A Beginners’ guide to Sex Determination and Differentiation

In humans as well as other species we can see some obvious differences between males and females. Differences such as the development of a penis or vagina, muscle mass, body size, the distribution of fat and the amount of distribution of body hair are all the differences that can be observed. These differences are a result of a long chain of events that begin during the early phases of embryology. It Involves Complex interaction between the environment and gene expression.

Chromosomes

Being male or female is determined in stages beginning at fertilization when the sex chromosomes carried by the sperms meet the egg. Females have XX chromosomes and Males have XY chromosomes. Having XX females and XY males provides a genetic framework for developmental events that eventually give us Male or female appearance.  The formation of reproductive structures we see in Males and females depend on several factors, this includes gene action and interaction within the embryo. Interactions with other embryos that may be in the uterus.

As mentioned this process is not straightforward, having XX and XY does not always mean that individual is either male or female. The outcome depends on the distribution of genes on the X and Y chromosome. All eggs that are produced by females carry the X chromosomes, in males about half of the mature sperms carry the X chromosomes. An egg that is fertilised by a sperm that has the X chromosomes will have XX Zygote and will develop into a female (Cummings, 2009).

Key Points

  1. Establishment of chromosomal sex happens at fertilization.  Although this is the case, external genitalia is neither male or female until about the third month of development.
  2. Before this, the embryo has two undifferentiated gonads present, along with both sets of the male and female duct systems.  The second phase begins at 8 to 9 weeks when gene expression activates the different developmental pathway. This then causes the undifferentiated gonads to develop into testicles or ovaries. It is important to point out that there are alternative routes that produce an intermediate outcome in gonadal sex and sexual phenotype ( appearance).
  3. For boys, the differentiation is not straightforward, If the Y chromosome is present, expressions of a gene on the Y chromosome will cause the undifferentiated gonads to become testis. A sex-determining gene that is found on the short arm of the Y chromosome called SRY activates the expression of other genes that play a part in the way testis develop. Once the testis development is initiated by the SRY genes, the testis then produces testosterone and anti-Mullerian hormone. The formation of the internal male duct system that carries sperm is caused by testosterone’s effects on the Wolffian duct system.  Further development of female structure is stopped developing testicles through the production of AMH.
  4. Embryos with two X chromosomes and the absence of the Y chromosomes causes the gonads to develop into ovaries. Ovaries are developed when the cells in the outer layers of the gonads divide and push into the inner layers. The Wolffian duct system then disintegrates because the ovaries do not produce testosterone (Cummings, 2009).

Hormones also shape the male and female appearance, after gonadal sex has been established the next phase in sex determination is the development of sexual appearance. In males, testosterone is converted into dihydrotestosterone, DHT directs the formation of external male features and influences brain development and size. In fact, recent studies have shown that boys with high levels of testosterone in the womb tend to be more interested in things than people. Gonadal sex and sexual appearance are produced by separate pathways in males and females.

As discussed, a lot can go wrong during this process, for example, differentiation pathway can lead to gonadal sex/ sexual appearance (phenotype) that is different to the one specified by XX  or XY. These outcomes that happen 1 in 2000 births, maybe due to several factors. Chromosomal events that exchange segments of the X and Y genes, mutations that affect the ability of the cells to respond to the Y chromosomes. In embryos with the Y chromosomes. An interesting one is ANDROGEN INSENSITIVITY SYNDROME, in this case, the chromosome sex is XY. And the action of SRY gene has stated the formation of the testis and the production of AMH.

AMH is what causes the degradation of the Mullerian duct system and in turn, no internal female reproductive system is produced. However, because of mutation, there are no receptors for male hormones. As a result of this, the development pathways continue along a default pathway which female even when DHT and testosterone are present. Individual affected are female in appearance but genetically male. Individuals like these have well-developed breasts and pubic hair but do not menstruate. They are raised as female and have a female gender identity  (Viner, Teoh, Williams, Patterson, & Hughes, 1997).

You also have mutations that give us pseudohermaphroditism where individuals have both male and female structure, but at different times in their lives. Other conditions include Congenital Adrenal Hyperplasia and 5 α–Reductase Deficiency. More on this some other time. This is a very edited information on this topic and if you want to more about sex determination and differentiation you can you can click here.

 

Cummings, M. R. (2009). Defining sex in Stages; Chromosomes Gonads and Hormones

Viner, R. M., Teoh, Y., Williams, D. M., Patterson, M. N., & Hughes, I. A. (1997). Androgen insensitivity syndrome: a survey of diagnostic procedures and management in the UK. Archives of Disease in Childhood, 77(4), 305-309. doi:10.1136/adc.77.4.305

 

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