Reproductive physiology Flashcards
How is the sex of an embryo determined ?
Male sex determination
- Determined by the male sperm carrying an XY chromosome
- The Y chromosome gene codes for testis determining factor (TDF) in the male
- TDF acts to make the mesonephric duct, rete testis and sex cords interconnect, and sertoli cells to form
- Sertoli cells secrete antimullerian hormone which causes the paramesonephric duct to degenerate, and Leydig cells to differentiate
- Leydig cells produce testosterone act on the sex cords and mesonephric duct to develop into the semififerous tubules, epididymis and vas deferens.
Female sex determination
- NO Y chromosome so no testis determining factor
- thus mesonephric duct regresses
- paramesonephric duct becomes oviduct and uterus.
Describe the classes of steroid hormones ?
Steroid hormones
all originate from cholesterol
- sex steroids, progestagens, oestrogens and testosterone
- diffuse through plasma membranes, cytoplasm and nuclear membrane of target cell
- bind to nuclear receptors that trigger mRNA production and synthesis of new proteins
- require carrier proteins
Principal properties of natural androgens ?
Androgens
- induce and maintain differentiation of male somatic tissues (primary sexual characteristics
- induce secondary sexual characteristics of the male
- influence sexual and aggressive behaviour in the male
- promote protein metabolism, somatic growth and ossification
- regulate secretion of gonadotrophins
Describe the principal properies of natural progestins ?
Progestins
- regulate secretion of gonadotrophins
- prepare uterus for conceptus
- stimulate growth of mammary glands and suppress milk secretion
- regulate secretion of gonadotrophins
- maintain uterus during pregnancy
- mild increase in sodium retention of the kidney
- general mild catabolic effect
Describe the general properties of natural oestrogens ?
Oestrogen
- stimulate secondary sexual characteristics of the female
- prepare uterus for sperm transport
- increase vascular permeability and tissue odema
- stimulate growth and activity of the mammary gland and endometrium
- prepare endometrium for the action of progesterone
- associated with female sexual behaviour in a number of species
- regulate secretion of gonadotrophs
Describe the general mechanism of action of natural Eicosamoids ?
Eicosamoids
- prostaglandins, (leukotrienes minor role in reproduction)
- PGS synthesised in most tissues of the body (type varies)
- PGF2alpha stimulate unterine contractions
- influences ovulation
- short half lives <3mins = local action
Describe the mechanism of action of protein hormones ?
Protein hormones
- these glycoproteins consist of an alpha and beta subunit
- water soluble means these hormones are impermeable to the plasma membrane
- protein hormones activate protein kinases Camp, which activates regulatory subunit R - allowing the construction of new proteins
- LH, FSH, growth hormone, prolactin
- inhibins and activins which act to inhibit or stimulate FSH respectively
The strength of hormone action depends on ?
Hormone strength
- pattern and duration of secretion - episodic, basal or sustained
- half life (short half life = more responsive to change)
- receptor density
- receptor hormone affinity
Note - hormones can be produced from both primary secretions and metabolic conversion eg 20-30% oestrone comes from the ovarie, the remaining 80-70% from metabolic conversion in the liver
In addition the removal of hormones is mainly by metabolic conversion.
Provide a definition for puberty in the male and female ?
Puberty
Defined as the ability to reproduce
There are a number of definitions for both males and females - as puberty requires time and is not defined by a singular event
Females
- age at first oestrus
- age at first ovulation
- age at which a female is first able to support a pregnancy without any deleterious effects
Males
- age at which reproductive behaviours are expressed
- age at first ejaculation
- age at which sperm first appear in urine
- age at which ejaculate contains a threshold number of spermatozoa = successful fertilization
How does the hypothalamus become altered during puberty in the male and female animal ?
The onset of puberty is regulated by the hypothalamus
- depends upon hypothalamic neurons producing sufficient quantities of GnRH to rpomote spermatogenesis
- the hypothalamus is functionally different inmales and females
Hypothalamus is inherently female
- testosterone released from the foetal testis “defeminises” the brain by inhibiting the surge centre
- oestrodiol is responsible for this process
- In females alpha-fetoprotein prevents oestrodiol from crossing the blood brain barrier
- no oestrodiol = surge centre development
How do GnRH neurons acquire the ability to release GnRH in high frequency doses after puberty ?
Puberty depends on the release of sufficient quantities of GnRH from the hypothalamus ?
This process is gradual
Before puberty
- the tonic centre is highly sensitive to negative feed-back by low levels of oestrogens or testosterone (ovaries / testes)
- the surge centre is not yet responsive to positive fedd-back by oestrogen in females
During the pubertal transition
- sensitivity begins to decline and increasing amounts of GnRH are produced by the tonic centre
- surge centre becomes responsive
What causes the loss in sensitivity of the tonic centre ?
A certain degree of fat reserves are needed before the brain “ allocates” energy to initiate reproductive processes
- fatness alone does not initiate puberty
- must also reach a threshold size
- infulenced by the concentrations of glucose, leptin and fatty acids
Describe what factors affect the onset of puberty ?
Factors which influence the onset of puberty
- quality and quantity of food - during growth and development
- genetics - breed differences
- social conditions - large group size, boar pressence in female pigs
- season of birth and photoperiod - seasonally breeding animals
- critical weight must be reached
Define the a simplex, Bicornuate and Duplex uterus ?
Uterus types
Simplex = single uterine body without horns and a single cervix eg humans
Bicornuate = poorly to moderately developed uterine horns eg mare, cow
Duplex = two distinct uteri and two cervixes eg rabbit, marsupial
What is the function of the cervix ?
Function cervix
- barrier to sperm transport
- isolate uterus during pregnancy
- cervical mucous = permmits sperm migration and lubricates the vagina under oestrogen
- plugs cervix under progesterone
Describe the structure of uterine tissue, and its function ?
Uterus
Comprised of
- Perimetrium - serosa
- Myometrium - muscularis
- Endometrium - mucosa and submucosa
Uterine glands develop from the endometrium mucosa penetrate the submucosa and become coiled - they secrete material into the lumen of the uterus.
Function
- sperm transport (motile contraction)
- environment for implantation
- expulsion of foetus at birth
- luteolysis PGF2A
- contributes to maternal placenta
Describe the structure and function of the oviduct ?
The oviduct
Function
- gamete transport to site of fertilisation
- temporary sperm storage
- faciliatae sperm capacitation
- support for pre-implanted embryo
Describe the structure and function of the ovary ?
The ovary
Function
- produces oocytes
- hormones oestrodiol, inhibin
- forms corpus luteum and produces progesterone
Structure
- infundibulum, isthmus, ampulla
- outer cortex contains follicles and CL
- innermedulla contains blood vessels, lymphatics and nerves
- exception the horse ovary is inside out eg. ovulation fossa
- in general all types of follicles are present within the ovary at any point in time (except CL only during the luteal phase)
Describe the different stages of follicle development ?
Follicle development
Primary follicle = single layer of cuboidal epithelium
Secondary follicle = mutiple stratified cuboidal epithelium, called granulosa cells and thecal cells begin to develop
Tertiary follicle / Antral follicle = oocyte develops a zona pellucida, outer thecal cell layer and an antrum begins to form
Preovulatory follicle = antrum expands isolating oocyte with surrounding ring of cummulus cells
Ovulating follicle = follicle ruptures and oocyte ovulated with cummulus cells
Corpus luteum = residual follicle luteinises to form CL
Describe the production and development of oocytes over a female animals lifetime?
Folliculogenesis
Oogonia
- divide extensively before birth but cese before birth (human, cow, sheep, goat), or shortly thereafter (rat, pig, rabbit)
Primordial follicles
- form when squamous cells surround primary oocytes
- finite reservoir at bith to last entire female reproductive life
- remain dormant until activated by puberty
- most primary oocytes degenerate before puberty
Pre antral = primordial, primary and secondary follicles
Antral = tertiary and preovulatory follicles
What events take place during folliculogenesispro ?
Folliculogenesis
Events from primordial to preantral transition
- Follicular diameter increases (20-400)
- growth of primary oocytes to final size
- formation of glycoprotein rich zona pellucida (polyspermy barrier)
- early development of granulosa and theca cells
Events from Preantral to Antral transition
- proliferation of granulosa cells
- fluid accumulation between cells
- formation of follicular antrum
- follicle size increases due to fluid accumulation and granulosa cell proliferation
Antral to ovulatory transition
- stimulated by FSH and LH
- continuously develop and are varous sizes at this point in the oestrous cycle
- small, medium and large according to species
Recruitment, selection and dominance
Describe the hormonal control of folliculogenesis ?
Hormonal control
Release GnRH from hypothalamic tonic centre
- FSH and LH receptors first appear on pre-antral and antral follicles
- Granulosa cells FSH receptors
- Theca cells LH receptors
- Antral follicles produce increasing amounts of oestrodiol and oestrone
What is recruitment, selection and dominance ?
Recruitment, selection and dominance
Recruitment = a cohort of small antral follicles start to develop and secrete small amounts of oestrogen
- FSH promotes follicular growth, LH is less important
Selection = some recruited follicles either degenerate or are chosen to grow and secrete moderate amounts of oestrodiol
- inhibin and oestrodiol inhibit FSH and LH secretions
Dominance = selected follicles that do not degenerate become dominant, and secrete large amounts of oestrodiol
- lead follicle secretes large amounts of oestrodiol
- oestrodiol reaches threshold and starts having positive feedback on the hypothalamus (through surge centre) - increases GnRH - surge of LH
There are several waves of follicular recruitment, selection and dominance but only after luteolysis can this result in ovulation. (removal of progesterone)
What is a Monotocous and polytocous species ?
Monotocous = one dominant follicle
Polytocous = species > 1 dominant follicle
Describe the effect of hormones during egg lay in chickens ?
Describe polyoestrous, seasonally polyoestrous and mono oestrous cycles ?
Different types of oestrous cycles
Polyoestrous cycle = uniform cycles through out the year (cow, pig, cat and rodents)
Seasonally polyoestrous = cluster of cycles within the year
Short day breeders - cycle as the length of day decreases in autumn sheep
long day breeders - cycle as daylength increases in spring horses
Monoestrous =one cycle per year (dog, wolf, bear and fox)
domestic dogs generally have 3 cycles in two years
Break the oestrous cycle down into six different stages ?
Oestrous cycle
Follicular phase = dominant follicle growth and increase in oestradiol (-20% of cycle)
- proestrous; preovulatory development and increase in oestradiol
- oestrous; peak in oestradiol and sexual receptivity/ovulation
Luteal phase = corpus luteum formation and increase in progesterone (80% of cycle)
- metoestrus; corpus luteum formation and increase in progesterone
- dioestrus; sustained luteal secretion of progesterone
Break the oestrous cycle down into six different stages ?
Oestrous cycle
Follicular phase = dominant follicle growth and increase in oestradiol (-20% of cycle)
- proestrous; preovulatory development and increase in oestradiol
- oestrous; peak in oestradiol and sexual receptivity/ovulation
Luteal phase = corpus luteum formation and increase in progesterone (80% of cycle)
- metoestrus; corpus luteum formation and increase in progesterone
- dioestrus; sustained luteal secretion of progesterone
Describe reproduction in the bitch ?
Reproductive cycle of the dog
Anoestrus = is a time of complete reproductive incompetance, marked by a reduction in GnRH secretion (5 months)
- consist prolonged anoestrous, proestrous, oestrous and diestrus
- length of dioestrus is the same in a pregnant and empty bitch
- Oestrogen peaks during proestrus but the bitch is receptive during the decrease in oestrogen and increase in progesterone
- ovulation 3 days p.c, fertilization 6 day p.c
Describe the reproductive cycle of the queen ?
Reproductive cycle of the queen
- induced ovulator
- consist anoestrous, proestros, oestrous, postestrous and dieestrous
- oestrous lasting nine days occurs every 17 days
- no mating - no ovulation - no CL- no P4 - induced ovulator
- female in constant follicular phase until mating occurs
What three things are required for ovulation ?
Hormonal regulation
- Follicular phase GnRH pulse frequency increases
- increase secretion of FSH and LH to be secreted from the anterior lobe of the pituitary
- act upon follicles to increase secretion of oestradiol
- oestradiol causes positive feed back on the neurons of the hypothalmic surge centre - GnRH neurons secrete a surge in GnRH
- additionally the follicle secretes inhibin which causes negative feed-back on FSH secretion from the anterior lobe of the pituitary.
- E2 also acts to inhibit the secretion of FSH
How does LH trigger ovulation
- acts to
- increase PGF2a and releases lysosomal enzymes
- increases blood flow to the ovary and dominant follicle
- increase contraction of ovarian smooth muscle, thus increasing pressure
- gap junction between cumulus cells and oocyte
- shift dominant follicle secretion from E2 to P4 from dominant follicle
- ⇡ P4 = ⇡collagenase = follicle wall breaks
What is the corpus luteum formed by and what is its function ?
What causes luteolysis, and what is the corpus albicans ?
What is the function of PGF2a ?
PGF2a
- main luteolytic agent
- derived from the endometrium - surgical removal of the uterus results in absence of luteolysis
- secreted 10-15 days after ovulation
If a conceptus is present it has to prevent PGF2a secretion if pregnancy is too be maintained
Action of PGF2a
- triggers Ca2+ influx initiating apoptosis
- activates protein kinase C - inhibiting protein synthesis
- PGF2a has a short half life
- secreted from endometrium in pulses 5 hr intervals and pass to the CL through a counter current system
Oxytocin
- secreted by the CL
- acts to increase PGF2a secretion, stimulation
- oxytocin receptors develop 10-15 days after ovulation in the endometrium under the influence of progesterone and oestrogens
Identify the ampulla, seminal vessicles, prostate and bulbourethral gland in the bull ?
Describe the relative sizes of the secondary sex glands in various animals ?
Temperature regulation of the testes
The cellular mechanisms of spermatogenesis require a temperature of 4-5 degrees lower than body temperatureAchieved
Achieved via
Pampiniform plexus
- wraps around the testicular artery in the spermatic cord / counter current exchange of heat from the arterial to venous blood occurs
- contractions of the cremaster muscle has a pumping effect on the pampiniform plexus to accelerate blood flow to and from the testes
- decreases 6 D through successive rounds of heat exchange
The scrotum
- protects the testes and epiidymis
- acts as a thermosensor - large numbers of thermosensitive nerves
- allows evaporative transfer - contains many sweat glands
Respiratory rate
Describe the testis, Leydig and Sertoli cells ?
Sertoli cells = fluid and support for spermatogenesis
Leydig cells = testosterone
Spermatozoa
- Basal compartment - spermatogonia divide to form more spermatogonia and primary spermatocytes.
- Deep adluminal compartment; primary spermatocytes move through “tight junctions” of adjacent sertoli cells then divide to form secondary spermtocytes and round spermatids
- Peripheral adluminal compartment; spermatids undergo elongation and maturation (spermiogenesis)
Blood testes barrier
- formed by tight junctions between adjacent sertoli cells
- barrier between basal and adluminal compartments of the seminiferous tubule
- prevents ‘naive’ immune system from mounting a response to ‘foreign’ spermatozoa
- controls composition of luminal fluid
Spermatogenesis
Three sequential phases
- mitotic prolifeeration
- meitotic division (genetic diversity haploid cell)
- cytodifferentiation (spermatids mature elongate)
- between 300 and 600 spermatozoa produced per gram of testis per second
- divisionof one spermatogonia yields up to 256 sperm
Describe the process of cytodifferentiation ?
Spermatogenesis is highly organised both temporally and spatially within the seminiferous tubule
- sertoli cells are the major means of control
- rounds of spermatogenesis occurs in waves down the seminiferous tubule, initiated at time intervals that are constant and characteristic of each species
- a cross section of a seminiferous tubule will have cells at one particular stage of development
Describe the hormone support of spermatogenesis ?
Describe what occurs as sperm matures within the epididymis ?
Sperm reservoir
- Sperm attach to the epithelial cells of the oviductal isthmus
- special cellular interactions occur, oviductal secretory products, oestrous associated glycoproteins
- beating of flagella slows down in attached sperm
- around the time of ovulation (highly hyperactivated motility, capacitation)
Capacitation
- Ejaculated sperm can not immediately fertilize the oocyte
- spend several hours within the female reproductive tract to attain the capacity to fertilise
- oestrogen and vaginal mucous destabilise sperm plasma membrane and triggers hypersensitivity
- reorganisation of plasma membrane to expose binding sites
