Gametogenesis Flashcards
1
Q
What happens to germ cells once they’ve colonised the gonad?
A
- They proliferate by mitosis
- Reshuffle genetically and reduce to haploid by meiosis
- Cytodifferentiate into mature gametes
2
Q
What determines the sex of the offspring?
A
- Male gamete contains either X or Y chromosome so determines the sex of offspring
- Female gamete always contains an X chromosome
3
Q
Summarise oogenesis in females
A
- Very few gametes (~400 in a lifetime)
- Intermittent production (~1/month)
- Each mature oocyte = 1/400th of reproductive potential
4
Q
Summarise spermatogenesis in males?
A
- Huge number of gametes produced (~200 million/ day)
- Continuous production
- Cells are essentially disposable
5
Q
What are the functions of meiosis?
A
- Reduce the chromosome number to ensure each gamete has 23
- Ensures each gamete is genetically unique
6
Q
How does genetic variation arise?
A
- Crossing-over
- Independent assortment (random orientation of each bivalent along the metaphase plate)
- Random segregation (random distribution of alleles among the four gametes)
7
Q
What is the role of the blood testis barrier?
A
- Separates different compartments
- Different environment is required for sperm development and differentiation
- Each sperm is genetically unique from father
- Prevents an auto-immune response
8
Q
What is the role of the seminiferous tubules?
A
- Contain germ cells that then differentiate
- Sertoli cells create spaces in which sperm can develop
- Provide nutrients and growth factors for sperm
- Transport sperm via peristalsis
9
Q
Spermatogonia divide by mitosis to give rise to what?
A
- Ad spermatogonium = reserve stock
- Ap spermatogonium = maintain stock and from puberty onwards produce type B spermatogonia (give rise to primary spermatocyctes)
10
Q
Outline how mature spermatozoa develop
A
- Primary spermatocytes divides by meiosis
- Secondary spermatocytes
- Spermatids
- Each primary spermatocyte forms 4 haploid spermatids which differentiate into spermatozoa
11
Q
What is the spermatogenic cycle?
A
- Time taken for reappearance of the same stage of spermatogenesis within a given segment of tubule
- ~16 days in a human
12
Q
What is the spermatogenic wave?
A
- The distance between the same stage of spermatogenesis within the same tubule
13
Q
What is spermiogenesis?
A
- Spermatids are released into lumen of seminiferous tubules (spermiation)
- Remodel as they move to epididymis to finally form spermatozoa
- Spermatids are non-motile until they reach epididymis
14
Q
What is the composition of semen?
A
- ~70% seminal vesicle secretions (amino acids, citrate, fructose, prostaglandins)
- ~25% secretions of prostate (proteolytic enzymes)
- ~2-5% sperm via vas deferens (200-500 million sperm released per ejaculate)
- <1% Bulbourethral gland secretions
15
Q
What is sperm capacitation?
A
- Final step before sperm become fertile
- Conditions in female genital tract stimulate:
1. Removal of glycoproteins and cholesterol from sperm membrane
2. Activation of sperm signalling pathways
3. Allow sperm to bind to zona pellucida of oocyte and initiate acrosome reactuin
16
Q
By what age has the female developed her entire stock of potential gametes?
A
- Before birth
17
Q
Describe how primary oocytes are formed
A
- Germ cells colonise gonadal cortex and differentiate into oogonia
- Oogonia proliferate rapidly by mitosis
- Some enter meiosis while majority continue to divide by mitosis
- Those in meiosis arrest in prophase I and are called primary oocytes
18
Q
When is the maximum number of female germ cells reached?
A
- By mid-gestation
19
Q
What happens once the maximum number of germ cells is reached?
A
- Atresia (many oogonia and primary oocytes degenerate)
- By 7th month gestation majority of oogonia have degenerated
- All surviving primary oocytes have entered meiosis I and are surrounded by follicular cells
- Each primary oocyte is now known as a primordial follicle
20
Q
Why is it thought that genetic defects are more likely as the mother’s age increases?
A
- The longer the primary oocyte is stationary, the more likely it is to have been damaged
- There is more time over which damage can occur to the oocyte
21
Q
When do oocytes continue to mature after birth?
A
- At puberty
- Most oocytes undergo atresia during childhood so ~40 000 remain by puberty
22
Q
What are the 3 stages oocytes pass through from puberty onwards?
A
- Preantral
- Antral
- Preovulatory
- ~15-20 oocytes mature each month
23
Q
Outline the preantral stage of oocyte development
A
- As primordial follicles begin to grow, surrounding follicukar cells change from flat to cuboidal and proliferate to produce a stratified epithelium of granulosa cells
- Granulosa cells secrete layer of glycoprotein = zona pellucida
24
Q
Outline the antral stage of oocyte development
A
- Fluid filled spaces appear between granulosa cells
- These coalesce to form the antrum
- Several follicles begin to develop with each ovarian cycle
- Only 1 should reach maturity
25
Outline the preovulatory stage of oocyte development
- Surge in LH induces preovulatory growth phase
- Meiosis I completed - now have 2 haploid daughters of unequal size
- 23 chromosomes per daughter cell
- One cell receives most cytoplasm
- The other becomes a polar body
- Cell then enters meiosis II but arrests at metaphase
26
What stimulates an oocyte to complete meiosis II?
- Fertilisation
- Otherwise oocyte degenerates ~24 hours after ovulation
27
Outline the process of ovulation
-FSH and LH stimulate rapid growth of follicle
- Mature follicle now known as graafian follicle
- LH surge increases collagenase activity
- Prostaglandins increase response to LH and cause local muscular contraction in ovarian wall
- Oocyte extruded and breaks free from ovary
28
What is the corpus luteum?
- Remaining granulosa and theca interna cells become vascularised
- Develop yellow pigment and change into lutein cells
- Form corpus luteum
29
What is the function of the corpus luteum?
- Secrete oestrogen and progesterone
- Stimulates uterine mucosa to enter secretory stage in preparation for embryo implantation
- Dies after 14 days if no fertilisation occurs
30
How is the oocyte transported out of the ovaries?
- 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 peristalsis and cilia
31
What happens if no fertilisation occurs?
- Corpus luteum degenerates
- Forms corpus albicans - a mass of fibrotic scar tissue
- Progesterone production decreases
- Menstrual bleeding
32
What happens if fertilisation does occur?
- Human chorionic gonadotrophin prevents degeneration of corpus luteum
- Corpus luteum grows and forms corpus luteum graviditatis
- Cells continue to secrete progesterone until ~4th month of pregnancy
- Placenta then takes over
