AVIAN

REPRODUCTION
 For Males
 Testes of birds are located inside the
body cavity (mammals outside; enclosed
by scrotum)
 Vas deferens in birds opens to the
Cloacal wall (mammals opens into the
seminal vesicle)
 No penis, but has rudimentary organ of
copulation known as papillae (used in
sex determination of chicks)
 No intromission during copulation;
sperm deposited in female through
“Cloacal Kiss”
 For females
 Produces young that are not suckled or fed with milk
 Egg is laid outside the body and much larger than in mammals
 No well-defined estrous cycle or pregnancy/gestation period
 Has sperm-host glands in the oviduct that store and maintain
viability of sperm for longer periods of time
 Sperm stored in the oviduct are capable of fertilizing the eggs
for about 10 days in chicken and 30 days in turkey.
 Reasons why chickens may be mated once every about 10 days
(or 1 week) or once a month for turkey and still produce
fertile eggs.
 Testes
 Internally placed
paired (intra-
abdominal testis)
rather scrotal testes
 Located just anterior
to the kidneys and are
attached to the dorsal
body wall
 Left gonad larger than
the right
Testes
 Soft and lack the CT septa commonly found in mammals
 Composed of numerous ST with less Leydig cells
 Body temperature – 41-42oc
 No accessory sex glands
 Male Reproduction
 Copulatory organ – phallus

 Small and does not

function as intromittent
organ except in geese
and ducks
 Phallus of many birds
(roosters, tom) are
small and do not
function as intromittent
organs
 Male Reproduction
 Semen is transferred
to the female by
touching the
rudimentary phallus
to the everted vagina
 Ducks and geese have
sizable penises and
mating is
accomplished by
intromission
Male Reproduction
 Cloacal kiss – able to transfer sperm to the female by
touching the phallus to the everted vagina
 Sperm storage – in vivo that survives from 10 days to
months
 Site – vagina of hen
 When the hen is about to ovulate, the sperm from the
vagina migrates
 Accessory glands
 Relatively undeveloped; no seminal vesicle, bulbourethral
gland, or prostate glands
 Epididymal region consist of:
1. Efferent tubules that carry sperm from the testis to a
single epididymal duct
2. Vas deferens tube from epididymis terminating at a small
papilla in the cloaca
o Enlarge terminal portion of the vas deferens serves as
storage site for spermatozoa
o The papilla ejects the semen into the cloaca
 Accessory glands
 Cock sperm (unlike those in mammals) are functionally
mature before they leave the testes
 Vasa deferentia of the cock have the ability to preserve
sperm trapped in them
Endocrine Regulation of the Testis
 Either LH or FSH stimulates Testicular growth but have
different target cells
 LH acts on Leydig cells to promote their development and
testosterone (steroidogenesis) production
 FSH acts on Sertoli cells and seminiferous tubule
 Full testicular production is brought by the combined action of
FSH and testosterone
 Testosterone is the major feedback regulator of LH secretion
 Levels of FSH decline more slowly than those of LH and do so
as the testes are approaching full size.
Female Birds
 Do not demonstrate estrous cycle
 Pullet lays her first egg (mated or not; fertile or not)
when sexual maturity is reached at about 19 – 21 weeks
of age
 Laying continues on daily basis until Gonadotropins
decreases (depending on the species/type, season, day
length nutritional status, presence, presence of stress,
etc.) and egg laying stops
Female Birds
 Prolactin is then produced stimulating broodiness or
the mothering instinct, and the hen will incubate or “set”
on the eggs until they hatch
 Ovary of birds
 Only the left ovary and oviducts
are normally functional
 Right oviduct degenerates
 Left ovary is located beneath the
left kidney attached by a short
mesovarium
 Hawk, falcon and kiwi –
possess a persistent right ovary
and oviduct
 Oviduct
1. Infundibulum
(15minutes)
 Engulf ovum before
ovulation
 Site of fertilization
2. Magnum (3 hours)
 Secretion of albumin
3. Isthmus (1.5 hours)
 Secrete keratinous shell
membrane (outer and
inner)
 Oviduct
4. Uterus (20 hours)
 Addition of water (plumping)
 Shell production + pigments
 ~ 5 hrs – plumping
 ~ 25 hrs – shell formation
 Calcium carbonate
 Ca metabolism, estrogen influence (Ca binding protein in intestines
<absorption> mobilize Ca from bones and diet
 Quality of shell – influenced by Ca
 Soft-shell eggs – temperature increase
 Bird pants → expire co2 → need CaCo3 (from co2) to form hard
shell
 Oviduct
5. Vagina (1 minute)
 Sperm storage
 Egg transport
Follicular hierarchy
 Group of follicles. With most mature called the f1, the
next most f2, and so on down to about f5
 Each differs from the adjacent one by about 1 day’s
growth (ovulation interval of 1 day)
 There are also few rapidly developing follicles;
intermediate-size follicles and many microscopic
 Estimates of the total number of follicles about 2,500
seen with naked eyes and about 12,000
Follicular hierarchy
Follicular structure
 Immature avian follicle consists of an oocyte surrounded
by follicular cells and contains the nucleus, Cellular
organelles and a membrane
 Mature follicle has grown to gigantic proportions by the
addition of yolk material, which is surrounded by a
proteinaceous membrane (perivitelline or vitelline
membrane, secreted by the follicular cells)
 Nucleus of the ovum lies beneath the perivitelline layer
and above the surface of the yolk
 Follicular structure
 Surrounding the
perivitelline layer is a
layer of granulosa cells,
which have cytoplasmic
processes that penetrates
it and are in contact with
the underlying yolk
(cytoplasmic processes
that extends to the yolk
is absent in mammalian
granulosa cells)
 Follicular structure

 Granulosa layer is
surrounded by thecal layers,
which are well vascularized
and innervated and are
active in steriodogenesis
 Follicular structure
 Avascular band, the stigma, is present on each follicle, and it is
along this band that follicular rupture occurs when oocyte is
released
 No structure analogous to the CL of mammals forms in these
postovulatory follicles
Follicular growth
 Characteristic stages of the growing follicle depend on
the amount of yolk
 Immature follicles – white
 Growing follicles – yellowish due to yolk
 Yolk <lipid, water, proteins, vitamins and minerals>
 Yolk proteins, lipids: liver + estrogen
 Yellow color: xanthophyll pigments – diet
Follicular growth
➢ Phase 1
 Slow deposition of neutral fat around oocyte
 Color – still whitish
 Happens several months
➢ Phase 2
 Deposition of the yellow yolk
 Follicles increase in size – 1 mm to 6 mm in diameter
 Lasts for several weeks
Follicular growth
➢ Phase 3
 Rapid growth phase
 Main mass yolk is deposited to the growing follicles
 Yolk synthesized by the liver under the influence of
estrogen
 Yolk protein is released to the blood stream →
sequestered by the ovary → and deposited around
the growing follicle
Oogenesis
 Persist for 2 – 3 days thereafter
 As the ovary matures and individual follicles begin to
grow, the blastodisck (a small round white disc,
Containing the chromosomal material of the ovum) is
seen under the vitelline membrane
 Before ovulation, the oocytes completes the first meiotic
division, but meiosis is not completed until the
Blastodisck is penetrated by a spermatozoon
 1st reduction division occurs 2 hrs before ovulation of
the follicle
Oogenesis
 After fertilization, the blastodisck is termed a
blastoderm
 By the time the egg is laid, the blastoderm is an embryo
(can be identified with the naked eye when examined)
 Meiosis II happen after fertilization
Hormonal regulation of oogenesis
1. Follicle-stimulating hormone (FSH)
➢ Functions:
 Growth and maturation of follicles
o Promotes uptake of yolk proteins by ovarian
follicle
 Stimulate estrogen secretion (with LH)
 Hypothalamus
Stimulate GnRH neurons
Increase synthesis and release of GnRH

Via the HHPV

Anterior Pituitary Gland

Stimulates Gonadotropes
Increase secretion of FSH

FSH transported via the circulation

Ovary
Hormonal regulation of oogenesis
2. Estrogen
 Principal estrogen produced by avian  estrone,
17β-estradiol, 17α-estradiol
 Stimulates the production of precursor of yolk in the
liver
Follicles (F1 to F5)
 Granulosa cells – main source of progesterone
 synthesis bound to growing follicles
 Synergistic actions of progesterone and LH necessary for
ovulation
 Hormonal Regulation of
Ovulation
 Luteinizing Hormone (LH)
o Regulated by progesterone produced by ovary
o LH is released 6-8 hrs prior to ovulation
Movement of Ovum Through
the Oviduct
 Initially when the ovum is released, the infundibulum
encloses the ovum in wave-like advances which may take
between 13-25 minutes
 Contraction of muscles of upper part of oviduct
 Peristaltic activity of remaining parts of oviduct
 Ovulatory cycle
 Ovulatory cycle of the domestic hen is about 24 – 26 hours
long, and cycles may occur for many days without
interruption. Eggs laid day after day without interruption are
called a clutch
 When hens are young and laying at a very high rate, the
interval between ovulations and the time required to form an
egg may approximate 24 hours.
 Very long clutches are possible
 As hens age, the interovulatory intervals within a clutch
approach 25 to 26 hours and shorter clutches result
Ovulatory cycle
 Ovulation and oviposition are not absolutely related

 Ovulated ova not picked up by the oviduct may be

released into the abdomen and are re-absorbed and many
may accumulate, but eggs escaped from the oviduct after
the shell membrane or shell is formed tend to be
retained
Egg production
 Some flocks will begin to lay at about 22 weeks of age
and lay continuously for about a year
 Average production of 260 eggs per hen
 Other flocks will lay for about 45 weeks, be forced into a
period of reproductive inactivity, and then be stimulated
to lay for an additional 35 weeks.
 Average of about 500 eggs
Regulation of the Ovulatory Cycle
1. Luteinizing Hormones
 Key to hen’s ovulatory cycle
 Hormone directly responsible for the induction of
ovulation
 Blood levels increases some 4 to 8 hours before each
ovulation
 Increase occur only during the dark period; LH surge
decreases during the day
Regulation of the Ovulatory Cycle
2. Progesterone
 Involved in a positive feedback loop
 Stimulates LH release from the pituitary into the blood
 LH release in turn stimulates more progesterone
synthesis and release
Regulation of the Ovulatory Cycle
2. Progesterone
 Relationship between LH and progesterone
 In most mammals, elevated plasma levels of
progesterone during luteal phase or pregnancy block the
release of LH from the pituitary
 In the hen:
o High level of progesterone can block the LH release
o Low level of progesterone may initiate pre-ovulatory
LH; high levels may serve to terminate LH secretion
Regulation of the Ovulatory Cycle
3. Testosterone
 Plasma level rise just before or concurrent with the
rise in LH and progesterone
 Can stimulate LH release at certain levels and suppress
it at high levels
Regulation of the Ovulatory Cycle
4. Estradiol
 Plasma level rise precedes that of LH, progesterone,
and testosterone
 Pre-ovulatory rise is small and does not stimulate
ovulatory surge of LH as does the greater rise in many
mammals (does not play a role in the regulation of the
ovulatory cycle)
Regulation of the Ovulatory Cycle
5. Prolactin
 Level increases during the period in the ovulatory
cycle when LH, progesterone, testosterone and
estradiol levels are decreasing
 Serve as turn off mechanism for the pre-ovulatory
LH-steroid hormone cascade
Pattern of Different Hormones
Prior to Ovulation
 Peak levels – 4-6 hours; progesterone and LH
 Testosterone and estradiol – 2 hrs
 Peak prior to the peak of P4 and LH
 Estradiol – no specific time
 Ovulation to oviposition – 25 - 26 hours (egg formation)
Oviposition patterns
 The first egg of a sequence is generally laid early in the
morning, at just about the time the light come on.
 Subsequent eggs in the sequence are laid later and later
in the day until the last egg in a sequence are laid in the
afternoon.
 Chickens rarely lay in a dark part of the photoperiod
Oviposition patterns
 Simply a reflection of the pattern of release of LH → the
ovulation-inducing hormone.
 LH release occurs about 32 hours before oviposition of
the egg whose ovulation it induced.
 LH releases are restricted to the dark phase of
photoperiod.
Factors that Trigger Oviposition
 Involve the hormones produced by the posterior
pituitary gland
 Prostaglandin
 PGE1 is a potent inducer of Oviposition
 PGF2a can induce Oviposition but higher doses
 Factors from the ruptures ovarian follicle in Oviposition
 AVT is potent stimulator of uterine smooth muscle
contraction
EGG LAYING OR OVIPOSITION
 Occurs about 30-40 hrs after ovulation
 Before egg is expelled from oviduct it rotates 180○ so
that the blunt end is expelled first
 Species difference in oviposition time and patterns
 Chicken – mid-morning
 Turkeys – midday
 Quail – late afternoon to early morning
 Duck – midnight to early morning
 Control of Oviposition
A. Hormones
1. Arginine Vasotocin (AVT) or Vasotocin
 Produced by PVN and SON
 Stored in the posterior pituitary gland
 Stimulates uterine contraction
 Pharmacological drugs that can induce oviposition
by stimulating uterine contractions:
 Oxytocin
 Prostaglandin
 Ergotoxin
 Control of Oviposition
A. Hormones
2. Progesterone
 First factor from the hen
 Second from the ruptured follicle
 Control of Oviposition
B. Bearing down reflex
 Involvement of abdominal muscle contraction and
smooth muscle contraction of the uterus or shell gland
to propel the egg to the vagina
 Factors that affect oviposition:
1. Number of eggs laid in a given time
2. Sequence in which eggs are laid
3. Interval or breaks in the sequence
4. Natural or artificial incubation of the eggs
 Control of Oviposition
B. Bearing down reflex
 Factors that affect oviposition:
5. Lighting
 Natural or artificial light stimulate oviposition
 12 hours or longer light period is required to
obtain maximum lay in birds especially
domesticated spp.
 Red or orange light stimulates oviposition by
stimulating pituitary and gonads
 Broodiness (incubator behavior)
 Prolactin is responsible for
broodiness in hen
 Trait to incubate the egg until
ready for hatching
 Important for wild species of
birds
 Ovaries – regressing to stop
laying eggs
 In doves and pigeon – PRL is
important in the production of
“crop milk
MOLTING
 Loosing of feathers which occurs once a year for 4 to 6 weeks
if exposed to natural lighting to temporarily stop egg laying
 Can be artificially induced by:
a) Stress
b) Inhibit LH release
 Characteristics of a molting chicken
 Inhibition of ovulation
 Inhibition of egg laying or oviposition
 Regression of reproductive tract
 Decrease blood levels of LH, progesterone
FEMALE REPRODUCTIVE CYCLES
 Ovulatory cycle – 24-26 hrs
 Luteal phase – absent
 No formation of CL
 Clutch – 1-30 eggs or more
 Eggs are laid continuously
 Interrupted by anovulatory days
 Long clutch – interrupted by short ovulatory days;
common in young hens
 Shorter clutch – in aged hens
FEMALE REPRODUCTIVE CYCLES
 Young hens:
 Ovulatory cycle - ~ 24 hrs thus longer clutches

 Mature hens
 Ovulatory cycle - ~ 25-26 hrs thus shorter clutches
FEMALE REPRODUCTIVE CYCLES
 “Internal layers” – uncatched ovum which remains
inside the abdomen (broad)
 Similar to ectopic pregnancy
 Surge of LH prior to ovulation  dictates egg formation
 In chickens, LH surge is limited only to darker part of
days (7 pm to 5 am)
 4-8 hrs prior to ovulation there is LH surge
Anterior Pituitary Hormones
 Secretes same hormones as the mammalian pituitary
gland
 LH
 FSH
 Prolactin
 STH
 ACTH
 Show common properties with mammalian hormones
but also have distinct differences in the their biological
activities and chemical properties
Anterior Pituitary Hormones
 Hypothalamic GnRH regulates the release of avian
gonadotropins (LH and FSH) from the anterior Pituitary
➢ cGnRH (Chicken GnRH)  stimulate secretion of
anterior pituitary gland
 Two active cGnRH isolate from chicken’s
hypothalamus
o GnRH 1 → has a glutamine residue rather than
arginine found in mammalian GnRH
o GnRH II → differs in 3 of the 10 amino acids
residues.
Anterior Pituitary Hormones
 Prolactin release is under stimulatory control of the
hypothalamus rather than the chronic inhibition
characteristic of mammals
 A number of peptide regulates its release:
1. Vasoactive intestinal polypeptide
 Prolactin releaser responsible for the
hyperprolactinemia associated with incubation
2. Thyrotropin releasing hormone
3. Arginine vasotocin
4. Neurotensin
Anterior Pituitary Hormones
 Neurotransmitters evoke the release of prolactin from the
avian pituitary
 Serotonin appears to be the most important,
 Dopamine plays a minor role in prolactin release in
birds and its effect is inhibitory as in mammals
Posterior Pituitary Hormones
 Neurohypophyseal hormones of birds have Antidiuretic,
oxytocic, and vasoactive actions

 Hormones have common composition as in mammals but

differ in amino acid composition
Posterior Pituitary Hormones
 Active principles released are:
1. Arginine vasotocin (AVT)
 Normal Antidiuretic hormone in birds and more
active in inducing uterine contraction
2. Mesotocin (MT)
 Oxytocin effect
 Vasoconstrictor effect
 Antidiuretic effect
 Gonadal Hormones
 Avian gonads produce steroid hormones that act though the
body
 Affecting development of the reproductive duct system
 Head furnishing such as comb and wattles
 Feathers, voice
 Nutrient absorption
 Behavior
 Predominant testicular steroid is testosterone
 Ovary secretes: estrogen, progesterone, and
testosterone
 Gonadal Hormones
 Estrogen is responsible for:
a) Enlargement of the oviduct to functioning size
b) Spread of the pubic bones and enlargement of the
vent
c) Mobilization of depot fat for yolk formation and of
calcium for shell formation
d) Controls the secondary sex characteristics such as
normal female plumage, sexual behavior
 Thyroid hormones
 Thyroxine
 Influence body growth, feather color and formation
 Secretion is controlled by TSH
 Partially responsible for seasonal changes in egg laying,
body weight and egg weight
 Thyroid gland increases in size and hormone during
molting
 Parathyroid hormone
 Parathormone → partially controls formation of the
egg shell
 PHOTOPERIODISM
 Reproductive activity in many avian species is controlled
by environmental stimuli that synchronize breeding
seasons with optimum time of the year
 Day length regulates breeding seasons (dictated mostly
by length of light period)
 Sexual activities increase with longer days and decreases
with shorter days
 Increasing light period – hastening the onset of
maturation of avian reproductive system photosensitivity
 PHOTOPERIODISM
 Length of photoperiod that stimulates the gonads under
natural conditions differs among species
 Light period of approximately 12 hours out of each 24
hours stimulates the gonads of many species
 Light causes the hypothalamus to release increased
amount of GnRH thus increasing the levels of FSH and
LH
 PHOTOPERIODISM
 Opsin
 Non-retinal photoreceptors regulating the photoseuxal
response are located in discrete areas of the
hypothalamus – GnRH neurons which contain the
visual pigment (opsin) has been identified in close
proximity to GnRH secreting cell bodies
 Hen never exhibit estrus and are never pregnant and no
corpus luteum