Endoderm Development

Introduction

The respiratory system does not carry out its physiological function (of gas exchange) until after birth. The respiratory tract, diaphragm and lungs do form early in embryonic development. The respiratory tract is divided anatomically into 2 main parts: 1. upper respiratory tract, consisting of the nose, nasal cavity and the pharynx; 2. lower respiratory tract consisting of the larynx, trachea, bronchi and the lungs.

The respiratory "system" usually includes descriptions of not only the functional development of the lungs, but also related musculoskeletal (diaphragm) and vascular (pulmonary) development.

Lecture Objectives

Understanding of embryonic lung development
Understanding of 4 stages of lung development
Understanding of diaphragm development
Brief understanding of respiratory vascular development
Brief understanding of respiratory abnormalities
Brief understanding of molecular mechanisms

Textbook References

Human Embryology Larson Chapter 9 p229-260
The Developing Human: Clinically Oriented Embryology (6th ed.) Moore and Persaud Chapter 12 p271-302

Additional Textbooks

Before We Are Born (5th ed.) Moore and Persaud Chapter 13 p255-287
Essentials of Human Embryology Larson Chapter 9 p123-146
Human Embryology Fitzgerald and Fitzgerald Chapter 19,20 p119-123
Anatomy of the Human Body 1918 Henry Gray 1. The Respiratory Apparatus

Developmental Overview

Week 4 - laryngotracheal groove forms on floor foregut.

Week 5 - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)

Week 6 - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes.

Week 7 - enlargement of liver stops descent of heart and lungs.

Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant.

Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and sacs.

Lung Development

week 4 - 5 embryonic
week 5 - 17 pseudoglandular
week 16 - 25 canalicular
week 24 - 40 terminal sac
late fetal - 8 years alveolar

Foregut

From the oral cavity the next portion of the foregut is initially a single gastrointestinal (oesophagus) and respiratory (trachea) common tube, the pharynx which lies behind the heart. Note that the respiratory tract will form from a ventral bud arising at this level.

Oral cavity
Pharynx (esophagus, trachea)
Respiratory tract
Stomach

Upper respiratory tract

In the head/neck region, the pharynx forms a major arched cavity within the phrayngeal arches.

Lower respiratory tract

The lungs go through an embryonic and 4 distinct histological phases of development and in late fetal development respiratory motions and amniotic fliud are thought to have a role in lung maturation.

Development of this system is not completed until the last weeks of fetal development, just before birth. Therefore premature babies have difficulties associated with insufficient surfactant (end month 6 alveolar cells type 2 appear and begin to secrete surfactant).

Stage 14

Diaphragm

Not respiratory tract but musculoskeletal development, there are 5 elements that contribute to the diaphragm

septum transversum- central tendon
3rd to 5th somite- musculature of diaphragm
ventral pleural sac- connective tissue
mesentry of oesophagus- connective tissue around oesophasus and IVC
pleuroperitoneal membranes- connective tissue around central tendon

Respiratory Tract Abnormalities

Images

UNSW Embryology Links

Lecture 10 2008
Respiratory Tract Notes Introduction
Movie links Endoderm
System Notes
Development Timeline

Links

Embryo Images by Drs. Kathleen K. Sulik and Peter R. Bream Jr. notes/images sections on respiratory development
Anatomy of the Human Body 1918 Henry Gray 1. The Respiratory Apparatus

Terms

amnion - An extraembryonic membrane]ectoderm and extraembryonic mesoderm in origin and forms the innermost fetal membrane, produces amniotic fluid. This fluid-filled sac initially lies above the trilaminar embryonic disc and with embryoic disc folding this sac is drawn ventrally to enclose (cover) the entire embryo, then fetus. The presence of this membane led to the description of reptiles, bird, and mammals as amniotes.

amniotic fluid - The fluid that fills amniotic cavity totally encloses and cushions the embryo. Amniotic fluid enters both the gastrointestinal and respiratory tract following rupture of the buccopharyngeal membrane. The late fetus swallows amniotic fluid.

buccal - (Latin, bucca = cheek) A term used to relate to the mouth (oral cavity).

buccopharyngeal membrane - (oral membrane) (Latin, bucca = cheek) A membrane which forms the external upper membrane limit (cranial end) of the early gastrointestinal tract (GIT). This membrane develops during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane lies at the floor of the ventral depression (stomadeum) where the oral cavity will open and will breakdown to form the initial "oral opening" of the gastrointestinal tract. The equivilent membrane at the lower end of the gastrointestinal tract is the cloacal membrane.

coelom - Term used to describe a space. There are extraembryonic and intraembryonic coeloms that form during vertebrate development. The single intraembryonic coelom will form the 3 major body cavities: pleural, pericardial and peritoneal.

foregut - The first of the three part/division (foregut - midgut - hindgut) of the early forming gastrointestinal tract. The foregut runs from the buccopharyngeal membrane to the midgut and forms all the tract (esophagus and stomach) from the oral cavity to beneath the stomach. In addition, a ventral bifurcation of the foregut will also form the respiratory tract epithelium.

intraembryonic coelom - The "horseshoe-shaped" space (cavity) that forms initially in the third week of development in the lateral plate mesoderm that will eventually form the 3 main body cavities: pericardial, pleural, peritoneal. The intraembryonic coelom communicates transiently with the extraembryonic coelom.

neuralation - The general term used to describe the early formation of the nervous system. It is often used to describe the early events of differentiation of the central ectoderm region to form the neural plate, then neural groove, then neural tube. The nervous system includes the central nervous system (brain and spinal cord) from the neural tube and the peripheral nervous system (peripheral sensory and sympathetic ganglia) from neural crest. In humans, early neuralation begins in week 3 and continues through week 4.

pharynx - uppermost end of gastrointestinal and respiratory tract, in the embryo beginning at the buccopharyngeal membrane and forms a major arched cavity within the phrayngeal arches.

somitogenesis The process of segmentation of the paraxial mesoderm within the trilaminar embryo body to form pairs of somites, or balls of mesoderm. A somite is added either side of the notochord (axial mesoderm) to form a somite pair. The segmentation does not occur in the head region, and begins cranially (head end) and extends caudally (tailward) adding a somite pair at regular time intervals. The process is sequential and therefore used to stage the age of many different species embryos based upon the number visible somite pairs. In humans, the first somite pair appears at day 20 and adds caudally at 1 somite pair/90 minutes until on average 44 pairs eventually form.

splanchnic mesoderm - Gastrointestinal tract (endoderm) associated mesoderm formed by the separation of the lateral plate mesoderm into two separate components by a cavity, the intraembryonic coelom. Splanchnic mesoderm is the embryonic origin of the gastrointestinal tract connective tissue, smooth muscle, blood vessels and contribute to organ development (pancreas, spleen, liver). The intraembryonic coelom will form the three major body cavities including the space surrounding the gut, the peritoneal cavity. The other half of the lateral plate mesoderm (somatic mesoderm) is associated with the ectoderm of the body wall.

stomadeum - (stomadeum) A ventral surface depression on the early embryo head surrounding the buccopharyngeal membrane, which lies at the floor of this depression. This surface depression lies between the maxillary and mandibular components of the first pharyngeal arch.

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Cite this page: Hill, M.A. (2024, April 30) Embryology 2009 Lecture 10. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2009_Lecture_10

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