Embryonic Development: Morula and Blastocyst Stages

• After fertilization, the zygote travels down the fallopian tube into the uterus.
• As the zygote travels, it undergoes cleavage via rapid mitotic cell division, without increasing in size.

Morula Formation

• The first stage of cellular differentiation begins with the cleavage of the zygote.
• The first mitosis division within the zona pellucida produces two daughter cells known as blastomeres, which are smaller.
• Blastomeres undergo further rounds of mitosis, becoming smaller with each division.
• A solid sphere of cells, resembling a mulberry, forms and is called a morula.

Blastocyst Stage

• Morula cells continue to divide, forming an outer and inner cell layer and a fluid-filled cavity.
• The outer cell layer, the trophoblast, will develop into the placenta.
• The inner cell mass consists of embryonic stem cells that will form the embryo.
• Each new daughter cell is pluripotential and can differentiate into cells of any lineage.

Zona Pellucida Dissolution

• The zona pellucida dissolves to allow the blastocyst to implant into the uterine wall and begin gastrulation.

Blastocyst Implantation

• The inner cell mass separates into a bilaminar embryonic disc after blastocyst implantation into the uterine wall.
• The bilaminar embryonic disc is composed of the epiblast, which forms the actual embryo, and the hypoblast, which generates extra-embryonic structures.
• The amniotic cavity forms within the epiblast.
• The cells of the hypoblast migrate to form the yolk sac.

Gastrulation

• In human development, gastrulation transforms the blastocyst into three embryonic germ layers, beginning after blastocyst implantation into the uterine wall.

Primitive Streak Formation

• At the midline of the embryonic disc, epiblast cells condense to form the primitive streak.
• Epiblast cells migrate inward through a process called invagination.
• The primitive node is formed and will be important for organizing neural tissue.

Endoderm Formation

• Migrating cells infiltrate and displace the hypoblast during invagination, creating the endoderm.
• The endoderm will form components of the respiratory and digestive systems.

Mesoderm Formation

• The process continues, adding a second layer manifest between the epiblast and primitive endoderm.
• The mesoderm contributes to the skeletal circulatory, and muscular systems.

Ectoderm Formation

• Invagination stops and the primitive streak vanishes.
• Cells that do not migrate and remain within the epiblast form the ectoderm.
• The ectoderm develops into the nervous system and skin.

Germ Layer Composition

• Human gastrulation results in an embryo composed of three stacked germ layers: ectoderm, mesoderm, and endoderm.
• Each layer contributes to unique components of the body.

Neurulation

• Neurulation is the process by which components of the post-gastrulation embryo transform to create the future nervous system.

Ectoderm Differentiation

• Centrally located ectoderm tissue differentiates into the neural ectoderm, which will produce the spinal cord and brain.
• Protein signals cause the midline ectoderm to form the neural plate.
• A condensed rod of mesodermal cells, called the notochord, forms.

Neural Plate Formation

• Neural plate cells twist to form edges.
• The entire neural plate bends upwards and inward.
• Plate edges become more prominent, generating neural folds.

Neutral Tube Formation

• Neural folds continue to move towards each other.
• The folds touch, coalesce, and detach from the abutting surface ectoderm.
• Result of the process is a hollow cylinder derived from the ectoderm called the neutral tube.
• The neutral tube will generate the spinal cord or brain.