Summary of Embryonic Development

Unfortunately, due to the volume of material that we must work through in this course, embryology is one of the topics that won’t be covered in great detail. Embryology is the study of the development of living organisms from fertilized egg, or zygote, to birth. This field of study is infinitely interesting and furthers the understanding of numerous congenital conditions. During the study of organ systems in this course, students will frequently come across embryologic terms such as blastocyst, mesoderm, endoderm, and ectoderm. For this reason, a brief summation is presented here.

The diagram below illustrates human development in the first week of life. Note that the fertilized egg is called a zygote. That cell is surrounded in a protective corona. Within the corona, the zygote cells divides multiple times. The cells then organize themselves into the blastocyst. The blastocyst must embed itself in the upper third of the lining (endometrium) of the uterus if the pregnancy is to be successful.

While buried in the endometrium, the blastocyte undergoes a series of cell migrations called gastrulation. This process begins when cells on the surface dive inward to become one of the three germ layers. A second and then third invagination of cells leads to development of the embryo. The chart on the next page illustrates the terminology, timeline, and development of the 3 germ layers: ectoderm, mesoderm, and endoderm.

Now that we have considered the development of the multilayered organization of the embryo, let us consider the fate of the three germ layers. Each germ layer is destined to produce a specific line of cells that becomes different organ systems. The diagram below sums up this process. (I would like to add that endoderm forms the organs of the digestive system.)

The development of the nervous system begins with the formation of the neural plate from the ectoderm layer at day 19. Two ridges form on the margin of the neural plate that will become the neural folds. The neural plate deepens to form the neural groove by day 20. By the 22^nd day, the neural folds have formed the neural crests which will become the cranial and spinal nerves. The neural groove closes to become the neural tube by day 26.

This early development of the nervous system, completed by the end of the 4^th week of fetal development, is illustrated in the diagrams below.

As soon as the neural tube forms, its anterior end begins to expand and constrictions appear. The resulting three primary brain vesicles are, from the anterior tip back, the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain).

In week 5, the primary vesicles will give rise to the secondary brain vesicles. The prosencephalon divides into the telencephalon and diencephalon. The mesencephalon remains undivided. The rhombencephalon divides into the metencephalon and myelencephalon. Below are two diagrams of a five week fetus, specifically showing further development of the nervous system.

The table below sums up what the embryonic structures of the nervous system become in adults.

Neural Tube	Primary Brain Vesicles	Secondary Brain Vesicles	Adult Brain Structures
Anterior End	Prosencephalon (forebrain)	Telencephalon	Cerebrum: Cerebral hemispheres (cortex, white matter, basal nuclei)
		Diencephalon	Diencephalon (Thalamus, hypothalamus, epithalamus)
	Mesencephalon (midbrain)	Mesencephalon	Brain stem: Midbrain
	Rhombencephalon (hindbrain)	Metancephalon	Brain stem: pons
			Cerebellum
		Myelencephalon	Brain stem: medulla oblongata
Posterior End			Spinal cord

This series of sonogram pictures shows fetal development from day 1 to day 60 (roughly through the first 2 months of pregnancy).