16.1 Embryonic and Fetal Development

4 min read•august 28, 2024

Embryonic and fetal development is a fascinating journey from a single cell to a complex human being. It's a process of rapid growth, intricate organization, and precise timing that transforms a zygote into a baby ready for life outside the womb.

This topic explores the key stages of development, from fertilization to birth. We'll look at how organs form, how the placenta supports growth, and the major milestones along the way. It's crucial for understanding human reproduction and early life.

Fertilization and Zygote Formation

The Fusion of Sperm and Egg

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  • Fertilization is the fusion of a sperm cell with an egg cell resulting in the formation of a diploid zygote
  • The acrosome reaction is a series of biochemical events that occur when the sperm cell reaches the zona pellucida of the egg allowing the sperm to penetrate the egg's outer layers
  • Cortical granules are released by the egg upon fertilization preventing polyspermy by altering the egg's zona pellucida and plasma membrane

Early Development of the Zygote

  • The zygote undergoes a series of rapid mitotic divisions called cleavage forming a solid ball of cells known as the morula
  • The morula develops into a hollow ball of cells called the blastocyst which implants into the uterine wall initiating the next stage of embryonic development
  • The inner cell mass of the blastocyst gives rise to the embryo while the outer layer of cells, the trophoblast, contributes to the formation of the placenta
  • The process of implantation involves the blastocyst attaching to and invading the uterine lining (endometrium) establishing a connection between the embryo and the mother's blood supply

Stages of Embryonic Development

Cleavage and Gastrulation

  • Cleavage is the rapid series of mitotic cell divisions that occur after fertilization resulting in the formation of the morula and blastocyst
  • Gastrulation is the process by which the blastocyst differentiates into the three primary germ layers: ectoderm, mesoderm, and endoderm with each germ layer giving rise to specific tissues and organs
    • The ectoderm forms the nervous system, sensory organs (eyes, ears), skin, hair, and nails
    • The mesoderm develops into the skeletal system, muscular system, cardiovascular system, and reproductive organs
    • The endoderm gives rise to the digestive system, respiratory system, and various glands (thyroid, liver, pancreas)

Neurulation and Organogenesis

  • Neurulation is the formation of the neural tube from the ectoderm which eventually develops into the brain and spinal cord
  • The neural crest cells, derived from the ectoderm, migrate and contribute to the formation of various structures (cranial nerves, pigment cells, adrenal medulla)
  • Organogenesis is the process by which the three germ layers differentiate and develop into the various organs and tissues of the embryo involving complex signaling pathways and gene expression patterns that guide cell fate and morphogenesis
  • The development of the heart begins with the formation of the primitive heart tube from the mesoderm which then undergoes looping and septation to form the four-chambered heart

Fetal Development Milestones

First Trimester (Weeks 1-12)

  • Rapid cell division and differentiation occur forming the major organ systems
  • The heart begins to beat around week 4, and the neural tube closes by week 6
  • Limb buds develop, and the face starts to take shape with the formation of the eyes, nose, and mouth
  • By the end of the first trimester, the fetus is approximately 3 inches long and weighs about 1 ounce with visible fingers and toes

Second and Third Trimesters (Weeks 13-40)

  • Second trimester (weeks 13-27):
    • The fetus grows rapidly, and the organs continue to develop and mature
    • The skeletal system begins to ossify, and the fetus starts to move with detectable kicks and movements
    • The skin develops layers, and fine hair (lanugo) and nails begin to grow
    • By the end of the second trimester, the fetus is approximately 14 inches long and weighs about 2 pounds
  • Third trimester (weeks 28-40):
    • The fetus continues to grow and gain weight, accumulating fat stores necessary for thermoregulation after birth
    • The lungs mature, producing surfactant to prepare for breathing outside the womb
    • The brain undergoes rapid development, forming complex neural connections essential for cognitive function
    • By the end of the third trimester, the fetus is approximately 20 inches long and weighs between 6 and 9 pounds, ready for life outside the womb

Placenta's Role in Fetal Development

Structure and Function of the Placenta

  • The placenta is a temporary organ that develops during pregnancy connecting the fetus to the mother's uterine wall
  • It is formed from both fetal and maternal tissues with the fetal portion derived from the chorion and the maternal portion from the endometrium
  • The placenta facilitates the exchange of nutrients, oxygen, and waste products between the maternal and fetal blood supplies through the chorionic villi
    • Nutrients and oxygen from the mother's blood diffuse across the placental barrier into the fetal blood while waste products and carbon dioxide diffuse from the fetal blood into the maternal blood

Endocrine Function and Placental Barrier

  • The placenta also produces hormones such as human chorionic gonadotropin (hCG), progesterone, and estrogen which help maintain the pregnancy and support fetal development
    • hCG is produced early in pregnancy and is the hormone detected by most pregnancy tests
    • Progesterone and estrogen help maintain the uterine lining, prevent uterine contractions, and stimulate breast development in preparation for lactation
  • The placental barrier protects the fetus from certain infections and harmful substances although some viruses (rubella, cytomegalovirus), drugs (alcohol, nicotine), and environmental toxins (mercury, lead) can cross the barrier and potentially harm the developing fetus
  • Placental abnormalities such as placental insufficiency or placental abruption can compromise fetal growth and development highlighting the crucial role of the placenta in ensuring a healthy pregnancy
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