Embryology of the Respiratory System Flashcards
How does development of the LRT begin?
It begins as a respiratory diverticulum (laryngotracheal diverticulum) from ventral surface of laryngopharynx (foregut) during 4th week.
Origin of LRT epithelium
Endoderm
What aspects of LRT is derived from mesoderm?
CT, cartilage and muscles.
How does the larynx, trachea and primary bronchi form?
- The oesophagotracheal ridges fuse to form oesophagotracheal septum, which separates the trache from the oeseophagus.
- The larynx remains in communication with laryngopharynx at laryngeal orifice (aditus).
- Branching begins - primaru bronchial buds (lung buds).
Problems with oesophagotracheal ridge/septum
Results in oesophageal atresia and/or tracheoesophageal fistula.
Most common form is proximal oesophageal atresia with distal tracheosophageal fistula.
Less commonly, atresia only, fistula only, or distal atresia with proximal fistula.
Draw a proximal oesophageal atresia with distal tracheoesopageal fistula.
Pharyngeal arches
Mesenchyme swellings of pharynx
Pharyngeal clefts
Also known as pharyngeal ‘grooves’: ectoderm, external surface between pharyngeal arches.
Pharyngeal pouches
Endoderm, internal surfaces between pharyngeal arches.
What gives rise to the caritlages and muscles of larynx?
Pharyngeal arches 4 and 6 (mesenchyme)
Development of the lungs
Primary bronchial buds (lung buds) divide repeatedly
- Secondary: lobar bronchi (week five)
- Tertiary: segmental bronchi (week six)
- Further branching week six onwards.
Bronchial branching abnormalities
Epithelial (endoderm): mesenchymal (mesoderm) interactions direct branching morphogenesis.
- Agenesis: lung fails to develop
- Aplasia: some rudemintary bronchi
- Hypoplasia: insufficient branching and development.
What occurs in week 5-16 of bronchial development?
Several bronchi generations, several bronchiole generations, to terminal bronchioles.
_The conducting zone is established. _
Respiratory zone development period
Weeks 16-28 is the initial development of the respiratory zone.
What occurs during respiratory zone development?
The terminal bronchioles divide into respiratory bronchioles, which divide into terminal sacs (future alveoli).
When does simple cuboidal epithelium come into direct contact with capillaries?
Around week 28.
When does alveoli develop?
In weeks 28-36 type I (squamous) and type II (cuboidal) of alveoli are develloping, and in contact with capillaries.
Alveoli continue to form by branching and septation postnatally, through about 10 years.
When do type II epithelial cells begin producing surfactant?
They begin producing surfactant in significant quantities beginning about week 35. The first production week is by week 20 by cuboidal epithelium of bronchioles.
Foetal breathing movement
- Occur before brith
- Cause lungs to be filled with amniotic fluid
- Exercises the respirtaory muscles in preparation for birth.
What can lead to reduced lung growth in utero?
Oligohydramnios (foetal renal insufficiency, prolonged premature rupture of membranes) can lead to reduced lung growth.
How is fluid removed from the lungs at birth?
- Expelled through nose and mouth.
- Absorbed into pulmonary capillaries and lymphatics - leaves behind a layer of surfactant.
How are the pleural cavities formed?
Begin as cavities in the lateral plate mesoderm. Lateral folding of embryo creates a single ventral body cavity (intraembryonic coelom), separate from extraembryonic coelom.
The ventral body cavity is divided into pleural, pericardiala nd peritoneal cavities by:
- diaphragm-forming tissues
- pleuropericardial folds.
Visceral pleura
Bronchial buds grow into ventral body cavity, evaginating part of the splanchnic layer of lateral plate mesoderm - forms visceral pleura.
Parietal pleura
Is derived from somatic layer of the lateral plate mesoderm.
Diaphragm development
The diaphragm is formed from several structures
- Septum transversum: a ridge of mesoderm
- Pleuropertoneal membranes
- Cervical somite myotomes (C3, C4, C5)
- Oesophagus dorsal mesentary
Descends from neck region
Congenital diaphragmatic hernias
The most common cause is failure of pleuroperitoneal membrane to close, leaving an opening in diaphragm.
Results in herniation of abdominal viscera into thoracic cavity.
Hypoplastic lung due to lack of room.
Nasal cavity origin
From nasal placode, pit and sac; and from stomedeum (primitive mouth)
Oral cavity origin
Stomodeum
Pharynx origin
Foregut
Development of the nose
Nasal placodes: ectodermal thickening, invaginate to form nasal pits.
Medial and lateral nasal processes around nasal pits form nose.
Stomedeum
Head fold creates a recess, stomodeum, initially separated from forgut by oropharyngeal membrane.
This membrane breaks down to create continuity between stomedeum and pharynx of foregut.
Development of nasal cavity
- Nasal placode invaginates to form nasal plt
- Nasal plt enlarged to form nasal sac, separated from stomedeum by primary palate and oronasal membrane.
- Oronasal membrane breaks down (nasal and oral cavities in open communication)
- Palate separates nasal cavity and oral cavity
- Turbinates (conchae) develop on lateral nasal wall
Development of palate
- Primay palate from intermaxillary segment (part of medial nasal processes)
- Secondary palates (palatal or palatine shelves) from maxillary processes.
- Primary palate and secondary palate fuse.
- Right and left secondary palates fuse on midline.
Errors in uRT and face development
Cleft lip: failure of intermaxillary segment to fuse with maxillary process/es.
Cleft palate: failure of first degree and second degree palates to fuse, and/or failure of right and left second degree palates to fuse.
May occur together or in isolation.
Panasal air sinuses
Outgrowths of nasal cavity, expand into surrounding bones with increasing age.
