Embryonic Development

• Fertilization

  Process by which a single sperm unites with a single egg cell of the same species of organism, resulting in the formation of a single diploid cell called the zygote

  
• Egg cell

  • Approximately 16 times larger than a human sperm cell
  • Contains varying amounts of yolk, nutrients to support growth of the developing embryo
  • Surrounded by a jelly layer composed of glycoproteins that release species-specific chemoattractants to guide sperm
  • In mammals, the jelly layer is called the zona pellucida

  
• Sperm cell

  • One of the smallest cells produced in most animal species
  • Consists of a head containing tightly packed DNA, a flagellar tail for swimming, and many mitochondria to provide power for movement
  • Plasma membrane contains proteins called bindin that recognize and bind to receptors on the egg plasma membrane
  • Contains an acrosome organelle that releases digestive enzymes to degrade the jelly layer/zona pellucida

  
• Steps of Fertilization

  1. Sperm is attracted to and contacts the jelly layer/zona pellucida

  2. Acrosome reaction releases digestive enzymes to destroy the jelly layer/zona pellucida

  3. Sperm reaches the egg plasma membrane and the sperm and egg plasma membranes fuse

  4. Fusion initiates electrical depolarization of the egg plasma membrane, temporarily preventing other sperm from fusing

  5. Calcium wave initiates the cortical reaction, releasing enzymes that degrade the bindin receptor proteins and the proteins holding the vitelline layer

  6. Cortical reaction results in the lifting away of the vitelline layer to create the fertilization envelope, a barrier to prevent additional sperm

  7. Egg activation occurs, causing the egg to recognize fertilization and initiate development

  
• Cleavage

  Rapid cell divisions of the zygote as it travels toward the uterus, resulting in more cells without increasing the total volume
• Cleavage Formation

  1. Zygote undergoes 5-6 rapid mitotic cell divisions, resulting in a 16-cell conceptus called a morula

  2. Conceptus continues to divide, creating a ball of approximately 100 cells and consuming nutritive endometrial secretions

  3. Ball of cells starts to secrete fluid and organize around a fluid-filled cavity, forming a blastocyst

  4. Within the blastocyst, a group of cells forms the inner cell mass that will become the embryo, and the outer cells form the trophoblast that will develop into the placenta

  
• Implantation

  Process where the blastocyst adheres to and embeds itself in the uterine wall, signaling the end of the pre-embryonic stage

  
• Implantation Process

  1. Blastocyst typically implants in the fundus of the uterus or on the posterior wall

  2. Trophoblast cells fuse to form the syncytiotrophoblast, which digests endometrial cells to secure the blastocyst

  3. Uterine mucosa rebuilds itself and envelops the blastocyst

  4. Trophoblast secretes human chorionic gonadotropin (hCG) to direct the corpus luteum to produce progesterone and estrogen, suppressing menses
• Extra-Embryonic Membranes

  • Amnion - a membrane that closely covers the embryo and fills with amniotic fluid to provide a protective environment
  • Yolk Sac - forms on the ventral side of the embryonic disc, opposite the amnion
• Implantation is complete by the middle of the second week
• Amnion

  A membrane that closely covers the human and various other embryos when first formed. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing embryo
• Yolk sac

  A structure that develops inside your uterus during early pregnancy. It provides an embryo with nourishment (food). It helps circulate gasses between you and the embryo. The yolk sac also produces cells that turn into important structures, such as the umbilical cord, blood cells and reproductive organs

  
• Allantois

  The fetal membrane lying below the chorion in many vertebrates, formed as an outgrowth of the embryo's gut. It has four functions: serves as an embryonic respiratory organ, receives the excretions of the embryonic kidneys, absorbs albumen which serves as nutriment for the embryo, and absorbs calcium from the shell for the structural needs of the embryo
• Chorion

  Also called Serosa, in reptiles, birds, and mammals, the outermost membrane around the embryo. It develops from an outer fold on the surface of the yolk sac
• Gastrulation

  A process where the embryonic cells transits from totipotency to multipotency. During this stage, cells in the blastula rearrange themselves to form three layers of cells and form the body plan

  
• Outcomes of gastrulation

  • Formation of the embryonic tissues (germ layers)
  • Formation of the embryonic gut (archenteron)
  • Appearance of the major body axes

  
• Germ layers

  The endoderm, ectoderm, and mesoderm. Each germ layer will later differentiate into different tissues and organ systems
• Diploplasts

  Animals with only two germ layers, like jellyfish and comb jellies, have radial rather than bilateral symmetry and have far fewer tissue types than triploplasts due the lack of a mesoderm
• Triploblasts

  Animals with three embryonic germ layers
• Gastrulation in triploblasts

  1. One group of cells moves into the blastocoel, forming the endoderm

  2. Another group of cells move to completely surround the embryo, forming the ectoderm

  3. A third group of cells move into the locations in between the outer and inner layers, forming the mesoderm

  4. The endodermal cells continue through the interior of the embryo until they reach the other side, creating a continuous tract through the embryo (the archenteron or embryonic gut)

  
• Germ layer derivatives

  • Endoderm: Epithelial lining of digestive and respiratory tracts, lining of urethra, bladder and reproductive system, liver and pancreas
  • Mesoderm: Notochord, musculoskeletal system, muscular layer of stomach, intestine etc, circulatory system
  • Ectoderm: Epidermis of skin, cornea and lens of eye, nervous system
• Fertilization

  Process by which a single sperm unites with a single egg cell of the same species of organism, resulting in the formation of a single diploid cell called the zygote

  
• Neurulation

  1. Specialized neuroectodermal tissues along the length of the embryo thicken into the neural plate

  2. Tissues on either side of the plate fold upward into a neural fold

  3. The two folds converge to form the neural tube

  
• Egg cell

  • Approximately 16 times larger than a human sperm cell
  • Contains varying amounts of yolk, nutrients to support growth of the developing embryo
  • Surrounded by a jelly layer composed of glycoproteins that release species-specific chemoattractants to guide sperm
  • In mammals, the jelly layer is called the zona pellucida

  
• Sperm cell

  • One of the smallest cells produced in most animal species
  • Consists of a head containing tightly packed DNA, a flagellar tail for swimming, and many mitochondria to provide power for movement
  • Plasma membrane contains proteins called bindin that recognize and bind to receptors on the egg plasma membrane
  • Contains an acrosome organelle that releases digestive enzymes to degrade the jelly layer/zona pellucida
• Steps of Fertilization

  1. Sperm is attracted to and contacts the jelly layer/zona pellucida

  2. Acrosome reaction releases digestive enzymes to destroy the jelly layer/zona pellucida

  3. Sperm reaches the egg plasma membrane and the sperm and egg plasma membranes fuse

  4. Fusion initiates electrical depolarization of the egg plasma membrane, preventing other sperm from fusing

  5. Calcium wave initiates the cortical reaction, releasing enzymes that degrade the bindin receptor proteins and the proteins holding the vitalline layer

  6. Cortical reaction results in the lifting away of the vitalline layer to create the fertilization envelope, a barrier to prevent additional sperm

  7. Egg activation occurs, causing the egg to recognize fertilization and initiate development

  
• The neural tube lies atop a rod-shaped, mesoderm-derived notochord, which eventually becomes the nucleus pulposus of intervertebral discs
• Cleavage

  Rapid cell divisions that occur after fertilization as the zygote travels toward the uterus

  
• Cleavage cell divisions can occur as frequently as every 10 minutes, although "typical" cell division occurs every 18-24 hours
• Block-like structures called somites form on either side of the neural tube, eventually differentiating into the axial skeleton, skeletal muscle, and dermis
• Blastomere

  Each daughter cell produced by cleavage
• Morula

  The 16-cell conceptus that reaches the uterus approximately 3-4 days after fertilization, where the cells are now compacted into a solid mass
• Embryonic folding

  1. The embryo folds laterally and again at either end, forming a C-shape with distinct head and tail ends

  2. The folding creates a tube, called the primitive gut, that is lined by the endoderm

  3. The amniotic sac, which was sitting on top of the flat embryo, envelops the embryo as it folds

  
• Blastocyst

  The ball of tightly bound cells that starts to secrete fluid and organize themselves around a fluid-filled cavity, the blastocoel
• Inner cell mass

  The group of cells within the blastocyst that are fated to become the embryo

  
• Trophoblasts

  The cells that form the outer shell of the blastocyst, which will develop into the chorionic sac and the fetal portion of the placenta

  
• Within the first 8 weeks of gestation, a developing embryo establishes the rudimentary structures of all of its organs and tissues from the ectoderm, mesoderm, and endoderm. This process is called organogenesis

  
• Implantation

  The process where the blastocyst comes in contact with the uterine wall and adheres to it, embedding itself in the uterine lining via the trophoblast cells
• Syncytiotrophoblast

  • The multinucleated body formed by the fusion of the superficial cells of the trophoblast, which digests endometrial cells to firmly secure the blastocyst to the uterine wall
• Like the central nervous system, the heart also begins its development in the embryo as a tube-like structure, connected via capillaries to the chorionic villi