Read Dr. JPJ’s lecture & foundation 1 Dr. Daw Khin Win. Also will be good to read from First Aid.

This is just a very short summary of the derivatives of the germ layers. Will update later on the embryogenesis process (3rd-8th week of development)

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Ectoderm

Tall columnar cells, it gives rise to external layer & organs that communicates with the external environment

  • skin
    • only epidermis
  • nervous system (neural crest cells)
    • CNS, PNS
  • Sensory epithelia
    • ear
    • nose
    • eye
      • corneal epithelium
      • conjunctiva
      • lens
      • retina
      • muscles of iris
  • Glands
    • invagination of epidermis
  • Ear
    • external auditory meatus
    • tympanic membrane (outer part)
    • membranous labyrinth
  • Lower part of anal canal
  • Terminal urethra
  • Teeth
    • enamel

Mesoderm

It gives rise to most muscles except the skin muscle (erector) & iris muscle. Derived from the invagination of ectoderm. It forms blood, connective tissue, bone, muscle, urogenital system (except bladder, prostate & urethra), gonads, cortex of adrenal gland (medulla is ectodermal), teeth (except enamel) and spleen.

Composed of 3 parts:

1) Paraxial mesoderm

  • At 3rd week segmented into somites
    • ventromedial part: sclerotome
      • forms vertebral column
    • dorsolateral part: dermomyotome
      • myotome: muscles
      • dermatome: skin

2) Intermediate mesoderm

  • In cervical & upper thoracic segments, it becomes segmental clusters: Nephrotomes
    • Caudally: forms unsegmented Nephrogenic cord
      • Kidney systems & gonads
    • Laterally: forms the ductal zone
      • Gives origin of the ducts of the kidney

3) Lateral plate mesoderm

  • Splits into 2 layers
    • Parietal
      • pain sensitive
      • lines the amnion
      • forms ventral & lateral body walls (together with ectoderm)
    • Visceral
      • pain insensitive
      • forms wall of gut
      • cells in the coelomic cavity form the mesothelial cells & serous membrane which lines the
        • pleural
        • peritoneal
        • pericardial cavities
  • Septum transversum
    • horseshoe shaped mesodermal plate at cranial end of embryo
    • adjoining the pericardial sac
    • forms
      • diaphragm
      • liver parenchyma
  • Somatopleura
    • parietal layer of peritoneal, pleura & pericardial sacs
    • dermis & subcutaneous tissue
    • skeletal & muscle elements of limb buds
  • Splanchnopleura
    • visceral layer of serous sac
    • muscles of heart & gut

Other than that: heart, blood cells (angioblasts)

Endoderm

It covers the ventral surface of the embryo and forms the roof of the yolk sac. Rapid growth of CNS causes lengthening of embryo causing cephalocaudal folding. Rapid development of somites causes broadening of embryo and lateral folding. Flat endoderm of the slipper shaped embryo is now tubular (primitive gut). Cavities incorporated by head & tail fold are the foregut & hindgut. Allantois is also derived from the endoderm.

Midgut is in communication with the yolk sac cavity (vitelline duct) which gets narrowed and constricted during the foldings.

There are 2 places where there is no intervening mesoderm

  • Buccopharyngeal membrane
  • Cloacal membrane

Derivatives

  • Epithelium
    • of the GIT
    • respiratory tract
    • urinary bladder & urethra
    • tympanic cavity & auditory tube (inner ear)
  • Duct system of liver
  • parenchyma of thyroid gland
  • pancreas

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Abnormal embryogenesis (3rd to 8th week)

  • Nutritional factors
    • Iodine deficiency
    • Hypervitaminosis
      • mental retardation
    • Mercuric fungicides
      • multiple neurological symptoms
  • Hormonal factors
    • Diabetes
      • skeletal defects
    • Progesterone
      • masculinising effects on female
    • Cortisone
      • cleft palate
  • Infections
    • German measles
      • cataract
      • deafness
      • PDA,ASD,VSD
    • Syphilis
      • deafness
      • mental retardation
    • Toxoplasmosis
      • hydrocephalis (big head)
      • microphthalmia
  • Drugs
    • Thalidomide (for leprosy)
    • Aminopterin
    • Diazepam
    • Thiouracil
    • Streptomycin
    • Alcohol
  • Xray
    • skull defect
    • spina bifida
    • cleft palate
    • microcephaly
    • mental retardation
    • blindness

_____________________________________________________________________

Gonadal sex

Indifferent gonads are composed of 3 cell types

  • Primitive germ cells (migrate to genital ridge)
    • lodge in the medulla
      • testes
      • cortex regresses
    • lodge in the cortex
      • ovary
      • medulla regreses
    • gonadal dysgenesis
      • non-migration of primitive germ cells
  • Cells of the coelomic epithelium
    • Sertoli cells of testes
    • Granulosa cells of the ovary
  • Mesenchyme of the gonadal ridge
    • interstitial cell
    • stroma

Phenotypic sex

Foetal testes secretes

  • Mullerian Regression Factor
    • by Sertoli cells
  • Androgenic steroids
    • by interstitial cells
    • testosterone (induce retension of Wolffian system)
    • DHT (induce formation of external genitalia)

Gonads

Male

  • Suspensory ligament degenerates
  • Gubernaculum attached
    • shortening of gubernaculum forms Processus vaginalis of peritoneum
    • brings testis and mesonephric duct from loin to goin
      • descent of testis
  • Mesonephric duct
    • forms duct of epididymis, vas deferens, seminal vesicle & ejaculatory duct

    Paramesonephric duct regresses

  • Urethral folds fuse to form urethral groove

Female

  • Medullary sex cords (Rete ovarii)
    • degerates & form vascular stroma
  • Cortical sex cords
    • broken down into fragments by mesenchyme
      • form primary ovarian follicles
      • follicular cells derived from surface epithelium
  • Cranial part of genital ridge
    • suspensory ligament
  • Caudal part of genital ridge
    • Gubernaculum
      • Cranial: Ovarian ligament
      • Caudal: Round ligament of uterus
  • Broad ligament
    • arrests descent of artery
  • Mesonephric duct regresses
  • Paramesonephric duct
    • Cranial : fimbriae
    • Middle: fallopian tube
    • Caudal: body of uterus
      • caudal tip: Mullerian tubercle –> vagina
  • Origin of vagina
    • Upper part: Paramesonephric duct
    • Lower part: Urogenital sinus (sinovaginal bulbs)
  • Urethral folds do not fuse, but form labia minora

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