FERTILIZATION AND EARLY EMBRYONIC DEVELOPMENT IN HUMANS

• The process of gametes (an egg and sperm) fusing to form a zygote is fertilization.
• In mammals, a layer of extracellular matrix composed primarily of glycoproteins called zona pellucida protects the egg.
• A series of biochemical processes, called acrosomal reactions, take place when a sperm binds to the zona pellucida.
• The acrosome produces digestive enzymes in placental mammals that initiate the degradation of the glycoprotein matrix that protects the egg and enables the sperm plasma membrane to fuse with the egg plasma membrane.
• To ensure that no more than one sperm fertilizes the egg, the egg releases proteins in other places until the acrosomal reactions occur at one position of the egg membrane to prevent other sperm from fusing with the egg.
• Multiple sperm can fuse with the egg if this mechanism fails, resulting in polyspermia.
• The embryo that results is not genetically viable and dies within a couple of days.
• In the Cleavage and Blastula Stage, a single-celled zygote, which undergoes rapid cell division to form the blastula, starts the growth of multi-cellular species.
• The rapid, multiple cell division rounds are called cleavage.
• After over 100 cells have been formed by cleavage, the embryo is called a blastula.
• The blastula is usually a spherical layer of cells around a fluid-filled or yolk-filled cavity (the blastoderm) (the blastocoel).
• Each cell is called a blastomere inside the blastula.
• A ball of cells is the typical blastula.
• Cells in the blastula are spatially rearranged to create three layers of cells.
• A ball of cells is the typical blastula. Cells in the blastula are spatially rearranged to create three layers of cells. This is called gastrulation.
• The blastula folds upon itself during gastrulation to form the three cell layers.
• Each of these layers is referred to as a germ layer, and various organ systems are distinguished by each germ layer.
• The endoderm, the ectoderm, and the mesoderm are the three germ layers.
• The ectoderm gives rise to the epidermis and the nervous system.
•  The mesoderm produces the body's muscle cells and connective tissue.
• The endoderm gives rise to columnar cells and several internal organs including the digestive system.
• Organogenesis - through the process of differentiation, organs form from the germ layers.
• Embryonic stem cells express unique sets of genes during differentiation, which will decide their ultimate type of cell.
• For example, genes unique to skin cells will be expressed by certain cells in the ectoderm. As a result, epidermal cells would be divided into these cells.
• In vertebrates, the development of the neural system is one of the key steps during organogenesis.
• Epithelial cells and tissues, and neural tissues, form the ectoderm.
• Special signaling molecules called growth factors during the development of the neural system signal those cells at the edge of the ectoderm to become epidermis cells.
• The neural plate is created by the remaining cells in the middle. If growth factors were to block the signaling, then the entire ectoderm would differentiate into neural tissue.
• The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube.
• The neural tube will give rise to the brain and the spinal cord in further development.
• The mesoderm lying on either side of the neural tube of the vertebrate will grow into the animal body's various connective tissues.
• A gene expression spatial pattern reorganizes the mesoderm into groups of cells with gaps between them, called somites.
• The somites, will grow further into the muscles of the ribs, lungs, and segmental (spine).