Download Homebox and Hox genes - Berkley School District

Homebox and Hox genes
A homeobox is a DNA sequence found within
genes that are involved in the regulation of patterns
of development (morphogenesis) in animals, fungi
and plants. Genes that have a homeobox are
called homeobox genes and form the homeobox
gene family.
The most studied and the most conserved group of
homeodomain protein are the Hox genes, which
control segmental patterning during development;
however, not all homeodomain proteins are Hox
proteins.
1) The vertebrate homeotic complex comprises four
distinct Hox gene clusters (Hox A, B, C, D) that are
organized into thirteen homology (or paralogue) groups.
2) The chromosomal organization of the genes in each Hox
cluster reflects its anterior-posterior expression in the
body plan (spatial colinearity). Unlike in Drosophila,
vertebrate Hox genes are also temporally colinear and are
expressed in an anterior to posterior direction. In general,
members of the same paralogue group are expressed at
the same time and have the same anterior boundary of
expression.
3) Homeotic genes are expressed within segmented and
unsegmented structures within the body plan. Hox gene
expression in some unsegmented structures arise from
segmented precursors. The specification of segmented
structures may be due to a specific combination of
homeotic genes (or Hox Code).
4) Modern genetic methods such as targeted
mutagenesis in mouse have begun to reveal the
function of homeotic genes in vertebrates. In
general, homeotic phenotypes observed in specific
hox gene mutations are restricted to the anterior
boundary of expression.
5) The genetic redundancy of the vertebrate
homeotic complex may have enabled the rapid
evolution of vertebrates.
Vertebrate Development
Embryological Development
Stages of Development
Fertilization
• Penetration
– glycoprotein-digesting enzymes in acrosome
of sperm head
• Activation
Fertilization
– events initiated by sperm penetration
• chromosomes in egg nucleus complete second
meiotic division
• triggers movement of egg cytoplasm
• sharp increase in metabolic activity
Frogs, reptiles and birds: >1 sperm
enters, but 1 succeeds
Mammals: 1 sperm prevents the 2nd
entry
Sperm penetration of sea urchin egg (20-30 sec)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 51.04(TE Art)
Sperm
Gray
crescent
Movement of pigment
opposite sperm entry
Frog eggs: Grey crescent formation
Stages of Development
• Nuclei fusion
– The third stage of fertilization is fusion of the
entering sperm nucleus with the haploid egg
nucleus to form the diploid nucleus.
Cell Cleavage Patterns
• Initial cell division, cleavage, is not
accompanied by an increase in the overall
size of the embryo.
– morula - mass of 32 cells
• Each cell is a blastomere.
– eventually a blastula is formed
Pattern of Cleavage Division:
Yolk location
• Pattern of cleavage division is influenced by
the presence and location of the yolk
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nucleus
Fig. 51.05(TE
Art)
Yolk
Lancelet
Nucleus
Frog
Yolk
Nucleus
Air
bubble
Yolk
Shell
Albumen
Chicken
Cell Cleavage Patterns
• Primitive chordates
– holoblastic cleavage - egg contains little or no
yolk, and cleavage occurs throughout the whole
egg
Cell Cleavage Patterns
Amphibians and advanced fish
– Eggs contain much more cytoplasmic yolk in one
hemisphere than the other.
• large cells containing a lot of yolk at one pole, and a
concentrated mass of small cells with very little yolk at
the other pole.
Cell Cleavage Patterns
• Reptiles and birds
– eggs composed almost entirely of yolk
– cleavage only occurs in polar cytoplasm
• meroblastic cleavage
Cell Cleavage Patterns
• Mammals
–
–
–
–
contain very little yolk
holoblastic cleavage
inner cell mass forms developing embryo
outer sphere, trophoblast, enters endometrium-fetal part
Inner
Blastocoel
of placenta
cell
mass
– other: decidua basalis
Trophoblast
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 51.09(TE Art)
Blastocoel
Inner cell
mass
Blastodisc
Yolk
Trophoblast
Mammalian blastula/blastocyst and Bird blastula
Cell Cleavage Patterns
• Blastula
– Each cell is in contact with a different set of
neighboring cells.
• Interactions are a major factor influencing
developmental fate.
Inner
cell
• ES cells
mass
Gastrulation
• Certain groups of cells
invaginate (dent inwards)
and involute (roll) from the
surface of the blastula
during gastrulation.
– By the end of gastrulation,
embryonic cells have
rearranged into three primary
germ layers:
• ectoderm
• mesoderm
• endoderm
•
Gastrulation
Gastrulation in primitive
chordates
– surface of blastula
invaginates into the
blastocoel
• eventually inward-moving
wall pushes up against the
opposite side of the
blastula
– produces embryo with
two cell layers:
» outer ectoderm
» inner endoderm
» mesoderm forms
later between the
ectoderm and
endoderm
Lancet
Gastrulation
• Gastrulation in most aquatic vertebrates
– Yolk-laden cells of the vegetal pole are fewer and
much larger than the yolk-free cells of the animal
pole.
Frog Gastrulation
Gastrulation
• Gastrulation in
reptiles, birds,
mammals
– no yolk separates
two sides of embryo
• lower cell layer
differentiates into
endoderm and
upper layer into
ectoderm without
cell movement
– primitive
streak
Blastodisc
Yolk
Blastocoel
Ectoderm
Endoderm
Ectoderm
Endoderm
Primitive
streak
Mesoderm
Developmental Processes
During Neurulation
•
Tissue
differentiation
begins with the
formation of the
notochord and the
hollow dorsal nerve
cord.
– neurulation
•
After the notochord
has been laid
down, ectodermal
cells above the
notochord
invaginate, forming
the neural groove
down the long axis
of the embryo.
– edges move
toward each other
and fuse creating
neural tube
Developmental Processes
During Neurulation
•
On either side of the
developing
notochord,
segmented blocks
of mesoderm tissue
called somites form.
– Ultimately,
somites give rise
to muscles,
vertebrae, and
connective
tissues.
• Mesoderm in the
head region
remains
connected as
somitomeres
and form
striated muscles
of the face, jaws,
and throat.
Developmental Processes
During Neurulation
•
Neural crest
–
Edges of neural
groove pinch off
and form the
neural crest.
•
•
Nearby
clusters of
ectodermal
cells thicken
into placodes.
Gill chamber
–
Some of the
neural crest cells
form
cartilaginous
bars between the
embryonic
pharyngeal slits.
•
forms efficient
pump
Developmental Processes
During Neurulation
• Elaboration of the
nervous system
– Some neural
crest cells
migrate
ventrally toward
the notochord
and form
sensory
neurons in the
dorsal root
ganglia.
• others
become
specialized
Schwann
cells
•
How Cells Communicate During
Development
Inductions
between
the three
primary
tissue
types are
referred to
as primary
inductions
.
Discard mesoderm
opposite dorsal lip
Dorsal lip
Primary
neural fold
Donor mesoderm
from dorsal lip of
another embryo
Primary notochord
and neural development
Secondary
notochord
and neural
development
Secondary neural development
How Cells Communicate During
Development
Inductions
between
tissues that
have
already
Ectoderm
been
Optic cup
Wall
of
forebrain
differentiat
Lens vesicle
ed are
called
Lens
Neural
secondary
inductions. cavity
Optic
Optic stalk
Lens
invagination
Lens
Sensory
layer Retina
nerve Pigment
layer
How Cells Communicate During
Development
• Nature of development decisions
– Some cells become determined early in
development.
– At some stage, every cell’s fate becomes
fixed (commitment).
• not irreversible, but rarely reverses under normal
conditions
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chordates Vertebrates
Fig. Zygote
51.17(TE Art)
Brain,spinal
cord,spinal
nerves
Neural
crest
Lining of
Blastula
Pharynx
respiratory
tract
Dorsal
Gastrula
Endoderm
Ectoderm
nerve
Lining of
Gill arches,
Epidermis,
skin,
cord
digestive
sensory
Major
hair,
epithelium,
tract
ganglia,
glands
inner ear, lens
Schwann
Liver
Pancreas
of eye
Mesoderm
Notochord cells,adrenal
Outer covering
medulla
Circulatory
Integuof internal
Heart
system
ments
organs
Vessels
Blood
Lining of
Skeleton
Gonads
Somites
thoracic and
Segmented
abdominal
muscles
Kidney
cavities
Dermis
•
Embryonic Development Ontogeny Vertebrate Evolution
recapitulates
phylogeny
– Embryological
development
(ontogeny)
involves the
same
progression of
changes that
have occurred
during evolution
(phylogeny).
Extraembryonic Membranes
•
Fluid-filled amniotic
membrane an
adaptation to
terrestrial life
–
amniotic
membrane an
extraembryonic
membrane
•
Extraembryon
ic
membranes,
later to
become fetal
membranes,
include the
amnion,
chorion, yolk
sac, and
allantois.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chorion
Fig. 51.20(TE Art)
Amnion
Umbilical
cord
Maternal
vein
Chorionic
frondosum
(fetal)
Decidua
Maternal Uterine basalis
artery
(maternal)
wall
Placenta
First Trimester
• First trimester
– fourth week - organ
development
• organogenesis
– most women not
yet aware of
pregnancy
» Fetal Alcohol
Syndrome
First Trimester
• Second month morphogenesis
– limbs assume adult
shape
– major organs become
evident
– embryo is about one
inch in length
First Trimester
• Third month completion of
development
– now referred to as
fetus
• nervous system and
sense organs
develop
• all major organs
established
Second and Third Trimesters
Second trimester growth
– bone formation occurs
– covered with fine hair
(lanugo)
– by the end of the sixth
month, baby is one foot in
length
Third trimester - pace of
growth accelerates
– weight of fetus more than
doubles
– most major nerve tracts
formed within brain
– by end, fetus is able to
survive on own
Uterus releases
prostaglandins
– begin uterine
contractions, but then
sensory feedback from
the uterus stimulates
the release of oxytocin
from the mother’s
pituitary gland
• rate of contraction
increases to one
contraction every two
or three minutes
– strong contractions,
aided by the
mother’s pushing,
expels the fetus
Increasing hormone concentration
•
Birth and Postnatal
Development
Intestine
hCG
Estradiol
Placenta
Umbilical
cord
Progesterone
Wall of
uterus
Cervix
Vagina
0123456789
Months of pregnancy
•
Nursing
Birth and Postnatal
Development
– Milk production, lactation,
occurs in the alveoli of
mammary glands when
they are stimulated by
prolactin.
– milk secreted in alveolar
ducts which are
surrounded by smooth
muscle and lead to the
nipple
• first milk produced after
birth called colostrum rich in maternal
antibodies
Rib
Adipose
tissue
Intercostal
muscles
Pectoralis
minor
Mammary
(alveolar)
duct
Pectoralis
major
Lactiferous
duct
– Milk synthesis begins
about three days
following birth.
Lobule
Lobe
Containing
mammary
alveoli
Birth and Postnatal
Development
Chimpanzee
• Postnatal
development
– Babies typically
double their birth
weight within a few
months.
– Neuron production
occurs for six
months.
– allometric growth
Fetus
Infant
Child
Adult
Human