10- The Gonads 1

Question 1

DEscribe how germ cells are activated in males and females

Answer 1

During embryogenesis - germ cells proliferate, reaching large numbers around 6-7 million
• in males, this high number of spermatogonia remains throughout life
• During childhood, these spermatogonia have a dormant period and they don’t become activated until puberty
• Similarly, in females, the oogonia multiply up and reach maximum levels at 24 weeks - after this, no more oogonia are produced
• They start off with around 6-7 milion however a process of ATRESIA (cell degeneration)commences
• Atresia is rapid at first - by birth there are only about 2 million oogonia left
• By puberty there are only about 400,000 oogonia left
• By menopause the ovary has been depleted of these cells
• In the female reproductive life, only about 300-400 of these cells reach
maturation and ovulation

Question 2

Describe spermatogenesis

Answer 2

germ cells in embryogenesis multiply and differentiate to produce spermatogonia which are DIPLOID
• Around puberty - when FSH starts to be released - the spermatogonia divide by mitosis to produce either more spermatogonia or to produce primary spermatocytes
• Primary spermatocytes are also DIPLOID
• Primary spermatocytes enter the first
meiotic division to give secondary spermatoctyes which are HAPLOID
• Secondary spermatocytes enter second meiotic division to give spermatids
• Spermatids then mature and differentiate into spermatozoa
• This whole process takes around 70 DAYS

Question 3

Describe oogenesis

Answer 3

Germ cells multiply up to produce oogonia
Primary Oocytes (first meiotic division)
at PROPHASE the development is HALTED so they stay in that stage of development
primordial follicles are formed around the oocytes
around the time of ovulation that they complete the first meiotic division to produce the Secondary Oocyte and a Polar Body (haploid)
The secondary oocyte enters the second meiotic division and it won’t complete that division until FERTILISATION
• If fertilised, it will produce the Ovum and the Second Polar Body

Question 4

Describe the testes

Answer 4

Spermatogenesis takes place in the SEMINIFEROUS TUBULES of the testes
• The testes develop in the abdomen but descend into the scrotum just before birth
• The scrotum is 2-3 degrees cooler than core temperature - which is critical for spermatogenesis
The seminiferous tubules are lined by layers of spermatogonia and layers of Sertoli cells
• The spermatozoa produced travel down to the collection in the Rete Testis where they are concentrated and drained by the Vasa efferentia into the epididymis where they are stored
• In the epididymis, nutrients are secreted for them - they mature in here and attain their motility
• Then they are propelled via the vas deferens (surrounded by smooth muscle) via the urethra

Question 5

Describe the structure of seminiferous tubules

Answer 5

There is a sheath of connective tissue and underneath that there is a layer of spermatogonia and under that there is a layer of elongated Sertoli cells
The Sertoli cells are connected at the periphery by tight junctions

Question 6

What is the function of thesertoli cells

Answer 6

The spermatogonia move into the Sertoli cells and they are enclosed in the cytoplasm of the Sertoli cells where spermatogenesis actually takes place
• The Sertoli cells provide structural support for spermatogonia and provide metabolic support as well

Question 7

What is the function of the leydig cells

Answer 7

Leydig cells contain enzymes so that they can make and secrete testosterone
• It is important that the Leydig cells are nearby because testosterone is needed for spermatogenesis to take place

Question 8

Which hormones do Sertoli cells respond to

Answer 8

respond to FSH + Androgen

Question 9

Which hormones do leydig cells respond to

Answer 9

LH

In response to LH it produces androgens (in particular, testosterone)

Question 10

Which hormones are produced in the adrenals

Answer 10

mineralocorticoids + glucocorticoids + (small amounts of)

androgens

Question 11

Which hormones are produced in the gonads

Answer 11

PROGESTOGENS (C21)
ANDROGENS (C19)
OESTROGENS (C18)

Question 12

Describe the first four reactions in steroidogenesis which are COMMON to both the gonads and the adrenals

Answer 12

Cholesterol, pregnenolone,progesterone, 17 OH progesterone, ANDROSTENEDIONE

Question 13

Why can cortisol and aldosterone only be produced in the adrenals

Answer 13

It is ONLY the adrenals, however, that contain the 21-hydroxylase enzyme that can convert progesterone to deoxycorticosterone and eventually to aldosterone
• Or the same enzyme can convert 17-hydroxyprogesterone to 11-deoxycortisol and down to cortisol

Question 14

Why is testosterone mainly produced in the testes

Answer 14

It is primarily the testes that contain the 17-hydroxysteroid dehydrogenase enzyme which converts androsteronedione to testosterone

Question 15

Why is oestrogen mainly produced in the ovaries

Answer 15

The ovaries will then aromatise androstenedione and testosterone in to oestrone which can then be converted to 17β-oestradiol

Question 16

What day does ovulation typically occur on

Answer 16

14

Question 17

What are the 2 cycles that make up the menstrual cycle

Answer 17

Ovarian Cycle
• Follicular Phase (first half)
• Ovulation (mid-cycle)
• Luteal Phase (second half)
Endometrial Cycle
• Proliferative
• Secretory

Question 18

How does oestrogen affect the endometrial cycle

Answer 18

Oestrogen causes proliferation of the endometrium - there is an increase in mitosis and an increase in the progesterone receptors (which acts later in the cycle) and an increase in oestrogen receptors
• The endometrium gets thicker, the glands get bigger and the blood vessels get longer

Question 19

What effects do progesterone and 17b-

oestradiol have in the luteal phase

Answer 19

The effect that these have is to induce a secretory phase
• Progesterone reverses the effects of oestrogen - it reduces the proliferation
that oestrogen causes by reducing oestrogen receptors
• Progesterone also increases the secretory activity of the cells in the
myometrium (middle layer of the uterus lining) - glands become wider and produce various substances which make the environment suitable for implantation to occur

Question 20

Describe how hormone levels vary during the menstrual cycle in the follicular phase

Answer 20

follicular phase (1st half) and the luteal phase (2nd half) 
• FSH levels are slightly raised at the beginning: rescue the dormant follicles so they continue their development
•follicles grow and  start producing 17b-oestradiol (has a negative feedback effect on gonadotrophin secretion)
• The dominant follicle will produce large amounts of oestrogen (other follicles undergo atresia)
• If oestrogen levels are high enough for long enough (36 hours), it will switch to a positive feedback effect stimulating LH and FSH. This stimulates ovulation
• After ovulation, the empty follicle is transformed into the corpus luteum which continues to produce 17β-oestradiol and produces large amounts of progesterone
•
•

Question 21

How do hormone levels vary during the luteal phase

Answer 21

PROGESTERONE is the dominant hormone
• Effect of oestrogen and progesterone together is a negative feedback effect on the gonadotrophins - levels fall
• If fertilisation does NOT take place - oestrogen and progesterone levels begin to FALL so NOT supporting the endometrium:
vasospasm in some blood vessels, necrosis of the tissue and contraction of the muscle and loss of the tissue with the blood - MENSTRUATION
• As a result of the decrease in progesterone and oestrogen, the negative feedback is reduced so FSH levels start to rise again so you get back to the start of the cycle

Question 22

What happens in the development of the follicle

Answer 22

The follicle will develop over embryonic life and will develop to this pre-antral stage
• To develop any further requires FSH (to get to early-antral stage)
• If FSH isn’t at a high enough level to continue the development they will
undergo atresia
• They will develop into Early-Antral Follicle (when exposed to sufficient FSH)
• The outer layers of the follicle are the Thecal cells and the inner layer are the granulosa cells
• Under the influence of FSH, the follicles will get bigger until they eventually get to their maximum size which is the Graffian Follicle which is ready for ovulation
• the LH surge occurs and causes rupture of the follicle and release of the egg
• After ovulation, the follicle is transformed into a corpus luteum which continues to secrete oestrogen and progesterone in the Luteal phase of the cycle
• The surge of LH will stimulate the egg to complete the first meiosis

Question 23

How are hormones produced during the cycle

Answer 23

nthesis and produce the Androgens
• Thecal cells do NOT have the aromatase enzymes to convert androgens to
oestrogen
• The Granulosa cells have FSH Receptors and stimulation of these cells will
stimulate the aromatase enzyme which convert these androgens into
oestradiol
• After ovulation, the corpus luteum is formed
• The Thecal cells and Granulosa cells will still be stimulated by LH and FSH and
they will continue to produce large amounts of 17β-oestradiol and Progesterone

Question 24

Summarise the endometrium cycle

Answer 24

• Proliferative - Oestradiol is the dominant influence - causes proliferation of the endometrium
• Endometrium thickens - it stimulates mitosis and the glands which are initially straight becomes enlarged and they coil - increased blood supply
• Secretory - Progesterone is the dominant influence
• In the secretory phase, oestrogen and progesterone are secreted
The glands become secretory, they dilate and produce large amounts of glycogen and mucopolysaccharides and other things
• The mucosa is engorged with blood
• If fertilisation doesn’t take place, oestrogen and progesterone levels fall so they
no longer stimulate the epithelium
• You get spasm of the blood vessels, tissue becomes necrotic and the blood is
shed