Embryologic derivatives

Overview

Mnemonic

Ectoderm: The outermost layer → develops into structures that interact with the external world (skin, nervous system)
Mesoderm: The middle layer → develops into supportive structures (muscles, bones, blood-related (heart, vessels, spleen, kidney))
Endoderm: The innermost layer → develops into internal organs and linings (digestive tract, lungs)

Ectoderm
- Surface ectoderm
  - Anterior pituitary (Rathke pouch)
    - Anterior pituitary gland (adenohypophysis): develops from oral ectoderm (Rathke pouch)
    - Posterior pituitary gland (neurohypophysis): develops from neural ectoderm
  - Lens & cornea
  - Inner ear sensory organs, olfactory epithelium
  - Nasal & oral epithelial linings, salivary glands
  - Epidermis, sweat & mammary glands
- Neural tube
  - Brain & spinal cord
  - Posterior pituitary, pineal gland
  - Retina
- Neural crest
  - Neural ganglia, adrenal medulla
  - Schwann cells; pia & arachnoid mater
  - Aorticopulmonary septum & endocardial cushions
  - Branchial arches (bones & cartilage)
  - Skull bones
  - Melanocytes
Mesoderm
- Muscles (skeletal, cardiac & smooth)
- Connective tissue, bone & cartilage
- Serosal linings (eg, peritoneum)
- Cardiovascular & lymphatic system
- Spleen & hemopoietic cells
- Kidney & ureters, internal genitalia
- Adrenal cortex
Endoderm
- Gastrointestinal tract, liver, pancreas
- Lungs
- Thymus, parathyroids, thyroid follicular cells
- Middle ear epithelium
- Bladder & urethra
- Parafollicular (C) cells*

Pharyngeal apparatus

Pharyngeal clefts—derived from ectoderm. Also called pharyngeal grooves.
- Just external ear canal
Pharyngeal arches—derived from mesoderm (muscles, arteries) and neural crest (bones, cartilage).
- Connective tissue
- Cartilage
- Muscle
- Bones
- Nerves
Pharyngeal pouches—derived from endoderm.
- Epithelium
- Glands

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Mnemonic

CAP covers outside to inside:

Clefts = ectoderm
Arches = mesoderm + neural crest
Pouches = endoderm

Pharyngeal arch derivatives

When at the restaurant of the golden arches, children tend to first chew (1), then smile (2), then swallow stylishly (3) or simply swallow (4), and then speak (6).

ARCH NERVES MUSCLES CARTILAGE 1st pharyngeal arch CN V₃ chew Muscles of mastication (temporalis, masseter, lateral and medial pterygoids), mylohyoid, anterior belly of digastric, tensor tympani, anterior 2/3 of tongue, tensor veli palatini Maxillary process → maxilla, zygomatic bone

Mandibular process → Meckel cartilage → mandible, malleus and incus, sphenomandibular ligament 2nd pharyngeal arch CN VII (seven) — smile Muscles of facial expression, stapedius, stylohyoid, platysma, posterior belly of digastric Reichert cartilage → stapes, styloid process, lesser horn of hyoid, stylohyoid ligament 3rd pharyngeal arch CN IX — swallow stylishly Stylopharyngeus Greater horn of hyoid 4th and 6th pharyngeal arches 4th arch: CN X (superior laryngeal branch) simply swallow

6th arch: CN X (recurrent laryngeal branch) speak 4th arch: most pharyngeal constrictors; cricothyroid, levator veli palatini

6th arch: all intrinsic muscles of larynx except cricothyroid Arytenoids, Cricoid, Corniculate, Cuneiform, Thyroid cartilage (used to sing and ACCCT)

Pharyngeal pouches derivatives

Ear, tonsils, bottom-to-top: 1 (ear), 2 (tonsils), 3 dorsal (bottom = inferior parathyroids), 3 ventral (to = thymus), 4 (top = superior parathyroids).

POUCH	DERIVATIVES	NOTES
1st pharyngeal pouch	Middle ear cavity, eustachian tube, mastoid air cells	1st pouch contributes to endoderm-lined structures of ear
2nd pharyngeal pouch	Epithelial lining of palatine tonsil
3rd pharyngeal pouch	Dorsal wings $\to$ inferior parathyroids Ventral wings $\to$ thymus	Third pouch contributes to thymus and both inferior parathyroids Structures from 3rd pouch end up below those from 4th pouch
4th pharyngeal pouch	Dorsal wings $\to$ superior parathyroids Ventral wings $\to$ ultimopharyngeal body $\to$ parafollicular (C) cells of thyroid	4th pharyngeal pouch forms para“4”llicular cells Developmental anomalies in 3rd and 4th pharyngeal pouch are present in DiGeorge syndrome

5、7、9、10为混杂

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