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