Biology: Understanding Life 231

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Chapter 23Development Before Birth

• Modern Biology

1. Most of the pictures used in this presentation
are adapted and/or modified from BIOLOGY
Understanding Life Third ed., Sandra Alters,
(2000) Jones and Barelett Publishers. 2. This
presentation is used for the course Modern
Biology only. 3. Please DO NOT distribute this
presentation without authorization.
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23.1 Patterns in Embryological Development

• The genetic control of development is universal
  among different species.
• The eyeless gene can be found in organisms from
  flatworm to human
• The master genetic control for the eye formation
  among organisms in the animal kingdom may have
  evolved from a single, common ancestor.

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• Homeotic genes control development of an organ or
  structure by producing transcriptional activators
  or inhibitors
• Three main processes control the development of
  multicellular organisms
• tissue differentiation
• morphogenesis
• cell division

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Tissue Differentiation

• Groups of cells become distinguished from other
  groups of cells.
• In some invertebrate organisms, tissue
  differentiation occurs very early in the
  development.
• In mammals, cells of very early embryos are
  capable of becoming any type of tissue.

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Cell Division and Morphogenesis

• Cell division is a process resulting in the
  growth of a developing organism.
• Morphogenesis (literally form creation) is a
  process to shape the individual during
  development by cell migration.

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Principal Evolutionary Lines

• Two principal evolutionary lines evolved from
  Coelomates
• Protostomes (first-mouth) ancestral development
  pattern found in mollusks, annelids, arthopods
• Deuterostomes (second-mouth) different branch
  found in echinoderms through vertebrates

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Fertilized egg (zygote) --gt morula (32 cells no
increase in mass) Morula --gt hollow ball of cells
(blastula) Blastula --gt three-layered and
elongated embryo (gastrula) by morphogenetic
movements
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• The morula of protosome forms though the cleavage
  of fertilized egg in a spiral pattern. The
  blastopore (first indentation) becomes mouth of
  the future organism.
• The morula of deuterostome forms though the
  cleavage of fertilized egg in a radial pattern
  cleavage. The blastopore becomes anus of the
  future organism.

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The Embryonic Period is Primarily one of
Development

• 23.2 Fertilization
• 23.3 The first and second weeks of development
  The pre-embryo
• 23.4 Early development of the extrembryonic
  membranes
• 23.5 Development from the third to eighth weeks
  The embryo

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Prenatal Development

• During prenatal development, human grows
  gradually and shows progressive changes in its
  morphology until matured enough to leave the
  mothers womb.

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The Egg
In egg-laying animals, the young develop totally
within the egg ? needs yolk to supply the
nutrient In human, the ovum contains a great deal
of cytoplasm but no yolk ? receives nourishment
from mother after implantation.
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23.2 Fertilization

• Sperm swims through vagina, cervix, uterus to
  uterine tube where it fertilizes egg

The traveling of sperms is also facilitated by
the mucin threads produced by female. Only one in
2 million sperm can make it to the egg.
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• The secondary oocyte released from ovary is not a
  matured ovum until a sperm gets into it and
  completes the second meiotic cycle.
• Once sperm meet egg, the enzymes at its tip allow
  it to penetrate the outer protective layer and
  membrane of the egg.
• Egg allows only one sperm to enter and blocks
  others.

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The Fusion of Nuclei
The union of a male gamete and a female gamete is
called fertilization or conception. Egg and sperm
nuclei fuse and form zygote.
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23.3 The First and Second Weeks of Development
The Preembryo

• Rapid division of zygote without growth forms
  morula (solid ball of cells)
• Radial cleavage pattern in humans

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Twins?

• Genetic identical twins a single. Fertilized egg
  spits into two cell clusters during cleavage, and
  each cluster continues developing on its own.
• Fraternal twins two secondary oocytes are
  released and fertilized by two separated sperm
  cells.

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The Journey of Preembryo

• The free floating preembryo or morula starts its
  3 days journey from uterine tube to uterus.
• If, occasionally, the preembryo get caught and
  implanted in the folded inner lining of the
  uterine tube, an ectopic pregnancy occurs.

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The Blastocyst
Once morula reaches uterus, it develops into
blastocyst, a hollow ball of cells (see below),in
two days.
Blastocyst consists of two layers tropho-blasts
and inner cell mass.
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• The inner cell mass of blastocyst destined to
  differentiate into various cell body tissues of
  the new individual.
• The trophoblast, the outer ring of blastocyst,
  give rise to most of the extraembryonic
  membranes, including much of placenta.

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Blastocyst Implants in Uterine Wall
About 7 to 8 days after fertilization, the
blastocyst imbeds itself into the wall of the
uterus.
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• Implanted balstocyst starts secreting a hormone
  called human chorionic gonadotropin (hCG), which
  maintains the structure of corpus luteum and
  allows it to keep releasing progesterone and
  estrogen.
• After the first three months development, the
  placenta begins to secrete the estrogen and
  progesterone that maintain the pregnancy.

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Three Primary Germ Layers

• Division and migration of inner cell mass causes
  morphogenetic changes to form three-layered
  gastrula
• ectoderm, endoderm, and mesoderm form various
  organs of embryo

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23.4 Early Development of the Extrembryonic
Membranes

• Extraembryonic membranes are structures which
  provide nourishment and protection for embryo
• Form from trophoblast (ring of cells surrounding
  inner cell mass of blastocyst) not part of
  embryo body
• Inner cell mass gives rise to pre-embryo itself

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Extraembryonic Membranes

• Amnion
• Chorion and chorionic villi
• Placenta
• Umbilical cord
• Allantois
• Yolk sac

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Amnion

• Amnion is a thin, protective membrane that
  develops during the third and forth weeks and
  encloses the embryo in a membrane sac.
• Amnion acts as a shock absorber while amniotic
  fluid helps to maintain the temperature of
  embryonic and fetal environment constant.

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Chorion

• Chorion is highly specialized to facilitate the
  transfer of nutrients, gases, and waste between
  the embryo and the mothers body.
• Chorion is the primary part of placneta, a flat
  disk of tissue about the size of a large, thick
  pancake that grows into uterine wall.

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Umbilical Cord

• Umbilical cord, containing two umbilical arteries
  and a single vein, is the developing embryos
  lifeline to the mother.
• It join the circulatory system of the embryo with
  the placenta.
• The umbilical cord develops from the body stalk,
  the yolk sac, and the allantois during the forth
  week of development.

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Yolk Sac

• Yolk sac is a structure established during the
  end of the second week, also becomes a part of
  the umbilical cord.
• The yolk sac produces blood for the embryo until
  its liver becomes functional during the sixth
  week of development.

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23.5 Development from the Third to Eighth Weeks
The Embryo

• It is a critical period for embryo formation by
  morphogenesis and for the development of major
  organ systems (internal organs, limbs) through a
  process called organogeneis.

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• Development from third to eighth weeks is the
  most sensitive period for embryos response to
  teratogens, such as alcohol, pollutants, and
  drugs, that can induce malformation in the
  rapidly developing tissues and organs.

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Human Gastrulation
Beginning of the third week, various cell groups
of the inner cell mass begin to divide, move and
differentiated, changing the two-layered
preembryo into a three-layered embryo.
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• At the beginning of the third week. the primitive
  streak run down the midline what will be the back
  side of the embryo
• The cells at the streak migrate inward and
  generate the mesoderm that develops during the
  gastrulation period.

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Formation of Notochord

• Cells at the head end of primitive streak grow
  forward and form the notochord.
• Notochord is a structure that forms the midline
  axis along which the vertebral column develops.

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Neurulation

• Neurulation is the development of a hollow nerve
  cord, which later develops into brain, spinal
  cord, and related structures such as eyes.
• Neurulation begins in the third week with
  indentation of the ectoderm that forms the neural
  groove.
• The either side of the groove are areas of tissue
  called neural folds.

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Neurula

• The neural fold of 3-week embryo have come
  together at one spot and have fused.
• Neurulation results in an embryo called neurula.

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Development of Cardiovascular System

• The embryo heart also begins to develop as a pair
  of heart tubes and provides a primitive
  circulation of blood.

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Human Embryo at 4.5 Weeks

• The paired somites can be seen in the embryo of
  41/2 weeks old.

Somites will give rise ot most of axis skeleton
with its associated muscles and most of the
dermis.
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• Fourth week head and tail end umbilicus
  branchial arches brain, eyes

Four gill arches (or branchial arches) evolved on
either side of head will develop into structures
such as middle ear, eustachian tube, tonsils,
thymus, and parathyroids.
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Fifth-eighth weeks

• In this period, embryo grows rapidly from 4 mm to
  30 mm and develops many organs, limbs, etc.

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Fifth Week

• During the fifth week, a nose begins to take
  shape as tiny pits.
• The limb buds (first arms and then the legs) are
  visible at fifth week.
• The brain grow rapidly this week, resulting in an
  embryo with a large head in proportion to the
  rest of the developing body.

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Sixth Week

• The fingers and ears are beginning to form.
• The retina of the eyes is now darkly pigmented.
• The heart beats 150 times per minute.

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Seventh Week

• Eyelids begin to partially cover the eyes, and
  ears developed more fully.
• Each arm develops an elbow.
• The legs gradually develop ankles and toes.

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23.6 Development from the ninth to thirty-eighth
weeks The fetus

• During the embryonic period, most of the body
  systems have been developed and functional.
• The job of the fetal period is the refinement,
  maturation, and growth of organ systems and of
  the body form of the developing individual.

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Fetus at Third Month

• The shape of face changes the eyes and eyelids
  are well developed the male or female sex organs
  appear
• Fetus grows rapidly to 85 mm long.

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Fourth through Sixth Months

• During the second trimester, the fetus grows to
  about 0.6 Kg and 0.3 m
• active movement inside the mother and sucks thumb
• audible heartbeat
• sensory organs developed completely

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Ossification

• The ossification, a process of bond formation,
  began at approximately 8 weeks of development and
  continue beyond birth to the age of 18 or 19
  years.

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Seventh through ninth months

• Predominantly growth rather than development
  major weight increases, including fat deposits
• Growth and development of central nervous system,
  including brain cells and nerve tracts
• By end of third trimester, fetus can survival
  independently

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Placental-fetal Circulation

• Fetus receives all its nutrients from its mother
  via the umbilical vein. Wastes are removed
  through two umbilical arteries.

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Hormonal changes in the fetus trigger the birth
process

• 23.7 Birth
• 23.8 Physiological adjustments of the newborn

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23.7 Birth

• Labor initiation
• changes in fetal hormone levels
• placenta makes prostaglandins for uterine
  contraction
• pituitary makes oxytocin for uterine contractions
• Initially only a few contraction occur each hour,
  later increasing to one every 2 to 3 minutes.

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• Birth
• positive feedback loop for oxytocin production
  ends with birth of newborn
• continual contractions expel fetus through birth
  canal (vagina) to outside
• After birth, uterine contractions continue and
  expel the placenta and associated membrane called
  after-birth.

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23.8 Physiological adjustments of the newborn

• Circulatory changes at birth
• lungs inflate with first breath
• oxygen enters baby via lungs, not umbilical cord
  and placenta
• Foramen ovale and ductus arteriosus close and
  seal allowing access of blood to lungs