Embryonic development begins with cleavage of the zygote. Monozygotic twins (identical) are derived from the same fertilized egg (in armadillos, the monovular fertilized ovum typically cleaves to yield monozygotic quadruplets). Dizygotic (fraternal/nonidentical) siblings descend from different fertilized ova.
(Sidebar: Asynchronous twin pregnancies (superfetation), most likely resulting from differential growth rates, have been reported in several mammalian species.)
The morula is a "mulberry-like" congregate of blastomeres resulting from early divisions of the zygote (Figure 5-25). Blastomeres secrete fluid forming a cavity (blastocoel). Cells of the blastula organize into an outer (trophoblastic) layer and polarized (inner) mass. In most species the blastocyst hatches from the zona pellucida within one week of fertilization (elongation of the conceptus in farm animals is quite rapid). The trophoblast forms the extrinsic membrane of the placenta (chorion) and the inner cell mass differentiates into the embryo proper (gastrulation) and its amniotic cavity (Figure 5-26).
(Sidebar: Parthenogenesis is the development of an embryo without fertilization; this can be induced by exposing oocytes arrested at the second meiotic metaphase to cytochalasin, a drug that prevents extrusion of the second polar body - maintaining diploidy. Mammalian embryos can only progress to the early limb bud stage because further development requires paternal gene expression.)
Embryology and reproductive biology are interrelated, but separable subjects. Detailed discussions of mammalian embryogenesis can be found in any standard embryology textbook. Here I will only consider the embryonic development of the reproductive organs.
Indifferent stage. Genotype is determined at conception. Males normally exhibit an XY and females an XX karyotype (some outstanding nondisjunctional sex chromosomal anomalies of humans are listed in Table 5-6). Phenotypic sex (masculinity or femininity) of the developing embryo cannot be distinguished for some time (about eight weeks in humans) - and thus an embryo becomes a fetus. Male and female organs develop from a primitive urogenital system common to both sexes (Figure 5-27).
Sexual differentiation. Phenotypic differentiation of the indifferent embryo begins with the genital ridges (gonadal anlage) of the mesonephros (a temporary embryonic kidney). Gonadal type is determined by genetic sex.
The testes form from the medulla of the genital ridge in the presence of the Y chromosome. A histocompatibility antigen associated with the Y chromosome (H-Y antigen), normally expressed as an integral membrane protein of cells of the male embryo, was thought for many years to be involved in testicular organization (in fact, the soluble form secreted by the testis is identical to MIH). It is now evident that testis formation is initiated by a sex-determining region located along the short arm of the Y chromosome (SRY). The SRY sequence codes for a protein with DNA-binding activity (Y carriers very few other functional genes). It appears that the X chromosome has little if anything to do with testicular development (there is a gene on the X chromosome of mice and humans that confers androgen sensitivity to tissues). Testicular differentiation occurs in XX males in which SRY is translocated to an X chromosome during meiosis. However, the presence of an extra (active) X chromosome impairs fertility (ie., is a phenocopy of 47,XXY). Autosomal translocations of X fragments are also associated with male infertility.
The ovaries develop from the cortex of the genital ridge in the absence of the Y chromosome. A normal XX karyotype is required for ovarian development to a functional adult state. So far, nearly 20 different sex chromosomal and autosomal genes have been implicated in sexual differentiation in man.
Primordial germ cells (gonocytes) migrate to the genital ridges from the endoderm of the yolk sac by ameboid movement along the dorsal mesentery. The gonads apparently produce a powerful gonocyte chemoattractant.
Development of the reproductive tract and presetting of adult modes of hypothalamic function follow gonadal differentiation. In the male, androgens of Leydig cell origin and MIH derived from Sertoli cells (induced by SRY molecules) stimulate maturation and regression of male and female homologs, respectively. Endocrine activity of the fetal testis is driven by gonadotropins of pituitary and placental origin (numbers of Leydig cells decline late in fetal life). The testes become displaced from the vestigial mesonephros and eventually are guided through the inguinal canals by a ligamentous gubernaculum. Would-be patterns of gonadotropin secretion and sexual behavior are imprinted around the time of birth. A male secretory mode of gonadotropins (ie., acyclic) is instilled by hypothalamic exposure to androgens or estrogens (under native conditions testosterone is probably aromatized within the CNS). In the female, female primordia develop and male primordia degenerate without testicular hormones (Table 5-7). Thus, processes of embryonic sexual morphogenesis are active in the male and passive/default in the female. Development of a female-like system occurs in either genetic sex when the embryonic gonads are removed before critical periods of differentiation.
In conclusion: chromosomal sex controls gonadal sex, which determines phenotypic sex.
Intersexuality. Genetics and environment are important factors in sexual development. Because gonadal sex is quite stable in mammals, true hermaphroditism (coexistence of ovarian and testicular tissues) is relatively rare. More common are pseudohermaphroditisms; these untidy syndromes include scenarios in which definitive ovaries or testes are present, but phenotypic gender is hormonally modified.
Testicular feminization is an example of a pseudohermaphroditic error of 46,XY human males in which the phenotypic sex is ambiguous or female; the condition is due to defects in testosterone synthesis or resistance of target tissues to androgen action (ie., lack of DHT receptors or 5a-reductase). Pseudohermaphroditism in 46,XX women is associated with virilization caused by androgen excess resulting from congenital adrenal hyperplasia - only the external genitalia are involved (internal organs usually differentiate before the onset of adrenal dysfunction).
Freemartinism is a classical intersex of cattle (free = barren, martin = bovine killed at Martinmas); it occurs when placental fusion takes place between male and female twins (~ 90% of cases) and testicular hormones are transferred to the circulatory system of the female (blood and germ cells also can be exchanged). The freemartin female is almost always sterile and exhibits varying degrees of virilization of the genitalia. Sex chromosomal chimerism (XX/XY) has been detected in both sexes (ovotestis is expressed only in the female). Freemartinism has been reported in sheep, goats, and pigs. Oddly, female spotted hyenas are masculinized (fused labia) in utero by maternal androgens, yet become reproductively competent (an enlarged urethra serves as a copulatory organ and birth canal).
Sperm/embryo sexing. No one technique of sorting sperm cells to control sex ratio has gained widespread acceptance; flow cytometry and immunoaffinity separations using antisera against Y-bearing cells are among the newer methods (Table 5-8).
Embryos can be sexed using DNA probes specific for the Y chromosome; this procedure requires removal of cells from the embryo and is time-consuming. Embryos also can be sexed using a fluorescent antibody technique directed against male-specific antigens (eg., H-Y) or assayed for X-linked enzymes.
Micromanipulation. Copies of genes can be inserted directly into fertilized ova by micromanipulation (Figure 5-28). Embryos also can be cut into segments using microsurgical techniques to produce identical offspring; fragments are placed into empty zona pellucida and transferred to recipient females (Figure 5-29). Oocyte activation and nuclear transfer also can be accomplished by electropulse fusion of a donor cell and enucleated recipient oocyte.
Toxicology/embryopathy. Reproductive toxicology is a topic of increasing relevance in a society that habitually abuses drugs and heavily pollutes the environment. Research concerning reproductive effects of toxic substances is in its infancy (the majority of clinical reports are retrospective and controlled experiments have been confined mainly to laboratory animals). The greatest risk is to the developing embryo.
Nearly 10% of the US population of child-bearing age uses illicit drugs or alcohol on a regular basis. Many drugs act by inhibiting neuroendocrine function (ie., neurotransmitter-mediated episodic secretion of GnRH). Alcohol suppresses gonadal production of steroid hormones, interferes with hormonal metabolism, and inhibits general protein synthesis and cellular division. Most drugs of abuse (Table 5-9) are small and lipid soluble and can freely cross the blood-brain and placental barriers.
Adverse effects of drugs on reproduction are often not manifested until fertility, pregnancy, and the neonate are severely and sometimes irreversibly compromised. Teratogenic effects in pregnancy are escalated with drug abuse (eg., fetal alcohol growth retardation syndrome). Neonatal complications include the narcotic abstinence syndrome (withdrawal) and an increased incidence of abnormal sleep respiratory patterns associated with the sudden infant death syndrome. Hypogonadism is typical with chronic drug abuse in adults. Dysfunctional pubertal development will surely become a serious dilemma with the trend of increased use of drugs and anabolic steroids by adolescents.
Medications of any kind should be avoided during the first trimester of pregnancy - a lesson well learned with thalidomide. Thalidomide is a hypnotic/sedative and immunosuppressive drug that causes birth defects (especially phocomelia, a condition in which the proximal portions of the limbs fail to develop); it was commonly prescribed in Europe in the 1950/60s to soothe "morning sickness" (treatment of slight to moderate symptoms consists of frequent, light, dry meals; severe vomiting [hyperemis gravidarum] can require intravenous therapy; off-label drugs [Zofran/ondansetron, promethazine, Reglan/metoclopramide, prochlorperazine] which have not demonstrated a risk to the conceptus have been prescribed if an expectant mother suffers from dehydration/malnutrition).
During the 1950s and 60s small amounts of diethylstilbestrol (DES), a synthetic nonsteroid with estrogenic activity, were sometimes used (under false perception) to treat threatened pregnancies. Subsequent observations indicated that in utero exposure of the fetus to DES caused undesirable effects manifested later in life (eg., structural defects of the reproductive organs, infertility, and clear cell adenocarcinoma of the cervix and vagina). Diethylstilbestrol was also used as a growth promotant in feedlot ruminants (it has a protein anabolic effect, possibly mediated by increased secretion of GH), but because of its connection with cancer was removed from the marketplace (and replaced by implants containing related compounds, such as Ralgro, Compudose, and Synovex).
Hormone-like substances that can alter embryonic development and cause abortion are present in some forages accessible to livestock. Subterranean and red clovers contain phytoestrogens (isoflavones). Certain grains (barley and oats) also exhibit estrogenic activity. Freshly growing grasses contain 6-methoxybenzoxaline, a melatonin-like phenol.
Nutrient restrictions (eg., energy, protein, iodine, manganese, phosphorus, selenium, vitamins A and E) can have detrimental effects on embryonic well-being. Too much of a good thing can also be a problem - complications in human pregnancy are related to obesity and diets high in concentrate predispose pigs and sheep to early embryonic losses.
Risk assessments of environmental/occupational hazards on human fertility and pregnancy outcome are just beginning to emerge. Altitude has been identified as a natural risk factor in pregnancy; low infant birth weights are associated with high-elevation pregnancies (a fall of ~ 100 grams/1000 meters rise). The current research emphasis is being placed on understanding mutagenic actions of ionizing radiation and xenobiotics (biologically foreign chemicals). A computerized database of physical and chemical agents known to have teratogenic effects is continually updated by the Reproductive Toxicology Center (Washington, D.C.).