Reproductive Physiology Flashcards
Genetic Sex
Chromosomal
Determined at fertilization
XX (homogametic, homomorphic) or XY (heterogametic) in mammals
ZZ (male) and ZW (female) in birds
Gonadal Sex
Decided by chromosomal- default is female
Testis determining genes; SRY (sex determining region of Y chromosome) and SOX9
There are also ovary-determining genes (RSPO1, WNT)- even though this is default
Phenotypic sex
What the animal looks like
Tubular and external structures
AntiMullerian Hormone, Testosterone, Dihydrotestosterone
Gonads
Initially indistinguishable (about 6 weeks in large domestic animals)
SRY
Testis Determining Factor
Synthesized in sex cord of male
Medullary sex cords differentiate into Sertoli cells; cortical sex cords degenerate (drive to male directon)
Sex cords differentiate into seminiferous tubules
Sex cords also give rise to rete testis
Male pre-sertoli cells also produce Anti Mullerian hormone
In absence of SRY cortical sex cords develop into follicles; medullary sex cords degenerate
AntiMullerian Hormone (AMH)
Secreted by Sertoli cells of fetal testis
Causes degeneration of paramesonephric (female) ducts
Absent from normal adult male
Secreted by granulosa cells of developing (antral) follicles in adult female
Amount proportional to number of developing follicles
Freemartinism
Bovine (camelids, occasionally other species)
Male and Female co-twins
Early fusion of placental circulation
-AntiMullerian hormone transferred from male to female (because of placental fusion)
-Mediates regression of paramesonephric (Mullerian) ducts
-vestigial development of vagina, cervix, uterus, uterine tubes
Almost normal vulva and vestibulum
-derived from urogenital sinus- aka not due to paramesonephric duct
Female is infertile and reproductive tract underdeveloped
Sex Reversal
Disagreement between chromosomal and gonadal sex
XX sex reversal: XX geneotype and some degree of testicular development
-XX male is bilateral testes
-xx tru hermaphrodite if testis and ovary present
Most human patients have SRY translocation to an autosomal chromosome
Hypothalamus: Male vs Female
Female hypothalamus contains two functional areas for secretion of GnRH: Tonic (arcuate ventromedial region; ARC) and Surge (preoptic area; POA) centers
Hypothalamus inherently female
-testosterone during development defeminizes the brain (T from fetal testes crosses BBB and converted to estradiol which defeminized hypothalamus, eliminating surge center
Fetal ovaries produce estradiol, but this does not cross BBB because it is bound to alpha-fetoprotein
Hormones
Signaling molecules produced in the body that regulate activity of certain cells and organs (Metabolism, sleep, lactation, growth, mood, reproduction, etc)
May be secreted by specific glands (e.g. thyroid)
Three main classes: steroid, peptides /proteins, eicosanoids
Bind specific receptors to induce response
Hormone delivery
Endocrine Neuroendocrine Paracrine Autocrine Pheromonal
Endocrine
Produced in one organ to the blood stream to find target organ elsewhere
Estradiol, FSH (pituitary to blood to testes)
Neuroendocrine
Produced and secreted by nerve cells and have direct effect on other nerve cells
GnRH (hypothalamus to pituitary)
Paracrine
From one cell and effects a neighboring cell
Testosterone from leydig cells influences Sertoli cells in testis
Autocrine
Cell produces a hormone that acts on itself
Estradiol from Sertoli cells hastens pubertal development of these cells
Pheromonal
Bruce effect, ram effects (bring ram to ewes=syncronized ovulation in all)
Supraphysiological Stimulation
Internalization of hormone receptors (face inside of cell) when overstimulated
Hormone becomes ineffective
(prolonged release, high dose GnRH as a contraceptive in dogs)
Hormone classification by structure
Peptides
Glycoproteins
Steroids
Prostaglandins
Peptides
Oxytocin, GnRH
Small molecules with few amino acids
Regulates pituitary gonadotropin secretion
Decapeptide secreted in a pulsatile manner
Both LH and FSH are secreted from the same gonadotrope cell after stimulation by GnRH
Glycoproteins
FSH, LH, TSH
Protein unit attached to carbohydrate unit. (changes half life so it degrades slower)
Alpha chain is common among hormones
Beta chain is unique for each hormone
Steroids
Estradiol, progesterone
Exactly the same between species
3 6C rings and 1 5C ring
Easily cross membrane
Prostaglandins
PGF2a, PGE2
Small fatty acid derivatives
Built out of molecules liberated from cell membrane
Huge variety- so lots of actions
Steroid biosynthesis
Cholesterol is always the starting point Needs to be transported into mitochondria for final component of steroidgenesis Then to pregnenolone To Progesterone To androstenedion (the male steroid) To estradiol and testosterone
Estradiol
Major female sex steroid
Produced mainly by follicular cells in ovary (and placenta- smaller amounts in adrenal, testis, fat, and other sites)
Two cell, two gonadotrophin synthesis
Metabolized in liver!!
Estradiol synthesis
Cholesteral -> Androstenedione (happens in theca and is stimulated by LH)
Androstenedione goes to granular cell layer and converts to estradiol with help of FSH
Estradiol Functions
Mediate sexual behavior and secondary sexual characteristics of female
Anabolic
Sexual behavior
Uterine development and function: Endometrium (cyclical fashion and estrogen dependent), Myometrium (estradiol increases myometrial activity)
Cervical Mucus (less viscose)
Increased progesterone receptors (uterus- nutrients from embryo -> anticipation of ovulation and pregnancy)
Vaginal Proliferation- tougher in anticipation of mating
Essential for mammary development
Mediates ovulatory surge of LH
Role in maintaining Pregnancy
Bone health
Estrogens
Generic term for molecules with action like estradiol
Endogenous (produced in the body): estrone, estradiol- synthesized from androgens
Exogenous (not immediately eaten up by liver): diesthystilbestrol
Progesterone
Produced mainly by ovary (specifically corpus leuteum)
CL produced after ovulation
Proliferation and differentiation of granulosa cells (large luteal cells) and theca interna cells (small luteal cells)
Both cell types produce progesterone
Metabolized by liver
Progesterone Functions
Development and function of uterus (endometrium, uterine glands)
Myometrium (quiescence-lessened activity)
Closure of cervix, increased, viscosity of cervical mucus
Maintenance of pregnancy
Development of mammary gland
Effects on brain
Effects on immune function
Progestagens
Class of hormones that bind to progesterone receptors and have progesterone-like actions
Critical for maintenance of equine pregnancy
Testosterone
Produced mainly by Interstitial Leydig cells of the testis
Metabolized mainly by liver
Testosterone Function
Masculinization
Maturation and function of male reproductive tract
Spermatogenesis
Male sexual behavior (beyond a (low) threshold)
Muscle mass, bone density
Osteoblastic
Erythripoietic
Increased BMR (less thyroid binding globulin)
Androgens
Class of compounds bind ing to androgen (testosterone) receptors
- testosterone
- dihydrotestosterone
- androstenedione
Hypothalamic-Pituitary Interrelationships
Nerves terminate in stalk and release GnRH
Anterior Pituitary releases of LH, FSH, ACTH (stimulated by GnRH from hypothalamus)
Nerve cells which release peptide hormones (oxytocin produced here)
Release of oxytocin into circulation from neurohypophysis
Hypothalamo-hypophyseal-ovarian axis
Hypothalamus
to GnRH
to FSH and LH (production of estradiol)
FSH stimulates follicular development
Estrogen-progesterone and androgen produced by ovary
These feed back negatively to the hypothalamus and anterior pituitary
Estrous Cycle Regulation
Regulated by: Pineal gland-melatonin Hypothalamus-GnRH Hypophysis-FSH and LH Ovary-estradiol, progesterone, and inhibin Uterus- prostaglandin, PF2a
Ovary
Medulla: vasculature, nerve, connective tissue
Cortex: Oocytes, follicles
Primordial Follicles
Oocyte surrounded by a single layer of squamous cells
Multiply in very early pregnancy
Arrest and number stays same- no new ones
Primary follicle
Oocyte surrounded by single layer of cuboidal cells
Activated periodically from pole- activation is a one way trip it either develops or stops
Secondary Follicle
Oocyte surrounded by two or more layers
Zona pellucida
Antral Follicle
Fluid accumulates within a cavity formed by follicular cells
Folliculogenesis
Primordial follicle: origin of oocyes, and origin of follicular (granulosa) cells Committed follicle (activated follicle from resting pool to go grow): 20-30 follicles commence to growth each day. Committed to gonadotrophin-independent growth phase Subsequent deveopment requires gonadotropin support
Most will degenerate long before they can ovulate
Oocytes penetrate early ovary and get surrounded by peripheral cells from ovary- become leydig granulosa cells
Resting Pool
Pool of inactive primordial follicles
Committed follicle
Follicle commences development from resting pool
Gonadotropin-independent growth
Follicular development up to development of antrum is independent of FSH or LH
Gn-independent
No requirement for LH and FSH
Primordial, Primary, Seconday
Gn-Responsive
Doesn’t need Gn but will be more responsive with it
Preantral
Antral
The preantral-early antral transition
Formation of the Theca cell layer
Most susceptible to follicular atresia- not developing any further
Gn-dependent
If you don’t have gonadotropin it won’t move on
Antral and preovulatory follicle
Antral follicle components
Theca externa: connective tissue
Theca interna: produce androgens under LH stimulation
Granulosa cells: produce estrogen, inhibin, and follicular fluid. Responsive to FSH
Corpus Hemorrhagicum
Structure left immediately after ovulation (after follicle)
Antrum collapses and is filled with blood
Corpus luteum
Theca interna and granulosa cells proliferate to fill the cavity, differentiate into luteal calls and produce progesterone
Large luteal cells (from granulosa; also secrete oxytocin and relaxin)
Small luteal cells (from theca interna)
Remains if pregnant
Corpus albicans
Remnant of old corpus luteum
Scar remnant
Tonic GnRH center
Frequency of GnRH pulses controlled by a pulse generator
Affected by internal and external signals
Surge center
Responsible for bursts of GnRH required to achieve preovulatory LH surge
Paraventricular nucleus
Oxytocin synthesis
Hypothalamo-pituitary portal system
Axons of hypothalamic neurons extend to blood vessels of portal system
GnRH affects anterior pituitary directly
Estrogen Function
Thickening of vaginal mucosa
Mucus secretion (cervix and cranial vagina)
Epithelial mitosis in endometrium
Increased secretory and ciliary beat activity in tubal cells
Endometrial edema
Myometrial activity
Estrous behavior
Ovulation
LH surge- precursor to ovulation
Theca interna cells produce progesterone rather than androstenedione
Collagenase (weakening follicle wall and changes to blood supply there-rupture of spot on wall)
Prostaglandins (lack of vasculature. Contractile- oocyte ejection)
CL: Progesterone
Increased endometrial secretion
Reduced myometrial activity
Priming effect in brain to enhance effects of estrogen
Luteolysis
Not pregnant, need CL to go away to have another cycle
PGF2a release from endometrium
Ipsilateral (from right uterine horn to right ovary) in ruminants
Systemic (uterus to ovary by general circulation) in mare and sow
Both systemic and counter-current in camelids (systemic in left uterine horn, local exchange in right uterine horn)
Luteolytic trigger not known in dogs (CL length is same whether pregnant or not)
Hormonal Feedback Female
GnRH in anterior pituitary. FSH and LH released and stimulate follicle growth in ovary.
Ovary released estradiol and there is a positive feedback: Graafian Follicle increases estrogen production will stimulate LH surge.
Estradiol can also stimulate inhibin and cause negative feedback to regulate FSH release.
LH can cause CL to be maintained which releases progesterone. Negative feedback will regulate the tonic release of LH to support the CL until PGF causes CL regression
Seasonality
Mediated by photoperiod
perceived by eye
melatonin produced by pineal gland during hours of darkness (synthesized from tryptophan)
Melatonin increases GnRH release in short day breeders (small ruminants)
Decreases GnRH release in long day breeders (Horses)
Puberty
The acquisition of capability of sexual reproduction Production of gametes Onset of cyclicity in females Mature sperm in males Distinguished from sexual maturity
Sexual maturity
Further body growth
Adequate size for pregnancy, delivery, lactation
Puberty mediation
No intrinsic immaturity of gonads and pituitary
Animals may be forced to ovulate by appropriate stimulation prepubertal ovaries
Hypothalamic and pituitary development
GnRH neurons originate outside brain in olfactory placode
Migrate through forebrain to hypothalamus
By midgestation the number of GnRH neurons and the Hypothalamic content of GnRH and mRNA are the same as adult
In contrast, LH and FSH content of anterior pituitary are low (as is encoding mRNA)
Juvenile pituitary unresponsive to acute dose of GnRH, but is easily upregulated by pulsatile infusion of GnRH
Pulse generator is dormant in juveniles
Neurobiologic Brake on prepubertal GnRH Pulsatility
y-amino butyric acid (GABA): negative effect of GnRH release and pulsatility
Neuropeptide Y (NPY): negative effect of GnRH release and pulsatility
Kisspeptin: positive effect on GnRH signaling. Most likely mediator of GnRH pulsatility
Leptin
Adipocyte hormone
Infusion of leptin can precipitate puberty
Leptin seems to sense somatic status: adequate body size and maturity (fat stores)
Increasing leptin may mediate onset of puberty
Preovulatory GnRH surge
Positive feedback of estradiol- enough estradiol to trigger LH surge and ovulation
Prepubertal hypothalamus exquisitely sensitive to estradiol: negative feedback on GnRH. Follicles do not develop to preovulatory size. Follicles do not produce preovulatory quantities of estradiol so surge centers are never triggered
Puberty vs Sexual Maturity
Puberty occurs at a stage when reproduction is not usually desirable
Further growth required before animal may safely and successfully undergo pregnancy, parturition and lactation
Although male animals may be capable of fertilization, maturity implies ability to be consistently fertile and impregnate an appropriate number of females
Estrous Cycle
Period from the beginning of one estrus to the beginning of the next (or from one ovulation to the next)
-follicular phase (proliferative phase in women)
-Luteal phase (secretory phase in women)
-Quiescent phase (anestrus)
Estrus is defined behaviorally
Polyestrous
Cows. Keep cycling again and again in absence of pregnancy- all year
Monestrous
Dogs. Each estrus is an individual event- lots of time (anestrus) between them
Seasonally polyestrous
horse, sheep
Spontaneous ovulators
cows, mares, dogs, etc
Induced Ovulators
Cats- spikes on penis to stimulate LH surge
Rabbits
Camelids
Ferrets- can die without ovulation- follicle just sits there making estradiol
Phases of Ovarian Cycle
Follicular Phase (proliferative) Luteal phase (secretory)
Follicular Phase
Proestrus and estrus
Developing follicles
Estrogen-dominated
Luteal Phase
From ovulation to CL regression
Progesterone-dominated
Male fertility requires
Competent Spermatogenesis -endocrine regulation -thermoregulation -spermatocytogenesis (mitosis, meiosis) -spermiogenesis (morphological transformation) Functional delivery system -accessory glands -erection, ejaculation -libido
Male Anatomy
Scrotum Testis Epididymis Ductus Deferens Accessory glands Penis Prepuce
Scrotum
Blood supply: external pudendal artery
Nerves: from 2nd and 3rd lumbar roots (genitofemoral nerve)
Lymph drainage: superficial inguinal lymphnode
Scrotal skin: thin, sweat glands, delicate
Testes
Oval, laterally compressed
Vertical in bulls and rams; more horizontal in stallions, dogs, cats and pigs
Testicular artery follows a tortuous route before entering the testis
Testicular vein: pampiniform plexus- important in thermal regulation
Lymphatic drainage: lumbar lymph nodes
Testicular thermoregulation
Testes approximately 4C lower than core body temperature
Scrotal skin is thin with numerous sweat glands. Tunica dartos
Pampiniform plexus
Cremaster Muscle (lifts and drops)
Whole body response to increase in testes temp
Tunica dartos
muscular layer that crinkles or relaxed skin to cool or heat up testes
Pampiniform plexus
Countercurrent arterio-venous exchange
Warm arteriolar blood entering loses its heat to the venous blood leaving the testis
So blood coming in cools off to correct temperature
Testosterone countercurrent: testosterone will leave, cross easily to artery and come right back
Reduced pulsatility: contorted and long
Testis
Seminiferous tubules: sertoli cells, spermatogenic cells Tubuli recti Rete restis Ductuli efferentes Interstitial cells of leydig
Blood-Testis Barrier
Maintains unique environment for development and maturation of germ cells
Protects germ cells from noxious agents
Prevents autoimmune response to sperm
Blood-testis Barrier structure
Specialized capillary endothelium-non-fenestrated (no pores for stuff to get through)
Basement membrane
Specialized junctions between neighboring sertoli cells
Epididymis
Head, body, and tail
Macroscopically, more or less cylindrical
Single tortuous duct- up to 15m long
functions in sperm maturation, storage, and transit
All epididymal functions are androgen dependent
Penis
Two basic variations of penile structure in domestic animals:
Musculocavernous
fibrous
Dorsal nerves and vessels:
-dorsal nerve responsible for sensory innervations of glans penis (essential for ejaculation)
-Dorsal vessels do not communicate functionally with corpus cavernosum or corpus spongiosum in ruminants
Musculocavernous penis
increase in size due to blood engorgement within corpus cavernosum penis, and to a lesser extent in corpus spongiosum penis
fibrous penis
strong tunica albuginea prevents substantial increase in diameter; increase in length involves relaxation of retractor penis muscle and extension of sigmoid flexure of penis
Endocrinology of male reproduction
Anti-Mullerian Hormone (AMH) Gonadotropin releasing hormone (GnRH) Luteinizing hormone (LH) Follicle stimulating hormone (FSH) Testosterone Estradiol Inhibin
Anti-Mullerian Hormone (male repro)
Protein
Inhibits development of paramesonephric (mullerian) ducts in male embryo
Low levels in postnatal male- elevated in cryptorchidism
GnRH (male repro)
Peptide hormone
Secreted by hypothalamus
Acts on anterior hypophysis (pituitary) to increase secretion of FSH and LH
LH (male repro)
A peptid hormone
Secreted by anterior pituitary
Binds and acts primarily on the leydig cells
Hence synonym interstitial cell stimulating hormone
Stimulates androgen production from leydig cells
Prolactin (PRL) (male repro)
Peptide hormone secreted by pituitary gland
Enhances LH-induced testosterone secretion
More important in dogs than other domestic species
Help support lifespan of corpus luteum
FSH (male repro)
Secreted by anterior pituitary gland
Acts on sertoli cells
Major action is to promote protein synthesis. At least four proteins produced
Androgen binding protein is secreted into the seminiferous tubules. It is chemically indistinguishable from testosterone binding globulin found in the serum of may species. Serves to ensure a constant high intratubular concentration of testosterone essential for the function of the epididymis, and possibly other accessory glands
Endocrine control of spermatogenesis is a complex process requiring the presence of FSH and high intratesticular androgen levels
Under FSH stimulation, sertoli cells produce inhibin, which has a negative feedback effect on pituitary release of FSH
Note that sertoli cells also produce significant quantities of estrogens
Under FSH stimulation Sertoli cells convert testosterone to dihydrotestosterone
Testosterone (male repro)
Steroid hormone
Produced by interstitial cells under LH influence
Crucial for spermatogenesis
In addition to circulating levels, testes and excurrent tract have increased concentrations
Testosterone excchange occurs in the pampiniform plexus so arterial blood entering the testes has high concentration
ABP maintains high intratubular levels
Negative feedback on hypothalamus decreases GnRH and therefore FSH and LH secretion by pituitary
Dihydrotestosterone (male repro)
More potent than testosterone
Local conversion in Sertoli cells an din distal tract (accessory glands, penis)
Mediator of sexual development and secondary sex characteristics
Mediator of BPH
Estrogen (male repro)
Steroid hormone
Produced by sertoli cells
Local support of spermatogenesis within wall of seminiferous tubule
Negative feedback on hypothalamus to decrease GnRH and therefore FSH and LH secretion
Inhibin (male repro)
Peptide hormone
Produced by sertoli cells
Negative feedback on pituitary decreases FSH secretion
Androgen binding Protein (ABP) (male repro)
Produced by sertoli cells
Stimulated by FSH
Structurally similar to circulating steroid binding globulin
Functions to bind testosterone (and DHT) to maintain high intratubular concentration
Seasonality (male repro)
Gonadotriptin concentrations decrease during the non breeding season
Driven by effect of photoperiod on pineal gland and subsequent production of melatonin (which is secreted mainly at night)
In long day breeders (stallion and tomcat) increased photoperiod results in decreased melatonin secretion and increased GnRH secretion
In short day breeders (small ruminants) melatonin has a stimulatory effect on GnRH secretion
Spermatogenesis-steps
Spermatogonia yield primary spermatocytes by mitosis
Primary spermatocytes enter meiosis I, yielding 2 secondary spermatocytes each
Each secondary spermatocyte undergoes meiosis II, yielding 2 spermatids
Spermatids undergo morphological changes to produce mature spermatozoa
Duration
Waves
Stages
Spermatogenesis
Formation of spermatid from spermatogonia
Spermiogenesis
Morphological change of spermatid into spermatid into spermatozoa
Sperm in Epididymis
Transit takes about 2 weeks
Sperm acquire capability for mortality and fertilization
Sperm are stored in tail of epididymis
Treating a bull with poor libido with testosterone injections
may cause liver damage
Erection
Corpus Cavernosum
Corpus spongiosum
Ischiocavernosus muscle (pudendal nerve)
Erection in Ruminants
Corpus cevernosum: vasodilation- BO here will rise and meet arterial BP. Retractor penis relaxes so penis is extended
Corpus spongiosum
Ischiocavernosus muscle: clamps down hard at root of penis when about to mount and prevents blood from draining out of cavernosum and so blood is under high pressure. Low additional blood volume
Emission
Ejection of sperm from epididymis
Ejactulation
Ejection of semen from urethra
Reflex afferents: glans penis to pudendal nerve to spinal cord
Efferent: hypogastric nerve to muscles. Neuro-endocrine loop (oxytocin)
Ejaculation (anatomy)
Epididymis Ductus deferens Accessory glands Penile muscles Urethra
Seminal Fluid
Testes
Accessory sex glands: ampullae, vesicular glands, prostate, bulbourethral glands, depending on species.
Prostaglandins, citrate, fructose, zinc, mucus
Important for sperm motility, metabolism, uterine contraction, DNA stabilization, control of capacitation, immunoregulation etc
B nerve growth factor: ovulation inducing factor
Copulation
Visual stimuli
Penile sensation
Musculoskeletal system
Inititate ejaculation
Ovum development in fetal life
Primary germ cells originate near yolk sac and migrate to developing ovary
Differentiate to oogonia and multiply by mitosis
Enter prophase of first meiotic division (primary oocytes)
Most mammals are born with oogenesis at primary oocyte stage
Postnatal Oocyte development
Diplotene nucleus remains in resting stage: germinal vesicle stage (reaches prophase of meiosis I)
Chromosomal complement is 4n
Meiosis resumed at ovulatory surge of LH
Oocyte development
When primordial follicle starts growing, both the oocyte and the follicle increase in size
Oocyte growth is complete at stage of antrum formation
LH peak stimulates GV breakdown: assembly of chromosomes into metaphase plate
During telophase first polar body is extruded: minimal cytoplasm
Oocyte enters second meiotic division: ovulation stage for most mammals
Dogs
Oocyte ovulate at prophase I- before resumption of meiosis I
Sperm penetration occurs at a stage earlier than ovulation in most other species
Cytoplasmic Maturation
Apart from nuclear maturation, cytoplasmic changes are necessary
Depends on close association between oocyte and corona radiata cells: cytoplasmic projections through zona pellucida
exchange of molecules and metabolites-helps prep for ovulation
Brake and start up meiosis depends on this communication
Ovulation
Oocyte and cumulus mass moved into infundibulum of uterine tube by fimbria
Fertilization occurs in ampulla of uterine tube
Short window of fertilizability
Cumulus cells expand so they can be fertilized
Sperm Transit
Sperm gain ability to fertilize during epididymal transit
Sperm move from site of deposition to uterine tube within minutes due to contractions of female tract
Many millions, or billions, of sperm ejaculated
About a thousand reach the uterine tube
Tens to hundreds present for fertilization
Functional Sperm Reservoir
Where sperm hang out waiting for an oocyte to fertilize
Isthmus of uterine tube near uterotubal junction
Sperm adhere to tubal epithelium: intimate junction with cell membrane interaction
Adherent sperm have prolonged lifespan (oocyte has short lifespan)
Sperm are released at time of ovulation
Capacitation
Biochemical changed required in female tract before sperm are capable of fertilization: removal of seminal plasma components, interaction with female secretions
Changes in membrane cholesterol, glycosaminoglycans (membrane fluidity)
Tyrosine phosphorylation
Sperm becomes capable of undergoing acrosome reaction
Hypermotility
Oocyte receptor expression
Acrosome reaction
Usually occurs upon contact of sperm with zona pellucida (in at least some species during cumulus penetration)
Fusion of sperm plasma membrane with outer acrosomal membrane
Extensive vesiculation over surface of sperm and acrosome
Calcium dependent
Allows orderly release of acrosomal enzymes
Acrosome enzymes are in viscous medium: can act sequentially
Penetration of zona pellucida
Sperm migrate between cumulus cells
Attach to and migrate through zona pellucida
Fusion of sperm and oocyte membrane
Zona pellucida has 3 major proteins: ZP3 is specific binding site for sperm, mediated by oligosaccharides
Only acrosome-intact sperm bind to zona
Sperm motility and enzymes required for zonar penetration
Gamete fusion
Sperm goes through cumulus and ZP and is in space between ZP and membrane of oocyte: fuse here
Oocyte membrane is less species-specific in binding than ZP
Completion of acrosome reaction is required before sperm can fuse with oocytes
Fusion involves postacrosomal sperm membrane- becomes fused with oolemma
Blocks to polyspermy
Mainly at level of ZP
Oocyte cortical granules released into perivitelline space- extensive reorganization of ZP structure
Fertilization
Oocyte completes meiosis and expels second polar body
Remaining maternal (haploid) chromosomes are enclosed in pronucleus
Sperm nuclear envelop disintegrates- DNA decondensation
New envelope: male pronucleus
Male and female pronuclei migrate to cell center
Nuclear envelopes disperse
Intermixing of chromosomes occurs
Chromosomes aggregate in prophase of first cleavage division
Proximal centriole of sperm forms one of the zygotes centrioles
Early Development
Several mitotic divisions
Early divisions occur without increase in cell mass (cleavage)
Metabolic support provideed by maternal secretions
Zygotic protein synthesis begins at 2 to 16 cell stage depending on species
Development stage
Ootid Zygote Morula Early blastocyst Hatching blastocyst Hatched blastocyst
Migration to uterus
Early cleavage occurs in uterine tubes
Zygote reaches uterus in about four days in cows and most other species but longer is horses and dogs
Tubal migration in mare
Only fertilized zygotes transported through uterine tubes
Zygote probably responsible for transport by secretion of PGE2
Hatched Blastocyst
Increased fluid in the blastocele
Proteolytic enzymes from the trophoblast
Blastocysts contraction
Important early events
Development inside zona pellucida
Hatching from ZP
maternal recognition of pregnancy
Formation of extraembryonic membranes: yolk sac, amnion, chorion, allantois
Maternal recognition of pregnancy
Critical process in which mother “recognizes” she is pregnant
Major consequence is to prolong period of function of corpus luteum
Transition from cyclic to pregnant state
Maternal recognition of pregnancy major strategies
Anti-luteolytic:
diversion/inhibition of PGF2a secretion
Luteotropic:
maintain CL
Maternal recognition of pregnancy in Ruminants
After hatching from ZP, blastocyst elongates very rapidly- contacts most of uterine luminal epithelium
Trophoectoderm produces interferon tau- IFNt
Inhibits oxytocin receptor synthesis
Inhibits prostaglandin F2a
Stimulates endometrial glands
Prevents luteolysis: makes CL lifespan several months
Maternal recognition of pregnancy in Swine
Blastocyst produces estradiol
Estradiol changes direction of PGF2a secretion by endometrium: from basal (in blood stream) to apical (into uterine lumen)
PGF2a is inactivated in lumen (so CL persists)
Maternal recognition of pregnancy in Horses
Pregnant and non pregnant cycle similar for first 14 days
Suppression of PGF2a release in pregnant mares
-mechanism not completely understood
-embryo mobility
-pre-implantation factor
Pregnancy Physiology
Fertilization in uterine tube
Embryo enters uterus around 6 days after fertilization (transport depends on PGE2)
Unfertilized oocytes not transported through uterine tube
Mobility Phase
Equine embryo migrates through both uterine horns and body several times a day (up to 18 horn changes per day)
Movement is mediated by uterus
Movement is maximal around days 10-12
Prevention of movement results in pregnancy loss
Embryo Capsule
Unique to equids
Polysaccharide-rich membrane between trophoectoderm and ZP
Forms soon after embryo reaches uterus
Probably responsible to preserving round shape of embryo and maintaining some rigidity, necessary for uterine migration
Disappears after day 23
Maternal recognition of pregnancy in Llama
Almost all pregnancies in left uterine horn
PGF2a synthesis or release is inhibited, but mechanism is not known
Chorionic gonadotrophins
eCG: Horses, Chorionic girdle-endometrial cups
hCG: Human, very early: luterotrophic
Placental lactogen/pregnancy-associated glycoproteins: Ruminants
Placenta
Temporary organ providing support and protection for developing embryo/fetus
Maternal and fetal components
Metabolically active
Placental Origin
Trophoblast
Allantoic cavity originates from here as well?
Placental General Structure
Chorion- outermost layer Amnion Allantois Yolk sac Umbilical cord
Chorion
Outermost layer
Amnion
Cavity immediately surrounding embryo/fetus
Allantois
Cavity originating as outpouching of hindgut
Continuous with urachus
Placental classification
Shape
Degree of invasiveness
Intimacy of attachement
Nature of fetal placental vasculature
Placental shapes
Diffuse (horse, pig, camelid)
Cotyledonary (cow, sheep, goat)
Zonary (dog and cat)
Discoid (human, mouse)
Epitheliochorial
Intact membranes both sides
Horse, pig
No erosion of baby side
Syndesmochorial
Fetal-maternal synctium
Sheep (cow)
Some cells on maternal side fuse together to form syncytium
Endotheliochorial
Dog and car
Chorion eats through maternal epithelial
Hemochorial
Human, mouse, guinea pig
Adeciduate
Horse, pig, ruminants
Deciduate
Dog, cat, human
When baby is born and placenta shed, some maternal tissue is lost
like deciduous tree
Bovine Placenta
Cotyledonary, chorioallantoic, epitheliochorial, and syndesmochorial, adeciduate (qualified)
Amniotic plaques
Allantoic calcification mineralization
Equine Placental
Diffuse, chorioallantoic, epitheliochorial, villous (microcotyledonary, adeciduate
Canine Placenta
Zonart, chorioallantoic, endotheliochorial, deciduate
Marginal hematoma
Yolk sac
Placental Hormone Production
Progesterone Estrogen Relaxin Placental lactogen Chorionic gonadotrophin Prolactin
Fetal Fluids
Protection
Nutrient reservoir (fetus continually swallows fluid)
Water reservoir
Maintenance of Pregnancy
Progesterone (or progestogen) is an absolute requirement for mammalian pregnancy
Where does the progesterone come from? Ovary and placenta
Progesterone in Pregnancy
C21 steroid Progesterone or prostagens essential for maintenance of pregnancy in mammals Placental development and function Myometrial quiescence Cervical closure Immune mediation
Physiology of Parturition
Studied mainly in sheep
To a lesser extent in laboratory species, non human primates
Understanding depends on fragmentary knowledge and extrapolation
Trigger for Parturition
Fetal hypothalamic-pituitary-adrenal axis
Sustained fetal hypoxemia in late pregnancy
Placental adequacy, rapidly growing fetus
Fetal maturation
CRH->ACTH-> Cortisol
High cortisol levels: highly bound;rapidly metabolized; cortisol receptors
Elevated Fetal Cortisol
Final maturation of fetal lung (mediates release of surfactants here), kidneys, brain
Induces P450 enzymes: 17a-hydroxylase and C17-20lyse
Progesterone-Androgens
Increased production of prostaglandins by placenta and uterus
Prostaglandin F2a
Further stimulates fetal secretion of CRH
Stimulates P450 system
Myotonic
Estrogen
Synergistic with relaxin
Breaks down disulphide bonds in collagen: softening of ligaments and cervical ripening
Relaxin: ovary (cow), Placenta (mare)
Fergusson’s Reflex
Pressure in cervix/vagina
Increased secretion of oxytocin (positive feedback)
Urge to push
Fetus moves into birth canal
Cervical dilation
Starts at vaginal end
Cone-shaped
Progressively shorter
Ultimately completely obliterated
Neonate
Adjustment to postnatal life
Respiration
Circulatory changes: fetal adaptation that are harmful in postnatal life
Thermoregulation
Fetus practices breathing amniote to strengthen muscles
First Breath
Fluid compressed from lung and airway during vaginal delivery
Amniotic fluid also absorbed from lung into pulmonary circulation
Cold, touch, sound stimulate respiration
Central chemoreceptors increase respiratory dive- stimulated by hypoxia, hypercarbia
Initial breathing
high negative inspiratory pressure: -airway resistance -fluid in airways -surface tension in alveoli Production of surfactant by type II pneumocytes: -cortisol -epinephrin -alveolar distension
Control of ventilation
Respiratory rhythm generated in ventrolateral medulla: modulated by central chemoreceptors (O2, pH, CO2)
Peripheral chemoreceptors (aortic and carotid bodies) are functional bu silent (high O2 in neonate): adaptation over 48h
Initial response to hypoxia is tachypnea followed by reversion to fetal response
Fetal Circulation
Oxygenated blood delivered preferentially to brain, myocardium
Returning oxygenated blood: ductus venosus (bypasses liver to vena cava) and foramen ovale (shunted from right to left atrium)
Deoxygenated blood: right atrium, right ventricle, pulmonary artery. Ductus arteriosus deviates 80% to aorta
At birth
Increased pulmonary blood flow
Removal of placenta and placental circulation
Closure of: foramen ovale (functional and rapid),
Ductus arteriosus (drop in pulmonary pressure, increase in systemic vascular resistance: reverse flow; increased PO2; PGE2 declines),
Ductus venosus (reverse flow)
Thermoregulation
Heat loss:
-high surface to volume (bodyweight) ratio
-Limited subcutaneous fat for insulation
-evaporative hear loss (wet neonate)
Thermogenesis
-limb movement
-stimulation of brown fat (sympathetic)- triglycerides and free fatty acids
Other considerations
High body water content (75%)
Hepatic function: gradually increases over first 3 months
Renal function: glomeruli and nephrons present at birth but immature
-lack of osmotic gradient (lack of concentrating ability)
-lower GFR than adult (neonate susceptible to both dehydration and volume overload)
Lactation
Progesterone, estradiol, prolactin, placental lactogens
Colostrum: immuneoglobulins, immune cells, nutritive value
Milk ejection reflex: sensory input, oxytocin release
