Transcript Development Biology (Ms. Lucky Juneja)

Development Biology
INTRODUCTION
Ms. Lucky Juneja
Lecturer, School of
Biotechnology, DAVV
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.
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.
Homologous 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.
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?
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.
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.
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.
Cleavage, 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.
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.
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
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.
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.
Patterns of embryonic cleavage
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:
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.
DROSOPHILA DEVELOPMENT
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 9:Adult fly/ Imago
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)
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:
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.
2:50 h
3:40 h
4:20 h
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
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.
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).
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.