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REPRODUCTIVE PHYSIOLOGY (CUNNINGHAM) (PART 5) The Mammary Gland (III)…
REPRODUCTIVE PHYSIOLOGY (CUNNINGHAM)
PART 1) Control of Gonadal and Gamete Development
II) Hypothalamopituitary control of reproduction
a) Hypothalamus and Anterior Pituitary (Adenohypophysis) Secrete
Protein
and
Peptide
Hormones = Control Gonadal Activity
gonadal activity is under the control of
hypothalamus
and the
anterior pituitary gland
The pituitary responds to the hypothalamic peptides - produce hormones (control of the gonads)
b) Adenohypophysis (pars distalis)
produces follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin
-they control reproductive processes.
3 parts of pituitary gland
ANTERIOR LOBE (adenohypophysis, or pars distalis)
Produces 2 gonadotrophins
LH
final stages of follicle maturation through ovulation.
FSH
role during growth of follicles
produce PROLACTIN
Other pituitary hormones
growth hormone (GH)
corticotropin (adrenocorticotropic hormone, ACTH)
thyroidstimulating hormone (TSH).
INTERMEDIATE LOBE (pars intermedia)
POSTERIOR LOBE (
neurohypophysis/ pars nervosa
)
release oxytocin
hypothalamus has direct connection with
neurohypophysis
(thru neural stalk - located @ hypothalamus)
supraoptic
and
paraventricular nuclei
,
responsible for the synthesis of vasopressin and oxytocin, respectively
coupled to larger peptide molecules, called
neurophysins
are transported from the site of synthesis in the hypothalamus (neuronal cell bodies) through axons to the site of storage and eventual release, the neurohypophysis.
does not involve the direct passage of axons through the neural stalk.
venous portal system connects the median eminence (rise/elevation) within the hypothalamus to the adenohypophysis.
GnRHassociated peptide (GAP).
can stimulate the release of FSH and LH,
critical hormone for
gonadotropin
release.
ability to inhibit prolactin secretion.
III) Modification of gonadotropin release
a) Pulsatile Release of GnRH
Induces the Critical Pulsatile Production of the Gonadotropins, FSH and LH
Pulsatile: main secretory pattern of gonadotropin
b) Gonadotropin Release
-Modulated by the
Process of Negative Feedback
from
Estrogen
and
Progesterone
importance is shown if GnRH is administered in a continuous (pharmacological) manner, the system can be
downregulated
continuous occupancy of GnRH interrupts the intracellular signal for the synthesis and release of gonadotropins
pulse generator system
increased in the follicular phase
decreased in the luteal phase of the estrous cycle
Estrogen
decreases the pulse amplitude
Progesterone
decreases the pulse frequency of gonadotropin secretion
important for nurturing the final growth phase of the developing antral follicle
Follicular phase
pulse frequency increases because of the absence of progesterone
pulse amplitude decreases because of the presence of estrogen.
Positive Feedback:
sustained increase in estrogen secretion by increased secretion of gonadotropins (by hypothalamus and adenohypophysis)
causes an increase in gonadotropin secretion by increasing the frequency of pulsatile release of GnRH and, as a result, gonadotropin secretion.
purpose of the gonadotropin surge is to induce changes within the follicle that lead to its rupture (ovulation).
positive-feedback stimulation of gonadotropin release
= preoptic anterior hypothalamic region.
negative-feedback inhibition of gonadotropin secretion
caused by estrogen- characterised by sensitivity (effective at low concentrations) and its rapid onset (within a few hours).
hypothalamic site for negative-feedback inhibition of gonadotropins
= arcuate nucleues
depression of FSH secretion
Control the number of follicles that are brought to final maturation
Control of gonadotropin secretion in the male
pulses of GnRH, ( @ hypothalamus) affect
the pulsatile secretion of the gonadotropins
-cause the
secretion of testosterone
, also in pulsatile form, from the testes.
MAJOR DIFFERENCE
positive-feedback release of gonadotropins
in males
does not exist
negates any need for a surge release of gonadotropins,
PROLACTIN
the control of secretion has more emphasis on inhibition than does stimulation of secretion.
prolactin secretion increases if the pituitary gland is disconnected from the hypothalamus by either cutting the pituitary stalk or transplanting the pituitary gland to another site
catecholamine dopamine,
which is produced by neurons in the ventral hypothalamus (arcuate nucleus), is a
potent inhibitor
Photo below: negative-feedback control of prolactin
Other inhibitors:
γ-aminobutyric acid (GABA) and GAP
thyrotropin-releasing hormone (TRH
) - prolactin-releasing factor
Estrogens
can i
ncrease prolactin secretion
by
lactotropes
by decreasing lactotrope sensitivity to dopamine and increasing the number of TRH receptors
I) Development of the reproductive system
a)
Organisation of Gonads under genetic Control (genetic sexual differentiation)
Development of Embryonic Ovary: Migration of GERM CELLS into gential ridge from yolk sac
Primodial germ cells populate sex cords:
coelomic epithelium
(germinal epithelium) of genital ridge
sex cords contribute cells
follicle cells --> granulosa cells (immediately surround oocyte)
Mesenchyme become
theca
Follicle: oocyte, granulosa, and theca cells.
No direct connection formed between oocytes and tubes that become oviducts (mullerian ducts)
oocytes released by
rupture of tissue elements that surround ovary
(
ovulation
)
Fimbria
: enable oocyte to be removed efficiently from surface of ovary`
some animals: oocytes funneled to fimbria through
bursa
Development of embryonic testis (similar to ovary)
Sertoli Cells:
male counterpart of granulosa cells - dev. sex cords
Leydig cells
: male counterpart of thecal cells - dev. from mesenchyme of genital ridge
fundamental diff.: invagination of the sex cords in the male continues into the medulla of the embryonic gonad, where connections are made with medullary cords from the mesonephros (primitive kidney)
wolffian duct:
epididymis, vas deferens, and urethra, which has a direct connection to the seminiferous tubules
male germ cells pass to exterios of animal through
closed tubular system
b) Sexual Orientation of the Genitalia and Brain Depends on the
Presence or Absence of Testosterone
if
female (ovary)
:
mullerian duct
develops to oviduct, uterus,cervix. vagina
external genitalia: folds of tissue called labia form the vulva, and a clitoris develops
if
male
:
rete testis
produces
müllerian-inhibiting factor,
-- causes regression of the müllerian ducts.
The
wolffian duct
is maintained in the male because of the influence of androgens produced by the testis.
External genetalia:
androgens from the testis
direct formation of the
penis
and the scrotum (the absence or presence of androgens is an important factor)
-
müllerian ducts
are permanent structures
-
wolffian ducts
are temporary structures
unless acted on by the presence of male hormones
Enzyme,
5α-reductase
,
important for the effect of the androgens
- testosterone must be converted intracellularly into
dihydrotestosterone
for
masculinization of the tissues
to occur.
Sexual differentiation of hypothalamus:
hypothalamus exposed to androgens at about the time of birth causes the hypothalamus to be organized as
male
.
no androgens: hypothalamus organized as
female
IV) Ovarian follicle development
a) Gamete development
occurs initially without gonadotropin support and with pulsatile gonadotropin secretion.
mitotic division
influence of
meiosis-initiating factor
, thought to be produced by the
rete ovarii.
Oocytes remain in meiosis I until the follicle begins its final development, an interval that can be as long as 50 years or more in humans.
reduction of chromosome to haploid in meiosis
External factors like gonadotropins, are not required because
preantral follicles
can develop in hypophysectomized (Removal of pituitary gland) animals.
In cattle and horses (perhaps sheep and goats as well)
several dominant follicles develop during the estrous cycle, likely that a few follicles begin to develop each day.
In animals which cohort of follicles develops synchronously (pigs, cats, dogs)
appears to be l
ess tendency
to have competing follicle growth waves during the luteal phase (pig)
and a tendency to have only one cohort of follicles during the preovulatory period (cat and dog)
the development of a cohort of follicles may limit follicle development from the primordial state, at least during the period of active follicle development leading to ovulation.
Initial follicle growth is
under genetic control,
and the pattern reflects the
needs of the particular species.
b) In the
preantral follicle
gonadotropin receptors for
LH
develop on the theca --> results in androgen synthesis
FSH
directs the granulosa to transform the
androgens to estrogens.
Formation of antral follicle: appearance of
fluid
that begins to divide the granulosa.
follicular fluid
: a secretory product of the granulosa, coalesces to form a larger fluid cavity (antrum) within the granulosa.
In later development: the oocyte remains surrounded by a layer of granulosa cells called the
cumulus oophorus
Proximity of the granulosa and theca cells allows
cooperative estrogen synthesis
theca
produces
androgens
(testosterone and androstenedione)-precursors of estrogen biosynthesis, under the
influence of LH
androgens
are transformed into
estrogen
(estradiol-17)
granulosa
is incapable of forming androgens and the
theca
has limited capacity for producing estrogens
two-cell mechanism
(cooperative effort) for estrogen secretion
this is the way most follicular estrogen are produced
Estrogens have a positive-feedback effect on the granulosa
stimulate the cells to undergo mitotic division
the follicle grows in size as the granulosa proliferates in response to its own secretory product (estrogen).
Effect of Estrogen
formation of additional receptors for FSH
the antral follicle becomes sensitive to FSH and is able to gorw under steady state of FSH secretion
c) Late in the ovarian follicular phase: LH receptors develop on the granulosa, permits the preovulatory surge of LH to cause ovulation.
FSH and estrogens start formation of LH receptors on the granulosa
FSH receptors begin to
diminish
Antral follicle increases secretion of estrogen
results in the initiation of the
preovulatory surge of gonadotropins.
last stages of development
follicle falls under control of LH, makes its last growth spurt to the point of ovulation.
PART 2) Control of Ovulation and the Corpus Luteum
II) corpus luteum
(CL)
a) CL
secretes progesterone
: essential for pregnancy.
Secretion of Progesterone
prepares the uterus for the initiation and maintenance of pregnancy
forms from the wall of the follicle, which is collapsed/ruptured and folded after ovulation.
Follicle ruptured: breakdown of the tissues that surround the granulosa (membrana propria)
hemorrhage into the cavity can occur from vessels in the theca
Although the granulosa cell is the dominant cell of the CL, theca cells also contribute significantly to the composition of the structure
Luteinization
: process of granulosa cells undergo during change from estrogen to progesterone secretion
begins with start of preovulatory LH surge and accelerates with ovulation
Progesterone produced by CL withing 24hrs of ovulation.
dog & primates: small amounts of progesterone produced @ preovulatory LH surge
sexual receptivity: estrogen levels decline while progesterone levels increase
b) Luteinizing hormone is important for the maintenance of CL
Domestic animals: LH is the important
Luteotrophin
CL maintained in nonpregnant/ pregnant animals by slow pulsatile pattern of LH release
Rodents: Prolactin is the important
Luteotrophin
Prolactin has been implicated as luteotropin in
sheep
and
dogs
Folliculogenesis (maturation of ovarian follicle) sets the stage for subsequent dev. of postovulatory CL
c) Regression of CL in nonpregnant large domestic animals is controlled by
uterine secretion of prostaglandin F2α
Important in non pregnant animals so that animals reenter a potentially fertile state as soon as possible
CL life span after ovulation must have sufficient duration
allow newly developing conceptus to synthesize and release factors allowing CL to be maintained
short enough that nonpregnant animals can return to potentially fertile state
Uterus
is responsible for
control of the duration of the life span of the CL
in large domestic species (& guinea pigs)
PGF2α ( a 20-carbon unsaturated FA), is the uterine substance that causes regression of the CL in large domestic animals
it has no known natural role in CL regression in cats and dogs or in primates
In large domestic species, regression of the CL is initiated by uterine synthesis and release of PGF2α @ 14 days post-ovulation
mode of transfer of PGF2α from uterus to ovary
local countercurrent transfer
involves the movement of molecules across the blood vascular
system from higher concentrations in the venous effluent (uteroovarian vein) to an area of lower concentration (ovarian artery)
General systemic transfer
involves passage of the molecules through the general circulatory system.
In cow and ewe: PGF2α
synthesis from uterine horn only influences life span of CL in
ipsilateral ovary
In sow and mare: PGF2α
synthesis from 1 horn is suffiecient for regression of CL in both ovaries
due to greater production of PGF2α
by uterine tissue
PGF2α is rapidly
metabolized systemically, with more than 90% changed by one passage through the lungs.
system involcing this as luteolytic agent in large domestic species requires PGF2α to be conserved thru special transfer system/ produced in large amounts
PGF2α synthesis and release is pulsatile
uterus must be exposed to estrogen and progesterone to synthesize and release PGF2α.
Estrogen from antral follicle cause initial synthesis & release of PGF2α.
PGF2α affects CL cause reduction in progesterone production and release of luteal oxytocin
oxytocin interact with receoptor within uterus0 initiate another round of PGF2α systhesis
d) Changes in luteal life span in large domestic animals occur
due to changes in prostaglandin F2α synthesis by the uterus.
the presence of an embryo
will block PGF2α synthesis & continuance in luteal activity.
non-preg large domestic animals
inflammatory responses of endometrium caused by bacterial infection results in synthesis and release of PGF2α leading to
premature luteolysis & shortened estrous cycle
Luteal activity is almost always normal in absence of uterine abnormality in large species
.
short estrous cycles in large species signal uterine infection pathologically.
I) OVULATION
a) Ovulatory follicles are selected at the onset of
luteolysis
(in large domestic animals).
Luteolysis- luteal regression; the structural and functional degradation of the corpus luteum (CL)
In cattle, several
dominant (large) antral follicles
to develop sequentially during the cycle
the cycles are distinct that
follicle regression
(luteolysis) begins (as indicated by follicle size) before the onset of the growth of the next follicle
Whether the second dominant follicle is the ovulatory follicle or if a 3rd one develops:
it depends on the stage of the follicle at the time of regression of the corpus luteum (CL).
If the 2nd dominant follicle regress at the time of CL regression, a third follicle develops
final growth phase of follicle development can be initiated during the
luteal phase
Luteal phase initiated under influence of a relatively slow pulse rate of gonadotropin release
Rapidly growing follicle requires exposure to a faster
gonadotropin pulse rate
by the 3rd, or 4th day so that the follicle(s) can complete the normal growth pattern through ovulation
Follicle development is dynamic once the rapid growth phase is achieved; the follicle(s) must be acted on through proper gonadotropin stimulation within a few days; if not the
follicle will die
- atressia (regression) of follicle
Follicles that regress are attacked by inflammatory cells
the empty space is then filled with connective tissue which is an ovarian scar
Ways
dominant follicle
maintains status
produce substances that
inhibit dev
of other antral follicles
Inhibin
: peptide hormone produced by granulosa
-inhibits secretion of FSH
b) Ovulation is caused by an
estrogen-induced preovulatory
(occurring during period immediately preceding ovulation) surge of gonadotropins.
preovulatory surge of LH
begins about 24 hours before ovulation in most domestic species
initiate critical changes in follicle and
will release oocyte
Fx of preovulatory surge release of LH:
cause granulosa to produce
relaxin
and
prostaglandin F2α
(PGF2α), that will affect the continuity of the CT of the thecal layers of the follicle
disrupt the theca through the
development of vesicles
(within fibrocytes) that contain
hydrolytic enzymes capable of breaking down the collagen matrix of CT.
Oocyte and the granulosa, have been kept under control by the
production of inhibitory substances
oocyte-inhibiting factor:
prevents the oocyte from resuming meiosis,
luteinizing-inhibiting factor
: prevents the granulosa from prematurely being changed into luteal tissue
LH surge
block production of both factors mentioned
Effect of LH surge on Granulosa: allow starting of
luteinization
- transforms the cells from estrogen to progesterone secretion.
estrogen
secretion
declines
simultaneously with the onset of progesterone secretion.
Summary: estrogen is used by the follicle(s)
to stimulate the growth and development of the granulosa
to signal the hypothalamus and anterior pituitary as to the readiness of the follicle(s) for ovulation.
III) ovarian cycles
a) In ovulating animals,
ovarian cycles
have two phases (ovulation separating the phases):
follicular
luteal
**animals that require copulation for ovulation can have only a follicular phase (induced ovulators)
Ovarian Cycle (in non-preg animal):
interval between successive ovulations composed of 2 phases mentioned above
Spontaneous ovulators
: ovulatory process governed by
internal mechanism
s;
estrogen
from the antral follicle initiates the
ovulatory release of gonadotropins.
Differences among animals in their relationship of the 2 cycles
Higher primates:
complete separation of follicular and luteal phases, with no significant follicle growth occurring until luteolysis is complete.
Large domestic animals:
ignificant follicle growth does occur during the luteal phase of the cycle.
much of the follicle growth is centered @ the luteal phase. Results in shorter cycles in large domestic animals compared to primates
induced ovulators
: animals that require copulation for ovulation
cats, rabbits, ferrets, mink, camels, llamas, and alpacas
Copulation replaces estrogen
as the
stimulus
that induces the ovulatory release of gonadotropins
require exposure to
elevated estrogen concentrations
before they can respond to copulation by the release of gonadotropins.
Have follicle growth patterns (in the absence
of coitus) in which groups of follicles develop, are maintained in a mature state for a few days, and then regress
Follicle growth patterns can be distinctly separated
Follicle waves can also have some overlap
b)
Luteal phase is modified by copulation
in some species
Rodents: luteal phase of ovarian cycle extended via copulation
PSEUDOPREGNANCY
: Copulation
initiates the release of prolactin
, which results in
prolongation of luteal activity
( up to 10 or 11 days) in the absence of pregnancy
Canine: spontaneous regression of the CL marking the end of diestrus occurs with increased levels of prolactin,
causing pseudopregnancy
queen can exhibit pseudopregnancy if copulation occurred with an i
nfertile tom
PART 3) Reproductive Cycles
I) Reproductive cycles
a) The two types of reproductive cycles are
estrual
and
menstrual
terminologies in order to use external characteristics for accurately identifying a particular stage of the reproductive cycle and, most importantly, relating it to the time of ovulation
First day of both cycles begins shortly after end of luteal phase
Anestrous period separates diestrus and proestrus
Time of ovulation differs for both cycles
Primates: luteal and follicular phases separate
Domestic animals: follicular phase overlap with luteal phase (vulation occurs relatively earlier in the estrous cycle)
Ovulation is easier to predict in domestic animals (versus primates) because
estrus is usually tightly coupled to the preovulatory release of gonadotropins and ovulation
follicular development in primates can be delayed for a variety of reasons (e.g., stress), making the time of ovulation less predictable for primates
Domestic animals:
limited periods of estrus (sexual receptivity)- term
estrus
cycle is used
Primates
: sexually receptive during most of reproductive cycle so
menstrual
cycle is used (with onset of
menstruation
(vaginal discharge of blood-tinged fluids and tissues) as start of cycle
Estrous cycle divided into stages that represent behavioral/gonadal events (guinea pig/rat/mouse)
Proestrus
- Period of follicle development, occurring subsequent to luteal regression and ending at estrus
Estrus
- period of sexual receptivity
Diestrus
- period of mature phase of CL
Terminologies for domestic animals
Behavioral activity
: indicating whether animals are in estrus (sexually receptive) or not, including the stages of proestrus, metestrus, and diestrus
gonadal activity
: if differentiation of follicles and the CL is possible. Animals can be in the follicular phase (proestrus and estrus) or the luteal phase (metestrus and diestrus).
Equine: classified by sexual behavior, estrus or nonestrus
IV) External factors controlling reproductive cycles
Photoperiod, lactation, nutrition, and animal interaction are important
factors
that
affect reproduction
.
PHOTOPERIOD
Controls occurrence of reproductive cycles in a number of domestic species,
including cats, goats, horses, and sheep.
annual period in which they have continuous (cyclical) ovarian activity,
And a period of no ovarian activity, termed
anestrus
.
Response to photoperiod differs among species
cats and horses are positively affected by increasing light,
goats and sheep are positively affected by decreasing photoperiod
Main translator of photoperiod is
pineal gland
-
produces melatonin in response to darkness
CNS pathway involved with the translation of light includes the retina, suprachiasmatic nucleus, superior cervical ganglion, and pineal gland.
melatonin
has been previously described as
antigonadal
, this is
not true
because both short and long phases of darkness, with resultant short and long duration of melatonin secretion, can have a positive effect on reproductive cycles.
suppressive effects of photoperiod can be
overcome
by
exposure to artificial lighting regimens
BUT it's
not possible
to place sheep and goats in light-tight
barns to increase their exposure to dark
oral or systemic (implant) administration of melatonin to sheep
LACTATION
Lactation can have suppressive effects on ovarian activity
importance of suckling
with its related stimulation of prolactin synthesis
sensory input from suckling suppresses the production of these prolactin-inhibiting factors
Reduced output of dopamine and GnRH-associated peptide results in reduced ovarian activity through decreased gonadotropin synthesis and release
Inhibiting factors for prolactin synthesis, including dopamine and the GnRH-associated peptide,
need to be suppressed
in order for prolactin synthesis to proceed
PHEROMONES
: chemical compounds that allow communication among animals through the olfactory system
Sex Pheromones- when sexual behavior is affected
Sources of pheromones:
sebaceous glands
the reproductive tract
the urinary tract.
Whitten effect:
synchronization of estrus in female mice through the sudden introduction of a male
effect of the pheromones in this case is to stimulate the synthesis and release of gonadotropins
Whitten effect has been used to manipulate the estrous
cycles of animals (like onset of puberty in pigs)
Bruce effect:
blockage of pregnancy development by the i
ntroduction of a different (strange) male
in proximity to a recently bred female.
Important for the attraction of the male to the
female at the time of sexual receptivity
Sexual attractiveness of the female is from the pheromones that she elicits on a limited, cyclical basis in association with estrus
Females are also influenced by male odors
Androgens can serve as pheromones/ they can influence the production of substances within the kidney that influence female sexual behavior.
Attractiveness of the female to the male involves a change in perception of the male by the female resulting from a changing physiological state within the female, not because of changes that are occurring in the male.
b) Inadequate nutrition results in ovarian inactivity, especially in cattle
Animals must have an adequate level of nutrition to initiate ovarian activity, so ovarian activity is suppressed until a positive energy balance is established.
If pregnant beef cows are
not returned to a positive nutritional balance by the last month of gestation,
the reestablishment of ovarian cyclicity, which usually occurs between days 45 and 60 postpartum,
will be delayed
II) Puberty and Reproductive senescence (deterioration with age)
a) Puberty is the time when animals initially release mature germ cells.
puberty is used to define the onset of reproductive life
The precise definition of puberty: is the time of
first ovulation
For all species: critical requirement for the attainment of a certain size in order for puberty to be initiated
If it is not met because of
inadequate nutrition, puberty is delayed
Physiological mechanisms for control of puberty best known in
SHEEP
fundamental concepts to start puberty: an increase in the synthesis and release of GnRH from the hypothalamus, which drives gonadotropin secretion (in pulsatile form) and follicle growth
Before puberty, GnRH and gonadotropin secretion are kept in check because the hypothalamus is
highly sensitive to negative-feedback inhibition by estrogens
.
PHOTOPERIOD
the response of an organism to seasonal changes in day length
Important for allowing lambs to enter puberty
exposure to long photoperiod during their prepubertal development
Termination of long photoperiod- allows the
sensitivity of the hypothalamus to decrease in response to negative estrogen feedback.
With appropriate growth and photoperiod exposure, the secretion of gonadotropins in lambs causes significant follicle growth.
1st endocrine event of puberty in the ewe lamb
appearance of a preovulatory-type surge of gonadotropins, probably induced by estrogens produced by developing follicles
Photoperiod can have a suppressive effect on the
timing of
puberty in animals whose ovarian cyclicity is controlled by light.
For Males: occurs during period of increasing/ long photoperiod, compared to ewe lamb.
Spermatogenesis
: begins at this time, but because of the length of the process, lambs are usually not capable of successful breeding until about 30 weeks of age or more, or
in concert with the onset of puberty in ewe lambs
.
puberty is a relatively gradual phenomenon in male sheep compared with the abrupt process in females.
b) Reproductive Senescence (deterioration with age) in Primates Occurs
Because of Ovarian Inadequacy
, Not Inadequacy of Gonadotropin Secretion
Menopause
end of ovarian activity
Due to depletion of oocytes- occurred throughout the reproductive life of the individual
in essence, it represents
ovarian failure
initiation of menopause: cyclical irregularity caused by failure of follicle development and ovulation.
Gonadotropin secretion can be increased, or can be normal, because of the lack of estrogen and therefore lack of negative feedback on gonadotropin secretion.
Ovarian follicle activity ceases, estrogen concentrations decline, and in the
absence
of negative-feedback inhibition, gonadotropin concentrations
increase
dramatically.
Reproductive senescence is not recognized in domestic animals
some domestic species have lives that are shortened for economic or humane reasons
III) Sexual Behavior
a)
Sexual receptivity
interaction of the hormones estrogen and progesterone, via GnRH (female) & testosterone (male)
sexual behavior depends on exposure, or lack thereof, of the
hypothalamus to testosterone
during the
early neonatal period
Testosterone
causes
masculinization
of the sexual centers in the hypothalamus;
absence
of it makes hypothalamus
feminized
.
Testosterone
: important for libido in female primates
theca layer from
degenerating follicles
- secretes the androgens
androstenedione
and
testosterone
Androgens
: essential for the maintenance of libido in
males
medial preoptic area
(in rat) : area that is
modified structurally by exposure to testosterone.
Principles regarding effects of hormones on
sexual behavior of domestic animals
i) magnitude of change in hormone concentration that affects sexual behavior is small
ii) synergism between hormones is often important for sexual receptiveness;
iii) sequence of exposure to hormones can be important
Estrogen
: required for sexual receptivity in all domestic animals.
Progesterone
: also important for estrus in some animals.
sheep and goats:
estrus occurs in response to estrogen
only
if the animal has been exposed previously to progesterone
Domestic species
: example- dogs are different in that sexual receptivity is keyed by progesterone
Prior exposure to
estrogen
makes the
female
attractive to males
but does not produce sexual receptivity
;
estrus requires the additional exposure to progesterone.
Other domestic species (i.e., cats, goats, horses) all
show estrus with the first ovulation
of the season with no requirement for progesterone priming.
onset of sexual receptivity in animals relates to the onset of the
preovulatory gonadotropin surge
increased GnRH secretory activity
affects sexual centers within the hypothalamus
for the promotion of sexual receptivity
PART 4) Pregnancy and Parturition
I) PREGNANCY
A) The development of an embryo involves fusion of an oocyte and spermatozoon within the oviduct.
spermatozoa are in a state of reduced metabolism.
Mature sperm are only able to metabolize a special sugar, fructose, within the reproductive tract.
movement of sperm through the cervix is aided by estrogen-induced changes in cervical mucus
thinning of mucus occurs just before ovulation, a factor that can be used to predict the time of ovulation.
Special reservoirs have evolved in the female tract to aid in the survival of sperm during transport; these include the cervix and oviduct, the latter involving areas at the uterotubule junction and within the ampulla
reservoir within the ampulla is able to release a few sperm on a continuous basis, so that fertilization can occur shortly after the arrival of oocytes within the oviduct
Capacitation
: removal of glycoproteins from the sperm cell surface (prereq. for fertilization)
acrosome reaction
: release of hydrolytic enzymes from the acrosomal cap- for penetration of the sperm through the granulosa and zona pellucida
acrosome reaction also changes the surface of the sperm, which allows it to fuse with the oocyte
prerequisite for fertilization of the oocyte- must undergo the first meiotic division before fertilization
in the horse and dog the first meiotic division does not occur until after ovulation (
embryo usually develops
to the morula, or early blastocyst stage, within the oviduct before moving into the uterus.
Allows the endometrial glands time to secrete nutrients under the influence of progesterone from the developing CL: the nutrients are essential for the development of embryos during their pre-implantation stage.
B) Extension of the life span of CL in large domestic species and cats is
essential for pregnancy maintenance.
embryo produces substances that modify uterine production of PGF2α
Estrogen synthesis
- a way the endometrium is informed regarding the presence of an embryo
Trophoblastin: has a close structural relationship to the molecule interferon.
absence of pulsatile secretion of PGF2α:
critical for the extension of the life span of the CL and the establishment of pregnancy in large domestic species.
interstitial implantation is essential to the development of pregnancy in primates
Domestic animals depend more on uterine secretions for the support of pregnancy than do primates.
The cervix forms an important barrier in both the non-pregnant and the pregnant animal (avoid contamination of uterine lumen)
C) The Placenta Acts as an Endocrine Organ
Fx of placenta: production of progesterone
In primates this function is established early in gestation
occurs later in domestic animals
production of estrogen, in contrast to that of progesterone,
requires interaction between the fetus and the placenta
a system has evolved in which the placenta supplies
pregnenolone
, the immediate precursor of progesterone, to the fetus, and the fetal zone of the adrenal cortex t
ransforms pregnenolone to a C-19 androgen, dehydroepiandrosterone
.
This is returned to the placenta, which is able to convert dehydroepiandrosterone to an estrogen
Placental Lactogen:
have both somatotropic and lactogenic effects on the basis of growth hormone–like and prolactin-like properties.
Prolactin
: important for alveolar development in prepartum period
increases during the latter part of gestation due to the effect of estrogen on its release from the adenohypophysis.
II) PARTURITION
Fetal cortisol initiates delivery through increased secretion of estrogen and thus prostaglandin F2α
key organ system of the fetus responsible for initiating the process is the
fetal adrenal cortex
Critical changes in cortisol secretion
by the fetus result in the synthesis and
release of PGF2α from the uterus
, which
produces muscle contraction and relaxation of the cervix.
maturation of the fetal adrenal cortex is of critical importance in the initiation of parturition
progressively sensitive to fetal adrenocorticotropic hormone (ACTH, corticotropin
The end result of increased estrogen secretion is the secretion of prostaglandins, particularly PGF2α
PGF2α: important hormone for the initiation of parturition
Effect of PGF2α: critical effect of PGF2α
release intracellular calcium ion, which binds to actin and myosin to initiate the contractile process.
Estrogen induces oxytocin receptor formation in the myometrium
Relaxin
: causes the ligaments and associated muscles surrounding the pelvic canal to relax, which
allows the fetus to expand the pelvic canal to its fullest potential
first stage of parturition:
presentation of the fetus
at the internal os of the cervix.
second stage of parturition:
actual delivery process
third stage of parturition:
the delivery of the fetal
membranes.
PART 5) The Mammary Gland
II) Control of mammogenesis
A) Initial development of the mammary gland is programmed by embryonic mesenchyme.
Fetal development of the mammary gland is under both
genetic
and
endocrine
control
fetal development of the mammary gland is under both genetic and endocrine control
B) Proliferation of the mammary duct system begins at puberty,
ducts under the control of estrogens, GH, & adrenal steroids
alveoli under the control of progesterone and prolactin.
The
development of alveoli from the terminal ends
of the ducts requires the addition of progesterone and prolactin
gland remains relatively undeveloped until the occurrence of pregnancy
end of pregnancy: mammary gland transformed to a structure that is filled with alveolar cells that are actively synthesizing and secreting milk
I) Anatomical aspects of the mammary gland
A) Milk-Secreting Cells of the Mammary Gland Develop
Through the Proliferation of Epithelium into Hollow Structures Called Alveoli
Parenchyma (milk-secreting cells) of the mammary
gland develops
through the proliferation of epithelial cells
that arise from the
primary mammary cord.
epithelial cells form hollow, circular structures called
alveoli
: fundamental milk-secreting units of the mammary gland
nipple
: the external connection to the internal milk-secreting system, develops on the surface
B) Most of the milk that accumulates before suckling or milking is
stored in the alveoli,
even though animals have enlarged milk-storage areas called
cisterns.
Duct systems connect alveoli with the nipple, or teat, enabling milk to pass from the area of formation to the area of delivery (nipple).
Both cow and the goat have specialized areas for holding milk, called
cisterns
, which are
located in the ventral part of the gland
and into which all main ducts empty
enabled them to synthesize and store larger amounts of milk
Mammary glands develop typically as paired structures
pairs of mammary glands that are closely apposed to each other is called an
udder
.
C)
A suspensory system- the udder
of the cow allows the animal to carry a large amount of milk.
This system is formed by the median suspensory ligament (formed between pairs of mammary glands) composed of elastic connective tissue that originates from the abdominal tunic.
IV) Lactogenesis
A)
Prolactin
is the most important hormone involved in the process of milk synthesis/ lactogenesis;
GH also important for lactogenesis.
Prolactin responses decrease as lactation period progresses
synthesis and release of
dopamine
is
blocked
while neurons in the
paraventricular nucleus
are stimulated to produce and release
vasoactive intestinal peptide
(a stimulator of prolactin release)
prolactin released through suckling/ milking process
B) The
Release of Fat into Milk from the Alveolar Cell
Involves
Constriction
of the Plasma Membrane Around the Fat Droplet
Fat droplets first accumulate in the basal cytoplasm of the cell and then move to the apex, where the droplet protrudes into the alveolar lumen.
cell membrane constricts about the base of the fat droplet, so fat is dispersed in milk in small droplets
C) Milk proteins and lactose are
released
from alveolar cells by the process of
exocytosis
.
Milk proteins are synthesized on the endoplasmic reticulum
casein molecules pass to the Golgi apparatus, where they are
phosphorylated
and formed into
micelles
Lactose is also synthesized within the Golgi vesicles and is released in conjunction with milk proteins
III) Colostrum
A) Prepartum milk secretion (without removal) results in the formation of colostrum.
milk formed before parturition: colostrum
formation represents a secretory process in which lactogenesis occurs in the absence of milk removal.
B) The ingestion of colostrum is important
- passive immunity it confers through the presence of high concentrations of immunoglobulins.
has an important function in temporary, or passive, protection against infectious agents.
immunoglobulins
are highly concentrated in colostrum- the neonate can receive passive immunity against pathogens after consuming colostrum
allows the young to receive immediate protection from environmental organisms
antibodies are acquired through the ingestion of colostrum
C) Time immunoglobulins can be absorbed through the neonatal gut is
limited to the first 24 to 36 hours of life.
feeding of colostrum within this period is important to ensure the presence of immunoglobulins in the newborn
antimicrobial factors found in milk: lysozymes, lactoferrin, and the lactoperoxidase system
D)
Lipids
(particularly vitamin A) and
proteins
(caseins and albumins) are
high
in concentration in colostrum;
carbohydrates (lactose) are low.
lactose synthesis is significantly inhibited by progesterone until about the time of delivery
V) MILK REMOVAL
Efficient milk removal requires the
release of oxytocin
-Causes contraction of muscle cells that surround the alveoli & movement of milk into the ducts and cisterns.
myoepithelial cells are particularly responsive to oxytocin- contract when exposed to hormone
Release of oxytocin occurs within seconds after the stimulus arrives in the hypothalamus
Milk Letdown: increased pressure within the mammary gland is evident within a minute of stimulation as
milk is forced out of the alveoli and ducts because of contraction of the myoepithelial cells
If drainage of milk is passive process: movement of milk into the gland cistern at suckling or milking would be slow, and less milk would be obtained
VI) FIRST NURSING
Carbohydrate Stores Are
Good
in Neonates Born as
Singles
or
Twins
,
Carbohydrate Stores Are
Low
in Neonates Born in
Litters
Domestic animals that have 1/2 offspring like cattle or horse- young need to stand to suckle
neonates have reasonably good carbohydrate stores,
Young born in litters are susceptible to hypoglycemia, and suckling delays are often detrimental to their survival
VII) COMPOSITION OF MILK
A) Fats are the most important energy source in milk.
Milk fat is composed of a number of lipids, including monoglycerides, diglycerides, triglycerides, free fatty acids, phospholipids, and steroids
Triglycerides are the main component of milk fat.
amount of fat produced varies greatly both within and among species
B) Lactose, composed of glucose and galactose, is the main carbohydrate of mammalian milk.
Blood glucose is the main precursor molecule for lactose,
propionate an important precursor for glucose in ruminants.
Lactose is formed under the direction of lactose synthetase, an enzyme composed of α-lactalbumin (a milk protein) and galactosyl transferase
Prolactin
- stimulatory for the formation of lactose synthetase
C) The main proteins in milk are called
caseins
and are found in
curd
.
Caseins can be removed (as a curd) from milk through a process called curdling or coagulation, with other milk proteins, such as albumins and globulins
VIII) THE LACTATION CYCLE
A) Milk production peaks at 1 month postpartum in dairy cattle, followed by a slow decline in production
milking usually stops at 305 days of lactation so that the animal can prepare the mammary gland for the next lactation
Involution
: The back pressure of milk within the alveoli gradually
inhibits the secretion of milk
by the alveolar epithelial cells- regression of the alveolar cells and small ducts.
B) Lactation can be induced by
hormone administration (estrogen and progesterone)
and
enhanced by GH & increased photoperiod
exposure
.
To induce lactogenesis by hormonal means: animals should not be lactating at treatment and should have mammary glands free of infection.
GH acts on the post-absorptive use of nutrients so that protein, fat, and carbohydrate metabolism in the whole body are changed, and
the nutrients are directed toward milk synthesis
GH treatment decreases the energy balance of cows; but is adjusted by a
voluntary increase in feed consumption
.
use of
thyroprotein
does not affect the efficiency of the lactational process as GH does.
manipulation of lactation- milk yield in cows can be increased by
exposing them to increased light
(photoperiod)
IX) DISEASE ASSOCIATED WITH MAMMARY GLAND
A) Main diseases:
mastitis (prevalent in dairy cattle and dogs)
neoplasia (prevalent in intact dogs and cats).
Mastitis
: inflammation of gland
Cause: injury to the teat canal from the repeated stretching that occurs with the milking process
Effect: Formation
of connective tissue within the udder as a result of the attempt of the gland to wall off the infection.
The presence of connective tissue limits the area into which ducts and alveoli can proliferate, thus reducing the milk-producing potential of the gland
Neoplasia
: abnormal growth of tissue
Dog is most susceptible to mammary tumors
Exposure of mammae to the ovarian hormones estrogen and progesterone greatly increases the chance of neoplasia
B) main conditions that involve the mammary gland indirectly
passive transfer of red blood cell
agglutinating antibodies
by the ingestion of colostrum (mare, queen)
hypocalcemia
caused by the transient drain of calcium that occurs with initiation of lactation (dairy cattle) or during the perinatal period (dog).
Passive transfer of RBC agglutinating antibodies commonly occur in horses
fetal RBC pass into the maternal system and elicit antibody formation against the fetal RBCs.
Foals go into hemolytic crisis
life threatening to the
dam
are hypocalcemia, puerperal tetany, or eclampsia
animals lose their ability to maintain normal muscle activity; cows are often unable to stand, and become prostrate with the appearance of being comatose
PART 6) Reproductive physiology of the Male
I) Functional Anantomy
A) The male reproductive system consists of many individual organs acting in concert to produce spermatozoa and deliver them to the female’s reproductive tract.
involves both the neuroendocrine (hypothalamus and anterior pituitary glands) and the genital system
consist of two testes, each suspended within the scrotum by a spermatic cord and external cremaster muscle;
two epididymides; two deferent ducts; accessory sex glands; and the penis.
accessory sex glands:
paired ampullae
paired seminal vesicles (vesicular glands),
a prostate gland
paired bulbourethral glands (Cowper glands)
B) Normal spermatogenesis requires maintenance of uniform testicular temperature
2° to 6° C lower than core body temperature.
Elevated testicular temperature reduces the numbers of live, normal spermatozoa
maintained by the pendulous scrotal sac, the testicular vasculature, the dartos and cremaster muscles, and the scrotal skin
C)
Emission
- the release of spermatozoa and accessory gland fluids into the pelvic urethra
ejaculation
- forceful expulsion of semen from the urethra.
Emission- as a result of sympathetically mediated thoracolumbar reflex contraction of the smooth muscle in the ductus deferens and accessory glands
Ejaculation: prompted by a parasympathetically mediated sacral reflex that induces rhythmic contractions of the bulbospongiosus, ischiocavernosus, and urethralis muscles.
all testicular functions are profoundly influenced by the neuroendocrine system
testis is responsible for steroidogenesis, primarily the production of androgens, as well as the generation of haploid germ cells by spermatogenesis.
testis is considered to have three compartments
Seminiferous tubule: basal compartment contains spermatogonia, which divide through mitosis
Seminiferous tubule: adluminal compartment represents a special environment where spermatocytes undergo meiosis and continue their meiotic divisions to differentiate into spermatids and finally into spermatozoa.
he interstitial tissue compartment, containing the Leydig cells, surrounds the seminiferous tubules and bathes them with testosterone-rich fluid.
Copulatory organ of the male is the penis- cylindrical in all species and extends
from the ischial arch
to near the umbilicus on the ventral abdominal wall
II) Spermatogenesis
A) Spermatogenesis- lengthy orchestrated process in which diploid stem cells divide by mitosis to maintain their own numbers and cyclically produce progeny that undergo meiotic division and differentiation into haploid germ cells.
Spermatogenesis is generally divided into three major events
2) meiosis
Secondary Spermatocytes
Spermatids
3) spermiogenesis- packaging
Spermatozoa
1) spermatocytogenesis - mitotic proliferation
Spermatogonium
Spermatogonia
Primary Spermatocytes
B) Testicular size can predict daily sperm production.
Within a species, both individual and breed variation in testicular size can also influence daily sperm production
indirect measure commonly used in ruminants is
scrotal circumference
other species with more horizontally oriented testes- total scrotal width
Each gram of normal testicular parenchyma produces the same quantity of spermatozoa according to the species but differs among species
III) Hypothalamic-pituitary-testicular axis
A) The reproductive system of the male is regulated by the
hypothalamus
- hormonally linked to the
anterior pituitary
and
testes
by LH and FSH
hypothalamus
synthesizes and secretes the
decapeptide gonadotropin-releasing hormone
(GnRH) in a pulsating manner
GnRH acts directly on gonadotropic cells in the anterior pituitary.
on stimulation of GnRH- gonadotropes synthesize and secrete FSH & LH
LH
binds to membrane receptors on the
Leydig cells and stimulates them to convert cholesterol to testosterone.
FSH
specifically targets receptors on the Sertoli cells within the seminiferous tubules.
FSH and testosterone
stimulate a variety of Sertoli cell functions
which are
the synthesis and secretion of ABP, inhibin, activin, estrogen, and several products (e.g., transferrin) that are involved in the transfer of nutrients to germ cells
meiosis
spermatocyte maturation
spermiation
Leydig cell function.
IV) PUBERTY
A) Puberty is not synonymous with sexual maturity.
Correctly defined when male is first able to produce
sufficient numbers of sperm
to impregnate a female
Can occur months to years later (depend on species)
B) Puberty results
from a continuous process of endocrine changes
that are initiated shortly after birth.
pituitary gland, gonads, and steroid-dependent target tissues are capable of
responding to stimulatory hormones before puberty
hypothalamus is considered to play a pivotal role in the initiation of puberty
Theory: puberty occurs when animal’s hypothalamic-pituitary complex becomes desensitized to the feedback inhibition of gonadal steroids.
Desensitization allows increased discharge of GnRH from the hypothalamus and a greater response of the pituitary to GnRH
Low level of
gonadotropin secretion
rapidly
decreases
when
sex steroids are administered
. Therefore, a
highly sensitive negative-feedback mechanism appears to exist
in prepubertal domestic animals.
V) ANABOLIC STEROIDS
Known as androgen Derivatives That Exert Negative Feedback on the
Hypothalamic-PituitaryTesticular Axis
many receiving anabolic steroids are peripubescent or prepubescent
Anabolic steroids are androgen derivatives that have been
altered to maximize their anabolic action
and to
minimize their androgenic side effects
adverse reproductive side effects observed are
similar to those associated with testosterone administration.
affects pituitary function and leads to long-lasting impairment of testicular endocrine function
Side Effect: incomplete development of the hypothalamicpituitary-gonadal axis.
colts and stallions in training/ racing
receive this drugs- they have smaller testicles vs. horses not receiving such drugs.
Alterations in seminal parameters are also observed, including depression of sperm concentration, sperm motility, and total number of sperm per ejaculate.
Histologically: reduction in the number of developing germ cells other than type A spermatogonia
Adverse effects more severe in younger stallions
presence of atrophic testicles and low LH, FSH, and testosterone levels
after drug withdrawal
indicates long-term androgenic/ anabolic steroid use
affects pituitary function and leads to long-lasting impairment of testicular endocrine function
indiscriminant use of anabolic steroids in males intended for breeding
should be strongly discouraged
