UNSW Embryology

Beginnings, Growth and Development

© Dr Mark Hill (2009)

Acknowledgements

Practical: Embryology of the Face and Ear

2. Early Embryo

Week 4

During week 4 a number of features appear visible on the embryo surface:

• At the level of the body somite bulges and limb buds appear.
• At the level of the head sensory placodes and pharyngeal arches appear.

Carnegie Stage 12 to 14

Stage 12	Stage 13	Stage 14

See also Week 4 section to find more about this stage.

Sensory Placodes

Sensory placodes develop as small patches of ectodermal thickenings. The placodes are laterally paired and contribute key components to sensory structures of the ear, eye and nose and are named by the sensory system and components they will form: otic placode, lens placode and nasal placode.

Pharyngeal Arches

	Each arch has initially similar components contributions from all 3 germ layers Ectoderm - outside surface (and neural crest of core) Mesoderm - core of mesenchyme Endoderm - inside pharyngeal surface
	Cranial neural crest (midbrain and hindbrain region) enters pharyngeal arch mesenchyme Forms connective tissues (cartilage, bone, ligaments): jaw cartilage and bone middle ear in mammals frontonasal process bones teeth dentine cranial nerve peripheral neurons and glia

Stage 13 Pharyngeal Arches

Look through the cross-sections of the stage 13 embryo observing and identfying structures of the face and ear visible at this stage.

Otic Vesicle (otocyst)

A2: Otocyst (R). Apex of otocyst (primordium of L endolymphatic sac).   See also Senses Notes	A3: Otocyst; surrounding mesenchyme = otic capsule. Note proximity of otocyst to wall of rhombencephalon. Superior glossopharyngeal ganglion. Vestibulo-cochlear-facial ganglion complex (R). Trigeminal ganglion (not in image excerpt).	A4: Indentation in rostral margin of otocyst = primordium of utriculosaccular canal. Note alignment of superior cardinal vein (LS) in relation to the nerve trunks (XS). Trigeminal ganglion (not in image excerpt). Note variation in height of cells of L otocyst wall. Dorsal end of 1st pharyngeal arch and groove (not in image excerpt).	A5: Facial ganglion (R). "Floor" of L otocyst. Note on R, the former position of the otocyst in relation to the 2nd pharyngeal arch.

Pharynx

A6: 1st, 2nd arches, grooves and pouches on R. Inferior glossopharyngeal and facial ganglia on left.	A7: 1st pharyngeal pouch opening into pharynx. A portion of which can be seen in the embryo midline.	B1: The pharynx is forming a tight arch sectioned twice. 1st pharyngeal pouch opening into pharynx. Hypopharyngeal eminence (=hypobranchial eminence) can be seen in middle of image.
	B2: The pharynx is forming a tight arch sectioned twice. 1st pharyngeal pouch opening into pharynx. Hypopharyngeal eminence (=hypobranchial eminence) can be seen in middle of image. Dark spots within the hypopharyngeal eminence are the "cords" of thyroid precursor cells descending from the pharynx floor to form the thyroid gland in the neck. Thyroid has an important endocrine role in later development. B3: Aortic sac with emerging 3rd arch artery. Thyroid cords. Arches l- 4. 4th arch arteries. Note at midline compression in the pharynx floor as it approaches the glottis.

B1: Rathke's pouch. Roof of pharynx can be seen on left of image. Developing diencephalon. Lens pit (arrowed, derived from lens placode).	B2: Rathke's pouch. Optic cup with optic ventricle.	B2: Note how close roof of pharynx is to the floor plate region of the neural tube (diencephalon).

Pharyngeal Arch Structures

ARCH	Nerve	Muscles	Skeletal Structures	Ligaments
1 (maxillary/mandibular)	trigeminal (V)		malleus, incus	ant lig of malleus, sphenomandibular ligament
2 (hyoid)	facial (VII)		stapes, styloid process, lesser cornu of hyoid, upper part of body of hyoid bone	stylohyoid ligament
3	glossopharyngeal (IX)		greater cornu of hyoid, lower part of body of hyoid bone
4 & 6	superior laryngeal and recurrent laryngeal branch of vagus (X)		thyroid, cricoid, arytenoid, corniculate and cuneform cartilages

Structures derived from Pouches

Each pouch is lined with endoderm and generates specific structures.

POUCH	Overall Structure	Specific Structures
1	tubotympanic recess	tympanic membrane, tympanic cavity, mastoid antrum, auditory tube
2	intratonsillar cleft	crypts of palatine tonsil, lymphatic nodules of palatine tonsil
3	inferior parathyroid gland, thymus gland
4	superior parathyroid gland, ultimobranchial body
5	becomes part of 4th pouch

Structures derived from Grooves

Only the first groove differentiates into an adult structure and forms part of the external acoustic meatus.

Structures derived from Membranes

At the bottom of each groove lies the membrane which is formed from the contact region of ectodermal groove and endodermal pouch. Only the first membrane differentiates into an adult structure and forms the tympanic membrane.

Next

Link to next page in this Practical - Late Embryo The next page will look at the late embryoic structures.

 

Glossary

Use the alphabetical list below to find definitions of terms that are new to you or use the Google search window to search UNSW Embryology site.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Terms

cervical sinus - An ectodermal depression visible externally during embryonic development (approx carnegie stage 14-16), which marks the region where 2nd pharyngeal arch has grown over 3rd and 4th arches.

diencephalon - (encephalon = brain) the embryonic neural tube region that will form the thalmus, hypothalmus and other nuclei in the adult brain. The diencephalon is the second most anterior of the 5 secondary brain vesicles formed from division of the prosencephalon (forebrain) primary brain vesicle (there are 3 primary brain vesicles). It is at the level of the diencephalon that the optic stalks (optic nerve) extends from the brain. The diencephalon lumen (cavity of the neural tube) will form the third ventricles. (More? Neural Notes | Ventricular System)

ectomesenchyme - The term used to describe ectoderm derived neural crest which contributes in the head region skeletogenic mesenchyme (cartilage, bone, and connective tissue) which in the body region are typically derived from mesoderm. (More? Neural Crest Notes)

eminence - An anatomical description for a narrow extension from a larger structure. Synonyms: tuberosity, a protuberance. (More? hypopharyngeal eminence | median eminence | Head Notes)

first trimester - Clinical term used to describe and divide human pregnancy period (9 months) into three equal parts of approximately three calendar months. The first trimester corresponds approximately to embryonic development (week 1 to 8) of organogenesis and early fetal. The second and third trimester correspond to the fetal period of growth in size (second trimester) and weight (third trimester), as well as continued differentiation of existing organs and tissues. (More? Embryo Stages | Human Fetal Period | Development Week by Week)

hypopharyngeal eminence - (hypobranchial eminence) An early embryonic structure in the developing head. A narrow midline mesodermal (mesenchymal) exension lying within the floor curve of the developing pharynx. Fusion of 3rd pharyngeal arches and precursor of root of tongue. Early developing thyroid cells also migrate into this structure as cords of cells. (More? Head Notes | Endocrine Development - Thyroid)

lingual - (Latin, lingua = tongue) Term used to describe structures and features related to the tongue: lingual papilla, lingual plate, lingual plexus, lingual thyroid nodule. (More? Tongue Notes | Head Notes)

mandible - Term used to describe the lower jaw of the face, which forms from the lower part pharyngeal arch 1, the mandibular process. The smaller upper part of pharyngeal arch 1 forms the two maxillary processes, which form the upper jaw. (More? Face Development | Face Abnormalities | Head Development)

mandibular process - In head and face development, lower part and majority of pharyngeal arch 1 which forms the lower jaw (mandible) of the face Smaller upper part of pharyngeal arch 1 are the maxillary processes. (More? Face Development | Face Abnormalities | Head Development)

maxillary process - In head and face development, upper part of pharyngeal arch 1 which forms as a pair of small lateral swellings which contributes the upper jaw and forms the palatal shelves. Larger lower part of pharyngeal arch 1 is the mandibular process. Associated abnormality is cleft palate/lip. (More? Face Development | Face Abnormalities | Head Development)

mesenchyme - Term used to describe the cellular organisation of undifferentiated embryonic connective tissue . Mesenchymal tissue is mainly derived from mesoderm and neural crest, which will form most of the adult connective tissues. This connective tissue organization contrasts with the other main form of cellular organization, epithelial tissue. (More? Musculoskeletal Notes)

olfactory - Refers to the sense of smell. (More? Senses Smell | Senses Notes)

olfactory epithelium - The specialised sensory epithelium that lines the nasal cavity associated with smell. Some cells within the adult epithelium remain as stem cells which can be isolated and purified. (More? Senses Smell | Senses Notes | Stem Cell Notes)

otocyst - (= otic vesicle) the sensory otic placode which sinks into mesoderm to form spherical vesicle (stage 13/14 embryo) that will form components of the inner ear. (More? Organs of Audition and Equilibrium | Senses Notes)

paraxial mesoderm - The two lateral strips of mesoderm lying beside the axial mesoderm (notochord). This mesoderm at the body level will segment into somites, at the head level it remains unsegmented. (More? Musculoskeletal Notes | Week 3 Notes)

pharyngeal arch - (branchial arches, Greek, branchial = gill) These are a series of externally visible anterior tissue bands lying under the early brain that give rise to the structures of the head and neck. In humans, five arches form (1,2,3,4 and 6) but only four are externally visible on the embryo. Each arch has initially identical structures: an internal endodermal pouch, a mesenchymal (mesoderm and neural crest) core, a membrane (endoderm and ectoderm) and external cleft (ectoderm). Each arch mesenchymal core also contains similar components: blood vessel, nerve, muscular, cartilage. Each arch though initially formed from similar components will differentiate to form different head and neck structures. (More? Head Notes | Face Notes |Neural Crest Notes | Endocrine Notes)

pharyngeal arch artery - Each early developing pharyngeal arch contains a lateral pair of arteries arising from the aortic sac, above the heart, and running into the dorsal aorta. later in development these arch arteries are extensively remodelled to form specific components of the vascular system. Pharyngeal Arch 1 arteries are mainly lost and forms part of maxillary artery. Pharyngeal Arch 2 arteries remains to form the stapedial arteries. Pharyngeal Arch 3 arteries forms the common carotid arteries, internal carotid arteries in the neck. Pharyngeal Arch 4 arteries will form part of aortic arch (left arch artery) and part right subclavian artery (right arch artery) Pharyngeal Arch 6 arteries form part of left pulmonary artery (left arch artery) and part of right pulmonary artery (right arch artery). (More? Cardiovascular Notes | Head Notes | Face Notes)

pharyngeal arch cartilage - Each early developing pharyngeal arch contains a horseshoe shaped band of cartilage that acts as a template and contributes to the development of head and neck bony and cartilagenous features, including the middle ear bones. Pharyngeal Arch 1 cartilage (Meckel’s cartilage) dorsal ends form malleus and incus midpart forms ligaments (ant. malleus, sphenomandibular) ventral part forms mandible template. Pharyngeal Arch 2 cartilage (Reichert’s cartilage) dorsal ends form stapes and Temporal bone styloid process, ventral part ossifies to form hyoid bone components, lesser cornu and superior body. Pharyngeal Arch 3 cartilage forms hyoid components, greater cornu and inferior part of hyoid. Pharyngeal Arch 4 and 6 cartilage forms laryngeal cartilages except epiglottis (from hypobranchial eminence). (More? Middle Ear Notes | Head Notes | Face Notes)

pharyngeal arch nerve - Each early developing pharyngeal arch contains the developing cranial nerves, as a pair, within the arch mesenchyme. Each cranial nerve is numbered (roman numeral) in rostrocaudal sequence and also has a specific name. The cranial nerve within each arch often relates to the other structures formed from taht arch. Pharyngeal Arch 1 contains the trigeminal nerve (CN V, cranial nerve 5). Pharyngeal Arch 2 contains the facial nerve (CN VII, cranial nerve 7). Pharyngeal Arch 3 contains the glossopharyngeal nerve (CN IX, cranial nerve 9) Pharyngeal Arch 4 and 6 contains the Vagus (CN X cranial nerve 10), forming the adult superior laryngeal and recurrent laryngeal branches. (More? Neural Notes | Neural Crest Notes | Head Notes | Face Notes)

pharyngeal arch pouch - An out-pocketing of the endoderm lined pharynx occurring between each developing pharyngeal arch. Each of the pharyngeal arch pouches contributes different components of the head and neck, either cavities or endocrine tissues. Pharyngeal Arch 1 pouch elongates to form tubotympanic recess tympanic cavity, mastoid antrum and auditory tube (Eustachian tube). Pharyngeal Arch 2 pouch forms the tonsillar sinus and is later mostly oblierated by palatine tonsil. Pharyngeal Arch 3 pouch forms the inferior parathyroid and thymus. Pharyngeal Arch 4 pouch forms the superior parathyroid, parafollicular cells of Thyroid. (More? Middle Ear Notes | Thymus Notes | Parathyroid Notes | Thyroid Notes | Endocrine Notes | Head Notes | Face Notes)

pharynx - (throat) Forms the initial segment of the gastrointestinal and upper respiratory tract divided anatomically into three regions: nasopharynx, oropharynx, and laryngopharynx (hypopharynx). Anatomically extends from the base of the skull to the level of the sixth cervical vertebra. (More? Respiratory Notes)

prosencephalon - (forebrain) the most anterior neural tube primary brain vesicle (there are 3 primary brain vesicles) that will form the two secondary brain vesicles, telencephalon and diaencephalon. These generate in the adult brain the cerebral hemispheres (neocortex, basal nuclei, palaeocortex, archicortex) and thalmus, hypothalmus and other nuclei respectively. The prosencephalon lumen (cavity of the neural tube) will form the lateral and third ventricle. (More? Neural Notes | Ventricular System) pruritus - (Latin, prurire = to itch) itching skin sensation which can have a number of different causes (diabetes, iron deficiency, hyperthyroidism, Hodgkin's Disease, hair loss, allergic drug reaction). (More? Skin Notes)

placode - (Greek, plax = plate, eidos = shape or form) A surface ectoderm thickening seen in the early embryo head region. Each pair of placodes will contribute a different component of each sensory system (otic placode, optic placode, nasal placode). (More? Head Notes)

Rathke's pouch - An ectodermal fold in roof of pharynx forming anterior pituitary (adenohypophysis) and pars intermedia. Named after German embryologist and anatomist Martin Heinrich Rathke (1793 — 1860). (More? Pituitary Notes | Endocrine Notes | Martin Heinrich Rathke)

rhombencephalon - (Greek rhombos = rhomboid, enkephalos = brain) The hindbrain, the most caudal neural tube primary brain vesicle (there are 3 primary brain vesicles) that will form the two secondary brain vesicles, metencephalon and the myeloncephalon. These will in turn generate in the adult brain the cerebellum, pons and medulla. The rhombencephalon lumen (cavity of the neural tube) will form the fourth ventricle. (More? Neural Notes | Ventricular System)

rhombomere - A segmentation of the hindbrain (rhombencephalon) reflects segemental (rostro/caudal) expresion of Hox gene. (More? Neural Notes)

stomadeum - (stomadeum) A ventral surface depression on the early embryo head surrounding the buccopharyngeal membrane, which lies at the floor of this depression. This surface depression lies between the maxillary and mandibular components of the first pharyngeal arch. (More? Gastrointestinal Tract Notes | Head Notes)

thyroid - (Greek, thyreos = sheild , eidos = form) endocrine gland located in the neck, the origin of the name reflects the organs anatomical structure. In the fetus, the thyroid gland has a role in neurological development. (More? Endocrine Notes | Thyroid Gland | Abnormal Development - Iodine Deficiency)

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