The lung bud sometimes referred to as the respiratory bud forms from the respiratory diverticulum, an embryological

endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...

al structure that develops into the

respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed in through the nose to th ...

organs such as the

larynx The larynx (), commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about ...

trachea The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...

bronchi A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi pronounced (BRAN-KAI) to branch from the trachea at the carina are the right main bronchus and the left main bronchus. ...

and

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s. It arises from part of the

laryngotracheal tube The laryngotracheal groove is a precursor for the larynx and trachea. The rudiment of the respiratory organs appears as a median longitudinal groove in the ventral wall of the pharynx. The groove deepens and its lips fuse to form a septum which gr ...

Early stage

In the fourth week of

development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is reshaped * Photograph ...

, the respiratory diverticulum, starts to grow from the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...

(front) side of the

foregut The foregut is the anterior part of the alimentary canal, from the mouth to the duodenum at the entrance of the bile duct. Beyond the stomach, the foregut is attached to the abdominal walls by mesentery. The foregut arises from the endoderm, deve ...

into the

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...

that surrounds it, forming the lung bud. Around the 28th day, during the separation of the lung bud from the foregut it forms the trachea and splits into two bronchial buds, one on each side.

Molecular signaling

The molecular signaling involved in the specification of the respiratory bud starts with the expression of the

Nkx2-1 NK2 homeobox 1 (NKX2-1), also known as thyroid transcription factor 1 (TTF-1), is a protein which in humans is encoded by the ''NKX2-1'' gene. Function Thyroid transcription factor-1 (TTF-1) is a protein that regulates transcription (genetics ...

gene, which determines the respiratory field – the area where the respiratory bud will begin to grow from. The signaling that makes the growth of the respiratory bud possible is complex and involves a number of interactions between the mesoderm and the respiratory bud

epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...

, in which members of the Fgf and Fgfr family of genes express.Cardoso, W.V. and Kotton, D.N.
Specification and patterning of the respiratory system
(July 16, 2008), StemBook, ed. The Stem Cell Research Community, StemBook, doi/10.3824/stembook.1.10.1

Separation of trachea and esophagus

At first, the posterior part of the

is open to the

esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...

, but as the bud elongates two longitudinal mesodermal ridges known as the laryngotracheal folds, begin to form and grow until they join, forming a wall between the two organs. An incomplete separation of the organs leads to a congenital abnormality known as a

tracheoesophageal fistula A tracheoesophageal fistula (TEF, or TOF; see spelling differences) is an abnormal connection (fistula) between the esophagus and the trachea. TEF is a common congenital abnormality, but when occurring late in life is usually the sequela of surgica ...

Larynx development

The

of the

is of endodermal origin, but the laryngeal cartilages, unlike the rest of the respiratory bud

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

, come from the

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every o ...

of the fourth and sixth

pharyngeal arches The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill arch ...

. The fourth pharyngeal arch, adjacent to what will be the root of the

tongue The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...

, will become the

epiglottis The epiglottis is a leaf-shaped flap in the throat that prevents food and water from entering the trachea and the lungs. It stays open during breathing, allowing air into the larynx. During swallowing, it closes to prevent aspiration of food int ...

. The sixth pharyngeal arch, located around the laryngeal orifice, will become the

thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thy ...

cricoid The cricoid cartilage , or simply cricoid (from the Greek ''krikoeides'' meaning "ring-shaped") or cricoid ring, is the only complete ring of cartilage around the trachea. It forms the back part of the voice box and functions as an attachment sit ...

and arytenoid cartilages. These structures are formed in a process in which the lining cells of the primitive larynx proliferate and occlude it. Later, it recanalizes leaving two membrane-like structures: the

vocal folds In humans, vocal cords, also known as vocal folds or voice reeds, are folds of throat tissues that are key in creating sounds through vocalization. The size of vocal cords affects the pitch of voice. Open when breathing and vibrating for speech ...

and the vestibular folds. In between, an enlarged space, the ventricle, remains. Failure in this process leads to a serious but rare condition called congenital

atresia Atresia is a condition in which an orifice or passage in the body is (usually abnormally) closed or absent. Examples of atresia include: *Aural atresia, a congenital deformity where the ear canal is underdeveloped. * Biliary atresia, a condition i ...

of the larynx.

Later development

After the lung buds have formed, they begin to grow and branch forming a primitive version of the bronchial tree, determining how the lobes of the lung will be arranged in the mature organ. The first stage of

alveolar Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * ...

development, spanning between the fifth and the 16th week of development, is called the ''pseudoglandular stage''.Phases of lung development
/ref> It is so called because of the

histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...

appearance of the primitive alveoli, which resemble glandular tissue. After the pseudoglandular stage, the lung enters the canalicular and saccular phases. During these stages, the terminal tubes narrow and give rise to small saccules, which become increasingly associated with

capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

as to make

gas exchange Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liqui ...

possible. The alveolar epithelium begins to differentiate into two distinct types of cells:

type I pneumocyte A pulmonary alveolus (plural: alveoli, from Latin ''alveolus'', "little cavity"), also known as an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where oxygen is exchanged for carbon dioxide. A ...

s and

type II pneumocyte A pulmonary alveolus (plural: alveoli, from Latin ''alveolus'', "little cavity"), also known as an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where oxygen is exchanged for carbon dioxide. A ...

s, as well as the

respiratory epithelium Respiratory epithelium, or airway epithelium, is a type of ciliated columnar epithelium found lining most of the respiratory tract as respiratory mucosa, where it serves to moisten and protect the airways. It is not present in the vocal cords o ...

of the trachea and bronchial tree.

References

External links

* * *
Histology at med.unsw.edu.au

{{Authority control Embryology Lung