Fertilization And Maturation

Exercise 2

Fertilization and Maturation Stage

Oogonium (2N) Growth / Maturation Meiosis I

Primary Oocyte (2N) Arrest at metaphase I until stimulated by hormone

ovulation Meiosis II

Secondary Oocyte (N) Arrest at metaphase II No Fertilization degeneration

Differentiation

Ootid (N)

Ovum (N)

First polar body

Fertilization MII continues

Second polar bodies

Slides to be studied: Ascaris uterus Ascaris Parasitic nematode Ascaris eggs – transparent – possible to study steps in maturation, fertilization and cleavage Sperm cells enter the eggs in the genital tract even before Meiosis I 2N = 4

Unfertilized eggs Vacuolated cytoplasm inconspicuous nuclei thin cell membranes Primary oocyte note: staining in the picture is different from the staining of the slides in the lab no pronuclei, no sperm head, just vacuolated cell

Stages of Maturation Sperm Penetration Stage Pronuclear Stage Fusion Stage Early Cleavage

1.

Sperm Penetration Stage

Stage of Development: Primary oocyte Presence of Bivalents Heads of spermatozoa – small, dense triangular bodies After fertilization: forms fertilization membrane and outer shell (chitinous shell)

Chitinous shell Fertilization Membrane Perivitelline space Cell membrane bivalents Germinal Vesicle or Female nucleus Sperm head

2.

Pronuclear Stage

Female Pronucleus – lightly stained/ transparent Male pronucleus – darkly stained; bigger Formation of First polar body Separation of the homologous pairs First polar body extruded Stage of development: Secondary oocyte Formation of Second polar bodies Separation of sister chromatids 2nd 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 Cell membrane

First Polar Body Female Pronucleus Male Pronucleus Stage of development: Secondary oocyte

First Polar Body Chitinous shell Fertilization Membrane Perivitelline space Cell membrane

Second Polar Body Female Pronucleus Male Pronucleus Stage of development: ootid

3.

Fusion Stage

Fusion 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 membrane Stage of development: Ovum

First Polar Body Chitinous shell Fertilization Membrane Perivitelline space Cell membrane

Second Polar Body Fused Male and Female Pronuclei 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 Stage Zygote Diploid Entire cell dividing (contrast telophase to fusion stage) Observe the phases of mitosis (prophase, metaphase, anaphase, telophase)

Prophase

Metaphase

Anaphase

Telophase