Th e three germ layers (ectoderm, mesoderm and endoderm) give rise to all the tissues and organs of the body. Most organs receive contributions from more than one germ layer. A summary of the derivatives of each germ layer is given below.

DERIVATIVES OF ECTODERM

Lining Epithelia

Th e epithelium lining the following is of ectodermal origin: 1. Skin, including its pigment cells (from the neural crest). 2. Mucous membrane of lips, cheeks, gums, part of the fl oor of the mouth, part of the palate,

nasal cavities and paranasal sinuses. 3. Lower part of the anal canal. 4. Terminal part of the male urethra. 5. Outer surface of labia minora and whole of labia majora. 6. Anterior epithelium of cornea, epithelium of conjunctiva, epithelial layers of ciliary body

and iris. 7. Outer layer of tympanic membrane. Epithelial lining of membranous labyrinth including

the special end organs.

Glands

1. Exocrine: Sweat glands, sebaceous glands, parotid gland (and other salivary glands?), mammary gland, lacrimal gland.

2. Endocrine: Hypophysis cerebri, adrenal medulla.

Other Derivatives

1. Hair. 2. Nails. 3. Enamel of teeth. 4. Lens of eye; musculature of iris; ciliary muscles (from neural crest?); vitreous(?).

Fate of the Germ Layers

Chapter CD 1

Human Embryology

5. Nervous system (brain and spinal cord) including all neurons, neuroglia (except microglia), and Schwann cells (from neural crest).

6. Pia-arachnoid (from neural crest). 7. Branchial cartilage (from neural crest?). 8. Substance of cornea, sclera and choroid (from neural crest?).

DERIVATIVES OF ENDODERM

Lining Epithelia

The following lining epithelia are of endodermal origin. 1. Epithelium of part of the mouth, part of the palate, tongue, tonsil, pharynx, oesophagus,

stomach, small and large intestines and upper part of anal canal. 2. Epithelium of pharyngo-tympanic tube, middle ear, inner layer of tympanic membrane,

mastoid antrum and air cells. 3. Epithelium of respiratory tract. 4. Epithelium of the gall bladder and of the extrahepatic duct system; epithelium of the

pancreatic ducts. 5. Epithelium of the urinary bladder except the trigone (mesoderm); female urethra except

part of its posterior wall (mesoderm); male urethra except part of the posterior wall of its prostatic part (mesoderm) and except the part of the penile urethra lying in the glans penis (ectoderm).

6. Epithelium of greater part of vagina, vestibule and inner surface of labia minora.

Fig. CD-1.1: Derivatives of ectoderm. Derivatives of surface ectoderm, of neural tube and of neural crest are listed separately.

Chapter CD 1 Fate of the Germ Layers

Glands

1. Endocrine: Thyroid, parathyroid, thymus, islets of Langerhans. 2. Exocrine: Liver, pancreas, glands in wall of gastrointestinal tract, greater part of prostate

(except inner glandular zone) and its female homologues.

DERIVATIVES OF MESODERM

1. All connective tissues including loose areolar tissue filling the interstices between other tissues, superficial and deep fascia, ligaments, tendons, aponeuroses, and the dermis of the skin.

2. Specialized connective tissues like adipose tissue, reticular tissue, cartilage and bone.

Fig. CD-1.2: Derivatives of endoderm are listed under three different heads. 1. From epithelial lining of the gut (including that of the urinary bladder). 2. From epithelial lining of the respiratory system and

of the pharynx. 3. Derivatives of the pharyngeal pouches.

Fig. CD-1.3: Derivatives of mesoderm. Structures derived from the paraxial, intermediate and lateral plate mesoderm are listed separately.

Human Embryology

3. Dentine of teeth. 4. All muscles (smooth, striated and cardiac) except the musculature of the iris (ectoderm)

and ciliary muscles (neural crest?). 5. Heart, all blood vessels and lymphatics, and blood cells. 6. Kidneys, ureters, trigone of bladder, posterior wall of part of the female urethra, posterior

wall of upper half of prostatic part of male urethra, and the inner glandular zone of the prostate.

7. Ovary, uterus, uterine tubes, upper part of vagina. 8. Testis, epididymis, ductus deferens, seminal vesicle, ejaculatory duct. 9. Lining mesothelium of pleural, pericardial and peritoneal cavities; and of tunica vaginalis. 10. Lining mesothelium of bursae and joints. 11. Substance of cornea; sclera; and choroid. (?) 12. Substance of ciliary body and iris. 13. Duramater; pia-arachnoid (?); microglia 14. Adrenal cortex.