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Animal Tissues — Introduction
Definition & History
- Tissue = Group of cells having similar structure, origin, and performing a common function, held together by intercellular matrix
- Term "Animal tissue" coined by Bichat (also called "Father of Animal Histology")
- Plant tissue given by N. Grew
- Histology = Study of tissues (comes from Greek: Histos = tissue, Logos = study)
- Histogenesis = Formation of tissues
- Only 2 types of cells are excitable (can generate action potential): Muscular tissue + Nervous tissue
| Tissue Type | Origin | Functions |
|---|
| Epithelial Tissue | Ectoderm, Mesoderm, Endoderm (all 3) | Protection, diffusion, absorption, secretion, reproduction |
| Connective Tissue | Mesoderm | Connection, support, transport |
| Muscular Tissue | Mesoderm | Movement and locomotion |
| Nervous Tissue | Ectoderm | Control and coordination |
Key Features
- Term given by Ruysch; Epi = upon/above, Thelial = grows
- Present on exposed external and internal surfaces of body (as protective covering)
- Cells tightly packed, less intercellular space, no blood vessels (avascular)
- Nutrition: depends on underlying connective tissue through diffusion
- Basement membrane (non-cellular, acellular) below all epithelial tissues:
- Basal lamina — secreted by epithelium (laminin + fibronectin)
- Fibrous lamina — secreted by connective tissue (collagen + reticular fibres)
- High regeneration/repair capacity
- Nerve endings may penetrate epithelial tissue
- Exception: Cochlea (special epithelium — Stria vascularis) has blood vessels
- Can evolve from all 3 germ layers: Ectoderm (skin), Mesoderm (mesotheliun — pleura, peritoneum), Endoderm (endothelium — inner lining of intestine, blood vessels)
Simple Epithelium (Single Layer)
| Type | Cell Shape | Nucleus | Location (Examples) |
|---|
| Simple Squamous | Flat, polygonal, scale-like | Flat, at centre | Bowman's capsule (podocytes), Loop of Henle (thin), Endothelium of blood vessels, Alveoli |
| Simple Cuboidal | Cube-shaped | Round, at centre | Germinal epithelium (gonads), Acini of pancreas, Thyroid follicles, Collecting duct, PCT (brush-bordered) |
| Simple Columnar | Pillar/elongated | Elongated, basal | Liver, Bile duct; with goblet cells: Colon, Stomach, Rectum; with brush border: Duodenum, Ileum (Succus entericus secretion) |
| Pseudostratified | Columnar, different heights — appears multilayered but is single layer | At different heights | Trachea, Bronchi (with goblet cells and cilia = Pseudostratified glandular ciliated columnar) |
Compound Epithelium (Multiple Layers)
| Type | Key Feature | Location |
|---|
| Stratified Squamous Keratinized | Keratin protein present (insoluble, dead cells on surface) | Skin (epidermis), Oral cavity, Nails, Hair |
| Stratified Squamous Non-keratinized | No keratin, cells alive with nuclei | Oral cavity, Oesophagus, Anal canal, Vagina (protective, moist lining) |
| Stratified Cuboidal | Uppermost layer cuboidal | Ducts of salivary gland, Pancreas, Mammary glands, Sebaceous glands |
| Transitional (Urothelium) | Stretchable — changes shape with stretching; relaxed = dome-shaped upper cells; stretched = flat; no basement membrane when stretched | Urinary bladder, Ureter, Renal pelvis, Part of urethra |
Intercellular Junctions in Epithelium
- Tight Junctions (Zonula Occludens) — seal between cells; prevent leakage; present in apical part of columnar epithelium; most common in columnar epithelium
- Adherens Junction (Zonula Adherens / Intermediate Junction) — protein band (actin microfilaments); cementing or anchorage; both anchor and commit
- Desmosomes (Macula Adherens) — strongest junction; protein plate with keratin filaments; most half-life; present in stratified epithelium; interlocking digit arrangement for strength
- Gap Junctions — cytoplasmic connections between adjacent cells; chemical exchange (communication); present in intercalated discs of cardiac muscle
Glandular Epithelium — Classification
- Endocrine glands — no duct; secrete directly into blood (hormones)
- Exocrine glands — have ducts; secrete saliva, enzymes, sweat, milk, oil, wax
- Heterocrine (Mixed) — e.g. Pancreas, Gonads
- Modes of secretion:
- Merocrine (Eccrine) — secretory product released by simple exocytosis; cytoplasm NOT lost; e.g. Goblet cells, Salivary glands, Sweat glands, Tear glands
- Apocrine — apical part of cell lost along with secretory product; e.g. Mammary glands
- Holocrine — whole cell disintegrates carrying secretion; e.g. Sebaceous glands
Q. What is transitional epithelium and where is it found?
✅ Answer: Transitional epithelium (Urothelium) — stretchable epithelium found in urinary bladder, ureter, and renal pelvis
Explanation
Transitional epithelium is a special type of stratified epithelium that can change shape depending on whether it is stretched or relaxed. When the urinary bladder is empty (relaxed), the cells appear dome-shaped or rounded at the surface, and there appear to be many layers. When the bladder fills with urine and stretches, the cells flatten out and the epithelium appears to have fewer layers. This ability to accommodate volume changes without tearing makes it perfectly suited for the urinary bladder. The basement membrane becomes less distinct when the tissue is stretched. It is also called urothelium because it specifically lines the urinary tract (ureter, urinary bladder, and part of urethra).
Components of Connective Tissue Proper
- Matrix — Mucopolysaccharides (Hyaluronic acid) + Glycoproteins
- Fibres — Collagen fibres, Elastic fibres, Reticular fibres
- Resident cells — Fibroblast, Macrophages, Mast cells, Adipocytes, Plasma cells
| Fibre Type | Protein | Properties | Location |
|---|
| Collagen (White fibres) | Collagen protein | Most abundant (2/3 of total); tough, rigid, non-elastic; form bundles (fascicles); unbranched; boils → Gelatin | Tendons (connect muscle to bone); dermis |
| Elastic (Yellow fibres) | Elastin protein | Single fibre, branched; network formation; does NOT dissolve in dilute acid; elastic; highly resistant to chemical reactions | Ligaments (connect bone to bone) |
| Reticular fibres | Reticulin protein | Highly branched; thin, non-elastic; present in lymphoid organs (largest = spleen) | Lymphoid organs, basement membrane |
Cells of Connective Tissue
- Fibroblast — major cell; primary role in fibre formation + secretion of matrix (Ground substance); irregular shape; mature form = Fibrocyte
- Macrophages — modified monocytes; phagocytosis; agranular but appear granular due to lysosomes; kidney-shaped nucleus
- Mast cells — modified basophils; release Histamine (vasodilation), Heparin (anticoagulant), Serotonin (vasoconstriction)
- Plasma cells — activated B-lymphocytes; secrete antibodies (immunoglobulins)
- Adipocytes (Fat cells) — White fat (unilocular, large fat globule, cytoplasm + nucleus pushed to periphery); Brown fat (multilocular, small fat globules, many mitochondria)
Types of Connective Tissue
| Type | Subtype | Features | Examples |
|---|
| Loose CT (Areolar) | Areolar tissue | Maximum intercellular space; most abundant CT; collagen + elastic fibres; below skin | Subcutaneous layer (below skin) |
| Adipose tissue | Blood vessels present; nucleus at periphery in white fat; brown fat more mitochondria | Below skin; around organs |
| Dense CT | White fibrous dense regular | Collagen fibres only; no elastic/reticular; thick, tough, non-elastic; cord-like | Tendons (muscle to bone) |
| White fibrous dense irregular | Sheet/sheath-like | Periosteum, Perichondrium, Gilson's capsule (liver), Renal capsule, Sclera |
| Yellow fibrous dense (elastic) | Elastic fibres; some collagen; cord-like; no reticular; elastic | Ligaments (bone to bone); elastic arteries |
| Fluid CT | Blood, Lymph | Fibres absent; matrix = plasma; colloidal osmotic pressure maintained by albumin | Blood vessels, lymph vessels |
| Skeletal CT | Cartilage, Bone | Fibres + matrix + mineral deposition (in bone) | Skeleton |
| Type of Cartilage | Key Feature | Examples |
|---|
| Hyaline (Articular) | Most abundant; translucent, glassy-white; collagen fibres in matrix; cells = chondrocytes in lacunae | Trachea rings, Nasal septum, Ribs (costal cartilage), Larynx, Fetal skeleton, Articular surfaces of joints |
| White Fibrous (Fibrocartilage) | White fibrous/collagen fibres in excess; no reticular fibres; stiff, resistant | Intervertebral discs, Pubic symphysis, Knee meniscus |
| Elastic Cartilage | More elastic fibres; recoil property; structural support with flexibility | Pinna (ear), Nose tip, Epiglottis, Eustachian tube |
| Calcified Cartilage | Calcification (mineralization) | End of long bones (Humerus), Temporary cartilage |
Bone — Key Facts
- Study of bone = Osteology; Formation of bone = Ossification / Osteogenesis
- Cells: Osteoblast (bone-forming, modified fibroblast) → mature = Osteocyte (matrix cell) → Osteoclast (bone-resorbing, macrophage-like)
- Structural unit = Osteon (Haversian system)
- Haversian canal — longitudinal canal containing blood vessels + nerves (central)
- Volkmann's canal — horizontal/transverse canal; connects Haversian canals
- Lacunae — spaces in bone matrix where osteocytes sit; connected by canaliculi
- Matrix: Inorganic (calcium phosphate, calcium carbonate) + Organic (collagen)
Bone Regions — Epiphysis, Metaphysis, Diaphysis
- Epiphysis — spongy (cancellous) bone; ends of long bone; contains Red Bone Marrow (RBM) — haemopoietic
- Metaphysis — between epiphysis and diaphysis; contains Epiphyseal plate (growth plate — site of elongation of bone)
- Diaphysis — shaft of long bone; compact bone (Haversian system); contains Yellow Bone Marrow (fat storage)
- Coverings: Periosteum (outer covering — White Dense Irregular CT + osteoblasts) + Endosteum (innermost lining)
- RBM sites in adults: Sternum, Ribs, Vertebrae, Skull bones, Scapula, Clavicle, Pelvic bones, Ends of long bones
Q. What is the difference between tendons and ligaments?
✅ Answer: Tendons connect muscle to bone (white fibrous dense regular CT); Ligaments connect bone to bone (yellow fibrous elastic CT)
Explanation
Tendons are made of white fibrous dense regular connective tissue — containing tightly packed, parallel bundles of collagen fibres only. They are extremely strong, non-elastic, and cord-like. Their rigidity transmits the full contractile force of muscle to move the bone. Ligaments are made of yellow fibrous dense connective tissue — mainly elastic (elastin) fibres with some collagen. They connect one bone to another at a joint. Their elasticity allows the joint to move while still holding bones together and preventing dislocation. At the microscopic level: both lack reticular fibres, but tendons have NO elastic fibres while ligaments do. This explains why tendons are non-stretchable and ligaments have some elasticity.
Basic Facts
- Class: Insecta; Phylum: Arthropoda
- Species: Periplaneta americana (American cockroach — most common)
- Size: 34–53 mm long; dark brown/black body
- Nocturnal omnivores; live in damp warm places
- Body: Segmented, bilateral symmetry; 3 distinct regions: Head + Thorax + Abdomen
- Body covered by hard chitinous exoskeleton (made of chitinous plates called sclerites)
- Sclerites: Tergum (dorsal) + Sternum (ventral) + Pleura (lateral) — connected by Arthrodial (Articular) membrane
Head
- Head capsule (Cranium) with: Vertex (top), Frons (front), Gena (sides), Clypeus (ventral)
- Head position: Hypognathous (mouth parts directed downward)
- Antennae — 1 pair; segmented; olfactory (sense of smell + maintaining equilibrium)
- Eyes: Simple eyes (Ocelli — 1 pair, not well developed) + Compound eyes (1 pair) — vision; each compound eye has ~2000 ommatidia
- Vision type: Mosaic vision; Images: Apposition images (non-overlapping)
- Mouth parts (biting and chewing type):
- Labrum (upper lip)
- Labium (lower lip) — also called 2nd maxilla; has sensory palps
- Hypopharynx (tongue)
- Mandibles (1 pair) — with denticles; for biting and cutting
- Maxillae (1 pair) — with maxillary palps (5 segments)
Thorax (3 segments)
- Prothorax — 1st segment; bears 1st pair of legs; Pronotum (largest plate)
- Mesothorax — 2nd segment; bears 2nd pair of legs + 1 pair Forewings (Tegmina/Elytra) — dark, chitinous, protective, NOT used for flight
- Metathorax — 3rd segment; bears 3rd pair of legs + 1 pair Hindwings — transparent, membranous, used for flight; largest thoracic segment
- Total: 3 pairs of legs + 2 pairs of wings
- Each leg: Coxa + Trochanter + Femur + Tibia + Tarsus (5 segments) + Pretarsus
Abdomen (10 segments)
- 10 segments in both male and female
- Anal cerci (1 pair, 15-segmented) — present in BOTH male and female; sensory (detect vibration)
- Anal styles (1 pair, unsegmented) — present in male only (sexual dimorphism)
- 7th sternum in female = boat-shaped — forms brood pouch/genital chamber anteriorly
- 9th sternum in male = carries anal styles
- Genital pouch: Female (sternites 7+8+9 = genital chamber) + Male (sternites 9+10)
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Cockroach — Anatomy (Organ Systems)
Digestive System (Alimentary Canal)
3 Parts — Origin from Different Germ Layers
- Foregut (Ectodermal origin) — has cuticle lining; Mouth → Pharynx → Oesophagus → Crop (storage + digestion) → Gizzard (6 chitinous teeth for grinding food)
- Midgut (Endodermal origin) — no cuticle; site of main digestion and absorption; has Hepatic caeca (6–8) at junction of foregut and midgut — secrete carbohydrate, protein, fat digestive enzymes; Peritrophic membrane (permeable for enzymes and digested products)
- Hindgut (Ectodermal origin) — has cuticle; Ileum → Colon → Rectum (with 6 Rectal papillae for water absorption)
- Salivary glands: 1 pair bilobed → release only carbohydrate digestive enzyme (amylase)
- Malpighian tubules (100–150) — at junction of midgut and hindgut → excretion
Circulatory System (Open Type)
Key Features
- Type: Open circulatory system
- Heart: Single, 13-chambered, neurogenic (dorsal vessel)
- Blood: Haemolymph (colourless — no respiratory pigment)
- Blood cavity: Haemocoel
- Alary muscles (13 pairs) — help in blood circulation by contraction/expansion
- 3 sinuses: Pericardial sinus (dorsal) + Perivisceral sinus + Perineural sinus (ventral)
Respiratory System
Tracheal System
- Network of tracheae — open outside through spiracles (on pleuron plate)
- Total spiracles: 10 pairs — 2 in thorax (on mesothorax and metathorax) + 8 in abdomen
- Spiracles have valves to regulate gas exchange and prevent water loss
- Tracheae → Tracheoles (fine branches) → directly supply O₂ to cells
Excretory System
Malpighian Tubules
- Main excretory organ: Malpighian tubules (100–150) — Enterocoelic (arise from junction of midgut and hindgut)
- Main excretory substance: Uric acid (uricotelic)
- Other excretory organs: Fat bodies, Nephrocytes, Uricose glands
- Uricose glands — present only in male cockroach (sexual dimorphism)
Nervous System
Ganglia & Nerve Cord
- Brain = Supra-oesophageal ganglion
- Nerve cord: Ventral nerve cord — 3 ganglia in thorax + 6 ganglia in abdomen
- Sense organs: 2 types of eyes (simple + compound), antennae, anal cerci
Reproductive System
| Feature | Male | Female |
|---|
| Gonads | Testes (1 pair, 2–4 lobes) in 4th–6th segment | Ovaries (1 pair) in 2nd–6th segment; each ovary = 8 ovarioles; total 16 ovarioles in 2 ovaries |
| Ducts | Vasa deferentia → Ejaculatory duct | Oviducts → Common oviduct (Vagina) → opens at female gonopore |
| Accessory glands | Phallic glands (outer layer of spermatophore); Conglobate gland (middle layer); Long tubules (inner layer + nutrition); Small tubules (nutrition for sperms) | Collaterial glands — secrete egg case (Ootheca) |
| Special structures | 3 Phallomeres (right, left, ventral) — external genitalia; Anal styles (sexual dimorphism) | Spermathecae (1 pair, in 6th segment) — store sperms |
| Egg case | — | Ootheca — each contains ~16 eggs (formed by collaterial glands); 9–10 oothecae per female |
Metamorphosis in Cockroach
Paurometabolous metamorphosis (Gradual metamorphosis) — Egg → Nymph (13 instars over 1 year) → Adult
Nymph resembles adult but lacks wings and gonads. No pupal stage. Also called Incomplete metamorphosis.
Q. How many chambers does a cockroach heart have and what type of blood does it contain?
✅ Answer: 13 chambers; Blood = Haemolymph (colourless, no respiratory pigment)
Explanation
The cockroach has an open circulatory system where blood is not always confined to vessels. The heart is a single elongated tube with 13 chambers located dorsally. It is neurogenic (requires nerve impulses to beat, unlike the myogenic human heart). The blood is called haemolymph and is colourless because it contains no haemoglobin or other respiratory pigments — oxygen transport in cockroach is done by the tracheal system directly to cells. The haemolymph fills the haemocoel (body cavity). Alary muscles (13 pairs, fan-shaped) pump the haemolymph by their rhythmic contractions, filling the heart chambers through ostia (small openings). The haemolymph carries nutrients, hormones, and waste products but NOT oxygen.
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Quick Revision — Must-Know Points
Animal tissue coined by → Bichat
Excitable tissues → Muscular + Nervous
Epithelium name by → Ruysch
Basement membrane secreted by → Both epithelium + CT
Holocrine gland → Sebaceous glands
Apocrine gland → Mammary glands
Transitional epithelium → Urinary bladder
Strongest junction → Desmosome
Most abundant CT fibre → Collagen (2/3 total)
Mast cells → Modified basophils; release Histamine, Heparin
Tendon = Muscle to Bone (collagen)
Ligament = Bone to Bone (elastic)
Most abundant cartilage → Hyaline
Intervertebral disc → White Fibrocartilage
Cockroach body region → Head + Thorax + Abdomen
Compound eye units → ~2000 ommatidia
Cockroach heart chambers → 13
Anal styles → Male cockroach only
Malpighian tubules → 100–150; excrete Uric acid
Ootheca eggs → ~16 eggs per ootheca