ANAT2341 Lab 10 - Early Embryo

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ANAT2341 Lab 10: Introduction | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | Online Assessment

Week 4

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

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

Carnegie Stage 12 to 14

Week 4

Stage 11

Stage11 sem13c.jpg Stage11 sem20c.jpg

This is a scanning EM of the embryo superior dorsal view showing the paired otic placodes sinking into the surface at the level of the hindbrain between day 24 and day 25

Stage 12

Stage12 sem1c.jpg Stage12 bf2c.jpgStage12 bf1c.jpg

Stage12 sem4.jpg Stage12 sem5.jpg

Vision - Week 4

(data based upon[1] and other sources).

  • Stage 10 - optic primordia appear.
  • Stage 11 - right and left optic primordia meet at the optic chiasma forming a U-shaped rim.
  • Stage 12 - optic neural crest reaches its maximum extent and the optic vesicle becomes covered by a complete sheath,

Week 5

Stage 13

Stage13 sem1c.jpg Stage13 bf1c.jpg Stage13 sem2c.jpg

Sensory Placodes

Stage14 sem2l.jpg

Stage 14

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.

Named by the sensory system and components they will form: otic placode, lens placode and nasal placode.

Stage 13

Identify the structure and position of the otic vesicle (otocyst) relative to other head structures.

Stage 13 image 051.jpg

Stage 13 image 052.jpg

Stage 13 image 053.jpg

Stage 13 image 054.jpg

Stage 13 image 055.jpg

Vision - Week 4 to 5

(data based upon[2] and other sources).

  • Week 4 - Stage 13 - By the end of the fourth week the optic vesicle lies close to the surface ectoderm. Optic evagination differentiation allows identification of optic part of retina, future pigmented layer of retina, and optic stalk. The surface ectoderm overlying the optic vesicle, in response to this contact, has thickened to form the lense placode.
  • Week 5 - Stage 14 - (about 32 days) the lens placode is indented by the lens pit, cup-shaped and still communicates with the surface by a narrowing pore. Stage 15 - (about 33 days) the lens pit is closed. The lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface.

Pharyngeal Arches

Head arches cartoon.jpg Pharyngeal arch structure cartoon.gif

Stage 13 Pharyngeal Arches

Stage13 pharyngeal arch excerpts.gif

A6L  A7L  B1L  B2L  B3L  B4L  B5L  B6L

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

Pharyngeal arch cartilages.jpg

Structures derived from Arches






trigeminal (V)

(temporalis, masseter, medial pterygoid, lateral pterygoid)

Meckel's cartilage
mandible modelmalleus, incus

maxillary(terminal branches)

facial (VII)

facial expression
(buccinator, platysma, stapedius, stylohyoid, digastric posterior belly)

Reichert's cartilage
stapes, styloid process, lesser cornu of hyoid, upper part of body of hyoid bone

stapedial (embryonic)
corticotympanic (adult)

glossopharyngeal (IX)


greater cornu of hyoid, lower part of body of hyoid bone

common carotid, internal carotid (root)

4 and 6
superior laryngeal and recurrent laryngeal branch of vagus (X)

intrinsic muscles of larynx, pharynx; levator palati

thyroid, cricoid, arytenoid, corniculate and cuneform cartilages

4 - aortic arch, right subclavian
6 - ductus arteriosus, pulmonary (roots)

Structures derived from Pouches


Overall Structure

Specific Structures


tubotympanic recess

tympanic membrane, tympanic cavity, mastoid antrum, auditory tube


intratonsillar cleft

crypts of palatine tonsil, lymphatic nodules of palatine tonsil


inferior parathyroid gland, thymus gland


superior parathyroid gland, ultimobranchial body


becomes part of 4th pouch

Additional Information

Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.
Cranial neural crest skeletal fate 01.jpg Cranial neural crest contribution to skeletal structures

The embryo figure shows colonization of the head and pharyngeal arches by diencephalic, anterior and posterior mesencephalic, and rhombencephalic neural crest cells (NCCs), as indicated by the colour code. The diagram is representative human embryos, although the NCC migratory pathways might differ slightly in different species. The skull drawings show contributions of NCC populations to cranial skeletal elements of humans, based on NCC fate-mapping studies and on extrapolation of avian and mouse data to known homologues in the human.

Some bones, including the squamosal (SQ), alisphenoid (AS), and pterygoid (PT), are shown with mixed contribution from different NCC populations. Note that in mammals the frontal (FR) and parietal (PA) bones have been reported to be of neural crest and mesodermal origin, respectively.

(text modified from original figure legend)

  • BA1-BA3 - pharyngeal arches 1 - 3
  • di - diencephalon
  • FNP - frontonasal process
  • HY - hyoid bone
  • IN - incus
  • MA - malleus
  • mes - mesencephalon
  • MX - maxillary bone
  • NA - nasal bone
  • R1–R7 - rhombomeres 1 – 7
  • ST - stapes
  • ZY - zygomatic bone

Review article[3]


  1. <pubmed>7364662</pubmed>
  2. <pubmed>7364662</pubmed>
  3. <pubmed>14523380</pubmed>| Nat Rev Neurosci.
ANAT2341 Lab 10: Introduction | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | Online Assessment

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Cite this page: Hill, M.A. (2024, June 24) Embryology ANAT2341 Lab 10 - Early Embryo. Retrieved from

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G