BIOLOGY Volume 1 - A Guide to General Biology - 2004

6. HISTOLOGY

Histology is the branch of science that studies Tissues. All Multicellular Organisms contain groups of Cells that are similar in Structure and function, known as tissues. A tissue can be defined as a group of physically interconnected cells and associated intercellular substances, specialized to perform one or more specific Functions. The cells forming a specific tissue typically share a common origin, meaning they develop from the same region of the embryo. The specialization, or adaptation, of each tissue to perform a particular function enhances the overall efficiency of the Organism through the division of labor.

Higher Levels of Organization beyond THE TISSUE LEVEL exist—primarily in animals. Various tissues combine into functional units called Organs, such as The Heart or Stomach. In animal organisms, organs are integrated into even larger functional units known as systems; Examples include the Digestive System (Pancreas, Liver, stomach, duodenum, etc.) or The Cardiovascular system (heart and Blood Vessels).

All cells within a given tissue may be of a single type. For instance, in plants, parenchyma, collenchyma, and cork are constructed from a single Cell type, while in animals, squamous epithelium is a similar example. Examples of tissues containing multiple cell types include xylem and phloem in plants, and certain types of Connective Tissue in animals.

The Study of tissue structure and function relies primarily on light Microscopy, employing various techniques for tissue fixation, staining, and sectioning (refer to the relevant methodologies in sections 5.11 and 5.12).

In this chapter, we will explore histology at a level accessible through the Light Microscope. In some cases, Electron microscopy data will be introduced for greater clarity. When examining the relationship between tissue structure and function, It is important to keep in mind the three-dimensional nature of cellular components and their interconnections. Information of this kind is assembled piece by piece by studying thin tissue sections, predominantly transverse and longitudinal. Neither type of section alone can provide all the necessary details, but in combination, they often yield a complete picture. Certain cells, such as xylem vessels and tracheids, can be observed intact after plant tissues undergo maceration; this process destroys the softer tissues while leaving the tougher, Lignin-impregnated histological elements of the xylem—vessels, tracheids, and wood fibers¹—intact.

Plant tissues can be divided into two main groups:

1. Consisting of a single cell type

— parenchyma (sec. 6.1.1)

— collenchyma (sec. 6.1.2)

— sclerenchyma (sec. 6.1.3)

2. Consisting of multiple cell types

— xylem (sec. 6.2.1)

— phloem (sec. 6.2.2)

Animal tissues can be divided into four groups:

1. Epithelial (sec. 6.3)

2. Connective, including loose (areolar), fibrous, adipose, Cartilage, and bone (sec. 6.4)

3. Muscular (sec. 6.5)

4. Nervous (sec. 6.6)

Table 6.1 provides a brief summary of certain plant tissues, along with their functions and distribution within the plant. Fig. 6.1 will help the reader visualize where these tissues are located.

Class="center">Table 6.1. Main characteristics, functions, and Distribution of plant tissues¹

Fig. 6.1. Introduction/19.html">Primary Structure of a leaf, stem, and root in a young dicotyledonous plant.

¹ Information on The structure of certain plant tissues can be found in various sections of this book. In particular, phloem structure is described in more detail in Chapter 13, which examines the relationship between tissue structure and its transport function. The Development of plant tissues from meristematic cells is discussed in Chapter 22, alongside topics such as secondary growth and the structure of wood (secondary xylem) and bast.

6.1. Simple Plant Tissues (Tissues Composed of a Single Cell Type)

6.1.1. Parenchyma

Structure

The structure of parenchyma is shown in Fig. 6.2. Parenchyma cells are largely rounded (isodiametric) in shape, though they may also be elongated.

Fig. 6.2. Structure of parenchyma cells. A. Transverse section. Cells are typically isodiametric (rounded), but can be elongated. B. Transverse section of a Helianthus stem pith. The pith is a packing and supportive tissue located in the center of dicot stems.

Functions and Distribution

1. Parenchyma is referred to as packing tissue because its unspecialized cells fill the spaces between more specialized tissues, as seen, for example, in the pith of a stem or the cortex of stems and roots (Fig. 6.1). Cells of this tissue make up the bulk of a young plant.

2. The osmotic properties of parenchyma cells play an important role because, in a turgid state, these cells are tightly packed and consequently provide support to the organs in which they are located. This is particularly crucial for herbaceous stems, where such support is essentially the sole mechanism. During dry periods, the cells of these plants lose water, causing the plants to wilt.

3. Despite being structurally unspecialized, parenchyma cells are metabolically active: many processes vital to the plant organism take place within them.

4. Air-filled intercellular spaces form a system that facilitates gas exchange between living cells and the external environment, connecting via Stomata (specialized leaf pores) or lenticels (specialized slits in woody stems). Through these intercellular spaces, oxygen for Respiration and carbon dioxide for photosynthesis are supplied to the living cells. This system of air spaces is particularly well-developed in spongy parenchyma.

5. Parenchyma cells often serve as nutrient storage sites, primarily in storage organs such as potato tubers, where starch is stored within the amyloplasts of these cells. A rare instance of reserves being deposited in the thickened walls of parenchyma cells is found in the date palm: here, hemicelluloses are stored as reserves in the seed endosperm in this manner.

6. The walls of parenchyma cells provide an important pathway for the movement of water and mineral salts within the plant (part of the 'apoplastic pathway' described in Chapter 13). Substances can also move via plasmodesmata connecting adjacent cells.

7. In certain PARTS OF THE plant, parenchyma cells undergo modification to become more specialized. Below, we list some of the tissues that can be regarded as modified parenchyma.

EPIDERMIS. The epidermis is a thin protective tissue consisting of a single cell layer (see Fig. 6.1); it completely covers the primary plant body. Its primary function is to protect the plant against desiccation and pathogen invasion. During secondary growth, the epidermis may rupture and be replaced by a layer of cork (Chapter 22). The typical structure of epidermal cells is shown in Fig. 6.3.

Fig. 6.3. Structure of epidermal cells. A. Epidermal cells in transverse section, longitudinal section, and three-dimensional rendering. B. Leaf epidermis of a dicotyledonous plant (top view). C. Leaf epidermis of a monocotyledonous plant (top view). D. A spider mite caught and killed by glandular hairs on a potato leaf. Some potato glandular hairs contain an enzyme capable of digesting animal matter, suggesting that potatoes may function as carnivorous plants. It is possible that many other plants not conventionally considered carnivorous possess this capability. E. Young leaf of hemp (Cannabis sativa) with glandular hairs and trichomes. F. Surface of a stinging nettle leaf (Urtica dioica).

Epidermal cells secrete a waxy substance called cutin. Cutin often impregnates the walls of epidermal cells and forms a film of varying thickness on their outer surface, known as the cuticle. This reduces water loss (limits Transpiration) and provides additional protection against pathogens (disease-causing organisms).

Examination of leaf surfaces under a light microscope reveals that in dicots, epidermal cells are irregular in shape with undulating walls (Fig. 6.3, B), whereas in monocots, their shape is more regular and approaches a rectangular form (Fig. 6.3, C). Distributed at regular intervals across the leaf surface are specialized epidermal cells known as guard cells. These always occur in pairs—two adjacent cells with an opening visible between them, referred to as a stoma (Fig. 6.1 and Fig. 6.3, B and C). Guard cells possess a characteristic shape distinct from other epidermal cells. Furthermore, they are the only epidermal cells containing chloroplasts; all other epidermal cells are colorless. The size of the stomatal pore depends on the turgor pressure of the guard cells (for details, see Chapter 13). Stomata facilitate gas exchange for photosynthesis and respiration, making them most abundant in the leaf epidermis, though they also occur on stems. Water vapor also escapes from the plant through the stomata as part of the overall process known as transpiration.

Certain epidermal cells bear outgrowths in the form of fine hairs (trichomes). These hairs may be unicellular or multicellular and serve diverse functions. On roots, in the region immediately behind the root tip, unicellular hairs develop to increase the surface area available for the absorption of water and mineral salts. In cleavers (Galium aparine), the stems and leaves bear hooked hairs (prickles) that help the plant cling to support structures and prevent it from sliding down.

Hairs frequently perform various protective functions as well. Along with the cuticle, they help reduce water loss by trapping a layer of moist air near the plant surface and reflecting sunlight. Some hairs, particularly in xerophytes (plants adapted to arid environments), have the capacity to absorb water. Short, prickle-like hairs can serve as mechanical defense for the plant. The stinging hairs of the stinging nettle (Urtica dioica) feature a rigid cell wall terminating in a brittle tip. When an animal brushes against such a Hair, the tip breaks off, and the barbed, sharp point pierces the Skin. This releases the Contents of the bulbous cell base, which contains irritating chemicals, into the wound. In some cases, hairs form a barrier around the floral nectaries. This barrier excludes crawling insects and thereby promotes cross-pollination by larger, flying insects.

Glandular cells, sometimes resembling hairs in shape, are also found within the epidermis. They may secrete a sticky substance used by the plant to trap insects—insects adhere to it and perish. This adaptation either serves a purely defensive function or, if the exudate contains Enzymes, enables the plant to digest and assimilate insect tissues. Such plants can be classified as carnivorous (Fig. 6.3, D). In some instances, such as in the leaves of lavender (Lavendula), the plant's aroma is also attributed to glandular hairs.

MESOPHYLL (SEE ALSO FIG. 7.3 AND 7.4). This packing tissue is located between the two layers of the leaf epidermis (Fig. 6.1) and consists of modified parenchyma cells that carry out photosynthesis. Photosynthetic parenchyma is sometimes referred to as chlorenchyma. The Cytoplasm of chlorenchyma cells contains A large number of chloroplasts, where the reactions of photosynthesis take place. In dicotyledonous plants, the mesophyll is composed of two distinct layers: the upper layer is palisade parenchyma, whose cells are columnar in shape, while the lower layer is spongy parenchyma, featuring irregularly shaped cells containing fewer chloroplasts. Photosynthesis occurs primarily in the palisade parenchyma, whereas the air spaces of the spongy parenchyma ensure intensive gas exchange.

ENDODERMIS. The endodermis refers to the layer of cells surrounding a plant's vascular tissue. It can be viewed as the innermost layer of the cortex (Fig. 6.1). Endodermal cells are typically parenchymatous, yet they may undergo both physiological and structural modifications. In roots, where the endodermis consists of a single cell layer, it is much more distinct than in stems because each such cell features a Casparian strip (Fig. 6.4)—a band of suberin (a lipid-like substance) encircling the cell wall. At a later developmental stage, further thickening of the cell wall may occur. For more on the structure and function of the root endodermis, see Chapter 13.

Fig. 6.4. STRUCTURE OF THE root endodermis. A. Cross section showing a young endodermis with Casparian strips. B. Cytology/practical/72.html">Cross section of an old dicot root.

In dicot stems, vascular bundles form a ring, while the endodermis—consisting of one or several cell layers—lies just outside this ring and directly adjacent to it (Fig. 6.1). Quite often, the endodermis is indistinguishable in appearance from the rest of the cortex; however, starch grains sometimes accumulate within its cells, turning it into a so-called starch sheath (endodermis), which is easily visualized by staining the preparation with iodine. These starch grains can sediment within the cells under The Influence of gravity, enabling the endodermis to play a crucial role in the geotropic response, much like the Cells of the root cap (Chapter 16).

PERICYCLE. In roots, situated between the central vascular tissue (stele) and the endodermis is the pericycle—a layer consisting of one or more rows of cells (Fig. 6.1). The pericycle retains meristematic activity: lateral roots originate here. In plants whose roots undergo secondary growth, the pericycle contributes to this process. In stems, a homologous layer is typically absent.

COMPANION CELLS. These are specialized parenchymatous cells associated with sieve tubes, playing an active role in their function. Companion cells are metabolically highly active; they are distinguished from ordinary parenchyma cells by a denser cytoplasm and smaller vacuoles. The origin, structure, and function of companion cells will be discussed in Section 6.2.2.