Transcript Reproduction & Development

Mammalian Reproduction
• All mammals have sexual reproduction
– Haploid gametes by meiosis
– Control of gamete production dependent on
hormones
• Mammals are viviparous (except the
Monotremes)
– Placental mammals retain their young in utero
– Most mammalian offspring require large
amount of parental care
– Males play a small role in parental care (in
most cases)
Mammalian Reproduction
• Female reproductive cycles involve
periodic release of a mature ovum
– Ovulation
• Most female mammals have estrous
cycles
– Females sexually receptive to males only at
time of ovulation (“estrus”)
• Primates (inc. humans) have menstrual
cycles
Human Reproductive System
• Sex differentiation is determined
genetically; presence of Y chromosome
determines maleness
• Developing embryo has wolffian ducts
and müllerian ducts
– Wolffian ducts – develop into male
epididymus, vas deferans, and accessory
organs (seminal vesicle and prostate)
– Müellerian ducts – develop into vagina,
uterus, cervix and oviducts
Human Reproductive System
• In males, genes located on Y chromosome
initiate differentiation of epithelial cells into
Sertoli cells
• Sertoli cells produce anti-Müllerian
hormone; causes the deterioration of the
Müllerian duct (defeminization)
• Mesenchyme cells differentiate into
Leydig cells
– Leydig cells produce the androgens (male sex
steroids)
Human Reproductive System
• Androgens
– Testosterone – primary male sex hormone,
responsible for the development and
differentiation of the Wolffian duct
• Sertoli cells require testosterone for
spermatogenesis
– 5-Dihydrotestosterone – responsible for
masculine development of external genitalia
• Guevodoces – genetic condition, delays male
sexual development until puberty
Human Reproductive System
• In females (XX), the absence of
testosterone leads to feminization (default
condition)
• All mammals, regardless of sex, begin life
with primordial (undifferentiated) gonads
• No androgens for Wolffian ducts to
develop, will degenerate. Müellerian ducts
will develop due to secretion of estrodial
• Gonads will become ovaries, vagina,
uterus, and cervix
Developmental disorders
• XO, Turner’s syndrome – infertile female
• XXY, Klinefelter’s – testes development
(male), but sterile
• Congenital Adrenal Hyperplasia, XX embryo
– ovarian development; adrenal gland
overproduces androgens; mascularization of
genitalia; development of Wolffian ducts and
Müellerian ducts (no sertoli cells!)
– At puberty, can get secondary characteristics,
ovary kicks in and secretes estrodial
Human Male Reproductive System
• When testes form in the male embryo,
they develop highly-convoluted
seminiferous tubules, the site of sperm
production
• In the wall of the seminiferous tubule, a
spermatogonium divides by mitosis
producing diploid cells
• Diploid cell known as the primary
spermatocyte undergoes meiosis
producing secondary spermatocytes
Human Male Reproductive System
• Seminiferous tubules contain Sertoli cells
• Sortoli cells help convert spermatocytes
into spermatozoa (sperm) by engulfing
their extra cytoplasm
• Shortly before birth, the testes descend
into the scrotum, because sperm need
cooler temperature to develop
Human Male Reproductive System
• Sperm are delivered into the epididymis
where they must remain for at least 18
hours for mobility to develop, then pass
into the vas deferens
• Seminal vesicles produce fructose-rich
fluid, while the prostate gland produces a
milky fluid  semen
Human Male Reproductive System
Human Female Reproductive
System
• Ovaries develop more slowly than testes
• Ovaries contain microscopic ovarian
follicles
• Each follicle contains a potential egg cell
called a primary oocyte and smaller
granulosa cells
– Granulosa cells secrete estradiol (estrogen)
Human Female Reproductive
System
• Primary oocytes begins meiosis, but
arrested in 1st prophase (meiosis I) before
birth; finite number of oocytes formed by
mitosis
• At puberty, granulosa cells proliferate and
secrete estrogen
– Triggers first menstrual cycle
– Stimulates secondary characteristics
Human Female Reproductive
System
• After puberty, oocyte completes first
meiotic division (meiosis I)
• However, the first meiotic division is very
uneven
– Produces large ‘secondary oocyte’ and a
small polar body (disintegrates)
– The secondary oocyte begins meiosis II, and
is arrested in metaphase II
– Fertilization stimulates continuation of meiosis
The Menstrual Cycle
Human Female Reproductive
System
• Development of a mature, or Graafian follicle
occurs monthly (~28 days) during the menstrual
cycle
• Follicles remaining in ovary after ovulation
develop into the corpus luteum
• The corpus luteum secretes progesterone,
which stimulates proliferation of the
endometrium
– usually short-lived unless oocyte gets fertilized
– When corpus luteum dies, loss of progesterone
results in shedding of endometreum
Human Female Reproductive
System
• After fertilization, the developing embryo
produces human chorionic gonadotropin
– Maintains the corpus luteum
– Prevents menstruation by keeping levels of
estrodial and progesterone high
– This hormone is tested for in pregnancy tests
Developmental Biology
• Fertilization – union of male and female
gametes; consists of three events:
– Sperm penetration and membrane fusion
– Egg activation
– Fusion of nuclei
• Development involves:
– Growth
– Differentiation
– Specialization
Fertilization
• A sperm must
penetrate to the
plasma membrane
of the egg for
membrane fusion to
occur
• Egg enveloped by
one or more
protective coats
– Zona pellucida in
mammalian eggs
Fertilization
• Membrane fusion activates the egg
– When sperm makes contact with the egg’s
plasma membrane, it triggers a release of
Ca+2 from internal organelles starting at the
point of sperm entry
– Changes membrane potential of egg,
prevents other sperm from fusing with egg
– Enzymes from cortical granules remove
sperm receptors
– prevents polyspermy!
Fertilization
• Sperm triggers egg to complete meiosis
(remember that the oocyte is arrested in
metaphase II)
• Restores the diploid state (haploid sperm
nuclei fuses with haploid egg nuclei)
• There are proteins on surface of the
acrosome that will bind to receptors on the
membrane of the egg – very specific
match, secures species specificity
Development
• Meiosis – formation of male and female
gametes
• Fertilization
• Cleavage – a period of rapid cell division
• Gastrolation – cell division slows, but cells
go through extensive rearrangement 
ectoderm, mesoderm, and endoderm
• Organogenesis – develop of specific organs
Development
• Cleavage – the rapid division of the zygote
into a larger and larger number of smaller
and smaller cells called blastomeres
• No overall increase in size of embryo
• Animal pole and vegetal pole
– Blastomeres of animal pole form external
tissues of the body; blastomeres of vegetal pole
form the internal tissues
Development
• A hollow ball of
cells, the
blastula,
contains a fluidfilled cavity, the
blastocoel
Development
• In bilateral phyla, all organisms have
developmental stage involving 3 tissue
layers: ectoderm (outside), mesoderm
(middle), and endoderm (innermost layer)
• In protostomes, the mouth develops first
– All worms, arthropods, molluscs
– Coelom formed by hollowing out of solid mass
of cells that make up mesoderm
– Undergo spiral cleavage
– Determinant cleavage – fate of cells
determined early
Development
• In deuterostomes, the mouth forms second
– Echinoderms and chordates
– Mouth forms from opening on opposite side of
blastopore
– Blastopore – opening to outside, first opening in
development
– Radial cleavage or rotational cleavage
– Indeterminant development - fate of cells
determined by interactions between other cells
Development
• Deuterostomes evolved from Protostomes
>500 million years ago
• Go back to Chapter 32 – pages 626-7
Development
• The pattern of development is dependent
on the amount of yolk
Development
• Egg contains yolk proteins, huge stores of
mRNA, tRNA, and ribosomes (powers egg
during cleavage), and morphogenic
determinants (decide what cell becomes)
• In eggs containing moderate to little yolk,
cleavage occurs throughout the egg –
holoblastic cleavage
• In eggs containing large amounts of yolk,
cleavage, incomplete or meroblastic
cleavage occurs
Development
• Holoblastic cleavage – eggs without very
much yolk; results in more evenly-sized
cells, cleavage furrows extend throughout
the egg
• Meroblastic cleavage – cleavage forrows
cannot make it through yolk; incomplete
cleavage
– In birds and reptiles, cleavage is restricted to
region of concentrated cytoplasm
Meroblastic cleavage
Development
• Compaction occurs at 8-cell stage
• Outer cells develop into trophoblast
– Becomes part of placenta
– Inner cell mass develops into embryo
– Random: no one cell is destined to be inner or
outer
– Blastocoel forms by secretion of fluid by
trophoblastic cells
Gastrulation
• Gastrulation – migration and
rearrangement of cells of the blastula
• Results in formation of three germ layers:
– Endoderm (internal organs)
– Mesoderm (bones, heart, blood vessels,
muscles, connective tissue, gonads)
– Ectoderm (skin and nervous system)
Gastrulation
• Cells move during
gastrulation, undergoing a
variety of cell shape
changes
• Cells attached tightly at
junctions will move as cell
sheets
• Invagination – cell sheet
dents inward
Gastrulation
• In mammals, embryo develops from inner
cell mass
• A thickening of cells forms the primitive
streak
• In the middle of the primative streak, cells
sink inward forming the primative groove
– Cells entering the sides of the primative
groove become mesoderm and endoderm
– Cells that stay on outside become ectoderm
Gastrulation in mammals
Extraembryonic membranes
• As an adaptation to terrestrial life, the embryos of
reptiles, birds, and mammals develop within a
fluid-filled amniotic membrane, or amnion
• Chorion – fetal contribution to placenta
• Yolk sac – critical role in the nutrition of birds and
reptiles, present in mammals, but not nutritive
• Allantois – in birds, fuses with chorion and
fascilitates gas exchange; in mammals, it
contributes blood vessels to developing umbilical
cord
Extraembryonic membranes
Extraembryonic membranes
Development
• Neurulation – formation of the nervous
system; the first ‘organs’ that develops in
embryo
• Folds of ectodermal tissue come together,
forming the neural tube
• Neural tube has to close off
• Neural tube becomes spinal cord
Development
• Cell migration – cells migrate to different
parts of the embryo to form distinct tissues
– Example – cells of neural crest form sense
organs
• Organogenesis – formation of organs in
their proper locations
– Occurs by interaction of cells within and
between the 3 germ layers
– Follows rapidly on the heels of gastrulation
Development
• At some point, every cell’s fate becomes
fixed; cell determination
• All of the cells in an animal’s body, with
the exception of a few specialized ones
that have lost their nuclei, contain the
same compliment of genetic information
• To a large degree, a cell’s location in the
developing embryo determines its fate (but
this is only true up until a certain stage in
development)
Development
• Ontogeny recapitulates phylogeny!
• In the early 1800’s, van Baer noted that
early embryos of all members of phylum or
subphylum appear fairly similar
• As development continues, it takes on
more specific characteristics of
classorderfamilygenusspecies
• Human development goes through
embryonic stages of other animals
Development
• Developmental patterns of more recently
evolved groups are built on more primative
patterns
• Ernest Haeckel (late 1800’s) surmised that
evolution of new species arise by adding
on an additional step to the end of the
previous species’ embryonic stage (we go
through adult stages of all previous
species)
Plant Development
• Development begins once the egg cell is
fertilized
Polar nuclei
Egg
Sperm
Micropyle Pollen tube
3n endosperm
2n zygote
Plant Development
• The first division of the fertilized egg is
asymmetrical
– One daughter cell becomes embryo, small
– Other larger cell becomes the suspensor,
which links the embryo to the nutrient tissue
of the seed
– Cells near the suspensor become root
– Cells at opposite end become shoots
Plant Development
• Tissue formation
– Protoderm – develops into dermal tissue that
protects plant (external surface of plant)
– Ground Meristem – develops into ground
tissue that stores food and water
– Procambium – develops into vascular tissue
that transports water and nutrients; xylem and
phloem
Plant Development
• Cotyledons
– One or two seed leaves (monocot or dicot,
respectively) develop
• May store, absorb food from endosperm
– Seed coat forms
• Seeds may exist in dormant state (for <thousands
of years)
• Resistant to harsh, undesirable conditions
Plant Development
• Seed formation
– Early in the development of an angiosperm
embryo, the embryo stops developing
– In many plants, development is arrested
following the differentiation of the meristems
and cotyledons
– Seeds protect the young plant, provides food
for the embryo (until it can produce its own),
and facilitates dispersal of the embryo
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Shoot apical meristem
Seed coat
(integuments)
Procambium
Root apical
meristem
Root cap
Endosperm
Cotyledons
Plant Development
• Germination cannot take place until water
and oxygen reach the embryo
• Some seeds only germinate when
sufficient water is available to leach
inhibitory chemicals from the seed coat
• Other seeds germinate only after passing
through the intestines of birds or mammals
• Other seeds germinate only after being
exposed to fire
First leaves
Plumule
Epicotyl
Cotyledon
Hypocotyl
Hypocotyl
First leaf
AdventiColeoptile Scutellum tious root
Withered
cotyledons
Seed coat
Primary roots Secondary roots
a.
Coleorhiza
Radicle
Primary root
b.
Plant Development
• Fruits
– Mature ovaries
– During seed
formation, the
flower ovary
begins to develop
into a fruit