Embryology: Background

Gametes are sex cells carrying genetic information in the form of chromosomes. Humans, for example, have 23 pairs of chromosomes, half of which are inherited from the mother and half are inherited from the father. Most of these chromosomes (22 pairs) are called autosomes. Autosomes carry the hereditary genetic information that produces our unique characteristics such as hair color, eye color, and height.

The last pair of chromosomes are the gonosomes. These are chromosomes that determine whether we become male or female. Two X chromosomes make an embryo genotypically female, while one X and one Y chromosome make an embryo genotypically male. Female gametes (the eggs), also called oocytes, carry only X gonosomes, but male gametes (sperm) can carry either an X or a Y gonosome. This means that the genotypic sex of the child is decided by the father.

There are three prenatal periods: the first two weeks are the pre-embryonic period, weeks three to eight are the embryonic period, and weeks nine to birth are the fetal period.

During the pre-embryonic period, there are several important phases of human embryology that lead up to and follow sex determination: fertilization, cleavage, gastrulation, and organogenesis fertilization occur when there's a successful union between two gametes. In our example, this occurs when Peg the Egg and Vern the Sperm unite to form a zygote, a fertilized egg. After successful fertilization, the zygote undergoes a rapid replication process called cleavage.

By the end of day two, a multicellular pre-embryo called a blastocyst is formed. The blastocyst begins to embed itself into the lining of the mother's uterus by the end of the first week through a process called implantation and is fully embedded in the uterine lining by the end of the second week. Upon successful implantation, Peg and Vern can collectively be called Romeo the Embryo.

Sperm need to surround the oocyte (Peg the Egg) until one sperm breaks through the egg's outer layer, contributing its DNA (aka Vern the Sperm).

At the time of ovulation, the oocyte enters the uterine (fallopian) tube where it is fertilized. The fertilized egg travels through the uterine tube, undergoing cleavage until it becomes a blastocyst and implants in the uterine lining.

The blastocyst embeds itself into the uterine lining. Once complete, cells begin to differentiate to form a bi-layer and then tri-layer embryonic disc. The tri-layer embryonic disc gives rise to the primary germ layers of the embryo. Germ layers are composed of cells that will undergo transformation to become the organs and structures of the developing embryo.

DNA from the sperm (23 chromosomes) and DNA from the oocyte (23 chromosomes) combine via meiosis, ensuring that the zygote contains precisely 46 chromosomes (23 pairs of chromosomes).

Next, Romeo the Embryo undergoes gastrulation, which is the formation of the primary germ layers. By the end of week two, a bi-layer embryonic disc forms, and then by the end of week three a tri-layer embryonic disc is formed. The tri-layer embryonic disc is comprised of ectoderm, endoderm, and mesoderm, which will become all the wonderful things that make Romeo unique.

Organogenesis, the development and differentiation of a fetus's organs, begins during week three and continues through week eight. Sex determination begins during organogenesis, but for the first six weeks Romeo is sexually indifferent.

How do we determine if Peg and Vern made a boy or a girl? We do a little genetic profiling and see that Romeo has both an X and a Y chromosome. That means Romeo is a boy, right? Not necessarily.

For Romeo to be both genotypically and phenotypically (outward expression of genes) male, there are a couple of things that must be present. First, the Y chromosome must have a gene called the Sex Determining Region Y, or Testis Determining Factor (TDF for short). The SRY gene produces a protein that activates the Androgen Receptor (AR for short) gene on the X chromosome. If the AR gene is present on the X chromosome and the SRY gene successfully triggers activation, the AR gene triggers the production of AR proteins.

What is androgen, by the way? It is a steroid-based hormone that is converted into testosterone in males and female sex hormones in females. Testosterone is attracted to AR proteins and testosterone is what makes men, men.

The sexually indifferent embryo has bipotential gonads, the primary sex organs that can become either ovaries or testes. What happens next is dependent upon the composition of and the interaction between the X and Y chromosomes. The urogenital sinus will become the bladder and associated urethra.

What does this all mean? It means that all the requirements necessary for Romeo to be male have been met. As you can see in this picture, Romeo is now on his way to becoming phenotypically male.

By week ten, Romeo's gonads are making testosterone, and by week twelve, the external genitalia begin to take the shape of a penis and scrotum. By the time Romeo is born, his internal and external reproductive organs are precisely where they should be for a newborn boy.

What would happen if Romeo's SRY gene did not properly activate the AR gene on the X chromosome? The default sex is female so Romeo would be genotypically male but phenotypically female. Biological sex is a complicated thing!

If the embryo is XX, the female duct system is selected. In the event any of the critical male genes and/or hormones are absent, the female duct system is selected.

Okay, that was quite a bit, so let's take a moment to review what we've learned. As we saw in this lesson, sex determination is just one facet of the complex process of embryonic development and largely the focus of embryology, which is the science that deals with the development and growth of an individual within the uterus. As we also learned, genetic sex is decided at the time of fertilization by our gonosomes, which are the chromosomes that determine whether we become male or female (either XX or XY, in other words).

However, genotypic sex is just part of what determines if we're male or female. Phenotypic sex is determined during organogenesis, which happens when there's a successful union between two gametes. It happens by either inhibiting the development of the female duct system through the synthesis of male sex hormones or by defaulting to the female duct system.

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Embryology: Definition & Development Stages - Study.com