CHAPTER ONE:
INTRODUCTORY ANIMAL HISTOLOGY

Kabir Mohammed Adamu

Department of Biology,

IBB University, Lapai

Maureen Chukwu

Department of Pure and Applied Sciences

National Open University of Nigeria, Abuja

1.1 Introduction

Animals are multicellular heterotroph that grow and develop through a series of stages and actively move about during all or part of their life. Histology is the scientific study of the fine detail of biological cells and tissues that have been carefully prepared using histological techniques. It is derived from the Greek word: Histos and logia means the study of tissues of living organisms. Therefore, Animal Histology is the study of the microscopic structures of cells and tissues of animals. Animal tissues are divided into four groups;

Epithelia Tissues,
Connective Tissue,
Muscular Tissue
Nervous Tissue.

1.2 Epithelia Tissue

Epithelium can be defined simply as "a layer of cells with a free surface". However, it is single or multiple layers of cells. In embryonic terms, they are derived from all the 3 germ layers: Ectoderm (mainly nervous system), Mesoderm (mainly muscle and the lining of body cavity) and Endoderm (mainly organs of digestive tract). Epithelial cells are exposed to toxic chemicals, pathogens and mechanical abrasion. - An epithelial cell of the small intestine may survive only a day or two before it is destroyed. - New epithelial cells are produced by division of stem cells (germinative cells) located near the basal lamina.

The Features/Characteristics (Fig 1) are;

a. they line the surface of the body: mainly located on the borders between the external and internal.

b. they have basal lamina: found where epithelium contacts the connective tissue.

c. they have basement membrane: a thin sheet of collagen and glycoprotein produced by epithelia and underlying connective tissue.

d. with few exceptions in stria vacularis of the cochlea, hypertrophied thyroid gland, it lacks blood vessels.

e. regeneration: a high rate of cell replacement by stem cells in the epithelium.

f. cellularity: tightly bound together by cell junction.

g. surface specializations of epithelia such as microvilli.

h. cilia/Flagella: movement of materials over epithelia. Can remove debris- e.g. tracheal cilia that direct mucus carrying particulates out of trachea.

a. they play role in homeostasis

Fig 1: Epithelial tissues showing features/characteristics.

The Functions of epithelial tissues are:

i. Protection: covering and lining surfaces, e.g. skin, epithelial cells (endothelium) lining blood vessels, body cavities = coeloms = peritoneal, pleural and pericardial coeloms). For example, the entire external body is covered by an epithelium.

ii. Absorption (tall columnar epithelium of intestine): by absorbing substances and preserving water and salts of the body.

iii. Secretion (epithelia of glands): by secreting fluids and chemicals substances necessary for digestion, lubrication, protection, excretion of waste products, reproduction and the regulation of metabolic processes of the body.

iv. Sensation (sensory cells, neuroepithelium - taste buds): by constituting parts of sense organs especially, of smell and taste.

v. Contractility (myoepithelium - often associated with glands such as sweat and mammary glands).

vi. Lubrication: by lining all the internal cavities of the body, including the peritoneum, pleura, pericardium, and the tunica vaginalis of the testis.

vii. Cleaning: Ciliated epithelium assists in removing dust particles and foreign bodies which have entered the air passages.

viii. Diffusion: Simple epithelium promotes the diffusion of gases, liquids and nutrients. Because they form such a thin lining, they are ideal for the diffusion of gases (e.g. walls of capillaries and lungs).

ix. Reduces Friction: The smooth, tightly-interlocking, epithelial cells that line the entire circulatory system reduce friction between the blood and the walls of the blood vessels.

x. Excretion: Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine. Sweat is also excreted from the body by the epithelial cells in the sweat glands.

The features (Fig 2) of epithelial tissue classification are;

a. Simple (one cell thick): such as squamous (flat) - nucleus flattened with long axis parallel to basement; cuboidal (square) - nucleus is spherical; columnar (column) - nucleus oblong, long axis perpendicular to the basement membrane and pseudo - has a stratified appearance, however, is actually a single layer of cells with unaligned nuclei and some cells which do not reach the basement membrane.

b. Compound (more than one cell thick): such as stratified (more than one cell layer) and transitional.

Fig 2: Features of epithelium tissue classification

The different types of epithelial tissue are;

1. Simple epithelia:

a. Simple squamous epithelium: the cells are thin and flattened. It has little mechanical strength. It is found in absorptive surface e.g. air sacs of the lungs; lining ventral body cavity called mesothelium e.g. pleura, peritoneum, pericardium; lining heart and blood vessels called endothelium. Its thinness permits diffusion of materials through it. It also provides smooth linings to hollow structures such as blood vessels and the chambers of the heart. The characteristics are as presented in Fig 3.

Fig 3: Simple squamous epithelial tissue.

b. Simple cuboidal epithelium: they are found in glands and in lining of the kidney tubules (Fig 4). In glandular tissues e.g. salivary, thyroid glands. They constitute the germinal epithelium which produces egg in the female ovary and the sperm cells in male testes. This is the least specialized of all epithelia. The cells are roughly cub-shaped and possess a central spherical nucleus.

Fig 4: Simple cuboidal epithelium

c. Simple columnar epithelium: their cells are elongated and column-shaped. The nuclei are elongated and are usually located near the base of the cells. They are found lining the stomach and intestines (Fig 5), function in absorption and secretion; secretion of mucus.

Fig 5: Simple columnar epithelium

d. Simple ciliated columnar epithelium: they possess in addition to the above, fine hair-like outgrowths, called cilia on their free surfaces (Fig 6). They are capable of rapid, rhythmic, wave-like beating in a certain direction. This causes mucus secreted by goblet cells to move in specific direction. Found in air passages like the nose. They are also found in the uterus and fallopian tubes which helps propel the ovum to the uterus.

Fig 6: Simple ciliated columnar epithelium

e. Pseudostratified epithelium: these are found lining the male reproductive system. They can be ciliated e.g. trachea (Fig 10) or non-ciliated e.g. male urethra. They are columnar in shape.

Fig 10: Pseudostratified columnar epithelium

2. Compound epithelia:

a. Stratified squamous epithelium: this is the main protective tissue of the body. It is found lining the mouth, esophagus, anus and exposed surfaces. There are two forms;

i. Keratinized stratified squamous epithelium (Fig 7a): this is packed with fibrous protein. Found in apical layers of skin cells such as the epidermis. It is tough and water resistant. It is made up of 3 layers: the top layer has no nuclei, are modified to form keratin – used to form hair, skin; middle layer has polymorphic cells – modified to form the epidermis; bottom layer, has basal cells, modified to form sole of feet, palms of hand.

ii. Non keratinized stratified epithelium (Fig 7b): they resist abrasion, dry out and must be lubricated. They are found to cover wet surfaces that have a lot of tears and wears e.g. oral cavity, pharynx, esophagus, anus, vagina


a-Keratinized	b-Non-Keratinized

Fig 7: Stratified squamous epithelium

b. Stratified cuboidal epithelium (Fig 8): these are found lining the ducts of sweat glands, mammary gland, ovarian follicle, seminiferous tubule. They function in secretion and production.

Fig 8: Stratified cuboidal epithelium

c. Stratified columnar epithelium: this is similar to stratified cuboidal epithelium but its superficial cells are columnar and may be ciliated. They rarely line the larger ducts of some large glands, forms the conjunctiva, and occurs in small, isolated patches in some mucous membranes. They sometimes cover the respiratory surface of the epiglottis.

Fig 9: Stratified columnar epithelium

d. Transitional epithelium: they have the ability to accommodate stretching. They are found in urinary tract as fluid pressures varies thus in ureters, urethra and bladder (Fig 11).

Fig 11: Transitional epithelium

1.3 Connective Tissue

These are tissues in which cells are usually separated by greater amounts of intercellular substance. They serve to bind and hold body structures together. The study of connective tissue is called MESENCHYMOLOGY. They form the framework upon which epithelial tissue rests; Within which nerve and muscular tissues are embedded. They are developed from the mesenchyms, foetal supporting tissue. Connective tissue is the major supporting tissue of the body. It includes the skeletal tissues, bone and cartilage.

The functions of connective tissues are;

a. They are largely responsible for the cohesion of the body as an organism, of organs as functioning units and of tissues as structural systems. This cohesive function is achieved through their permeation of other tissues of the body.

b. They are essential for the protection of the body both in the elaborate defense mechanisms against infection and in repair from chemical and physical injuries.

c. Nutrition of nearly all cells of the body and the removal of their waste products in both medial is through the connective tissues.

d. They are also important in the development and growth of many structures.

e. Constitutes the major contributor to the homeostatic mechanism of the body so far as salts and water are concerned.

f. They act as the great storehouse for the body salts and minerals, as well as of fat.

g. They determine in most cases the pigmentation of the body.

h. The skeletal system (cartilage and bones) plus other kinds of connective tissue (tendons, ligaments, fasciae and others) make motion possible.

The components of connective tissues are;

a. Cellular Components such as;

Fibroblast: they are elongated, spindled shaped cells with many cell processes. They active cells. The more mature form with slow activeness is called FIBROCYTE.
Macrophages (Histiocytes): Shows pronounced phagocytotic activity. Originates from monocytes which are precursor cells in bone marrow. They function in phagocytosis of microorganism such as bacteria.
Mesenchymal cells: They are stem cells. They response to injury or infection by differentiating into fibroblast, macrophages etc.
Mast cells: They are large amoeboid mesenchymal cells. They have central nucleus and often packed within granules. The granules are made up of histamine & heparin which are released in inflammatory responses.
Plasma cells: Ovoid, weakly amoeboid with eccentric nucleus. Responsible for antibody production. Found in sites of high risk of invasion.
Melanocytes: Cytoplasm filled with minute granules. There granules may either be yellow or brown. They synthesize and store melanin.
Yellow fat cells: Yellowish in colour. Generally spherical with a thin shell of protoplasm enclosing a single enlarged fat drople
Brown fat cells: Moderately large and spherical. Have small droplets of variable size scattered in the cytoplasm.

b. Extracellular Components such as;

i. Fibrillar components:

a. Reticular fiber: these have affinity to silver thus ARGYROPHILIA. Abundant in lymphatic organs (spleen & lymph node), stomach muscle, endoneurium, liver, endocrine gland.

b. Collagenous fiber: they have longitudinally striated appearance. Synthesized with cells such as fibroblast, osteoblast, chondroblast, odontoblast, reticular cells, epithelial cells etc. They are the most abundant protein in the body.

c. Elastic fiber: Highly refractile and appear slightly yellowish. Prominent in elastic tissue such as elastic ligament.

ii. The non-fibrillar component of connective tissues appears amorphous with the light microscope and is the matrix in which cells and fibres are embedded. It consists of two groups of substance:

a) Those probably derived from secretory activity of connective tissues cells including mucoproteins, protein-polysaccharide complexes, tropocollagen and antibodies, and

b) Those probably derived from the blood plasma, including albumin, globulins, inorganic and organic anions and cations and water. In addition, the ground substance contains metabolites derived from or destined for blood.

Connective tissues can be classified as;

1. Connective Tissue Proper: They have many protein fibers. Have syrupy ground substance. It is divided into two categories based on above;

a) Loose Connective Tissue Proper: they are very common; they fill the space between muscle fibers. It has more ground substance and less fiber. There are three types;

i. Areolar loose connective tissue proper (they have little space; it is the least specialized with open framework distort without damage; underlying all epithelia; surrounding nerves, blood vessels, esophagus, trachea; it is the least specialized with open framework distort without damage; they have viscous ground substance to absorb shocks; they have elastic fiber that returns to original shape; they hold blood vessels and capillary beds (Fig 12)).

Fig 12: Areolar loose connective tissue proper

ii. Adipose loose connective tissue proper (this is similar to areolar but contains adipocytes for storing fats (Fig 13). They also absorb shocks and slow heat loss. There are two types: white fat tissue (the most common adipose tissue) and brown fat tissue (adipocytes contain many mitochondria).

Fig 13: Adipose loose connective tissue proper

iii. Reticular loose connective tissue proper (this has 3-dimensional network of supportive fiber called STROMA. The stroma supports functional cells – PARENCHYMA. They are found in the spleen, liver, lymph nodes and bone marrow (Fig 14)).

Fig 14: Reticular loose connective tissue proper

b) Dense Connective Tissue (Collagenous Tissue): They have more fibers, less ground substances. They are dense because they have high numbers of collagen fibers. They are divided into three categories;

i. Dense regular connective tissue (they have tightly packed, parallel collagen fibers; they attach muscle to bone (Tendon); they connect bone to another bone or stabilize organ (Ligament)).

ii. Dense irregular connective tissue (they have interwoven networks of strengthening fibers; found in skin around the cartilage (Perichondrium), around bones (Periosteum) and forms capsules around liver and kidney).

iii. Elastic tissue (though both dense regular and dense irregular connective tissues contain elastic fibers, elastic tissue is mostly elastic fibers. e.g. elastic ligaments of the spinal column).

2. Supportive connective tissue: these are soft tissues that provide the weight of the body. There are 2 types of supportive connective tissues thus:

a) Cartilage: they are composed of chondrocytes. Chondrocytes are located in lacunae surrounded by an intercellular matrix. It is an avascular tissue. It has low metabolic activity & turnover except the embryo. Receive nutrients from perichondrium. Chondrogenesis is the process of cartilage development. There are different types of cartilage thus:

· Hyaline/joint cartilage (they lack bundles of fibers. Has collagen fibrils; Derived from Greekword ‘hyalos’ = glass; fresh sample is milky-white. they are flexible and resilient to mechanical forces. found in the respiratory tract (nose, larynx, trachea, bronchi); in embryo it plays role in long bone development).

· Fibrocartilage/Patella (found in areas subjected to high mechanical stress; found in intervertebral disk, pubic symphysis, temporo-mandibular joints; it is characterized by large numbers and concentrations of collagen fibers in the matrix with relatively little amorphous matrix).

· Elastic cartilage (found in areas subjected to high flexibility & elasticity; they provide the yellowish color in the fresh tissue).

b) Bone: osteogenesis is the development of bone. The first bone to develop is spongy bone called woven, immature or primary bone. Woven bone lacks order of lacunae (osteocytes). Primary bone is found in developing embryo. Composed of cells and extracellular matrix in which fibers are embedded. Extracellular matrix is calcified. They provide protection. The hematopoietic bone marrow is protected by the surrounding bony tissue. There are two main categories;

· Spongy/trabecular/cancellous bone (composed of lattice or network of branching bone spicules or trabeculae which contains bone marrow) and

· Compact/corticol bone (lack spaces visible to unaided eyes).

Bones have different cells that includes;

a) Osteoprogenitor cells: derived from mesenchyme cells.

b) Osteoblast: first cell to develop from osteoprogenitor cells. It is involved in formation of bone. Found on the boundaries of developing and growing bones.

c) Osteocytes: mature bone cells that developed from osteoblast: Located in the lacunae within bony matrix. Have cytoplasmic process located in canaliculi.

d) Osteoclasts: highest bone cells. It is involved in bone resorption. Found in eroding surfaces often in cavities called Howship’s lacunae.

3. Fluid connective tissue: they have watery matrix of dissolved proteins. There are two types:

a. Blood: this consist of variety of cells suspended in a fluid medium called PLASMA. It has different components that include;

· Blood cells:

Ø Erythrocytes (circular biconcave discs; non-nucleated in mammals; primarily involved in respiration).

Ø Leucocytes (nucleated amoeboid cells; constitute important part of defense and immune system). Leucocytes are differentiated into;

§ Neutrophil: accounts for about 55-70%; segmented nucleus, cell diameter about 12µm; cytoplasm packed with small specific granules; they are phagocytic cells; they are important in inflammation and at sites of injury or wound; does in site of infections are mostly dead neutrophil.

§ Eosinophil: they represent about 1-4% of the blood cell; have bilobed nuclei with acidophilic granules; involve in selective phagocytosis; increase in the cell, its associated with allergic reaction or helminth parasitic infection.

§ Thrombocytes: they are small and colourless disc; they play role in blood clotting; they occur only in mammals.

§ Basophil: they represent about 1%; the nucleus is bilobed or s-shaped; there are large irregular basophilic granules; the granules are similar to those of the mast cells.

§ Lymphocytes: they represent about 25-30%; they are involved in immune response; they have little cytoplasm with rounded nucleus; classified as small or large depending on size of the cytoplasm.

§ Monocytes: they represent about 5%; have oval or kidney shaped eccentric nuclei; they are non-terminal cells i.e. can differentiate into phagocytic cells such as macrophages.

· Blood Platelets (they are derived from cytoplasm of megakaryocytes of the bone marrow. They initiate the process of blood clotting. They initiate the plugging up and sealing of damaged blood vessels).

b. Lymph: They are transparent intercellular or lymphatic vessel fluid. They include; lymphatic vessel fluids (lymphates). Important proteins such as immunoglobulin, from which protective antibodies are derived.

Fig 15: General characteristics of connective tissues

Table 1: Summarized characteristics of connective tissues

Tissue Type	Cells Present	Fibers Present	Matrix Characteristics
Connective Tissue proper
Loose Connective Tissue:
Areolar	fibroblasts macrophages adipocytes mast cells plasma cells	collagen elastic reticular	loosely arranged fibers in gelatinous ground substance
Adipose	adipocytes	reticular collagen	closely packed cells with a small amount of gelatinous ground substance; stores fat
Reticular	reticular cells	Reticular	loosely arranged fibers in gelatinous ground substance
Dense Connective Tissue:
dense regular	fibroblasts	collagen (some elastic)	parallel-arranged bundles of fibers with few cells and little ground substance; great tensile strength
dense irregular	fibroblasts	collagen (some elastic)	Irregularly arranged bundles of fibers with few cells and little ground substance; high tensile strength
Supportive Tissue:
Cartilage:
hyaline (gristle)	chondrocytes	collagen (some elastic)	limited ground substance; dense, semisolid matrix
Fibrocartilage	chondrocytes	collagen (some elastic)	limited ground intermediate between hyaline cartilage and dense connective tissue
Elastic	chondrocytes	Elastic	limited ground substance; flexible but firm matrix
Bone (osseous tissue):
compact (dense)	osteoblasts osteocytes	Collagen	rigid, calcified ground substance with (canal systems)
spongy (cancellous)	osteoblasts osteocytes	Collagen	rigid, calcified ground substance (no osteons)
Fluid Connective tissue
Blood	erythrocytes leukocytes thrombocytes	“fibers” are soluble proteins that form during clotting	“matrix” is liquid blood plasma
Lymph	leukocytes	“fibers” are soluble liquid proteins that form during clotting	“matrix” is blood plasma

1.4 Muscular Tissue

Muscle (from Latin musculus, diminutive of mus "mouse") is a contractile tissue of animals. They are primarily concerned with contractility. They are responsible for movement of body parts. It is developed from embryonic mesoderm (except myoepithelium). The cells contain contractile filaments that moves pass each other thereby changes the size of the cell. The contractile filaments are bound by connective tissue. The cytoplasm is called sarcoplasm. The endoplasmic reticulum is called sarcolemma. Whilst muscle cells are called myocytes.

Muscular tissue is classified into three based on morphological and physiological functions thus;

a) Smooth: this is known as involuntary muscle (Fig 16). They lack cross striation. Have ability to undergo hyperplasia and hypertrophy as in the uterus of pregnant woman. They can regenerate. Found in the walls of hollow internal organs such as blood vessels, dermis, iris diaphragm, uterus

Fig 16. Smooth muscle

b) Skeletal: Make up about 40-50% of body mass (Fig 17). Myofibers are bound together in bundles or fascicles. Connective tissue in the muscle serves to bind and integrate the action of the various contractile units. The end of the muscles is attached to bone, cartilage or ligaments by tendons. The muscle functions in; skeletal movement; maintenance of body position; support soft tissues; guard body openings and maintenance of body temperature.

Fig 17. Skeletal muscle

c) Cardiac: It is a striated muscle (Fig 18). It has fiber branches (bifurcate). Each myocyte has one or two central nuclei. The fibers have more sarcoplasm. They possess a system of T-tubules.

Fig 18. Cardiac muscle

Table 2: The characteristics of the three types of muscles.

Character	Skeletal	Smooth	Cardiac
Location	Attached to skeleton	Walls of stomach, intestine etc	Walls of heart
Types of control	Voluntary	Involuntary	Involuntary
Shape of fibres	Elongated, cylindrical blunt ends	Elongated spindle – shape pointed ends	Elongated, cylindrical fibres that branch and fuse
Striations	Present	Absent	Present
No of nuclei per fibre	Many	One	One or two
Position of nuclei	Peripheral	Central	Central
Speed of contraction	Most rapid	Slowest	Intermediate
Ability of remain contracted	Least	Greatest	Intermediate

INTRODUCTORY ANIMAL HISTOLOGY

1.5 Nervous Tissue

Nervous tissue components are concerned primarily with rapid conduction of impulses; their cells are specialized for conduction. They therefore serve as the complex telecommunications network of the body. These tissues act in a sensory capacity, to receive, disseminate, and store information collected from receptors. In a motor capacity, nervous tissues provide response potential by controlling effectors such as muscles or glands. Nervous tissue carries electrical signals from one part of the body to another. Thus, the functions include;

i. allows an organism to sense stimuli in both the internal and external environment.

ii. analyzed and integrated to provide appropriate, coordinated responses in various organs.

iii. the afferent or sensory neurons conduct nerve impulses from the sense organs and receptors to the central nervous system.

iv. internuncial or connector neurons supply the connection between the afferent and efferent neurons as well as different parts of the central nervous system.

v. efferent or somatic motor neurons transmit the impulse from the central nervous system to a muscle (the effector organ) which then react to the initial stimulus.

vi. autonomic motor or efferent neurons transmit impulses to the involuntary muscles and glands.

vii. autonomic nervous system (ANS): Is formed from two chains of ganglia found along the spinal cord and scattered among body tissue, concerned chiefly with regulation of visceral activity.

Nervous tissue contains densely packed nerve cells called neurons. Neurons are cells that are specialized for irritability and conduct. The neuron consists of a cell body called soma or perikaryon and few - many attenuated cytoplasmic processes which radiate out of the cell body. Depending upon their functional role, these processes are termed axons or dendrites. Depending on the length, a fibre which carries impulse towards a cell body is called a dendron (long) or dendrite (short). An axon carries impulses away from cell body. Neurons can be classified based on its morphological and physiological properties.

Morphologically, they are classified into three (Fig 19);

a. Unipolar/pseudo unipolar neurons have a single process (axon); they have one process that emerges from the cell body, T-fashion, into two processes; these are found in sensory ganglia of dorsal roots of spinal nerves.

b. Bipolar neurons have two processes (one dendrite and one axon); these are very rare and have a limited distribution in the body; they are present in special sensory structures including the retina, olfactory epithelium, and vestibular and cochlear nerves).

c. Multipolar neurons possess several processes (several dendrites and a single axon); most neurons belong to this category.

Fig 18: Morphologically classified neurons

Physiologically, neurons can be classified into four thus;

a. Sensory neurons these receive sensory stimuli from the environment (from receptors) and from within the body (e.g. unipolar neurons).

b. Motor neurons (Fig 19) these control the effector organs (muscles, exocrine glands, endocrine glands); a motor neuron has many processes (cytoplasmic extensions) called dendrites, which enter a large, grey cell body at one end; a single process, the axon, leaves at the other end, extending towards the dendrites of the next neuron or to form a motor endplate in a muscle; dendrites are usually short and divided while the axons are very long and does not branch freely; the axon is surrounded by the myelin sheath, which forms a whitish, non-cellular, fatty layer around the axon; outside the myelin sheath is a cellular layer called the neurilemma or sheath of Schwann cells; the

myelin sheath together with the neurilemma is also known as the medullary sheath; this medullary sheath is interrupted at intervals by the nodes of Ranvier.

c. Interneurons/intermediate neurons these are typically found in the CNS and connect between other neurons (often between sensory and motor neurons).

d. Neurosecretory neurons these are specialized neurons that synthesize and secrete hormones.

Fig 19: motor neuron

References

Ø Berry, A.K (2015). A Textbook of Animal Histology, Emkay Publications, Delhi

Ø Sharma, A.K. Concepts in Animal histology, Anmol publications PVT, Ltd, India

Ø Bancroft, John; Stevens, Alan, eds. (1982). The Theory and Practice of Histological Techniques (2nd ed.). Longman Group Limited.

Ø Bracegirdle B (1977). "The History of Histology: A Brief Survey of Sources". History of Science. 15 (2): 77–101.

Ø Maximow, Alexander A.; Bloom, William (1957). A textbook of Histology (Seventh ed.). Philadelphia: W. B. Saunders Company.

Ø Ross, Michael H.; Pawlina, Wojciech (2016). Histology: a text and atlas: with correlated cell and molecular biology (7th ed.). Wolters Kluwer. pp. 984p.

Ø Ulric Dahlgren (2012). A text-book of the principles of Animal histology. Hardpress publishing, 542pp.

CHAPTER ONE: INTRODUCTORY ANIMAL HISTOLOGY