organogenesis

• Organogenesis


  1. Cells of each germ layer proliferate

  2. Migrate

  3. Reaggregate

  4. Differentiate into various tissues that form the organs
• Germ layers in organogenesis

  • Differ by three processes: folds, splits, and condensation
• Internal organs initiate development in humans

  Weeks 3-8 in utero
• Development of organs in humans

  1. Week 3: brain, heart, blood cells, circulatory system, spinal cord, digestive system

  2. Week 4: bones, facial structures, limbs, heart, brain, nervous tissue

  3. Week 5: eyes, nose, kidneys, lungs, heart, brain, nervous tissue, digestive tract

  4. Week 6: hands, feet, digits, brain, heart, circulatory system

  5. Week 7: hair follicles, nipples, eyelids, sex organs, kidney, brain waves

  6. Week 8: Facial features, internal organs, brain, heart, external sex organs
• By the end of the embryonic stage all essential external and internal structures have been formed, the embryo is now referred to as fetus

  
• Critical period of development


  Specific time during which the environment has its greatest impact on an individual's development
• Critical period example


  • Fetal alcohol syndrome
• Why critical period is critical

  • It will disturb the process of: cell division, apoptosis, gene expression, cellular metabolism
• Apoptosis


  The process of programmed cell death, used during early development to eliminate unwanted cells and in adults to rid the body of damaged cells and prevent cancer
• Effects of maternal drug use on fetal development

  1. Pre-embryonic stage: highly resistant to birth defects, but high dose teratogens could result in deaths, stopping teratogens before embryonic stage could result in normal development

  2. Embryonic stage (weeks 3-8): increasingly detrimental, can result in miscarriage and structural abnormalities

  3. Fetal stage (week 9 onwards): can result in improper organ functioning and delayed growth, but seldom birth defects
• Nervous system development


  Begins as a simple, straight neural tube that develops into the brain and spinal cord
• Nervous system development


  1. Fertilized egg divides to form cells that belong to 3 germ layers: endoderm, mesoderm, ectoderm

  2. Ectoderm differentiates into neuroectoderm, which forms neuroepithelium and neural plate

  3. Neural plate buckles and folds inward to form neural groove and neural folds

  4. Neural folds converge to form neural tube, with neural crest cells separating to form parts of peripheral nervous system

  5. Anterior end of neural tube develops into brain, posterior into spinal cord

  6. Brain develops from 3 primary vesicles into 5 secondary vesicles that form different brain structures
• Embryonic development establishes a framework on which more complex nervous system structures can be built
• Embryonic development


  Helps in understanding the structure of the adult brain by establishing a framework on which more complex structures can be built
• Establishment of the neuraxis

  1. Neural tube establishes the anterior-posterior dimension of the nervous system

  2. Embryonic nervous system in mammals has a standard arrangement

  3. Humans and other primates complicate this by standing up and walking on two legs

  4. Anterior-posterior dimension of the neuraxis overlays the superior-inferior dimension of the body

  5. There is a major curve between the brainstem and forebrain called the cephalic flexure

  6. Neuraxis starts in an inferior position (end of spinal cord) and ends in an anterior position (front of cerebrum)
• Primary vesicles


  • Establish the basic regions of the nervous system: forebrain, midbrain, and hindbrain
  • These divisions are useful but not equivalent regions
• Secondary vesicles


  Establish the major regions of the adult nervous system
• Telencephalon


  The cerebrum, which is the major portion of the human brain
• Diencephalon


  The region between the cerebrum and the rest of the nervous system, through which all projections have to pass between the cerebrum and everything else
• Brainstem


  Includes the midbrain, pons, and medulla
• Cerebellum


  A large portion of the brain, considered a separate region
• The retina, which began as part of the diencephalon, is primarily connected to the diencephalon
• The eyes are just inferior to the anterior-most part of the cerebrum, but the optic nerve extends back to the thalamus as the optic tract, with branches into a region of the hypothalamus
• There is also a connection of the optic tract to the midbrain, but the mesencephalon is adjacent to the diencephalon, so that is not difficult to imagine
• The cerebellum originates out of the metencephalon, and its largest white matter connection is to the pons, also from the metencephalon
• There are connections between the cerebellum and both the medulla and midbrain, which are adjacent structures in the secondary vesicle stage of development
• In the adult brain, the cerebellum seems close to the cerebrum, but there is no direct connection between them
• Ventricles


  Open spaces within the CNS where cerebrospinal fluid circulates, remnants of the hollow center of the neural tube
• Skin development


  1. Ectoderm forms at 4 weeks

  2. Between 4 and 12 weeks, stratified epithelium forms and mesoderm forms blood vessels and connective tissue

  3. At 16 weeks, basement membrane folds and melanoblasts start producing melanin

  4. At 20 weeks, hair begins to grow from sebaceous glands and sweat glands form
• Sebaceous gland


  A gland of the skin that secretes an oily substance, sebum, usually into a hair follicle near the surface of the skin
• Melanoblast


  A transient, multipotent, migratory cell population that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons, and glia
• Integumentary system


  Formed from ectoderm, mesoderm, and neural crest cells, the largest organ system in the human body responsible for protection from physical and environmental factors
• Components of the integumentary system

  • Skin (the largest bodily organ)
  • Appendages
  • Sweat and sebaceous glands
  • Hair
  • Nails
  • Arrectores pillorum (tiny muscles at the root of each hair that cause goose bumps)
• Intramembranous ossification


  1. Replacement of sheetlike connective tissue membranes with bony tissue, forming intramembranous bones like certain flat bones of the skull and some irregular bones

  2. Osteoblasts migrate to the membranes and deposit bony matrix around themselves, becoming osteocytes when surrounded by matrix
• Endochondral ossification


  1. Replacement of hyaline cartilage with bony tissue, forming most of the bones of the skeleton

  2. Future bones first formed as hyaline cartilage models

  3. Perichondrium becomes infiltrated with blood vessels and osteoblasts, changing into periosteum

  4. Osteoblasts form a collar of compact bone around the diaphysis

  5. Cartilage in the center of the diaphysis disintegrates, replaced by spongy bone forming a primary ossification center

  6. Osteoclasts break down newly formed bone to open up the medullary cavity

  7. Cartilage in the epiphyses continues to grow, increasing bone length

  8. Secondary ossification centers form in the epiphyses, retaining spongy bone
• Bone growth at the epiphyseal plate

  1. Cartilage in the region next to the epiphysis continues to grow by mitosis

  2. Chondrocytes in the region next to the diaphysis age and degenerate

  3. Osteoblasts move in and ossify the matrix to form bone

  4. This process continues until cartilage growth slows and stops, epiphyseal plate ossifies into an epiphyseal line
• Appositional growth


  Increase in bone diameter throughout life in response to stress from increased muscle activity or weight, by osteoblasts forming compact bone around the external bone surface and osteoclasts breaking down bone on the internal surface around the medullary cavity