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The loop of Henle is also called ansa nephroni. It is lined by both the squamous and cuboidal epithelium at different parts.

The tubular epithelial cells grow progressively taller throughout the segment until those in the collecting duct form a simple columnar epithelium.

The given histological image shows a cross-section of the kidney showing the thin and thick loops of Henle.

Microvilli are absent in collecting ducts. They are lined by simple columnar epithelium without microvilli.

The simple cuboidal or columnar epithelium may undergo certain specializations such as:

• Microvilli - long finger-like projections arising from the surface of the epithelium, which increases the surface area of absorption. If the microvilli are long and are arranged regularly, parallel to one another, it gives a striated appearance on light microscopy. However, if the microvilli are arranged irregularly, it gives a brush border appearance. Microvilli are located at:
• Small intestine - columnar cells with striated microvilli.
• Gall bladder - columnar cells with brush border microvilli.
• Proximal convoluted tubule - cuboidal cells with brush border microvilli
• Stereocilia - long and branching microvilli. Unlike the cilia, the stereocilia are non-motile and hence, the preferred name is stereovilli. They are located at:
• Epididymis (increase the surface area of absorption)
• Cochlear and vestibular receptor cells (function as sensory transducers)
• Cilia - hair-like projections on the epithelial cells that are concerned with the movement. The ciliated columnar epithelium is seen in the lining of the following structures:
• Respiratory tract except in lower pharynx and vocal folds.
• Tympanic cavity and auditory tube.
• Uterine tube and efferent ductules of the testis.
• Secretory - columnar cells may be specialized for secretions where they are referred to as glandular cells. Such cells form the mucin secreting and chief cells of the gastric epithelium.

Sweat glands are simple coiled tubular glands.

Reticular fibers are stained black with silver impregnation method. Reticular fibers are also called argentophil fibers, due to their specific affinity for silver salts.

The fibers of connective tissue are of three main types. These include:

• Collagen fibers
• Reticular fibers
• Elastic fibers

The staining characters of each of these fibers are as follows:

The image below shows reticular fibers in reticular connective tissue, stained black with silver impregnation method.

Connective tissue fibers

Elastic fibers Aldehyde fuchsin

Stain

Unaided dye Light pink Blue

Red Brown

Silver impregnatio n

Verhoeff-Van Gies on

Purple

Appeara nce

White

Black Black

Connective tissue fibers Stain Appeara nce

Orcein Purplish brown

The images below show specimens of glans penis showing collagen fibers (Cf) stained blue and muscle fibers (Mf) stained red using Masson trichrome stain, and elastic fibers (Ef) stained black using Verhoeff-Van Gieson stain.

Mast cells are not part of the intrinsic component of the connective tissue.

The various types of cells that are present in connective tissue are classed into two distinct categories that are namely:

• Cells that are intrinsic components of connective tissue:
• Fibroblasts
• Undifferentiated mesenchymal cells
• Pigment cells
• Adipocytes
• Cells that belong to the immune system:
• Macrophage cells (or histiocytes)
• Mast cells
• Lymphocytes
• Plasma cells
• Monocytes
• Eosinophils

Sudan III and Sudan IV dyes are used for the staining of adipose tissue. The image below shows adipose tissue stained with Sudan dye.

However, during the routine preparation of slides, the tissues are treated with fat solvents (like xylene or benzene), which dissolve the fat. Therefore, in such preparations, fat cells look like rounded empty spaces, as shown in the image below.

The cytoplasm of the cells of brown adipose tissue contains numerous mitochondria. The adipose tissue is mainly of two types:

• Yellow or white adipose tissue.
• Brown adipose tissue (embryonic type).

Features Seen in Cell size

Fat droplet(s) Also called

Cytoplasm and nucl eus

Mitochondria

White adipose tissue Adults

Larger Single

Unilocular adipose tissue Pushed to periphery

Few

Brown adipose tissue Newborn

Smaller Multiple

Multilocular adipose tissue Not pushed to periphery

Numerous

Calcium phosphate in the form of calcium hydroxyapatite is the major constituent of the inorganic salts present in the bone.

The ground substance of the bone (bone matrix) is predominantly made up of inorganic mineral salts, which constitute about 60-70 of the dry weight of the bone. These inorganic salts are deposited within the organic matrix.

• Inorganic mineral salts - makes the bone hard and rigid, and thus provides resistance to the compressive and impact forces as in weight-bearing or jumping respectively. Its constituents are:
• Calcium phosphate in the form of calcium hydroxyapatite (85 of total salts)
• Calcium carbonate (10 of total salts), magnesium, chloride, fluoride, citrate, sodium, and potassium.
• Organic matrix - makes the bone tough and resilient (flexible) and thus provides resistance to the tensile forces. Its constituents are:
• Type I collagen (predominantly)
• Glycosaminoglycans, proteoglycans, and water.
• Osteonectin and osteocalcin - predominant glycoproteins present in bone. These bind to calcium and help in the mineralization of the bone.
• Other substances include chondroitin sulfates, phospholipids, and phosphoproteins.

Note: Histological preparation of bone sections involves a process called decalcification, wherein the inorganic mineral salts of the bone are removed by treating the bone with weak acids or chelating agents such as EDTA or citrate. The decalcification causes the bone to lose its rigidity.

During the formation of bone, the mesenchymal cells and osteoblasts secrete type I collagen and gelatinous matrix (unmineralized bone matrix), which together forms the osteoid.

The collagen fibers swell up within the gelatinous matrix and become indistinct. Calcium salts are deposited into the gelatinous matrix under the influence of osteoblasts and this process is called mineralization.

The below image shows the microscopic appearance of osteoid in a patient with osteoid osteoma.

Membranous bones (and not membranocartilagenous bones) are also known as dermal bones. The bone is classified into three categories on the basis of pattern of development as follows:

• Membranous bones - These bones ossify from highly vascular membranes of primitive mesenchyme. At centres of ossification, mesenchymal stem cells differentiate into osteoprogenitor cells which give rise to osteoblasts. These osteoblasts help in the formation of bone. e.g., bones
• of the skull and facial bones.
• Cartilaginous bones - These bones ossify from hyaline cartilage. The deeper layers of the perichondrium surrounding the cartilage contain osteoprogenitor cells which help in bone formation. e.g., bones of limbs, vertebral column and thoracic cage.
• Membranocartilaginous bones - These bones ossify partly from cartilage and partly from mesenchymal condensations. e.g., clavicle, mandible, occipital, temporal and sphenoid.

The elastic cartilage is also known as yellow fibrocartilage.

The elastic cartilage resembles the hyaline cartilage in that it contains individual or small groups of chondrocytes that are surrounded by type II collagen fibrils. However, it differs from hyaline cartilage in that, much of the interterritorial matrix consists of fine yellow elastic

fibres containing elastin protein.

On microscopy, elastic cartilage can be identified by:

• Presence of chondrocytes within lacunae
• Presence of elastic fibres in the matrix
• Perichondrium with outer fibrous layer and the inner cellular layer

The elastic fibers present in the elastic cartilage can be visualised by Verhoeff's stain. The fibers stain dark purple/black with Verhoeff’s stain. The Verhoeff's stain binds to the elastin protein, which is the main constituent of elastic fibers.

The below image shows dark purple/black coloured elastin fibres stained with Verhoeff's stain.

Smooth muscles consist of spindle-shaped myocytes.

Smooth muscles are specialised for slow steady contraction under the influence of autonomic nerves and various hormones. They are also called as visceral muscles.

They are the major component of blood vessels, digestive, respiratory, urinary and reproductive tracts.

In microscopy, smooth muscles can be identified by:

• In longitudinal section, the spindle-shaped muscle cells are seen. They are elongated, tapering and unstriated cells with a centrally placed elongated nucleus.
• In transverse section, the spindle-shaped cells are cut at different places along its length and hence are seen as a group of cells with different shapes and sizes. The nucleus is seen only in those cells, which are cut through the centre.

The neurites of the neurons constitute the dendrites and the axon.

Neurites or neuronal processes refer to any projection arising from the cell body (soma) of a neuron. They are of two types:

• Axon - a single, long projection arising from the cell body that carries the impulse away from the cell body. The axon ends by dividing into several branches called axon terminals, which synapses with the dendrites of another neuron. An axon may also end in relation to an effector organ such as muscle or a gland. It is free of Nissl granules.
• Dendrites - multiple short projections arising from the cell body, that carry the nerve impulse towards the neuron. Nissl granules are present here.

The neuronal cytoskeleton consists of neurofilaments and microtubules. These are abundantly seen in the cell body, dendrites and the axon. The neurofilaments are heteropolymers of proteins that give tensile strength to the axon. Bundles of neurofilaments constitute a neurofibril.

Microtubules assist in axonal transport.

The Nissl-free zone of a neuron from where an axon arises is called an axon hillock.

Nissl bodies or granules are aggregates of polyribosomes and rough endoplasmic reticulum. They are present in the cell body of a neuron and in large dendrites. Under light microscopy, they appear as basophilic granules. The affinity towards the basic dyes is due to the presence of a large amount of RNA which is acidic in nature.

The image below shows a photomicrograph of bluish-coloured Nissl bodies (indicated by arrows) in the cytoplasm of motor neurons.

Option A: The initial segment refers to the proximal part of the axon, that lies just beyond the axon-hillock. Thus, an axon-hillock is part of the cell body while the initial segment is part of an axon.

Option B: Mesaxon refers to the area where the plasma membrane of the Schwann cell fuses with itself around the axon. During the formation of the myelin sheath, the axon invaginates the Schwann cell. The plasma membrane of the Schwann cell fuses with itself and continues to wrap

around the axon to form multiple layers of the plasma membrane called the myelin sheath. The cytoplasm and nucleus of the Schwann cell are pushed to the periphery. The thin layer of the cytoplasm of the Schwann cell surrounding the myelin sheath is called neurilemma.

Option D: Each Schwann cell myelinates only a short segment of the axon. Hence, along

the length of the axon, there are several Schwann cells myelinating it. The unmyelinated portions of the axon present in between myelinated segments are known as nodes of Ranvier.

The outer band of Baillarger is seen in the internal granular layer of cerebral cortex.

The cerebral cortex contains nerve fibers that run radially and tangentially. The tangential fibers are concentrated in the internal granular layer (layer IV) referred to as the outer band of Baillarger and in the ganglionic layer (layer V) referred to as the inner band of Baillarger. These bands are well developed in the sensory areas because of the concentration of terminal parts of the thalamocortical fibers. In the visual cortex, the outer band of Baillarger can be seen with the naked eye and is known as the stria of Gennari. It is because of this stria that the visual cortex is also referred to as the striate cortex.

The given image shows a micrograph of the visual cortex. The blue, horizontal band in the lower half of the image are the bands of Baillarger/the stria of Gennari. Subcortical white

matter (predominantly blue) is seen at the very bottom of the image.

The most likely cause for the opacity at the site of the pineal gland is corpora arenacea.

The pineal gland has a structure called the corpora arenacea (brain sand) located within the gland. These bodies are prone to an increase in calcification with advancing age. They are visible on x-ray and can be used as landmarks. Chemical analysis of the calcium deposits shows the different compositions, including calcium phosphate, magnesium phosphate, calcium carbonate, and ammonium phosphate.

Option A: The cells of the pineal gland are called pinealocytes that produces the hormone melatonin which works with the hypothalamus to regulate sleep-wake cycles.

Option D: Corpora amylacea are small eosinophilic inclusions frequently seen in histological sections of the brain, prostate, lung and uterus.

The image below is a histology section of the pineal gland showing dark stained corpora arenacea within the gland.