Principle of Germ Layer Segmentation

Ectoderm gives further rise to neuroectoderm and neural crest cells.

Endoderm remains intact.

Mesoderm gives further rise to paraxial mesoderm (somitomeres and 35 pairs of somites), intermediate mesoderm, and lateral mesoderm:

The somites segment into the sclerotome (forms axial cartilage and bone), myotome (forms axial muscle), and the dermatome (forms the dermis of skin).

The intermediate mesoderm forms the urogenital system.

The lateral mesoderm is split into two layers by the formation of the intraembryonic coelom called the somatic layer and the splanchnic layer. The somatic layer of the lateral mesoderm and the ectoderm form the embryonic body wall or somatopleure. The visceral layer of the lateral mesoderm and the endoderm form the embryonic gut tube or splanchnopleure.

General Rule for Germ Layer Derivatives

Ectodermal derivatives:

1. Everything that makes you attractive: Skin, hair, nail, breasts, teeth enamel etc.

2. Nervous system: CNS, PNS, Sensory parts of eye, ear and nose

3. Epithelial linings that can be touched with your finger: Oral cavity, lower anal canal, external ear canal, terminal part of male urethra

4. Exocrine glands: Sweat, sebaceous, mammary, parotid, lacrimal, etc.

5. Heart: Aorticopulmonary septum and Endocardial cushion

Endodermal derivatives:

1. Lining of tube from nose, mouth and ear to anus and urethra and vagina except those that can be touched with your fingers.

2. Internal organs:

Gastrointestinal tract: except spleen

Renal and genitourinary system

Mesodermal derivatives:

1. All stuffs between skin and internal organs

2. Genitourinary and renal organs

3. Spleen

4. Adrenal cortex

5. Duramater

Derivatives of Ectoderm

a. Surface ectoderm:

Skin Hair Nails Enamel of teeth Oral epithelium: Lip, cheeks, gums, part of floor of mouth

Embryologic attachment with oral epithelium: Rathke’s pouch (Adenohypophysis) Lower third of anal canal below pectinate line Terminal (Glanular) part of male urethra Labia majora and outer surface of labia minora Epithelium of conjunctiva, cornea, ciliary body and iris External ear, outer layer of tympanic membrane and internal ear (sensory) Lens of eye Exocrine glands: Sweat, sebaceous, mammary, parotid, lacrimal, etc.

b. Neuroectoderm: CNS and brain

Central Nervous System (CNS) Retina and Optic nerve Epithalamus (Pineal gland) Neurohypophysis Astrocytes Oligodendrocytes Ependymal cells

c. Neural crest: PNS and nearby non-neural structures

Neuroendocrine: Adrenal medulla and chromaffin cells

Enterochromaffin cells

Parafollicular C cells of thyroid

Melanocytes Ganglia: Sensory, cranial and autonomic Cranial nerves Celiac ganglion Schwann cells Meninges: Pia and arachnoid mater Pharyngeal arch chartilage Odontoblasts Aorticopulmonary septum Endocardial cushions

Ectodermal Derivative’s Mnemonic: 7 E Epidermis Epithelial linings of external orifices Ear, eye and nose – sensory part like olfactory epithelium, retina, etc. Enamel of teeth Exocrine glands Encephalon (CNS) Eye lens

Derivatives of Mesoderm

Connective tissues: Loose areolar tissue

Superficial and deep fascia

Ligaments

Tendons

Aponeuroses

Dermis of skin Specialized connective tissue: Adipose tissue

Reticular tissue

Cartilage

Bone Muscles: except musculature of iris Smooth

Cardiac

Skeletal All serous membranes Blood, lymph, cardiovascular organs Substance of cornea, sclera, choroid, ciliary body and iris Adrenal cortex Gonads and internal reproductive organs Spleen Kidney and ureter Trigone of bladder Duramater

Mesodermal Derivative Mnemonic: GONADS Genitourinary and Renal Others – Muscle, bone, connective tissue, serous lining of body cavities, cardiovascular system, parenchyma Notochord – Nucleus pulposus Adrenal cortex Duramater Spleen

Derivatives of Endoderm

Epithelial lining of:

Respiratory: Larynx, Trachea, Bronchi and Lungs Tonsils, Pharynx and GI tract Thymus Urinary: Urinary bladder (except trigone)

Female urethra (except part of posterior wall)

Male urethra (except posterior part of prostatic urethra and glanular part) Biliary system Lower 2/3rd of vagina and Inner surface of labia minora Ear: Inner layer of tympanic membrane

Middle ear cavity

Auditory tube

Mastoid antrum and air cells

Parenchyma: