Early Human Development Notes

Early Human Development

Development Overview

  • Development is an amazing process.
  • Second Law of Thermodynamics: Entropy will increase over time.

Foetal Development Timeframe

  • Spans from 1-4 weeks to 35-38 weeks (referencing Table 29.2, Tortora, 2019).

Learning Objectives

  • Major developmental events in week 1:
    • Fertilisation.
    • Cleavage of the zygote.
    • Formation of the morula and blastocyst.
    • Implantation.
  • Developmental events in weeks 2-4:
    • Trophoblast development and embryonic disk.
    • Gastrulation.
    • Structures derived from the three primary germ layers.
  • Stem cells:
    • Understanding of what stem cells are and why they are needed.
    • Listing the hierarchy of stem cells.
    • Describing one clinical application of stem cells.

Embryogenesis

  • Patterning: Cells acquire identity in space and time.
  • Major axes defined: anterior/posterior and dorsal/ventral.
  • Three germ layers defined.
  • Rudiments of major organs are formed.

Gastrulation and Germ Layers

  • Gastrulation generates three germ layers: ectoderm, mesoderm, and endoderm.
  • Ectoderm:
    • Contributions: Epidermis, hair follicles and hairs, nails, sweat glands, mammary glands, sebaceous glands.
    • Pharyngeal cartilages and their derivatives (portion of sphenoid, auditory ossicles, styloid processes of temporal bones, cornu and superior rim of hyoid bone).
    • All nervous tissue, including brain and spinal cord.
    • Pituitary gland and adrenal medullae.
    • Mucous epithelium of nasal passageways.
    • Mucous epithelium of mouth and anus, salivary glands.
  • Mesoderm:
    • Contributions: Dermis and subcutaneous layer (hypodermis).
    • All skeletal structures except some pharyngeal derivatives.
    • All muscular structures.
    • Adrenal cortex, endocrine tissues of heart, kidneys, and gonads.
    • All cardiovascular structures.
    • All lymphatic structures.
    • The kidneys, including the nephrons and the initial portions of the collecting system.
    • The gonads and the adjacent portions of the duct systems.
    • The lining of the pleural, pericardial, and peritoneal cavities and the connective tissues that support all organ systems.
  • Endoderm:
    • Contributions: Thymus, thyroid gland, and pancreas.
    • Respiratory epithelium (except nasal passageways) and associated mucous glands.
    • Mucous epithelium (except mouth and anus), exocrine glands (except salivary glands), liver, and pancreas.
    • Urinary bladder and distal portions of the duct system.
    • Distal portions of the duct system, stem cells that produce gametes.
  • Mnemonic:
    • Ecto - outside.
    • Meso - between.
    • Endo - inside.

Fertilisation

  • Sperm cell penetrates the corona radiata and zona pellucida.
  • Sperm enters oocyte, triggering completion of Meiosis II.
  • Male and female pronuclei form.

Cleavage and Blastocyst Formation

  • Cleavage of zygote into two-cell stage (day 1), four-cell stage (day 2).
  • Morula formation (day 4).
  • Blastocyst formation with embryoblast (inner cell mass), blastocyst cavity, and trophoblast (day 5).

Implantation Timeline

  • Fertilization: Occurs within the uterine tube 12-24 hours after ovulation.
  • Cleavage: First cleavage completed about 30 hours after fertilization.
  • Morula: 3-4 days after fertilization.
  • Blastocyst: 4.5-5 days after fertilization.
  • Implantation: Occurs about 6 days after fertilization.

Trophoblast Development

  • Trophoblast differentiates into syncytiotrophoblast and cytotrophoblast.
  • Chorionic villi form the fetal placenta, interacting with the uterine endometrium.

Bilaminar Embryonic Disc

  • Formation of the bilaminar embryonic disc consisting of the hypoblast and epiblast within the uterine endometrium.

Gastrulation Significance

  • Gastrulation: "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life." - Prof. Lewis Wolpert
  • Formation of the primitive streak.
  • Development of the connecting stalk, amnion, yolk sac, embryonic disc, mesoderm, ectoderm, and endoderm.

Notochord and Embryonic Folding

  • Formation of the notochord and neural tube.
  • Embryonic folding at the head end (oropharyngeal membrane, foregut, future pharynx), tail end (cloacal membrane, hindgut), and lateral folds (midgut).
  • Development at 22 days, 24 days, 26 days, and 28 days.

Human Embryonic Development

  • Described as