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Lecture
Reproduction & Development
Sex in the animal kingdom
• both sexual and asexual
• sexual – fusion of haploid gametes to form a
diploid zygote
– gametes – egg and sperm
– zygote can itself give rise to gametes via meiosis
– zygote develops into an embryo via mitosis
• asexual – generation of a new individual
without the fusion of gametes
– relies upon mitotic division
Asexual reproduction
• mechanisms:
• 1. fission
– separation of the parent into two equal sized
individuals
• 2. budding
– new individuals arise as outgrowths from the parent
• 3. fragmentation & regeneration
– breaking of the body into several pieces
• 4. parthenogenesis
– egg develops without being fertilized
– progeny is either haploid or diploid
Sexual reproduction
• sexual reproduction must enhance reproductive
success
• if not successful – it would not have evolved
because asexual reproduction produces more
individuals
• advantages of sex?
– diversity due to recombination of parental genes
during meiosis and fertilization
• sexual reproduction is advantageous when
environmental conditions change rapidly and
frequently
Reproductive Cycles
• most animals’ reproductive cycles coincide with the change
of seasons
– give birth when resources are plentiful and conditions favor the
survival of their offspring
• ovulation – mid-point in the reproductive cycle corresponding
to the release of eggs
• reproductive cycles not just restricted to animals who
reproduce sexually
• asexual reproduction cycles – some fishes, amphibians and
reptiles can undergo periods of parthenogenesis
– double their chromosome number to produce diploid offspring
• reproductive cycles found in animals that have sexual and
asexual cycles
Mammalian Reproductive Cycles
– most females undergo estrus = time during which the
female is behaviorally and physiologically receptive to the
male (fertile egg is produced)
• complex series of hormonal changes that affect egg production,
uterine and vaginal environments and sperm production
– many mammals are monoestrus
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only once a year
strictly regulated by the environment
e.g. wild dogs, bears and sea lions
e.g. domestic dogs – biestrus
other animals have cycles that can repeat on a cyclical basis
– e.g. rats – every 4 to 6 days
Hermaphrodism
• happens when finding a sexual partner is
difficult
• rather common in sessile, aquatic animals and
in parasites
• both male and female reproductive organs
(monoecious)
• this means any two individuals can mate
– one acts as a male, one acts as a female
• some can self-fertilize
Fertilization
• union of egg and sperm
• can either be: external or internal
• external fertilization requires a moist habitat
– prevents dessication of gametes
– also allows the sperm to swim to the egg
– clustering of animals in the same area for
fertilization = spawning
– when not synchronized within a population –
mating pairs will undergo courtship behaviors
to stimulate release of gametes
– e.g. frogs – external fertilization with courtship
behavior – locates breeding sites and potential
mates
Internal Fertilization
• sperm deposited in or near the female reproductive tract
– male copulatory organ deposits sperm
– female can have a seminal receptacle for receipt and storage of sperm – then
delivers it to the egg when needed
– e.g. sharks – modified pelvic fins called claspers that are inserted into the
female cloaca for sperm transfer
• followed by internal fertilization – thus ensuring success
• some sharks are viviparous – give birth to live young
• fertilization occurs within the female’s reproductive tract
– usually followed by internal embryonic development
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evolved because of dry, terrestrial habitats
requires cooperative behavior
requires compatible reproductive structures
fewer gametes produced vs. external fertilization because the chances of
successful fertilization are higher
• mammalian fertilization:
– in a few mammals – fertilization is delayed
after coitus (delayed fertilization)
• e.g. bats – coitus in fall, fertilization in spring
– most mammals have direct fertilization after
coitus
• but development of the embryo can be arrested for
one to two weeks = embryonic diapause
• e.g. sea lions, bears and marsupials
• may be due to presence of resources – allows
mother time to feed before having to nurse
• also allows young to be born during a time of good
resources
Care of Offspring
• internal fertilization and gestation ensures
better protection of offspring
– against desiccation and physical damage
• internal gestation done by placental mammals =
eutherians
– receive nourishment from the mother through the
placenta
• internal gestation for long periods of time is not
necessary
– marsupials – embryos crawl out of the uterus and
complete development attached to a mammary gland
located in a pouch
Care of Offspring
• alternative to internal gestation and
development: development of the amniotic
egg
– egg laying animals – monotremes
– birds and reptiles
– protection of the developing embryo by a
protective layer of calcium and proteins in the
form of the shell
– eggs of fish and amphibians are quite different –
surrounded by a gelatinous material and they lack
internal membranes
Gamete production
• gametes = egg and sperm
• develop from a set of precursor cells called germ cells
– develop during embryonic development and remain
dormant until sexual maturity
• e.g. humans
• typical site for these germ cells and gamete
production are the gonads
– not required in simpler animals
• more complex reproductive systems also have
accessory organs in association with the gonads
– seminal vesicles, prostate gland
– seminal receptacles or spermathecae (in insects)
Reproductive adaptations
• same basic plan for all vertebrates
• in many non-mammalian vertebrates – formation of a
cloaca
– combined opening for digestive, urinary and reproductive
tracts
– female mammals have separate openings for all three
– male mammals combine the urinary and reproductive opening
• many vertebrates have a uterus that is divided into two
chambers
• in mammals that produce only a few offspring at a time –
uterus is a single structure
• differences in the male reproductive system – in the
copulatory organs
– human penis is quite distinct from other mammals
Male reproductive system
• three accessory organs
– seminal vesicle
– prostate
– bulbourethral glands (Cowper glands)
• reproductive ducts
– epididymus
– vas deferens (ductus deferens)
– urethra
Reproductive Ducts
-vas deferens: conducting tube from testis to urethra
-epididymus – connection between the testis and the vas deferens
-stores immature sperm
-vas deferens + blood vessels + nerve = spermatic cord (passes through inguinal
canal)
-urethra: 3 sections:
A. prostatic - runs through prostate
B. membranous - between prostate and penis
C. spongy - through penis
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Reproductive glands
seminal vesicles, prostate, bulbourethral glands
-produce fluid that combine with sperm to make semen
-semen: alkaline, activates sperm cells
1. prostate: surrounds the urethra
-secretes a thin, milky fluid that enhances sperm motility and neutralizes vaginal
fluid
2. seminal vesicles: connect to urethra via the ejaculatory ducts
-secretes an alkaline fluid that contains sugars and prostaglandins (stimulates
uterine contractions)
3. bulbourethral glands: 2 glands behind the prostate
-secrete a fluid that lubricates the penis
Bladder
Rectum
Prostate
Prostatic
Urethra
Membranous
Urethra
Spongy/
Penile
Urethra
Testes
Corpus
spongiosum
Corpus
cavernosum
Glans
Penis
External
• scrotum = supportive
structure for the testes
• hangs from the root of the
penis
• externally- single pouch
separated at the midline by
a raphe
• internally – divided by a
scrotal septum into two
sacs each containing 1 testis
• the scrotum contains two
muscles
– dartos muscle (smooth
muscle) & the cremaster
muscle – skeletal muscle that
is a continuation of the
internal oblique
pampiniform
plexus
Vas
deferens
testicular
artery
testicular
artery
branches
Epididymus
Seminiferous
Tubules
Tunica Albuginea
Tunica Vaginalis
-testis: develop internally near the kidneys and descend during the latter half of the seventh
month gestation
-covered by two protective membranes:
1. tunica vaginalis –derived from the peritoneum
2. tunica albuginea – internal to the TV
-extends inward to divide the testes into lobules (200-300)
-each lobule contains 1 to 3 coiled seminiferous tubules for sperm production
- lined with spermatogenic cells that produce sperm
-contain sperm stem cells = spermatogonium
-testis: also contain two additional cell types:
-Sertoli cells = sustenacular cells
-found in the seminiferous tubule
-secretion of growth factors that mediate spermatogenesis
-interstitial cells – located between the seminiferous tubules
-for testosterone production
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sperm development – from sperm
stem cells called spermatogonium
these spermatogonium develop in the
embryonic testes
the spermatogonium remain dormant
in the testes until puberty
mature sperm cells = spermatozoa
1. differentiation of the
spermatogonium into primary
spermatocytes (2n)
2. start of meiosis
3. formation of secondary
spermatocytes (n) – 23 chromosomes
comprised of two chromatids
4. completion of meiosis and
formation of four spermatids (23
chromosomes each made up of one
chromatid)
5. spermiogenesis – development of
spermatids into a sperm cell (head,
acrosome and flagella development)
Spermatogenesis
Sperm
• 300 million made each day
• 60 um long
• major parts
– 1. head: contains the nucleus with 23 highly
condensed chromosomes (one chromatid)
– 2. acrosome: covers the anterior 2/3 of the
head
• contains digestive enzymes to dissolve the
protective barriers of the egg (hyaluronidase
and proteases)
– 3. tail or flagellum
• neck - constricted region just behind the head
• iddle piece – contains mitochondria arranged in
a spiral
• principal piece – longest portion of the tail
• end piece – terminal portion of the tail
principal
piece
end
piece
made up of a:
1. root
2. body (or shaft)
3. glans
-conveys urine and semen
-body is found externally, root is found internally (attached to the pubic ramus)
-body is comprised of two tissue types: erectile tissue surrounded by connective tissue
A. corpus cavernosum – larger spaces
B. corpus spongiosum - smaller spaces & surrounds the urethra
-corpus spongiosum enlargens at the tip - glans penis (sensory receptors)
-glans penis covered with a loose fold of skin = prepuce
-release of Gonadotropic releasing hormone (GnRH) from the hypothalamus
which stimulates the anterior pituitary gland
-anterior pituitary releases two gonadotropins (FSH and LH)
1. Follicle stimulating hormone - stimulates spermatogenesis
2. Leutinizing hormone - stimulates male hormone production
-male hormones: androgens (e.g. testosterone)
-testosterone: secondary sex characteristics, reproductive organ
development and maintenance
-rise in testosterone causes a negative feedback: hypothalamus release of
GnRH is inhibited
-uterus: receives and nourishes the embryo
-comprised of a body, a curved portion (fundus) and the cervix
-uterine wall outer perimetrium, muscular myometrium and inner
endometrium
-endometrium: mucosal layer covered with epithelium
-rich blood supply, sloughed off during menstruation
-comprised of a deeper basal layer and an outer functional layer
Fundus
Fornix
Body
Cervix
Bladder
Rectum
Vaginal canal
Urethra
Labia minora
Labia majora
External
urethral orifice
Vaginal orifice
-uterine tubes or oviducts (Fallopian tubes): conduction of egg from ovary to uterus
-expands at end near the ovary = infundibulum with fimbrae (fingers) for the “catching”
of the released egg
-fertilization occurs in the uterine tubes
-cervix: projects into the vaginal canal
-vaginal canal broadens to surround the cervix – fornix
-site for the insertion of the penis and deposition of sperm
-opens at the exterior as the vulva (female external genitalia)
-known as the vulva
-mons pubis – mass of adipose tissue that lays on top of the pubic bone
-mons pubis divides into the labia majora and minora
-labia majora: folds of adipose tissue that enclose and protect the labia minora
-labia minora: surrounds the external urethral orifice and the vaginal orifice
-clitoris: erectile tissue with a glans, body and a root
-ovary: production of oocytes
-inner medulla and outer cortex
-medulla - connective tissue with blood & lymphatic vessels and nerves
-cortex - granular tissue due to the presence of tiny ovarian follicles
containing egg stem cells (oogonium)
-ovarian follicles mature and contain the developing oocyte = oogenesis
1. growth of primary follicles into secondary follicles containing a primary
oocyte
2. secondary follicle matures -> vesicular /tertiary follicle with a large
secondary oocyte (for ovulation)
3. ruptured follicle degenerates into a corpus hemorrhagicum and then into a
corpus luteum  female sex hormone production (estrogen & progesterone)
• early fetal development – primordial germ
cells in the developing ovaries differentiate
to form oogonia (egg stem cells)
• at birth: meiosis in the oogonia results in
the development of primary follicles
containing primary oocytes – stops at
prophase I of meiosis
• at puberty - release of FSH and LH each
month causes the development of a primary
oocyte into a secondary oocyte followed by
ovulation
• coincides with the development of the
primary follicle (primary oocyte) into a
secondary follicle (primary oocyte) and
finally into a tertiary follicle (secondary
oocyte)
• tertiary follicle – ovulation of secondary
oocyte
• fertilization is required for the completion of
meiosis II
• therefore during oogenesis, ONE oocyte
merges with the sperm and THREE polar
bodies form
Oogenesis
-GnRH causes release of FSH and LH from anterior pituitary
-FSH causes maturation of follicles
-LH results in development of corpus luteum
-corpus luteum produces and releases estrogen and progesterone
-estrogen and progesterone: regulate pregnancy, menstruation, secondary sex
characteristics & development of sex organs at puberty
-divided into two cycles
1. uterine cycle
2. ovarian cycle
-Ovarian cycle:
-rise in FSH matures the follicle
- day 9 - follicle begins to produce
follicular estrogen
-day 14 - sudden increase in FSH and
LH plus follicular estrogen coincides
with ovulation
-the increase in follicular estrogen is
what causes the spike in LH
-day 14 - ovulation = LH spike is the
trigger
-Ovarian cycle:
-drop in LH, FSH and
follicular estrogen after
ovulation
-but FSH will begin to climb
at the end of the cycle
because of an increase in
GnRH release from the
hypothalamus
Uterine cycle:
-menstrual cycle initiated by increased
FSH at Day 1
-rise in follicular estrogen starts
thickening of uterine lining = known as
the proliferative phase
-following ovulation and the
development of the corpus luteum =
increased secretion of luteal estrogens
+ synthesis of progesterone
-increased progesterone results in
thickening of the stratum functionalis =
known as the secretory phase
-Uterine cycle:
-estrogen and progesterone
begin to drop about day 25
to 28 - disintegration of the
stratum functionalis
-Menstruation (3 to 7 days)
Fertilization
-fertilization in the upper third of the oviduct/fallopian tube
-fertilization = union of egg and sperm to produce the zygote
-sperm must undergo capacitation after ejaculation – increase rate of tail beating
-happens in the female reproductive system
-plasma membrane of the
ovulated secondary oocyte is
surrounded by an extracellular
matrix (glycoproteins) = zona
pellucida and a ring of follicular
cells = corona radiata (for
nourishment in the follicle)
-sperm must penetrate these
layers before docking onto the
plasma membrane
1. several sperm penetrate
corona radiata and enter zona
pellucida
2. Sperm bind to the zona
pellucida & trigger release of
acrosomal contents  zonal
digestion
3. ONE sperm contacts the
plasma membrane of the
oocyte
4. changes to the zona
pellucida result (hardens) –
blocks polyspermy
5. entry of sperm nucleus into
egg
6. fusion of the sperm’s
pronucleus with the
pronucleus of the egg = zygote
embryonic stage:
1st week to week 8
-first cell division – within 24 hrs & takes 6 hrs
to complete
-second day – four cells
-end of third day – 16 cells
-fourth day – morula stage
-fourth to fifth day – blastocyst stage
-end of fifth day – hatching of blastocyst from
zona pellucida
-6th day – implantation of blastocyst into
enodmetrium
OVIDUCT:
-union of sperm and egg nuclei (zygote) -> first cell division within 24 hours to form
the embryo
- cell division continues -> formation of the morula at day 4
- morula = a mass of tiny, uniformly sized cells with equal amounts of cytoplasm
- cells of the embryo = blastomeres
embryonic stage:
1st week to week 8
-first cell division – within 24 hrs &
takes 6 hrs to complete
-second day – four cells
-end of third day – 16 cells
-fourth day – morula stage
-fourth to fifth day – blastocyst
stage
UTERUS:
-day 4 – 5: morula forms a blastocyst (blastula) = assymetrical ball of cells with a
cavity called a blastocoel
-day 6: -> implantation of blastocyst into the stratum functionalis of the
endometrium
-blastula = blastocyst - hollow ball of cells/blastomeres
-outer layer = trophoblast - forms extraembryonic tissues (e.g. placenta,
yolk sac)
-inner mass of cells at one end = totipotent embryonic stem cells
-second week of development (day 7 – 14) :
-amniotic cavity forms between the inner cell mass and the trophoblast
-the inner cell mass flattens = embryonic disc
Gastrulation
• third week of gestation in
humans (days 14 – 21)
• prior to gastrulation – the embryo
is composed of an upper and
lower layer that form an
embryonic disk
Fertilized egg
Primitive
streak
Embryo
Yolk
– the epiblast and hypoblast
– epiblast = embryo
– hypoblast (‘roof’ of the yolk sac)
= supports embryology & forms
part of the yolk sac
Primitive streak
Epiblast
Future
ectoderm
Blastocoel
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
Gastrulation
• during gastrulation - epiblast cells
move toward the midline of the
blastoderm – toward a structure
called the primitive streak
• epiblast cells migrate through the
primitive streak toward the yolk
– some cells push the hypoblast to
the side to become the endoderm
– those remaining cells 
mesoderm
– leftover non-migrating epiblast
cells  ectoderm
Fertilized egg
Primitive
streak
Embryo
Yolk
Primitive streak
Epiblast
Future
ectoderm
• animals with three germ layers are
known as Triploblastic
Blastocoel
Migrating
cells
(mesoderm)
Endoderm
Hypoblast
YOLK
-three germ layers:
1. ectoderm 
integumentary system &
nervous system
2. mesoderm 
mesenchyme which forms
the bones, fat, cartilage,
blood, muscle of the
animal
3. endoderm  digestive
organs and their linings
Endometrial epithelium
(uterine lining)
1 Blastocyst reaches uterus.
Uterus
Inner cell mass
Trophoblast
Blastocoel
2 Blastocyst implants
(7 days after fertilization).
Expanding region of
trophoblast
Maternal
blood
vessel
Epiblast
Hypoblast
Trophoblast
3 Extraembryonic membranes
start to form (10–11 days),
and gastrulation begins
(13 days).
Amniotic cavity
Epiblast
Hypoblast
Yolk sac (from hypoblast)
Chorion (from trophoblast)
4 Gastrulation has produced a
three-layered embryo with
four extraembryonic
membranes.
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
yolk sac
chorion
amnion
placenta
-yolk sac: forms blood cells, gives rise to sex cells and the stem cells of the immune
system
-portion of it becomes part of the umbilical cord
-embryo’s first connection is via a connecting stalk
-allantois: tube from the yolk sac that projects into the connecting stalk - gives rise to
the umbilical artery and vein
yolk sac
placenta
amnion
chorion
-chorion – forms four weeks after implantation as slender projections that grow
out from the trophoblast = chorionic villi (becomes the placenta)
-produces human chorionic gondadotropin hormone – hormone of
pregnancy
-amnion – surrounds the amniotic cavity (amnionic fluid + developing embryo)
-amnion enfolds the connecting stalk and remnants of the yolk sac to
form the umbilical cord
-fifth week (days 28 – 35): formation of lens, beginnings of maxilla and mandible &
paddle-shaped forelimb
-embryo is 10-12 mm long
-35 + 2 days: formation of eye, ear, forebrain, nasal pit, tail
-35 + 5 days: formation of midbrain, heart, external ear, primitive fingers
-sixth week: formation of primitive toes (22-24 mm)
-seventh week: formation of eyelids, webbed fingers
-eighth week: separation of toes and fingers (34-40 mm)
-fetal stage : end of the eighth week -> birth
-third month: body lengthens and head growth slows
-ossification of bones
-fourth month: reproductive organs appear
-rapid body growth
-lower limbs lengthen
-development of hair, eyebrows, lashes, nipples
-fifth month: growth slows
-skeletal muscles are active
-fetus curls into fetal position
-sixth to ninth month: weight gain
-skin smoothens as fat is deposited beneath skin
-eyelids open
-organs elaborate and grow (digestive and respiratory are last)
-neuronal networks form