D E V E L O P M E N T S I N SEXUAL O R G A N O G E N E S I S
303
placental mammals. Thus the exact nature of the fetal androgens remains
to be established. With the facts now known in regard to the biosynthesis
of derivatives from the hormones administered, it may be that, in mammals, the method of injecting steroid hormones to the pregnant female does
not exactly reproduce the physiological conditions established in the
freemartin itself. This question has just been analyzed by Price and Ortiz
(1965). The most notable facts are related to the biochemical identification
of the androgens, the mechanisms of the biosynthesis and metabolism of
the steroid hormones in the fetus. The results are set forth in a succeeding
section for the vertebrates as a whole.
2. Relations between Genetic Structures and Sexual Organogenesis
Improvement in techniques permitting an exact knowledge of the karyotype of mammals (Tjio and Levan 1956; Ford and Hamerton, 1956), and
advances in cytogenetics are the sources of the progress made in this area.
This is particularly noteworthy in the case of man. In mice, the combination
of karyotypic analysis and suitable crosses became possible with the discovery of about 20 sex-linked genes. This very important body of facts
is beyond the scope of the present article (reviewed in Beatty, 1957, 1964;
Russell, 1961, 1962; Armstrong, 1964; Bruner-Lorand, 1964; Gallien,
1962c, 1965; Turpin and Lejeune, 1965; Overzier, 1963).
Two facts predominate in the results which have been obtained : (a) the
role of the Y chromosome as a carrier of genes for maleness; (b) the role of
aneuploidy in decreased fertility, sterility, dysgenesis, and intersexuality
of the genital tract.
The interest in these studies lies in the fact that they attack the problem
of the relationship between genetic constitution and endocrine factors in
the control of sexual organogenesis in mammals. This question has been
investigated in amphibians also (reviewed in Gallien, 1965).
3. Early Parabiosis of Mouse Blastocysts
The problem of genetic and epigenetic factors controlling the sexual
differentiation of the gonads in mammals is beginning to be investigated
by placing two very young developing eggs in parabiosis, (Tarkowski,
1961, 1963, 1964a,b; Mintz, 1964). Tarkowski fused two mouse eggs at
the 8-celled stage. After culture in vitro he obtained a double blastocyst that
was large, but of normal morphology. This double blastocyst was implanted in the uterus of a pseudopregnant female, where its development
continued. Regulation of size occurred. Sixteen chimeric embryos were
obtained at parturition, two of which have survived. These two were fertile
males. The sex ratio of the chimerae was: 2 females: 3 intersexes: 11 males.
According to Tarkowski, the small number of intersexes and the preponderance of male combinations suggests that in some of these latter chimerae, it
303
placental mammals. Thus the exact nature of the fetal androgens remains
to be established. With the facts now known in regard to the biosynthesis
of derivatives from the hormones administered, it may be that, in mammals, the method of injecting steroid hormones to the pregnant female does
not exactly reproduce the physiological conditions established in the
freemartin itself. This question has just been analyzed by Price and Ortiz
(1965). The most notable facts are related to the biochemical identification
of the androgens, the mechanisms of the biosynthesis and metabolism of
the steroid hormones in the fetus. The results are set forth in a succeeding
section for the vertebrates as a whole.
2. Relations between Genetic Structures and Sexual Organogenesis
Improvement in techniques permitting an exact knowledge of the karyotype of mammals (Tjio and Levan 1956; Ford and Hamerton, 1956), and
advances in cytogenetics are the sources of the progress made in this area.
This is particularly noteworthy in the case of man. In mice, the combination
of karyotypic analysis and suitable crosses became possible with the discovery of about 20 sex-linked genes. This very important body of facts
is beyond the scope of the present article (reviewed in Beatty, 1957, 1964;
Russell, 1961, 1962; Armstrong, 1964; Bruner-Lorand, 1964; Gallien,
1962c, 1965; Turpin and Lejeune, 1965; Overzier, 1963).
Two facts predominate in the results which have been obtained : (a) the
role of the Y chromosome as a carrier of genes for maleness; (b) the role of
aneuploidy in decreased fertility, sterility, dysgenesis, and intersexuality
of the genital tract.
The interest in these studies lies in the fact that they attack the problem
of the relationship between genetic constitution and endocrine factors in
the control of sexual organogenesis in mammals. This question has been
investigated in amphibians also (reviewed in Gallien, 1965).
3. Early Parabiosis of Mouse Blastocysts
The problem of genetic and epigenetic factors controlling the sexual
differentiation of the gonads in mammals is beginning to be investigated
by placing two very young developing eggs in parabiosis, (Tarkowski,
1961, 1963, 1964a,b; Mintz, 1964). Tarkowski fused two mouse eggs at
the 8-celled stage. After culture in vitro he obtained a double blastocyst that
was large, but of normal morphology. This double blastocyst was implanted in the uterus of a pseudopregnant female, where its development
continued. Regulation of size occurred. Sixteen chimeric embryos were
obtained at parturition, two of which have survived. These two were fertile
males. The sex ratio of the chimerae was: 2 females: 3 intersexes: 11 males.
According to Tarkowski, the small number of intersexes and the preponderance of male combinations suggests that in some of these latter chimerae, it