Origins Of Gametes Flashcards
After colonising the gonad, germ cells…
Proliferate by mitosis
Reshuffle genetically and reduce to haploid by meiosis
Cytodifferentiate into mature gametes
What are the sex chromosomes of the different genders?
XX = female XY = male
Which part of the gonad do the germ cells colonise in males and females?
XY = colonise medulla XX = colonise cortex
Approximately how many gametes has a woman got per lifetime?
400
How many sperm do men produce per day?
~ 200 million
What are the 2 main functions of meiosis?
Reduce to haploid (chromosome number = 23)
Ensure every gamete is genetically unique
What happens to the 4 daughter cells of meiosis in females?
1 becomes the mature oocyte
3 become polar bodies
When does crossing over occur?
Prophase 1
In meiosis, genetic variation arises from …
Crossing over
Random segregation
Independent assortment
Describe crossing over
Exchange of regions of DNA between 2 homologous chromosomes
In prophase 1
Describe the passage of sperm from inner testis to vas deferens
Seminiferous tubules > rete testis > ductus efferentes > epididymis (head, body, tail) > vas deferens
What do we call male germ cells?
Spermatogonia
What is the difference between Ad and Ap spermatogonia?
Ad = maintains stock of spermatogonia Ap = gives rise to primary spermatocytes
Describe the order of cells from spermatogonium to sperm
Spermatogonium > primary spermatocyte > secondary spermatocyte > spermatid > spermatozoa (sperm)
What is the spermatogenic cycle?
Time taken for reappearance of the same stage within a given segment of tubule
(~ 16 days in humans)
What is the spermatogenic wave?
The distance between parts in the same stage
Define spermiation
Spermatids released into lumen of seminiferous tubules
Where does the final stage of spermiogenesis occur?
Epididymis
Describe how transport occurs in epididymis
Via Sertoli cell secretions assisted by peristaltic contractions
How much semen is there per ejaculate?
2ml
What percentage of semen volume comes from the seminal vesicles?
70%
What is in the seminal vesicle secretions?
Amino acids
Citrate
Fructose (main energy for sperm)
Prostaglandins
What percentage of semen volume comes from the prostate?
25%
What is in the prostatic secretions?
Proteolytic enzymes eg. PSA
Zinc
What percentage of semen is sperm?
2 - 5%
How many sperm are there per ejaculate?
200 - 500 million
What percentage of the volume of semen comes from bulbourethral glands?
< 1%
What is the other name for bulbourethral glands?
Cowper’s glands
What is secreted from bulbourethral glands?
Mucoproteins to help lubricate urethra and neutralise acidic urine in distal urethra
What is sperm capacitation?
Final maturation step required before sperm become fertile
Occurring in the female genital tract
Removal of glycoproteins and cholesterol to activate signalling to allow sperm to bind to zona pellucida
When are female gametes produced?
All before birth
Approximately what age is menopause?
55 years
Where do germ cells arise from?
Yolk sac
Primary oocytes are arrested in which phase?
Prophase 1
How many oocytes remain by puberty?
40,000
Others have undergone atresia
Approximately how many oocytes start to mature every month?
15 - 20
What are the 3 stages of maturation an oocyte must pass through?
Pre-antral
Natural
Pre-ovulatory
Describe the pre-antral stage
Primordial follicles begin to grow
Surrounding follicular cells get more cuboidal and proliferate
Produce a stratified epithelium of granulosa cells which secrete glycoprotein
Describe the antral stage
Fluid filled spaces appear between granulosa cells
These coalesce to form the antrum
What do we call the granulosa cells surrounding the oocyte directly?
Cumulus oophorus
What are the 2 layers of thecal cells?
Outer fibrous
Inner secretory
Describe the pre-ovulatory stage
Surge in LH gives growth phase
Meiosis I completed
Cells enter meiosis II and arrest in metaphase
Meiosis II only completed if fertilised
Describe ovulation
FSH and LH stimulate rapid growth of follicle
Mature follicle 2.5 cm diameter (Graafian follicle)
Ruptures to release oocyte - LH surge (to increased collagenase activity)
What is the corpus luteum?
Remaining granulosa and theca interna cells become vascularised
Develop yellowish pigment and change into lutein cells
Secrete oestrogens and progesterone
Stimulate uterine mucosa to enter secretory stage - prep for embryo implantation
Dies after 14 days if no fertilisation
What is the corpus albicans?
When no fertilisation occurs the corpus luteum degenerates forming a mass of fibrotic scar tissue
Progesterone production decreases - menstrual bleeding
What happens to the corpus luteum if fertilisation occurs?
Degeneration prevented via hCG
CL continues to grow and forms the CL of pregnancy
Cells continue to secrete progesterone until 4th months until secretion by placenta is adequate
What are the main differences between spermatogenesis and oogenesis?
4 spermatids vs 1 ovum 200 million/day vs 1 per 28 day cycle Starts at puberty vs starts in foetus Motile gametes vs non-motile Continues vs ends at menopause
Why do males have a blood-testis barrier?
To prevent autoimmune attack on sperm (as will be genetically different to the father’s cells)
