Reproductive Systems and Gametogenesis

What does HPG stand for?

Hypothalamic Pituitary Gonadal axis

What does it mean that the HPG is an ‘axis’?

• Involves multiple endocrine glands

• Work together as a system to regulate development, reproduction, and aging in animals

Where is the Hypothalamus and what croe bodily functions does it regulate?

• Component of the forebrain (sits above the anterior and posterior pituitary gland)

• Regulates many core homeostatic functions: metabolism, growth, reproduction, stress

Which peptide hormone does the Hypothalamus secrete which is involved in the HPG?

• GnRH

• Gonadotrophin releasing hormone (10aa long)

What cells within the Anterior Pituitary Gland are stimulated by GnRH?

• Gonadotrophs

How does GnRH travel from the Hypothalamus to the Anterior Pituitary Gland, where does it then bind?

• Through the Hypophyseal portal system

• Binds to the GnRHr receptor

What is the released from a Gonadotroph when stimulated by GnRH?

Signaling cascades culminate in expression of:

• LH - Lutenising Hormone

• FSH - Follicile-Stimulating Hormone

Where are LH and FSH transported to?

• The Gonads, (Testes or Ovaries)

What are the 3 main types of Sex Steroids, and what is the common precusor?

• Progestagens (e.g. progesterone)

• Androgens (e.g. testosterone, dihydrotestosterone, androstenedione)

• Oestrogens (e.g. oestradiol, oestrone)

Common precursor: Cholesterol, (Interconverison occurs via a biosynthetic network)

What is the role of the Sex Steroids?

• Essential for normal reproductive development, gametogenesis and the maintinece of secondary sex characteristics

• Action of the steriods is dictated by tissue-specific receptors

How do Progesterone, Testosterone and Oestrogen regulate the production of LH and FSH?

• Negative Feedback loop

• P,T and O enter the Gonadotroph and combine with respective receptors, these hormone-receptor complexs mediate negative transcriptonal control on target genes

• Thus regulating the production of FSH and LH

Whare are Activin and Inhibin expressed and what is their action with Gonadotrophs?

• Gonads (Testes and Ovaries)

• Bind to specific receptors on the cell surface

• Exert opposing actions on FSH expression, (inhibin also modulates Activin activity)

Both protein families have other diverse biological roles

What are 3 layers of control on Hormone Production and Action?

• Level of production of a given hormone, dictated by a balance of signals elsewhere

• Presence of cellular receptors, how many?, which cells/tissues?

• Intracellular signals, impacted by multiple contributing factors can dictate cellular response to hormone binding

In what manner are GnRH and Anterior Pituitary hormones released?

Pulsatile

• GnRH ~1 per hour

• Pulse generator resides in the hypothalamus

• Alterations in the output of LH and FSH can be achieved by:

  • Increasing or decreasing the amplitude or frequency of GnRH pulses

  • Modulating the response of the Gonadotrophs to the pulses

When does the Pulse generated by the Hypothalamus start?

At pubperty

How does Kisspeptin Positively Regulate GnRH?

• It binds to the GPR receptro found in GnRH neurons, is a potent GnRH stimulator, its receptor is found in many tissues

• KiSS-1 _ve neurons are direct targets of steroid hormones: binding results in negative feedback of Kisspeptin and therefore GnRH

How does RFRP-3 negatively control GnRH secretion?

• RFamide-related peptide 3 is a mammalian orthologue of Gonadotroph-inhibitory hormone (GnIH)

• Acts on the gonadothrophs, KiSS-1 neuronsand GnRH neurons on the hypothalamus, receptors also found in Gonads

• Results in reduced GnRH output and also suppresses the signalling cascade within gonadotrophs that are responsible for LH and FSH expression

What is the deffinition of Spermatogeneis?

The production of Spermatozoa (mature male gametes) frmo spermatogonial stem cells

What is the role of the Testes in the male reproductive system?

Houses seminiferous tubules; site of spermatogenesis, temperature important

What is the role of the Epididymis in the male reproductive system?

Sperm storage ans maturation

What is the role of the Vas Deferens in the male reproductive system?

Transport of sperm from epididymis to urethra during ejaculation

What is the role of the Seminal Vesicle in the male reproductive system?

Produces a mucus secretion wich aid the mobility of sprem

What is the role of the Prostate gland in the male reproductive system?

Produces an alkaline secretion that neutralises acidity of any urine in the urethra and ids the mobility of sperm

What is the role of the Urethra in the male reproductive system?

Tube that carries urine and cum out of the body

What are the 2 main overlapping functions of the Testes?

1. To produce androgens and other hormones for sexual differentiation and secondary sexual characteristics

2. To produce sperm for sexual reproduction: occurs in the semineferous tubules with maturation in the epididymis

Where are Leydig cells found and what do they do?

• Found as part of stroma in the Testes (outside of semineferous tubes or epididymis)

• Synthsise and secrete hormones

What is a PGC?

Primordial Germ Cell

• Gamete precursors,

  • first identafiable at about 3 weeks gestation

• The PGC population expands by mitosis

  • migrates to the genital ridge primordium by six weeks gestation

What migrates to the Genital Ridge at 6 weeks gestation alongside PGCs?

• The Germinal Epithelium

• Eventually becomes:

  • Sertoli cells (male)

  • Granulosa cells (female)

A brief overview of Spermatogenesis, when does it start, what does it generate?

• Begins at puberty (100 million produced per day)

• Involves mitosis and meosis

• Final differentiation step is called spermiogenesis

• Generates 4 mature spermatozoa - identical in size but not genetically

What is the fate of spermatogonial stem cells (SSCs)?

• Adult stem cells, self-regenerating pool undergo rounds of mitosis

• At intervals, groups of morphologicaly distinct cells emerge: type A spermatogonia

• Produce a clone of 16 cells which enter further rounds of mitosis with some differentiations in between

• This produces type B spermatogonia which will go on to become primary spematocytes

Within the Semi-neferous Tuble, which direction does develpment occur in?

• Centripetal (outside in)

• Spermiation: fully differentiated sperm released into the lumen

What is the morphology of a Sperm?

• Acrosome cap - important for fertilisation

• Tail - required for motility

• Mid-piece - contains the mitochondria

• Central axenome - made up of bundles of fibers

What occurs during Spermatogenesis?

• Golgi apperatus forms the acrosome cap

• Nucleus changes shape to fit into the sperm head

• One of the centrioles of the spermadid elongates to become the tail

• The remaining cytoplasm and organelles (residual body) are removed by sertoli cells via phagocytosis

What happens to chromatin during Spermatogenesis?

• X and Y chromosome transcription stops before meotic divisions

• Autosomal transcriptional activities ceases later, during spermatogeneis

• Massive re-packaging of the DNA: histones are replaced by protamines; tightly compressed chromatin with no gene expression

What are Sertoli Cells?

Nurse cells

• Maintain close contact with each other via ‘tight junctional complexes’. Form a barrier to macromolecules called the blood-testis barrier. This separates teses into basal and adluminal compartments

• Spermatogonia become spermatocytes once they enter meosis and move away from the basement membrane. They cross into the adluminal compartments where they recieve all their nutrients from the Sertoli cells

Where in the Testes is an ‘immune-privileged site’?

The adluminal compartment of the semi-neferous tubles, protects the haploid cells from potential immune rejection

What is the duration of spermatogenic cycle?

• 16 days

• At any given time there are four clones of developing sperm cells (within a region of seminiferous epithelium) at different stages of maturation

• Every section of seminiferous tuble produces sperm every 16 days

  • There are pools of cells at different points in the testes entering the cycle at different times to ensure a constant production of sperm

How is Spermatogenesis regulated by Sertoli Cells?

• There is a set lag time between development of clones in order to ensure a constant production

• High degree of spatial and temporal organisation of spermatogenesis; gap junctions between adjacent Sertoli cells provide means for communication

How is Sperm stored and Matured?

• 90% fluid is absorbed in vasa efferentia; dependant on oestrogen

• Passage through epididymis takes 5-11 days; sperm acquire potential to swim and to fertilise oocyte; dependant on androgens

• Mature sperm are stored in the tail end of epididymis ready for ejaculation via the vas differens

What effect does Activin A have on Spermoatogenesis?

Positive feedback on pituitary FSH production, autocrine effects on Sertoli cells; paracrine effects on spermatogenic cells

What effect does Prolactin have on Spermoatogenesis?

Enhances LH stimulation of Leydig cells

What effect does Oestrogen have on Spermoatogenesis?

Required for fluid absorbtion in Testes

What effect does Inhibin B have on Spermoatogenesis?

Stimulates effects on Leydig cells, Negative feedback on FSH production, acts as Activin antagonist, indicative of sperm count

What effect does Oxytocin have on Spermoatogenesis?

Stimulates Seminifirous tuble motility

What effect does Progesterone have on Spermoatogenesis?

Supports spermiogenesis capacitation and testosterone synthesis

What Sterodis are converted in the Testes?

• Some Testosterone and Androstenedione from Leydig cells enter the sertoli cells

• They may be converted to the more potent Dihydrotestosterone or to the oestrogens, Oestrone and Oestradiol17B

• These hormones act to stimulate the sertoli cells but are also secreted into the blood or the tubular fluid

What role does Testosterone play in spermatogenesis?

• Testosterone acts on Sertoli cells where it is essential for; maintinence of the blood-testis barrier, sertoli-spermatid adhesion; spermatid elongation; spermatid adhesion; spermiation; production of testicular fluid

• Testosterone is also essential for the development of male seccondary sex characteristics

• Exerts feedback control on the hypothalamus

• Travels to the tuble lumen, binds to androgen binding proteins (ABP) secreted by the Sertoli cells. Testosterone-ABP travels to and stimulates the ducts of testes

• Testostrone is converted within the Sertoli cells to dihydrotestestosterone and oestrogen which further stiumlates Sertoli cells and support spermatogenesis

What is the deffinition of oogenesis?

The production of oocytes (mature female gametes) from primordial germcells

What is the role of the Uterus in the Female reproductive system?

Suports pregnancy

What is the role of the Ovaries in the Female reproductive system?

Produce oocytes and secrete hormones

What do the Ovarian Stroma consist of?

Connective tissue, smoothmuscle, stromal cells, developing follicles

What is the role of the Uterine (fallopian) tube in the Female reproductive system?

Connects ovary and uterus ; imortant for transport of oocyte/embryo

What are the two main reproductive funtions of the Female Genital Tract?

1. Gamete production and transportation

2. Site of implantatio; support foetal development

How does the episodic nature of the menstral cycle support these two functions?

• During the first (oestrogenic) half, a mature oocyte is produced and made ready for fertilisation

• In the second (progestagenic) half, the uterus is made ready to allow implantation and to support pregnancy

How does Adult Ovaian function differ from Testicular function?

• Far fewer oocytes are produced (arounf 400 in a lifetime vs millions each day)

• Ovulation occurs episodically rather than being contionously produced

• Ovulation stops at menopause. sperm production declines with age but continues into old age

What is a brief outline of Oogenesis?

• Mitotic divisions all occur during foetal development

• Females are born with primary oocytes arrested at prophase 1

• Resumption of meiosis and development of the oocyte occurs after puberty

• Asymetric divisions produce only pne mature oocyte and two polar bodies that contain chromosomes but very little cytoplasmic material

What is the follicle and how long is spent in each stage of development?

The follicle provides the appropriate supportive environment for a developing oocyte in the same way that the seminiferous tubles support sperm development

• Primordial follicile (up to 50 years)

• Pre-antral (primary) follicle (77-85 days)

• Antral (secondary) follicle (8-12 days)

• Pre-ovulatory (tertiary) follicle (30-36 days)

What changes occur between the Primordial to Pre-Antral follicle?

• Growth (20um diameter to around 200-400um)

• Primary oocyte finishes growth to 60-120um, still arrested in prophase 1 of meosis

• Large amounts of mRNA and rRNA produced to build organelles and generate protein stores

• Oocyte secretes glycoproteins which condense to form the Zona Pellucida

• Granulosa cells proliferate to form thick layer around oocyte, contact between granulosa cells and oocyte in maintained through cytoplasmic processes

• Ovarian stromal cells condense to form thecal layer separated from granulosa layer by membrana propria

What happens during Antral Follicle development?

• Thecal layer expands and furthr develops to form two layers. teca interna and theca externa

• Granulosa cells secrete fluid

• Antrum: Fluid filled cavity, this stage is charcterized by increasing follicle size (due to Antrum)

• Oocyte surrounded by granulosa layer ‘cumulus oophorus’ is susspended in follicular fluid by a thin stalk which connects to ‘mural’ granulosa

• Oocyte continues to synthesize RNA and make proteins

How do the Oocyte and Granulosa cells comunicate?

• Granulosa cella are connected to oocyte through cytoplasmic processes

• Gap junctions form between adjacent granulosa cells and the oocyte surface

• Extensive network of communication which allows transfer of amion acids and nucleotides to growing oocyte

• Granulosa-oocyte complex resembles Sertoli cell-spearmatogenic complex

How is Follicle development regulated?

• Very early primordial follicle development is stimulated locally via growth factors and cytokines, a few folliciles recomence growth everyday

• Further follicile development is dependant on the pituitary, abcence of the pituitary input results in Atresia

• FSH - KO mice arrest follicular development at the pre-antral stage; LH KO mice stop at the antral phase

• During follicular phase, only cells in theca interna bind LH, only granulosa cells bind FSH

What do Thecal cells generate as a result of LH stimulation?

• Androgens

  • Androstenedione

  • Testosterone

What hormones from the Pituitary are required for Follicle growth and egg maturation?

• FSH

• LH

• (also low levels of oestrogens)

What happens to the amount of Hormones released by Antral Follicles as they grow?

It increases

What do the Granuloasa cells convert the Anreogens released by the Thecal cells into?

• Oestrogens

  • Oestradiol 17B

  • Oestroen (upon FSH stimulation)

What happens later in the Menstral cycle when Granulosa cells express LH receptors?

When stimulated with LH, Progensterone is synthesised

Generaly how does the HPG axis work in Females?

• Increase in Androgens case granulosa cell proliferation and increase in oestrogen procuction

• Oestrogens themselves also promote granulosa proliferation

• Causes an oestrogen surge which then exerts positive feedback to stimulate LH surge

What are the effects of the LH surge?

• Entry into preovulatory phase of growth

• Nuclear membrane breaks down and meosis resumes up to metaphase 2. Half of the chromosmes but majority of the cytoplasm go into one cell (the seccondary oocyte), rest is the first polar body (dies)

• Cytoplasmic maturation occurs (synthesis of specific set of proteins, reorganisation of microtubles)

• Follicle rutures and oocyte is carried out in follicular fluid

What happens in the Luteal phase?

Granuloas cells switch from oestrogen production to progesterone production under LH stimulation. Forms positive feedback loop where granulosa cells are then stimulated by their own progesterone resulting in exponential increase in progesterone release

What happens as a result of the Progesterone surge in the Luteal phase?

• Depresses the growth of less mature follicles

• Promotes transition to progestagenic phase of ovarian cycle

What is the Corpus Lutem?

• Following ovulation, the empty follicle collapses and beomes highly vascularised - the corpus lutem

• Large lutein cells (granulosa cells) synthesise progestagens, small lutein cells (thecal cells) produce progesterone and androgens

• Also secretes inhibin and oxytocin: important in maintaingin the corpus lutem and for luteolysis

• LH is required for luteinisation; prolactin, oestrogen and progesterone are required to maintain corpus lutem

• Corpus lutem undergoes luteolysis (life ranges from 12-14 days in humans)

How does Oestradiol regulate the HPG axis in Females?

Acts in different ways at different points in the menstrual cycle: at low concentrations it acts to negatively regulate LH expression but at high levels it acts to positively regulate expression

How does Progensterone regulate the HPG axis in Females?

• High concentration seen in luteal phase of menstural cycle (after ovulation) enhances negative feedback of Oestradiol

• Additionaly, the positive feedback effect of Oestradiol is blocked

• Progesterone acts on both the hypothalamus and the pituitary

• The result is a fall in FSH and LH after ovulation

How does Activin regulate the HPG axis in Females?

Positively regulates FSH secresion at the level of the pituitary

How does Inhibin B regulate the HPG axis in Females?

Selectively negatively regulates FSH secretion at the level of the pituitary; inhibins also act as Activin antagonsits