Gonads Flashcards
The gonads: explain the stages of gametogenesis in male and female gonads; recall the principle structures of the testes and ovaries and their function Sex hormones: explain the process and regulation of steroidogenesis in male and female gonads, recall the physiological actions of male and female gonadal steroids; recall the hypothalamo-pituitary gonadal axis and its regulation in males and females Menstrual cycle: recall the phases of the menstrual cycle and the physiological changes that oc
What is the definition of a gonad?
Organ that produces the gamete.
What are the two gonads?
Testes and ovaries.
What is the basic embryology of the gonads?
Develop from the same undifferentiated structure. Genes on the Y chromosome (SRY gene) are key to the differentiation of the gonads into testes. Without it, ovaries will develop.
What are the two major functions of the gonads? Alternative name for each.
PRODUCTION OF GAMETES for reproduction. Also called GAMETOGENESIS. PRODUCTION OF STEROID HORMONES. Also called STEROIDOGENESIS.
Gametogenesis: what’s the process in males and females?
IN MALES: Spermatogenesis – production of mature spermatozoa. IN FEMALES: Oogenesis – production of ripe ova.
Steroidogenesis: what’s the process in males and females?
IN MALES: Androgens, and small quantities of oestrogens and progestogens. IN FEMALES: Oestrogens and progestogens, and small amounts of androgens.
What is an androgen?
Any steroid hormone that regulates the development and maintenance of male characteristics by binding to androgen receptors.
Examples of androgens? (x1)
Testosterone
What is the stem cell that gives rise to gamete cells in both sexes?
Primordial germ cells.
Describe how the number of germ cells in MALES fluctuates throughout life – from conception? What is the name of the immature germ cell we are looking at – explain what they are? Mention one key number.
Spermatogonia – immature germ cell produced at an early stage during embryogenesis, in the wall of seminiferous tubule in the testes.
They proliferate by mitosis in these tubules – number around 6 to 7 million. Levels remain fairly consistent throughout life but declines slightly in later life.
Describe how the number of germ cells in FEMALES fluctuates throughout life – from conception (including one specific timing)? What is the name of the immature germ cell we are looking at – explain what they are? Mention two key numbers.
Oogonia – immature germ cell produced at an early stage during embryogenesis.
In females, these germ cells proliferate my mitosis, hence there’s a quick increase. The numbers peak to around 5-6 million at around 24 weeks gestation.
Before birth, most oogonia have either degenerated (atresia) or differentiated into primary oocytes, hence the number of oogonia fall. Atresia is initially very fast.
Primary oocytes will then undergo meiosis but are then arrested at prophase I until puberty.
Numbers have fallen to around 2 million by birth.
Numbers continue to deplete from birth until there is none left, and menopause is induced.
What happens in the process of spermatogenesis? Time-scale?
Germ cell differentiates into spermatogonia in very early life. This germ cell is diploid (SO WILL DEVELOP INTO 4 sperm).
Spermatogenesis doesn’t actually start UNTIL PUBERTY. Spermatogenesis– process occurs in conjunction with the immature germ cell dissociating from the basal lamina of the tubule.
In puberty, release of hormones including testosterone means spermatogonia mitotically divide – half are kept as spermatogonia, the other half become primary spermatocytes.
Primary spermatocytes undergo their first meiotic division to secondary spermatocytes which are HAPLOID.
These undergo their second and final meiotic division to give spermatids which are haploid and contain 23 chromosomes in each.
The spermatid undergoes further maturation to form Spermatozoa. The process is called spermiogenesis and when the spermatid develops a tail, acrosome, and mitochondria neck region…
Alternative name for spermatogenesis?
Can also be referred to as male gametogenesis.
How many sperm are produced a second?
300-600.
What happens in the process of oogenesis? Time-scale?
Germ cell differentiates into oogonia in embryogenesis. Oogonia are diploid.
Mitosis of oogonia produces some oogonia which undergo atresia, while others form primary oocytes – also diploid. Primary oocyte is an immature ovum that undergoes growth and some maturation (some follicle cells grow around the oocyte to form a primary follicle. During embryogenesis, the primary oocyte also begins meiosis, but stopped during prophase I.
Ovaries become inactive. During puberty, the primary oocytes continue their meiotic division and the follicle continues to grow as a result of hormonal changes.
The first meiotic division occurs asymmetrically – this still produces two cells with the same number of chromosomes in each, it’s just the actual cell that is split differently so one is bigger than the other.
This produces a ‘first polar body’ (which immediately degrades), and a secondary oocyte.
The secondary (haploid) oocyte is the precursor of an egg. They complete their development into an ovum if fertilised by a sperm cell. In their final meiotic division, they divide asymmetrically to produce a second polar body and an ovum that has a large cytoplasm because of the asymmetrical process.
The rest of the oocytes remain dormant, and supplies remain until they run out and menopause happens.
Where are gonadotrophins produced?
Anterior pituitary gland.
What hormones in men increase during puberty?
At puberty, there is increased secretion of gonadotrophins in the pituitary and testosterone in the testis.
What does testosterone do? (x3)
Gives secondary male sexual features e.g. facial hair. Maturation ad development of the testis. Maturation of the seminiferous tubules which allows spermatogenesis.
What is the structure of the male gonad?
Note that tubules are coiled. Note also that the epididymis is highly coiled. Both for higher surface area and greater storage. Remember that for the tubules, the immature germ cells are associated to the basal lamina, so need a large surface area.
Where do the male germ cells go in relation to the testes anatomy, as they develop and mature?
Spermatids collect into the lumen of the seminiferous tubules and drained by the vasa efferentia into the epididymis. Epididymis is where spermatids are stored and nutrients are secreted for maturation into spermatozoa. Eventually expelled by vas deferens by the smooth muscle into the urethra.
What is the structure of a seminiferous tubule – cross section?
Layered into: lumen, Sertoli cell, basal lamina. Sertoli cells form gap junctions.
Where are TESTICULAR androgens produced specifically? (x2 points)
Leydig cells. Found adjacent to the seminiferous tubules.
How are Leydig cells stimulated to produce AND secrete testicular androgens (mainly?)?
Stimulation of LH receptors on the Leydig cells by LH (a gonadotrophins) – stimulates production and secretion of testicular androgens, mainly testosterone.
How do spermatogonia move within the seminiferous tubules during maturation and division?
Spermatogonia migrate towards the lumen via their own special mechanism. They develop partly in the Sertoli cell metabolism – this is how they pass the tight junction. They emerge as spermatids into the lumen, and cannot re-associate with the basal membrane because of the tight junctions between the Sertoli cells.
What is the significance of the tight junctions between the Sertoli cells? (x2)
Keep spermatids in the lumen of the tubules. Forms a blood-testes barrier.
What receptors do the Sertoli cells contain? (x2)
FSH receptors and androgen receptors.
What are the purposes of the receptors of Sertoli cells? (x2)
Control spermatogenesis. FSH receptors detect when FSH high and produce inhibin as well as other molecules. Inhibin reduces FSH levels in the blood by affecting the pituitary (and hypothalamus). This is a negative feedback system that control FSH levels – FSH decreases levels of sperm cells.
In the ovaries, what would it look like in terms of the processes going on?
Ovaries also – obviously – contain oocytes from birth with one or some layers of follicular cells around them.
In gametogenesis, the dominant follicle is selected and it grows into the GRAAFIAN follicle. All remaining follicles that are stimulated to grow from FSH undergo atresia (degradation). Once the ovum is released, the remaining follicular cells turn into a corpus luteum
What is the structure of the GRAAFIAN follicle? (x4) Characteristics? (x2)
Closest point to ovulation. Maximum size has been reached. Follicle contains ovum, located inside a follicular fluid which is very large in the Graafian follicle. This is surrounded by granulosa cells (follicular cells), and thecal cells (endocrine follicular cells that have roles including synthesising androgens and signal transductions).
What does the corpus luteum do? (x1)
Temporary endocrine function that produces high levels of progesterone.
What are steroid hormones synthesised from?
Cholesterol.
What is the process of steroidogenesis in the male and female gonads? Points to be made about testosterone and relative androgen strengths.
NB: only refer to the processes involving the gonads. No need to know processes involving the adrenals. Androstenedione is a weak androgen. Testosterone more potent androgen. Dihydrotestosterone also more potent.
How does the ovarian cycle influence the endometrial cycle? (x2)
In follicular phase, oestrogen rises (17B-oestradiol), which has a proliferative effect on the endometrium. It does this by causing implantation of more oestrogen and progesterone receptors into the endometrium (progesterone receptors are for later in the menstrual cycle). In the luteal phase, 17B-oestradiol and progesterone levels increase (again). Progesterone removes oestrogen receptors from the endometrium and create a more ideal environment for egg implantation.
What happens in the early follicular phase?
Menstrual bleeding. FSH levels are raised at this time. Between 8 and 10 follicles start to mature and grow. They compete to become the dominant follicle. They are growing under the regulation of FSH.
What happens in the early-mid follicular phase?
FSH and LH stay the same, while levels of oestrogen (oestradiol) rise. In this stage, one follicle gets bigger than the rest – dominant follicle. Higher levels of oestradiol are therefore produced. These increased levels have a positive feedback effect on the granulosa cells around it. Oestradiol results in increased growth of the granulosa cells, so my oestrogen can be synthesised.
What happens in the mid-follicular phase?
We now have a dominant follicle called a Graafian follicle that has developed. This causes very high levels of oestrogen. This has a negative feedback effect on the pituitary and hypothalamus which results in reduced levels of gonadotrophins (FSH and LH). Graafian follicle no longer relies on the FSH and LH, so remaining follicles undergo atresia.
What happens in the late-follicular phase?
High oestradiol – at a point – results in POSITIVE feedback on the GnHR (in the hypothalamus), and LH (in the pituitary). There is also a small surge in FSH. Results in ovulation and release of the ovum from the follicle, into the Fallopian tube. There is a small increase in the progesterone 17OH-progesterone, which enhances the positive feedback effect that results in the surge of gonadotrophins.
What happens in the luteal phase?
Follicle that remains after ovulation transformed to corpus luteum. Continue to produce oestrogen, and lots of progesterone. These both negatively feedback on the gonadotrophins to keep those levels low. The progesterone prepares the endometrium for implantation by a fertilised egg.
What happens to hormone levels if fertilisation does not occur?
Indirect and direct negative feedback. If fertilisation DOES NOT occur, oestrogen, progesterone and inhibin exert a negative feedback effect on the LH and FSH (in the pituitary and hypothalamus). LH and FSH rise and the cycle starts again. The corpus luteum involutes (called luteolysis), progesterone and oestrogen levels fall, and menstruation occurs.
What happens to body temperature in the menstrual cycle – what causes the change?
There is an increase in body temperature because of the increase in progesterone levels in the luteal phase.
Recall the hypothalamo-pituitary-testicular axis. (x5 points about how the axis works in general, x2 points about how the axis is regulated).
Hypothalamic pulse generator releases GnRH in pulses every hour or two. This triggers pulses in the production and secretion of FH and LSH in the anterior pituitary. LH stimulates ANDROGEN PRODUCTION in the testis: LH binds to receptors on the Leydig cells which stimulates them to make testosterone (virilisation = development of male characteristics). FSH stimulates GAMETE PRODUCTION in the testis: binds to receptors on the Sertoli cells which line the seminiferous tubules, which stimulates spermatogenesis. Testosterone also interacts with the FSH pathway – it is needed for the final stage of spermatogenesis.
Testosterone negatively feedbacks on the hypothalamus and anterior pituitary. Inhibin (produced by the Sertoli cells (stimulated by the FSH) negatively feedbacks on the hypothalamus and anterior pituitary. SO, BOTH INHIBIT FSH AND LH.
Recall the hypthalamo-pituitary-ovarian axis.
Hypothalamic pulse generator releases GnRH in pulses every hour or two. This triggers pulses in the production and secretion of FH and LSH in the anterior pituitary.
CASE STUDY: Patient is female, has headaches, loss of peripheral vision, tiredness, oligomenorrhea, occasional expression of milk from the breasts. Oestrogen, progesterone, FSH and LH levels are all low. What does the patient have? How does this explain the symptoms?
Patient has pituitary adenoma (tumour) affecting prolactin production. Prolactin inhibits LH and FSH secretion.
Headaches: overgrowth (even small) can increase pressure enough to cause pain.
Galactorrhoea (lactating): stimulated by excess prolactin.
Loss of peripheral vision: see photo.
