General Embryology Notes

General Embryology Clinical Considerations

Cleft lip and cleft palate:

Cases of cleft of the lip may occur individually or with associated cleft of the secondary palate.

Two major groups of cleft lip and cleft palate are seen based on the aetiology and development:

• Due to deficiency of the medial nasal process – larger group
• Underdeveloped maxillary prominence – smaller group

Read And Learn More: General Embryology Important Notes

Factors responsible for cleft lip and cleft palate:

• Genetic factors
• Environmental factors such as infectious agents, radiation, hormones, Drugs and nutritional deficiencies
  • Exposure to teratogens
    • If the exposure is within the 4th week, then it can lead to the death of the embryo.
    • When the exposure is in between the 4th and the 8th weeks, then malformations are produced as histodifferentiation and organ differentiation occurs during this period.
    • In addition, any factor that prevents the downward movement of the tongue can also prevent the elevation of the palatine processes and lead to cleft palate.

Types of facial cleft lip and cleft palate:

Disturbances between the maxillomandibular relations may lead to:

• Macrostomia or an enlarged oral orifice
• Microstomia or a small oral orifice
• Astomia – lack of an oral orifice (very rare)

First arch syndromes:

Most of the structures derived from the first pharyngeal arch are affected. It is due to the inadequate migration of the neural crest cells in the area of the first arch during the 4th week of intrauterine life due to lack of cell proliferation or increase in cell death.

• Pierre robin syndrome:
  • X-linked
  • Micrognathia (smaller mandible), glossoptosis (abnormal backward or downward positioning of the tongue leading to problems in respiration and feeding)
  • Defects in the ear
  • Defects in speech
  • Cleft lip and palate
• Treacher collins syndrome:
  • Downward slanting of the eyes
  • Notch-like defect in the inferior part of the iris
  • Micrognathia (smaller mandible)
  • Underdeveloped ear
  • Cleft lip and palate

Lingual anomalies:

• Ankyloglossia (tongue tie) – occurs when lingual frenulum extends up to the tip of the tongue. In this condition, tongue protrusion is prevented causing difficulty in the speech.
• Microglossia – too small tongue.
• Macroglossia – too large tongue. It can lead to feeding and breathing problems in neonates.
• Hemiglossia (half tongue) – occurs when one of the lingual swellings fails to develop.
• Aglossia (absence of tongue) – occurs due to complete agenesis of tongue primordia.
• Bifid tongue (anterior part of tongue split into two parts) – occurs when two lingual swellings fail to fuse. In this condition, frequent accumulations of debris on the tongue is observed.

Developmental cysts:

Cysts lined by epithelium, formed due to the epithelial rests along the lines of fusion of pharyngeal arches/facial processes

• Branchial cysts/fistulas
  • Branchial cysts are ectodermal lined cavities formed due to the failure of obliteration of cervical sinus. They are seen in the neck along the anterior border of sternocleidomastoid muscle.
  • Branchial fistula is formed when a branchial cyst ruptures and opens on the surface of the neck.

Globulomaxillary cysts:

Cysts formed from the epithelial cells after the fusion of the medial nasal, lateral nasal and the maxillary processes

• Anterior palatine cysts: Occur in the midline of the maxillary alveolar
  Prominence.
• Nasolabial cysts: Arise from the base of the ala of the nose and then protrude into the nasal and the oral vestibule and the root of the upper lip. These may be retention cysts of the vestibular nasal glands, or may develop from the epithelium of the nasolacrimal duct or may be derived due to the excessive proliferation of the epithelium.

General Embryology And Development Of Face Oral And Paraoral Structures Synopsis

• Fertilization is a process by which a mature male gamete (spermatozoon) fuses with a mature female gamete (ovum) to form a zygote with diploid number of chromosomes. It occurs in the ampulla of uterine tube.
• Embryonic period is from the 1st week of intrauterine life to 8th week of intrauterine life.
• Fetal period is from the 9th week of intrauterine life till the termination of pregnancy.
• At the beginning of 3rd week, embryo is in the form of bilaminar disc.
• Gastrulation is a process that converts the bilaminar germ disc in to a trilaminar germ disc. It occurs during the 3rd week of intrauterine life.
• Notochord is a craniocaudally directed midline structure. It extends from the cranial end of primitive streak to caudal end of the prochordal plate. It acts as a guide for the formation of axial skeleton of the embryo. It also induces the formation of neural tube.
• Neurulation is the formation of the neural tube. The neural tube develops into the brain and the spinal cord.
• The mesenchyme in the walls of the pharyngeal part of the foregut undergoes segmentation in a cranio-caudal direction to differentiate into six pairs of mesodermal bars called pharyngeal arches/branchial arches. Each pharyngeal arch consists of an outer covering of ectoderm, a central core of mesoderm and an inner lining of endoderm. The corresponding endodermal grooves on the inner side between the arches are called pharyngeal pouches.
• These pharyngeal arches, clefts and pouches are numbered from the cranial to caudal end and together referred to as the pharyngeal apparatus. It is responsible for the development of face, neck, mouth, tongue, pharynx and larynx.
• Tongue develops from three different sources.
  • Extrinsic and intrinsic muscles of the tongue develop from the occipital myotomes.
  • Connective tissue stroma of the tongue develops from the mesenchyme of The branchial arches.
  • The mucous membrane (epithelium) of the tongue develops in relation with the pharyngeal arches from the endodermal cells covering the floor of the mouth.
• Development of face takes place around the stomodeum by the contribution of five processes: A frontonasal process, right and left maxillary processes, right and left mandibular processes.
• Development of mouth: From the stomodeum after the rupture of the Buccopharyngeal membrane.
• Development of lower lip: From fused right and left mandibular processes.
• Development of upper lip:
  • Lateral parts of the lip are formed by corresponding (right and left) Maxillary processes.
  • Median part of lip (philtrum) is formed by the two medial nasal processes which are the parts of frontonasal process.
    Development of cheek: The maxillary and the mandibular processes forming the upper and lower lips fuse with each other lateral to the developing mouth to form the cheeks.
• The epithelium covering the facial processes differentiates into odontogenic epithelium or the tooth forming epithelium. The epithelium proliferates and thickens to form the primary epithelial bands on two surfaces:
  • Inferior surface of the maxillary process
  • Medal nasal process
  • Superior surface of the mandible
• Developmentally there are two stages of formation of palate:
  • Formation of primary palate
  • Formation of secondary palate
• The mandible develops from the mandibular process of the first pharyngeal arch.
• The events in the development of the mandible are as follows:
  • Appearance of meckel’s cartilage
  • Condensation of the mesenchyme and first point of ossification
  • Formation of the rudimentary mandible
  • Development of the secondary cartilages of the mandible
  • Formation of the alveolus