Embryogenesis and Development on the MCAT


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The development of the human individual is without a doubt one of the most amazing wonders that the world has to offer (even more than Lebron James breaking the NBA’s all time scoring record!). To go from a single cell to a whole organism just illustrates the astounding complexity of the human body!

It’s without a doubt that embryogenesis and development is one of the more complex topics on the MCAT, but it’s luckily not as heavily tested on the exam! As such, we’ll go ahead and leave out any of the unnecessary “fluff” that you’ll find in in-depth textbooks!

Let’s dive into an introductory overview of the concepts and processes involved in embryogenesis and development. 

Embryogenesis & Development on the MCAT: What You Need to Know

Topics on embryogenesis and development will be tested on the biology section of the MCAT and can appear both as passage based and fundamental discrete questions. 

Don’t worry if you can’t get a real in-depth review of these topics, as only about 2-3 questions are likely to appear on the MCAT!

Introductory biology account for 65% of the content covered in the Biological and Biochemical Foundations of Living Systems section (Bio/Biochem), 5% of content tested in the Chemical and Physical Foundations of Biological Systems (Chem/Phys), and 5% of material on the Psychological, Social, and Biological Foundations of Behavior (Psych/Soc)

Important Sub-Topics: Embryogenesis and Development

One of the things that might scare students about embryogenesis and development is the vast amount of steps that are a part of the overall process. 

However, like we always stress and emphasize, starting out slow from the bottom and working our way up is a great way to simplify the entire process!

1. Embryogenic Processes

As we introduced in our reproduction overview, embryogenesis begins with the formation of the zygote, the diploid cell formed from the fertilization of an egg cell by sperm. When moving down to undergo implantation, the zygote undergoes a series of cell divisions called cleavage.

In this process, the cell divides rapidly, generating multiple daughter cells. However, the resulting daughter cells are actually smaller in size than the zygote but are still genetically identical. Once there are about 16 daughter cells, a structure called the morula is formed. 

After further division where the morula has about 60 cells, blastulation occurs to form a structure called the blastocyst which can be implanted in the uterine. An important group of cells in the blastocysts is the inner cell mass, which is a group of pluripotent stem cells which can differentiate into any of the organism’s cells!

Afterwards, gastrulation occurs to form the gastrula, a stage of embryogenesis where the blastocysts cells rearrange to give rise to the 3 germ layers: the ectoderm, mesoderm, and endoderm.

Embryogenic Processes

The cells of the 3 germ layers will go on to differentiate into the more specialized cells of the human body. 

The ectoderm (outermost layer) goes on to develop the nervous system and the skin. The mesoderm (middle layer) will develop into the body’s stromal (or support) structures such as musculoskeletal and circulatory system. 

The endoderm (innermost layer) develops into most of the internal organs and glands such as those in the digestive and respiratory tract.
Gastrula

(Coming Soon!) Full Study Notes : Embryogenic Processes 

For more in-depth content review on the process of embryonic development, check out these detailed lesson notes created by top MCAT scorers. 

2. Cell Determination, Differentiation, and Migration

In the process of organogenesis and embryo development, the cells of the germ layers must become specialized so that they fit a specific role whichever organ or tissue occupies in the fully developed organism!

When a cell commits to becoming a specific type of cell, this process is called cell determination. In more technical terms, the cell has committed itself towards a specific cell lineage and begins to express the genes specific to that cell lineage.

Cell differentiation describes the morphological cell changes that occur when entering the cell lineage. This includes the production of the cell’s specific proteins, organelles, shape, etc. Note that all of this arises from the changes in gene expression that occurred in cell determination!
Cell Determination, Differentiation, And Migration

Another key component of embryogenic development is cell migration which is exactly what it sounds like! It involves the movement of the embryonic cells to their eventual location where they’ll undergo further differentiation. 

This is a constantly ongoing process during embryonic development. To give one example, the cells of the inner cell mass during gastrulation will migrate as layers to eventually give rise to the 3 germ layers!

(Coming Soon!) Full Study Notes : Cell Differentiation, Differentiation, and Migration

For more in-depth content review on how cell determination, differentiation, and migration play a role in embryonic development, check out these detailed lesson notes created by top MCAT scorers. 

3. Cell Senescence, Death, and Regeneration

These topics take a little bit of a tangent as they’re usually associated when embryogenesis is complete. However, they still serve important roles in both embryogenesis and the maintenance of the human body!

To explore cell senescence, we have to go back just a little in DNA structure. Recall that at the ends of the chromosomes are a repeated sequence of nucleotides called telomeres, which functions to act as “caps” to the DNA chromosomes. 

This prevents the loss of the coding genetic information as a small amount of DNA is lost from the chromosome ends after each round of cell division. 

However, at some point, the telomeres eventually deplete which marks cellular senescence, where the cell is no longer looking to actively divide and is one of the main cellular bases behind aging.
Cell Senescence, Death, And Regeneration

A step further from cell senescence is cell apoptosis, which is defined as programmed cell death in response to various stimuli. Though intuitively viewed as a bad thing, cell death is actually a crucial part in our physiology! 

Just look at the constant death and shedding of our skin cells! As the top layer of keratinocytes are exposed to many damaging factors, most notably UV radiation, they undergo apoptosis and eventually shed from our skin. 

However, this is where the body’s regenerative capacity comes into play thanks to the underlying stem cells! Because the stem cells still have the capacity to differentiate, they can differentiate into the keratinocytes and replace the dead keratinocytes which have been shed off!

Stem Cells

More specifics on cell senescence, apoptosis, and regeneration can be found here in this article!

(Coming Soon!) Full Study Notes : Cell Senesence, Death, and Regeneration

For more in-depth content review on cell senescence, apoptosis, and regeneration, check out these detailed lesson notes created by top MCAT scorers. 

Important Definitions and Key Terms

Below are some high yield definitions and key terms to refer to when reviewing the immune system!

Term

Definition

Zygote

Diploid, somatic cell formed from the fertilization of an egg cell from sperm

Morula

A 32 cell structure which is formed from the cleavage of the original zygote

Blastocyst

Spherical embryonic structure composed of trophoblast outlining the sphere and the inner cell mass

Inner Cell Mass

A group of pluripotent stem cells which can differentiate into any of the organism’s cells (except extraembryonic cells)

Gastrula

Embryonic structure which forms the 3 germ layers: ectoderm, mesoderm, and endoderm

Determination

The commitment of a cell towards a specific cell lineage

Differentiation

The biochemical and cellular processes that results in the specialized cell with unique structures and functions

Senescence

Term to describe a cell no longer actively looking to divide; one of the cellular basis behind aging

Apoptosis

Term to describe programmed cell death in response to a variety of stimuli

Additional FAQs - Embryogenesis and Development on the MCAT

What are the 5 Stages of Embryogenesis – MCAT?

Though there a many phases of embryogenesis, they can most aptly be subdivided into fertilization (zygote formation), cleavage (morula formation), blastulation (blastocyst formation), gastrulation (gastrula formation), and neurulation (formation of neural tube).

What is the Process of Embryogenesis – MCAT?

The process of embryogenesis is a complex series of steps and stages where a single celled zygote undergoes development which results in a fully functioning fetal organism.

What is the Difference between Embryology and Embryogenesis – MCAT?

Embryology is a more overall term referring to the entire study of developmental biology while embryogenesis refers to the actual processes and stages of embryogenesis.

What is Embryogenesis in Developmental Biology– MCAT?

As the name implies, embryogenesis refers to how an embryo develops from fertilization all the way to organogenesis, eventually developing the mature embryo. There are many processes and stages that go into embryogenesis and the best way to approach this is to take it one step at a time!

Additional Reading Links (Coming Soon!) – Embryogenesis and Development on the MCAT

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