Factors Affecting Primary and Secondary Sex Determination in Mammals

Sexual reproduction is the basis for the continuity of human life. Thus, processes that determine the sex of an organism are of core importance. The gender of the developing embryo of a mammal is determined by two important processes secondary and primary sex determination. These two processes are sequential. Sex determination refers to the mechanisms by which off springs of two different sexes are produced by an organism. The process of sex determination commences during the early developmental stages of an embryo with the organization of two cell groups that eventually develop into either the testicles or ovaries. This takes place in the gonad rudiment. This process is determined by genetic factors related to the sex chromosomes of an embryo (Palmer 247). On the other hand, additional development of sex-specific characteristics (secondary sex determination) is determined by hormones that are sex-specific and which are secreted by the differentiated gonad. Sex differentiation in other body parts such as the development of Wolfian and Mullerian ducts in developing embryos is influenced by hormones.

Factors that affect primary sex determination
Primary sex differentiation in mammals is solely determined by chromosomes. Studies indicate that various genes are responsible for this differentiation (Stephen, et al. 4). The gene WNT4 plays a key role in determining ovary formation.WNT4 is usually not expressed in XY gonads and is thus undetectable as would be necessary for the development of testis. However, the gene is maintained in XX gonads as the process of ovary formation begins. The autosomal SRY gene also plays a great role in influencing the primary sex determination process in mammals (Palmer 257). SOX9 encodes a high mobility group (HMG) box that contains a transcription factor. Therefore, humans with XX and an extra SOX9 copy develop to be males. Research indicates that presence of only a single functional copy of SOX9 leads to a syndrome referred to as Campomelic Dysplasia and that close to 75 of individuals with this syndrome develops as hermaphrodites or as phenotypic females. Hence SOX9 is responsible for the formation of testis.

The Y chromosomes short arm accommodates the SRY (sex-determining region). SRY is responsible for encoding the testis determining factor in humans (Ball, et al. 405). Hence, individuals who posses the short arm of the Y chromosome rather than the long one are males while individuals possessing the long arm develop to be females. Studies have found out that SRY is present in normal XY males as well as in rare XX males. SRY either directly or indirectly activates the SF1 (steroidogenic factor 1) transcription factor which is necessary for the development of the bipotential gonad. This transcription factor declines in the XX mammals genital ridge while it persists in the developing testis in XY mammals. SF1 also works in collaboration with the SOX9 sex determining gene.

The X chromosome contains a gene Dax1 that is vital in directing the development of the ovary (Ball, et al. 406). The gene usually competes with the SRY factor such that in the process of testicular development, it is suppressed. Dax1 is usually expressed in the genital ridges of the embryo and is expressed in the same cells as Sry. Dax1 antagonizes Sry function and also down-regulates the expression of SF1. The translocation of the Y chromosome can as well affect the primary sex determination process. In this, genetic information that initiates the differentiation of the testicles and which is usually found on the Y chromosomes is transferred to an X chromosome or an autosome (Stephen, et al. 17). This leads to the development of males who are sterile or hermaphrodites. Primary chimerism also affects primary sex determination. Chimerism refers to a condition in which the cells from a set of two or more embryos combine and develop as one individual during the early developmental stages. Incase the embryos involved are of the opposite sex, the resulting individual contains both sets of XX an XY chromosomes (Ball, et al 406). Presence of both XX and XY leads to the differentiation of both the ovaries and the testicles depending on the proportionate distribution of the XY and XX-bearing cells.              

Factors that affect secondary sex determination
Secondary sex determination is affected by the work of hormones secreted by the testes and ovaries (Capel 97). This is because these hormones influence the development of both male and female phenotypes. However, secondary sex determination takes place in two key temporal phases with the first occurring during organogenesis in the embryo and the second one during adolescence.

Testosterone is one of the primary mascularnizing hormones and it promotes the formation of the reproductive structures in male. These structures include vesicles, epididymis and vas deferens and these develop form the Wolfian duct. Secretion of testosterone is thus an important factor determining secondary sex differentiation (Jost, et al. 122). On the other hand, Dihydrotestosteron hormone controls the mascularnization of the male prostate, scrotum, penis and urethra. The effect of estrogen hormone is responsible for the full development of the Wolfian and Mullerian ducts. Thus, estrogen is am important factor affecting secondary sex determination. In females, estrogen is secreted by the fetal ovaries and is responsible for inducing the Mullerian duct differentiation into the various components such as oviducts, uterus and cervix. In men, estrogen only plays a role in fertility and not sex determination. The Sertoli cells-secreted Anti-Mullerian duct hormone (AMH) is responsible for the degeneration of the Mullerian duct (Jost, et al. 126). AMH thus influences the development of male sexual phenotypic structures. It may cause sex reversal thus affecting secondary sex differentiation.    


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