VT 106
Comparative Anatomy and Physiology
Reproductive System
REPRODUCTIVE SYSTEM
Functions of Reproductive System
production of offspring – continuation of species
genetic recombination – individual variation, adaptation
SEXUAL REPRODUCTION
gametogenesis – production of gametes (reproductive cells)
female – oogenesis produces oocytes (ovum)
male – spermatogenesis produces spermatozoa (sperm)
copulation – male gametes transmitted to female
fertilization – ovum and sperm united
gestation – development of offspring from fertilization till birth
parturition – giving birth
GAMETOGENESIS
gonads – site of gamete production
female – ovaries
male – testes
somatic cells – all cells throughout body
diploid (2N) – contain pairs of chromosomes (N depends on species)
one member of pair from each parent
1 pair of sex chromosomes
X chromosome, Y chromosome
XX = female
XY = male
gametes – ovum and sperm
haploid (1N) – contain unpaired chromosomes from sire or dam
ovum always contains X chromosome
sperm – 50% contain X chromosome
50% contain Y chromosome
sperm determines sex of offspring
zygote – cell produced by fertilization; divides and differentiates into offspring
diploid – pairs of chromosomes
combined chromosomes from ovum and sperm
Mitosis – cell division which produces 2 identical, diploid daughter cells
1
MEIOSIS – 2 stage cell division which produces 4 haploid gametes
occurs in gonads
gametes produced are not genetically identical to parent cell
Stages of Meiosis:
meiosis I – a diploid cell divides and forms 2 haploid cells
genetic recombination
haploid cells randomly receive 1 chromosome from each
chromosome pair
crossing-over – random exchange of genes between
chromosome pairs
meiosis II – the 2 haploid cells divide again, forming 4 haploid cells
each is genetically unique due to genetic recombination
MALE REPRODUCTIVE SYSTEM
TESTES – male gonads
site of spermatogenesis – production of spermatozoa
produce androgens – masculinizing hormones
testes develop in abdominal cavity near kidneys
normal sperm production requires a temperature below body temperature
Descent of the Testes – testes move out of abdomen and into scrotum
scrotum – sac of integument and superficial fascia
inguinal canals – tunnels through anterior abdominal wall
gubernaculum – connective tissue cord that pulls testis through
inguinal canal into scrotum
spermatic cord – ductus deferens, blood vessels, and nerves
running through inguinal canal to testis
Regulation of Testicular Temperature
vascular heat exchange mechanism cools testes
pampiniform plexus (venous network) cools testicular artery
cremaster muscle – skeletal muscle in spermatic cord
elevates testes close to body when too cold
drops testes away from body when too warm
cryptorchidism – one or both testes fail to descend completely
animal is sterile – can’t produce normal sperm
still produces androgens – normal male appearance and behaviors
ANATOMY OF TESTES
vaginal tunics – layers of peritoneum covering testes
visceral vaginal tunic – on surface of testis
parietal vaginal tunic – continuous with parietal peritoneum
forms a fibrous sac around testis and spermatic cord
open vs. closed castration
2
tunica albuginea – dense, tough connective tissue forming surface of
testes and dividing it into lobules
seminiferous tubules – coiled tubules within lobules
sites of spermatogenesis
spermatocytes – form spermatozoa
Sertoli cells – support and protect spermatocytes
secrete nutrients and hormones (some estrogen)
protect spermatocytes from immune system
(genetically different than parent animal)
interstitial cells – lie between seminiferous tubules
secrete androgens – mainly testosterone
SPERMATOGENESIS – occurs continuously
spermatogonia – diploid stem cells
undergo mitosis to constantly produce new spermatocytes
1) primary spermatocyte – diploid cell on peripheral seminiferous tubule
undergoes meiosis I
2) 2 secondary spermatocytes – haploid cells
undergo meiosis II
3) 4 spermatids – haploid cells
undergo differentiation (grow flagellum, etc.)
4) 4 spermatozoa (sperm)
released into lumen of seminiferous tubule
rete testis – network of tubules that collect sperm
take sperm to epididymis to mature
Anatomy of Sperm
head
acrosome – tip containing lysosome-like vesicle
contains enzymes to penetrate oocyte
nucleus – contains haploid chromosomes
midpiece
mitochondria – produce ATP for movement
tail
flagellum (microtubules) – propeller-like movement
Hormonal Regulation of Spermatogenesis
Hypothalamus – Gonadotropin Releasing Hormone (GnRH)
stimulates anterior pituitary
Anterior Pituitary
Luteinizing Hormone (LH) – stimulates interstitial cells
interstitial cells produce testosterone
Follicle Stimulating Hormone (FSH) – stimulates Sertoli cells
synergism of FSH and testosterone stimulates spermatogenesis
3
Functions of Testosterone
prenatal – male reproductive development
puberty – maturation of testes and genitals
secondary sexual characteristics
anabolic effects – increase mass of tissues (bone, muscle)
behavior and libido (sex drive)
MALE REPRODUCTIVE TRACT
EPIDIDYMIS – long, comma-shaped, coiled tube on posterior testis
site of sperm maturation and storage
secretes nutrients, reabsorbs wastes
old sperm digested and reabsorbed
DUCTUS (VAS) DEFERENS – muscular tubule that carries sperm from
epididymis to urethra
ampulla – dilated distal end in some species
may produce seminal secretions
peristalsis expels sperm rapidly during copulation
URETHRA – carries semen and urine to exterior
pelvic urethra – in pelvic cavity
passes through prostate gland
collects sperm and seminal secretions
penile urethra – in penis
transmits semen to female
ACCESSORY REPRODUCTIVE GLANDS – secrete seminal fluid
alkaline fluid – neutralizes acid in female reproductive tract
fructose – ATP source for sperm motility
prostaglandins – stimulate smooth muscle contractions in female
reproductive tract
Seminal Vesicles – posterior to bladder
none in cat or dog
Prostate – surrounds urethra
enlargement can compress urethra
benign hyperplasia – testosterone stimulates growth
Bulbourethral Glands – near pelvic outlet
none in dog
secrete mucus – lubricates urethra and end of penis
4
PENIS – organ of copulation
transmits semen (sperm + seminal secretions) to female
Anatomy of Penis
stratified squamous epithelium
tough fibrous connective tissue encloses internal structures
roots – connective tissue attachment to pubis
body – main portion
corpus cavernosum – large, dorsal columns of erectile tissue
cause penile erection
corpus spongiosum – tube of erectile tissue around urethra
prevents collapse of penile urethra during erection
glans – distal tip containing erectile tissue and many sensory receptors
appearance varies in species
prepuce (sheath) – protective skin flap covering flaccid penis
smegma – waxy secretion from glands of prepuce
skeletal muscles – bulbospongiosus & ischiocavernosus muscles
cause pumping contractions of penis during copulation
Function of Penis
erectile tissues – contain extensive blood sinuses
erection – sexual stimulation causes a parasympathetic reflex
dilates arteries to penis
blood sinuses of erectile tissues fill
expansion of erectile tissues compresses veins
penis enlarges and stiffens (vasocongestion)
emission – continuing stimulation triggers contraction of glands and
ductus deferens
discharges semen into urethra
ejaculation – peak stimulation (orgasm) triggers contraction of penile
muscles and urethra
ejects semen into vagina or uterus
Variations
dog – os penis – bone in penis
bulb of glans – enlarged erectile region at base of glans
“tie” – penis remains stuck in vagina following copulation
sigmoid flexure – S-shaped curve of proximal penis (bull, ram, boar)
erect penis straightens
flaccid penis is withdrawn into sheath by retractor penis muscle
urethral process – tube-like extension of urethra beyond the glans
(prominent in ram)
5
FEMALE REPRODUCTIVE SYSTEM
OVARIES – female gonads
site of oogenesis
produce estrogens, progestins – female hormones
caudal to kidneys
usually ovoid
anchored to dorsal body wall by suspensory ligament
ANATOMY OF OVARIES
Ovarian Cortex – outer region
developing oocytes
all oocytes are already present around the time of birth
surrounded by cells that nourish oocytes and secrete hormones
follicular cells – single layer of squamous cells surrounding
inactive oocytes
granulosa cells – 1 or more layers of cuboidal cells around
developing oocytes; produce secretions
Ovarian Medulla – inner region
loose CT, blood vessels and nerves
OOGENESIS AND FOLLICULAR DEVELOPMENT
hormone levels periodically stimulate development of some oocytes and their
surrounding follicles
atresia – degeneration of some oocytes and their follicles
1) primary oocytes (diploid)
primordial follicles – single layer of squamous follicular cells
contain inactive primary oocytes
ESTROUS CYCLE – begins at puberty
hormones activate a group of primordial follicles periodically
growing follicles
follicular cells divide, forming multiple layers of granulosa cells
granulosa cells secrete hormones and nutritional secretions
antrum – fluid-filled pocket in center of follicle
mature follicle – blister-like swelling on surface of ovary
corona radiata – crown of granulosa cells around oocyte
2) secondary oocyte
oocyte in mature follicle undergoes meiosis I, forming 2 haploid cells
secondary oocyte – large cell; receives most of cytoplasm
first polar body – small packet of chromosomes with no
cytoplasm
ovulation – mature follicle ruptures and releases ovum and its corona
radiata
number of ova ovulated varies with species
6
uniparous species – one offspring at a time
multiparous species – multiple offspring at once (litters)
Fate of Ovum
1) sperm in uterine tube (within 24 hr of ovulation)
fertilization – sperm penetrates ovum
ovum completes meiosis II – unequal cytokinesis
ovum (haploid) – mature egg; large cell
second polar body – small, discarded cell
nuclei of sperm and ovum unite to form a zygote
2) no sperm in uterine tube
degeneration of ovum and end of estrus cycle
OVIDUCTS (uterine tubes, Fallopian tubes)
normal site of fertilization
convoluted tubes that carry sperm to ovum and zygote to uterus
Anatomy of Oviducts
infundibulum – funnel-shaped portion surrounding ovary
fimbriae – fringe on border; sweep oocyte into infundibulum
attached to dorsal body wall by mesosalpinx
Histology of Oviducts
ciliated columnar epithelium – sweeps oocyte through tube
secretions nourish ovum, sperm, and zygote
smooth muscle – peristalsis helps move ovum and zygote to uterus
UTERUS – nourishes and protects the fetus during gestation period
attached to dorsal body wall by mesometrium
Anatomy of Uterus – varies with species
body – caudomedial portion
cervix – caudal smooth muscle sphincter; normally closed
opens into vagina during estrus and parturition
uterine horns – 2 craniolateral arms
main sites of fetal development
Histology of Uterus
1) perimetrium (outer layer) – visceral peritoneum
2) myometrium (middle layer)
3 layers of smooth muscle
contracts during parturition
3) endometrium (inner layer) – supports developing fetus
columnar epithelium
highly vascular
endometrial glands – secrete mucus, nutrients, hormones
7
Types of Uteri
bicornuate – 2 long, narrow horns
species that have litters (eg. bitch, queen, sow)
bipartite – 2 short, thick horns
species that have 1 fetus or 1/horn (eg. mare, cow, ewe)
simplex – no horns
species that have 1 fetus (eg. primates)
duplex – 2 long horns, each with its own cervix
species that have large litters (eg. rabbit)
VAGINA
canal from cervix to vulva
site of copulation
birth canal
Histology of Vagina
stratified squamous epithelium
mucous glands lubricate during copulation
normal bacterial flora – create an acidic environment
inhibits other microbes (eg. yeast)
harmful to sperm – semen buffers acid
epithelium thickens and becomes keratinized (cornified) before copulation
cytology can be used to time breeding
VULVA – external genitals
labia – lips of vulva
vestibule – entrance to vagina
urethral orifice – urethra opens into floor of vestibule
clitorus – small erectile structure homologous to penis
located just caudal to urethral orifice
ESTROUS CYCLE – hormone cycle
causes periodic development of ovarian follicles
coordinates ovulation with development of the uterus
induces behavioral changes and sensory cues that coordinate copulation with
ovulation
polyestrous – cycle continuously (one cycle ends and the next begins)
non-seasonal polyestrous – cycle all year (eg. cow, sow)
seasonal polyestrous – only cycle in certain seasons
mare, queen – spring/summer
ewe – fall/winter
diestrous – 2 cycles/year (eg. bitch)
8
HORMONAL REGULATION
Hypothalamus – Gonadotropin Releasing Hormone (GnRH)
stimulates anterior pituitary
Anterior Pituitary
Follicle Stimulating Hormone (FSH)
stimulates growth of ovarian follicles
stimulates estrogen secretion by granulosa cells
Luteinizing Hormone (LH) – secreted as estrogen increases
stimulates follicle maturation
LH peak triggers ovulation
promotes differentiation of follicle after ovulation
corpus luteum – granulosa cells differentiate
secretes progestins, mainly progesterone
Functions of Estrogen
development of female reproductive tract in fetus
stimulates physical and behavioral changes associated with estrus
Functions of Progesterone – secreted by corpus luteum after ovulation
supports physical and behavioral changes associated with pregnancy
prevents further follicle development during pregnancy
PHASES OF ESTROUS CYCLE
1) Proestrus
follicles developing
growing follicles produce increasing estrogen levels
estrogen stimulates growth of oviducts, uterus, vagina, and vulva
2) Estrus (heat)
estrogen peak prepares animal for breeding
LH peak causes ovulation near end of estrus
physical and behavioral changes attract males
“standing heat” – female receptive to male
3) Metestrus
LH stimulates formation of corpus luteum
corpus luteum secretes progesterone
endometrium prepares for pregnancy
vascularity and glandular secretions increase
follicle growth inhibited
4) Diestrus – corpus luteum reaches full size
No Fertilization – corpus luteum degenerates to corpus albicans
progesterone levels decline
endometrium degenerates
new estrous cycle begins
or
Anestrus – hormone production shuts down and estrous cycles stop
temporarily
9
Fertilization
hormone produced by implantation of embryo prevents degeneration of
corpus luteum
progesterone secretion continues and maintains pregnancy
pseudopregnancy – prolonged diestrus in some animals (eg. bitch)
corpus luteum fails to degenerate after failure of fertilization
increasing progesterone level causes signs similar to pregnancy
FERTILIZATION
COPULATION – male ejaculates semen into vagina or uterus
oxytocin and prostaglandins in semen stimulate contractions of female
reproductive tract
pumps sperm up into uterine tubes
capacitation – male and female secretions activate sperm
sperm require time to prepare for fertilization
become more motile, prepare to release acrosome enzymes
FERTILIZATION – union of ovum and sperm
normally occurs in uterine tubes
ovum is viable for a limited time following ovulation
standing heat is timed to provide capacitated sperm in uterine tubes when
ovum arrives
sperm swim towards ovum
acrosomal enzymes dissolve corona radiata and penetrate ovum
male pronucleus – haploid
female pronucleus – haploid
pronuclei fuse to form diploid nucleus of zygote
FETAL DEVELOPMENT
zygote divides and differentiates to form a fetus
CLEAVAGE – rapid mitotic divisions of zygote without growth
occurs as zygote travels into the uterus
morula – ball of cells
monozygotic (identical) twins – cells separate into 2 separate embryos
[dizygotic (fraternal) twins – develop from 2 different zygotes]
blastocyst – fluid-filled ball of cells
blastocoel – fluid-filled cavity in center
trophoblast – wall of blastocyst
develops into fetal placenta
inner cell mass – cluster of cells near one wall
develops into embryo
10
IMPLANTATION – blastocyst attaches to uterine endometrium
ectopic pregnancy – abnormal site of implantation
PLACENTAL DEVELOPMENT
site of exchange with uterine endometrium
provides oxygen and nutrients
eliminates wastes
protects growing fetus – cushions, prevents dessication and adhesions
develops from 3 embryonic membranes
Amnion – fluid-filled sac surrounding embryo
Allantois – fluid-filled sac attached to urinary bladder
contains fluid formed by fetal kidneys
Chorion – outermost layer formed from the trophoblast
umbilical cord – vascular cord connecting fetus to chorion
Functions of Chorion:
protects embryo from maternal immune system
attaches to regions of uterine endometrium – forms functional
placenta (site of exchange)
diffuse placenta – loose attachment to entire uterine surface (horse, pig)
zonary placenta – attached to belt-like region encircling fetus (dog, cat)
discoid placenta – attached to a relatively small, circular region (primates, rabbit)
cotyledonary placenta – many snap-like attachments to uterus (ruminants)
placentome = fetal cotyledon + uterine caruncle
form tight attachments that sometimes fail to detach (retained placenta)
retained placentas occur less commonly in other types of placentas
maternal and fetal blood doesn’t mix – diffusion occurs across the chorion
diffusion of oxygen and nutrients to fetal blood
diffusion of wastes and carbon dioxide to maternal blood
FORMATION OF 3 PRIMARY GERM LAYERS
ectoderm – dorsal epithelial layer
forms nervous tissues and epidermal structures (skin, hair, etc.)
mesoderm – middle loose connective tissue layer
forms:
all connective tissues, including blood
all muscles
kidneys and reproductive organs
linings of body cavities
endoderm – ventral epithelial layer
forms linings of GI, respiratory, urinary tracts, digestive glands
11
PREGNANCY
gestation period – fertilization to parturition
varies widely between species
3 trimesters (3 months each in humans)
1st trimester – embryonic stage
rapid division and differentiation of all basic tissues and organ
systems
all embryos look very similar
2nd trimester – fetal development
growth and differentiation of systems into distinct species
3rd trimester – fetal growth
dramatic growth and maturation of most systems
PARTURITION – birth
progesterone (hormone of pregnancy) – supports endometrium and inhibits uterine
contractions
early pregnancy – corpus luteum produces progesterone
later in pregnancy – placenta produces progesterone
late pregnancy – progesterone level declines
fetus becomes stressed (limited space, uterine support no longer adequate?)
secretes stress hormones – glucocorticoids (cortisol)
stimulates uterine secretion of prostaglandin F2a
prostaglandins + decreased progesterone increase uterine receptors for oxytocin
oxytocin stimulates uterine contractions
Signs of Parturition
enlarged mammary glands (2-6 days prior)
may drip colostrum
mucous discharge from vulva
large animals – dropping of belly, relaxation of pelvis
dog – temperature drop of 1-2o F 24 prior
behavior changes – restlessness and discomfort
nesting behavior
large animals – kicking at belly
STAGES OF PARTURITION
1) dilation stage – stretching of cervix stimulates oxytocin release
positive feedback increases strength of contractions
dilates cervix
2) expulsion stage – delivery
powerful uterine contractions assisted by abdominal contractions
expel fetus
stress to neonate causes sympathetic responses
prepares infant for extrauterine life
12
3) placental stage
powerful contractions expel placenta (afterbirth)
dystocia – difficult parturition
small dam/large fetus (eg. bred too young)
abnormal presentation
Caesarian section – surgery to remove fetus from uterus
involution – uterus returns to original size (takes weeks)
lochia – discharge during sloughing of endometrium
MAMMARY GLANDS and LACTATION
specialized skin glands
location and number varies among species
thoracic - inguinal regions
2-4
lactation – secretion of milk beginning shortly before parturition and continuing
as long as young nurses
proper levels of many hormones required
FSH, LH, estrogen, progesterone, prolactin
GH, thyroid hormones, glucocorticoids, insulin
oxytocin to stimulate letdown of milk
Cow Udder
4 “quarters” (separate glands) in inguinal region
dairy cow – full udder can weigh over 100 pounds
suspensory ligaments support weight of mammary gland
medial ligament – tough but elastic
2 lateral ligaments – tough and inelastic
compound alveolar glands
sac-like glands with highly branched ducts
drain into large lactiferous ducts
lactiferous ducts drain into sinuses (cavities that store milk)
glandular sinus in quarter
teat sinus in teat (large papilla that young nurses on)
streak canal – single canal through teat to outside
sphincter muscle normally closes to prevent leaking
myoepithelial cells – smooth musle cells around alveoli and ducts
milk letdown – contraction expels milk into sinuses
stimulated by oxytocin in response to suckling
mammary glands are undeveloped until puberty
estrous cycles estrogen and progesterone stimulate hyperplasia of alveoli
and ducts
near parturition, hormone levels stimulate prolactin release, which
stimulates secretions
13
mastitis – infection of mammary gland
usually isolated to one quarter unless spread by poor hygiene
(milk infected quarter last)
colostrum – first milk
high levels of proteins, lipids, vitamins, antibodies
gives passive immunity to neonate
laxative effect – helps clear neonate GI tract
meconium – dark, sticky first feces
accumulation of dead, degenerate cells from GI tract
14
AVIAN REPRODUCTIVE SYSTEM
MALE
testes – in abdominal cavity near kidneys
enlarge during breeding season
seminal fluid – secreted by testes
seminal vesicle – sperm storage pouch at distal end of ductus deferens
site of sperm maturation – temperature is lower than body temperature
no penis in most species
sperm is passed by “cloacal kiss”
FEMALE
female gamete determines sex of offspring
only one side of reproductive system develops in most species (usually left)
left ovary – grape-like clusters of follicles
ovulation – one ovum at a time
ovum is attached to a yolk sac containing stored nutrients
oviduct
infundibulum – catches ovum
magnum – secretes albumin (egg white)
isthmus – secretes cell membranes
uterus – secretes shell and pigment
vagina – muscular tube, lays egg
clutch – a usual number of eggs are laid one at a time
incubation – parent holds eggs against body and or feet to maintain a specific
temperature range
EGG
albumin – protects embryo and stores water
yolk sac – vascular sac full of nutrients for embryo
amnion – similar to mammals
chorioallantois – chorion and allantois fuse and line inner shell
exchanges gas across the shell
allantois stores urinary waste
HATCHING
chick absorbs nutrients in yolk as it grows
fluid in egg decreases during incubation
air space in egg increases – chick begins breathing
eggshell thins during incubation – chick uses calcium from shell to grow
softens shell, making hatching easier
egg tooth – sharp point on upper beak used to crack through shell
15