Embryology Summary PDF

**[EMBRYOLOGY SUMMARY]** **[DEVELOPMENT OF PHARYNGEAL APPARATUS (ARCHES AND POUCHES) ]** **Introduction:** The development of the pharyngeal apparatus begins on the 4^th^-6^th^ week of embryonic development. Here, bars of mesoderm form six (6) pharyngeal arches in a craniocaudal fashion. Between...

**[EMBRYOLOGY SUMMARY]** **[DEVELOPMENT OF PHARYNGEAL APPARATUS (ARCHES AND POUCHES) ]** **Introduction:** The development of the pharyngeal apparatus begins on the 4^th^-6^th^ week of embryonic development. Here, bars of mesoderm form six (6) pharyngeal arches in a craniocaudal fashion. Between the pharyngeal arches, four(4) pharyngeal clefts cover each arches external part with ectoderm and four (4) pharyngeal pouches line each arches internal part with endoderm. Each pharyngeal arch carries its own cranial nerve. The first pharyngeal arch is innervated by the mandibular branch of the trigeminal nerve (CN V3) The second pharyngeal arch is innervated by the facial nerve (CN VII) The third pharyngeal arch is innervated by the glossopharyngeal nerve (CN IX) The fourth is innervated by the superior laryngeal branch vagus nerve (CN X) The sixth is innervated by the recurrent laryngeal branch of CNX By the 6^th^ to 8^th^ week, several muscles and bones originate from the pharyngeal arches. They include: +-----------------------+-----------------------+-----------------------+ | **ARCH** | **BONE** | **MUSCLE** | +=======================+=======================+=======================+ | **FIRST ARCH** | Mandible, Maxilla, | **Muscles that help | | | Zygomatic bones, | with chewing**: | | | Temporal Bones, Incus | Temporalis, Masseter, | | | and Malleus. | Pterygoid muscles and | | | | Tensor tympani | | | | muscles. | | | | | | | | **Muscles that help | | | | with swallowing**: | | | | Tensor veli palatini, | | | | Mylohyoid muscles, | | | | Anterior belly of | | | | Digastric muscle. | +-----------------------+-----------------------+-----------------------+ | **SECOND ARCH** | Lesser horns, Upper | Stylohyoid muscle, | | | portion of the hyoid | Posterior belly of | | | bone, Styloid process | Digastric muscle and | | | of temporal bone and | Stapedius muscle of | | | Stapes | the middle ear. | +-----------------------+-----------------------+-----------------------+ | **THIRD ARCH** | The rest of the hyoid | Stylopharyngeus | | | bone | muscle in the throat | +-----------------------+-----------------------+-----------------------+ | **FOURTH ARCH** | --- | Levator palatini, | | | | Pharyngeal | | | | constrictors and | | | | Cricothyroid muscle | +-----------------------+-----------------------+-----------------------+ | **SIXTH ARCH** | --- | The rest of the | | | | intrinsic muscles of | | | | the larynx | +-----------------------+-----------------------+-----------------------+ **[Pharyngeal Pouches and Clefts]** ** **The first pharyngeal pouch and cleft form the ear. The pouch gives rise to the internal auditory meatus (Middle ear and Eustachian tube) while the cleft gives rise to the external auditory meatus (Ear drum) The second to fourth clefts fade as cells grow and cells lining the second pharyngeal pouch multiply and migrate to form primitive tonsils. The third and fourth pouches both divide into dorsal and ventral portions. \- The dorsal portion of the THIRD pouch becomes the inferior parathyroid gland. \- The ventral portion of the THIRD pouch becomes the primitive thymus and later descends down to the chest. \- The dorsal portion of the FOURTH pouch becomes the superior parathyroid gland. \- The ventral portion of the FOURTH pouch becomes the ultimo-pharyngeal body which contain cells that differentiate into parafollicular/C-cells that migrate into the thyroid. By the 8^th^ -- 12^th^ week of development, the pharyngeal apparatus development is completed. **ANOMALIES OF THE PHARYNGEAL APPARATUS** Pierre Robin sequence (Underdevelopment of the mandible (first arch) and tongue) Treacher Collins syndrome (Underdevelopment of the zygomatic bone and mandible (first arch)) Hemifacial microsomia (Underdevelopment of the mandible and surrounding tissues (first arch)) Branchial cleft cysts (Abnormal development of the second pharyngeal cleft, resulting in cysts or sinuses.) Branchial cleft fistulas Abnormal development of the second pharyngeal cleft, resulting in fistulas.) Thymic hypoplasia (Underdevelopment of the thymus (third pouch)) Parathyroid gland abnormalities (Abnormal development of the parathyroid glands (third and fourth pouches)) **[DEVELOPMENT OF THE FACE]** **Introduction**: The development of the face begin at the 4^th^ -- 5^th^ week of embryonic development with the formation of the pharyngeal arches from the mesoderm layer. At the 6^th^ -- 7^th^ week, development of facial prominences begin. These facial prominences include: The frontonasal prominence which is formed from the fusion of medial nasal prominences. The maxillary prominences which develop from the first pharyngeal arch. The mandibular prominences which develop from the first pharyngeal arch. At the 8^th^ -- 10^th^ week, the frontal prominences merge. The frontonasal prominence and maxillary prominences merge to form the upper jaw and nose while The maxillary prominences and mandibular prominences fuse to form the lower jaw and cheeks. At the 11^th^ -- 12^th^ week, the facial prominences differentiates into various tissues including skin, muscle and connective tissue and the formation of facial structures such as the nose, eyes and mouth begins. At the 13^th^ -- 16^th^ week, the facial structures continue to grow and refine their shape. Also, formation of facial features such as the eyelids, eyebrows and lips is completed. By the 16^th^ week, the basic structure of the face is complete but will continue to grow and refine its shape after birth. **SUMMARY**: 1\. Formation of pharyngeal arches (4^th^-5^th^ week) 2\. Development of facial prominences (6^th^-7^th^ week) 3\. Merging of facial prominences (8^th^-10^th^ week) 4\. Differentiation of facial tissues (11^th^-12^th^ week) 5\. Formation of facial structures and features (13^th^-16^th^ week) **ANOMALIES OF THE FACE** Cleft Lip and Palate (A gap in the upper lip and/or palate) Craniosynostosis (Premature fusion of skull bones, leading to abnormal head shape) Hemifacial Microsomia (Underdevelopment of one side of the face) Treacher Collins Syndrome (Underdevelopment of facial bones, particularly the cheekbones) **Development of the Cheeks** The development of the cheeks begins with the formation of the pharyngeal arches from the mesoderm layer. The first and second pharyngeal arches contribute to the development of the cheeks. At the 6^th^ -- 7^th^ week, the maxillary process, a growth from the first pharyngeal arch, develops and expands to form the upper jaw and cheeks. At the 8^th^ -- 10^th^ week, the maxillary process gives rise to the cheek prominences, which are the precursors to the cheeks. These prominences grow and merge with the mandibular prominences (lower jaw) to form the facial contours. At the 11^th^ -- 12^th^ week, the cheek prominences differentiate into various tissues, including skin, muscle, and connective tissue. These form the characteristic structure of the cheeks. At the 13^th^ -- 16^th^ week, the cheeks continue to grow and refine their shape which is influenced by the development of surrounding facial structures, such as the nose, eyes, and mouth. After birth, the cheeks continue to grow and develop, influenced by facial bone growth, fat deposition, and muscle development. **Anomalies of the Cheeks** Hemifacial Microsomia (Underdevelopment of one side of the face, including the cheek) Treacher Collins Syndrome (Underdevelopment of facial bones, including the cheekbones) **[DEVELOPMENT OF THE NOSE]** The development of the nose begins on the 4^th^ -- 5^th^ week of Embryonic development with the fusion of the medial nasal prominence to from the frontal nasal prominence. This prominence will eventually give rise to the nose, upper lip and primary palate. At the 5^th^ -- 6^th^ week, nasal placodes are formed from the ectoderm on either side of the frontal nasal prominence. The placodes will eventually invaginate to form nasal pits. At the 7^th^ -- 8^th^ week, The nasal septum forms and eventually separates and deepens to form two nasal cavities which will eventually develop into nasal passages in the 9^th^ -- 12^th^ week. After the 12^th^ week, the nasal structure continues to refine and develop and the nasal, passage, septum and conchae develop and mature. **ANOMALIES OF THE NOSE** Cleft nose ( Cleft or groove in the nose.) Nasal aplasia (Absence or underdevelopment of the nose.) Nasal proboscis (Tubular structure protruding from the nose.) Choanal atresia (Congenital blockage of the nasal passages.) Nasal septal deviation (Deviation of the nasal septum, causing breathing difficulties.) Nasal polyps (Growths in the nasal passages.) **[DEVELOPMENT OF THE PALATE ]** The development of the palate begins at the 6^th^ to 7^th^ week of IUL with the fusion of the medial nasal prominences to form the primary palate. The primary palate gives rise to the; Upper lip Philtrum (the groove between the nose and upper lip) Primary palate (anterior portion of the hard palate) At the 7^th^ -- 10^th^ week, the secondary palate forms from the fusion of the maxillary prominences. The secondary palate gives rise to: Hard palate (posterior portion) Soft palate Uvula At the 10^th^ -- 12^th^ week, the palatine shelves elevate and fuses to form the secondary palate. The fusion f the palatine shelves completes the formation of the palate **[DEVELOPMENT OF THE EYE (Oculogenesis)]** **Introduction** The embryonic development of the eye begins at the 3^rd^ to 4^th^ week of intrauterine life with the formation of the optic vesicles from the neuroectoderm layer. These vesicles are outpouchings of the forebrain which will eventually give rise to the retina and optic nerve. At the 4^th^ -- 5^th^ week, the optical vesicles invaginate to form optic cups. These cups are composed of two layers: The outer (pigmented) layer which will form the retinal pigment epithelium The inner (neural) layer which will form the neural retina. At the 5^th^ -- 6^th^ week, ectoderm cells elongate and forms the lens placode. The lens placode invaginates into the optic cup to form the lens vesicle which will eventually give rise to the lens. **Development of the Lens** At the 5^th^ -- 8^th^ week, ectoderm cells elongate and forms the lens placode. The lens placode invaginates into the optic cup to form the lens vesicle. At the 8^th^ -- 12^th^ week, the lens vesicle differentiates into two (2) main layers Lens epithelium (single layer of cells on the anterior side of the lens) Lens fibers (long thin cells that make up the bulk of the lens. After the 12^th^ week, the lens continues growing and develop until it reaches adult size at ages 5 -6 **Development of the Cornea** The development of cornea begins at the 5^th^ -- 6^th^ week of embryonic development. Here, the corneal epithelium is formed from the surface ectoderm and thickens and differentiates into a stratified squamous epithelium. At the 6^th^ -- 7^th^ week, the corneal stroma is formed from the mesenchyme and is composed of collagen fibres, keratocytes and other cells. At the 7^th^ -- 8^th^ week, the corneal endothelium is formed from the mesenchyme. The endothelium lines the posterior aurface of the cornea At the 8^th^ -- 12^th^ week, the corneal epithelium, stroma continue to develop and differentiate, making the cornea to become more organized and structured After the 12^th^ week, the cornea continues to grow and mature and becomea mot transparent and avascular **Development of the Choroid** The development of the choroid begins after the formation of optic cups at the 4^th^ -- 5^th^ week of embryonic development. At the 6^th^ -- 7^th^ week, mesenchymal cells migrate BETWEEN the Retinal Pigment Epithelium (RPE) and the sclera to form the choroidal mesenchyme. At the 7^th^ -- 8^th^ week, the choroidal mesenchyme differentiates into two layers: Choriocapillaris (capillary layer) Larger vessels (arterioles and venules) At the 8^th^ -- 12^th^ week, the choroid continues to develop and mature becoming more vascularized and organized. **Development of the Sclera** The development of the sclera begins after the formation of the optic cup at the 4^th^ -- 5^th^ week of embryonic development. At the 6^th^ -- 7^th^ week, mesenchymal cells migrate AROUND the optic cup to form the scleral mesenchyme. At the 7^th^ -- 8^th^ week, the scleral mesenchyme differentiates into scleral fibroblasts which produce collagen. At the 8^th^ -- 12^th^ week, the sclera continues to develop and mature becoming more dense and organized. **Development of the Iris** The development of the iris begins in the 3^rd^-4^th^ week of embryonic development with the formation of the optic vesicle which will then invaginate to form the optic cup. At the 5^th^-6^th^ week, the iris develops from the anterior portion of the optic cup and forms two layers: The stroma which is derived from the mesenchyme. The epithelium which is derived from the neuroectoderm. At the 7^th^-8^th^ week the iris differentiates into two layers: The anterior border layer which forms the stroma The posterior pigment epithelium At the 9^th^-12^th^ week, the dilator pupillae muscle forms from the anterior border layer and the sphincter pupillae muscle forms from the posterior pigment epithelium. The iris muscle also develop and mature. After the 12^th^ week till birth, the iris continues to mature and develop and the iris muscles become functional (controlling the pupil size) **Development of the Retina** The development of the retina begins after the formation of optic vesicles which are precursors to the retina. The vesicles invaginate to form the optic cups which has two layers: The outer (pigmented) layer which will form the retinal pigment epithelium The inner (neural) layer which will form the neural retina. At the 6^th^ -- 7^th^ week, the neural retina differentiates into distinct layers namely: The Photoreceptor layer The Bipolar cell layer The Ganglion cell layer. At the 7^th^ -- 8^th^ week, the retinal layers continue to differentiate and mature, becoming more organized and structured. At the 8^th^ -- 12^th^ week, the retina continues to develop and mature becoming more complex and organized. **Development of the Optic nerve** The development of the optic nerve begins after the formation of the optic vesicles which are precursors to the optic nerve. At the 6^th^ -- 7^th^ week, the optic stalk is formed from the proximal part of the optic vesicle. The optic stalk connects the retina to the brain. At the 7^th^ -- 8^th^ week, the optic stalk differentiates into the optic nerve which is composed of retinal ganglion cell axons. At the 8^th^ -- 12^th^ week, the optic cell continues to develop and mature becoming more organized and structure. The optic nerve continues to develop and mature after birth and reaches its adult structure and function by around age 2-3. **Development of the Aqueous and Vitreous body** Thd development of the aqueous and vitreous body begins after the formation of optic cups at the 5^th^ -- 6^th^ week of IUL **Aqueous Body** At the 6^th^ -- 7^th^ week, formation of an anterior chamber between the corneas and the lens occurs At the 7^th^ -- 8^th^ week, the aqueous body differentiates into two: Aqueous fluid (clear fluid) Aqueous vessel (blood vessel) At the 8^th^ -- 12^th^ week, the aqueous body continues to develop, mature and starts to circulate and nourish the lens and cornea. **Vitreous Body** At the 6^th^ -- 7^th^ week, vitreous space forms between the lens and retina and vitreous body starts to form in the vitreous space. At the 8^th^ week, the vitreous body differentiates into two layers: Vitreous humor (clear gel-like substance) Vitreous fibers (collagen fibers) At the 8^th^ -- 12^th^ week, the vitreous body continues to develop and mature and starts to provide cushioning and support to the retina. By the 9^th^ week of Embryonic development, the eyelids along with the lacrinal glands and ducts begin to form from the surface ectoderm and mesenchyme (only eyelids) At the 13^th^ week, the eye continues to grow and refine its shape. Also, the formation of the iris, pupil, and sclera is completed. After Birth, the eye continues to grow and develop after birth, influenced by visual experience and neural maturation until it reaches its full size and functional maturity by around age 5-6 **ANOMALIES OF THE EYE** Small eyes (Microphthalmia). Absence of one or both eyes (Anophthalmia) Hole or defect in the iris, retina, or optic nerve (Coloboma). Drooping eyelids (Ptosis) Thinning and conical shape of the cornea (Keratoconus). Fuchs' endothelial dystrophy (Degeneration of the corneal endothelium) Corneal ectasia (Thinning and bulging of the cornea) Retinal detachment (Separation of the retina from the underlying tissue) Retinitis pigmentosa (Genetic disorder causing progressive retinal degeneration) Cataract (Clouding of the lens) Ectopia lentis (Dislocation of the lens) Aphakia (Absence of the lens) Optic nerve hypoplasia (Underdevelopment of the optic nerve) Optic nerve atrophy (Degeneration of the optic nerve) **[DEVELOPMENT OF THE EAR]** **Introduction** The development of the ear, also known as Otogenesis, is a process that involves the formation of the outer ear, middle ear, and inner ear. **DEVELOPMENT OF THE INNER EAR** Around the 22^nd^ day of embryonic development, thickening of the ectoderm occurs on the lateral surface of the hindbrain. This thickening invaginates to form the OTIC PLACODE. The otic placode invaginates forming the otic vesicle (AKA otocyst). The vesicle then divides to form two components: The ventral component The dorsal component The ventral component forms the saccule and the cochlear duct while the dorsal component forms the utricle, semicircular canals and the endolymphatic duct. **Development of the Cochlea** Around the 6^th^ week of embryonic development, the cochlear duct forms as the saccule forms a tubular outgrowth. The cochlear duct spirally penetrates the mesenchyme and completes 2.5 turns at the 8^th^ week of development. **Development of the Organ of Corti** At the 7^th^ week of embryonic development, epithelial cells of the cochlear duct forms two ridges: Inner epithelial ridge Outer epithelial ridge The inner ridge gives rise to the spiral limbus while the outer ridge gives rise to the sensory hair cells of the auditory system. Tectorial membrane covers the sensory cells while attached to the spiral limbus. Both ridges will then merge and differentiate to form the organ of corti. **Development of the Semicircular Canals** On the 6^th^ week of embryonic development, flattened outpouchings appear on the dorsal component of the otic vesicle. These outpouchings will then dilate to form 3 semicircular canals: \- Superior canal \- Posterior canal \- Lateral canal Each canal expands at one end to form the ampulla which contain sensory hairs responsible for detecting rotational movement. **ANOMALIES OF THE INNER EAR** Absence of cochlea (Cochlea Aplasia) Underdevelopment of the cochlea (Cochlear Hypoplasia) Abnormal development of the cochlea (Cochlea Dysplasia) Absence of one or more semicircular canal Underdevelopment of one or more semicircular canal Abnormal development of one or more semicircular canal. **DEVELOPMENT OF THE MIDDLE EAR** The development of the middle ear begins at the 4^th^ week of embryonic development with the formation of the first pharyngeal arch from the mesoderm layer At the 5^th^ week, the endoderm lined pharynx invaginates between the first and second pharyngeal archs to form the first pharyngeal pouch. At the 6^th^ week, the pouch expands and differentiates into 2 parts: Proximal part (Tubotympanic recess) Distal part (Tympanic cavity) The tubotympanic recess gives rise to eustachian tube while the tympanic cavity develops into the middle ear cavity **Development of the Ossicles** Embryonic development of the ossicles also begin with the formation of the pharyngeal arches from the mesoderm layer at the 4^th^ -- 5^th^ week of embryonic development. At the 6^th^ week, cartilage templates are formed for the malleus, incus and stapes. The first pharyngeal arch (mandibular arch) forms the cartilage template for the malleus and incus while the second pharyngeal arch (hyoid arch) forms thw cartilage template for the stapes. At the 8^th^ week, the cartilage templates begin to ossify forming the ossicles. **ANOMALIES OF THE MIDDLE EAR** Underdevelopment of the (malleus/incus/stapes) Fusion of two (2) or more ossicles Absence of one or more ossicle Otosclerosis (abnormal bone growth in middle ear) **DEVELOPMENT OF THE OUTER EAR** Development of the outer ear begins at the 4^th^ -- 5^th^ week of embryonic development with the formation of the pharyngeal arches from the mesoderm layer. At the 5^th^ -- 6^th^ week, six (6) auricular hillocks which are precursors to the outer ear form around the 1^st^ and 2^nd^ pharyngeal arch. At the 7^th^ -- 8^th^ week, the hillocks fuse to form the outer ear. The mandibular arch gives rise to the tragus, the crus of helix and the helix. The hyoid arch gives rise to the antihelix, the antitragus and the lobule. At the 9^th^ -- 12^th^ week, the fused hillocks undergo morphogenesis and forms the characteristic shape of the outer ear i.e the pinna (auricle). The external auditory canal also begins to take shape as it forms a tube that connects the outer ear to the ear drum. By the 16^th^ week, the basic structure of the outer ear is complete but will continue to grow and refine its shape after birth. **ANOMALIES OF THE OUTER EAR** Underdeveloped or absent outer ear (Microtia) Complete absence of the outer ear (Anotia) Abnormally large outer ear (Macrotia) Blockage of the external auditory canal (Atresia) **[DEVELOPMENT OF THE MOUTH]**