Fertilization

Spermatozoa Transport and Viability

In most cases, fertilization occurs in the uterine tube next to the ovary; during natural copula­tion, spermatozoa are deposited in the vagina (most species) or uterus via the cervix (mare,

sow, and bitch).

Oxytocin, a peptide hormone from the neu­rohypophysis (see Chapter 12), promotes mus­cular activity of the female tubular genitalia to assist with spermatozoa transport. it is released in the cow during natural mating and during artificial insemination, presumably as a result of a neural reflex initiated by physical stimula­tion of the female tract.

spermatozoa must remain in the female reproductive tract for some period after ejacula­tion before they are capable of fertilization. The process that occurs here to convert nonfertile spermatozoa to fertile spermatozoa is termed capacitation. Capacitation includes changes in or removal of components of the outer acrosome and plasma membranes so that acrosomal enzymes can later be released and activated. Part of the natural capacitation process requires exposure of the spermatozoa to female reproductive tract secretions, but capacitation of spermatozoa can be done in vitro using experimentally derived protocols and solutions.

Under normal conditions, viability and sur­vival times of spermatozoa in the female repro­ductive tract are only a matter of hours.

Gamete Fusion and Early Embryonic Development

At ovulation, a zona pellucida, a relatively thick membranous structure consisting of cross­linked glycoproteins, surrounds the vitelline membrane (cell membrane or plasma mem­brane) of the ovum. in most cases, a variable number of granulosa cells surround the zona, and this layer is termed the cumulus oopho- rous. The zona is believed to be a product of the innermost layer of granulosa cells. Micro­villi from the vitelline membrane of the ovum penetrate the zona, as do processes from the granulosa cells. The first polar body, which results from the first meiotic division, also accompanies the ovulated ovum within the zona (see Chapter 27).

The zona pellucida is a semipermeable mem­brane that helps protect the ovum and that has receptor sites for attachment of spermatozoa during fertilization. A specific protein, ZP3, in the zona serves as a binding site for spermato­zoa. The precise structure of this protein varies among species, and this variation is the primary reason that spermatozoa from one species cannot bind to and fertilize ova from other species.

During or just after binding and attachment to the zona, spermatozoa undergo a series of events termed the acrosome reaction (Fig. 28­1). As part of this reaction, locally released acrosome enzymes digest a passage through the zona. This passage permits spermatozoa to swim their way to the vitelline membrane of the ovum, which is accomplished in a matter of minutes. Multiple spermatozoa may attach to the zona of a single ovum, even though only one spermatozoon will ultimately be responsi­ble for fertilization.

After penetration of the zona, the cell mem­brane of the single spermatozoon that will accomplish fertilization attaches to and fuses with the vitelline membrane of the ovum (Fig. 28-1). This initiates the second meiotic division by the ovum, which results in formation of the second polar body. The fusion of the sperma­tozoon with the ovum also stimulates release of cytoplasmic granules by the ovum, whose con-

Figure 28-1. Sequence of events from initial binding of spermatozoon to zona pel­lucida to fusion between plasma membranes of spermatozoon and oocyte: A, spermato­zoa arrive at site of fertilization; B, initial binding of spermatozoon to zona pellucida; c, acrosome reaction; D and E, penetration of zona pellucida; F, fusion of plasma mem­branes of spermatozoon and oocytes; G, the cumulus matrix. (Reprinted with permission of Wiley-Blackwell from Hafez E.S.E. and Hafez B. Reproduction in Farm Animals. 7th ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)

tents bring about changes in the chemical nature of the zona pellucida. These changes act to prevent penetration by other spermatozoa. This, together with changes in the vitelline membrane, prevents polyspermy, entry of more than one spermatozoon into the ovum. When these mechanisms fail and polyspermic fertil­ization occurs, the typical result is early embry­onic death. This is believed to be rare in most domestic species when fertilization occurs in vivo, but the incidence of polyspermy is higher when fertilization is done in vitro.

A maternal pronucleus is formed in the ovum by the enclosure of the maternal chro­mosomes in a nuclear membrane. The head of the sperm enlarges, becoming the male pronu­cleus. The two pronuclei come together and fuse membranes, forming one cell with the genetic material of both parents. The new cell is ready for cleavage and formation of a morula. cleavage and the subsequent development into a morula and blastula are described in detail in Chapter 3.

During early development, the embryo is not attached to the epithelium lining the female reproductive tract. During this period, embryos obtain their nourishment from fluids and nutri­ents secreted by glands in the walls of the reproductive organs (e.g., endometrial glands in the walls of the uterus). Progesterone stimu­lates secretion by these glands, and blood levels of progesterone are relatively high during this period, as it is being secreted from ovarian corpora lutea.

Maternal recognition of pregnancy is detec­tion of a developing embryo, which prevents regression of the progesterone-secreting corpora lutea. Several mechanisms have been identified in different species, but in general, the mecha­nisms involve secretory products from the developing embryo. These products (e.g., pro­teins or steroids) act locally within the repro­ductive tract. in most cases, the embryonic secretory products inhibit the uterine secretion of prostaglandin F_2α (PGF_2α). Recall that uterine secretion of PGF_2α is the key hormonal signal that causes Ieutolysis in most domestic species. In litter-bearing animals, a minimal number of developing embryos appears to be required to recognize pregnancy and prevent regression of the corpora lutea. This number is about four for the sow.

Embryos develop to the blastula stage while still enclosed in a zona pellucida (see Chapter 3). The zona is shed (hatching) prior to attach­ment of the embryo to the uterine wall for placentation. The outermost layer of cells of the blastula is the trophoblast, and it is from these cells that the fetal membranes will be formed.

Early embryonic death (death of the embryo prior to attachment to the uterine wall) is responsible for a significant number of reproductive failures in domestic animals. Some studies report that up to 30% of fertil­ized embryos die before developing into fetuses. Possible causes of embryonic death include inherited lethal factors, infections, nutritional deficiencies, inappropriate levels of maternal hormones, and defects in the ovum or spermatozoa before fertilization.