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
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.
Mesenchyme swellings of pharynx
Also known as pharyngeal 'grooves': ectoderm, external surface between pharyngeal arches.
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.
Bronchial buds grow into ventral body cavity, evaginating part of the splanchnic layer of lateral plate mesoderm - forms visceral pleura.
Is derived from somatic layer of the lateral plate mesoderm.
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
Development of the nose
Nasal placodes: ectodermal thickening, invaginate to form nasal pits.
Medial and lateral nasal processes around nasal pits form nose.