Human Development

• Key components of human development

  • Egg
  • Sperm
  • Fertilization

  
• Sex cells


  Packages of genetic material
• Sperm cell


  • Simple, only role is to get the DNA to the egg
  • Head contains the nucleus (housing the DNA), and the acrosome (a collection of enzymes, important for fertilization)
  • Middle section is rich with large mitochondria (75-100 Mt per sperm cell) to provide energy to the tail to propel the sperm cell
  • Tail is just a simple flagellum

  
• Egg Cell


  • Not made for mobility
  • Huge (egg cell is a single cell that is visible to the human eye) (10,000 times larger than sperm cells)
  • Zona pellucida - thick outer layer of glycoproteins that lines the outside of the plasma membrane of the egg
  • Cytoplasm contains nucleus (housing the DNA), mitochondria (100,000-200,000 Mt per cell – extremely Mt rich), cortical granules (right under the plasma membrane. Contain enzymes that digest the zona pellucida during fertilization), and other organelles

  
• Fertilization


  1. Sperm binding

  2. Acrosome reaction

  3. Cortical reaction

  4. Genetic transfer
• Sperm binding


  Sperm first binds to zona pellucida
• Acrosome reaction


  Enzymes from the acrosome are released into the zona pellucida, digesting it. This allows the sperm to get closer to plasma membrane of egg.
• Cortical reaction


  Enzymes contained in the cortical granules are ejected into zona pellucida to digest the glycoproteins that are required for sperm binding (the 'sperm receptors'). This, along with hardening of the rest of the zona pellucida, prevents additional sperm from binding (and therefore puts a block to polyspermy - multiple sperm penetrating the a single egg cell). If polyspermy does somehow occur (i.e. egg receives DNA from multiple sperm), the zygote will fail.
• Genetic transfer


  Once the acrosome is gone (released all its enzymes) and the cortical granules are done releasing enzymes too, the plasma membranes of the sperm and the egg begin to fuse, and all the genetic material of the sperm gets released into the egg. The genetic material of both the egg and sperm will now fuse, marking the point of fertilization. The sperm mtDNA is also released into egg, but since the egg mtDNA is in such a higher abundance, the sperm mtDNA doesn't have much effect – explains why 'you get your mtDNA from your mother'!

  
• Early embryogenesis


  1. Cleavage

  2. Blastulation

  3. Gastrulation

  4. Neurulation

  
• Cleavage


  Division without growth of the zygote, occurring within the zona pellucida: 1 cell (zygote) -> 2 cells -> 4 cells -> 8 cells -> 16 cells -> 32 cells (morula)
• Morula


  The 32 cells contained within the zona pellucida (following 5 rounds of cleavage). The morula undergoes compaction (cells 'clump' and get packed closer together) and differentiation (outer layer of cells become different). Trophoblasts - cell type of the outer layer. Embryoblast - cell type of the inner layer.
• Blastulation


  Blastocyst forms as a result of the embryoblast cells clustering even more such that there is an inner cell mass and a blastocoel (a hollow cavity). Zona pellucida disintegrates completely. Amniotic cavity forms within the inner cell mass. Further differentiation of the bottom layer of embryoblasts produces hypoblasts, with the cells just above them becoming epiblasts. Bilaminar disk = Epiblasts + Hypoblasts. Primitive streak (migration of epiblast cells) forms on the bilaminar disk, marking the transition into the next phase of embryogenesis (gastrulation).

  
• Gastrulation


  As the epiblast cells migrate (primitive streak), they differentiate further, and the structure and number of layers changes, producing a trilaminar disk: Ectoderm (top), Mesoderm (middle), Endoderm (bottom). These are referred to as the three 'germ layers'.
• Neurulation


  Formation of neural structures through further differentiation of the cells. Mesodermal cells in the core of the mesoderm begin differentiating into a notochord. The notochord induces a change in the cells of the above ectoderm, causing the cells to thicken and become the neural plate. Cells of the neural plate next begin to 'dive into' the mesoderm, forming the neural tube. As this process is occurring, ectodermal cells are breaking off and going into the mesoderm, forming neural crest cells (which will eventually differentiate into distinct tissues).

  
• Implantation


  1. Apposition (blastocyst makes first contact with endometrium)

  2. Adhesion (trophoblasts begin to divide and 'invade' into the endometrial tissue)

  3. Endometrial cells divide and surround the blastocyst

  4. Blood vessels of the endometrium grow and fuse, forming pools of blood

  5. Formation of syncytio-trophoblasts (large, multinucleated cell conglomerations formed from trophoblasts that have fused with each other)

  6. Synciotrophoblasts continue to grow, and grow out as villi towards the fusing blood vessels

  7. Within these villi, little fetal blood vessels begin to form, which are in really close contact with the uterine blood, divided only by a membrane of trophoblasts

  8. This villi-based structure continues to grow with the developing embryo, and more and more nutrients and waste can be transferred

  9. Eventually, this structure will line almost the entire inside of the uterine cavity, forming a structure called the placenta – the organ that connects the developing fetus to the uterine wall to allow nutrient uptake and waste elimination
• Germ layer derivatives


  • Ectoderm: Outer layer of skin and related structures, Nervous system
  • Mesoderm: Inner layers of skin, Bone + all three muscle types, Circulatory system, Kidneys, Gonads
  • Endoderm: GI tract tube and accessory organs, Lungs, Bladder

  
• Mnemonic for germ layer derivatives

  • Ectoderm - top layer. Think: any epithelium that you can touch with your finger, and the nervous system (top – brain). What makes you attractive – looks, skin, brain (intelligence) etc
  • Mesoderm - the meat – muscles, bones, inner layers of skin
  • Endoderm - innermost, anything that has connections to the GI tract in some way but that we can't touch with our finger (e.g. pancreas and liver both feed into SI, lungs and esophagus sprout off of the pharynx, etc.), plus bladder (associate bladder and LI as the same role – storage of waste – derived from same layer) (also think CAVITIES)

  
• Gestation timeline


  1. t=0 - Last menstrual period

  2. t=2 - Fertilization

  3. 2<t<10 - Embryogenesis (cells divide and differentiate, organogenesis)

  4. t>10 - Fetal development

  5. t=24 - 50% chance of survival outside of the womb

  6. t>24 - Complication rate progressively declines

  7. t<37 - Pre-term

  8. 37<t<42 - Full term

  9. t>42 - Post-term

  10. Birth - End of gestation
• Major motor milestones


  • 2-4 months - Heads up/chests up
  • 2-5 months - Roll over
  • 5-8 months - Sit up
  • 5-10 months - Stand with support
  • 6-11 months - Pull up to standing position
  • 7-12 months - Crawl
  • 7-13 months - Walk while holding onto furniture
  • 10-14 months - Stand on their own
  • 11-15 months - Walk alone
• Reflexes


  Pre-programmed motor skills, involuntary motor responses. Present in infants from the day they are born.
• Voluntary movements


  Rapidly develop in the period of a year, progressing from simplest to more complex.
• Factors influencing motor development

  • Nature (genetic factors and anatomical/neurophysiological/biological traits)
  • Nurture (environmental factors and experience)
• Identical twins learn to walk on the same day
• Children all over the world tend to develop motor skills in the same time frame and in the same order (including blind children)
• Infants need to physically develop the necessary brain regions (e.g. cerebellum) and muscles to allow for motor milestones, and these milestones won't happen without the biological support, regardless of how much practice or experience the parent tries to give
• Motor development


  • Progresses from simplest to more complex (logical) movements
  • Relevant to nature vs nurture
• Nature


  Genetic factors and anatomical/neurophysiological/biological traits drive motor development
• Evidence of nature driving motor development

  • Identical twins learn to walk on the same day
  • Children all over the world tend to develop motor skills in the same time frame and in the same order (including blind children)
• Infants need to physically develop the necessary brain regions (e.g. cerebellum) and muscles to allow for motor milestones (e.g. crawling or walking, or even bladder and bowel control) and these milestones won't happen without the biological support, regardless of how much practice or experience the parent tries to give (e.g. toilet training literally can't happen until the muscles required for bladder and bowel control physically develop)
• Nurture


  Environment/culture definitely does definitely still play an important role in the development of motor skills
• Evidence of nurture driving motor development

  • Motor development can be sped up by providing a child with space and time to practice motor skills, and by providing an enriched environment to allow for motor exploration
  • Sleeping on your back reduces the chance of sudden infant death syndrome (SIDS), but also slows the time it takes for an infant to start crawling
  • Children who wear diapers slows the time it takes for an infant to start walking
• Gross motor skills


  Movements involving large muscle groups (e.g. arms, legs)
• Fine motor skills


  Movements involving small muscle groups (e.g. fingers)
• Gross motor skills develop before fine motor skills
• Head to toe development

  Development is said to go 'from head to toe' (e.g. infant can lift head before they can crawl)
• Children that are late in developing one motor skill may be faster at developing another
• Neonatal reflexes


  Involuntary, automatic motor responses that are 'built in' and allow an infant to interact with the world
• Permanent reflexes


  • Breathing reflex
  • Eyeblink reflex
  • Pupillary reflex
  • Swallowing reflex
• Neonatal reflexes


  • Rooting
  • Galant
  • Babinski
  • Moro
  • Tonic neck (aka fencing posture)
  • Palmer grasp
  • Sucking
  • Stepping
  • Swimming