Chapter 7 Ovigensis and Fertilization

7-1 Ovigenesis

* Definition:
a. Ovigenesis= Oogenesis= ³­ÀÚÇü¼º: Formation and maturation of female gamete
b. Folliculogenesis= ³­Æ÷Çü¼º: Formation and maturation of follicle

7-1-1 Process of ovigenesis: See FIGURE 7-1

A. Source of oogonium : Oogonium <--- extension of yolk sac <--- hind gut of embryo
B. Fetal period:

a. Proliferation by mitotic division within parenchyma of ovary
b. Maturation to primary oocytes by meiotic division

C. Shortly after birth:

a. No mitotic division of oogonium, and maximum number of oocytes
b. Growth of primary oocytes, and formation of zona pellucida
c. Matured to and arrested in the late prophase of Meiosis I =Dictyate stage(diffuse diplotene)

D. From puberty:

a. Some of primary oocytes start to development, of which most are atretic, but some reach matrurity and the mature oocytes are released during ovulation under preovulatory LH surge, in the stage of primary oocytes in mare, but secondary oocytes in other farm animals. b. Meiosis I is completed, but Meiosis II is arrested at metaphase II.

E. After ovulation : Sperm penetration occurs and Meiosis II resumes and zygogte is formed (= fertilized ova).

7-1-2 Process of folliculogenesis

A. Fetal period : Primary follicles with single layer of follicular cells = granulosa cells (g.c.)---> Secondary follicles with two or more layers of g.c. and the oocytes surrounded with zona pellucida

B. At birth in most animals, and about 70 days after birth in pig : Tertiary follicles with antrum and multi layers of g.c.,with corona radiata around zona pellucida: FSH stimulates their development.

C. From puberty :

a. Follicular growth to Graafian follicles with follicular fluid in antrum, formation of cumulus oophorus, membrana granulosa, theca interna and externa: stimulated by FSH, but then most of them are atretic, and one or two dominant follicles in monotocous animals, a few in sheep or goat, or 20 or more follicles in sows : matured to preovulatory follicles by preovulatory FSH and LH surge.
b. Proliferation of g.c. by FSH and less pronounced proliferation of thecal cells by LH.
c. Ovulation occurs under LH surge.

7-2 Ovulation

A. Ovulation time: At 24 to 45 hours after preovulatory LH surge

B. Mechanism:

a. Physiological change around ovulation
1) LH induces acute inflammatory reaction.
2) Inc.P4, followed by inc. E2 and PGs in follicular fluid
3) Inc. blood flow
4) Inc. muscle contraction by PGF2@.

b. Six cellular layers surrounding the follicles, which should be dissociated or decomposed for ovulation are as follows:.

1) Membrana granulosa: Dissociated only at follicular apex and finally disappeared by proteolytic enzymes in response to LH, P4 and PGF2@.

2) Basement membrane (= basal lamina) : Degenerated by plasmin. (a proteolytic enzyme from activation of plasminogen by plasminogen activator stimulated by inc. PGF2@)

3) Theca interna

4) Theca externa

5) Tunica albuginea (collageneous)

6) Surface epithelium

* Localized degeneration by proteolytic enzymes as collagenase from lysosomes¡çrupturing lysosome-like vesicles between tunica albuginea and surface epithelium in the apex of follicle by PGF2¥á.

C. Oocytes released: See FIGURE 7-2

a. Extruded into peritoneal cavity near infundibulum
b. The surrounding g.c. (cumulus and corona radiata) will be shed quickly in some species,but persist for about as long as the egg remains fertilizable in rabbit,rat and mouse.

7-3 Gamete Transport

* Definition:
Movement of viable gametes to the site of fertilization(= Ampullary portion from ampullary-isthmic junction:AIJ); Timing of transport: important.

7-3-1 Oocyte transport

A. The oocyte with its associated cumulus mass: picked up by ciliated epithelial cells of the infundibulum.

B. Time for oocyte to pass through AIJ:See TABLE 7-1

Cattle: 90 hrs.
Sheep: 72
Horse: 98
Swine: 50

C. Mechanism:

a. Cilia beating toward uterus, with directional flow of oviductal fluids.
b. Segmented, peristaltic contractions of oviduct: milking the oocyte down.
c. E₂ causes retention of the oocyte and P₄ hastens transport of oocyte.

7-3-2 Spermatozoa transport

A. Time and mechanism for sperm to reach AIJ: two phasic

a. Rapid phase:
Dead and alive, uncapacitated sperm: within a few minutes: By peristaltic contractions of the tract stimulated by:

1) PGF2@ from semen
2) Oxytocin from brain-sensory nerve-copulation

b. Slow and sustained phase:
Only alive capacitated sperm: By 8 hrs. after natural mating in cows and ewes by:
1) Barriers to sperm transport: Cervical mucus (filteration and
selection), endometrial glands, and uterotubal junction (UTJ)*
2) Sperm reservoirs: Cervical crypts, endometrial glands and isthmus
3) PGF2@ and E2 speed up sperm transport:
  * The relationships of steroid hormones and prostaglandins to oviductal
motility (frequency and amplitude of circular or longitudinal muscle:

See FIGURE 7-3) or oviductal motility to gamete or zygote movement have not been established.

* Species variation in number of spermatozoa ejaculated and reaching ampulla.

Species	Average no. spermejcaulate (millions)	Site of sperm deposition	No. of sperm in ampulla of oviduct
Mouse Rabbit Cattle heep Pig Man	50 280 3000 1000 8000 280	Uterus Vagina Vagina Vagina Uterus Vagina	100# 250- 500## 4200-27500## 600- 5000## 1000# 200#

# From M.J.K.Harper. In Reproduction in Mammals: 1 (Germ Cells and Fertilization), 2nd ed. Ed. C. R. Austin and R. V. Short, pp.102-127. Cambridge Univ.Press; Cambridge (1982).
## From R. J. Blandau. In Handbook of Physiology, Section 7: Endocrinology, Vol.II, Female Reproductive Systems, Part 2. Ed. R. O. Greep and E. B. Astwood, pp. 153-163. American Physiological Society, Washington, D.C.(1973)

7-4 Fertilization: See FIGURE 7-4

A. Sperm penetration

a. Cumulus oophorus : Hyaluronidase
b. Corona radiata : Corona radiata penetrating enzyme¡çSperm head*
c. Zona pellucida : Acrosin (a trypsin-like enzyme)
   * Acrosomal enzymes¡çvesicles¡çfusion of outer acrosomal membrane and plasma membrane of sperm head<--acrosomal reaction¡çsperm capacitation

B. Oocyte reaction

a. Zona reaction: Guards against penetration of Z.P.by other sperm.*
* By spilling of the contents of cortical granules into perivitelline space.
b. Sperm penetration of vitelline membrane: By phagocytosis
c. Vitelline block: Guards against fertilization by other sperm.
d. Extrusion of 2nd P.B. into perivitelline space: After sperm entery and degenration of sperm tail by mitochondria

C. Pronucleus formation and syngamy

a. Formation of female (small) and male (large) pronucleus
b. Syngamy ---> zygote

7-5 Polyspermy: Òýïñí­ áôïñ

A. Definition: Fertilization by more than 1 spermatozoon: Polyploidy ---> Early embryonic mortality; Triploidy(3n): Shortly develop normally ---> Die and degenerate

B. Incidence:

a. Generally 1-2% in natural breeding of mammals, but usual in birds.
b. High incidence in mammals from the secondary oocytes:
1) Aged from too late breeding
2) Heated from elevated body temperature by fever or ambient temprature

C. Preventive mechanism:

a. Zona reaction: If not--->Supplementary sperm: Not harmful
b. Vitelline block: If not--->Supernumery sperm: Harmful to fertilization

7-6 Aging of gametes

A. Fertile life of gametes: See TABLE 7-2

a. Sperm: About 24 hrs postmating in cattle, swine and sheep 72-120 hrs in horse 32 days in hen 62 days in turkey a few months in bats
b. Ovum : Less than 12 hrs in cattle , swine and horse 16-24 hrs in ewe

B. Problem of aged gametes: See TABLEs 7-3 and 7-4

a. Low fertilization rate
b. High embryo and fetal mortality