Spermatogenesis & Oogenesis
Chapter 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
chromosomes as the parent cell.

diploid or (2N chromosome #)

haploid or (N chromosome #)

homologous chromosomes or homologues
__ & each has
as many
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
 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
Oogenesis
in 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

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