Embryonic development is a fascinating journey from a single cell to a complex organism. It starts with fertilization and progresses through stages like cleavage, blastocyst formation, and implantation. These early steps lay the foundation for the entire human body.

As development continues, three germ layers form, giving rise to all body tissues and organs. The placenta develops, supporting the growing embryo. Organogenesis then shapes major body systems, setting the stage for fetal growth and eventual birth.

Stages of Embryonic Development

Stages of pre-implantation development

• Fertilization occurs when a sperm penetrates the ovum, forming a zygote (fertilized egg)
• Cleavage involves rapid mitotic divisions of the zygote, resulting in a solid ball of cells called a morula (resembling a mulberry)
• Blastocyst formation involves the development of the inner cell mass (embryoblast) which will give rise to the embryo, and the trophoblast which will form the placenta and other extraembryonic membranes
  • A fluid-filled cavity called the blastocoel forms within the blastocyst

Process and significance of implantation

• The blastocyst attaches to the uterine wall (endometrium) around 6-7 days after fertilization, a process known as implantation
• Trophoblast cells secrete enzymes that enable the blastocyst to invade and embed into the endometrium
• Implantation establishes a connection between the embryo and the mother's blood supply, allowing for the exchange of nutrients, gases (oxygen and carbon dioxide), and waste products
• Implantation triggers hormonal changes (increased progesterone and estrogen) that help maintain the pregnancy by preventing menstruation and preparing the uterus for embryonic development

Embryonic Membranes and Germ Layers

Functions of embryonic membranes

• Amnion is a fluid-filled sac that surrounds and protects the developing embryo, cushioning it against mechanical injury (acts as a shock absorber)
• Yolk sac serves as a primitive circulatory system and is the site of early blood cell formation (hematopoiesis), and contributes to the formation of the gut
• Allantois contributes to the formation of the urinary bladder and forms the umbilical cord along with the yolk sac, connecting the embryo to the placenta
• Chorion is the outermost extraembryonic membrane that contributes to the formation of the placenta, facilitating maternal-fetal exchange

Gastrulation and germ layer formation

• Gastrulation begins around 14-16 days after fertilization and involves the formation of the primitive streak on the epiblast (the upper layer of the embryonic disc)
• Epiblast cells migrate through the primitive streak and differentiate into the three primary germ layers:
  1. Ectoderm gives rise to the nervous system (brain, spinal cord), epidermis (skin), and various glands
  2. Mesoderm gives rise to the musculoskeletal system (bones, muscles), circulatory system (heart, blood vessels), and connective tissues
  3. Endoderm gives rise to the digestive system (liver, pancreas), respiratory system (lungs), and several glands (thyroid, thymus)
• The formation of germ layers is a crucial step in differentiation, where cells begin to specialize into specific tissue types

Placenta Formation and Organogenesis

Placenta formation and functions

• The placenta develops from the chorion (fetal tissue) and the maternal uterine tissue called the decidua
• Chorionic villi extend into the decidua, forming the fetal portion of the placenta, which is rich in blood vessels
• The placenta performs critical functions during pregnancy:
  • Facilitates the exchange of nutrients, gases, and waste products between maternal and fetal blood (acts as a selective barrier)
  • Produces hormones such as human chorionic gonadotropin (maintains corpus luteum), progesterone, and estrogen (support pregnancy)
  • Provides immune protection for the fetus by preventing the entry of maternal immune cells that could attack the fetus as a foreign body

Embryonic shape transformation

• Neurulation is the process by which the neural tube (precursor to the brain and spinal cord) forms from the ectoderm
• Body folding involves a series of folding events that transform the flat embryonic disc into a three-dimensional shape:
  1. Lateral folding brings the edges of the embryo together, forming a cylindrical shape
  2. Cephalocaudal folding (head-to-tail) establishes the head and buttocks regions
• Somite formation involves the development of blocks of mesoderm that give rise to the vertebrae, ribs, and skeletal muscles, contributing to the segmented appearance of the embryo

Key events in organogenesis

• Organogenesis is the development of specific organs from the three primary germ layers (ectoderm, mesoderm, and endoderm)
• Ectoderm-derived organs:
  • Nervous system structures such as the brain, spinal cord, and peripheral nerves
  • Epidermis (outer layer of skin), hair, and nails
  • Sensory receptors (eyes, ears, nose)
• Mesoderm-derived organs:
  • Musculoskeletal system components like bones, cartilage, and muscles
  • Circulatory system structures including the heart and blood vessels
  • Urogenital system organs such as the kidneys and gonads (ovaries, testes)
• Endoderm-derived organs:
  • Digestive system structures including the liver, pancreas, and gastrointestinal tract (esophagus, stomach, intestines)
  • Respiratory system components such as the lungs and trachea (windpipe)
  • Glands like the thyroid (regulates metabolism) and thymus (plays a role in immune system development)

Fetal Development

• After the eighth week of development, the embryo is referred to as a fetus
• The fetal period is characterized by rapid growth, continued organ development, and the refinement of body systems
• During this stage, the fetus begins to move, and external genitalia become distinguishable