Gonads Flashcards
What are the gonads?
Testes in males
Ovaries in females
What are the main functions of the gonads?
Gametogenesis
Steroidogenesis of Androgens (more in males) and Oestrogens/Progesterones (more in females)
What are the names for immature sperm and egg cells?
Spermatogonia
Oogonia
When does production of Oogonia ‘stop?’
After 24 weeks
When does spermatogenesis begin?
Puberty - due to testosterone and gonadotrophin releasing hormone
How long does spermatogenesis take?
~70 days in total
Describe the steps of Spermatozoa production:
Germ Cells 44+XY
Differentiate to spermatogonia 44+XY
Mitotic division to primary spermatocytes 44+XY (50:50 to produce spermatogonia as well)
First meiotic division to produce secondary spermatocytes 22X/Y
Second meiotic division to produce spermatids 22X/Y
Spermatids become spermatozoa 22X/Y
Describe the spermatogonia pool:
Undergo differentiation and self-renewal, so pool remains throughout life; males retain some capability for whole life - producing 300-600 sperm/gm testis/second (1000 sperm for every heartbeat)
Describe Oogenesis:
Germ Cells 44XX
Differentiate to oogonia 44XX
Mitotic division to primary oocytes 44XX - early in embryogenesis, also develop a layer of cells around self to form a primordial follicle
Primordial follicles undergo arrest of meiosis until puberty
First meiotic division to produce secondary oocytes 22X and first polar body (just membrane and chromosomes - with no cytoplasm or resources)
Second meiotic division occurs to produce ovum 22X (retains all cell resources) with second polar body (that has no cell resources)
What is the initial number of primordial follicles and how many remain by puberty?
6 million to 0.5 million
Due to atresia
Describe the anatomy the testes:
Coiled seminiferous tubules connect to the rete testis.
Semen leaves the testis through the Vasa efferentia through the epididymis into the vas deferens
What happens to the testis during puberty?
Testosterone causes maturation of coiled seminiferous tubules to allow spermatogenesis
How does the Vas Deferens expel fluid?
Surrounded with smooth muscle
What are the cells in the testes that produce Testosterone and androgens?
Leydig cells
What are the cells present in the Seminiferous tubules, and how are they arranged?
Sertoli cells
Elongates, connected at periphery by tight junctions forming the blood-testes barrier.
What does the blood-testis barrier do?
Stops blood molecules entering the tubules
Where are spermatogonia present in seminiferous tubules?
Around the outside
How do spermatogonia undergo maturation?
Cross membrane of sertoli cells, in which they mature and develop before release into the lumen of the tubule
What receptors do Sertoli cells contain and why?
FSH/Androgen receptors which act as the primary control of spermatogenesis
What happens when FSH binds to sertoli cells?
Inhibin and androgen binding protein produced which are associated with developing spermatocytes and forming their protective enviroment
What is contained in the ovaries?
Graafian follicles
Follicles undergoing atresia
Corpus luteum
What is a Graafian follicle?
Ovum in follicular fluid surrounded by Granulosa and Thecal cells
What is the corpus luteum?
After an egg is released, the follicle converts to an empty corpus luteum
When does the menstrual cycle start?
On the first day of bleeding.
What are the two parts of the menstrual cycle?
Ovarian cycle - Follicular phase followed by luteal phase
Endometrial cycle - Lining of uterus with superficial and deep stromal layer; proliferative phase followed by secretory phase
What is the Proliferative phase of the Endometrial cycle?
Mitosis of cells lining uterus, causing wall thickening inducing oestrogen and progesterone receptors
What is the secretory phase of the menstrual cycle?
Progesterone and oestradiol produced in the luteal phase causes the endometrium to stop proliferating by reducing oestrogen receptors whilst increasing secretory activity to make it suitable for implantation
What are the four main hormones active in the menstrual cycle?
FSH
LH
Oestradiol
Progesterone
Describe the levels of FSH during the cycle:
Increased at beginning to stimulate follicles to start growing and developing oestradiol.
Peaks around 13 days
Looks like ‘W’
Describe levels of oestradiol during the cycle:
Negative feedback decreases of FSH levels
Low at beginning
Peaks at 12 days, peaks again (less) around 20 days
(Looks like ‘M’)
Describe the levels of LH during the cycle:
Surge of gonadotrophins on day 14 triggers ovulation of dominant follicle
Describe the levels of Progesterone during the cycle:
Corpus luteum secretes progesterone (and a little oestrogen) to have a negative feedback effect, keeping LH/FSH low; decline if no fertilisation, so negative feedback lessons so LH/FSH rise to cause follicle maturation
Describe the four stages of the ovarian cycle:
Follicles spontaneously selected in groups for development to the pre-antral stage with no gonadotrophin stimulation
FSH causes development to early antral follicles, growing and developing to late antral follicles, and producing oestrogen
Dominant follicle selected, and others die in atresia, while dominant grows to Graafian follicles for ovulation
After ovulation, forms corpus luteum; granulosum cells grow, enlarge and produce a lot of progesterone for release to the blood for 13-14 days
Which days of the cycle are the proliferative phase and secretory phase?
Proliferative: 1-14
Secretory: 15-28
Where does reduction of testosterone occur?
Reduction to Dihydrotestosterone: Prostate Testes Seminal vesicles Skin Brain Adenohypophysis
Where does aromatisation of testosterone occur?
Aromatisation to oestrogens: Adrenals Testes Liver Skin Brain
What are the principle uses of androgens?
Development of genitalia and promotion of foetal growth
Spermatogenesis and growth of secondary sex characteristics in adults
What are the main effects of Oestrogens?
Stimulate endometrial mitosis and LH surge in menstrual cycle
Decreases renal salt reabsorption by competitively inhibiting aldosterone
Negative feedback on gonadotrophin releasing hormone
What are the main effects of progestogens?
Stimulates secretory activity in endometrium/cervix ready for implantation of eggs
(Negatively regulates Gonadotrophin releasing hormone)
Describe the Hypothalamo-Pituitary-Testicular Axis:
GnRH released in pulses every hour from hypothalamus
Causes pulses of LH/FSH release from APG
LH = endocrine, FSH = gametes
LH stimulates Leydig cells to make testosterone
FSH stimulates Sertoli cells to support spermatogenesis
Testosterone negatively feedbacks hypothalamus/APG
Inhibin produced by Sertoli cells also has negative feedback on hypothalamus/APG
Describe the Hypothalamo-Pituitary-Ovarian Axis:
GnRH released in pulses every hour from hypothalamus
Causes pulses of LH/FSH release from APG
LH/FSH release oestradiol (feedback on LH/FSH)
Inhibin release also has negative feedback on hypothalamus
Name the 5 phases of the Menstrual cycle:
Early Follicular Early-mid follicular Mid follicular Late follicular Luteal Phase
What happens in the early follicular phase?
Day 1 - Day 5;
5-10 eggs start to enlarge and grow, competing with each other to grow, so only one (Graafian) follicle will become dominant; FSH produced by APG causes follicles to grow and produce oestradiol
What happens in the early-mid follicular stage?
Follicles growing release oestradiol (begins to negatively feedback on LH/FSH to decrease levels; autocrine feedback loop arises, where more oestrogen production stimulates increased granulosa cell growth, in turn leading to more oestrogen
What happens in the Mid follicular stage?
Oestrogen and inhibin reduces FSH to LH to very low levels, so all follicles regress except the Graafian follicle
What happens in the Late follicular stage?
Oestrogen reaches high level and when bypasses threshold, causes a positive feedback loop on GnRH/LH secretion to cause an LH surge and ovulation
What happens in the Luteal phase?
Corpus luteum produces progesterone (men should NOT produce) to prepare the endometrium for implantation
What happens if the egg is not fertilised by the end of the Luteal phase?
If no fertilisation then progesterone, oestrogen and inhibin have a direct negative feedback on the APG to reduce LH/FSH production, and indirectly on GnRH
What is Amenorrhoea?
Absence of menstrual cycle
What is the difference between primary and secondary amenorrhoea?
Primary: periods never start
Secondary: periods stop after having some
What is Oligomenorrhoea?
Infrequent periods; less than every six weeks
What is Infertility defined as?
Couple unable to get pregnant following 12 months of regular unprotected sex; can be caused by pituitary failure, prolactinoma (prolactin inhibits LH/FSH), testicular failure (e.g. Klinefelter), ovarian failure (e.g. Turner’s), polycystic ovarian syndrome (most common - 1in8)
