Plant and Animal Tissues: Cellular Structure and Function

1. Plant Tissues

Plant tissues appear exclusively in cormophyte plants and are divided into two main categories: embryonic (meristematic) and adult tissues.

1.1. Meristematic Tissues

Meristematic tissues originate from undifferentiated embryonic cells and retain their capacity for continuous cell division. They allow the plant to grow continuously, and from them, all adult tissues derive.

Meristematic cells are typically small, have thin cell walls, large nuclei, and few vacuoles. When they divide, one daughter cell remains undifferentiated while the other differentiates into a specialized cell type.

There are two primary types of meristems:

• Primary meristems: Derived directly from embryonic cells. They are responsible for growth in length (elongation) and are located at the growing tips (apices) of roots and stems.
• Secondary meristems: Derived from adult cells that regain their capacity for division. There are two lateral meristems: the cambium (vascular cambium) and the phellogen (cork cambium). They trigger secondary growth, increasing the thickness of stems and branches.

1.2. Adult Plant Tissues

Adult tissues are formed through cellular differentiation from meristems. Based on their function, they are classified into several groups:

A) Parenchymal Tissues (Ground Tissue)

Parenchymal tissue, also known as fundamental tissue or parenchyma, is the most abundant tissue in the plant. Its primary functions are storage, metabolic processing, and providing cushioning between other tissues. Its cells are relatively undifferentiated, feature thin primary cell walls, contain plastids, leave intercellular spaces between them, and possess a large central vacuole.

Depending on their specialized physiological role, several distinct types exist:

• Storage parenchyma (Reserve parenchyma): Stores reserve substances, predominantly starch. It is abundant in organs such as seeds, tubers, fleshy roots, and certain leaves.
• Chlorophyll parenchyma (Chlorenchyma): Abundant in green stems and leaves. Its cells contain numerous chloroplasts responsible for photosynthesis. Two main arrangements exist: palisade parenchyma and spongy (lagunar) parenchyma.
• Aquiferous parenchyma: Stores water. It is abundant in the stems and leaves of plants adapted to dry environments, such as cacti and succulents.
• Aeriferous parenchyma (Aerenchyma): Cells leave large intercellular spaces and internal chambers where air is stored. It is typical of aquatic plants.
• Vascular parenchyma: Closely accompanies conductive (vascular) tissues to provide metabolic and structural support.

B) Protective Tissues

Protective tissues cover external surfaces, shield the plant from desiccation, and protect against external mechanical or biological damage, as well as covering some internal regions. The three most prominent protective tissues are:

• Epidermis: Covers young parts of plants that are less than one year old. It consists of a single layer of living, flattened cells that leave no intercellular spaces between them. On aerial stems and leaves, the outer surface is covered by a cuticle, composed of cutin (a lipid) and surface waxes, which renders it impermeable to water and gases. For respiration and photosynthetic gas exchange, specialized structures called stomata are embedded in the epidermis. Each stoma consists of two kidney-shaped guard cells that open or close a central pore (the stoma or ostiole).
• Endodermis: Located internally within the root, separating the vascular bundles from the inner cortex parenchyma. It consists of a single layer of living cells whose radial and transverse walls are impregnated with waterproof substances (lignin and suberin), forming the Casparian strip. This strip blocks apoplastic transport and forces water and solutes to cross through the living cytoplasm of the endodermal cells into the vascular cylinder.
• Suber (Cork): Replaces the epidermis in stems and roots when the epidermis is shed during secondary growth. To communicate with the exterior and permit the exchange of gases and water vapor, the cork layer is perforated by lenticels, which are open pores and fissures filled with loosely arranged, living parenchymal cells.

C) Support Tissues

Support (mechanical) tissues provide mechanical strength and rigidity. There are two primary types:

• Collenchyma: Present in growing, young parts of woody plants and acts as the sole mechanical support tissue in herbaceous plants. It consists of living, elongated, prismatic cells with unevenly thickened primary walls that may contain chloroplasts.
• Sclerenchyma: Found in mature and woody parts of plants. Its cells die upon reaching maturity, developing very thick, rigid, and lignified secondary cell walls that offer high resistance. Sclerenchyma cells can be elongated (fibers) or rounded/irregularly shaped (sclereids or sclera).

D) Conductive (Vascular) Tissues

Conductive tissues transport sap throughout the plant. They consist of highly specialized cells arranged in continuous columns that form tubular transport pathways. There are two primary systems:

• Xylem (Woody Vessels): Transports raw sap (water and dissolved inorganic minerals) upward from the roots to the photosynthetic sites. There is primary xylem, formed by the apical meristem, and secondary xylem, derived from the vascular cambium to renew transport capacity annually. It consists of conductive vascular elements (vessels, tracheae, and tracheids) as well as non-vascular elements (parenchyma and fibers).
• Phloem (Liberian Vessels / Sieve Tubes): Conducts elaborated sap (organic nutrients, sucrose, and amino acids) from the leaves and green photosynthetic stems to the rest of the plant. It is divided into primary phloem and secondary phloem. Both consist of conductive elements (sieve cells and sieve tube members) supported by non-vascular companion cells, parenchyma, and fibers.

2. Animal Tissues

Animals possess four primary fundamental tissue types: epithelial, connective, muscular, and nervous tissue.

2.1. Epithelial Tissue (Epithelia)

Epithelial tissue consists of tightly packed cells that are flat (squamous), cubic, or prismatic (columnar). They are relatively undifferentiated, have a short lifespan, and are joined together with virtually no intercellular spaces. Epithelia rest upon an extracellular basal lamina (basement membrane) supported by underlying connective tissue. Epithelial tissue is avascular (lacks blood vessels) and has no direct nerve supply; it receives nourishment via diffusion from the underlying connective tissue. Its functions include protection, secretion, and absorption. Depending on cell layers, it can be simple (monostratified) or stratified (pluristratified).

Functionally, epithelia are classified into two groups:

• Covering and Lining Epithelia: Coat external body surfaces and line internal organs and cavities.
• Glandular Epithelia: Formed by invaginated epithelial cells grouped together to synthesize and secrete specialized substances. These form organs called glands, categorized into three types:
  • Exocrine glands: Maintain a duct through which their secretions are released either to the external body surface or into internal cavities that connect to the exterior (e.g., salivary, sweat, and mammary glands).
  • Endocrine glands: Have lost their ducts; their secretions, called hormones, are released directly into the bloodstream (e.g., thyroid and pituitary glands).
  • Mixed glands: Possess both an exocrine and an endocrine component (e.g., the pancreas).

2.2. Connective Tissue

Connective tissue is the most abundant tissue in the animal body. It connects, supports, nourishes, and protects other tissues. Unlike epithelia, its cells are widely separated from one another, embedded within an abundant extracellular matrix. Several specialized varieties exist:

A) Connective Tissue Proper

Acts primarily as a packing and filling tissue. It is rich in blood vessels and nerves. Its principal cellular components include:

• Fibroblasts: Irregular cells with extensive cytoplasmic projections responsible for synthesizing and secreting the extracellular matrix. The matrix contains structural protein fibers—collagen fibers (providing tensile strength), elastic fibers (providing elasticity), and reticular fibers—embedded within a ground substance rich in glycoproteins.
• Macrophages: Immune-derived cells that actively phagocytize cellular debris, foreign particles, and pathogens.
• Lymphocytes and Plasma Cells: Immune cells responsible for active defense against systemic and localized infections.

B) Adipose Tissue

Located under the skin (subcutaneous layer) and surrounding vital internal organs. It serves as mechanical protection, provides thermal insulation, and acts as the primary energetic reserve of the body. Its specialized cells, adipocytes, are large and spherical, storing large droplets of neutral lipids (fats) within their cytoplasm.

C) Cartilaginous Tissue

Forms cartilage, constituting an essential component of the skeletal system. It provides a semi-rigid framework for soft body parts, covers articular surfaces in joints to facilitate frictionless movement, and plays an integral role in bone growth and development. Its characteristic cells are called chondrocytes, which reside within small cavities termed lacunae. Chondrocytes secrete a solid, resilient, and elastic extracellular matrix heavily reinforced with collagen and elastin fibers. Cartilage is an avascular tissue that receives nutrients via diffusion from an external dense connective tissue envelope known as the perichondrium.