Exercise 45

Physiology of Reproduction

1

Meiosis

GametesHaploid complement (n)

GametogenesisProcess of gamete formation with

the reduction by half of the chromosome number

Homologous chromosomeCarries genes for the same traits

2

Meiosis

Meiosis IProphase

• 23 tetrads• Homologues chromosomes

synapse• Chiasmata or crossover

3

Meiosis

MetaphaseAnaphase

• Homologues separates• Dyads stays together

Telophase• 2 cells with diploid number of

chromosomes

4

Meiosis

Meiosis IIProphase

• No chromosome replication• Cells are diploid

Metaphase

5

Meiosis

Anaphase• Breaks of the centromeres will result

each cell having one chromatid• 4 daughter cells• Crossover and the way in which the

homologues align on the spindle equator during the first maturation division introduces a great variability in the resulting gametes

6

Spermatogenesis

It is the male gamete formation Starts at puberty and continues

throughout life

7

Spermatogenesis

8

Spermatogenesis

Steps of sperm formation: Spermatogonia (2n)

Stem cellFound in the seminiferous tubulesIt undergoes mitosis and forms one

primary spermatocyte and one spermatogonium

Phase regulated by FSH

9

Spermatogenesis

Primary spermatocyte (2n)It undergoes to a growth phaseIt undergoes to meiosis I after the

growth phase and forms the secondary spermatocyte

Secondary spermatocyte (n)It undergoes meiosis II and produces

4 spermatids

10

Spermatogenesis

Spermatids (n)Nonfunctional gametesNo motileThey undergo spermiogenesis and

form the sperm

11

Spermatogenesis

SpermMotile Smaller than the spermatidHead of the sperm

• Contain the genetic material • Acrosome

12

Spermiogenesis

13

Spermatogenesis

Midpiece• Centriole that gives rise to the

filaments that forms the tail• Mitochondria present

Tail• flagellum

14

Spermatogenesis

Sertoli cellsSupport cells responsible for sperm

nourishment

15

Oogenesis and ovarian cycle It is the female gamete formation Steps of the oogenesis: Before birth

Oogonia• Primitive stem cell• it undergoes mitosis many times• It grows in size and becomes a

primary oocyte

16

Oogenesis and ovarian cycle

Primordial follicles• Capsule formed by a single layer

of squamouslike cells that surround the primary oocyte

At birthOogonias no longer existsAll cells are oocytes encapsulated by

a primordial follicleNo more oocytes will be produced

17

Oogenesis and ovarian cycle

From birth to pubertyPrimary oocytes are quiescent

From puberty and onIncrease of FSH levels cause:FollicleOne or more primordial follicles to

undergo maturation (every 28 days)

18

Follicle

19

Oogenesis and ovarian cycle

Formation of primary follicle• From the maturation of the primordial

follicle• Follicle cells

• The epithelium changes from squamous to simple cuboidal

• Many layers of cuboidal cells• Granulosa

• It secrets estrogen• It contains the primary oocyte

20

Oogenesis and ovarian cycle

Secondary follicle• Follicle with several layers of

granulosa cells• Presence of antrum

Tertiary follicle (Graafian)• Presence of granulosa cells• Corona radiata

21

Oogenesis and ovarian cycle

• It is a mature follicle that continues secreting increasing amounts of estrogen

• It contains the secondary oocyte• It expels the secondary oocyte

accompanied by the corona radiata at the middle of the 28-day cycle

22

Oogenesis and ovarian cycle

Corpus luteum• It comes from the rupture Graafian

follicle• It produces mainly progesterone• If pregnancy does not occur it will

degenerate and will be replaced by scar tissue

23

Oogenesis and ovarian cycle

Corpus albicans• Scar tissue resulted from the

disintegration of the corpus luteum Oocyte Primary oocyte

Meiosis I is completed forming one secondary oocyte and one first polar body

24

Follicle

25

Oogenesis

26

Oogenesis and ovarian cycle

Secondary oocyteLarge cell (contains most of the

cytoplasm of the primary oocyte)HaploidArrested in metaphase of meiosis IIIt is expelled by the Graafian follicle

(ovulation)It is captured by the uterine tube

27

Oogenesis and ovarian cycle

If penetrated by the sperm, it will complete meiosis II and will produce a large ovum and one small secondary polar body

OvumChromosomes of the ovum and

sperm will combine to form a diploid cell

28

Oogenesis and ovarian cycle

First polar bodySmall cell (almost no cytoplasm)Produced by the division of the

primary oocyteIt will disintegrate

Secondary polar bodyProduced by the division of the

secondary oocyteIt will also disintegrate

29

Oogenesis and ovarian cycle

Hormonal cycle Beginning of the menstrual cycle

FSH is the dominant hormoneLow levels of LHLow levels of estrogen and

progesterone

30

Oogenesis and ovarian cycle Before ovulation

Estrogen levels increaseProgesterone level remain low

OvulationCaused mainly by a peak of LH

After ovulationEstrogen levels decreaseProgesterone levels increase FSH and LH levels decrease

31

Menstrual cycle

Controlled by ovarian hormones Menstrual phase

Lasts 1-5 daysSloughing off of the functional layer

of the endometrium with bleeding

32

Oogenesis and ovarian cycle

Proliferative phaseFrom day 6-14Estrogen is the main hormoneOvarian follicle growsEndometrium thickens with

proliferation of its glands and blood vessels

Ovulation occur at day 14

33

Oogenesis and ovarian cycle Secretory phase

From day 15-28Progesterone produced by the

corpus luteum is the main hormoneEndometrium thickens even more

with further proliferation and coiling of the glands

Uterus is getting ready for the embryo implantation

34

Oogenesis and ovarian cycle

If no implantation occurs the corpus luteum will deteriorate causing lack of ovarian hormones that will cause deterioration of the functional layer of the endometrium

35

Menstrual cycle

36