Phases of Embryonic Development

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Phases of Embryonic Development

• Abdelalim Gadallah (Ph.D.)
• Lecturer of Comparative Anatomy and Embryology
• Zoology Department, Faculty of Science,
• MANSOURA UNIVERSITY
• EGYPT

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7 steps

• 1. gametogenesis
• 2. fertilization
• 3. cleavage
• 4. blastulation
• 5. gastrulation
• 6. neurulation
• 7. organogenesis

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• 1.Gametogenesis
• is a process by which the diploid germ cells
  undergo a number of chromosomal and morphological
  changes to form mature haploid gametes.
• Animals produce gametes directly through meiosis
  in organs called gonads. Males and females of a
  species that reproduces sexually have different
  forms of gametogenesis
• spermatogenesis (male) in testes produce sperms.
• oogenesis (female) in Ovary produce ova.

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Structure of sperm
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• Structure of ovum of sea urchin

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Common terms

• Animal Pole the pole (end) of the egg where yolk
  is least concentrated.
• Animal hemisphere the hemisphere of the egg
  where animal pole is located.
• Vegetal pole the pole (end) of the egg where
  yolk is the most concentrated.
• Vegetal hemisphere the hemisphere of the egg
  where vegetal pole is located.

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• 2.Fertilization
• is the process whereby two sex cells (gametes)
  fuse together to create a new individual with
  genetic potentials derived from both parents.
• Have two separate activity
• Combining of genes derived from the two parents.
• Creation of new organisms.
• Thus
• The first function is Transmit genes from
  parents to offspring.
• The second is initiate reactions in the egg
  cytoplasm that proceed development. Also,
• Restoration of the diploid number of chromosomes
  reduced during meiosis.

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It have 4 major steps

• Sperm contacts the egg
• Sperm or its nucleus enters the egg, and
  regulation of sperm entry .
• Sperm and egg nuclei fuse
• Egg becomes activated and developmentalchanges
  begin

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3. Cleavage

• Is the process of repeated rapid mitotic cell
  divisions of the zygote (unicellular structure)
  to form the Blastula (multicellular structure).
• The produced cells named Blastomeres.
• During this stage the size of the embryo does not
  change, the blastomeres become smaller with each
  division.
• The type pattern of cleavage differ from
  species to species.
• continues divisions to form a ball of 32 cells
  called the morula.
• The morula continues divisions to form the hollow
  blastula with up to several hundred cells.
• The cavity of the blastula is the blastocoel.

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Figure 47.8x Cleavage in a frog embryo
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4. Blastulation

• The result (end period) of cleavage.
• The production of a multicellular blastula
• Blastula cells are called blastomeres.
• A cavity forms within the ball of the cells
  called the blastocoel.

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Blastula of frog
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Sea urchin blastula
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Human blastula
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Starfish development, unfertilized egg.
4 blastomeres.
2 blastomeres.
Starfish development, nonmotile blastula.
16 blastomeres.
32 blastomeres. morula
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5. Gastrulation

• The morphogenetic process called gastrulation
  rearranges the cells of a blastula into a
  three-layered (triploblastic) embryo, called a
  gastrula, that has a primitive gut.
• It means rearrangement of blastula cells that
  transforms the blastula into a gastrula.
• The blastula develops a hole in one end and cells
  start to migrate into the hole this forms the
  gastrula
• Characterized by cell movement.
• Blastocoel is gradually disappear and a new
  cavity is formed Gastrocoel.

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• The gastrula is a three-layered embryo
• The formation of three primary embryonic germ
  layers
• Endoderm (inner)
• Mesoderm (middle)
• Ectoderm (outer)
• The pattern of gastrulation is affected by the
  amount of yolk.
• The cells at the vegetal pole invaginate,
  initiating gastrulation.

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Gastrulation in a Frog Embryo
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Figure 47.10 Gastrulation in a frog embryo
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6. Neurulation
Chordates Only

• formation of a dorsal, hollow neural tube
• by
• ectodermal cells flatten into neural plate
• the center of the plate sinks forming neural
  groove
• edge of plate is elevated to form neural folds
• neural folds fuse and form neural tube
• anterior end develops into brain
• posterior end develops into spinal cord

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Neurulation
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The Neural Crest

• The neural crest is a critical structure that
  guides formation of several organ systems
• The neural crest forms on either side of the
  point of fusion
• Its cells migrate to form the dorsal root
  ganglia, the postganglionic sympathetic neurons,
  many sense organs and all pigment-forming cells

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Organogenesis

• Organogenesis is the formation of the organs
• The layers are germ layers they have specific
  fates in the developing embryo
• Endoderm
• The innermost layer
• Goes on to form the gut
• Mesoderm
• The middle layer.
• Goes on to form the muscles, circulatory system,
  blood and many different organs
• Ectoderm
• The outermost
• Goes on to form the skin and nervous system

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Organogenesis Begins With Development of the
Nervous System

• The nervous system is the first organ system to
  develop.
• The notochord grows and induces overlying
  ectoderm to form the neural plate.
• Cells of the neural plate fold to form the neural
  groove and the surrounding neural folds fuse to
  form the neural tube.
• The anterior portion forms the brain the rest
  forms the spinal cord.

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7. Organogenesis

• Development of organs from three primary germ
  layers
• Ectoderm forms
• skin and associated glands, nervous system.
• Mesoderm forms
• muscles, skeleton, gonads, excretory system,
  circulatory system.
• Endoderm forms
• lining of digestive tract, liver, pancreas, lungs.

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Figure 47.11 Organogenesis in a frog embryo
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Somatic Germ Cells

• Somatic Cells
• Found in all body tissues except gonads.
• Contain diploid numbers of chromosomes (2N).
• Replacement of dead cells
• Reproduce by mitotic division.
• Functions
• Responsible for formation of different system and
  organs.
• Have other specific functions
• e.x. muscular system have myoplast for
  contraction and relaxation .
• nervous system have neurons for
  transmission of impulses.

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• digestive system have secretory cells for
  secretion of enzymes for digestion.
• bone have osteoplasts make hardening of bone.
• Lung have cells for respiration.
• Germ Cells
• Found only in gonads (testes ovary)
• Contain haploid number of chromosomes (1N)
• Reproduce by meiotic division (meiosis).
• Function Formation of gametes (male female)

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Primordial germ cells

• Appears in the wall of the endodermal layer of
  the yolk sac due to their large size and high
  content of alkaline phosphatase , and migrate by
  amoeboid movement toward the hind gut epithelium
  and then through dorsal mesentery reach to the
  primordia of the gonads ( primitive sex glands).
• Become recognizable at 24 days post-fertilization.
  
• Invading the genital ridges in the 6th week of
  development.
• The primordia of the gonads give a chemostatic
  attractant for the germ cells.

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• Also may be the germ cells directed toward the
  gonads by the mature of cellular and non-cellular
  microenvironment that surround them.
• Or by tissue rearrangement that occur in the
  early embryos.
• If they fail to reach the ridges, the gonads will
  not develop.
• So, it have the inductive influence on the
  development of gonads into ovary and testes.
• Determine sex of the embryo.
• They will form different stages of
  spermatogenesis and oogenesis in the future.
• Found in adult in gonads (testes and ovary)

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Basic Developmental Vocabulary

• Fertilization activates egg brings together
  the nuclei of the egg and sperm.
• Cleavage partitions the zygote into many smaller
  cells.
• Gastrulation rearranges the blastula to form a
  three-layered embryo with a primitive gut, the
  archenteron.
• Organogenesis is the process by which the organs
  in the animal body form from the three embryonic
  germ layers.

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Basic Developmental Vocabulary

• Blastula a hollow sphere of cells (128 cells)
  formed by cleavage of the morula. The blastula
  contains the blastocoel that is fluid-filled. The
  blastopore is the place where gastrulation
  begins.
• Gastrulation the process leading to the
  creation of the primitive gut or archenteron.
  Invagination at the blastopore results in the
  gut.
• Gastrula transformation of the blastula into an
  embryo possessing 3 germ layers, ectoderm,
  mesoderm, endoderm.
• Morula a solid mass of 16-64 cells formed by
  cleavage.

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Thank you for your time.

• Questions?
• 
  Dr. Abdelalim Gadallah