Introduction

What could be more fundamental to the success of a species than reproduction? Understanding homeostasis and physiological controls is of limited use unless we can maintain our animals and replenish stocks. Natural selection has produced an astounding variety of successful reproductive strategies. From an evolutionary viewpoint, the most favored pattern is one that results in the largest number of offspring reaching sexual maturity. Most population biologists have recognized two general patterns, namely, r‐selection or K‐selection. These terms were coined from logistic equations that were used to model the growth of populations of animals in the wild (Bearden et al., 2024; Randall et al., 2002).

For r‐selection animals, the investment in individual animals is small both in a physical and energetic sense. Examples of this pattern of reproduction include fishes or amphibians that can produce extremely large numbers of eggs. The trade‐off in producing so many small offspring is that there is little if any care of the offspring. They are produced and released into the environment to fend for themselves. For example, it is estimated that only six out of a million mackerel fry survive to also reproduce. On the other hand, K‐selection species produce fewer but larger offspring. Because these offspring are cared for—at least to some degree—during development, the chance that they will successfully reach reproductive age is far greater. Compared with the mackerel, individual K‐selection animals may have a 50% chance of success. Clearly, for most agricultural operations the K‐selection pattern predominates.

Energetic costs of parental care occur in two categories. First, there is the cost that relates to the transfer of nutrients to the developing neonate. For mammals, the demands of lactation can be very large. The lactating mother can expend 40% of her energy producing milk. This can be even greater, in high‐producing dairy cows that have been selected for copious milk production. Other animals, for example, crop milk produced by pigeons and doves also represent a direct energetic expenditure cost. In addition, there are costs of behavioral responses to protect and nurture neonates.

As with most of the chapter topics, entire courses are devoted to these physiological specialties. Many animal science majors will take courses devoted to nutrition, lactation, and reproduction. Our goal for this chapter is not to substitute for a specialized course in reproduction but rather to provide some basics and stimulate your interest in a more detailed study.

Overview of the Female Reproductive Tract

The structures of the female reproductive tract include the ovaries, oviducts, uterus, cervix, vagina, and external genitalia. In farm animal species, the reproductive tract is positioned below the rectum. For cows and mares, this is helpful because it allows the producer or veterinarian to evaluate the reproductive tract by manipulation per rectum. This provides a practical means to determine the functional status of the ovary, pregnancy status, or manipulation of the tract for artificial insemination (AI). The female tract is essentially a series of interconnected tubes with distinct layers of varying thickness. The outer serosa is a thin layer of simple squamous epithelial cells. The next layer, the muscularis, is composed of an outer layer of smooth muscle cells arranged longitudinally and an inner circular layer of smooth muscle cells. This arrangement allows for the generation of muscle contractions to aid the transport of fluids and secretions, movement of ova and spermatozoa, passage of the early embryo, and expulsion of the fetus and fetal membranes at the time of parturition. Just under the muscularis, the submucosa provides connective tissue space for blood and lymphatic vessels, nerves, and glands to support and nourish the mucosa. The lumen of all the regions of the mucosa is lined by epithelial cells but the structural and functional attributes of these cells vary from region to region to reflect different activities and variations in the reproductive cycle. To illustrate, a simple layer of columnar epithelial cells lines the oviduct but some of the cells are also ciliated. Fluids that coat the surface allow for the beating of the cilia to propel ovulated eggs from the ovary into the oviducts and into the uterus for implantation if fertilization occurs. In contrast, a layer of stratified squamous epithelial cells provides for increased protection and lines the lumen of the reproductive tract in the posterior region of the vagina. Figure 19.1 illustrates the general histology of a cross section through the oviduct of a cow as well as the specialization of the epithelial cells, that is, presence of cilia.

As you might suspect there is substantial deviation in the surface epithelium at different locations of the reproductive tract. There are also marked variations associated with the stage of the estrous cycle. For example, Figure 19.2 illustrates the vaginal epithelium along the surface of the posterior vagina as well as differences associated with the stage of the estrus cycle. Unlike the oviduct, here the epithelial lining is composed of stratified squamous epithelial cells. During the follicular phase of the estrus cycle the layer of cells is thicker compared with the luteal phase.

For primates that express menses, variation in the uterine epithelium and in uterine glands is pronounced. Figure 19.3 shows the histological appearance of the uterus. Compared with other regions of the reproductive tract, the muscularis is extensive and the submucosa area has abundant glands that supply secretions that are especially important during pregnancy.

The reproductive tract develops in a retroperitoneal position so that it is positioned against the peritoneum. With continued growth, the tract becomes enveloped. This new connective tissue sheath forms a continuous drape around the reproductive tract. This functions to suspend and maintain the position of the ovaries, oviduct, uterus, and the anterior vagina. In this mature state, this supporting tissue is called the broad ligament.

The ovaries, analogous to the testes in the male, are the primary reproductive organs because they produce the female gametes (ovum, singular or ova, plural). However, the ovaries produce not only gametes they are also critical endocrine organs. Release of mature ova occurs with the rupture of follicles on the surface of the ovary. The ovum or ova enters the funnel‐like open end of the oviducts (infundibulum) to be directed to the uterus. Fertilization usually occurs within the oviduct during transit to the uterus. Figure 19.4 illustrates the gross anatomy of the primary structures of the reproductive tract of a cow and Figure 19.5 is a dissected view of the vagina and cervix.

The paired ovaries are in the lumbar region near the kidneys. In sexually mature animals, the ovary undergoes dramatic but predictable cyclic development. For example, within a window of about three weeks in cows, pigs, or horses, ovulation occurs, and the selected ovarian follicles are transformed into corpus luteum (singular) or corpora lutea (plural) that produce large quantities of progesterone. If fertilization does not occur, the corpus luteum regresses and a new crop of follicles matures. These follicles produce estrogen and selected follicle(s) proceed to undergo ovulation. This pattern constitutes an estrus cycle. The ovary is oval to round with distinct regional differences and blister‐like structures—the follicles—near the outer surface. A connective tissue layer called the tunica albuginea covers and protects the ovary. This connective tissue band supports the surface layer of epithelial cells, which is unfortunately called the germinal epithelium. This is unfortunate because, despite the promising name, these epithelial cells do not produce the gametes. Instead, beneath the tunica albuginea, within the ovarian cortex, there are populations of oocytes, which are recruited, to develop into the mature follicles. The ovarian cortex also houses the corpus luteum as well as older degenerated corpora lutea called corpora albicantia. The center of the ovary is called the ovarian medulla and contains the blood, nerves, and lymphatic vessels that supply the ovary. The structure of the bovine ovary is illustrated in Figure 19.6.

In domestic species, the uterus consists of a body, a cervix (or neck), and two uterine horns. However, there are substantial variations in the shape and arrangement of the horns. For example, the body of the bovine uterus appears larger because of the intercornual ligament, which acts to obscure the individual nature of the two uterine horns and covers the caudal region of the uterus. Among mammals, there are three distinct types of uteri. The duplex uterus has two cervical canals, which act to separate each uterine horn into distinct compartments. However, there are two types of duplex uteri. In one there is a single vaginal canal opening to the outside. On the interior, it divides to produce two vaginas and two cervices. This occurs in marsupials. In the North American opossum, for example, the male accommodates this circumstance by having a forked penis. The rabbit has a less complex arrangement; there are two uterine horns and two cervical canals but a single vaginal canal. The bicornuate uterus is characterized by the presence of two uterine horns and a small uterine body. In all these cases, the uterus opens into the vagina via a single cervical opening. Cows, mares, and pigs all have this type of uterine structure. Primates on the other hand have a simplex uterus. There is a large uterine body but essentially no uterine horns.

The appearance of the internal lining of the uterus, the endometrium, varies during the estrous or menstrual cycles and pregnancy. The tissue is also highly glandular with a rich blood supply. The epithelial surface is a simple columnar epithelium in the mare but stratified columnar epithelial cells in ruminants. In addition, simple branched tubular glands provide secretions—called uterine milk—that are especially important during estrus and pregnancy. In many animals, the uterine glands are scattered throughout the endometrium. But in ruminants, the internal uterine surface is punctuated by caruncles that are not glandular. These mushroom‐like caruncles provide sites for the attachment of fetal membranes in these animals. The smooth muscle of the uterus (the muscularis) is frequently called the myometrium.

The most caudal region of the uterus leads to the cervix. The cervix is a tough, connective tissue and smooth muscle sphincter that is tightly closed except during estrus and parturition. During estrus, the slight loosening of the cervix allows spermatozoa to enter the uterus. In ruminants, the inner surface of the uterus is oriented in a series of circular folds or ridges called annular folds. Learning to traverse these folds can be a challenge to beginning artificial breeding technicians.

The vagina is the region of the reproductive tract within the pelvis positioned between the cervix on the cranial end and the vulva on the caudal end. The vulva or external genitalia is comprised of the right and left labia, which join at the midline to produce a commissure or union. The ventral commissure of the vestibule (the posterior vagina) houses the clitoris, the female homolog of the glans penis in males. It contains erectile tissue and is covered by stratified squamous epithelium. It is well supplied with sensory nerve endings. The functional significance is not well established in domestic animals but clitoral stimulation at the time of insemination has been shown to increase conception rates in beef cows. Some comparative features of the female reproductive tract of various nonpregnant farm animals are given in Table 19.1.

Animal
Organ	Mare	Sow	Cow	Ewe
Oviduct	20–30 cm	15–30 cm	25 cm	15–19 cm
Uterus
Type	Bipartite	Bicornuate	Bipartite	Bipartite
Horn	15–25 cm	40–65 cm	35–40 cm	10–12 cm
Body	15–20 cm	5 cm	2–4 cm	1–2 cm
Endometrium	Prominent longitudinal folds	Slight longitudinal folds	40–120 Caruncles	88–96 caruncles
Cervix
Lumen	Conspicuous folds	Corkscrew‐like	2–5 Annular rings	Annular rings
Opening to uterus	Clearly defined	Ill‐defined	Small and protruding	Small and protruding
Vagina	20–35 cm	10–15 cm	25–30 cm	10–14 cm
Vestibule	10–12 cm	6–8 cm	10–12 cm	2.5–3 cm

Puberty in Females

On the surface, the definition of puberty seems simple enough, the age at which reproductive competence is achieved. However, in farm animals, several criteria have been used to define puberty in females. For example, age at first estrus or heat can be a defining moment in cattle. However, this may or may not be associated with ovulation. Thus, this may not necessarily reflect reproductive competence. Ovulation can be evaluated by palpation of the ovary or ultrasound imaging in larger animals. However, in practical terms, the age at which a female can conceive and support pregnancy without harm to herself is probably an excellent definition. The physiological demands to complete follicular development, ovulation, and transport of the fertilized ova to the uterus for implantation are relatively minor. The metabolic requirements to maintain the pregnancy and initiate lactation to support a rapidly growing neonate can be daunting. Thus, from a husbandry standpoint, it is rarely an advantage for females to become pregnant at the earliest possible time.

Estrous Cycles, Estrus, and Ovulation

The ovaries are the source of the female gametes (ova) and estrous cycles, in postpubertal animals, these cycles delineate periodic development or maturing of follicles, which results in the release of eggs or ova that have the possibility of being fertilized. The cellular events required to create eggs are called oogenesis. The period when the female is receptive to sexual activity is called estrus or more commonly heat. An estrous cycle is simply the time from the beginning of one estrus period to another. It is worth taking a moment to clarify some of this confusing terminology. Estrus is a noun (e.g., A cow displays estrus or heat). Estrous is an adjective (e.g., the average length of an estrous cycle in the Holstein dairy cow is 21 days). A review of British and European scientific literature shows that oestrus and estrous are the equivalent terms for estrus and estrous.

Animals that exhibit only one estrous cycle per year are monoestrous and estrus lasts for several days (dogs, wolves, foxes, and bears). As a reproductive strategy, having an extended period of estrus increases the chance of a successful mating. In contrast, polyestrous species exhibit multiple cycles each year. However, these cycles are not necessarily uniformly distributed throughout the year. There are windows of inactivity when the animals are anestrous. These animals are seasonally polyestrous. Even among animals that are not seasonal breeders, there can be interruptions in the usual regular pattern. For example, in cattle that breed year around, there is typically an anestrous period for several weeks or months after calving. This can be especially pronounced in beef cows that are suckling their calves. Table 19.2 provides a summary of selected reproductive measures in some common farm animals.

It is important to appreciate that there can be substantial differences between breeds. For example, the onset of puberty in cattle can vary from a low of about 9 months in Holsteins to 19 months in Brahman cows. Similarly, Meishan pigs can show estrus as early as 3 months but Yorkshire gilts average about 7 mos. In the case of the onset of puberty, several external factors are also important. In sheep and goats, the season of birth or photoperiod can act to hasten or delay the onset of puberty. The presence or absence of the opposite sex during the peripubertal period impacts cattle and swine as does the density of housing in swine. In a large grouping of gilts normal puberty is 28 weeks but in a smaller group (~3) puberty is often delayed until 32 weeks. Age at the onset of puberty is also greatly influenced by body size and condition. The onset of puberty is a particularly important aspect of farm animal management. In dairy cattle, it is advantageous to breed females as soon as practical so that the animals enter the milking herd earlier. This must be coupled with the animals also having the necessary body size and condition to avoid calving difficulties and successfully compete with older stronger cows at the feed bunk. From a genetic viewpoint, minimizing the age at puberty in males likely has the greatest benefit. In other words, like many aspects of animal production—growth, lactation—reproduction is also markedly impacted by the management of the animals.

We will use the dairy cow as our model species to describe the estrous cycle and process of ovulation. The general development of follicles is similar in most mammals. The ovary contains thousands of primary or dormant follicles. Essentially these are the ova, each of which is surrounded by a thin layer of cuboidal follicular or granulosa cells. In response to tonic secretion of FSH and LH, some of these primary follicles enlarge to become small antral follicles. Antral follicles exhibit the appearance of a fluid‐filled space between the oocyte in the center and the surrounding granulosa cells. These follicles provide the source of follicles that are eligible for activation during the next estrous cycles.

Based on convention, the estrous cycle is divided into two phases, named after the dominant structures that are present on the ovary. The follicular phase is relatively short ~20% of the estrous period and the preovulatory follicles, which produce estradiol are in control. The follicular phase encompasses the time from the regression of the corpora lutea to the time of ovulation. This is not to say that there is no follicular activity at other stages of the estrous cycle. Indeed, there are populations of follicles that sequentially develop throughout the estrous cycle. However, as in many things, timing is everything. As described below, it is only the dominant follicle(s) that are part of a wave of development occurring near the time when the regression of the corpus luteum occurs that can be qualified for ovulation.

It is now known that several waves (typically 3) of follicular development occur during the estrous cycle in cattle. Beginning after ovulation, groups or clusters of small or medium antral follicles become sensitive to gonadotropins. These follicles are described as recruited follicles. Among this group of recruited follicles, several are selected and begin to mature. However, typically only one of these selected follicles will win the maturity race so that it becomes the dominant follicle. Other selected follicles in this class begin to undergo regression or atresia. Even the dominant follicle is destined to undergo atresia if it is so unlucky to have been recruited in the first or second wave of follicular development during the estrous cycle. This developmental process is illustrated in Figure 19.11. Figure 19.12 illustrates changes in major reproductive hormones during a bovine estrous cycle.

For a moment, let us consider the development of the ovary and specifically the relationship between the follicle and the oocytes. In the fetus, primordial germ cells migrate from the yolk sac to the immature ovary. These germ cells (now called an oogonium) become surrounded by a single layer of follicular cells. As the ovary matures, it is usual to find clusters or nests of these germ cells in the cortex of the ovary (Fig. 19.9A). It is worth remembering that these cells undergo meiosis, but they stop in the first prophase before the first division. In most animals, the first of the two meiotic divisions is completed, producing the first polar body, at about the time of ovulation. The essential point is that unlike spermatogenesis, where each primary germ cell produces four spermatozoa, the maturation of the oocyte creates only one mature ovum and three polar bodies. Thereafter some of these primary or primordial follicles are stimulated to growth progressing first to a secondary follicle stage. This is characterized by an increase in the number of granulosa cells surrounding the oocyte (Fig. 19.7B). A tertiary follicle or antral follicle is characterized by the coalescence of fluid that appears between the granulosa cells so that a prominent space is evident (Fig. 19.7C). With progressive development, the stroma cells surrounding the follicle segregate into two distinct layers—the theca interna (closest to the layer of granulosa cells) and outer theca externa. These layers give additional structural substance to the rapidly developing follicle. Moreover, the theca interna, if the follicle is a dominating ovulatory follicle, is destined to be a major source of steroid hormones. The granulosa cells are separated from the theca cells by the basement membrane. At this time, the oocyte is typically displaced to one side of the follicle and antrum where it is surrounded by a cluster or cloud of granulosa cells called the cumulus oophorus (Fig. 19.8). Other granulosa cells remain in layers surrounding the oocyte adjacent to the theca interna. These cells are referred to as the membrana granulosa. The granulosa cells adjacent to the ovum secrete glycoproteins that form a protective layer called the zona pellucida. Figure 19.9 shows examples of developing follicles.

While we have used the cow as our primary model, there are dramatic differences between species. The LH surge and ovulation in most farm animals (cow, sow, ewe, and mare) take place regularly independent of copulation. These animals spontaneously ovulate. In contrast, in rabbits, mink, camels, llamas, and alpacas ovulation requires copulation. Such animals are induced ovulators. In these cases, the appearance of the preovulatory LH surge depends on neural reflexes produced by vaginal simulation. These animals (often depending on the season) have typical estrous cycles and associated follicular development, but the mature follicles undergo atresia if copulation does not occur.

Once ovulation occurs the granulosa cells lining the now empty follicular cavity begin to divide, fill the space, and begin a process called luteinization in response to high levels of LH. The result is the formation of a structure called the corpus luteum (yellow body or CL). The CL is a powerful endocrine tissue that produces large amounts of progesterone. The secretion of progesterone prepares the uterus to receive the ovulated ovum or ova. If fertilization and implantation of the ova is successful, this produces maternal recognition of pregnancy, and the usual regression of the CL does not occur. Progesterone concentrations in the blood are maintained throughout pregnancy. Some species are dependent on the CL for all the progesterone needs but in others, the CL can actually be removed and pregnancy maintained because of progesterone produced by the placenta. Progesterone, especially in the later stages of pregnancy, is critical for mammary development needed to support lactation after the birth of the young.

If pregnancy is not established, the CL must regress for the animal to continue estrous cycles. In most domestic species, the signal to induce CL regression is prostaglandin F₂α (PGF₂α). Secretion of PGF₂α begins to increase after ovulation with timing that corresponds with estrous cycle length in the species.

When PGF₂α secretion is sufficient this stimulates a series of biochemical and cellular changes in the CL so that it regresses or undergoes luteolysis. This then allows for another estrous cycle to begin.

Certainly, successful breeding depends on the ability to detect candidate animals in estrus. In situations where cattle are bred naturally, herd bulls handle these demands very well. However, in dairy operations, it is not usual to maintain bulls for breeding. Dairy bulls are exceptionally aggressive and dangerous. Thus, the physical need for adequate handling and housing facilities can be substantial. Perhaps more importantly, genetic progress in dairy has depended on the ability to breed many cows to genetically superior bulls. This is only possible by the use of AI. Without bulls to detect females in heat, this means that producers had to learn other methods of heat detection. Fortunately, behavioral cues in cattle are dramatic and if animals are housed so that interactions between females are possible, that is, appropriate pastures or lots with good footing and sufficient time for observation. Specifically, as the cow enters estrus, she gradually begins to display activity that signals approaching sexual receptivity. These include increased physical activity (locomotion), bellowing, nervousness, and attempts to mount other females. As the period progresses the female’s willingness to accept the male increases. During this time, the cow will display a mating posture called lordosis. Such animals stand still so that herd mates will periodically engage in mounting behavior. The animal ready for breeding is said to be in standing heat. Observation of standing heat is the major cue for managers to breed these animals. To be most efficient, it is important to have the inseminate deposited very near the time of ovulation. This means that most operations would need to observe candidate animals at least twice daily likely 30–45 minutes per session. Because labor is an important part of farm costs, it is not surprising that multiple techniques to better automate heat detection have been devised. For example, patches placed on the rump of candidate animals that contain packets of dye that rupture when the animal is mounted, mounted pressure sensors that send radio signals to a base station on the farm, or monitoring of activity via a daily reading of pedometers are all methods used to decrease the costs and increase the efficiency of heat detection. One of the more interesting experimental approaches was to train dogs to identify cows in heat. Other advances in biotechnology including AI and machine learning are adding to the tools for reproductive improvements (Hoorn et al., 2024; Shakweer et al., 2023).

In recent years, control of estrous cycles to more efficiently manage the breeding of cattle has become a reality using estrus synchronization schemes. This has become possible because of the availability of commercial formulations of PGF₂α, GNRH, and progesterone. Indeed, there are now effective schemes to allow the timed breeding of cows and heifers without the need for heat detection. As an example, Peeler et al., 2004 reported results from a trial in which heifers were synchronized using intravaginal progesterone inserts also called controlled internal drug release devices (CIRD), coupled with GNRH treatment, and timed AI. Briefly, heifers were assigned to the protocol irrespective of the stage of the estrous cycle. At this time, they received a CIDR containing 1.38 g of progesterone and a 1 mg injection of estradiol cypionate (ECP). After 7 day the CIDR was removed, and the heifers were given a 25 mg injection of PGF₂α. On day 9, the heifers were given an injection containing 100 μg GNRH. The heifers were bred 48, 56, or 72 hours after the CIDR was removed. The first service pregnancy rates averaged 57.6%. There was no estrus detection in the study but more importantly the success rate was like that obtained with laborious heat detection systems. Some of the advances in reproductive technologies and the history of research to create these tools have been reviewed (Stevenson and Britt, 2017). Perry 2012; Wiltbank et al., 2011; Sakaguchi 2011).

Fertilization and Pregnancy

Assuming a successful ovulation and insemination have occurred, the reproductive story is just beginning. As spermatozoa ascend through the cervix, uterine body, and oviduct, many are lost. However, those that remain must undergo a process called capacitation. These biochemical changes to the sperm cells are induced by secretions of the female reproductive tract. It is generally accepted that exposure of spermatozoa to seminal fluid during maturation in the testes or at the time of ejaculation leads to the coating of the cell surface with a complex layer of proteins and carbohydrates. Removal of these materials via the capacitation process in the female tract is essential for the spermatozoa to bind to the oocyte. Fertilization typically occurs when the oocyte and spermatozoa meet in the ampulla region of the oviduct.

Interestingly, when the spermatozoa reach this area, swimming patterns change from very regular linear movements to more erratic or frenzied motions. This change is induced by molecules secreted in this region of the oviduct and is thought to increase the opportunity for contact between sperm and oocyte. Fertilization depends on a complex series of steps which are outlined in Figure 19.11.

Sperm cells have very specific proteins associated with the acrosome portion of the head of the spermatozoa, which have an affinity for the zona pellucida of the oocyte. This outer layer of the oocyte is composed of three glycoproteins called zona protein 1, 2, and 3 or ZP1, ZP2, and ZP3 for short. ZP1 and ZP2 are primarily structural proteins that maintain the space and organization of the zona pellucida. ZP3 in contrast, acts as an anchor or receptor for proteins found in the membrane of the sperm cell. Two binding sites are present. The primary zona binding region allows for the close adherence of the oocyte and sperm cell. A second site induces the acrosome reaction when ZP3 from the zona pellucida binds.

The acrosome is a lysosome‐like membrane‐bound structure oriented around the outer portion of the spermatozoa where it partially encapsulates the condensed nucleus of the spermatozoa. It contains enzymes that are important for events associated with fertilization. The acrosome reaction is essential because it allows the spermatozoa to penetrate the zona pellucida and second it exposes the nucleus of the sperm cell, that is, now surrounded by the former inner membrane of the acrosome. After binding, the acrosomal reaction begins as the plasma membrane surrounding the sperm head forms fusion points with the acrosomal membrane. As small vesicles appear, this produces a morphological cellular landmark called vesiculation that is used to identify cells that have undergone the acrosome reaction. Among the enzymes released, proacrosin has a very high affinity for the zona pellucida. This supports the close adherence of the oocyte and spermatozoa undergoing the acrosome reaction. When activated to acrosin, its hydrolytic action degrades the zona pellucida in the very local region where the sperm cell is attached. This is important because the continued presence of the zona pellucida is necessary for subsequent development. The continued beating of the tail of the spermatozoa supplies the mechanical force needed for the activated spermatozoa to penetrate through the zona pellucida to the perivitelline space, which surrounds the plasma membrane of the oocyte. When the spermatozoon gets to the perivitelline space, it binds to microvilli on the surface of the oocyte. The plasma membrane of the oocyte then fuses with the membrane surrounding the spermatozoon and engulfment occurs. After this event dense vesicles—the cortical granules—previously produced inside the oocyte, migrate to the cell surface and release their contents by exocytosis. The proteases, mucopolysaccharides, plasminogen activator, acid phosphatases, and peroxidases contained in the granules alter the biochemical properties of the zona pellucida so that in addition sperm cannot enter. This is called zona block. In some species, these substances also reduce the capacity of the oocyte plasma membrane to fuse with additional spermatozoa, the vitelline block. This provides a second mechanism to prevent polyspermy (fertilization by more than one spermatozoon), which results in embryonic death. After the sperm nucleus is freed into the cytoplasm of the oocyte it becomes the male pronucleus. However, this requires a major decondensation of the sperm nucleus. During the maturation of the spermatozoa in the testis the nucleus becomes very highly compacted and ordered due to the creation of many disulfide crosslinks. In this state, the chromosomes are essentially inert. Fortunately, the oocyte cytoplasm is rich in glutathione, which allows for the loosening of the chromosomes so that interaction can take place. The last step in fertilization is the fusion of male and female pronuclei. This is called syngamy. After this point, the zygote enters the first stages of embryo genesis. Structures of the oocyte and spermatozoa are illustrated in Fig. 19.12 (see also follicular development).

Implantation and Placentation

After a successful fertilization, the embryo then develops into a blastocyst and erupts or hatches from the surrounding zona pellucida. It then develops a functional trophoblast and secretes signaling proteins that allow maintenance of the corpus luteum. After the fusion of the male and female pronuclei, the single‐celled egg is called a zygote. It can also be referred to as an embryo (defined as an organism in the early stages of development). Embryos typically have not acquired features that allow for recognition of a particular species. By contrast a fetus—a potential offspring still within the uterus—can generally be recognized as a member of the species. A conceptus, defined as the product of conception, is a commonly used term but is poorly descriptive. It is really a catchall term because it includes early and late embryos, the embryo + extra embryonic membranes during preimplantation, as well as the fetus and placenta.

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