Abstract
The development of sex organs is the consequence of cellular and morphological alterations triggered by molecular and endocrine mechanisms initiated during early embryonic life.
In the first phase of sexual differentiation, the bipotent gonad is developed, followed by the differentiation of gonads, either testis or ovary. The differentiation of internal and external genital organs then undergoes a long maturation process until puberty. The primary determinants of male gonads are the sry gene in the sex (Y) chromosome and DMRT1 gene in homologous ZZ chromosomes, respectively, in mammals and birds. Later stages of gonadal development are mostly under endocrine control. The development of infertile or sterile animals results from either developmental anomaly during sexual differentiation or due to interspecies mammalian hybridisation. The detailed knowledge of sex determination and its modulation through assisted reproduction techniques is the key to cloning and transgenesis to generate economically competent farm animals, wildlife conservation and avoidance of developmental disorders like freemartins.P. K. Das (X) ∙ J. Mukherjee ∙ D. Banerjee
Department of Veterinary Physiology, West Bengal University of
Animal & Fishery Sciences, Kolkata, West Bengal, India
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023
P. K. Das et al. (eds.), Textbook of Veterinary Physiology, https://doi.org/10.1007/978-981-19-9410-4_18
Graphical Abstract
Description of the graphic: Sex development is initiated during the embryonic stage and continues entire foetal life. The bipotent gonad (1) is developed. In the presence of the Y chromosome (2) and with the influence of a group of genes (3), including the sry gene and testosterone (4), the male sex reproductive system (5) develops in mammals.
The sex differentiation process continues with forming phenotypic male sex characters (6) until puberty. Similarly, in male birds (7), testes are developed in the presence of ZZ homologous chromosomes (8). In the absence of Y chromosome (9) (presented single dotted line) under the influence of ovarian determining factor (3), the female reproductive system (10) is developed and continued till puberty with various phenotypic female sex characters (11). Female bird (12) is developed due to the effect of the ZW chromosome (13). In coordination of the sex-determining factors leads to disorders of sex development (DSD) such as hermaphrodite (14). Various tools (15) are used to determine sex differentiation and to modulate the sex ratio (16) as well as to conserve the endangered animals (17)Keywords
Bipotent gonad ∙ Development of sex organs ∙ Disorders of sex development ∙ Sex determination in mammals and birds ∙ Sex ratio
Factors and causes for the disorders of sex development (DSD) and impaired fertility (IF)
Theories of sex ratio and its alteration to conserve the economic farm animals and wildlife
Learning Objectives
• The physiological phenomenon of development of bipotent gonad or bisexuality in early embryonic life
• Involvement of sex chromosome-specific genes, endocrines and biomolecules in the differentiation of sex in mammals and birds
• Principles of sex determination and its modulation through assisted reproduction techniques
The central dogma of sexual differentiation states the sequential process of sex determination in three distinct steps. The chromosomal or genetic sex determined at fertilization triggers the development of gonadal sex (testis or ovary) from a bipotent gonad during embryogenesis. The gonadal sex determines the phenotypic sex (secondary sexual characteristics). In sexual differentiation, the male phenotype is induced, and the female phenotype is passively developed due to the absence of male determinants.
During the early embryonic stage, the development of sex organs is the consequence of cellular and morphological changes triggered by genetic and endocrine events. The embryonic development of the gonads can be categorized into two distinct phases. In the first indifferent phase (as it is common in both sexes) of gonadal development, the bipotent genital ridge comprises primordial germ cells (PGCs) and other supporting cells having characteristics of both male and female gonads are developed. The second phase is called the sex determination phase, in which the bipotent genital ridge undergoes differentiation to develop either testis or ovaries. The determination of phenotypic sex through the differentiation of internal and external genital organs commenced during embryonic life until puberty was influenced by genetic and endocrine control.
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