Frog Early Development

• Fertilization

  • The acrosome of the sperm contains enzymes that help it penetrate the egg
  • The middle piece of the sperm contain mitochondria.
  • The sperm absorbs high-energy nutrients (fructose) in the semen so that the mitochondria can produce ATP for movement of flagella.
  • The one successful sperm that has reached the egg still have enough energy to penetrate the egg and deposit its nucleus into the egg cytoplasm
• Steps during Fertilization

  1. The contact of a sperm with the jelly coat of the egg triggers the release from the acrosome of a cloud of enzymes via exocytosis
  2. The enzymes digest a cavity into the jelly
  3. When the sperm head reaches the vitelline layer, species-specific proteins on its surface bind with specific receptor proteins on the vitelline layer. The binding between these proteins ensures the sperm of other species cannot fertilize the egg (especially in external fertilization)
  4. After specific binding occurs, the sperm’s plasma membrane fuses with that of the egg
• Steps during Fertilization

  5. Fusion of the two membranes makes it possible for the sperm nucleus to enter the egg

  6. About 1 sec after the membranes fuse, the entire egg plasma membrane becomes impenetrable to other sperm cells. Shortly thereafter, the vitelline layer hardens and separates from the plasma membrane. The space quickly fills with water so that the vitelline membrane becomes impenetrable to sperm

  7. About 20 mins after the sperm nucleus enters the egg, the sperm and egg nuclei fuse. The first cell division occurs after about 90 mins, thus marking the end of fertilization phase.
• Cleavage

  • Rapid succession of cell divisions that produce a ball of cells, a multicellular embryo, from the zygote.
  • DNA replication, mitosis, and cytokinesis occurs rapidly, but gene transcription virtually (nearly) shuts down
  • The embryo does not enlarge significantly. Instead, cleavage partitions the cytoplasm of the one-celled zygote into many smaller cells, each with its own nucleus
• Cleavage

  • As cleavage continues, a fluid-filled cavity called blastocoel forms in the center of the embryo.
  • Thus, at the completion of cleavage, there is a hollow ball of cells called blastula
• Gastrula

  • In the blastula, cells toward the animal pole are smaller than those at the vegetal pole. This is because the animal pole contains less yolk.
  • During gastrulation, cells take up new locations that will allow later formation of all the organs and tissues
  • As gastrulation proceeds, the embryo is organized into a three-layer stage called gastrula
  • The 3 layers produced by gastrulation are embryonic tissues called ectoderm, mesoderm, and endoderm
• Gastrula

  • Gastrulation begins when a small groove called blastopore appears on one side of the blastula
  • In the blastopore, cells of the future endoderm (yellow) move inward from the surface and fold over to produce a simple digestive cavity
  • Gastrulation is complete when cell migration has resulted in a three-layered embryo
• Neurula

  • Once the ectoderm, mesoderm, and endoderm forms, cells in each layer begin to differentiate into tissues and embryonic organs
  • An organ called notochord has developed from the mesoderm. Also, a structure that will become the hollow nerve cord is beginning to form in the ectoderm
  • The notochord and dorsal hollo nerve cord are hallmarks of chordate development
• Neurula

  • Later in development, the notochord will function as a core around which mesodermal cells gather and form the backbone
  • A thickened region of the ectoderm called neural plate is formed. From it arises a pair of ectodermal ridges (called neural folds)
  • The neural plate rolls up and forms a tube, which sinks beneath the surface of the embryo and is covered by an outer layer of ectoderm
• Neurula

  • The neural plate rolls up to form a tube called neural tube, which will eventually become the brain and spinal cord
  • Therefore, the neural lies directly above the notochord and digestive cavity. This gives a preview of the basic body plan (of a frog in this case)
  • The spinal cord will lie within extensions of the dorsal (upper) surface of the backbone, whereas the digestive tract will be ventral to (beneath) the backbone
• Neurula

  • Other changes in the embryo:
  • The embryo becomes elongated
  • The beginnings of eye and tail become visible
  • Internal ridges called somites form. These somites are blocks of mesoderm that will give rise to segmental structures, such as the vertebrae and associated muscles of the backbone
  • The mesoderm next to the somites is developing a hollow space called the coelom (body cavity)
• Frog Oogenesis

  • During the growth of the oocyte, animal-vegetal polarity arises.
  • In the previtellogenic (pre-yolky) stage, a special cytoplasmic region rich in mitochondria is formed. This is called germ plasm (or Balbiani body). This determined the future vegetal pole.
  • The hemisphere containing the germinal vesicle (large nucleus of the oocyte) is destined to become the future animal pole.
• Frog Oogenesis

  • In mid-oogenesis, the animal pole appears dark due to accumulation of pigment granules, whereas the vegetal pole remains light colored.
  • When the germinal vesicle breaks down, meiosis I occurs, forming the secondary oocyte (unfertilized egg) and the first polar body.
  • The secondary oocyte gets arrested again in metaphase II. The eggs are shed into the body cavity (of the frog), travel down the oviducts where they get wrapped with jelly
  • ▪ When the secondary oocyte becomes fertilized, the sperm causes rise in intracellular Ca2+ enabling the continuation of meiosis II
• Frog Fertilization

  • The point of sperm entry determines dorsal-ventral polarity
  • The point of sperm entry marks the ventral (belly) side
  • The point 180-degrees opposite the point of sperm entry marks the dorsal (spinal) side
• Frog Fertilization

  • The centrioles of the sperm (that entered along with the nucleus) initiates a cytoplasmis rearrangement by organizing the microtubules of the egg into parallel tracks in the vegetal cytoplasm, separating the outer cortical cytoplasm from the inner yolky cytoplasm
  • In the zygote, the cortical cytoplasm rotates 30- degrees with respect to the internal cytoplasm. This exposes a region of gray-colored inner cytoplasm directly opposite the point of sperm entry.
  • This region is called the gray crescent, where gastrulation will begin
• Cleavage Patterns

  • Isolecithal - evenly distributed yolk
  • Mesolecithal - moderate vegetal yolk disposition
  • Telolecithal - dense yolk throughout most of cell
  • Centrolecithal - yolk in the center of egg
• Frog Cleavage and Blastula

  • Cleavage in from embryo is radially symmetrical and holoblastic (complete)
  • The large cells resulting from early cleavage are called blastomeres
  • By the 3rd cleavage, the frog embryo becomes divided into 4 small blastomeres (micromeres) and 4 large blastomeres (macromeres)
  • Cleavage continues rapidly for 12 divisions. The amphibian embryo containing 16-64 cells is called morula
• Frog Cleavage & Blastula

  • At the 128-cell stage, the blastocoel forms in the center of the animal hemisphere. In this stage, the embryo is called blastula.
  • Then the mid-blastula transition (MBT) or late blastula stage occurs
  • During the late blastula, the rate of cleavage slows down and transcription of zygotic genome commences.
• Frog Early Cleavage

  
• Frog Late Cleavage

  
• Identify the parts of a frog early blastula

  A) animal pole

  B) micromeres

  C) blastocoel

  D) macromeres

  E) vegetal pole

  F) yolk platelets

  
• Identify the parts of a frog early blastula

  A) animal pole

  B) micromere

  C) blastocoel

  D) macromere

  E) vegetal pole

  
• Identify the parts of a frog late blastula

  A) animal pole

  B) blastocoel

  C) vegetal pole

  
• Invagination - infolding of a sheet of cells, much like the indentation of a soft rubber ball when it is poked.
• Involution - inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells.
• Involution - inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells.
• Ingression - migration of individual cells from the surface of into the embryo's interior. Individual cells become mesenchymal and migrate independently.
• Delamination - splitting of one cellular sheet into two more or less parallel sheets. While on a cellular basis it resembles regression, the result is the formation of a new epithelial sheet of cells.
• The fate of 3 germ layers

  • Ectoderm - Epidermis, nervous system, lens and ear, cement gland
  • Mesoderm - Head mesoderm, notochord, somites, kidney, lateral plate, blood, blood vessels, heart, limbs, gonads
  • Endoderm - Epithelial lining of the gut, lungs, liver, pancreas, bladder
• Important Events During Frog Gastrulation

  • The bottle cells of the marginal zone move inward to form the dorsal lip of the blastopore, and the mesodermal precursors involute under the roof of the blastocoel.
• Important Events During Frog Gastrulation

  • The archenteron forms and displaces the blastocoel. The cells migrate from the lateral and ventral lips of the blastopore into the embryo
• Important Events During Frog Gastrulation

  • The cells of the animal hemisphere migrate down toward the vegetal region, thereby moving the blastopore to the region near the vegetal pole
• Important Events During Frog Gastrulation

  • Toward the end of gastrulation, the blastocoel is obliterated (destroyed). The embryo becomes surrounded by ectoderm.
• Important Events During Frog Gastrulation

  • On the other hand, the endoderm has been internalized and the mesodermal cells have been positioned between the ectoderm and endoderm.
• Identify the parts of frog early gastrula

  A) animal pole

  B) blastocoel

  C) vegetal pole

  
• Identify the parts of a frogs late gastrula

  A) archenteron roof

  B) outer ectodermal layer

  C) inner ectodermal layer

  D) archenteron

  E) direction of epiboly

  F) dorsal lip

  G) yolk plug

  H) ventral lip

  I) direction of epiboly

  J) blastocoel

  
• Frog Neurula

  • The formation of neural plate marks the start of neurulation; hence the embryo is now a neurula
• Frog Neurula

  • The neural plate becomes visible covering much of the dorsal surface of the embryo, delimited by raised neural folds
  • The neural folds rise and move together to form the neural tube.
• Frog Neurula

  • After closure of the neural tube, it becomes covered by ectoderm from beyond the folds, now known as epidermis
  • During and after neurulation, there is striking elongation of the body, which means that the trunk and tail region are derived from the posterior quarter of the neurula
• Frog Neurula

  • This elongation of the body is due to convergent extension of the notochord and other tissues
  • The roof of the archenteron is formed by endoderm