Trachea, Esophagus and Thymus

THYMUS

• The thymus is an asymmetrical, bilobed primary lymphoid organ involved in the development and maturation of T lymphocytes.
• The two primary lymphoid organs are the thymus and bone marrow.

Location

– The thymus is situated mainly in the superior and anterior mediastinum, with extensions into the lower neck, particularly in children.

Size and thymic involution

• At birth: The thymus is well developed and weighs approximately 10–15 g.
• At puberty: It reaches its maximum size, weighing approximately 20–40 g.
• After puberty: It gradually undergoes involution, with much of the lymphoid tissue replaced by fibrofatty tissue.
• Functional thymic tissue may persist throughout adult life.

Structure

• The thymus consists of two unequal lobes connected by fibroareolar tissue.
• Each lobe is enclosed by a thin connective tissue capsule.

Vascular Supply and Innervation

Blood supply

– The thymus receives arterial branches mainly from:

1. Internal thoracic artery.
2. Inferior thyroid artery.

Venous Drainage

– Venous blood drains into the left brachiocephalic, internal thoracic, and inferior thyroid veins.

Nerve supply

• Sympathetic fibres: They arise from cervical and upper thoracic sympathetic pathways and provide vasomotor innervation.
• Parasympathetic fibres: They reach the gland through branches of the vagus nerve.
• Sensory fibres: They are carried partly by the phrenic nerves.

Development of Thymus

– The thymus develops mainly from two embryological components:

1. Endoderm of the third pharyngeal pouch: It forms the thymic epithelial framework.
2. Haematopoietic precursor cells: They originate from haematopoietic tissues, including the developing bone marrow, and form thymocytes.

Developmental Stages

• The ventral part of the third pharyngeal pouch forms the thymic primordium.
• The thymic primordium descends with the developing inferior parathyroid gland, which arises from the dorsal part of the same pouch.
• Both structures lose their connection with the pharyngeal wall.
• The inferior parathyroid gland separates from the thymic primordium, while the thymus continues its descent into the superior mediastinum.
• Thymic epithelial cells and Hassall’s corpuscles develop from the pharyngeal endoderm.
• Haematopoietic precursors enter the thymic primordium and differentiate into T lymphocytes.

Histology of Thymus

• The thymus is enclosed by a thin connective tissue capsule.
• Septa extend from the capsule and divide the gland into numerous incomplete lobules.
• Each lobule presents:
  1. Outer cortex: It stains darkly because of densely packed immature T lymphocytes.
  2. Inner medulla: It stains lightly because it contains fewer lymphocytes.
• The medullary regions of adjoining lobules are continuous.

Hassall’s Corpuscles

• They are characteristic structures present in the thymic medulla.
• They consist of concentric layers of type VI epithelioreticular cells surrounding a central keratinised or hyalinised region.

Epithelioreticular Cells

• Six types of epithelioreticular cells (types I–VI) are recognised.
• They form a supporting cellular network for developing thymocytes.
• Type I cells contribute to the blood–thymic barrier.
• They produce factors such as thymosin and thymopoietin that support T-cell development.

Functions of Thymus

• It provides a specialised environment for the proliferation, differentiation, and maturation of T lymphocytes.
• It supports the selection of functional T cells and elimination of many self-reactive lymphocytes.
• It secretes thymic factors, including thymosin and thymopoietin, that contribute to T-cell maturation.
• It establishes central immunological tolerance, helping prevent autoimmune responses.
• It is particularly important for the development of cell-mediated immunity during early life.

Blood–thymic Barrier

• The blood–thymic barrier separates developing T lymphocytes in the thymic cortex from many circulating antigens.
• It maintains a controlled environment for T-cell maturation.

Components

1. Continuous capillary endothelium: Endothelial cells are connected by tight junctions and rest on a basal lamina.
2. Perivascular connective tissue: It contains macrophages that help remove antigens and cellular debris.
3. Type I epithelioreticular cells: They surround cortical capillaries and form an additional barrier through their tight junctions.

Functions

• It restricts the entry of circulating antigens into the thymic cortex.
• It protects immature thymocytes during their development and selection.

TRACHEA

• The trachea (windpipe) is a flexible, fibrocartilaginous tube that conducts air from the larynx to the principal bronchi.
• Its wall contains 16–20 C-shaped hyaline cartilage rings that maintain the patency of the airway.

Location

– The trachea lies in the lower part of the neck and superior mediastinum.

Extent

• Beginning: It starts at the inferior border of the cricoid cartilage, opposite the C6 vertebra.
• Termination: It divides into the right and left principal bronchi at the carina, usually near the T4–T5 level. The level varies with respiration, posture, and individual anatomy and may extend to T6.

Dimensions

• Length: Approximately 10–11 cm in adults.
• External diameter: About 2 cm in males and 1.5–2 cm in females.
• Internal diameter: It varies with age, body size, and respiratory movements.

Relations of Trachea

Relations of cervical part of trachea

Anteriorly

1. Skin.
2. Superficial fascia containing anterior jugular veins.
3. Investing layer of deep cervical fascia.
4. Jugular venous arch in the suprasternal space.
5. Sternohyoid and sternothyroid muscles.
6. Isthmus of thyroid gland covering the second to fourth tracheal rings.
7. Anastomosis between the anterior branches of superior thyroid arteries.
8. Inferior thyroid veins.
9. Thyroidea ima artery, when present.
10. Cervical extension of thymus, particularly in children.

Posteriorly

1. Oesophagus.
2. Recurrent laryngeal nerves in the tracheoesophageal grooves.

On each side

1. Lateral lobe of thyroid gland.
2. Common carotid artery.
3. Carotid sheath.

Relations of thoracic part of trachea

Anteriorly

1. Manubrium sterni.
2. Left brachiocephalic vein.
3. Brachiocephalic trunk.
4. Left common carotid artery.
5. Arch of aorta.
6. Deep cardiac plexus near the tracheal bifurcation.
7. Thymus or its remnants.

Posteriorly

1. Oesophagus.
2. Vertebral column, separated by the oesophagus.

On the right

1. Right lung and mediastinal pleura.
2. Right vagus nerve.
3. Arch of azygos vein.

On the left

1. Arch of aorta.
2. Left common carotid artery.
3. Left subclavian artery.
4. Left lung and mediastinal pleura.
5. Left recurrent laryngeal nerve.

Vascular Supply, Lymphatic Drainage, and Innervation

Arterial Supply

• Cervical part: It receives branches mainly from the inferior thyroid arteries.
• Thoracic part: It receives branches from the bronchial arteries.

Venous Drainage

– Venous blood drains through the inferior thyroid and bronchial venous systems.

Lymphatic Drainage

– Lymph drains into the pretracheal, paratracheal, and tracheobronchial lymph nodes.

Nerve Supply

Parasympathetic fibres

• They arise from the vagus and recurrent laryngeal nerves.
• They provide sensory and secretomotor innervation and stimulate contraction of the trachealis muscle.

Sympathetic fibres

• They arise from cervical and upper thoracic sympathetic pathways.
• They provide vasomotor innervation and influence tracheal smooth muscle and glandular activity.

Histology of Trachea

– The wall of the trachea consists of four principal layers:

1. Mucosa.
2. Submucosa.
3. Fibromusculocartilaginous layer.
4. Adventitia.

Mucosa

• It is lined by pseudostratified ciliated columnar epithelium containing:
  1. Ciliated cells: They propel mucus towards the pharynx.
  2. Goblet cells: They secrete mucus that traps inhaled particles.
  3. Basal cells: They act as progenitor cells for epithelial renewal.
  4. Brush cells: They have sensory functions.
  5. Small granule cells: They contain neuroendocrine secretory granules.
• The epithelium rests on a prominent basement membrane.
• The lamina propria contains elastic fibres, blood vessels, and lymphoid cells.

Submucosa

– It consists of loose connective tissue containing blood vessels, nerves, and seromucous glands.

Fibromusculocartilaginous Layer

• It contains approximately 16–20 C-shaped rings of hyaline cartilage.
• The rings are connected by fibroelastic tissue.
• Their posterior free ends are joined by the trachealis smooth muscle, forming the membranous posterior wall.

Adventitia

– It is the outer connective tissue layer that anchors the trachea to surrounding structures.

OESOPHAGUS

• The oesophagus is a muscular tube lined by mucous membrane that connects the pharynx to the stomach.
• It is divided into three parts:
  • Cervical part.
  • Thoracic part.
  • Abdominal part.

Measurements

• Length: Approximately 25 cm (10 inches).
• Diameter: Approximately 2 cm in adults.
• The oesophageal lumen normally remains collapsed and expands during swallowing.

Course

• Beginning: It starts at the lower border of the cricoid cartilage, opposite the C6 vertebra, as a continuation of the pharynx.
• It descends anterior to the vertebral column through the superior and posterior mediastina.
• It passes through the oesophageal hiatus of the diaphragm at T10.
• Termination: It joins the cardiac end of the stomach approximately at the T11 vertebral level.

Curvatures

Anteroposterior curvatures

– The oesophagus follows the natural curvatures of the cervical and thoracic vertebral column.

Lateral curvatures

– It deviates slightly towards the left at two levels:

1. Near the root of the neck.
2. In the lower thorax, around the T7 vertebral level.

Constrictions

– The oesophagus has four commonly described anatomical constrictions.

1. First constriction (pharyngoesophageal): It lies at C6, approximately 15 cm from the upper incisors, and is produced by the cricopharyngeus muscle.
2. Second constriction (aortic): It occurs where the arch of aorta crosses the oesophagus, approximately 23 cm from the upper incisors.
3. Third constriction (bronchial): It occurs at the crossing of the left principal bronchus, approximately 27–28 cm from the upper incisors.
4. Fourth constriction (diaphragmatic): It lies at the oesophageal hiatus of the diaphragm (T10), approximately 38–40 cm from the upper incisors.

Clinical significance: These constrictions are important sites where swallowed foreign bodies may become lodged and where narrowing may be encountered during oesophagoscopy.

Relation of Cervical Part of Esophagus

Anterior Relations

• Trachea.
• Recurrent laryngeal nerves in the tracheoesophageal grooves.

Posterior Relations

• Longus colli muscle.
• Prevertebral fascia.
• Cervical vertebral column.

Lateral Relations

• Common carotid arteries and carotid sheaths.
• Lateral lobes of the thyroid gland.
• Thoracic duct near the root of the neck on the left side.

Blood Supply, Lymphatic Drainage, Innervation

Arterial supply

• Cervical part: It receives branches from the inferior thyroid arteries.
• Thoracic part: It is supplied by oesophageal branches of the thoracic aorta and bronchial arteries.
• Abdominal part: It receives branches from the left gastric and left inferior phrenic arteries.

Venous drainage

• Cervical part: It drains into the inferior thyroid veins.
• Thoracic part: It drains mainly into the azygos and hemiazygos venous systems.
• Abdominal part: It drains into the left gastric vein and systemic venous tributaries.

Note: The lower oesophagus is an important site of portosystemic venous anastomosis between tributaries of the left gastric vein (portal system) and azygos venous system (systemic circulation).

Lymphatic drainage

• Cervical part: It drains mainly into the deep cervical and paratracheal lymph nodes.
• Thoracic part: It drains into the paratracheal, tracheobronchial, and posterior mediastinal lymph nodes.
• Abdominal part: It drains into the left gastric and coeliac lymph nodes.

Nerve supply

Parasympathetic fibres

• They arise from the vagus and recurrent laryngeal nerves.
• They provide motor innervation to the oesophageal musculature and secretomotor supply to its glands.

Sympathetic fibres

• They arise from upper and middle thoracic sympathetic pathways.
• They provide vasomotor fibres and carry many visceral pain afferents.

Histology of Esophagus

– The oesophageal wall consists of four principal layers:

1. Mucosa.
2. Submucosa.
3. Muscularis externa.
4. Adventitia.

Mucosa

• It is lined by non-keratinised stratified squamous epithelium, which protects against abrasion during swallowing.
• The lamina propria contains loose connective tissue, blood vessels, and occasional mucous glands.
• The muscularis mucosae consists predominantly of longitudinally arranged smooth muscle fibres.

Submucosa

• It consists of fibroelastic connective tissue containing blood vessels, nerves, and compound tubuloalveolar mucous glands.
• These glands produce mucus that lubricates the oesophageal lumen.

Muscularis Externa

• It consists of inner circular and outer longitudinal muscle layers.
• The type of muscle varies along the oesophagus:
  • Upper third: Predominantly skeletal muscle.
  • Middle third: Both skeletal and smooth muscle.
  • Lower third: Predominantly smooth muscle.

Adventitia

• Most of the oesophagus is surrounded by fibroelastic connective tissue forming the adventitia.
• The short abdominal segment is partly covered by serosa (visceral peritoneum).