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Reproductive systems > Gametogenesis > Flashcards

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
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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
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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
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4
Q

Summarise spermatogenesis in males?

A
  • Huge number of gametes produced (~200 million/ day)
  • Continuous production
  • Cells are essentially disposable
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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
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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)
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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
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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
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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)
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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
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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
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12
Q

What is the spermatogenic wave?

A
  • The distance between the same stage of spermatogenesis within the same tubule
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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
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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
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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
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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
  1. Preantral
  2. Antral
  3. 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
Q

Outline the preovulatory stage of oocyte development

A
  • 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
Q

What stimulates an oocyte to complete meiosis II?

A
  • Fertilisation
  • Otherwise oocyte degenerates ~24 hours after ovulation
27
Q

Outline the process of ovulation

A

-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
Q

What is the corpus luteum?

A
  • Remaining granulosa and theca interna cells become vascularised
  • Develop yellow pigment and change into lutein cells
  • Form corpus luteum
29
Q

What is the function of the corpus luteum?

A
  • 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
Q

How is the oocyte transported out of the ovaries?

A
  • 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
Q

What happens if no fertilisation occurs?

A
  • Corpus luteum degenerates
  • Forms corpus albicans - a mass of fibrotic scar tissue
  • Progesterone production decreases
  • Menstrual bleeding
32
Q

What happens if fertilisation does occur?

A
  • 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

Reproductive systems (20 decks)

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