Embryonic Development, Gastrulation, and Neurulation Vocabulary

Maternal to Zygotic Transformation

• The embryo's genome becomes active early in development.
• Maternal to Zygotic Transformation: Transition from relying on maternal mRNA to the embryo's own genome.
• Eutherians undergo rotational cleavage relatively early.
• Shift in dependence from maternal instructions to the embryo's genome.

Mammalian Development

• Two layers: epiblast and hypoblast.
• These two layers are found in the inner cell mass.
• Green cells (hypoblast) and pink cells (epiblast).

Inner Cell Mass and Trophoblast

• Inner cell mass develops into the embryo.
• Outer cells are supportive and aid in nutrient release.
• Inner cell mass consists of epiblast and hypoblast.
• Trophoblast cells form the outer circle.
• Trophoblast: Outer cells with various functions.
• Two subcategories of trophoblasts (not required for the exam).
• Trophoblast cells break down endometrial cells in the uterus lining to release nutrients for the inner cell mass (the cells wrapping around the outer margins of the uterus).

Placenta Development

• Early placenta develops around the trophoblast.
• Placenta becomes the primary endocrine and exchange center.
• Hormonal control.
• Acquisition of oxygen and nutrients from maternal blood.
• Waste product expulsion into maternal blood.

Gastrulation and Neurulation

• Following cleavage, embryos enter gastrulation.
• Blastomeres move, migrate, and are induced to become three tissue types.
• Triploblastic organisms: endoderm, ectoderm, and mesoderm.
• Zygote genome takes over protein synthesis.
• Maternal mRNA is no longer significant.
• Embryo's genome coordinates early developmental events.

Gastrulation Overview

• Cells differentiate and become different tissue types.
• Sea urchin, frog, and chick gastrulation will be discussed.

Sea Urchin Gastrulation

• Vegetal pole becomes the archenteron.
• Invagination: cells migrate upwards.
• Mesenchyme cells: specialized cells that detach from the outer layer of blastula cells.
• Mesenchyme cells form contractile filaments that attach to the roof of the blastula.
• Contraction of filaments pulls the archenteron upwards.
• Urchins are deuterostomes: blastopore becomes the anus.
• Archenteron contacts the ectoderm: the contact point becomes the mouth.

Sea Urchin Gastrulation - Visual

• Vegetal hemisphere at the bottom.
• Mesenchyme cells (red) break away from the blastula.
• Blastopore invaginates into the hollowed-out cavity.
• Mesenchyme cells attach to the roof of the cavity/archenteron.
• Filaments with contractile units (similar to sarcomeres).
• Contraction pulls the archenteron upwards towards the ectoderm.
• Point of contact forms the mouth; anus forms first (deuterostome).

Sea Urchin Gastrulation - Simplicity

• Sea urchin gastrulation is relatively simple compared to frogs and chicks.
• Invagination aided by mesenchyme cells and contractile filaments.
• Archenteron becomes the gut cavity.

Frog Gastrulation

• Frogs have a moderate amount of yolk.
• Smaller cells divide in the animal hemisphere (less yolk).
• Cells divide, but without significant cell growth, resulting in smaller cell size.
• Vegetal hemisphere has more yolk, which slows down mitosis. Fewer cells are produced, and they are larger.

Gray Crescent

• Gray crescent forms.
• Point of sperm entry determines frog polarity.
• Cells divide around the blastula, but yolk slows down division at the vegetal hemisphere.
• Animal hemisphere cells divide faster and move around, caving inwards through the blastopore.
• Animal hemisphere cells move faster and differentiate into mesoderm and endoderm.

Frog Gastrulation - Visual

• Fewer, larger cells in the vegetal hemisphere; more, smaller cells in the animal hemisphere.
• Yolk slows down cell division.
• Animal pole cells divide more rapidly and spread outward.
• Cells curve around and move inward into the blastopore.
• Dorsal lip of the blastopore: a ledge on top of the blastopore.

Frog Gastrulation - Archenteron Development

• Animal hemisphere cells divide more quickly and go around the dorsal lip.
• Archenteron forms as cells move inward, pushing the blastocoel aside.
• Vegetal hemisphere cells make their way toward the blastopore.
• Archenteron replaces the blastocoel, becoming the future digestive tract.
• Cell signaling tells mesoderm to develop.
• Three germ layers form: endoderm (yellow), mesoderm (red), and ectoderm (blue).
• Moving, what determines how fast the cells are moving/speed determined by amount of yolk, what's happening once invaginate inwards, and dorsal lip.

Neurulation

• Formation of the central nervous system.
• The view is from the side of a blastula cut in half.

Chicken Gastrulation

• Very yolky eggs go through incomplete cleavage.
• Cleavage is restricted to a small portion of the egg; the rest remains yolk.
• Instead of a blastula, a flattened structure called a blastodisc forms.
• Blastodisc has two primary cell layers: epiblast and hypoblast.
• Undifferentiated ectoderm cells divide and move inward.
• Primitive streak/groove: cells migrate down into it.
• Henson's node: a dark spot at one end of the groove that contains determinants to cause formation of anterior structures, mostly associated with the head of the organism.."
• Cells moving through the primitive streak differentiate into mesoderm and endoderm.

Chicken Gastrulation vs. Mammals

• Similar to placental mammals with epiblast and hypoblast.
• In chicken development, the hypoblast is displaced.
• In placental mammals, the hypoblast contributes to placenta development.

Primitive Groove

• Streak becomes longer during early embryonic development, forming the primitive groove.
• Cells migrate towards the groove and invaginate.
• Cells going through the primitive streak or groove are induced to become mesoderm or endoderm.
• Henson's node is at the top, forming the head of the chicken.

Germ Layers and Body Cavities

• At the end of gastrulation, three germ layers (ectoderm, endoderm, mesoderm) are neatly organized.
• Germ layers undergo structural changes to differentiate into specific tissues and organ systems.
• Deuterostomes have the anus forming first, while protostomes have the mouth forming first.

Protostomes vs. Deuterostomes - Mesoderm

• Deuterostomes: Archenteron forms after cell migration.
• Outpouches from the endoderm are signaled to become mesoderm.
• Outpouches fold and pinch off to produce mesoderm layers.
• Protostomes: Specialized blastomeres create mesoderm.
• Cells dissociate from the blastula wall and grow through mitosis around the archenteron.

Germ Layer Fates

• Ectoderm:
  • Epidermis.
  • Hair.
  • Claws.
  • Sweat glands.
  • Brain.
  • Nervous system.
  • Some neural crest cells.
• Endoderm:
  • Lining of the gut/digestive tract.
  • Accessory organs (liver, gallbladder, pancreas).
  • Lungs.
• Mesoderm:
  • Notochord.
  • Connective tissues.
  • Muscles (including heart).
  • Blood/blood vessels.
  • Urogenital system.
  • Bones.
  • Dermis.

Neurulation: Neural Tube Formation

• Cells move through the blastopore (or primitive groove in chicks).
• Populations of cells separate from the archenteron.
• Notochord induces overlying ectoderm to become the neural plate.
• Edges of the neural plate roll up and pinch off, forming the neural tube.
• Neural tube becomes major aspects of the central nervous system.

Brain Development

• In vertebrates, the neural tube forms three distinct areas: forebrain, midbrain, and hindbrain.
• Anterior end of the neural tube forms three pouches that develop into these brain regions.
• The rest of the neural tube becomes the spinal cord.

Neural Tube Formation – Visual

• Early neurogenesis: cells migrate over the dorsal lip.
• Dorsal lip cells induce a cluster of cells to become the notochord.
• Notochord signals to the overlying ectoderm to develop into the neural plate, which folds inward to create the neural tube.

Notochord

• Notochord is a solid mass of cells.
• Neural tube is a hollowed-out tube forming brain and spinal cord.
• Spina bifida: neural tube defect where the tube doesn’t fully close, often in the lower spinal column.

Neural Crest Cells

• Neural crest cells detach from surrounding tissue when the neural tube closes.
• Multipotent and wandering cells (migrate long distances).
• Can differentiate into:
  • Ectoderm.
  • Mesodermal cells.

Neural Crest Cells - Fates

• Various aspects of the sensory system and autonomic nervous system (peripheral nervous system).
• Skull bones (osteoblasts, osteoclasts, chondrocytes).
• Pigment cells (melanocytes).
• White streak down the middle of fur is an example of neural crest cell migration pattern.

Mesoderm Differentiation

• Mesoderm gives rise to muscle and connective tissue.
• Clusters of mesoderm cells are somites.
• Somites are repeating segments that give rise to vertebrae, ribs, muscles, dermis, and cartilage.

Mesoderm Types

• Somites: vertebrae, ribs, muscles, dermis, cartilage.
• Intermediate mesoderm: reproductive and urinary structures.
• Lateral plate mesoderm: lining of body cavities, muscles of the digestive tract, circulatory system, heart, and vessels.

Cell Signaling and Morphogens

• Cell position and morphogens determine cell fate.
• Morphogens are inducers that affect surrounding cells through diffusion.
• Concentration of morphogen determines tissue development.

Vertebrate Limbs

• Limb bud: a cluster of cells that will become the limb.
• ZPA (zone of polarizing activity): a cluster of cells that secretes morphogens.
• High morphogen concentration develops into the little finger, while lower concentration develops into the thumb.
• Morphogen example: Sonic Hedgehog.