Lecture 5 - Gametogenesis Flashcards
What do the germ cells do after colonising the gonad?
- Proliferate by mitosis, reshuffle genetically and reduce to haploid via meiosis. Cytodifferentiate into mature gametes.
What occurs in meiosis & what is its 2 main functions?
- Meiosis used for production of sperm and eggs, has two successive cell divisions (Meiosis 1 & 2) to produce 4 daughter cells (only 1 mature oocyte develops in females, others form polar bodies).
1) Reduces chromosome number in gamete to 23
2) Ensures every gamete is genetically different
What are the 3 ways in which genetic variation occurs during gametogenesis?
1) Crossing over = exchange of regions of DNA between 2 homologous chromosomes which occurs during prophase l in meiosis.
2) Independent assortment = random orientation of each bivalent along the metaphase plate (2^23 possibilities)
3) Random segregation = random distribution of alleles among the four gametes
Where does spermatogenesis occur?
What are the 2 compartments of this area and what are they separated by?
What is the role of the rete testis?
- Spermatogenesis occurs in the seminiferous tubules of the testis.
- Basal compartment & Adluminal compartment separated by blood testis barrier (sertoli cell barrier). Protects sperm which are genetically different from parent cells from triggering immune response.
- Rete testis concentrates the spermatozoa, they accumulate here.
Describe the entire process of spermatogenesis
1) Spermatogonia (male germ cells) divide by mitosis to form Ad (resting) spermatogonium and Ap (active) spermatogonium which produce type B spermatogonia which give rise to primary spermatocytes.
2) Primary spermatocytes divide via meiosis to form 2 secondary spermatocytes.
3) Secondary spermatocytes divide via meiosis again to form 4 spermatids
4) So 1 spermatocyte form 4 haploid spermatids, which differentiate into spermatozoa via spermiogenesis
Define:
- The spermatogenic cycle
- The spermatogenic wave
Not all stages in spermatogenesis visible in cross section of seminiferous tubules, cells appear in groups with same maturation stage, so …
1) Spermatogenic cycle = time taken for reappearance of the same stage within a given segment of tubule (roughly 16 days)
2) Spermatogenic wave = distance between the same stage
What occurs during the process of spermiogenesis? (spermatids to spermatazoa)
- Spermatids released into lumen of seminiferous tubules in process called spermiation
- Remodel as they pass down seminiferous tubule, through rete testis and into epididymis to form spermatozoa
- Non-motile, so assisted by peristaltic contractions until they reach epididymis
Describe the journey of spermatozoa during ejaculation and state the 4 area of secretions, their components and what % their contribution to semen is.
- Spermatozoa from seminiferous tubules to rete testis into epididymis in testis, then into vas deferens
1) Seminal vesicle secretions (70%) in vas deferens, containing AA”s, citrate, fructose and prostaglandins
2) Prostatic secretions (25%) in urethra, containing proteolytic enzymes & zinc
3) Sperm (2-5%) in urethra (via vas deferens), 200-500 million per ejaculate
4) Bulbourethral gland secretions (<1%) - in urethra, mucoproteins to lubricate and neutralise acidic urine in distal urethra
What is the final step in maturation before sperm becomes fertile?
- Sperm capacitation
- Occurs in female genital tract, causing removal of glycoproteins & cholesterol from sperm membrane
- Activation of sperm signalling pathways, allows sperm to bind zona pellucida and initiate acrosome reaction.
How does the stock of potential gametes in the female change over a lifetime?
- Female has developed entire stock of potential games before birth (around 400)
- Stock declines with age due to atresia until menopause (none left) - around 50 Y.O
Describe the process of oocyte maturation that occurs before birth
1) Germ cells colonise gonadal cortex, differentiate into oogonia
2) Oogonia proliferate rapidly by mitosis, and arrange in clusters
3) Most continue to divide by mitosis, some enter meiosis and arrest in prophase 1, these are now primary oocytes
4) Max number germ cells reached by mid gestation, cell death begins and many oogonia and primary oocytes generate by atresia
5) All surviving primary oocytes enter meiosis one, are surrounded by follicular cells and now called primordial follicle.
From puberty onwards, 15-20 oocytes start to mature each month passing through what 3 stages?
1) Preantral
2) Antral
3) Preovulatory
What occurs during the preantral stage of oocyte maturation?
- As primordial follicles begin to grow, surrounding follicular cells change from flat to cuboidal and proliferate to produce stratified epithelium of granulosa cells.
- Granulosa cells secrete layer of glycoprotein on oocyte forming the zona pellucida
What occurs during the antral stage of oocyte maturation?
- Fluid filled spaces appear between granulosa cells, which coalesce to form the antrum
- Several follicles begin to develop with each ovarian cycle, usually only one reaches maturity
What occurs during the preovulatory stage of oocyte maturation?
- Surges in LH induces this phase
- Meiosis one completes resulting 2 haploid daughter cells of unequal size (each with 23 chromosomes and 46 chromatids)
- One cell receives most cytoplasm, other (first polar body) receives virtually none
- Cell enters meiosis ll but arrests in metaphase 3 hours before ovulation
- Meisosis ll only completed if oocyte is fertilised, otherwise cell degenerated roughly 24 hours after ovulation
