Session 1 - Gametogenesis Flashcards
Give three hormones released by the hypothalamus
Gonadotrophin Releasing Hormone (GnRH)
Prolactin Releasing Hormone (PRH)
Prolactin Inhibiting Hormone (PIH)
What is ST?
Seminiferous tubules
What is R?
The rete testis
What is DE?
Ductus efferentes
What is HBE?
Head, body & tail of the epididymus
What is V?
The vas deferens
What are the two main products of the testes?
Spermatazoa
Testosterone
What are the testis suspended by?
Spermatic cords
What is the surface of each testis covered by?
The visceral layer of the tunica vaginalis
What is the parietal layer of the tunica vaginalis attached to?
The internal spermatic fascia
What is the tough, fibrous outercoating of the testes called?
Tunica albuginea
Where does the tunica albunginea form a ridge?
On the posterior aspect of the testes, forming the medisastinum of the tesits
What is the main substance of the testes?
Seminiferous tubulues
What are the seminiferous tubulues joined to and how?
Rete testis by straight tubules
What cell type makes up the seminiferous tubules
Sertoli cells
What occurs in sertoli cells?
Spermatazoa development
What cell types make up the interstitium of the testes?
Leydig cells
What do leydig cells do?
Secrete testosterone
What do germ cells first colonise in the primordial gonad?
Sex cord
What three things do sex cords connect with?
Rete testis
Epididymis
Vas deferens
What do germ cells morph into?
Spermatogonia stem cells
How long do spermatogonia stem cells last?
Up to and beyond the age of 70, by self regeneration
What happens to the sex cords at puberty?
They hoolow out to form the seminiferous tubules
How many seminiferous tubules in each testes?
250 - 750
What marks the beginning of spermatogenesis?
The prouduction of distinct A1 spertogonia cells
What do A1 spermatogonia divide to prouduce?
More type A cells or type B cells (type B cells are commited to differntiation to spermatozoa
What happens to each type B spermatogonium produced?
Undergoes a fixed number of mitotic divisions ot produce a clone group (64, normally) of primary spermatocytes all linked together by cystoplasmic bridges
What happens to the chain of primary spermatocytes produced from type B spermatogonium in the sertoli cells?
Push towards the lumen of the seminiferous tubule and begins meiosis
Outline the series of divisions a type B clone (64) of primary spermatocytes undergoes when they reach the lumen of the seminiefrous tubule
Two divisions
First division produces two haploid secondary spermatocytes
Second division produces four spermatids in total
What is the maximum each A1 spertogonium can yield in terms of spermatids?
256
What happens to spermatids once they are formed?
Re-modelled to form sperm by spermiogenesis. Cytoplasmic bridge between them are broken down before they are released into the tubule lumen.
Where do spermatids get washed to, and how?
Rete testis by fluid produced from sertoli cells
How long does spermatogenesis take?
70 days
How many spermatogenic processes are going on simultaneously if new group of A1 spermatogonia arise every 16 days?
Four spermatogenic processes occuring at the same time
How is production of sperm continous?
Because different sections along the length of the tubule begin the process at different times
Where does final maturation of spermatatozoa occur?
Epididymis
Outline the constituent parts of semen
Secretion of seminal vesicle – 60%
Secretion of Prostate – 20%
Sperm – Via vas deferens
How many sperm in 3.5ml of typical ejaculate?
350 million
What is the spermatogenic cycle?
The development of A1 spermatogonia through to 256 sperms. The amount of time it takes for reappearance of the same stage of the cycle within a given segment of the tube.
What is the spermatogenic wave?
Different parts of the tube begin the spermatogenic cycle at different times, in a ‘wave’, so the production of sperm is constant. The distance on the tube between parts that are in the same stage is the spermatogenic wave.
What are the rete testis?
A network of canals in the mediastinum of the testis that seminiferous tubules drain into.
What is the epididymis?
A convoluted duct, in which sperms are stored and continue to mature.
What is the vas deferens?
A continuation of the epididymis, the vas deferens has relatively thick muscular walls and a minute lumen. During copulation these muscular walls contract, forcing sperm along the tube to be mixed with other components of ejaculate.
What are the seminal vesicles?
The seminal vesicles secrete a thick, alkaline fluid that is rich with fructose (energy source for sperms) and a coagulating agent. This fluid makes up ~60% of the volume of semen.
The duct of the seminal gland joins the ductus deferens to form the ejaculatory duct.
What role does the prostate play in sperm production?
Prostatic fluid makes up ~20% of the volume of semen, and plays a role in activating sperms.
What are the bulbourethral glands?
two pea-sized bulbourethral glands lie posterolateral to the intermediate part of the urethra, largely embedded within the external urethral sphincter.
The ducts of the bulbourethral glands open into the proximal part of the spongy urethra in the bulb of the penis.
Their mucus-like secretion enters the urethra during sexual arousal.
What are the two main roles of the ovaries?
Allow oocyte development
Produce reproductive hormones
What is each ovary suspended by?
A mesentery known as the mesovarium
What lies within the mesovarium which tethers the ovary to the uterus?
Ovarian ligament
What is the ovarian ligament an embryological remnant of?
Gubernaculum
What is the connective tissue capsule of the ovary called?
The tunica albuginea of the ovary
Why does the tunica albuginea of the ovary become progressively scarred and distorted?
Due to repeated rupture of ovarian follicles and discharge of oocytes during ovulation
What is the function of the fallopian tubes?
Conduct the oocyte from the ovary from the periovarian peritoneal cavity to the uterine cavity
What do primordial germ cells become when they colonise the cortex of the female primordial gonad?
Oogonia
When is the peek number of oocytes in a female ovary?
20 weeks gestation, at 7 million
What is the oohonias entry into meiosis 1 stimulated by?
Mesonephric cells, also called folicular cells
What forms the primordial follicle?
Primary oocyte and the surrounding granulosa cells
When is meiosis arrested in the female gamete? Why?
Diplotene stage of prophase due to Ooxyte Maturation Inhibitor secreted from the follicular cells
Why do chromosomal defects in children become more likely the later a woman has children?
Oocytes remain in stage of arrested development, increasing the chance of cells damage
What does formation of a mature gamete in women require?
The follicle to go through three stages
What is ovulation?
The release of ovum resulting from follicle development and start of a very short (36hr) period of fertility
What is the menstrual cycle? When does it begin?
Starts on first day of bleeding
Break down the three stages of the menstrual cycle
Preparatory phase - 0-12
Ovulation at days (follicular/proliferative) - 12-14
Waiting phase (luteal/secretory) - 14-28
Name the three stages of follicle development
Pre-antral or primordial follicle
Antral or secondary follicle
Pre-ovulatory follicle
What occurs in the pre antral/primordial follicle stage?
o The primary oocyte grows dramatically, but does not re-start meiosis
o Flat follicular cells become cuboidal Granulosa cells
o Granulosa cells secrete glycoprotein to surround the oocyte with a Zona Pellucida
o Surrounding connective tissue (stroma) cells form a Theca Folliculi
Inner Theca Interna that is vascular and endocrine
Outer Theca Externa that is a fibrous capsule
o Theca and Granulosa cells collaborate to secrete oestrogens
What occurs in the antral stage?
o Granulosa cells continue to proliferate and a fluid appears between them, forming the antrum
o As more fluid forms, this secondary or Graafian follicle expands
o Expands to 2mm diameter without stimulation from reproductive hormones
o Continued development depends on reproductive hormones.
FSH – Binds only to Granulosa cells
LH – Binds only to Thecal cells
o Under the influence of LH, Thecal cells secrete androgens, which are converted to oestrogens by the Granulosa cells under the influence of FSH
o Grows to 20mm
What occurs in the pre-ovulatory follicular stage?
o Phase begins 37 hours before ovulation
o Oestrogen causes receptors for LH to appear on outer Granulosa cells
o LH surge stimulates these receptors, leading to rapid changes in the follicle
o Within 3 hours of the LH surge, the follicle restarts meiosis, and the first meiotic division is completed. This division is asymmetric; cytoplasm remains with one daughter cell and the other forms a condensed polar body.
o The secondary follicle then enters meiosis II and arrests again 3 hours prior to ovulation.
o Follicle size increases dramatically by increase in antral fluid volume to 25mm diameter
o Structure begins to weaken
o LH stimulates collagenase activity leading to follicle rupture
o Ovum is carried out in the fluid and gathered up into the fallopian tube by fimbria
o Meiosis is not completed unless the ovum is fertilised. Second division not until after fertilisation.
Unfertilised cells degenerate 24 hours after ovulation
What happens to follicle after ovulation?
Forms a corpus lutem
What does the corpus lutem do?
The remains of the follicle re-organise themselves into a corpus luteum, which secretes progesterone and oestrogen under the influence of LH.
How long does the corpus lutem last in humans?
14 days before regressing spontaneously (in the absence of a fertilised ovum).
What are the four functions of the reproductive system?
- Make gametes
- Allow the gametes to meet
- Allow the new individual to develop
- Control the process of development
- Gonadotropins
- Sex steroid hormones
Give five male secondary sexual characteristics
Body size – (rel to ♀) • Body composition & fat distribution • Hair & skin • Facial hair, male pattern baldness Central nervous effects • Smell
Give five female secondary characteristics
• Body size – (rel to ♂) • Subcutaneous fat distribution • Hair & skin • Breast development • Central nervous effect
What do both genders develop from?
•One tissue
- Urogenital ridge (From intermediate mesoderm)
•Two pairs of ducts
- Mesonephrics and paramesonephric
What do differences in genital development stem from?
The presence or absence of a Y chromosome
What are gonads derived from?
- Indifferent - Urogenital ridge (intermediate mesoderm)
* Extragonadal - Primordial germ cells
What are primordial germ cells?
- Special population allocated shortly after initation of current generation
- Arise in yolk sac and migrate in the retroperitoneum, along dorsal mesentery
What does the Y chromosome cause the expression of?
The SRY gene
Explain what happens for gonads, internal genitalia and external genitalia in a male
- Gonad – Testis
- Internal genitalia – Duct system
- External genitalia – Penis/Scrotum
Explain what happens to medullary cords, cortical cords and tunica albuginea in 44+XY genetic make up
44 + XY -> Medullary cords develop, no cortical cords, thick tunica albuginea
Explain what happens for gonads, internal genitalia and external genitalia in a female
Female gamete – XX Primordial germ cells do not carry Y chromosome. Causes development of • Gonal – Ovary • Internal genitalia – Duct system • External genitalia
Explain what happens to medullary cords, cortical cords and tunica albuginea in 44 + XX individual
44 + XX -> Medullary cords degenerate, cortical cords develop, no tunica albuginea
