Bio 127 - Section IV
Late Development
Development of the Tetrapod Limb
Gilbert 9e – Chapter 13
Pattern Formation
• Limbs show an amazing aspect of development
• Similarities:
– Architectural symmetry in all four limbs
– Size symmetry in the opposite limb
– Symmetry in growth rate and development
• Differences:
– Forelimb-Hindlimb: rostral-caudal asymmetry
– Mirror imagery in opposite limbs: left-right asymmetry
– Structural polarity in 3 axes per limb:
• Proximal-Distal, Anterior-Posterior, Dorsal-Ventral
• Pattern Formation occurs in four dimensions
Skeletal pattern formation
Femur  Tibia-Fibula  Metatarsals-Tarsals
add Dorsal-Ventral
The pattern extends to muscles,
tendons, cartilage, vessels, nerves
Figure 13.18 Deletion of limb bone elements by the deletion of paralogous Hox genes
Molecular Specification of the Pattern
• “Morphogenetic Rules” cross species
– Grafts of reptile or mammal can induce avian limb
– Frogs limb buds can pattern salamander limbs
– Regeneration of salamander limbs follows the pattern
• Axis formation is driven by specific molecules
– Proximal-Distal axis governed by FGF family
– Anterior-Posterior axis governed by Shh
– Dorsal-Ventral axis governed by Wnt7a (mostly)
Formation of the “Limb Field” in the Mesoderm
The black ring of cells will not
contribute to the limb unless
the other cells are lost.
Emergence of the limb bud
Bulge is due
to proliferation
and migration
of somatic
mesoderm
for bone and
abaxial
myotome
for muscle
Multilimbed Pacific tree frog (Hyla regilla), the result of infestation
of the tadpole-stage developing limb buds by trematode cysts
All cells in
“Limb Field”
are competent
for all limb cells
Parasite
infestation
splits the
“Limb Field”
Limb Formation is a Reciprocal Induction between Mesoderm and Ectoderm
Proximal-Distal
Patterning
As mesodermal mesenchyme
forms the limb bud it tells
overlying ectoderm to form
Apical Ectodermal Ridge
The AER then directs
changing limb development
in nearby mesoderm called
the Progress Zone (PZ)
Figure 13.11 The AER is necessary for wing development
Removal of the AER at
progressively later times
Anterior-Posterior Patterning
The “Zone of Polarizing Activity” is a piece of mesoderm
at the junction of the young limb bud and the body wall
When a ZPA is grafted to anterior limb bud mesoderm,
duplicated digits emerge as a mirror image of the normal digits
Anterior-Posterior Patterning
The same thing happens
when you build a ZPA by
making cells express Shh
Anterior-Posterior Patterning
Concentration
and timing of
Shh is critical
Anterior-Posterior Patterning
The Shh pattern is made more intricate by response in the webbing
Secondary BMP signal makes all digits different
Dorsal-Ventral Patterning
The knuckles vs palm
axis is determined by
the overlying ectoderm
Reverse the covering,
reverse the patterning
Figure 13.26 Patterns of cell death in leg primordia of (A) duck and (B) chick embryos
Figure 13.28 Inhibition of cell death by inhibiting BMPs
Figure 13.30 Tiktaalik, a fish with wrists and fingers, lived in shallow waters ~375 million years ago
Joints are complex:
- ligaments
- tendons
- fibrous capsule
- lubrication
- immune system
Movement and
muscular force
are required to
form the joints
Bio 127 - Section IV
Late Development
Sex Determination
Gilbert 9e – Chapter 14
Section 4 Encompasses :
•
•
•
•
Development of the Tetrapod Limb
Sex Determination
The Saga of the Germ Line
Post-Embryonic Development
Chromosomal Sex Determination
• Mammals: XX = female, XY = male
• Avians: ZZ = male, ZW = female
• Flies: 2X = female, 1X = male (Y = no role)
• Bees, Ants: 2(n) = female, 1(n) = male
Chromosomal Sex Determination
Mammals
Mammalian Sex Determination Overview
• Primary Sex Determination in Gonads
– First form bipotential “indifferent” primordia
– From intermediate mesoderm near mesonephros
– Default is female, Y is crucial for SRY gene
• Secondary Sex Determination Elsewhere
– Genitalia, mammaries, size, voice, musculature, hair
– Endocrine, paracrine secretions from gonads
– Absence of gonads produces female phenotype
Remember: Development of the vertebrate kidney
Nephric duct
is the primitive
organizer:
Wolffian Duct
Bipotential gonad
starts as genital
ridge epithelium
in intermediate
mesoderm
~ week 4 in humans
Differentiation of gonads in transverse section
Genital ridge forms as
epithelium mixed with
interstitial mesenchyme
Germline stem
cells arrive
from elsewhere
(Ch. 16)
Mullerian duct
develops parallel
to Wolffian duct
Back to the larger view....
Differentiation of the male gonad
Cell Fates:
- Epithelium becomes Sertoli cells
- Mesenchyme becomes Leydig cells
- Germ cells will become spermatogonia
- Sertoli cells line up to form solid cords
Ductal fates:
- Sertoli AMF degenerates Mullerian duct
- Leydig testosterone maintains Wolffian duct
- Testis cords will become seminiferous tubules
- Rete testis canals join testes cords to the
vas deferens and then epididymous.
Differentiation of the female gonad
Cell Fates:
- Epithelium becomes granulosa cells
- Mesenchyme becomes thecal cells
- Germ cells will become meiotic oogonia
- Together they form the follicles
Ductal fates:
- No AMF allows devo of Mullerian duct
- oviduct, uterus, cervix, upper vagina
- No testosterone degenerates Wolffian duct
Back to the larger view...
Secondary Sex Determination
• Gonadal determination is done by ~20 wks
– True hermaphroditism is very rare in humans
– Can result from translocation of Y onto X
• Secondary determination starts at puberty
–
–
–
–
Secondary characteristics usually match gonadal
The dominant influence is gonadal steroid hormones
0.4-1.7% of population have mixed traits
This called pseudohermaphroditism or
“Intersex Conditions”
Gynandromorph finch with ZZ (male) cells on its right
side and ZW (female) cells on its left side
True
Hermaphrodite
We’ve also seen
them in worms...
Androgen insensitivity syndrome
Gonadal male
with a mutation
in testosterone
receptors –
estrogen from
adrenal glands
dominates puberty
AMF still causes
Mullerian duct
degeneration
Congenital adrenal hyperplasia in females
• Androgens accumulate because of a
mutation in the adrenal enzyme to make
corticosteroids
• Allows male secondary characteristics in a
gonadal female
Testosterone must be converted to dihydrotestosterone to finish male system.
Failure to convert causes
external genitalia to fail to
form prior to puberty
The children are often
raised as girls until the
high testosterone levels
of puberty finish the job!
Sex Determination in the Brain
• Interestingly, the brain doesn’t wait for the
gonads like most tissues do
– More than 50 genes are expressed in a sexually
dimorphic pattern prior to gonad differentiation
– These include SRY
– Song centers in male birds can form w/o testosterone
– Gonadal hormones refine an already present sex
Sex Determination in the Brain
Testosterone must be converted to
estrogen to masculinize male brain
Temperature-dependent sex determination in reptiles: American
alligator, red-eared slider turtle, and alligator snapping turtle
Increased estrogen can override temperature
• Percentage of males crashes
• We put three key things into environment
– Atrazine weed killer
– Estrogen/Progesterone birth control
– Estrogenic plastics
Figure 14.24 Possible chain of causation leading to the feminization of male frogs and the decline
of frog populations in regions where atrazine has been used to control weed populations (Part 2)
Figure 14.24 Possible chain of causation leading to the feminization of male frogs and the decline
of frog populations in regions where atrazine has been used to control weed populations
Other Environmental Sex Determinants
• Physical position
– Bonellia
• Larva drops to sea floor - female
• Larva finds proboscis of adult female – male
• Population conditions
– Goby fish
• School is one male, many females
• If male is lost, top females race to change
• Most aggressive becomes dominant male
Figure 14.25 Sex determination in Bonellia viridis