Test Five Outline
Sex Determination
Mammalian Sex Determination Overview
 Primary Sex Determination in gonads
o First form bipotential “indifferent” primordial
o From intermediate mesoderm, near the mesonephros
o Default sex if female; Y chromosome is crucial for SRY gene
 Secondary Sex Determination Elsewhere
o Genitalia, mammaries, size, voice, musculature, hair
o Endocrine and paracrine secretions from the gonads
o Absence of gonads (Females)
Remember the development of the kidney (Vertebrates)
 Nephric/Wolffian duct is the primitive organizer of the kidneys
Differentiation of Gonads
 The bipotential gonad starts as genital ridge epithelium in intermediate mesoderm (at about 4
weeks in humans)
 The genital ridge forms as epithelium mixed with interstitial mesenchyme
o The germline stem cells arrive from elsewhere
o The Mullerian duct will develop parallel to the Wolffian duct
Differentiation of the Male Gonad
 Cell Fates:
o Epithelium becomes Sertoli cells
 Sertoli cells will line up to form solid cords
o Mesenchyme becomes Leydig cells
o Germ cells become Spermatogonia
 Ductal Fates:
o Sertoli AMF degenerates Mullerian duct
o Leydig testosterone maintains Wolffian duct
o Testis cords will become seminiferous tubules
o Rete testis canals join testes cord to the vas deferens and the epididymous
Differentiation of the Female Gonad
 Cell Fates:
o Epithelium becomes granulosa cells
o Mesenchyme becomes thecal cells
o Germs cells will become meiotic oogonia
o Together they form the follicles
 Ductal Fates:
o No AMF allows development of Mullerian duct
 Oviduct, uterus, cervix, upper vagina
o With no testosterone, the Wolffian duct will degenerate
Secondary Sex Determination
 Gondadal differentiation is done by around 20 weeks.
 True hermaphroditism is very rare in humans and can result from a translocation of Y onto the X
chromosome
o *Example of the finch* *Also seen in worms*
Secondary determination starts at puberty
 Secondary characteristics usually match gonadal
 The dominant influence is gonadal steroid hormones
 0.4-1.7% of the population can have mixed traits – this is called pseudohermaphroditism or
“intersex conditions”
o Androgen insensitivity syndrome: Gonadal male with mutation in testosterone
receptors. Estrogen from adrenal glands will dominate puberty. And AMF will still cause
the Mullerian duct to degenerate.
o Congenital adrenal hyperplasia in females: Androgens accumulate because of a
mutation in the adrenal enzyme to make corticosteroids. This allows male secondary
characteristics in females.
Finishing the male system requires testosterone to be converted to dihydrotestosterone
 Failure of this conversion cause external genitalia to fail to form prior to puberty
Sex Determination in the Brain
 The brain does not wait for the gonads to form like most tissues do
o
More than 50 genes are expressed in a sexually dimorphic pattern prior to gonad
differentiation
 These include SRY genes
o Gonadal hormones will refine the sex that is already present
 Testosterone must be converted to estrogen in order to masculinize the male brain
Some Reptiles have Temperature Dependent Sex Determination
 American Alligator, Red-eared Slider Turtle, Alligator Snapping Turtle
o Increased estrogen can override the temperature, cause the percentage of males to
crash
 Atrazine weed killer
 Estrogen/Progesterone birthcontrol
 Estrogenic plastics
Other Environmental Sex Determinants
 Physical Position
o Bonellia
 If the larva drops to the sea floor, we get female
 If the larva finds the proboscis of adult female, we get male
 Population Conditions
o Goby Fish
 School is one male, and many females. If the male is lost, top females race to
change, and the most aggressive female will become the new dominant male.
Germ Line Development – The Saga of the Germ Line
Learning Objectives:
 Understand that sexual reproduction requiring the fusion of gametes from male and female
gonads occurs in specific organisms
 Understand that the primordial germ cells that give rise to gametes arise outside of the gonads
and must migrate to them.
 Understand that in most organisms the primordial germ cells are specified conditionally, while in
some they are specified autonomously by cytoplasmic determinants in the egg.
 Understand that migration of the germ cells from their site or origin to the gonads is an essential
part of reproductive success.
Two Processes of the Germ Line Saga
 Primordial germ cells are determines in a specific location in the embryo
 Primoridal germ cells migrate to the gonad and become the progenitor population for eggs and
sperm
Two Methods of Germ Cell Determination
1. Autonomous Specification (nematodes, frogs, flies)
a. Egg cytoplasmic determinants
2. Conditional Specification (Majority of sexually reproducing organisms, including mammals)
a. Signals come from surrounding cells
C. elegans
 *Remember cleavage and gastrulation – asymmetrical divisions produce a stem cell (P-lineage)
o P4 is has the germ line specification
 PIE-1 blocks all transcription, therefore, there is no differentiation
o
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Germ plasm also has blocks to translation, stem cell factors, controls for asymmetric
division and meiosis inducing agents
Synctitial cleavage is follow by cellularization.
o During cellularization we have the formation of the pole plasm
 Mitochondria
 Fibrils
 Polar Granules
*Also entails no transcription, no translation and stabilization of the germ line*
X. laevis
 Germ plasm is at the vegetal pole of frog embryos
 The frog’s cells that take up the granules will become PGS’s and migrate to the gonads as the
kidney forms.
*No transcription. No translation. No differentiation*
Conditional Specification – Mammals
 Posterior of epiblast at the junction of the primitive streak and extraembryonic ectoderm.
o The cells start off no different from other epiblast, but are specified in gastrulation
before the 3 germ layers form.
 WNTs from endoderm make the competent
 BMPs from extraembryonic ectoderm finish it.
 Differentiation is repressed by repressing gene expression
 Specified outside embryo forming cells
o Once expression is shut down they can go back into the embryo and won’t respond to
signals.
Germ Cell Migration – Drosophila
 As the endoderm invaginates, the ectoderm and mesoderm extend and converge to wrap
around the dorsal side to form the “germ band”
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Cell migration of germ cells  attatchment to endoderm and migration through midgut 
attachment to mesoderm  endoderm expresses repellent molecules  germ band retracts 
PGC’s and gonad progenitors in 2 migration streams  combination of Chemoattraction and
repulsion drive them to gonad E-cadherin MET forms epithelium around PGC’s.
*Germ cells passively ride the endoderm*
Both mesoderm and PGC’s divide through the larval stage and differentiate at metamorphosis
At the larval-pupal transition, anterior PGC’s in gonad become germ line stem cells
In the ovaries, the cells attach to the top stromal cap
In testes, the cells attach to the hub cells
Germ Cell Migration – Zebrafish
 Germ cell specification is the in the germ plasm, and are determined by 32 cells.
 Migration of bilateral clusters into developing gonad follows signal Sdf-1 using receptor CXCR4
o CXCR4 is a soluble growth factor
Germ cell Migration – Frogs
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During cleavage, the germ plasm rises up until it ends up in the endoderm at top and back near
lip
The endoderm below the mesoderm are PGCs
Migration anterior to gonads at endoderm-mesoderm boundary
o About 30 PGC’s reach gonads by fibronectin and Sdf-1
 Sdf-1 is a critical chemoattractant
Germ Cell Migration – Mice
PGCs are formed in the extraembryonic epiblast  Once formed, they migrate directly into the hindgut
endoderm and migrate anteriorly through day 9, dividing the entire time  They leave the gut by the
dorsal mesentery and enter the genital ridges by day 12 as 2500-5000 PGCs.
 The traveling stem cell niche
o Support cells travel with PGC’s to maintain the undifferentiated stem cell phenotype
o They secrete stem cell factor (SCF)
o The cells follow fibronectin trail
o Sdf-1 is also required.
Germ Cell Migration – Birds and Reptiles
Germ line cells determined in the area pellucida migrate to hypoblast  they then migrate to gonads via
the blood stream when extraembryonic vessels form  Sdf-1 from intermediate mesoderm draws them
out of vessels and through the mesodermal tissues to the gonad
Post Embryonic Development
Learning Objectives:
 Understand that development never stops during the life of the organism and that three major
processes occur in the post-embryonic animal; metamorphosis, regeneration and aging.
 Understand that Direct Development involves young organism with the same body plan as the
adult, whereas indirect development involves major body changes to form the adult body plan
 Indirect Development, or metamorphosis, is a hormonal reaction
 Understand that regeneration is the reactivation of developmental processes to restore missing
tissues
 Understand that aging and physiological senescence are an interplay of genetic and
environmental influence.
Metamorphosis
 Development of a larval stage and an adult stage specialized for different functions
o Larvae often specialized for growth, dispersal etc.
o Adults usually specialized for reproduction
 Two major types of larvae
o Primary: little to no similarity to adult (sea urchin)
o Secondary: add and subtract parts from similar form (insects, amphibians)
Metamorphosis (Amphibians)
 Four Major Morphological Processes
o Growth of new structures
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Tadpole eyes are on the sides of their head, frog eyes are on top in the front,
giving them binocular vision as adults. New neurons differentiate and form new
ipsilateral tracts
o Cell death in existing structures
 T3 induces apoptosis in tail and gills
 Apoptosis occurs in gut epithlium due to ECM loss
o Remodeling of existing structures
 Eyes
 Skull
 Skeleton
 Gut
 Sensory
o Biochemical respecification
 Shift in the genes express and the physiological functions they control
Hormones(s) T-3 and T-4 are very important
o Thyroid receptors are found in the nucleus where they are bound to inactive promoters
o When thyroid hormone enters such a cell, it will bind to receptors and the combination
is an active transcription factor for that specific gene.
o Cells that make high levels of deiodinase II are more responsive to thyroid stimulation
o Cells that make high levels of deiodinase III are less responsive
o What genes you have your thyroid receptors on is very important to your function
 Limb muscle cells grow in response to thyroid hormones
 Tail muscle apoptoses in response to thyroid hormones
o How much deidinase II you make is very important
 Limb buds make a lot and respond to early low levels of T4
 Tails make very little and wait for later very high levels of T4
o How a tissue is organized before T4 is very important
 Thyroid hormones make skin apoptose
 Head and body have basal stem cells, the tail does not
Heterochrony – alternatives to metamorphosis
o Neoteny: Normal gonadal maturity, retention of juvenile form
 Mexican axolotl (salamander)
o Progenesis: Accelerated gonadal maturity, retention of juvenile form
o Direct Development: No larval Form
 Puerto Rican frog – frogs hatch from eggs, not tadpoles
Metamorphosis (Insects)
Three types of Development
 Ametabolus development
o Structure of insect does not change much from larvae to adult
 Hemimetabolous development
o Develop new structures through molts; but not full blown metamorphosis
 Holometabolous development
o Full blown metamorphosis with completely different structures
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Larva are eating machines in order to provide energy for non-feeding pupal development
They have both doomed larval cells and rudiments of imaginal cells for adults
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The larval cells will apoptose in the pupa
Imaginal disc cells will form wings, legs, antennae, eye, head, thorax and genitalia
Histioblasts will form adult abdomen
Imaginal cell clusters in each organ will form adult organ as the larval organ degenerates
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Like amphibians, control is hormonal in insects
o Presence of juvenile-hormone makes a larval molt
o Shift to steroid 20-hydroxyecdysone gives pupal molt
o Differential timing in the development of pupal structures is due to 20E receptor
expression timing
Regeneration
 Restoration of missing tissues
 Post-embryonic reactivation of development
 Occurs in some form in all species
o Stem cell-mediated regeneration
o Epimorphosis: adult cell de- and re- differentiation
 Surviving cells lose their specification and form an undifferentiated tissue bulge
 Slat worms and salamanders (salamander limb growth)
o Morphallaxis: adult cell repatterning
 They hydra is able to constantly regenerate cells that are sloughed off.
 Any cells along the length can become head, body or foot
o Compensatory regeneration: adult cell division
 No dedifferentiation occurs in liver regeneration
 All five cell types produce more of themselves
 Progenitor cell back up plan: Oval cells
Aging
 Time related deterioration of physiological functions necessary for survival and fertilization
o Live span vs senescence
 Combination of:
o Mutations
o Environmental factors
o Random epigenetic changes