Session 3 - Neuroendocrine timing of puberty + Gametogenesis Flashcards
What is puberty?
A stage in puberty when sexual maturation and growth are completed and result in ability to reproduce.
Which basic features change in puberty in boys and girls?
- Accelerated somatic growth
- Maturation of primary sexual characteristics (gonad/genitalia)
- Appearance of secondary characteristics (pubic + axillary hair, female breast development, male voice changes)
- Menstruation and Spermatogenesis begin.
What is the average age of menarche?
Menarche = First menstruation
<13 years old
What is the critical weight in girls?
Weight thought to be needed to achieve first menarche. Around 47kg
What can happen in a woman of reproductive age with significant weight loss?
Menstruation can stop, as reproductive cycles cease.
What is thelarche, and when does it occur?
Development of breast bud
9-13 years
First sign of puberty in girls
What is adrenarche?
Increased levels of androgens from adrenal cortex, causes growth spurt and onset of menstruation.
When does puberty begin in boys, when do various stages happen?
10-14 genital development.
Begins:
- pubic hair growth
- spermatogenesis
What is the tanner scale?
A scale used to asses the stage a person is in puberty. From I to V Assesses: - Genital size - Pubic hair - Testicular volume - Breast size
Which hormone is responsible for pubic hair growth?
Testosterone - in both male and female.
What are the levels of oestrogen and testosterone in girls and boys before puberty?
Similar levels of both in Girls and Boys.
may be on average slightly more testosterone in boys, and vice versa
When does puberty start in girls, and boys?
Girls 9-13
Boys 10-14
Boys later
Why are men generally larger than women?
Boys start somatic growth phase later, but it is rapid, and achieve more growth each year.
The epiphyseal growth plates close later.
What is the first recognised sign that a boy has entered puberty?
Increased testicular volume.
What is the first recognised sign that a boy has entered puberty?
Increased testicular volume.
What is the duration of puberty for men and women, what features show this stage?
Men (3.2 +/- 1.8) years - Adult size of testis
Women (2.4 +/- 1.1) years - Menarche
What is the HPG axis, and its role in puberty?
Hypothalamo-pituitary-gonadal axis.
Gradual activation with release of GnRH from hypothalamus.
This is responsible for switching this axis on, and initiating puberty.
Which gene is important in initiating the cascade of hormones which control puberty?
GnRH- 1 gene
gonadotrophin releasing hormone
What are the characteristics of hypothalamic releasing hormones?
- Secreted in pulses, tied to internal biological clock.
- Act on specific membrane receptors.
- Transduce signals via second messengers.
- Stimulate release of stored pituitary hormones.
- Stimulate synthesis of pituitary hormones
- Stimulates hyperplasia + hypertrophy of target cells.
regulates its own receptor
What does GnRH stimulate in the pituitary?
Stimulates anterior pituitary to secrete:
1) Luteinizing hormone LH
2) Follicle-stimulating hormone FSH
(both made in anterior pituitary)
Which factors in the body affect the release of GnRH in the hypothalamus?
- Leptin (to do with fat levels)
(enough body mass- sexually active life) - Photoperiod (time of light in day)
What are the properties of leptin? How does it affect puberty?
- Adipose derived regulatory hormone
- Important in onset of puberty (weight)
- Can cause menstrual cycles to stop if underweight female.
- Is a pulsatile hormone
How does growth hormone initiate the growth spurt?
GH secretion from pituitary. - Increases TSH > Metabolic rate > tissue growth > Androgens = retention of minerals to support bone and muscle growth.
= Growth spurt
How is GnRH released?
- Pulsatile
- Every 2-3 hours (from hypothalamus)
In portal system - to anterior pituitary
Cause LH, FSH release.
What is the anterior pituitary?
Hormone producing glandular tissue, connecting to hypothalamus by portal system (NOT neurone)
Which hormones does the anterior pituitary produce?
6 Peptide hormones
- Prolactin
- Growth hormone
- Thyroid stimulating hormone(TSH)
- Adrenocorticotropic hormone (ACTH)
- FSH
- LH
When does LH start to rise in males? what time?
Starts in the night.
- Initial pulsitility of LH. (follows GnRH pulses)
Which other axes are there from the pituitary? What do they control?
HPT = thyroid
HTA = adrenal
HPG - gonadal
Growth, lactation, milk secretion, water metabolism
How is testosterone signalled to be produced in males?
Hypothalmus > GnRH
Anterior pituitary > LH
LH stimulates leydig cells to produce steroid hormone testosterone.
What are the leydig cells?
- Cells in the interstitial tissue
- Produce testodterone (separate to seminiferous tubules)
How is spermatogenesis controlled so that it is not constant/ too much?
Sertoli cells secrete inhibin in response to FSH, which negatively feedsback on the hypothalamus.
How do levels of oestrogen affect GnRH from the hypothalamus?
- Low levels oestrogen = reduce GnRH secretion
- High levels oestrogen = promote GnRH secretion
How many gametes does a woman release over her lifetime?
Around 400.
1 per month x number of years in reproductive age
How is genetic variation achieved in gametes?
- Crossing over of regions of DNA between 2 homologous chromosomes.
- Independent assortment - of each bivalent chromosome along the metaphase plate.
- Random segregation - of alleles among the four gametes.
Describe the basic process of spermatogenesis.
- Produced in seminiferous tubules.
(sertoli cells support developing spermatid to spermatozoa) - Travel down tubules to gather in Rete testis
- Here they become condensed, and move to ductili efferentes, for further condesing.
- Move through to head of epididymis (storage).
- Upon ejaculation, move through epididymis, through vas deferens and through the urethra.
Why is the blood testis barrier (sertoli barrier) important?
To ensure spermatid/ spermatozoa do not come into contact with blood, as they are genetically different and could be detected as non- self. This could cause immune response against it.
What are spermatogonia?
- Raw material for spermatogenesis (male germ cells)
What is spermiation?
Process by which spermatids are released into seminiferous tubules from sertoli cells.
What are the three types of spermatogonia in the testes?
Ad Spermatogonium - “Resting”: reserve stock
Ap Spermatogonium - “Active”: Maintain stock + from puberty onwards produce type B spermatogonia, which give rise to primary spermatocytes.
Ad - dark
Ap - pale
What is spermiogenesis?
Differentiation/ maturation of the spermatids (4 daughter cells 1n) into spermatozoa.
What is the spermatogenic cycle?
Defined as time taken for reappearance of the same stage within a given segment of tubule (~16 days).
What is the spermatogenic wave?
The distance between the same stages of the spermatogenic cycle along the length of the seminiferous tubule.
(distance to a section in the same stage of development)
What occurs in spermiogenesis?
- Moved through seminiferous tubules to epididymis.
- Secretions and perstalsis move them (non motile)
- Remodel as they pass through, to finally become motile.
What are the components of the ejaculate? Where do they come from?
- Seminal vesicles (70%) AA, citrate, fructose, prostaglandins - Prostate (25%) Proteolytic enzymes, zinc - Sperm (via vas) (2-5%) - Bulbourethral gland (<1%) Mucoproteins = lubricate, neutralise acidic urine.
What is sperm capacitation?
- Final maturation step before sperm are fertile, happens in female genital tract.
- Removal of glycoproteins and cholesterol from membrane.
Allow sperm to bind to zona pellucida of oocyte.
How is sperm capacitation signalled to happen?
Signalling pathways, (involving soluble adenylyl cyclase and PKA)
How do oogonia develop into oocytes in the female in development?
- Oogonia from yolk sac, stud the cortex of ovary.
- Rapidly proliferate through mitosis.
- Some cells will enter meiosis, but arrest in meiosis 1 = Primary oocytes.
- Maximum ~7 million germ cells made.
- Cell death occurs, and many oogonia and oocytes degenerate.
- All surviving ~2 million oocytes have entered meiosis 1, and are surrounded by flat epithelial cells
= Primordial Follice!
What happens to the number of primordial follicles during childhood?
- Most oocytes undergo atresia during childhood.
- Only ~40,000 remain by puberty.
How many oocytes start to mature each month in the female?
Around 15 - 20
What are the three stages of oocyte maturation?
- Preantral
- Antral
- Preovulatory
What happens in the preantral stage of maturation?
- Surrounding follicular cells become cuboidal in shape, and form a stratified epithelium of granulosa cells around the oocyte.
- Secrete glycoprotein coat called Zona Pellucida. (around oocyte)
What is the antral stage of maturation of oocyte?
- Fluid filled spaces form between granulosa cells, and join forming an antrum.
- Surrounding outer Thecal cells.
How does LH affect theca cells?
Causes theca cells to secrete androgens, which the follicular cells convert to oestrogen.
What is the preovulatory stage of the follicle?
- Surge in LH induces growth phase, Meiosis 1 completes. (polar body too)
- Cell enters meiosis II but arrests at metaphase.
(complete if fertilised)
What happens to the follicles after ovulation?
Becomes corpus luteum.
Remaining granulosa/theca cells become vascularised.
Change into lutein cells (yellowish).
Secrete oestrogen + progesterone to maintain uterine wall.
What is the name of the follicle after growth, several days before ovulation?
Graafian follicle.
How long does the corpus luteum remain for?
degrades after 14 days.
What happens if the oocyte is not fertilised?
- Breakdown of corpus luteum. (forms fibrotic scar mass called corpus albicans)
- Less progesterone secreted
- Menstrual bleeding occurs.
What happens to the corpus luteum if fertilisation occurs?
- Human chorionic gonadotropin is released by embryo.
- Corpus luteum continues to grow: forms corpus luteum of pregnancy.
- Cells secrete progesterone until 4 months. (placenta secretion then becomes adequate)
