Gametes and the HPG axis Flashcards
What are gametes?
What are they formed from?
What do they contain?
Gametes is a generic term referring to sperm or eggs:
- Specialised cells that allow us to pass our genes on to the next generation
- Haploid cells containing 23 individual chromosomes rather than 23 pairs
- Produced by sexually mature adults in the gonads
- Arise from diploid cells in the embryo
Describe the proces of gametogenesis
Germ cells that produce gametes are separated in early embryonic life
Germ cells migrate to the developing gonads at about 6 weeks of embryonic life
After colonising the gonad, the germ cells:
- Proliferate by mitosis
- Reshuffle genetic material
- Reduce to haploid by meiosis
- Mature into a sperm/ova
Describe the phases of mitosis
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Interphase (G2)
- Centrosomes containing centrioles
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Prophase
- Centrosomes migrate to opposite poles of the cells, spindle poles and microtubules develop from the centrosomes.
- Breakdown of nuclear membrane- chromosomes in the nucleus condense.
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Prometaphase
- Chromosomes separate, joined to the centrosome by the kinetochore
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Metaphase
- Chromosomes align along metaphase plate in the centre of the cell
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Anaphase
- Sister chromatids separate so each daughter cell gets identical copy of genetic material
- Cell elongates and narrows in the middle.
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Telophase
- Cytokinesis- cell narrows in the middle to form cleavage furrow
- 2 nuclei form, one in each daughter cell
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Interphase (G1)
- 2 daughter cells are formed by the separation of the parent cell.
Describe the process of meiosis I
In meoisis I genetic information is halved:
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Prophase I
- Breakdown of the nucleus occurs
- Spindles form from centrosomes as they migrate towards opposite poles of the cell
- 3 chromosome pairs form to form tetrads in which chromatids from different pairs are joined by chiasmata
- This allows genetic material to shuffle, allowing each individual gene to be inherited separately
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Metaphase I
- Tetrads align in centre of the cell along the metaphase plate.
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Anaphase I
- Pairs of homologous chromosomes split up, (sister chromatids remain attached) and migrate to opposite poles of the cell and the cell splits.
2 secondary spermatocytes are produced in this process
Describe the process of meiosis II
Two haploid cells form in which chromosomes are still double (from meiosis I)
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Prophase II
- Cell begins division similar to mitosis: spindle poles form and centrosomes migrate to opposite poles of the cell
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Metaphase II
- Chromosomes align in the centre of the cell along the metaphase plate
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Anaphase II
- Sister chromatids separate and migrate to opposite poles of the dividing cell
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Telophase II
- The 2 haploid cells formed in meiosis I divide forming 4 haploid cells, each containing different genetic material.
These 4 haploid cells are called spermatids.
What is spermatogonia
XY germ cells in the male colonise the medulla of the testes in the embryo
They proliferate by mitosis forming spermatogonia
During puberty, the sex chords hollow out to form the seminiferous tubules
Spermatogonia cluster around the edges of the seminiferous tubules
Describe the process of spermatogenesis
In puberty, a group of spermatogonia begin dividing by mitosis a fixed number of times to form clones of around 64 linked diploid cells called primary spermatocytes
Meiosis I produces two secondary spermatocytes from each primary spermatocyte.
Meiosis II produces 4 haploid spermatids (2 from each seconday spermatocyte). Spermatids are close to the lumen of the seminiferous tubule.
Spermatogonia are constantly replaced at the edges of the seminiferous tubules by mitosis
What is spermiogenesis?
Describe this process
What are the major morphological changes?
The maturation of spermatids into fully differentiated spermatozoa
- Spermatids are released into the tubule and undergo remodelling/maturation as they pass down the rete testes, efferent duct and epididymis.
Major morphological changes are:
- Formation of a head containing the nucleus and a mid-piece containing the mitochondria
- Formation of the acrosome from the golgi apparatus
- Elongation of one centriole to form a tail
- Loss of excess cytoplasm
What does the acrosome contain?
Digestive enzymes that break down the zona pellucida; the glycoprotein layer surrounding the ovum
What are spermatogenic waves?
Spermatogenesis occurs in sequential waves along the length of the seminiferous tubules
The process takes around 70 days
New spermatogonia are recruited every 16 days
All stages of the process occur at the same time in different areas of the tubule
What does semen contain?
- Secretions of the seminal vesicle (60%)
- Secretions of the prostate (20%)
- Sperm (via das deferens)
- Secretions of bulbo-urethral glands
The contents are mixed via emission from ejaculation
What are oogonia?
In the female, XX germ cells colonise the cortex of the ovaries
They proliferate by mitosis, forming oogonia
Females develpo their entire stock of gametes before birth
What 6 hormones are produced and released from the anterior pituitary gland?
What hypothalamic hormone stimulates their release?
What pituitaru cells are they produced by?
What are their targets?
In the female, when does the process of meiosis begin and end? What does it produce?
When does it resume?
Begins before birth and ends before birth/shortly after- produces primary oocytes wrapped in a single layer of granulosa cells in a primordial follicle.
Resumes in puberty:
- Meiosis I produces 1 secondary oocyte and a first polar body
- Meiosis II produces a mature haploid ovum and a second polar body.
What does fertilisation require?
Requires the transfer of a viable male and female gametes to the right place at the right time
Describe the HPG axis
Hypothalamus produces and secretes gonadotrophin releasing hormone which acts on the anterior pituitary
Anterior pituitary secretes luteinising hormone and follicle stimulating hormone
LH and FSH act on the gonads to produce either testosterone and inhibin (male) or oesotrogen and progesterone (female)
Describe the phases of the gonadotrophic hormone levels, uterine cycle, ovarian cycle and ovarian hormone levels
Gonadotrophic hormone levels:
- Follicular phase (preovulation) FSH and LH levels low until they increase (LH surge) just before ovulation.
- Luteal phase post ovulation, LH and FSH levels decrease.
Ovarian hormone levels:
- Pre-ovulation: oestrogen high and continues to rise before ovulation, progesterone low
- Post-ovulation:
- Oestrogen reduces then rises again as corpus luteum expands.
- Progesterone rises post ovulation then falls towards the end of the luteal phase
Uterine cycle:
- Menses phase (pre-ovulation), shedding of uterine lining.
- Proliferative phase: (begins pre-ovulation) driven by oesotrogen - proliferation of uterine lining in preparation for implantation
- Secretory phase: uterus secretes glycogen into the lining in preparation for implantation, driven by progesterone produced by corpus luteum.
