1. Embryologic origin of digestive system, main stages of development.
Digestive system derives from endodermal germ layer. Formation depends on cephalocaudal & lateral folding of embryo. By beginning of 4th week develop head & tail folds. As a result, large portion of endoderm-lined cavity form primitive duct. 2 other portions of endoderm-lined cavity, yolk sac & allantois, remain outside embryo.
Primitive duct give rise to:
Foregut, from anterior part of endoderm Hindgut in tail region Midgut between foregut & hindgut (midgut temporarily connected with yolk sac by vitelline duct).
- Endoderm forms epithelial lining of digestive tract & gives rise to parenchyma of glands, such liver & pancreas. - Muscle, connective tissue & peritoneal components of gut wall derived from splanchnic mesoderm.
The foregut: Lies between brain & heart, separated by oropharyngeal membrane from stomodeum. Gives rise to:
• Primordial pharynx (oral cavity, pharynx, tongue, tonsils, salivary glands, upper respiratory system) • Lower respiratory system • Esophagus & stomach • Part of duodenum • Liver, gallbladder, bile ducts & Pancreas
The midgut:
• Small intestine, including most of duodenum • Cecum • Appendix • Ascending colon • Part of transverse colon
The hindgut:
• Part of transverse colon • Descending colon • Sigmoid colon • Rectum • Upper part of anal canal • Epithelium of urinary bladder • Most of urethra
2. Histological structure and development of teeth.
Structure:
- Crown - Neck - One or more roots
Structural components:
Pulp → Tooth pulp consists of loose connective tissue. main components are:
Fibroblasts Dendritic cells Non differentiated cells Lymphocytes Macrophages
Have: big number of blood vessels, very dense in crown region, Intensive network of lymphatic vessels & innervated.
Dentin → composed of 70% inorganic materials, 20% organic materials, 10 % water. Makes largest portion of tooth volume. Crown region covered by enamel, root region covered by cementum. No blood vessels & nerves.
Dentinal tubules:
- Directed - from pulp to dentin-enamel junction or dentin-cementum junction - Diameter 2.5μm near pulp & 1μm at periphery
The dentinal tubules contain:
- Processes of odontoblasts - Afferent nerve endings - Antigen presenting cells processes from pulp
Primary dentin is older. Later formed secondary dentin.
Odontoblasts - compose single cell line between pulp & dentin, in older age - several lines. Pulp cavity diminishes with age.
Have processes, (Tomes fibers), they branch & located in dentinal tubules. Body of odontoblasts & processes innervated with afferent nerves. Number of odontoblasts higher in crown than in root. They have developed ER & Golgi. They release predentine. In remaining part of tubulae located collagen & tissue fluid. Sensitivity of dentin connected with movement of tissue fluid.
Cementum → Components: collagen fibers, minerals.
Very thin near teeth neck, thicker at the way to apical foramen. External layer similar to bone.
