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FERTILIZATION

Joyce LearyJoyce LearySeptember 5, 2001September 5, 2001

THE SPERM:

must reach and penetrate the ovum.

must activate the ovum to resume nuclear and cytoplasmic division necessary for embryonic development.

contributes the paternal complement of chromosomes to the new embryo.

determines the sex of the embryo.

SPERM MATURATIONFrom stationary round cell (spermatogonia) to motile, tadpole-like cell (spermatid):Meiosis: 1 diploid cell 4 haploid gametesCentriole flagellumGolgi apparatus acrosomal vesicleMitochrondria collect at junction of flagellum

and head (at midpiece)Head shape streamlines, cytoplasm is stripped

off, and nucleus condenses

Spermiogenesis

From Wheater’s Functional Histology, 4th edition, 2000.

SPERM MATURATIONAcrosomal vesicle - contains enzymes that digest proteins and complex sugars. Important for egg recognition Lyses outer covering of egg

Flagellum – whip-like structure that propels the sperm using ATPATP is produced by the mitochrondriaThe enzyme dynein hydrolyzes ATP to release

energy Dynein is attached to the microtubules of the

flagellum

The Sperm Head

Head: EM, LS × 14 000AC = acrosome; PM = plasma membraneFrom Wheater’s Functional Histology, 4th ed., 2000.

The head of a mature sperm, lengthwise, is approximately 1/20 the diameter of a mature ovum.

The Sperm Neck

Neck (middle piece and principal piece): EM, LS × 17 000Mi = mitrochondria; C = cytoplasm; F, Rn, Rb, & An = all parts of flagellum structureFrom Wheater’s Functional Histology, 4th ed., 2000.

OvulationAt ovulation, the secondary oocyte is released:The secondary oocyte is frozen at metaphase of

the 2nd meiotic division

Accompanying the secondary oocyte:Corona radiata (same as cumulus oophorus) –

follicular cells that accompanied the ovum during ovulation

Zona pellucida – right next to ovum; extracellular matrix proteins that form a barrier assuring species-specific fertilization only

Ovum with corona radiata (arrow): (in vitro)

Advanced Fertility Center of ChicagoAdvanced Fertility Center of Chicagohttp://www.advancedfertility.com/http://www.advancedfertility.com/

THE OVUMUnlike the sperm, the ovum is accumulating nutrients and material throughout maturation, not eliminating it.The ovum must provide food reserves until the

embryo is able to feed on exogenous materials from the mother.

The ovum contributes the maternal complement of chromosomes to the new embryo.The ovum must reject all sperms but one.

The OvumGraafian follicleAzan × 75

FA = follicular antrum

CR = corona radiata

O2 = secondary oocyte

ZG = zona granulosa

TI = theca interna

The oocyte is released along with the corona radiata.

Notice the clear ring just around the ovum (arrow tip): this is the zona pellucida.

From Wheater’s Functional Histology, 4th ed., 2000.

Fertilization:

Location: usually the ampulla of the oviduct (fallopian tube/uterine tube) Fertilization can take place in any part of the tube, and even

in the abdominal cavity, but not in the uterus.

From Wheater’s Functional Histology, 4th ed., 2000.

Getting to the right site:Sperm: During ejaculation, the sperms are propelled

from the urethra by peristaltic contractions of the vas deferens musculature.

Sperm use their flagellum to move through the cervical canal.

The enzyme vesiculase (from seminal vesicles) coagulates some of the semen to prevent backflow into the vagina.

Fructose (from seminal vesicles) is an energy source for the sperm.

Getting to the right site:Helping the sperm: At ovulation, the cervical mucus increases in amount

and becomes less thick, allowing easier sperm transport. Passage of the sperm through the uterus and oviduct

occurs mainly due to muscular contractions of these organs.

Oocyte: The ends of the oviducts come into close contact with

the ovary during ovulation. Fimbriae of oviduct ends “sweep” the ovulated ovum into

the oviduct. Peristaltic waves of oviduct musculature bring the ovum

into the ampulla of the oviduct.