Embryology

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Embryology
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Embryology the study of the development of an
organism from the time an egg is fertilized until
the time when all major organ systems are formed
A. Historical Aspect
1. Preformation this was a concept held during
the 17th and early 18th century which stated that
sex cells (gametes) contained a completely
formed, but greatly miniaturized human inside
them. This miniaturized person was termed a
homunculus
2. By the mid-18th century, another view began to
become popular known as epigenesis. This concept
stated that the egg contained material from which
the embryo was built and directed the formation
of the individual by some unknown creative
principal.
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3. Rouxs experiment (using amphibian eggs) 1888
4. Drieschs experiment (using echinoderm eggs)
1892
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V. Steps in Embryonic Development
A. Definitions
1. Gamete sex cell
2. Sperm male gamete

• motile, flagellated
• usually much smaller than the female gamete
• haploid (contains only one set of chromosomes for
  that species) 1n

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3. Egg or ovum female gamete
a) non-motile b) up to 1,000 times larger
than a sperm cell
c) haploid 1n also d) within the egg are
most of the enzymes, proteins, and other
information needed for development for the new
embryo
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4. Fertilization union of sperm and egg
(fertilization is also called syngamy)

• the result of fertilization is a diploid zygote
• diploid having two complete sets of chromosomes
  resulting from syngamy
• zygote the cell produced from the union of egg
  and sperm that will divide mitotically, forming a
  new organism

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B. First Step Fertilization (syngamy)

• Sperm must penetrate the gel coat surrounding the
  egg
• 2. Enzymes called lysins, located in an
  organelle in the head of the sperm called the
  acrosome, dissolves the gel coat membrane around
  the egg

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C. Second Step Egg Activation
1. Egg activation begins when the acrosomal
membrane of one sperm fuses with the egg membrane
2. Early changes to the egg ensures that only one
sperm will fertilize that egg

• the first sperm that unites with the egg membrane
  is surrounded by the microvilli located on the
  membrane, which wraps around the penetrating
  sperm
• the nucleus of this one sperm is drawn into the
  center of the egg (contains the DNA)
• within milliseconds of the initial penetration of
  this sperm, ionic changes occur in the egg
  membrane making the membrane unresponsive to any
  other sperm
• a protective barrier called the fertilization
  membrane prevents any other sperm from entering

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e. In certain species, especially amphibian eggs,
a structure called the gray crescent forms on the
side opposite of the site where the sperm
initially penetrated
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D. Third Step Metabolic and Nuclear Events

• Very little mRNA synthesis occurs within the
  early zygote. Almost all of the mRNA directing
  early development and division was stored within
  the egg itself. This original mRNA is called
  maternal mRNA since it was formed by the mothers
  cells. (This is the main reason an egg cell is so
  much larger than a sperm cell)
• The maternal mRNA directs protein synthesis early
  on in embryonic development
• 3. The DNA only has to be concerned about
  replicating, not transcribing, and rapid
  replication occurs (mitotic divisions)
• 4. Egg regions
• a) animal pole area of the egg that
  contains less yolk, more mitochondria, more
  ribosomes and is more metabolically active

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b. vegetal pole contains more yolk (food) and
is less metabolically active
E. Fourth Step Cleavage and Egg Types
1. Definitions

• cleavage cell divisions (the result of
  cytokinesis)
• blastomeres the cells resulting from cell
  divisions

c) Yolk lipids, glycogen, proteins,
carbohydrates and other organic compounds used
for food
d) Synchrony term used for simultaneous
division of cells
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2. Cleavage patterns
a) Holoblastic cleavage division of a cell that
divides it into two equal daughter cells,
uniformly across the whole ball of cells
b) Meroblastic cleavage egg cells with a lot of
yolk divide only at one end (not equal)
i) examples include bird eggs and reptilian eggs
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c) spiral cleavage cleavage patterns occur at
obtuse angles to one another i) this type
is typical to lower invertebrates (determinant
development) d) radial cleavage cleavage
patterns oriented directly over other cells at
right angles to each other i) typical of
highest invertebrates and chordates
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• Determinant development zygotes, even by the
  four cell stages, have become specialized (lower
  invertebrates)
•     f) Indeterminant development cells become
  specialized much later in the gastrula stage
• i) unspecialized cells are called
  totipotential cells or undifferentiated cells
• ii) undifferentiated cells are also
  called stem cells

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3. Embryo development as a result of cleavages

• The one cell (zygote) divides by mitosis to
  become two cells, two cells divide to become
  four, four to eight, and so forth
• Soon a hollow fluid filled ball of cells forms
  called the blastula (the cavity within is called
  the blastocoel)

c) Some cells on the blastula begin to grow
inward (invaginate) which forms a double walled
structure called the gastrula (process is called
gastrulation) d) The opening created by this
invagination of cells is called the blastopore
and the new cavity is called the archenteron e)
The outer layer of cells will become the ectoderm
and the inner layer of cells will become the
endoderm
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VI. Primary Germ Layers beginnings of
differentiation formed during gastrulation
A. There are three primary germ layers
1.Ectoderm (outer tissue)- origins of the
following tissues

• nervous tissue including brain
• epidermis, including skin, nails, hair, fur,
  teeth, etc.
• sensory organs associated with the skin

2. Endoderm (inner tissue) origins of the
following tissues
a) gut tract lining
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b) respiratory tract lining c) digestive glands
including pancreas and liver
3. Mesoderm (middle tissue) -origins of the
following tissues

• connective tissues (bones, cartilage, blood,
  tendons, ligaments)
• b) muscle tissue (smooth, striated, and
  cardiac)
• c) notochord in chordates
• d) dermis (inner layer of skin)
• e) outer covering of internal organs
• f) excretory organs
• g) reproductive organs

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Patterns of Development

• Protostomes
• Blastopore Mouth
• Spiral-Determinate Cleavage
• Ectodermal Skeleton

• Deuterostomes
• Blastopore Anus
• Radial-Indeterminate Cleavage
• Mesodermal Skeleton

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Tissue Organization

• Acellular
• Single-celled, No tissues
• Diploblastic
• Body parts organized into layers that are derived
  embryologically from two tissue layers, Ectoderm
  and endoderm
• Triploblastic
• Body parts organized into layers that are derived
  embryologically from three tissue layers,
  Ectoderm, mesoderm, endoderm

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VII. Vertebrate Embryology
A. Chordate characteristics during some stage
of the development or life of a chordate, it has

• a hollow, dorsal nerve cord
• a stiff, but flexible, rod-like structure used as
  support in their back called a notochord
• post-anal tail
• pharyngeal gills slits

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B. Formation of a hollow tube (the neural tube)
in the back of vertebrates occurs after
gastrulationthis process is called neurulation.
It occurs by the invagination of the ectoderm
C. Avian and Reptilian Embrylogy

• Both of these groups undergo meroblastic
  development
• The egg in these groups is the true egg plus
  several protective membranes

• vitelline membrane surrounds and protects the
  true egg
• albumin egg white nutritional and consumed
  along with yolk during development

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• shell composed of calcium carbonate and is
  porous for the exchange of gases
• yolk food reserve
• amniotic sac encloses developing embryo in a
  fluid for protection
• chorion sac found near the shell and aids in
  gas exchange
• allantois waste sac, helps to accumulate uric
  acid wastes and break those toxic compounds into
  non-harmful products