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Embryonic Phase of Development (4-8 Weeks)

The embryonic period of development extends from fourth to eighth week of gestationand is also called period of organogenesis. During this period the main organ systems are developed from three germinal layers. However, the function of most of them is negligible except for cardiovascular system, i.e. cardiovascular system is the earliest to function. During 'embryonic phase, folding of embryo (folding of trilaminar disc) also occurs, which starts in the beginning of 4th week and is the key event in formation of body form and gastrointestinal tract.

Changes in germinal layers and their derivatives

During week 4th to 8th the three germinal layers (ectoderm, mesoderm and endoderm) differentiate into various tissues and organs.
  1. Derivatives of ectoderm
    Ectoderm is the outer most layer of trilaminar germ disc. It gives rise to those organs and structures that maintain contact with the outside world. During neurulation some of the ectodermal cells form neuroectoderm and the rest form surface ectoderm.
  2. Derivatives of surface ectoderm
    1. Skin and appendages: Epidermis, hair, nails, sweat glands, sebaceous glands.
    2. Mucous membrane of: Oral cavity, nasal cavity, paranasal sinuses, lower part of anal canal, terminal part of urethra, outer surface oflabiaminora, whole labia majora, lower part of vagina.
    3. Eye: Crystalline lens, epithelium of cornea and conjunctiva, lacrimal gland,
    4. Ear: Outer layer of tympanic membrane, epithelial lining of membranous labyrinth.
    5. Glands: Exocrine glands (sweat glands, sebaceous glands, salivary glands, mammary glands, lacrimal glands), endocrine gland (anterior part of pituitary).
    6. Teeth: Enamel.
    7. Pharyngeal Clefts
  3. Derivatives of neuroectoderm
    From neural tube: CNS (brain, spinal cord, astrocytes, oligodendrocytes, ependymal cells, retina, pineal gland, neurohypophysis (posterior pituitary),all cranial and spinal motor nerves.
    From neural crest: Neural crest derivatives are:-
    1. Neural derivatives
      1. Sensory neurons of 5th, 7th, 8th, 9th, 10th cranial nerve ganglia (trigeminal, geniculate, sphenopalatine, submandibular, cochlear, vestibular, otic and vagal parasympathetic ganglia).
      2. Sensory neurons of spinal dorsal root ganglia.
      3. Sympathetic chain ganglia and plexus (celiac/Preaortic/renal ganglia, enteric plexus in GIT, i.e. Auerbach’s and Meissner's.
      4. Parasympathetic ganglia and plexus of GIT.
      5. Schwann cells of peripheral nerves, satellite cells of all ganglia.
      6. Adrenal medulla, chromaffin cells, para follicular C-cells of thyroid gland.
      7. Melanocytes and melanoblasts.
    2. Mesenchymal derivatives
      1. Dermal bones of skull: Frontal, parietal, temporal, nasal, vomer, palatine, mandible, maxillae.
      2. Leptomeninges: arachnoid and pia mater (Durameter is mesodermal).
      3. Dentine of teeth (odontoblasts).
      4. Eye: choroid, sclera, iris epithelium, pupillary muscles (sphincter and dilatorpupillae, ciliary muscles.
      5. Pharyngeal arch cartilages.
      6. Retinal pigmented epithelium.
      7. Connective tissues of head including dermis, tendon, ligaments.
      8. Bulbar and conal ridges of heart.
  1. Derivatives of mesoderm
    Mesoderm is divided into three parts:-
    1. Paraxial mesoderm: It is organized into somites which in turn gives rise to:-
      1. Sclerotomes: Form axial skeleton including vertebrae, and parts of neurocranium.
      2. Myotomes: Form all voluntary (skeletal) muscles of head, trunk and limbs.
      3. Dermatomes: Form dermis of skin, especially over dorsal regions.
    2. Intermediate mesoderm: It gives rise to major portion of urogenital system:-
      1. Urinary organs: Kidney, ureters, trigone of bladder, posterior wall female urethra, posterior wall of upper half of prostatic part of male urethra, inner glandular zone of prostate.
      2. Reproductive organs: Gonads (testis and ovary, epididymis, ducts deferens, seminal vesicle, ejaculatory duct, uterus, uterine tube and upper part of vagina.
    3. Lateral plate mesoderm: It forms-
      1. Somatopleuric mesoderm (parietal layer)
        • All connective tissues including specialized connective tissues like boner, cartilage, adipose tissue.
        • Dermis of skin over ventrolateral body wall and limbs.
        • Superficial and deep fascia.
        • Ligaments, tendons, aponeurosis.
        • Parietal pleura, parietal peritoneum and tunica vaginalis of testis.
        • Dura-mater.
        • Lid muscles, extraocular muscles.
        • Sclera, choroid, vitreous.
        • Corneal stroma, irisandciliarybody(except epithelium).
      2. Splanchnopleuric mesoderm (visceral layer),
        • Smooth muscle and connective tissue of respiratory tract, gut, blood vessels and heart.
        • Adrenal cortex.
        • Mesothelium (visceral layer) of pleural, peritoneal and pericardial cavities.
        • Mesenchyme surrounding pericardial coelom gives rise to myocardium and serous pericardium.
        • Spleen and lymph nodes.
      3. Septumtransversum
        • Diaphragm
        • Fibrous pericardium
      4. Angiogenic mesoderm
        • Endocardium of heart
        • Endothelium of blood and lymphatic vessels
        • Microglia, tissue macrophages
        • Circulating blood cells
  2. Derivatives of endoderm
    The endoderm gives rise to:-
    1. Lining epithelium of:
      1. Gastrointestinal tract including gall bladder, extrahepatic duct system and pancreatic duct.
      2. Pharyngotympanic tube (eustachian tube), middle ear, inner layer of tympanic membrane, mastoid antrumand air cells.
      3. Respiratory tract including larynx, trachea, bronchi and lungs (alveoli)
      4. Genitourinary tract urinary bladder except trigone (mesodermal), female urethra except part of posterior wall (mesodermal), male urethra except posterior wall of prostatic urethra (mesodermal) and except part of penile urethra lying the glans (ectodermal), part of vagina, vestibule and inner surface of labia minora.
    2. Glandular tissue
      1. Endocrine glands, e.g. thyroid, parathyroids, islets of langerhans of pancreas.
      2. Exocrine glands such as liver, pancreas, glands in walls of GIT, prostate (except inner glandular zone).
    3. Reticular tissue
      1. Of thymus and tonsils.

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