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7/28/2019 Spermatogenisis Notes
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Spermatogenesis & OogenesisChapter 28 (p. 1036-1038 & 1049-1052):
Gametes – male and female reproductive cells
• sperm & egg fuse to form a zygote containing
Spermatogenesis:
•begins during puberty
• every day healthy male makes
Oogenesis: produces female egg
• the total supply of eggs a female can release has been determined by the time she is
born
Both of the above involve meiosis
• while mitosis distributes replicated chromosomes equally to the daughter cells, meiosis
consists of 2 consecutive nuclear divisions
• this results in __ daughter cells rather than __ & each has as many
chromosomes as the parent cell.
• diploid or (2N chromosome #)
• haploid or (N chromosome #)
• homologous chromosomes or homologues
o synapsis
• one member from each pair
• the 2 nuclear divisions of meiosis are called meiosis I and meiosis II
• meiosis consists in phases: prophase, metaphase, anaphase & telophase.
Meiosis I• DNA replication occurs within the nucleus, chromosomes condense, and become
visible (each chromosome consists of two duplicate chromosomes)
A. Prophase I:
• the replicated chromosomes seek out their homologous partners and become
aligned with them. This is called synapsis
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• each chromosome is composed of 2 chromatids in groups
consisting of 4 chromatids called tetrads.
• also during synapsis crossovers occur
B. Metaphase I:• tetrads line up at the center randomly
C. Anaphase I
• centromeres don't break
• the homologous chromosomes are distributed to opposite ends of the cell.
o Maternal to one side paternal to the other
D. Telophase I
• nuclear membranes
• chromatin
• 2 daughter cells formed
Meiosis II
• mirrors meiosis I except the _______ are not replicated
• the chromatids present in the 2 daughter cells are separated out among 4 cells.
Meiosis accomplishes 2 things:1. reduces the chromosome number by
2. introduces genetic variability:
• the random orientation of the homologous pairs during meiosis provides
variability in the resulting gametes
Spermatogenesis:
spermatogonium (stem cell) undergoes mitosis to form:
primary spermatocyte which then undergoes meiosis I to form:
(2) secondary spermatocytes which complete meiosis II to form:
(4) spermatids which mature to form:
• Spermiogenesis – the last step of spermatogenesis
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Oogenesisin fetus: oogonia (stem cells) undergo mitosis to form:
primary oocytes then begin meiosis I but stalls in prophase
Atresia -
In the ovarian cycle
after puberty one egg per month continues meiosis I to form:
first polar body and secondary oocyte which is ovulated(the first polar body continues meiosis II to produce two more polar bodies)
Differences of oogenesis and spermatogenesis at this point:
• Cytoplasm of the cell divisions is uneven.
• Ovary releases a secondary oocyte suspended in metaphase II
The ovulated secondary oocyte's fate depends on if it is fertilized:
• if it is fertilized it completes meiosis II to form the egg and another polar body
to end up with 3 polar bodies and one ovum (fertilized)
• if it is not fertilized it degenerates so end up with only 2 polar bodies and
degenerating oocyte
Ovarian Cycle
• a monthly stimulation of a group of primordial follicles
Step 1: Formation of Primary Follicles
• granulosa cells develop around the primary oocyte
• zona pellucida –
• thecal cells –
Step 2: Formation of Secondary Follicles (8-10 days)
• granulose cells secrete small amounts of follicular fluid
• follicle enlarges
Step 3: Formation of Tertiary Follicle
• 10-14 days after the start of ovarian cycle the follicle has developed into a tertiary
follicle
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• Antrum
• LH levels rise stimulating the primary oocyte to complete Meiosis I yielding
• Corona radiata
Step 4: Ovulation
•
Marks the end of the follicular phase
Step 5: Formation and Degeneration of the Corpus Luteum
• Empty tertary follicle collapses, granulose cells invade and divide to create an
endocrine structure known as
• Cholesterol in the CL is used to manufacture progesterone
Step 6: Unless fertilization occurs, the CL begins to degenerate roughly 12 days after ovulation
• Corpus albicans
• Disintergration (involution) marks the end of the ovarian cycle