Mammalian Development: Formation of blastocyst to development of organs

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II - Early Development
A. Cleavage

Cleavage is when the fertilized egg divides several times to create a multicellular embryo. This process occurs as the embryo travels along the oviduct towards the uterus (figure 1). Cleavage ends when a blastocyst is formed.

1. Events in the ampulla

The first cleavage starts when "arrested" meisos in the egg resumes. At this time sperm DNA are involved. The first cleavage is usually meriodional, producing 2 cells (figure 2a). Production of four cells is achieved through rotational cleavage where one blastomere divides meriodionally and the other one equatorially (figure 2b). 

2. Events in the Isthmus

The blastomeres divide further to produce 8 cells. At this point compaction happens. During compaction, blastomeres adhere to each other forming a compact ball of cells. These compacted cells divide further to produce a morula (figure 2d). The 8-cell morula is composed of a small group of internal cells which will give rise to the inner cell mass (ICM), and a larger group of external cells which will give rise to the trophoblast (figure 2e). The cells further divide to form a 32-cell morula.
3. Events in the uterus

At some point, the compacted morula undergoes cavitation. The trophoblast cells secrete fluid into the morulla creating a hollow portion called the blastocoel. As the blastocoel expands, the ICM is positioned to the side inside the ring of trophoblast cells.

The zona pellucida prevents the blastocyst from attaching to the uterine wall. When the embryo reaches the uterus, it must hatch from the zona pellucida and attach itself on the uterine wall. Hatching happens when a small portion of the zona pellucida is digested. The blastocyst then squeezes through the hole as it expands. The blastocyst will now make a direct contact to the epithelial lining of the uterus called the endometrium.

B. Gastrulation

1. Development of the trophoblast

During gastrulation, the trophoblast will develop into chorion. The trophoblast will also induce the the maternal portion of the placenta called the decidua. The decidua becomes rich in blood vessels that will provide the embryo with oxygen and nutrients.

2. Development of the ICM

The inner cell mass will give rise to the primitive endoderm and the epiblast. Together, they form the bilaminar germ disc.

*primitive endoderm
  • Generates yolk sac cells.
  • Used for position the site of gastrulation.
  • Used for regulating cell movements in epiblast.
  • Used for promoting maturation of blood cells.

*epiblast
  • Generates the embryo, amnion, and allantois.
  • Epiblast cell layer splits separating the embyonic epiblast and the other epiblast cells that form the amnion.
  • Once amnion is complete, the amniotic cavity fills with amniotic fluid.

3. Axis formation and the development of the Ectoderm, Mesoderm, and Endoderm

Gastrulation is the proces by which the bilaminar germ disc reorganizes to form a trilaminar disc.  A thick structure forms at the midline of the epiblast. This structure is called the primitive streak.

The formation of the primitive streak establishes the majox body axis of the embryo: cranial end, caudal end, left side, and right side of the embryo. Axis formation is important in determining proper positioning of organs during development.

At the cranial end of the embryo, the primitive streak expands creating a primitive node containing a circular depression called the primitive pit. This pit continues towards the mid-line of the epiblast forming a primitive groove. When the formation of the primitive groove is complete, cells of the epiblast migrate towards the streak. This event is know as invagination. The first cells to invaginate invade and displace the hypoblast cells, effectively replacing them. These cells are now known as the definitive endoderm. The remaining cells of the epiblast are now know as the ectoderm. Cells that remain in between the ectoderm and the endoderm form a germ layer known as the mesoderm. Upon completion of the definitive endoderm and mesoderm, migration of epiblast cells ceases. Gastrulation is now complete.

III- Organ Development

The three germ layers develop into different organs.

A. Ectoderm

The vertebrate ectoderm will develop into three tissues:
1. Neural plate
The neural plate develops into a neural tube, which is a precursor of the Central Nervous System (the brain and the spinals cord)
2. Epidermis
The epidermis forms an elastic, waterproof, and constantly regenerating barrier between the organs and the outside world.
3. Neural crest
The neural crest generates the Peripheral Nervous System.

B. Mesoderm
From the mesoderm develops the muscular, skeletal, and circulatory system.

C. Endoderm
The endoderm gives rise to the linings of the digestive and respiratory tracts as well as organs such as the liver and pancreas. 

References:
Scott G.F. 2016. Developmental Biology 11th Ed. Sunderland, Mass: Sinauer Associates.


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