Golgi Complex or Golgi bodies : Cell Organelles

Golgi Complex or Golgi bodies : Cell Organelles

This cytoplasmic organelle is named after its discoverer Golgi. The structure was discovered in 1898. 

The golgi bodies are also called lipochon- dria. For several years there was considerable disagreement about the existence of that organelle. 

Most of the early biologists believed that it was an artifact of fixation or staining procedures. 

Studies with phase contrast micro- scopes in the early 1940's also indicated the existence of golgi bodies. 

The study of electron micrographs of thin sections of cells in 1950's finally proved beyond doubt the existence of golgi bodies in all the cells of eukaryotes. 

The golgi apparatus does not exist in the prokaryotes.

Shape and Size:

Morphology of this complex is variable depending upon the type of on in which these are present. 

In fully mature and functional cells this complex is well developed (e.g., secretory cells and neurons), where it is clearly a reticulate structure, while in non-functional cells it is poorly developed. 

In plant cells Golgi bodies are about 1 to 3 u (mu) in leng and about 0.5 μ (mu) in height. These are basically identical in structure to animal cell's Golgi complex.

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Structure of Golgi bodies:

The electron microscopic studies have revealed that this organelle consists of series of compactly grouped smooth contoured membrane limited vesicles of variable shapes and dimensions and variable number of small vacuoles.

 They are selectively stained with Neutral red stain and differ from mitochondria in staining property because they do not take Janus green stain (special stain for mitochondria). 

 When the cytoplasm is centrifuged the mitochondria settle down first and golgi bodies afterwards. 

 This indicates that the golgi bodies are lighter than the mitochondria. 

 The presence of golgi bodies in plant cells has been denied by some early cytologists, but the electron micrographs in recent years have revealed that these bodies are of universal occurrence in both plant and animal cells.

The vesicles of golgi bodies are chiefly of two types:

1. Small and spherical vesicles.

2. Broad flattened vesicles in parallel or often in semicircular array, the cisternae (Singular-cisterna).

The cisternae are characterized by their dilated edges. They are compactly arranged in parallel fashion. 

The stack of flattened cisternae or saccules is known as 'dictyosome. 

The dictyosome has a polarity: its convex side forming the outer face and concave side forming the inner face. 

The cisternae on the outer face are very flat and thin whereas those on the inner face or concave side are comparatively much dilated and thick. 

The cisternae on the outer face react only with silver salt and osmic acid while those on the inner face do not react with silver salt and osmic acid. 

Thus the outer and inner faces are accordingly known as osmic or argentophilic and osmic or argentophobic.

 The number of vesicles per dictyosome varies presumably because of differ- ent functional stages of golgi complex. 

 The unit membrane of these cisternae are about 35 Å thick, smooth surfaced, and not associated with ribosome granules.

On its outer surface the dictyosome is often bounded by canaliculae or cisternae of endoplasmic reticulum. 

Numerous spherical vesicles found in the vicinity of dictyosome are budded off by the cisternae at their ends. 

Palade (1956-58) shown that the golgi bodies originate from smooth surfaced endoplasmic reticulum .

The smaller vesicles are aggregated around, the stacks of cisternae. 

These are also bounded by membranes.

 The central space of vesicle is very clear but frequently it becomes condensed and appears as small granule.

The golgi apparatuses are of usual occurrence in the secretory cells where they are involved in secretory process. 

In plant cells these bodies secrete mucoproteins, slime, mucilage, lipoproteins and other proteinaceous substances.

 Besides they also synthesize hemicelluloses and pectic group of polysaccharides especially during cell division. 

In nonsecretory cells the golgi apparatus is assigned some other functions:

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Function of Golgi complex:

Golgi apparatus appears to play an important role in the storage, packaging and secretion of certain cell products. It is involved in the formation of lysosomes and other enzyme-containing cellular inclusions, and in the formation of secretory granules in cells such as those found in the pancreas, pituitary and mammary glands, and mucous-secreting glands of the intestine and in many other cell types.

In general, Golgi complex is of vital importance and serves many functions:

1. Absorption of compounds:

Hirsch et al., have discovered that. when iron sugar is fed to an animal, iron becomes absorbed on Golgi bodies (Kedrowsky). Van Teel has shown that Golgi systems also absorb compounds of Cu and Au (gold). Kedrowsky has shown that Golg bodies of Opalina can absorb bismutose (compound of albumin and bismuth) and protargol (compound of albumin and silver). Palay and Kartin (1956) have indicated that Golgi complex are concerned with the storage and absorption of lipids. Thus, Kirkman and Severinghaus stated that Golgi apparatus acts as a condensation membrane for the concentration of products produced elsewhere into droplets or granules by losing water. which are transported to the cell surface for export. These products may be lipids, yolk, bile compounds, enzymes and hormones, etc. The proximity of Golgi and contractile vacuoles in protozoa may confirm the concentration behaviour of Golgi membranes. Golgi membranes may remove water from the products of synthesis during the formation of secretory granules.

2. Formation of secretory vesicles and secretion:

 The principal function of Golgi complex is secretion. In several types of cells, synthetic products from the rough endoplasmic reticulum are transferred to Golgi region, from where they are liberated from the cell through plasma membrane by pinocytosis. Secretory function of Golgi seems to be well founded experimentally.

3. Helps in enzyme formation:

 Bowen speaks of Golgi apparatus as a great intracellular centre of enzyme formation. Moricard has shown that Golgi body helps in the production of follicular fluid from granulosa cells of ovary. It may release zymogen granules (inactivated pancreatic enzymes) which arise from cisternae into secretory vesicles. Secretory vesicles containing granules migrate to cell surface where the membrane of the secretory vesicle and the cell membrane merge, releasing the contents into pancreatic ducts from where they pass into intestine as active digestive enzymes.

4. Production of hormones:

 Golgi body in endocrine cells helps in secretion of hormones. Cowdry has suggested that any harm to Golgi apparatus in thyroid gland cells will result in decline in secretion of its hormone.

5. Storage of protein:

 Vacuoles and vesicles which are the main components of Golgi complex become filled with protein-lipoid material for storage. These stored products help in secretory action. 6. Formation of acrosome. It forms the acrosome of sperm during sperm maturation as shown in figure 

6. The cisternae of Golgi complex are arranged in a cup-shaped pattern, the lamellae stacked in parallel.

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From the periphery of lamellae, small vesicles or vacuoles are pinched off. Gradually the system of cisternae is replaced by more vesicles and tubules and within some of these vesicles small granules appear. These granules represent secretory products within Golgi complex. Some of the granule-containing vesicles coalesce to form a single acrosome within a large vesicle, which comes to lie on the surface of the sperm nucleus.

The vesicle gradually spreads over more of the nuclear surface and eventually collapses on the nuclear membrane to form the cap material,acrosome.

 The dictyosomes release substances in the form of tiny vesicles budded from the saccules on the inner face. 

 These vesicles having limiting membrane of their own fuse together to form secretion granules of ever increasing sizes. 

 These granules are generally transported to the cell boundary where their membranes fuse with plasma membrane and finally their contents are discharged out of cell by ectocytosis . 

 Some proteins synthesized in granular endoplasmic reticulum, particularly those which need be coupled in varying proportions with polysaccharides to form muco or glucoproteins, pass through the cavities of the reticulum into the dictyosomes and from there they are discharged as secretion granules. 

 This transfer of proteins appears to take place through small vesicles budded from smooth cisternae of endoplasmic reticulum in the vicinity of dictyosomes. 

 The loss of membranes due to the budding of vesicles on the inner face of the dictyosomes appears to be compensated by contribution of vesicles by the smooth surfaced endoplasmic reticulum on the outer face. 

 This indicates that new saccules are formed on the outer face of dictyosomes while the internal saccules break up into secretion vesicles. 

This hypothesis is supported by the following observations:

(i) The number of saccules in dictyosome changes according to the physiological condition. 

(ii) The number decreases in starving cells and they completely disappear in enucleated cells.

The membranes of cisternae offer surfaces for enzymatic activities. 

Sometimes, the vesicles act as a system of channels collecting intracellular metabolites and fluids.

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