Fertilization and Maturation Stage

Exercise 2

First polar body

Oogonium (2N)

Primary Oocyte (2N)

Secondary Oocyte (N)

Growth / Maturation

Meiosis I

Meiosis II

Second polar bodiesOotid (N)

Ovum (N)

Differentiation

ovulation

Arrest at metaphase I until stimulated by hormone

FertilizationNo Fertilization

degeneration MII continues

Arrest at metaphase II

Slides to be studied: Ascaris uterus

Ascaris Parasitic nematodeAscaris eggs – transparent – possible to study steps in maturation, fertilization and cleavageSperm cells enter the eggs in the genital tract even before Meiosis I2N = 4

Unfertilized eggsVacuolated cytoplasminconspicuous nucleithin cell membranesPrimary oocytenote: staining in the picture is different from the staining of the slides in the labno pronuclei, no sperm head, just vacuolated cell

Stages of Maturation

Sperm Penetration StagePronuclear StageFusion StageEarly Cleavage

1. Sperm Penetration StageStage of Development: Primary oocytePresence of Bivalents Heads of spermatozoa – small, dense triangular bodiesAfter fertilization: forms fertilization membrane and outer shell (chitinous shell)

Chitinous shell

Fertilization Membrane

Perivitelline space

bivalents

Germinal Vesicle

or Female nucleus

Sperm head

Cell membrane

2. Pronuclear StageFemale Pronucleus – lightly stained/ transparentMale pronucleus – darkly stained; biggerFormation of First polar body

Separation of the homologous pairsFirst polar body extrudedStage of development: Secondary oocyte

Formation of Second polar bodiesSeparation of sister chromatids2nd polar body extruded ( 2 polar bodies will be seen; the outer one is the first and the inner one is the second polar body)Stage of development: Ootid

Chitinous shell

Fertilization Membrane

Perivitelline space

First Polar Body

Female Pronucleus

Male Pronucleus

Cell membrane

Stage of development: Secondary oocyte

Chitinous shell

Fertilization Membrane

Perivitelline space

Second Polar Body

Female Pronucleus

Male Pronucleus

Cell membrane

First Polar Body

Stage of development: ootid

3. Fusion StageFusion of the male and female pronuclei. (observe closely; it must be flat in the middle; there are cases when the male and female pronuclei seem to be fused but are actually just overlapping)

Presence of 2 polar bodies (inner- 2nd, outer- 1st) or 1 polar body (2nd polat body near the outer covering; 1st polar body already extruded from the shell)Perivitelline space – space between the egg and the fertilization membraneStage of development: Ovum

Chitinous shell

Fertilization Membrane

Perivitelline space

Second Polar Body

Fused Male and Female Pronuclei

Cell membrane

First Polar Body

Note: Discernment is needed to be able to distinguish the 1st and 2nd polar bodies. It is always the first polar body that is first extruded. The speed of the extrusion of polar bodies from the egg cell may vary, so in the fusion and early cleavage stage there may be 2, 1 or no polar body that can be observed.

Early Cleavage StageZygoteDiploidEntire cell dividing (contrast telophase to fusion stage)Observe the phases of mitosis (prophase, metaphase, anaphase, telophase)

Prophase

Metaphase

Anaphase

Telophase