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Insect Physiology
Developmental Systems
Department of Entomology
National Chung Hsing University
Preembryonic development 胚胎前發育
Insect eggs
– Egg membranes 卵膜
– Pattern formation within the oocyte 卵決定作用
Embryonic development
Blastoderm formation 囊胚形成
Formation of the germ band 胚帶形成
Blastokinesis and dorsal closure 轉胚動
Formation of the gut 腸化作用
Formation of the nervous system 神經系統形成
Formation of internal organs 內部構造形成
Formation of the reproductive system 生殖系統形成
Endocrinology of embryonic development 內分泌學
Preembryonic Development
All multicellular organisms begin their lives as
single-celled zygotes that divide to form groups
of cells that show progressive morphological
Two main processes occur in the embryonic
– Determination決定作用: the commitment process that
establishes the later differentiated state
– Differentiation分化作用: the development of
morphological differences and the generation of
cellular diversity
Insect Egg Structure
Egg Membranes
Chorion卵殼: synthesized within the
ovariole by the follicular epithelium
– Vitelline envelope卵黃膜: an inner noncellular
membrane with a thickness of about 0.3 mm
– Wax layer腊層: 5 nm to 2 mm; to provide
protection against desiccation
– Chorionic layers:
Inner chorionic layer
Endochorion: inner and outer endochorion
Cross-Section of an Drosophila Egg
Specialized Structures of
Insect Eggs
Micropyle受精孔: an opening of chorion
that allows a single sperm to enter. (some
with many micorpyles)
Respiratory appendages: structures that
serve as a plastron to extract oxygen from
Operculum卵蓋: a cap that is surrounded
by hatching regions and opens to allow
the larva to exit.
Cross-Section of an Insect Egg
Respiratory Horns of Drosophila Egg
Pattern Formation
within the Oocyte
There are distinct regions with an
– Periplasm: cytoplasm immediately
beneath the plasma membrane
– Spatial gradient of protein that result from
the transcription of specific genes to
create a pattern that provides the
developing embryo with positional
information within the egg.
Polarity Gradients of Morphogens
Polarity gradients of these materials
are established early during oogenesis,
while the oocyte resides in the female
even before fertilization and oviposition
occur (next slide)
– Maternal effect genes: establish an
anterior-posterior polarity in the egg, such
as bicoid, nanos, hunchback, and caudal
Cross-Section of an Insect Egg
Fig. The mechanism of Gurken signaling to establish anterior/posterior
and dorsal/ventral commitment of follicle cells.
Fig. The gradients of bicoid, nanos, hunchback, and caudal mRNA that
establish position in the Drosophila oocyte.
Insect Egg Prior to Fertilization
The oocyte, which has
been arrested in
metaphase of the first
meiotic division,
continues to mature after
oviposition occurs.
Shortly after the sperm
penetrates the egg and
oviposition take place, the
oocyte completes meiosis.
Insect Egg Prior to Fertilization
Syngamy: the union of
sperm pronucleus and
oocyte pronucleus;
occurs at the interior of
the egg.
reproduction without
sperm involvement; a
haploid polar nucleus
combines with the haploid
oocyte nucleus to restore
the diploid.
Egg Cleavage
Holoblastic cleavage:
animals that have relatively
little yolk are able to
undergo complete cleavage.
(e.g. mammals)
Meroblastic cleavage: the
relatively large amount of
yolk prevents the first
cleavage divisions from
cutting through the entire
egg, and their cleavage is
more superficial cleavage.
(e.g. insects)
Blastoderm Formation
Energid: each nucleus surrounded by a small island of
cytoplasm during the meroblastic cleavage of most
After a series of mitoses, the energids migrate to the
egg periphery and continue to divide there.
Syncytial blastoderm: lacks any membranes, with all the
cleavage nuclei contained within the common cytoplasm.
Pole cells: some of the energids migrate to the posterior
of the egg; give rise to the germ cells.
Vitellophages: some energids remain in the yolk;
involved in the digestion of yolk and the formation of the
midgut epithelium.
Fig. Migration of energids to the periplasm of an insect oocyte, forming the syncytial
Fig. Formation of the cellular blastoderm from the
syncytial blastoderm.
Formation of the Germ Band
Embryonic primordium: the thickened
portion of the blastoderm; develop into the
Extraembryonic ectoderm: the other cells
of the blastoderm beside the primordium
Germ line: the embryonic primordium
increases in length; represents the ventral
region of the future body
Fig. Development of the embryonic permordium and
extraembryonic ectoderm from the blastodermal
Gastrulation: the germ band develop to become
a double-layer embryo
The cells of the germ band migrate inward into
yolk to form a multicellular layer
Gastral groove: the longitudinal furrow that is
formed by the elongation and migration upward
of the cells along the ventral line midline of the
germ band
Mesoderm: the inner layer of cells of the gastal
The germ band constitutes the trunk of the
developing insect
Fig. Formation of mesoderm from the migration of
ectodermal cells.
Origin of the Mesoderm from Cells
Expressing Twist
Segmentation begins as the maternal effect
genes activate or repress the gap genes to
establish regions of segment identity.
The concentrations of Bicoid, Hunchback, and
Caudal proteins determine the transcription
patterns of the gap genes.
The produces of the gap genes regulate the
expression of the pair-rule genes.
The produces of the pair-rule genes further
divide the broad gap gene regions into
individual parasegments. (next slide)
The parasegments will ultimately produce the
anterior compartment of one segment and the
posterior compartment of the next.
The segment polarity genes assure that certain
repeated structures appear in each segment,
establishing the cell fates within each of the
Then, subsequent interactions of the gap and
pair-rule genes regulate the homeotic genes
that establish the identity of each segment and
its characteristic structure. (next slide)
Blastokinesis and Dorsal Closure
Amniotic folds: the folds which are formed while the
germ band elongates and widens, carrying the
margin of the extraembryonic ectoderm with it.
– Amnion: the inner walls of the amniotic folds.
– Amniotic cavity: the amnion enclosure
– Serosa: the ectoderm that is detached from the germ
Blastokinesis: the movements of the germ band
within the yolk; reverse the relative positions of the
yolk and the embryo
– Dorsal closure: after blastokinesis and growth of the
embryonic ectoderm over the dorsal portion and end up
with to enclose the yolk within the embryo.
Fig. The formation of the amnion and the invagination of the germ band to free
it from the serosa.
Fig. The formation of neuroblasts from ectodermal tissue and the
proliferation of mesoderm.
Fig. Dorsal closure of the embryo.
Formation of the Gut
Cells at the anterior and posterior ends of the
embryo migrate inward to form the foregut,
hindgut, and midgut.
– Foregut and hindgut: raise from ectoderm cells.
– Midgut: from the endoderm (some of the invaginating
The endodermal cells form sheets that enclose
the yolk in a tube that creates the midgut
epithelium. Vitellophages contained within the
yolk that are integrated into the endoderm to
form the definitive midgut epithelium.
Fig. Formation of the foregut and hindgut from ectodermal invaginations and
the development of endoderm that forms the midgut.
Formation of the Nervous System
The nervous system arises from ectodermal
cells in the ventral region of the germ band.
Proliferation of the neuroblasts in portions of the
embryonic ectoderm produces a neural groove
and neural ridges.
Three groups of neuroblasts that proliferate in
the anterior region ultimately produce the
procerebrum, deutocerebrum, and tritocerebrum
of the brain. Those in other segments give rise
to the subsesophageal and abdominal ganglia.
Fig. Formation of the ventral nerve cord from neuroblasts.
Formation of Internal Organs
Mesodermal tissue gives rise to most of the
internal organs of the insect.
– Splanchic mesoderm: forms the visceral muscles
– Somatic mesoderm: gives rise to the skeletal muscles
Formation of the Reproductive System
The reproductive system is constructed around
the pole cells.
The pole cells become the germ cells of the
future adult.
The genital ridge of mesoderm that forms the
reproductive organs, germaria of the ovarioles
and the follicles of the testes.
Mesoderm also gives rise to the lateral oviducts
and vasa deferentia.
Invaginations of the ectoderm form the median
oviduct and ejaculatory duct.
Fig. Cell lineages and derivation of tissues in the mature insect.
Endocrinology of
Embryonic Development
Hormones required during embryogenesis
are packaged into the egg largely as
inactive hormone conjugates and
released from the conjugates as active
hormones when they are required.
– Ecdysteroids: involved in embryonic molting.
– Juvenile hormone: less known about its
contributions during embryogenesis
– Diapause hormone in Bombyx mori eggs
Two Way to Create
a Morphogen Gradient
The Origins of the Drosophila Body Segments
During Embryonic Development
A Drosophila oocyte in Its Follicle
The Organization of the Four EggPolarity Gradient Systems
Morphogen Gradients Patterning the
Dorsoventral Axis of the Embryo
Examples of the Phenotypes of Mutations
Affecting the Three Types of Segmentation
The Regulatory Hierarchy of Egg-Polarity,
Gap Segmentation and Homeotic Selector