Gametogenesis Flashcards
What happens t othe germ cells after they colonise the gonad
- Proliferate by mitosis
- Reshuffle genetically and reduce to haploid by meiosis
- Cytodifferentiate into mature gametes
Where do the germ cells colonise
If X - colonise cortex - ovary will develop
If Y - determines - colonise medulla - development of testes
How many gametes in a femal
• Very few gametes (~400 in lifetime) • Intermittent production (~1/month) • ~1/400th of reproductive potential
How many gametes in a male
• Huge number (~200 million/day) • Continuous production • Essentially “disposable” cells
Where does genetic variation arise from
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)
Where does spermatogenis occur
Seminiferous tubules - between Sertoli cells
What are the fucntions of the rete testis, ductili efferentes and epididymis
Concentrate sperm as they are emerging from tubules. It anastomoses with ductili efferentes - also concentrate sperm. Then attached to epiphysis which id storage
What is the blood testis barrier
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.
Give an overview of spermatogenesis and what is the difference between Ad and Ap spermatogonia
• 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
Wheat is spermatogenesis?
Differentiation process - spermatocytes -> spermatids -> spermatozoa
What are the spermatogenesis cycle and wave?
• 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.
What is spermiation
• Spermatids released into lumen of seminiferous tubules (spermiation)
What is spermiogenesis
• 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
What is sperm capacitation
• 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
Describe the number of female gametes with age
See slide
Describe the maturation of oocytes before birth
• 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)
What is atresia befor ebirth
• 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
What are 3 stages that the oocytes pass through after puberty
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
What is the preantral stage
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
What is the antral stage stage
• 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).
What is the preovulatory stage
• 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.
How does ovulation occur
• 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
What is the corpus Luteum
• 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
Describe oocyte transport
- 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
What happens to the corpus Luteum if fertilisation occurs
- If fertilization does not occur corpus luteum (CL) degenerates
- Forms mass of fibrotic scar tissue, the corpus albicans.
- Progesterone production decreases, precipitating menstrual bleeding.
What happens to the corpus Luteum is fertilisation occurs
- 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
Give a overview ofthe ovarian cycle
• 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.
