Gametogenesis

Question 1

What happens t othe germ cells after they colonise the gonad

Answer 1

• Proliferate by mitosis
• Reshuffle genetically and reduce to haploid by meiosis
• Cytodifferentiate into mature gametes

Question 2

Where do the germ cells colonise

Answer 2

If X - colonise cortex - ovary will develop

If Y - determines - colonise medulla - development of testes

Question 3

How many gametes in a femal

Answer 3

• Very few gametes (~400 in lifetime) • Intermittent production (~1/month) • ~1/400th of reproductive potential

Question 4

How many gametes in a male

Answer 4

• Huge number (~200 million/day) • Continuous production • Essentially “disposable” cells

Question 5

Where does genetic variation arise from

Answer 5

Arises from:
• Crossing-over (exchange of regions of DNA between 2 homologous chromosomes)
• Independent assortment (random orientation of each bivalent along the metaphase plate with respect to other bivalents)
• Random segregation (random distribution of alleles among the four gametes)

Question 6

Where does spermatogenis occur

Answer 6

Seminiferous tubules - between Sertoli cells

Question 7

What are the fucntions of the rete testis, ductili efferentes and epididymis

Answer 7

Concentrate sperm as they are emerging from tubules. It anastomoses with ductili efferentes - also concentrate sperm. Then attached to epiphysis which id storage

Question 8

What is the blood testis barrier

Answer 8

Compartments creaedd - tight junctions between cells. Form the blood testes barrier. The cellular environment required for maintenance for germ cells line is iifferent for the environment for development.. also prevents immune response from being mounted against superm.

Question 9

Give an overview of spermatogenesis and what is the difference between Ad and Ap spermatogonia

Answer 9

• Spermatogonia (male germ cells) are ‘raw material’ for spermatogenesis
• Available for up to 70 years
• Divide by mitosis giving rise to:
• Ad spermatogonium (“Resting”: reserve stock)
• Ap spermatogonium (“Active”: Maintain stock & from puberty onwards produce type B spermatogonia which give rise to primary spermatocytes)
• Primary spermatocytes divide by meiosis giving rise to secondary spermatocytes and then to spermatids
• Each primary spermatocyte forms 4
haploid spermatids which differentiate
(spermiogenesis) into spermatozoa

Question 10

Wheat is spermatogenesis?

Answer 10

Differentiation process - spermatocytes -> spermatids -> spermatozoa

Question 11

What are the spermatogenesis cycle and wave?

Answer 11

• Not all stages in spermatogenesis are visible in a single cross section of seminiferous tubule
• Cells tend to appear in groups with same maturation stages.
• Spermatogenic cycle defined as time taken for reappearance of the same stage within a given segment of tubule
(~16 days in human).
• Different stages in spermatogenesis are also ordered in space as well as time
• Each stage follows in an orderly
sequence along the length of the
tubule.
• The distance between the same stage is called the spermatogenic wave.

Question 12

What is spermiation

Answer 12

• Spermatids released into lumen of
seminiferous tubules (spermiation)

Question 13

What is spermiogenesis

Answer 13

• Spermatids released into lumen of seminiferous tubules (spermiation)
• Remodel as they pass down seminiferous tubule, through rete testis and ductuli efferentes and into the epididymis to finally form spermatozoa.
• Non-motile (transport via Sertoli cell
secretions assisted by peristaltic contraction) until they reach epididymis
• Whole process = spermiogenesis

Question 14

What is sperm capacitation

Answer 14

• Final maturation step required before sperm become fertile
• Conditions in female genital tract stimulate:
• Removal of glycoproteins
and cholesterol from sperm membrane
• Activation of sperm signalling pathways (atypical soluble adenylyl cyclase & PKA involved)
• Allow sperm to bind to zona pellucida of oocyte and initiate acrosome reaction
Note: Human sperm for in vitro fertilisation must first be incubated in capacitation media

Question 15

Describe the number of female gametes with age

Answer 15

See slide

Question 16

Describe the maturation of oocytes before birth

Answer 16

• Germ cells (arise from yolk sac) colonise the gonadal cortex and differentiate into oogonia (single = oogonium).
• Oogonia then proliferate rapidly by mitosis
• By end of 3rd month oogonia arranged in clusters surrounded by flat epithelial cells
• Majority continue to divide by mitosis but some enter meiosis (these arrest in prophase of meiosis I and are called
primary oocytes)

Question 17

What is atresia befor ebirth

Answer 17

• Max number germ cells
reached by mid gestation (~7 million)
• Cell death then begins and many oogonia and 1o oocytes degenerate (atresia)
• By 7th month gestation majority of oogonia have degenerated
• All surviving (~ 2 million) primary oocytes have now entered meiosis I and are individually surrounded by
layer of flat epithelial cells called follicular cells
• Now called primordial follicle

Question 18

What are 3 stages that the oocytes pass through after puberty

Answer 18

Most oocytes undergo atresia during
childhood so ~40,000 remain by puberty • From puberty onwards ~15-20 oocytes start to
mature each month passing through 3 stages:
1. Preantral 2. Antral 3. Preovulatory

Question 19

What is the preantral stage

Answer 19

As primordial follicles begin to grow, the surrounding follicular cells change from flat to cuboidal and proliferate to produce a stratified epithelium of granulosa cells
Granulosa cells secrete layer of glycoprotein on oocyte forming the zona pellucida

Question 20

What is the antral stage stage

Answer 20

• As development continues, fluid filled spaces appear between granulosa
cells. These coalesce to form the antrum • Several follicles begin to develop with each ovarian cycle. Usually only one
reaches maturity (rest become atretic).

Question 21

What is the preovulatory stage

Answer 21

• Surge in LH induces preovulatory growth phase.
• Meiosis I is now completed resulting 2 haploid daughter cells of unequal size
• Each daughter cell now has 23 chromosomes and 46 chromatids
• One cell receives most of cytoplasm the other the (first polar body) receives practically none.
• Cell then enters meiosis II but arrests in metaphase ~3 hours before ovulation.
• Meiosis II is only completed if oocyte is fertilized, otherwise cell degenerates
~24 hours after ovulation.

Question 22

How does ovulation occur

Answer 22

• FSH & LH stimulate rapid growth of follicle several days before ovulation occurs
• Mature follicle now ~2.5cm in diameter and called graafian follicle
• LH surge increases collagenase activity. Prostaglandins increase
response to LH & cause local muscular contractions in ovarian wall
• Oocyte extruded & breaks
free from ovary

Question 23

What is the corpus Luteum

Answer 23

• Remaining granulosa and theca interna cells become vascularized
• Develop yellowish pigment and change into lutein cells, which
form the corpus luteum
• Secrete oestrogens & progesterone
• Stimulates uterine mucosa to enter secretory stage in preparation for embryo implantation
• Dies after 14 days if no fertilisation occurs

Question 24

Describe oocyte transport

Answer 24

• Shortly before ovulation, fimbriae sweep over surface of ovary
• Uterine tube begins to contract rhythmically.
• Oocyte carried into tube by sweeping movements of fimbriae and by motion of cilia on epithelial lining.
• Oocyte then propelled by peristaltic muscular contractions of the tube and by cilia in the mucosa
• If fertilized, oocyte reaches uterine lumen in ~3 to 4 days

Question 25

What happens to the corpus Luteum if fertilisation occurs

Answer 25

• If fertilization does not occur corpus luteum (CL) degenerates
• Forms mass of fibrotic scar tissue, the corpus albicans.
• Progesterone production decreases, precipitating menstrual bleeding.

Question 26

What happens to the corpus Luteum is fertilisation occurs

Answer 26

• Degeneration of CL prevented by human chorionic gonadotropin, secreted by developing embryo.
• The corpus luteum continues to grow and forms the corpus luteum of pregnancy (corpus luteum graviditatis). • Cells continue to secrete progesterone until ~ 4th month
• Secretion of progesterone by placenta then becomes adequate

Question 27

Give a overview ofthe ovarian cycle

Answer 27

• Under influence of
hypothalamic GnRH,
anterior pituitary
releases FSH, and LH. • Follicles stimulated to
grow by FSH and to
mature by FSH and LH. • Ovulation occurs on LH
surge • LH also promotes
development of the
corpus luteum.