Embryonic Period: Prenatal Development
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Questions and Answers

During gastrulation, which germ layer replaces the hypoblast?

  • Paraxial Mesoderm
  • Ectoderm
  • Mesoderm
  • Endoderm (correct)

Which of the following structures induces neurulation in the overlying ectoderm?

  • Intermediate mesoderm
  • Somites
  • Neural crest cells
  • Notochord (correct)

What is the origin of sensory nerve cells, ganglia, and melanocytes?

  • Lateral mesoderm
  • Intermediate mesoderm
  • Neural crest cells (correct)
  • Somites

Somites are formed from which type of mesoderm?

<p>Paraxial mesoderm (A)</p> Signup and view all the answers

The urogenital system develops from which embryonic structure?

<p>Intermediate mesoderm (A)</p> Signup and view all the answers

During which week of development does lateral folding occur, leading to the formation of the future gut?

<p>Week 4 (C)</p> Signup and view all the answers

By the end of the second month (8 weeks) of development, what significant event has occurred?

<p>All major organs have appeared. (C)</p> Signup and view all the answers

During which period of prenatal development do organs primarily grow in size and complexity?

<p>Fetal period (A)</p> Signup and view all the answers

In what month of fetal development do ossification centers begin to appear in bones?

<p>Third month (B)</p> Signup and view all the answers

During which months of fetal development does the fetus begin to resemble a human, with the formation of fingers, eyes, and ears?

<p>3rd and 4th (D)</p> Signup and view all the answers

During which prenatal period do the organs form from the three primary germ layers?

<p>The first 8 weeks of the embryonic period. (B)</p> Signup and view all the answers

What key event characterizes cleavage during the first week of embryonic development?

<p>Cell division that decreases the size of individual cells, with no overall growth of the zygote. (B)</p> Signup and view all the answers

Why is implantation delayed until approximately day 6 after fertilization?

<p>To allow time for the developing embryo to reach the uterus and for the endometrium to become receptive. (A)</p> Signup and view all the answers

If a fertilized egg implants in the fallopian tube and begins to develop, why is this dangerous?

<p>The Fallopian tube cannot support the growth of the embryo and may rupture, causing life-threatening complications. (D)</p> Signup and view all the answers

What is the primary reason for the maternal body to calcify around a lithopedion (stone baby)?

<p>To isolate foreign genetic material from the mother's system. (C)</p> Signup and view all the answers

During week 2 of embryonic development, the inner cell mass differentiates into which two layers?

<p>Epiblast and hypoblast. (D)</p> Signup and view all the answers

Which of the following is derived from the epiblast?

<p>The amnion. (C)</p> Signup and view all the answers

What is the primary function of the amniotic fluid?

<p>To cushion and protect the embryo. (A)</p> Signup and view all the answers

What is the primary role of the yolk sac in human embryonic development?

<p>To be incorporated as a small part of the digestive tube. (C)</p> Signup and view all the answers

Which structure gives rise to the extraembryonic membranes?

<p>The trophoblast. (D)</p> Signup and view all the answers

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Flashcards

Embryology

The study of the origin and development of single individuals.

Prenatal period

The time from conception to birth, including embryonic and fetal periods.

Embryonic period

First 8 weeks post-fertilization where major organ systems form.

Fetal period

The remaining approximately 30 weeks of development after the embryonic period.

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Zygote

The fertilized oocyte, which is the first cell after conception.

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Blastomeres

Daughter cells formed from the zygote through cleavage.

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Morula

A cluster of 12-16 blastomeres that forms before creating a blastocyst.

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Blastocyst

A fluid-filled structure containing ~60 cells; consists of trophoblast and inner cell mass.

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Bilaminar embryonic disk

Structure formed in week 2, dividing the inner cell mass into two layers: epiblast and hypoblast.

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Implantation

The process of the blastocyst embedding itself into the endometrium of the uterus, occurring on DAY 6.

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Primitive streak

A structure that forms during gastrulation indicating future body axis.

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Gastrula

An early embryonic stage formed after the gastrulation process.

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Three germ layers

Endoderm, mesoderm, and ectoderm formed during gastrulation.

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Endoderm

The innermost germ layer that replaces the hypoblast.

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Mesoderm

The middle germ layer that forms between ectoderm and endoderm.

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Ectoderm

The outermost germ layer that forms skin and nervous system.

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Notochord

A rod-like structure from mesoderm, important for development.

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Neurulation

Process where ectoderm forms the neural tube, leading to the nervous system.

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Somites

Segments of mesoderm that develop into vertebrae and muscles.

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Study Notes

Prenatal Development

  • Embryology studies the origin and development of individuals
  • Prenatal period encompasses the embryonic and fetal periods
  • Embryonic period: first 8 weeks after fertilization (in utero)
  • Fetal period: remaining ~30 weeks after fertilization (in utero)
  • No direct connection exists between the oviduct and ovary
  • Fertilization: sperm meets and fuses with the egg (oocyte)
  • Zygote: the fertilized egg (single cell)
  • Early development includes: 4-cell stage, morula, blastocyst, implanting blastocyst

Embryonic Period (First 8 Weeks)

  • Major events:
    • Organs form from three primary germ layers
    • Basic body plan emerges
  • Conception: occurs in the lateral third of the uterine tube
  • Zygote: fertilized oocyte (one cell)
  • Moves towards the uterus via cleavage (cell division without growth)
  • Blastomeres: daughter cells produced from zygote
  • Morula: a cluster of 12-16 blastomeres
  • Blastocyst: fluid-filled structure containing approximately 60 cells.
    • Trophoblast: outer layer of cells (becomes extraembryonic membranes)
    • Inner cell mass: inner clump of cells (becomes the developing organism)
  • Implantation: into the endometrium of the uterus occurs on day 6

Week 1 Events

  • Zygote formation, cleavage to blastomeres, morula to blastocyst.
  • No growth during cleavage, blastocyst growth to implantation
  • Blastocyst implants in uterine lining by day 7
  • Implantation occurs in uterine wall day 6
  • Ectopic pregnancy: fertilized egg implanting outside the uterine wall (dangerous)
  • Stone baby: rare viable ectopic pregnancy (developed to 32 weeks)

Week 2 Events

  • Blastocyst forms bilaminar embryonic disk
    • Epiblast (top layer)
    • Hypoblast (bottom layer)
  • Amniotic sac: formed from an outgrowth of the epiblast
  • Yolk sac: formed from the hypoblast
  • Yolk sac is crucial in other animals but placenta mammals do not need it

Week 3 Events

  • Primitive streak: migration of epiblast cells, forms gastrula.

  • Gastrulation: cells migrate inward forming three layers: ectoderm, mesoderm, and endoderm

  • Notochord (from mesoderm)

    • Forms the basis of the vertebral column, important signaling structure
  • Neurulation: ectoderm forms the neural tube (future brain and spinal cord)

    • Neural plate: thickened ectoderm
    • Neural folds: inward movement creates a groove
    • Neural tube: hollow tube
  • Somites (paraxial mesoderm): form body segments; vertebrae, ribs, muscles.

  • Intermediate mesoderm: forms urogenital system

Week 4 Events

  • Lateral folding: forms the gut tube
  • Yolk sac pinched off

Major Layers of the Embryo

  • Ectoderm: epidermis, hair, nails, glands of skin; brain, spinal cord, nervous system.
  • Mesoderm: nucleus pulposus, kidneys, gonads, skeleton, muscles, circulatory system
  • Endoderm: lining of digestive and respiratory tracts

Week 5 Events

  • Head accounts for 50% of body length
  • Limbs form as rudimentary appendages
  • Placenta fully functional

Fetal Period

  • Starts at week 9 post-fertilization until birth

  • Organs grow in size and complexity

  • 3rd month: head growth slows and body increases in length

  • Bone ossification begins

  • Sex can be determined by the end of the third month

  • 5th to 7th months: Human-like features appear

  • 8th and 9th month: Fetal preparation for birth

  • Mother’s perception of fetal movement: begins in the 3rd or 4th month.

  • Fetal viability: 22-24 weeks, lungs developed, ability to survive outside the womb.

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Description

The prenatal period includes the embryonic and fetal periods. The embryonic period covers the first 8 weeks after fertilization, during which organs form and the basic body plan emerges. Conception occurs in the uterine tube, forming a zygote that develops into a blastocyst.

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