LECTURE 24 - puberty and sex differentiation

Question 1

What is the gonads?

Answer 1

The organ that produces gametes - ovary or testes

Question 2

What are gametes?

Answer 2

Mature male or female reproductive cell with a haploid set of chromosomes (gametogenesis)
Female = oocyte (egg - produced by oogenesis)
Male = spermatozoa (sperm - produced by spermatogenesis)

Germ cell = sperm or oocyte or their developmental precursors

Question 3

What are primordial germ cells?

Answer 3

• male and female embryos are indifferent until week 7
• diploid germ cell precursors
• arise during gastrulation ~ week 3
• epiblast derived
• during weeks 3-7
  
  • proliferate by mitosis
  • migrate by amoeboid movement to region of Doral wall that will form the gonads (gonadal ridges)
  • migration is guided by chemotaxis

Question 4

What happens to PGCs from week 7 onwards?

Answer 4

• can now differentiate between male and female
• the Y chromosome is unable to encode enough to form a testis
• instead the Y chromosome encodes a controller gene = testis determining factor

Question 5

What is SRY?

Answer 5

Sex-determining Region Y gene = Testis Determining Factor (TDF)

• discovered to be conserved across mammals
• female mice with SRY transgene show male development
• XY females have SRY mutations or deletions and XX males have SRY translocations
• SRY is a transcription factor - controller gene

Question 6

How do testes develop from week 7 onwards?

Answer 6

3 sets of cells that start to arrive at gonadal ridges

1. columns of cells from coelomic epithelium - proliferate and penetrate deeply to from primitive sex cords - express SRY –> sertoli cells
2. PGCs arrive, become associated with primitive sex cords, sex cord cells surround PGCs and form seminiferous tubules (basic structural unit)
3. migratory cells from mesonephric primordial (form the vascular system of testes) and form Leydig cells

Question 7

Describe the structure of the testes

Answer 7

WITHIN seminiferous tubules

• prospermatogonia
• sex cord cells form Sertoli cells

BETWEEN seminiferous tubules
- vascularisation and clusters of Leydig cells form

• SRY no longer expressed; testes produce hormones => development of male reproductive system

Question 8

How are the ovaries formed?

Answer 8

• sex cords do not penetrate deeply, they cluster around PGC (oogonia) => primordial follicles –> form somatic cell type called granulosa
• mesonephric cells - form vasculature and thecal cells (second important cell type)
• there is no endocrine activity from the fetal ovary during development of the female reproductive system
• further development is dependent on the presence of normal germ cells

Question 9

What is Turner’s Syndrome?

Answer 9

• genetically XO (only has e1 X chromosome)
• normal oocyte development requires both X chromosomes –> oocyte death
• normal ovary development requires normal germ cells –> ovarian dysgenesis

Question 10

How does the internal reproductive system develop from weeks 8-12?

Answer 10

• gonads formed from bipotent structure
• internal reproductive formed from 2 separate unipotent structures
  Wolffian duct = male
  Mullerian duct = female

Question 11

How is the male internal reproductive system formed?

Answer 11

• testis produces androgens (Leydig) that maintain Wolffian ducts
• testis also produce Mullerian Inhibitory Substance (MIS) aka Anti-Mullerian Hormone (AMH) that causes regression of Mullerian ducts (produced by Sertoli cells)
  Females follow default pathway

Question 12

How do male external genitalia form?

Answer 12

• androgens and dihydrotestosterone (DHT) produced from fetal testes to promote male development
• testosterone –> dihydrotestosterone by enzyme called 5alpha-reductase
• genital tubercle –> penis
• uro-genital fold –> spongy urethra
• labio-scrotal swelling –> scrotal sac

Question 13

How do female external genitalia develop?

Answer 13

• female development = default pathway
• if androgens are not produced female genitalia develop
• genital tubercle –> clitoris
• labio-scrotal swelling –> labia majora
• uro-genital fold –> labia minora

Question 14

What is puberty defined as?

Answer 14

• menarche (female) at ~12.9yrs
• first ejaculation (male) at ~13.4yrs
• activation of the HPG axis
• secondary sexual characteristics
• growth spurt
• reproductive maturity

Question 15

What is the gonadotrophin releasing hormone (GnRH)?

Answer 15

• peptide hormone produced by hypothalamus
• produced from 92 aa prepropeptide
• stimulates the release of gonadotrophins from anterior pituitary
• at puberty, gonadal activation is triggered by activation of pulsatile GnRH secretion
• continuous GnRH leads to down regulation of gnRH receptor on surface of gonadotroph cells - no stimulation of FSH/LH release

Question 16

What is the GnRH pulse generator?

Answer 16

• 54 aa neuropeptide transmitter - kisspeptin 1 (KISS1) binds to KISS1 receptor (aka GRP54)
• kisspeptin-expressing neurones are intimately associated with GnRH-secreting neurones
• kisspeptin pulses match GnRH pulses
• kisspeptin expression rises at puberty
• exogenous kisspeptin administration induced puberty

Question 17

How is there indirect communication from the hypothalamus to the anterior pituitary?

Answer 17

• indirect communication through blood system
• GnRH secreted into capillaries in the primary portal plexus
• transport GnRH into anterior
• binds to GnRHR on gonadotroph cells within anterior
• produce FSH and LH and released into blood system where it can be transported to ovary and testis

Question 18

What are the gonadotrophins?

Answer 18

• HPG axis: FSH and LH
• hCG
• glycoproteins
• binds receptors on cell surface and signal via G-proteins

Question 19

What is Luteinising Hormone (LH)?

Answer 19

Secreted by: gonadotrophs (anterior pituitary)
Acts on: Leydig, theca and granulosa cells
Structure: common a-chain (116 aa), unique B-chain (121 aa), 1 carbohydrate chain
Receptor: LHCGR

Question 20

What is follicle stimulating hormone (FSH)?

Answer 20

Secreted by: gonadotrophs (anterior pituitary)
Acts on: Sertoli and granulosa cells
Structure: common a-chain (116 aa), unique B-chain (111 aa), 2 carbohydrate chains
Receptor: FSHR

Question 21

What is chorionic gonadotrophin (hCG)?

Answer 21

Secreted by: Trophoblast cells (embryo)
Acts on: luteal cells (corpus luteum)
Structure: common a-chain (116 aa), unique B-chain (145 aa), 2 carbohydrate chains
Receptor: LHCGR

Question 22

What are sex steroids?

Answer 22

All derived from cholesterol (via acetate)
3 families
1. Progestagens - pregnancy (+placenta)
2. Androgens - maleness (+adrenals)
3. Oestrogens - femaleness (+liver, adrenals, adipose and placenta)

• lipid soluble - act via nuclear receptors, act via steroid response elements (SREs)
• bound to carrier proteins - often bound to albumin or sex hormone binding globulin (SHBG), in testis there is androgen binding protein (ABP)

Question 23

What are the different progestagen forms?

Answer 23

1. Progesterone (P4)
2. 17a-hydroxyprogesterone (17a-OHP)
3. 20a-hydroxyprogesterone (20a-OHP)

Receptors = PR-A and PR-B

Question 24

What are the different androgen forms?

Answer 24

1. 5a-dihyrodtestoterone (DHT)
2. Testosterone (T)
3. Androstenedione (A4)
4. Dehydroepiandrosterone (DHEA)

Receptor = AR

Question 25

What are the different oestrogen forms?

Answer 25

1. Oestradiol 17B (E2) (puberty to menopause) 2. Oestriol (E3) (pregnancy) 3. Oestrone (E1) (post-menopause) Receptors = Era and ERB (alpha and beta)

Question 26

What does potency depend on?

Answer 26

- how well steroid fits binding site on receptor - varies within each class - some bind receptor from other classes e.g. synthetic progestagens in contraceptive pill can be androgenic - some bind but don't activate e.g. act as antagonists e.g. Mifepristone (RU486) - anti-progestagen (abortion pill)

Question 27

What are secondary sexual characteristics (SCC)?

Answer 27

Timing varies but sequence the same (Tanner staging) Girls - ovarian estrogens => growth of breasts and genitalia - ovarian and adrenal androgens => pubic and axillary hair Boys - testicular androgens => growth of pubic, facial and axillary hair, genitalia, enlargement of larynx and laryngeal muscles, lengthening and thickening of vocal cords

Question 28

What is a growth spurt?

Answer 28

- girls grow about 25cm, boys about 28cm - boys begin 2 years later - height at take off is ~10cm greater - average growth spurt lasts 24-36 months

Question 29

How does a growth spurt occur?

Answer 29

- during development, cartilage becomes ossified except for a small region of cartilage at each end, the epiphyseal growth plate - the key endocrine regulator of postnatal growth is growth hormone (GH) from the anterior pituitary

Question 30

How does GH promote growth development?

Answer 30

- GH stimulates production of insulin-like growth factor-1 (IGF-1) - IGF-1 directly simulates chondrocyte cell division and bone growth - IGF-1 is produced by the liver and locally by chondrocytes in the growth plate - endocrine and paracrine - in early stages of puberty, low levels of oestrogen, stimulates GH-IGF-1 axis and androgens stimulate chondrocyte activity - later puberty (high levels of oestrogen) causes fusion of epiphyseal growth plates - cartilage entirely replaced with bone- statuary growth ceases

Question 31

Describe the timings of puberty?

Answer 31

- research based mainly on females - some genetic impact, African and Hispanic descent tend to start earlier, also similar to family - environmental influences: endocrine disruptors - age at menarche has decreased from ~17yrs in 1840s to ~13yrs by 1950s - weight at menarche has remained constant ~47kg for females

Question 32

What is Leptin?

Answer 32

- produced by white adipocytes - patients with mutations in obsession or LepR gene fail to enter puberty - Leptin receptors are in hypothalamus - involved in puberty - unsure if it is trigger or permissive