S3 Puberty and the HPG axis

Question 1

what is puberty ?

Answer 1

a stage of human development when sexual menstruation and growth are completed and result in ability to reproduce

Question 2

what are the phases of puberty

Answer 2

girls begin and end puberty before boys
Phases :
accelerated somatic growth - occurs earlier and is less pronounced in girls (age 11-13), is longer and faster in boys and starts between 13-16 so boys are taller. ends when the epiphyseal plates fuse (oestrogen closes plates early in girls)
maturation of primary sexual characteristics : gonads and genitals (e.g penis enlargement)
appearance of secondary sexual characteristics : pubic hair , breast (F), voice change by growth of larynx (M)
menstruation and spermatogenesis. Menarche can only occur after the critical weight of 47 kg is reached, is about 12 years

Question 3

why is puberty now earlier in girls ?

Answer 3

<13 years, in girls as critical weight 47 Kg, any less and puberty shall stop
contributing factors : nutrition, body weight

Question 4

describe the appearance of secondary sexual characteristics in females

Answer 4

9-13 years 
- Breast bud (thelarche)
pubic hair growth (T)
begins (adrenarche)
- growth spurt
- onset of menstrual
cycles (menarche)
-pubic hair adult
- breasts adult

Question 5

describe the appearance of secondary sexual characteristics in males

Answer 5

10-14 years 
- genital development
begins
- pubic hair growth
- spermatogenesis
begins
- growth spurt
- genitalia adult 
- pubic hair growth

Question 6

what is adrenarche

Answer 6

increased testosterone secretion causing growth spurt in males and females

Question 7

what is menarche

Answer 7

first menstrual period

Question 8

what is the tanner scale

Answer 8

monitors development
girls : start at 9-13 years (av 11.5) breast bud development first
Boys : start at 10-14 years (av 12.5) genital development first

Question 9

describe the mechanism underlying such changes at puberty

Answer 9

gradual activation of GnRH, increased frequency and amplitude of LH pulses,gonadotrophins stimulate oestrogen and androgen secretion
GnRH 1 gene is needed for GnRH. Lack of gonadotrophin secretion and reproductive development occurs if GnRH is blocked

Question 10

describe the characteristics of hormones released by the hypothalamus

Answer 10

AP is regulated by H by releasing hormones and by the hormones produced by target tissues feeding back to the AP
these hormones are: secreted in pulses, act on specific receptors, transduce signals via second messengers, stimulate hypertrophy and hyperplasia of target tissue.
GnRH from H stimulates LH and FSH secretion from AP. In the absence of GnRH gonadotrophs secret little or no FSH LH

Question 11

describe the feedback loop of hormones released by the hypothalamus

Answer 11

• testosterone decreases GnRH secretion therefore decreases LH and FSH (-ve feedback)
• moderate amounts of oestrogen decrease GnRH secretion (-ve feedback)
• high amounts of oestrogen increase GnRH secretion(+ve feedback) causing the LH surge
• Progesterone increases the inhibitory effects of moderate oestrogen and prevents the positive feedback of high oestrogen ( so stops the LH surge)
• in both sexes, inhibin decreases FSH secretion (-ve feedback)

Question 12

what is leptin

Answer 12

signals information about energy stores to CNS. Reproductive dysfunction associated with leptin deficiency. Leptin can regulate GnRH levels - the fat levels of leptin tell the hypo if there is enough body mass for puberty

Question 13

describe the hypothalamic control of FSH and LH

Answer 13

GnRH release is pulsatile - every 1-3 hours. Intensity of GnRH stimulus is affected by the frequency of release. GnRH travels to the pituitary in the hypophysial portal system

Question 14

how is the AP connected to the hypothalamus

Answer 14

superior hypophysial artery

Question 15

what hormones do AP release

Answer 15

AP produces LH,FSH, prolactin, also ACTH, GH,TSH
NB when boys sleep LH increases causing a rise in testosterone , possibly accounting for some of the pubertal changes in males

Question 16

how is puberty initiated

Answer 16

initiated by hypothalamus and its onset is associated with rise in LH and FSH secretion

Question 17

what is percocious puberty

Answer 17

puberty at an early age (8-9), possibly caused by tumours of the pineal gland

Question 18

describe how LH and FSH initiate gonadal function

Answer 18

in young children LH and FSH are insufficient to initiate gonadal function, but between 9-12 years, blood levels of LH and FSH increase and amplitude of pulses increases> High levels of LH and FSH initiate gonadal development

Question 19

what does the hypothalamus -pituitary unit regulate

Answer 19

function of reproductive glands (HPG axis)
somatic growth, lactation and water metabolism

Question 20

describe how the HPG axis works in the Male

Answer 20

LH acts on leydig cells to produce testosterone – spermatogenesis
seminiferous tubules made up of sertoli cells and germ cells. Sertoli cells ;
- provide nutrition and hormonal support to germ cells allowing sperm formation
- sensitive to FSH (increase sperm production)
- secreted inhibin (-) feedback on AP FSH

Question 21

describe how the HPG axis works in females

Answer 21

FSH and LH act primarily on gonads via Ga s PCR —> adenylate cyclase
Female target cells : ovarian granulose cells (produce FSH), theca interna (LH)
these stimulate sex hormone synthesis (oestrogen, progesterone,inhibin) and control gamete production (folliculogenesis and ovulation)
oestrogen produced by granulosa cells, inhibin by granulosa cells of corpus luteum

Question 22

after colonising the gonad, what do the germ cells do

Answer 22

proliferate by mitosis
reshuffle genetically and reduce to haploid by meiosis
cytodifferentiate into mature gametes

Question 23

describe spermatogenesis

Answer 23

in the seminiferous tubule, primary spermatocytes divide by meiosis, giving rise to 2 secondary spermatocytes and then to 4 haploid spermatids which differentiate (spermiogenesis) into spermatozoa

Question 24

what is the spermatogenic cycle

Answer 24

defined as the time taken for reappearance of the same stage within a given segment of tubule (~16 days in human). The distance between the same stage is called the spermatogenic wave

Question 25

what is sperm capacitation

Answer 25

final maturation step required before sperm become fertile, conditions in female genital tract stimulate

• removal of glycoproteins and cholesterol from sperm membrane
• activation of sperm signalling pathways. Tail movement changes from beat to whip like action
• allow sperm to bind to zona pellucida of oocyte and initiate acrosome reaction

Question 26

describe oocyte maturation

Answer 26

• germ cells (arise from yolk sac) colonise the gonadal cortex and differentiate into oogonia
• oogonia then proliferate rapidly by mitosis
• majority continue to divide by mitosis but some neter meiosis and are called primary oocytes
• cell death begins around mid-gestation and many oogonia and primary oocytes degenerate (Atresia)
• all surviving primary oocytes enter meiosis 1 and are surrounded by layer of flat epithelial cells called follicular cells. they are now called primordial follicles
  NB most oocytes undero atresia during childhood so 40,000 remain by puberty.

Question 27

what the stages oocytes undergo from puberty

Answer 27

From puberty onwards ~15-20 oocytes start to mature each month passing through 3 stages :
preantral - surrounding follicular cells become granulosa cells
antral - fluid filled space appear between granulosa cells forming the antrum
preovulatory

Question 28

what is the preovulatory phase

Answer 28

surge in LH induces preovulatory growth phase
Meiosis 1 is now completed resulting 2 haploid daughter cells of unequal size
each daughter cells now has 23 chromosomes and 46 chromatids
one cell receives most of the cytoplasm the other (first polar body) recieves practically none
cell then enters meiosis II but arrests in metaphase ~ 3 hours before ovulation
meiosis II is only completed if oocyte is fertilised otherwise cell degenerates ~ 24 hours after ovulation

Question 29

describe the process of ovulation

Answer 29

FSH and LH stimulate rapid growth of follicle
LH surge increase collagenase activity. Prostaglandins increase response to LH and cause muscular contractions in ovarian wall
oocyte extruded and breaks free from ovary

Question 30

what does the corpus luteum do ?

Answer 30

remaining granulosa and theca interna cells become vascularised and form the corpus luteum
secrete oestrogen and progesterone
stimulates uterine mucose to enter secretory stage in preparation for embryo implantation

Question 31

what happens to the corpus luteum if fertilisation doesn’t occur

Answer 31

CL degenerates into corpus albicans, PG production decreases and menstruation occurs

Question 32

what happens to the corpus luteum if fertilisation does occur

Answer 32

the syncytiotrophoblast from embryo releases HGC preventing CL degeneration then CL forms CL graviditatis

Question 33

how is oocyte transported ?

Answer 33

carried into tube by sweeping movements of fimbraie and by motion of cilia on epithelial lining

Question 34

what are the differences in spermatogeneis and oogenises

Answer 34

SG -200 millions sperm per day OG - 1 ovum per 28 day menstrual cycle
SG - 4 spermatids formed with no polar body formation and equal division of cytoplasm OG - one ovum with unequal division of cytoplasm and 3 polar bodies formed
SG - starts at puberty OG - starts in fetus
SG - continues throughout adult life OG - ends at menopause
SG - motile gametes OG - non motile gametes
SG - all stages completed in testes OG - last stage of meiosis 2 occurs in oviduct