevis, transcription of new messages is
not activated until after 12 divisions. The
transition from fertilization to cleavage is
caused by the activation of mitosis promoting
factor (MPF). MPF continues to play a role after
fertilization, regulating the biphasic cell cycle
of early blastomeres. Blastomeres generally
progress through a cell cycle consisting of just
two steps M (mitosis) and S (DNA synthesis) The
MPF activity of early blastomeres is highest
during M and undetectable during S.
10
What causes this cyclic activity of MPF?
Mitosis-promoting factor contains two subunits.
The large subunit is called cyclin B (component
that shows a periodic behavior, accumulating
during S and then being degraded after the cells
have reached M) (Evans et al. 1983 Swenson et
al. 1986). Cyclin B is often encoded by mRNAs
stored in the oocyte cytoplasm, and if the
translation of this message is specifically
inhibited, the cell will not enter mitosis
(Minshull et al. 1989). Cyclin B regulates the
small subunit of MPF, the cyclin-dependent
kinase. This kinase activates mitosis by
phosphorylating several target proteins,
including histones, the nuclear envelope lamin
proteins, and the regulatory subunit of
cytoplasmic myosin. This brings about chromatin
condensation, nuclear envelope depolymerization,
and the organization of the mitotic spindle.
Without cyclin, the cyclin-dependent kinase will
not function. The presence of cyclin is
controlled by several proteins that ensure its
periodic synthesis and degradation. In most
species studied, the regulators of cyclin (and
thus, of MPF) are stored in the egg cytoplasm.
Therefore, the cell cycle is independent of the
nuclear genome for numerous cell divisions
11
The embryo now enters the mid-blastula
transition, in which several new phenomena are
added to the biphasic cell divisions of the
embryo. First, the growth stages (G1 and G2) are
added to the cell cycle, permitting the cells to
grow Second, the synchronicity of cell division
is lost, as different cells synthesize different
regulators of MPF. Third, new mRNAs are
transcribed. Cleavage is actually the result of
two coordinated processes. The first of these
cyclic processes is karyokinesis the mitotic
division of the nucleus. The second process is
cytokinesis the division of the cell.
12
Process Mechanical agent Major
protein composition Location
Karyokinesis Mitotic spindle
Tubulin microtubules Central
cytoplasm Cytokinesis Contractile ring
Actin microfilaments Cortical
cytoplasm
The mitotic spindle and contractile ring are
perpendicular to each other, and the spindle is
internal to the contractile ring. The contractile
ring creates a cleavage furrow, which eventually
bisects the plane of mitosis, thereby creating
two genetically equivalent blastomeres.
13
Patterns of embryonic cleavage
14
(No Transcript)
15
Gastrulation Gastrulation is the process of
highly coordinated cell and tissue movements
whereby the cells of the blastula are
dramatically rearranged. The cells that will
form the endodermal and mesodermal organs are
brought inside the embryo, while the cells that
will form the skin and nervous system are spread
over its outside surface. Thus, the three germ
layers outer ectoderm, inner endoderm, and
interstitial mesoderm are first produced during
gastrulation. In addition, the stage is set for
the interactions of these newly positioned
tissues. TYPES OF CELL MOMENTS DURING
GASTRULATION
16
(No Transcript)
17
Cell Specification and Axis Formation Embryos
must develop three very important axes that are
the foundations of the body the
anterior-posterior axis, the dorsal-ventral
axis, and the right-left axis. The
anterior-posterior (or anteroposterior) axis is
the line extending from head to tail (or mouth to
anus in those organisms that lack heads and
tails). The dorsal-ventral (dorsoventral)
axis is the line extending from back (dorsum) to
belly (ventrum). For instance, in vertebrates,
the neural tube is a dorsal structure. In
insects, the neural cord is a ventral structure.
The right-left axis is a line between the two
lateral sides of the body.
18
(No Transcript)
19
DROSOPHILA DEVELOPMENT
20
Embryonic development Fertilization. Day 1
development ,embryo hatches out of the egg shell
to become a larva Day 2,03 and so larva- three
stages/instars, separated by molting end of third
stage, Pupa forms. Inside a pupa a radical
remodeling of the body - a process called
metamorphosis After Day 9Adult fly/ Imago
21
(No Transcript)
22
Egg- series of nuclear division (every 8 minute)
without cell division creates a Syncytium 1st 9
divisions cloud of nuclei is formed- from
middle to surface of egg moment- form monolayer
called Syncytial blastoderm



4 nuclear divisions Few nuclei to
extreme posterior end few cycles (9)- pole
cells- give rise to germ cells. When the
nuclei reach the periphery of the egg during the
tenth cleavage cycle, each nucleus becomes
surrounded by microtubules and microfilaments.
The nuclei and their associated cytoplasmic
islands are called energids. (up to 12th cycle)
23
Plasma membrane grow inward converting syncytial
blastoderm into cellular blastoderm (13th
cycle) Cell division slows down,
asynchronous, and transcription rate is
increased. Blastoderm fate map
24
(No Transcript)
25
Transcription from the nuclei (which begins
around the eleventh cycle) is greatly enhanced at
this stage. This slowdown of nuclear division and
the concomitant increase in RNA transcription is
often referred to as the mid blastula
transition. Gastrulation The first movements
of Drosophila gastrulation segregate the
presumptive mesoderm, endoderm, and ectoderm. The
prospective mesoderm about 1000 cells
constituting the ventral midline of the embryo
folds inward to produce the ventral furrow. This
furrow eventually pinches off from the surface to
become a ventral tube within the embryo. It then
flattens to form a layer of mesodermal tissue
beneath the ventral ectoderm.
250 h



340 h


420 h
26
The prospective endoderm invaginates as two
pockets at the anterior and posterior ends of the
ventral furrow. The pole cells are internalized
along with the endoderm. At this time, the embryo
bends to form the cephalic furrow. The
ectodermal cells on the surface and the mesoderm
undergo convergence and extension, migrating
toward the ventral midline to form the germ
band, a collection of cells along the ventral
midline that includes all the cells that will
form the trunk of the embryo. The germ band
extends posteriorly and, perhaps because of the
egg case, wraps around the top (dorsal) surface
of the embryo
27
(No Transcript)
28
While the germ band is in its extended position,
several key morphogenetic processes occur
organogenesis, segmentation, and the segregation
of the imaginal discs. Imaginal discs are those
cells set aside to produce the adult
structures. In addition, the nervous system
forms from two regions of ventral ectoderm.
Neuroblasts differentiate from this neurogenic
ectoderm within each segment (and also from the
Non segmented region of the head ectoderm).
Therefore, in insects like Drosophila, the
nervous system is located ventrally.
29
Body plan of Drosophila Head Three thoracic
segments (T1 T2 T3) Eight abdominal segments (A1
to A8) The first thoracic segment, has only
legs the second thoracic segment has legs and
wings and the third thoracic segment has legs
and halteres (balancers).
30
The anterior-posterior and dorsal-ventral axes of
Drosophila form at right angles to one another,
and they are both determined by the position of
the oocyte within the follicle cells of the ovary.