SEX AND BABIES
46.1: BOTH ASEXUAL AND SEXUAL REPRODUCTION OCCUR IN THE ANIMAL
KINGDOM


Sexual reproduction: the fusion of haploid gametes forms a diploid cell (zygote)
o Egg: female gamete, large, nonmotile cell
o Sperm: smaller, motile cell
Asexual reproduction: generation of new individuals without the fusion of egg and sperm
o Usually relies on mitotic cell division
ASEXUAL REPRODUCTION


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

Fission: separation of a parent organism into two individuals of equal size
Budding: new individuals arise from outgrowths of existing ones.
Fragmentation: breaking of the body into several pieces, followed by…
Regeneration: regrowth of lost body parts
Parthenogenesis: the egg develops without being fertilized
SEXUAL REPRODUCTION

Advantages of sex:
o Enhance reproductive success of parents where environmental factors change rapidly
o New phenotypes
REPRODUCTIVE CYCLES AND PATTERNS


Ovulation: release of mature eggs
Hermaphroditism: each individual has both male and female reproductive systems
46.2: FERTILIZATION DEPENDS ON MECHANISMS THAT BRING TOGETHER
SPERM AND EGGS OF THE SAME SPECIES

Fertilization: union of sperm and egg
o External: female releases eggs into the environment where the male fertilizes them
 Allows mate selection, triggering release of both sperm and eggs, increase
probability of fertilization
o Internal: sperm deposited in or near the female reproductive tract
 Enables sperm to reach egg effectively
 Copulation, sophisticated and compatible reproductive systems
o Spawning: individuals in the same area release their games into the water at the same time
ENSURING THE SURVIVAL OF OFFSPRING

Species generally make more than necessary for survival
o External fert: produce large # of gametes small fraction of survivors
o Internal fert: produce fewer, protect and care for young.
 Marsupials: mammary glands
 Eutherian mammals: fetuses remain in the uterus, nourished by mother’s blood
supply through the placenta
 Offspring are fed by adult birds and mammals
 Parental care  important to survival
GAMETE PRODUCTION AND DELIVERY



Gonads: organs that produce gametes in most animals
Spermatheca: sac in which sperm may be stored for extended periods
Cloaca: reproductive systems have a common opening to the outside
46.3 REPRODUCTIVE ORGANS PRODUCE AND TRANSPORT GAMETES
FEMALE REPRODUCTIVE ANATOMY
OVARIES
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
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Follicles: made up of oocytes, partially developed eggs
Oogenesis: formation and development of an ovum
Corpus luteum: residual follicular tissue that grows within the ovary, secretes additional estrogen, as
well as progesterone
OVIDUCTS AND UTERUS
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Oviduct: fallopian tube, extends from uterus to each ovary
Uterus: womb, thick muscular organ that can expand during pregnancy to accommodate for big fetus
Endometrium: lining of the uterus
Cervix: opening into the vagina
VAGINA AND VULVA
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Vagina: muscular but elastic chamber that is the site for insertion of the penis and deposition of sperm
Vulva: external female genitals
Labia majora: fatty ridges that protects the vulva
Labia minora: slender skin folds that border the urethra and the vaginal opening
Hymen: covers the vagina opening at birth until sex/physical activity breaks it
Clitoris: short shaft supported by the glans, covered by a small hood of skin, the prepuce
o During arousal, these, the vagina and the labia minora all engorge with blood
MAMMARY GLANDS

Mammary glands: present in both sexes, but normally produce milk in only females
o Important to reproduction
MALE REPRODUCTIVE ANATOMY
TESTES
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

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Testes: consist of many highly coiled tubes surrounded by several layers of connective tissue
Seminiferous tubules: where sperm form, make up the testes
Leydig cells: scattered between the seminiferous tubules, produce testosterone
Scrotum: fold of the body wall that maintains the testis temperature
DUCTS
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

Epididymis: where the sperm goes after the seminiferous tubes, 6-ft long
Ejaculation: sperm are propelled from each epididymis through the muscular duct, vans deferens,
which extends around and behind the bladder, where it joins a duct from the seminal vesicle, forming
a short ejaculatory duct.
Urethra: outlet tube for both excretory system and reproductive system
ACCESSORY GLANDS
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Semen: fluid that’s ejaculated
Seminal vesicles: contribute 60% of the semen
Prostate gland: secretes products directly into the urethra through several small ducts, source of
some of the most common medical problems of men over 40
Bulbourethral glands: small glands along the urethra below the prostate, makes precum
PENIS

Penis: contains the urethra, three cylinders of spongy, erectile tissue
HUMAN SEXUAL RESPONSE


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Vasocongestion: filling of tissue with blood
Myotonia: increased muscle tension
Coitus: sexual intercourse
The Stages of Sex
o Excitement: vasocongestion, myotonia
o More excitement: plateau phase: continue as a result of direct stimulation of the genitals
o Orgasm: rhythmic, involuntary contractions of reproductive structures in both sexes
o Resolution: reverses responses of earlier stages
46.4: TIMING AND PATTERN OF MEIOSIS DIFFERS FOR MALES/FEMALES
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Gametogenesis: production of gametes
o Sperm are motile and small
o Eggs are large and stay stationary, have to provide initial food stores for the embryo
Spermatogenesis: formation and development of sperm, continuous and prolific in adult males
o Cell division and maturation occurs throughout the seminiferous tubules coiled within testes
Oogenesis: development of mature oocytes, prolonged process in female
o Immature eggs form in the ovary, but do not complete their development until later
Differences
o Spermatogenesis: all four products form mature gametes
 Oogenesis: cytokinesis during meiosis is unequal, almost all cytoplasm is segregated
to a single daughter (secondary oocyte)  polar bodies degenerate
o Spermatogenesis: occurs throughout adolescence and adulthood
 Oogenesis: mitotic divisions are thought to be complete before birth, production of
mature gametes ceases at age 50
46.5: INTERPLAY OF TROPIC AND SEX HORMONES REGULATES MAMMALIAN
REPRODUCTION


Principle sex hormones are steroid hormones
o Males: androgens (testosterone)
o Females: estrogens and progesterones (estradiol)
Sex hormones serve many functions
o Responsible for male vocalizations
o Development of primary sex characteristics of males, structures directly involved in
reproduction (seminal vesicles and other ducts)
o Males: voices deepen, facial and pubic hair to grow, mucsles to grow
 Promote sex drive and general aggressiveness
o Females: influences female sexual behavior, induces fat (breasts and hips), increases water
retention and alters calcium metabolism
HORMONAL CONTROL OF THE MALE REPRODUCTIVE SYSTEM

GnRH from Hypothalamus makes FSH (Sertoli Cells) and LH (Leydig cells)
o FSH  Spermatogenesis (inhibin)
o LH  testosterone  Spermatogenesis
REPRODUCTIVE CYCLE OF FEMALES
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Menstruation: cyclic shedding of endometrium from uterus
Menstrual cycle (uterine cycle): changes in the uterus
Ovarian cycle: changes in the ovaries
OVARIAN CYCLE
1.
2.
3.
4.
5.
6.
7.
8.
Release of GnRH from hypothalamus
Stimulates anterior pituitary to secrete FSH and LH
FSH stimulates follicle growth
Cells of growing follicles start to make estradiol  follicular phase, follicles grow and oocytes mature
Estradiol levels rise b/c excreted by growing follicle
FSH and LH levels increase, LH triggers ovulation
LH surge causes rupture of the follicle wall and release of the secondary oocyte
Luteal phase: follows ovulation: LH stimulate follicular tissue left behind to transform into corpus
luteum secretes progesterone and estradiol promote thickening of the endometrium
a. Corpus luteum degenerates, sharp decrease in estradiol and progesterone concentrations
UTERINE (MENSTRUAL) C YCLE
9.
Proliferative phase: coordinated with follicular phase, estradiol and progesterone secreted by corpus
luteum stimulate continued development and maintenance of uterine lining
a. Secretory phase: coordinated w/ luteal phase
10. Menstrual flow phase: disintegration of the corpus luteum, estradiol and progesterone levels drop
and blood
 Endometriosis: disorder: some of the cells of the uterine lining migrate to an abnormal location
(ectopic- abnormal location) ectopic tissue swells and breaks down each ovarian cycle, resulting
random bleeding and other shitty shit
MENOPAUSE


Menopause: cessation of ovulation and menstruation- ovaries lose responsiveness to FSH and LS,
resulting in a decline in estradiol production
Why do we have it? Nobody knows for sure. Awesome, bio book. Thanks for wasting 30 seconds of my
life.
MENSTRUAL VS ESTROUS C YCLES

Estrous cycles: in the absence of pregnancy, the uterus reabsorbs the endometrium (FUCK YOU WHY
DON’T WE HAVE THAT?!)
46.6: IN PLACENTAL MAMMALS, AN EMBRYO DEVELOPS FULLY WITHIN A
MOTHER’S UTERUS
CONCEPTION, EMBRYONIC DEVELOPMENT AND BIRTH

Conception: Your bed is the scene of the crime: when sperm fuses with an egg in the oviduct
o Cleavage: 24 hours, zygote divides
o Blastocyst: sphere of cells surrounding central cavity 1 weekish
o + a few days: embryo implants into the endometrium, starts developing into a fetus
o Human chorionic gonadotropin (hCG): acts like LH and maintains secretion of progesterone
and estrogens by corpus luteum through the first months of pregnancy
o Pregnancy/gestation: condition of carrying one+ embryos in the uterus, averages 266 days for
humans

Sometimes pregnancies spontaneously terminate
o Chromosomal/developmental abnormalities
o Ectopic pregnancy (egg lodges in the oviduct, may rupture the oviduct and result in internal
bleeding, etc)
FIRST TRIMESTER
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
Weeks 2-4: Embryo obtains nutrients from the endometrium
o Trophoblast: outer layer of the blastocyst, grows outward and mingles with endometrium in
this period
o Placenta: disk-shaped organ, containing embryonic and maternal blood cells
 Supplies nutrients, provides immune protection, exchanges respiratory gases,
disposes of wastes for the embryo, blood travel
o Monozygotic (one egg) twins – identical
o Dizygotic (two eggs) twins – fraternal
o Organogenesis: development of body organs
 8 weeks: all major structures are present, embryo is now a fetus
 Heart beats by 4th week, heartbeat detected at 8-10 weeks
Mother is…
o Makin’ more progesterone  cervix plus with mucus, maternal placenta grows, uterus gets
larger and ovulation/menstruation stops
o Momma is getting sick, getting bigger and having larger boobs
SECOND TRIMESTER
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
Pregnancy is obvious, baby starts moving
hCG levels declining, corpus luteum and placenta takes over production of progesterone
THIRD TRIMESTER
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
Fetus grows, mother feels the pain
Labor: process by which childbirth occurs; series of strong, rhythmic uterine contractions during the
three stages of labor bring about birth (parturition)
o Dilation, opening up and thinning of the cervix
o Expulsion/delivery of the baby (ow ow ow ow)
o Delivery of the placenta

Hormones
o Regulators (prostaglandins) and hormones (estradiol and oxytocin)

Lactation: hypothalamus signals anterior pituitary to secrete prolactin, which stimulates mammary
glands to produce milk
o Suckling also triggers release of oxytocin, which triggers release of milk
CONTRACEPTION AND ABORTION

Contraception: deliberate prevention of pregnancy
o Rhythm method: temporary abstinence
o Natural family planning: refraining from intercourse with conception is most likely
o Coitus interruptus: withdrawal method
o Barrier methods: condoms, diaphragms, female condom, cervical cap
o Sterilization: tubal ligation (females) cauterizing/tying oviducts
 Vasectomy: cutting/tying off a small section of each vas deferens
o IUD: implanted, interferes with fertilization and implantation
o Hormonal contraceptives: birth control pills
 Synthetic estrogen and progestin
 Mimics negative feedback of the ovarian cycle, stopping the release of GnRH and FSH
and LH  blocks ovulation
 Progestin: causes thickening of woman’s cervical mucus so that it blocks sperm from
entering the uterus, decreases frequency of ovulation and changes endometrium
o Abortion: termination of pregnancy in process mifepristone: drug that allows a woman to
terminate pregnancy nonsurgically
MODERN REPRODUCTIVE TECHNOLOGIES


Ultrasound, amniocentesis and chorionic villus sampling
Fetal blood cells  genetic disorders
TREATING INFERTILITY
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Assisted reproductive technologies: procedures that involve surgically removing eggs, stimulating
them with hormones and returning them to her body
In vitro fertilization: oocytes are mixed with sperm in culture dishes and fertilized eggs are incubated
until they have formed at least 8 cells  uterus
Intracytoplasmic sperm injection: head of spermatid/sperm is drawn into a needle and injected
directly into an oocyte.
CHAPTER 47: PREVIEW
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Cytoplasmic determinants: molecules that are placed into the egg by the mother, as the zygote
divides, differences arise between early embryonic cells due to uneven distribution of cytoplasmic
determinants
Cell differentiation: process of cell specialization in structure and function
Morphogenesis: process by which an organism takes shape and differentiated cells occupy their
appropriate locations
Model organism: species that lends itself to the study of a particular question, that is representative of
a larger group and is easy to grow in a lab
47.1: AFTER FERTILIZATION , EMBRYONIC DEVELOPMENT PROCEEDS THROUGH
CLEAVAGE, GASTRULATION AND ORGANOGENESIS
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Cleavage: cell division creates a blastula
Gastrulation: rearranges the blastula into a three-layered embryo, the gastrula
Organogenesis: create rudimentary organs
FERTILIZATION

Model: Sea urchins
ACROSOMAL REACTION


Eggs are fertilized externally; the egg is coated with jelly that exudes soluble molecules that attract
the sperm
Acrosomal reaction: begins when a specialized vesicle at the tip of the sperm, the acrosome,
discharges hydrolytic enzymes
o The enzymes digest the jelly coat, enabling a sperm structure to elongate, penetrating the
coat
o Molecules of the protein on the tip of the acrosomal process adhere to specific sperm
receptor proteins that extend from the egg plasma membrane through the surrounding
meshwork of extracellular matrix (vitelline layer)
  lock and key: recognition of molecules ensures that eggs will be fertilized only by
sperm of the same species
o Contact of tip of the acrosomal process with the egg membrane leads to fusion of sperm and
egg plasma membranes
 Sperm then enters the egg cytoplasm
 Change in membrane potential  fast block to polyspermy, only one sperm can
fertilize the egg
CORTICAL REACTION
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

b/c membrane depolarization only lasts for a short time
cortex: rim of cytoplasm just beneath the plasma membrane
cortical granules: vesicles that fuse with egg plasma membrane, initiating cortical reaction
o contain a lot of molecules that are secreted into the perivitelline space (between the plasma
membrane and the vitelline layer)
o
o
o
the enzymes and other macromolecules push the vitelline layer away from the egg and
hardens the layer, creating a protective fertilization envelope that resists the entry of
additional sperm
slow block to polyspermy: takes longer, but is a longer-term ordeal
due to high concentration of Ca2+ ions
ACTIVATION OF THE EGG

egg activation: increase in rates of cellular respiration and protein synthesis by the egg, caused by
sharp rise in Ca2+ concentration
FERTILIZATION IN MAMMALS


we have internal fertilization
the egg is cloaked by follicle cells released along w/ the egg during ovulation, the sperm must travel
through this layer of follicle cells before it reaches the zona pellucida, the extracellular matrix of the
egg
o zona pellucida functions as a sperm receptor
CLEAVAGE
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cleavage: rapid cell division, cells carry out the synthesis and mitosis phases of the cell cycle, but skip
the growth phases and little protein synthesis
blastomeres: smaller cells that make up the blastula
blastocoel: fluid-filled cavity of the blastula
blastula: hollow ball of cells

some animals have definite polarity, established as the egg developed within the mother during
oogenesis
o yolk: stored nutrients, influences the pattern of cleavage
 vegetal pole: more concentration of yolk
 animal pole: yolk concentration decreases towards this end, the polar bodies of
oogenesis bud from the cell

animal hemisphere is dark gray because it has dark-colored melanin granules
o determines anterior/posterior (head-tail) of the embryo
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
lack of melanin granules in the vegetal hemisphere allows the yellow color of the yolk to be visible
cortical rotation: movement where the plasma membrane and associated cortex rotate with repsect
to eh inner cytoplasm
o animal hem cortex moves toward the vegetal inner cytoplasm on the side where the sperm
entered
o allows molecules in the unpigmented vegetal cortex on the side opposite sperm entry to
interact with molecules in the inner cytoplasm of the animal hemisphere, which activates
previous inactive proteins of the vegetal cortex
 which leads to formation of cytoplasmic determinants that will affect gene
expressions in the cell
 gray crescent: light gray region of cytoplasm that was previously covered by the
pigmented animal cortex near the equator of the egg
 serves a marker for future dorsal side of the embryo


holoblastic cleavage: complete cleavage, cleavage furrow passes all the way through the cells
meroblastic cleavage: incomplete division of yolk-rich egg cells (birds, reptiles, fishes and insects)
GASTRULATION
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gastrulation: morphogenetic process, taking up new locations that will allow the later formation of
tissues and organs
gastrula: embryo is called this, two-layered body plan  three layered embryo with primitive digestive
tube
germ layers: collective name for three layers produced by gastrulation
ectoderm: outer layer
endoderm: lines embryonic digestive tract
mesoderm: fills space between endoderm and ectoderm
S EA URCHIN
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
1.
2.
3.
4.
5.
blastula  gastrulation at vegetal pole (invagination)  archenteron (inside the gastrula) 
blastopore: open end of the archenteron
produces an embryo with a primitive digestive tube with three germ layers
blastula consists of single-layered ciliated cells surrounding blastocoel, gastrulation begins with
migration of mesenchyme cells from vegetal pole into the blastocoel
vegetal pole invaginates, mesenchyme cells migrate throughout the blastocoel
endoderm cells form the archeteron, new mesenchyme cells at the tip of the tube begin to send out
thin extensions (filopodia) towards the blastocoel wall
filopodia then contract, dragging the archenteron across the blastocoel
fusion of archenteron with the blastocoel wall completes formation of the digestive tube
F ROG
1.
2.
3.
Gastrulation begins when a small indented crease, the blastopore, appears on the dorsal side of the
late blastula. The crease is formed by cells changing shape and pushing inward from the surface.
Sheets of outer cells then roll inward over the dorsal lip (involution) and move into the interior, where
they will form endoderm and mesoderm. Meanwhile, cells at the animal pole, the future ectoderm,
change shape and begin spreading over the outer surface
Blastopore extends around both sides of the embryo, as more cells invaginate. When the ends finally
meat on the other side, the blastopore forms a circle that becomes smaller as ectoderm spreads
downward over the surface. Internally, continued involution expands the endoderm and mesoderm,
and the archenteron begins to form; as a result, the blastocoel becomes smaller
Late in gastrulation, the endoderm-lined archenteron has completely replaced the blastocoel and the
three germ layers are in place. The circular blastopore surrounds a plug of yolk-filled eggs

Primitive streak: is like the blastopore, but it runs along the embryo’s anterior-posterior axis
ORGANOGENESIS
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Organogenesis: creation of organs
Notochord: skeletal rod characteristic of all chordate embryos, formed from dorsal mesoderm that
condenses when the cells associate tightly as a group just above the archenteron
Neural tube: runs along the anterior-posterior axis of the embryo, will become the animal’s central
nervous system, brain and spinal chord
Neural crest cells: migrate to various parts of the embryo, forming peripheral nerves
Somites: separated blocks of the notochord, arranged serially on both sides along the length of the
notochord, dissociate into mesenchyme cells, which migrate to new locations.
o Some mesenchyme cells gather around the notochord and forme the vertebrae, others
persist as inner portions of vertebral disks
o Other somite cells can form muscles associated with vertebral column and ribs
The three germ layers
DEVELOPMENTAL ADAPTATIONS OF AMNIOTES


Amniotes: animals that have their embryos surrounded by fluid within a sac, the membrane is called
the amnion
Extraembryonic membranes: membranes located outside the embryo
o Chorion: completely around the embryo and the yolk, serves as gas exchange
o Amnion: closes embryo in a protective fluid-filled amniotic cavity
o Yolk sac: encloses the yolk, which provides nutrients until the time of hatching
o Allantois: disposes of waste
MAMMALIAN DEVELOPMENT

Eggs are typically quite small, most fertilization occurs in the oviduct, and the earliest stages of
development occur while the embryo completes its journey down to the uterus
1. blastocyst: embryonic version of a
blastula. Clustered at one end of the
blastocyst cavity is a group of cells called the
inner cell mass, which will develop into the
embryo proper and form/contribute to all
extraembryonic membranes
2. trophoblast: outer layer of the
blastocyst, does not contribute to the embryo
itself but provides support services
a. Initates implantation
b. Extends fingerlike projections
into surrdounding maternal titssue
3. as implantation is completed,
gastrulation begins: cells move inwards from
the epiblast through a primitive streak and
form the mesoderm and endoderm, placenta
starts forming
4.
Formation of germ layers
47.2 MORPHOGENESIS IN ANIMALS INVOLVES SPECIFIC CHANGES IN CELL
SHAPE, POSITION AND ADHESION
CYTOSKELETON , CELL MOTILITY , AND CONVERGENT EXTENSION
1. Cuboidal ectodermal cells
form a continuous sheet
2.
Microtubules
help
elongate the cells of the
neural plate.
3. Actin filaments at the
dorsal end of the cells may
then contract, forming the
cells into wedge shapes
4. Cell wedging in the
opposite direction causes
the ectoderm to form a
dinge
5. Pinching off of the neural
plate forms the neural tube


Cells “crawl” within the embryo by using cytoskeletal fibers to extend and retract cellular protrusions
During gastrulation in some organisms, invagination begins with cuboidal cells on the surface of the
blastula become wedge shaped

Convergent extension: involves cell crawling, it’s a type of morphogenetic movement in which the
cells of a tissue layer rearrange themselves so that the sheet becomes narrower while it becomes
longer
ROLE OF CELL ADHESION MOLECULES AND THE EXTRACELLULAR MATRIX

Cell adhesion molecules (CAMs): key group of proteins that contribute to cell migration and stable
tissue structure, glycoproteins
o Transmembrane cell-surface proteins that bind to CAMs on other cells
o Cadherins: require calcium ions outside the cell for proper function, there are many of these
o Process of cell migration and tissue organization involves ECM, meshwork of secreted
glycoproteins and other macromolecules lying outside the plasma membranes of cells
 Helps to guide cells in many types of morphogenetic movements, such as migration
of individual cells/shape changes
47.3: DEVELOPMENTAL FATE OF CELLS DEPEND ON THEIR HISTORY AND ON
INDUCTIVE SIGNALS
1. During early cleavage divisions, embryonic cells must somehow become different form one another:
differences in cells’ cytoplasmic composition help specify the body axes and influence expression of genes that
affect the cells’ developmental fate
2. Once initial cell asymmetries are set up, subsequent interactions among the embryonic cells influence their
fate, usually by causing changes in gene expression—induction: eventually brings about differentiation, may
be mediated by diffusible signally molecules
FATE MAPPING

Fate maps: territorial diagrams of embryonic development
o 1920s: Walther Vogt: charted fate maps for different regions of early amphibian embryos
o  Sydney Brenner, Robert Horvitz and Jon Sulston
o Every adult hermaphrodite of the worm Caenorhabditis elegans has 959 somatic cells
ESTABLISHING CELLULAR ASYMMETRIES
AXES OF BASIC BODY PLAN


Nonamniote vertebrates: established during oogenesis/fertilization: locations of melanin and yolk in
the unfertilized egg definite the animal and vegetal poles
o Animal-vegetal axis indirectly determines the anterior-posterior body axis, fertilization
triggers cortical rotation, establishes dorsal-ventral axis – gray crescent (dorsal side)
Amniotes: body axes are not established until later
o Chicks: gravity, pH differences
RESTRICTION OF DEVELOPMENTAL POTENTIAL OF CELLS



Totipotent: only the zygote is capable of developing into all the different cell types of the species
o First cleavage is asymmetrical, two blastomeres receiving different cytoplasmic determinants
o Fates of embryonic cells can be affected not only by distribution of cytoplasmic determinants,
but also by how distribution relates to the zygotes characteristic pattern of cleavage
Mammalian embryos: remain totipotent until the morula stage
General feature of development in all animals is the progressive restriction of developmental potential
CELL FATE DETERMINATION AND PATTERN FORMATION BY INDUCTIVE SIGNALS


Spemann’s organizer/gastrula organizer/organizer: scientists are using this method to figure out how
to identify the molecular basis of induction
Ex: induction by the BMP-4 of lateral and ventral structures the organizer inactivates BMP on the
dorsal side of the embryo by binding proteins to BMP-4, rendering it unable to signal
FORMATION OF THE VERTEBRATE LIMB


Pattern formation: development of an animal’s spatial organization, arrangement of organs and
tissues in their characteristic places in 3D space
Positional information: molecular cues that control pattern formation, tell a cell where it is in respect
to the animals’ body axes and help determine how the cell and its descendants will respond to
molecular signaling
a. Organizer regions: vertebrae
limbs develop from protrusions called limb
buds, each consisting of mesoderm cells
covered by a layer of ectoderm. Two regions in
each limb bud, the AER and the ZPA play key
roles as organizers in limb pattern formation
b. Wing of chick embryo: as the
bud develops into a limb, specific patterns of
tissues develop: chick wing: digits are always
present in the arrangement. Pattern formation
requires the embryonic cell to receive positional
info indicating location along the three axes of
the limb.



Apical ectodermal ridge (AER): thickened area of ectoderm at the tip of a bud  removal blocks
outgrowth of the limb along the proximal-distal axis
Zone of polarizing activity (ZPA): block of mesodermal tissue located underneath the ectoderm where
the posterior side of the bud is attached to the body, necessary for proper pattern formation along
the anterior-posterior axis of the limb
Hox genes: important to limb pattern formation