Embryonic Development

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Embryonic Development

• By Lauren and Jocelyn

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Fertilization

• The first step in embryonic development is
  fertilization (the joining of male and female
  gametes)
• Fertilization has three parts
• Penetration
• Activation
• Nuclei fusion

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Penetration

• The sperm has swam up the female reproductive
  tract and meets the egg
• In this stage, the sperm begins to tunnel into
  the eggs plasma membrane

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Activation

• Once a sperm penetrates the membrane, changes
  occur allowing no other sperm to enter
• Stimulates second meiotic division making two egg
  nuclei
• Establishes bilateral
• symmetry when the
• cytoplasm moves
• around the entry spot

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Nuclei Fusion

• The sperm nucleus fuses with the haploid egg
  nucleus and forms the diploid nucleus of the
  zygote.

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Cell Cleavage

• The second step in development is the division of
  the zygote into a large number of small cells
• The overall volume of the zygote cytoplasm does
  not increase
• The result is a mass of about 32 cells called a
  morula. Each cell in the morula is called a
  blastomere
• Eventually, a hollow ball of 500 to 2,000 cells
  is formed called the blastula

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Blastula

• Each cell in the blastula is in contact with
  neighboring cells. These interactions determine
  what type of cell it will become.
• Just before implant in the uterus, the blastula
  cells can develop into 210 different types of
  cells in the body (embryonic stem cells)

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Gastrulation

• Gastrulation is when certain groups of cells dent
  inward and roll inward from the surface of the
  blastula
• Determine the basic developmental pattern of the
  embryo
• By the end, the cells of the embryo have arranged
  into three primary germ layers
• Ectoderm
• Mesoderm
• Endoderm

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Gastrulation Cont.

• The ectoderm forms the epidermis, central nervous
  system, sense organs, and neural crest
• The mesoderm forms the skeleton, muscles, blood
  vessels, heart, and gonads
• The endoderm forms the lining of the digestive
  and respiratory tract, liver, and pancreas

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Neural Crest

• It is a small strip of cells formed when the
  neural groove is pinched off. Incorporated into
  the roof of the neural tube
• Differentiation of the neural crest cells depend
  on the location
• For example At the anterior end of the embryo,
  they merge with the anterior portion of the
  brain. This helps in the development of skeletal
  and connective tissues of the head

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Human Development

• Human development, from fertilization to birth
  takes an average of 266 days
• Divided into three periods called trimesters

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First Month

• 30 hours- first cleavage occurs
• 60 hours- second cleavage occurs
• 6-7 days- embryo reaches the uterus
• 2 weeks- the placenta forms, this provides
  nourishment for the embryo. Gastrulation also
  takes place
• 3 weeks- neurulation occurs. By the end of the
  third week, over a dozen somites, which give rise
  to muscles, vertebrae, and connective tissues,
  are present and the blood vessels and gut have
  begun to develop. Embryo is about 2 millimeters
  long
• 4 weeks- organogenesis, which is the formation of
  body organs takes place. Eyes form, 30 somites
  are visible and the arm and leg buds have begun
  to form. Embryo is about 5 millimeters long

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Second Month

• Morphogenesis which is the formation of shape
  takes place
• The miniature limbs of the embryo begin to take
  their adult shape
• The arms, legs, knees, elbows, fingers, and toes
  can all be seen. Also, a short bony tail!!!!
• The embryo has grown to about 25 millimeters and
  weighs about one gram
• The eighth week marks the transition from embryo
  to fetus, all major organs of the body have been
  established

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Third Month

• The nervous system develops
• The arms and legs begin to move
• The embryo begins to show facial expressions and
  can suck

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Second Trimester

• Fourth month- the bones begin to enlarge and the
  mother can feel kicking
• Fifth month- the head and body grow a fine
  covering of hair called lanugo, lost later in
  development
• By the end of the fifth month- rapid heartbeat of
  the fetus can be heard with a stethoscope. It
  has grown to be about 175 millimeters in length
  and is 225 grams
• Sixth month- the baby weighs 600 grams and is
  over a foot long, still cannot survive outside
  the uterus without special medical attention

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Third Trimester

• The weight of the fetus doubles several times
• Most of the nerve tract in the brain as well as
  many new nerve cells are formed
• The brain is not complete though. That has to
  happen outside the uterus because the baby would
  get to be too big

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BIRTH!!

• The uterus releases prostaglandins which begin
  the uterine contractions
• Oxytocin is also released and that works with
  prostaglandins to to further stimulate the
  contractions, forcing the fetus down
• When they start, contractions occur a couple
  times per hour and then they become more frequent
• The placenta is then expelled and the umbilical
  cord, still attached to the baby is cut

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Postnatal Development

• The baby continues growing after birth, at a very
  fast rate
• Babies heads are disproportionate to the rest of
  the body because different parts grow at
  different speeds

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Diseases

• Atrial septal defect- a hole between the left
  and the right atrium in the heart
• Cleft lip deformity- incomplete closures in the
  area between the lip and the mouth
• Anencephaly- part of the brain is absent

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Why is it necessary?

• It is needed to form babies. Without this
  system, there would be no way for fertilization
  to form into a human

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Evolutionary Development

• The primitive chordates gave rise to vertebrates,
  they were initially slow-moving,
  filter-feeding-animals
• The vertebrates became fast-swimming predators
• Also, some fish release sperm into the water
  instead of straight into the female

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Interdependency

• Female and male reproductive system
• Without the sperm and the egg, there would be no
  baby
• The female carries the fetus and provides the egg
• The male provides the sperm