Veterinary Histology and Embryology

BASED ON THE DUCTULAR SYSTEM

• Glands are classified into Simple and compound glands in this view.
• The ductular system of a gland may consist of simple epithelial tube draining a secretory unit and opening at the epithelial surface or it may be in the form of an extensively branched system, the initial smallest ducts draining the secretory units, uniting to form larger ducts at various levels.
• Finally forming the main duct, which conveys the secretion from the gland to an epithelial surface. Based on the ductular system glands are classified as simple glands, which has an unbranched duct system, and compound glands which has a branched duct system.

Simple glands:

• Simple tubular glands
  • The glands consist of simple tubular secretory unit which opens on the surface. All the cells may be secreting or only the deeply situated cells alone. In the later case the more superficial portion of the tubule serves merely as a duct. The more highly developed simple tubular glands consist of a mouth opening upon a surface, a neck usually somewhat constricted and a fundus or deep secretory portion of the gland.
  • Simple tubular glands are divided according to the structure of the fundus into
    • Straight
      •  A Straight tubular gland is one in which the entire tubule runs in a straight unbranched course. e.g. glands of the large intestine.
    • Coiled
      • A coiled tubular gland is one in which the deeper portion of the tubule is coiled or convoluted. The sweat glands of the skin are the most typical examples.
    • Branched tubular glands
      • A branched tubular gland is a simple tubular gland in which the deeper portion of the tubule branches, the several branches being lined with secreting cells and opening into a superficial portion which serves as a duct e.g. fundic, pyloric and cardiac glands of the gastric mucous membrane.
• Simple alveolar glands
  • These are found in branched form, e.g., Sebaceous and Meibomien glands.
• Simple tubulo-alveolar or tubulo–acinar glands
  • These are found only in the branched form e.g. Brunner’s glands of the duodenum.

Compound glands

These contain a large number of branching duct systems, which finally join to form a common or main duct, which coveys the secretion from the glands. Based on the nature of secreting units compound glands are classified into:

• Compound tubular glands
  • The terminal ends of the duct systems end in tubular secretary units, which are usually tortuous or coiled e.g. kidney, Testis.
• Compound alveolar glands
  • The terminal part of the duct system ends in alveoli with dilated sac-like lumen e.g. mammary gland.
• Compound tubulo-alveolar or (tubulo-acinar) glands
  • In these the secretary units are with terminal or lateral alveoli. E.g. parotid gland, pancreas etc.
  • Large amount of connective tissue present in a compound gland, which starting off from the capsule divides the gland into lobes by interlobar connective tissue septa. Each lobe is subdivided by connective tissues into lobules, which are usually microscopical in size called interlobular connective tissue septa. Each lobule is made up of a number of secreting units or acini, with the ducts and inter acinar connective tissue.

Parenchyma

• The secreting units may be tubular, alveolar or tubulo alveolar and are made of secretory cells. The acinar duct is the smallest duct continuous with the lumen of the acinus and is usually lined by flattened or cuboidal epithelium. The acinar ducts unite within the lobule to form intralobular ducts and several of this type unites to form an Interlobular duct, located in the connective tissue between the lobules. Interlobular and interlobar ducts are similarly formed, the latter uniting to form the main duct of the gland.
• Interlobular ducts are lined by columnar epithelium but have a thin connective tissue coat. As the duct increases in caliber the connective tissue coat also proportionately increases and the epithelium may become two layered columnar.