Exocrine glands arise from epithelial surfaces during embryogenesis. A stalk of cells grows into the underlying connective tissue, forming the gland. The connecting cord of cells remains attached to the gland cells and forms duct. Spaces develop within the cord which coalesce into either a straight or branching duct.
Endocrine glands arise from epithelial surfaces, too. Downgrowths of ectoderm or endoderm into underlying connective tissue form gland primordia. The developing gland is connected to the parent epithelium by a stalk or cord of cells. During differentiation the connecting cord disappears leaving an "island" of cells embedded within the connective tissue.
Classification of exocrine glands
Exocrine glands can be classified by their cell number, by the character of their ducts and by the nature of their secretions. By the cell number glands may be:
- Unicellular. E.g.: Goblet cell of small and large intestine and of respiratory tract (portions).
- Multicellular:
1. Without ducts - a ductless sheet of secretory epithelial cells. E.g.: surface mucous cells (mucous lining cells) of the stomach; cervical lining epithelium
2. With ducts - describes most exocrine glands; E.g.: salivary glands.
By the type of secretion glands may be:
Mucous. These cells secrete a carbohydrate-rich, viscous and slimy secretion. The nuclei are dense, darkly stained, and pressed down against the base of the cell. The cytoplasm appears pale unless the cells are stained for carbohydrate (e.g. PAS stain). Mucous glands usually have a more prominent lumen as compared to serous glands. E.g.: some minor salivary glands, glands of esophagus, cardiac and pyloric glands of stomach, Brunner's glands of duodenum (alkaline mucus produced), colonic crypts (mostly goblet cells).
Serous. Pyramidal-shaped cells surround small central lumen of acinus (alveolus). These secrete enzyme-rich products as in the pancreas or parotid salivary glands. Enzymes are produced by RER and packaged into apical zymogen granules, containing proenzymes, which become activated after secretion. Usually the secretion is watery. Typically, the unit of organisation is an alveolar or acinar form. The nuclei are rounded or oval and the cells are basophilic due to the abundance of RNA in the basal cytoplasm, which is necessary for protein synthesis. Apical cytoplasm contains acidophilic (eosinophilic) (red) protein-containing granules. Granules may or may not be seen in the apex of the cell, depending on the type of staining.
Mixed (Seromucous). Mixed, mucous and serous glands. Gland may contain mixture of all-mucous and all-serous acini (alveoli), or mixed seromucous acini composed of both types of secretory cells. Seromucous acini often organised as mucous acini capped by serous "demilunes". E.g.: Submandibular and sublingual salivary glands; mixed glands of the nasal cavity, paranasal sinuses, nasopharynx, larynx, trachea and bronchi.
By structure of duct system glands may be with:
- Unbranched duct - simple glands. E.g: sweat glands.
- Branched duct - compound glands. E.g: exocrine pancreas.
By structure of secretory unit glands may be with:
- Unbranched sectetory unit – unbranched glands.
- Branched secretory unit – branched glands.
By shape of secretory unit glands may be
- Tubular- cylindrical lumen surrounded by secretory cells. E.g: sweat glands.
- Acinar (alveolar)- dilated saclike secretory unit. E.g: sebaceous glands, mammary glands.
- Tubuloacinar (tubuloalveolar)- intermediate in shape, or having both tubular and alveolar secretory units. E.g: exocrine pancreas, submandibular salivary gland.
BLOOD
Blood is a specific fluid connective tissue. Like the other connective tissues, blood consists of cells and intercellular substance, which is called plasma.
The relative volume of cells and plasma is about 45% and 55%, respectively. This value is called a hematocrit.
Functions of blood are the followings:
1. Conveying nutrients and oxygen to cells;
2. Carrying wastes and carbon dioxide away from the cells;
3. Carrying hormones and other regulatory agents to – and from – the cells;
4. Homeostatic role. It means keeping the constancy of composition of inner medium of organism.
Plasma. Most of the plasma consists of water (90–93%). The rest of the plasma – 7–10% consists of lipids, proteins(albumins for normal blood pressure, globulins (antibodies), protrombin) and carbohydrates.
Blood cells include: red blood cells, also called erythrocytes; white blood cells, also called leukocytes; platelets, also called thrombocytes.
Erythrocytes are specialized for the transport of oxygen and carbon dioxide. Erythrocytes are biconcave discs having diameter of about 7 μm. They are called normocytes. Large erythrocytes (more than 8 μm in diameter) are called macrocytes. Less than 6 μm in diameter are called microcytes. Predominance of macro- and microcytes is called anisocytosis. Erythrocytes don’t have a nucleus and organelles.
Hemoglobin fills the cytoplasm and is the specific component of the red blood cells, which is essential for their function, that is, plays an important role in carrying oxygen.
Erythrocytes maintain their normal shape only if suspended in an isotonic solution. In a hypotonic solution erythrocytes burst. This process is called hemolysis. In a hypertonic solution their surfaces become irregular (crenation), what is called plasmolysis. Such cells are called echinocytes, small amound of round shaped erythrocytes may be found. Condition followed by big amound of echinocytes and round shaped cells is called poikilocytosis.
One cubic millimeter of blood contains about 5 million erythrocytes.
Immature or young erythrocytes are called reticulocytes. One cubic millimeter of the blood contains about 1 – 2% of reticulocytes.
The erythrocytes have a life span about 120 days. Increase of amount of erythrocytes is called erythrocytosis. Decrease of amount of erythrocytes is called erythropenia.
White blood cells – leukocytes. Unlike erythrocytes leukocytes contain a nucleus and organelles. All leukocytes can move out of vessels to surrounding tissues. Most leukocytes have a relatively short life span (from several hours to several days). Some leukocytes live many years.
There are several types of leukocytes. Some of leukocytes have granules in their cytoplasm and are, therefore, called granulocytes.
Depending on the staining of their granules granulocytes are divided into neutrophils, eosinophils and basophils.
The other types of leukocytes haven’t granules and are therefore called agranular leukocytes – agranulocytes. Among them there are lymphocytes and monocytes.
There are about 7000 leukocytes (range 5000 – 10 000) in every cubic millimeter of blood. Of these about two third (60 – 70%) are neutrophils. About one fourth (20 – 30%) are lymphocytes. The eosinophils are about 3 - 5%, the basophils - about 0,5 – 1,5%, and the monocytes - about 5 – 9%.
The increased amount of leukocytes is called leukocytosis, decreased – leukopenia.
