Embryonic Germ Layers: Ectoderm, Mesoderm, Endoderm

Overview

• The development of the embryo starts with three major germ layers, each contributing structures.
• The three layers are: Ectoderm, Mesoderm and Endoderm

Ectoderm

• Differentiates into three key structures.
• Neuroectoderm: gives rise to the central nervous system (brain and spinal cord).
• Neural Crest: forms structures such as peripheral nerves, melanocytes, facial cartilage, and parts of the skull.
• Epidermis: develops into the skin, hair, nails, and glands.

Mesoderm

• Divides into three regions.
• Paraxial Mesoderm: forms the somites, which develop into the vertebrae, skeletal muscles, and parts of the dermis.
• Intermediate Mesoderm: gives rise to the urogenital structures (kidneys, gonads, and their ducts).
• Lateral Plate Mesoderm: splits into two layers, forming the body wall, limbs, and cardiovascular structures.

Endoderm

• Develops into the lining of the gastrointestinal tract, respiratory tract, liver, pancreas, and other internal organs.

Additional Information to the Development of Head and Neck

• This region is primarily formed by:
• Paraxial Mesoderm contributes to muscles and bones in the head.
• Lateral Plate Mesoderm: helps in the formation of the connective tissues in the neck.
• Neural Crest: gives rise to craniofacial cartilage and bone, along with peripheral nerves.
• Ectodermal Placodes: contribute to sensory structures, such as parts of the ear, nose, and eyes.
• Most structures arise from the pharyngeal region of the embryo, which develops into the pharyngeal arches that contribute to various facial and neck structures.

Pharyngeal Arches

• They begin to form during the 4th week of embryonic development, playing a critical role in the development of the head and neck.
• They develop as blocks of tissue separated by pharyngeal clefts (lined by ectoderm on the outer surface).
• Initially, there are six, but the 5th arch regresses, leaving five functional arches.
• Pharyngeal pouches (formed by the endoderm) extend toward the pharyngeal clefts, with boundaries formed by pharyngeal membranes.
• The pharyngeal arches help shape 4 out of the 5 main embryonic swellings (primordia) of the face
• Two mandibular processes (formed from the 1st pharyngeal arch).
• Two maxillary processes (also derived from the 1st pharyngeal arch).
• One frontonasal prominence (not derived from the pharyngeal arches but from the frontonasal area).
• The arches consist of the following:
• External surface: formed by ectoderm.
• Internal surface: lined by endoderm.
• Central core: containing lateral plate mesoderm, paraxial mesoderm, and neural crest cells.
• Skeletal components and connective tissue in the arches are derived from neural crest cells.
• Muscular structures are formed by mesodermal cells that migrate into the arches.
• Each arch is associated with a specific cranial nerve, which migrates along with the developing muscles, allowing for the innervation of the muscles and structures that form from each arch.

Derivatives of the Pharyngeal Arches

• Arch 1, is named the Mandibular arch and develops into the Maxillary and Mandibular process
• Its muscles from mesoderm include: Masseter, Temporalis, Lateral pterygoid, Medial pterygoid, Mylohyoid, Anterior digastric, Tensor tympani, Tensor veli palatini
• Its cartilage structures From the Neural Crest include: Malleus, Incus (both from Meckel's cartilage, which degenerates in adulthood)
• Its connective tissue from Neural Crest include: Sphenomandibular ligament, Anterior ligament of the malleus (both from Meckel's cartilage, which degenerates in adulthood)
• Arch 1 is innervated by Trigeminal nerve
• Arch 2, Muscles from Mesoderm: Muscles of facial Expression, Posterior digastric, Stylohyoid, Stapedius
• Lesser cornu of the hyoid and superior part of the body, are cartilage structures from the Neural Crest
• Stapes(all from Reichert's cartilage) is a cartilage structure from the Mesoderm
• The connectice tissue structure from the neural crest is the stylohyoid ligament
• The Nerve of arch 2 is the Facial nerve
• Arch 3 only has the stylopharyngeus muscle
• Arch 3s cartilage structures are: Greater cornu of the hyoid and the Inferior part of the hyoid body
• The Neural crest connective tissue is the thymus and inferior parathyroid tissue
• The nerve is Glossopharyngeal
• Arch 4 muscles are: Musculus uvulae, Levator veli palatini, Palatopharyngeus, Palatoglossus, Superior constrictor, Middle constrictor, Inferior constrictor, Salpingopharyngeus, Cricothyroid
• Its Cartilage structures from Mesoderm include: Epiglottis, Thyroid (both from lateral plate mesoderm)
• Its Neural Crest connective tissue is the paratiroid gland
• Arch 4s nerve is the Vagus
• Arch 6s muscles from mesoderm are: Thyroarytenoid, Vocalis, Lateral cricoarytenoid, Oblique arytenoids, Transverse arytenoids, Posterior cricoarytenoid, Aryepiglottis, Thyroepiglottis
• Its cartilage structures are: Arytenoid, Cricoid, Cuneiform, Corniculate (all from lateral plate mesoderm)
• The nerve to arch 6 is the vagus

Pharyngeal Pouches

• All are derived from endoderm.
• There are four primary pouches that give rise to specific structures in the head, neck, and thoracic regions.
• Pharyngeal Clefts originate from ectoderm
• Pharyngeal Membranes are located between the pharyngeal pouch and pharyngeal cleft
• Membranes are made up of ectoderm externally, mesoderm/neural crest cells in the core and endoderm internally

Cranium

• Lateral Plate Mesoderm contributes to structures in the neck region.
• Paraxial Mesoderm forms much of the cranium and vertebral column.
• Neural Crest contributes to various cranial bones and facial structures.
• The cranium can be divided into the viscerocranium and the neurocranium
• Viscerocranium
• Forms the bones of the face, developed from neural crest cells.
• Neurocranium
• Forms the bones of the cranial base and cranial vault; protects brain
• Further divided into:
• Membranous Neurocranium: forms cranial vault bones through neural crest and paraxial mesoderm.
• Cartilaginous Neurocranium: forms cranial base bones, also from neural crest and paraxial mesoderm.
• Two ways for ossification
• Intramembranous Ossification: Occurs mostly in the cranial vault bones.
• Endochondral Ossification: Mostly common in the cranial base

Fontanelles

• Anterior fontanelle closes 4-26 months
• Posterior fontanelle closes 1-2 months
• Sphenoidal fontanelle closes 2-3 months
• Mastoid fontanelle closes 12-18 months

The Face

• Three main swellings (prominences) are around the stomodeum:
• Frontonasal Prominence
• Maxillary Prominence (from the 1st pharyngeal arch)
• Mandibular Prominence (also from the 1st pharyngeal arch)
• Nasal placodes and nasal pits: located lateral to the frontonasal prominence
• The medial nasal prominences fuse at the midline, forming the intermaxillary segment.

Derivatives of the Adult face Structures

• Forehead :Frontonasal Prominence
• Upper Lip: Maxillary prominence
• Lower Lip: Mandibular Prominence
• Lacrimal Sac Nasolacrimal Duct: A nasolacrimal groove that separates the lateral nasal prominence and the maxillary prominence
• Nose: Frontonasala, Medial and Lateral Nasal Prominencse
• Cheeks: Maxillary prominence
• Philtrum: Medial Nasal Prominence
• Primary Palate: Intermaxillary segment

Palate

• Primary palate formed from the intermaxillary segment
• Secondary palate arises from protrusions of the maxillary prominences
• The primary palate, secondary palate, and nasal septum fuse at a midline landmark called the incisive foramen/fossa.

Tongue

• GSA fibers supplying the tongue's epithelium align with its formation and are associated it’s it the pharyngeal arches.
• 1st Arch is the anterior two-thirds of the tongue (via the lingual nerve, CN V3).
• 3rd Arch: posterior one-third (via the glossopharyngeal nerve, CN IX).
• 4th Arch: root of the tongue (via the vagus nerve, CN X).
• the 2nd pharyngeal arch provides SVA fibers for taste to the anterior two-thirds of the tongue.
• fibers travel via the chorda tympani .
• The chorda tympani joins the lingual nerve in the infratemporal fossa, allowing the SVA taste fibers to be distributed to the anterior two-thirds of the tongue.
• the muscles develop from mesoderm of the occipital somites.
• The palatoglossus muscle is an exception
• it arises from mesoderm of the 4th pharyngeal arch.
• Consequently, it is innervated by the vagus nerve (CN X).

Lingual Nerve

• A branch of the mandibular division.
• Provides general sensation to the anterior two-thirds of the tongue.
• Carries SVA:
• The chorda tympani joins the lingual nerve in the infratemporal fossa. Allows taste fibers to travel to the anterior two thirds
• Parasympathetic innervation (chorda tympani fibers to the submandibular and sublingual salivary glands)

Thyroid Gland

• Originates in the floor of the pharynx then descends inferiorly along the midline to its final position near the larynx.
• May be connected to the foremen Cecum via the thyroglossal duct
• Divided into two lateral lobes connected by an isthmus.
• Has a pyramidal lobe as well
• Follicular Cells: Produce Thyroid Hormones, derived from endoderm.
• Parafollicular Cells: C cells, derived from the ultimobranchial body. Produce calcitonin.