📚 Question Bank

Blockage of the medial and the lateral apertures of the 4th ventricle is seen in Dandy-Walker syndrome (DWS).

DWS is congenital hydrocephalus associated with atresia of the median aperture (foramen of Magendie) and lateral aperture (foramen of Luschka) of the 4th ventricle. This causes enlargement of the 4th ventricle and hydrocephalus.

The other associated defects seen in DWS include:

• Agenesis of the cerebellar vermis
• Agenesis of the splenium of corpus callosum

The image given below shows the normal 4th ventricle.

The hypophysis cerebri or pituitary gland develops from two sources i.e. surface ectoderm and neurectoderm.

The adenohypophysis or anterior pituitary, develops from the surface ectoderm. An ectodermal diverticulum, called Rathke's pouch, grows upward from the roof of the stomatodeum (primitive mouth). This becomes the anterior lobe of the pituitary (pars distalis).

The neurohypophysis or posterior pituitary, develops from the neuroectoderm. A downward extension of the diencephalon, called infundibular process, grows down from the floor of the 3rd ventricle, during the 6th week. It gives rise to the stalk and the posterior lobe of pituitary (pars nervosa).

The anterior lobe of pituitary gives rise to a small extension called the pars tuberalis. It grows along the stalk of the infundibulum and eventually surrounds it.

The posterior wall of the Rathke pouch develops into the pars intermedia. The image given below shows the development of pituitary gland.

The cells of the adrenal cortex and medulla are respectively derived from the mesoderm and neural crest cells.

The adrenal glands consist of a superficial cortex and deeper medulla.

• Adrenal cortex - formed by proliferation of the coelomic epithelium (mesodermal origin)
• Adrenal medulla - formed by the invading neural crest cells from somite levels 18-24

The urogenital ridge develops from the intermediate mesoderm.

After the formation of the peritoneal cavity, the intermediate mesoderm is seen as a bulge in the posterior abdominal wall. This bulge lateral to the attachment of the dorsal mesentery is known as the urogenital ridge.

The urogenital ridge has 2 parts:

• The genital ridge, which is medial and gives rise to the genital system
• The nephrogenic cord, which is lateral and contributes to the urinary system

The proximal convoluted tubules develop from the metanephric blastema. The kidney develops from 3 sources as follows:

• Metanephric blastema gives rise to the excretory system of the kidney. It forms the Bowman's capsule, proximal convoluted tubule, the loop of Henle, and distal convoluted tubule.
• Ureteric bud, which is an outgrowth of the mesonephric duct, gives rise to the collecting system of the kidney. It forms the collecting ducts, minor and major calyces, and renal pelvis.
• Angiogenic mesenchyme, which migrates into the metanephric blastema, gives rise to the glomeruli and the vasa recta.

The ureteric bud develops from the mesonephric duct (Wolffian duct).

The tip of the ureteric bud penetrates the metanephric blastema, which forms a cap over the distal end of this bud. The ureteric bud gives rise to the ureter and the collecting system of the kidney.

Structures arising from the metanephric blastema and ureteric bud are as follows:

Metanephric blastema Bowmans capsule

Proximal convoluted tubu le

Loop of Henle

Distal convoluted tubule

Ureteric bud Collecting ducts Renal papillae

Major and minor calyc es

Renal pelvis Ureter

The horseshoe kidney is the most common congenital anomaly of the kidney.

During development, the kidney is initially located in the pelvic region. Later, the kidneys ascend due to the growth of the body in the lumbar and sacral regions. Sometimes, the kidneys are pushed so close to each other during their ascent that their lower poles fuse, forming a horseshoe kidney.

The inferior mesenteric artery prevents the ascent of the horseshoe kidney.

The horseshoe kidney is usually found at the level of the lower lumbar vertebrae. Its ascent is halted by the root of the inferior mesenteric artery. The connecting isthmus of the horseshoe kidney may be present either in front or behind the aorta and the inferior vena cava.

Pelvic kidney: Occasionally, one kidney fails to ascend. It is usually found in the pelvis near the common iliac artery.

The seminiferous tubules are derived from the sex cords.

The epithelium in the gonadal ridge proliferates to form the primitive sex cords. They multiply further under the influence of the SRY gene on the Y chromosome and penetrate deep into the medulla to form testis cords. Near the hilum of the testis, these cords break into a network of tiny cell strands, which later give rise to rete testis.

Various parts of the testis are derived as follows:

Structure in Testis Primordial germ cells Sertoli cells

Interstitial cells of Leydig

Seminiferous tubules, rete te stis

Tunica albuginea

Efferent ducts (ductuli effere ntes)

Ductus deferens

Origin Epiblast

Surface epithelium of the earl y gonad

Mesenchyme of the gonadal r idge

Sex cords (testis cords)

Mesenchyme of the gonadal r idge beneath the coelomic ep ithelium

Mesonephric tubules (Wolffia n tubules)

Mesonephric duct (Wolffian duct)

The appendix of the epididymis is derived from the Wolffian duct (mesonephric duct).

It is the most cranial portion of the Wolffian duct (mesonephric duct) and remains as a vestigial structure.

The prostatic utricle is derived from the paramesonephric duct (Mullerian duct).

In males, under the influence of the anti-Mullerian hormone (AMH) produced by the Sertoli cells, the paramesonephric ducts degenerate.

Only two structures are derived from paramesonephric ducts in males:

• The appendix of the testis (also known as hydatid of Morgagni) is derived from the cranial end of the paramesonephric duct.
• The prostatic utricle (also known as utriculus prostaticus) is a small indentation in the
• prostatic urethra. It is located at the apex of the urethral crest, on the seminal colliculus (verumontanum), laterally flanked by openings of the ejaculatory ducts.

The given condition is hypospadias and it is due to the failure of the urethral folds to fuse completely.

The cloacal folds fuse:

• Cranially to form the genital tubercle
• Caudally they constitute -
• Urethral folds anteriorly
• Anal folds posteriorly

At the end of the third month, the two urethral folds close over the urethral plate forming the penile urethra.

In hypospadias, there is an incomplete fusion of the urethral folds. The urethra opens abnormally on the inferior aspect of the penis usually near the glans, but it can open along the shaft or near the base of the penis. In rare instances, the urethral meatus opens in the scrotal raphe. If the urethral folds fail to fuse completely, a wide vertical slit can be present along the entire length of the penis and the scrotum. The two scrotal swellings then closely resemble the labia majora.

In epispadias, the urethral meatus is found on the dorsum of the penis. It is most commonly seen with exstrophy of the bladder and defects in the closure of the anterior body wall.

Appendix of the epididymis is a derivative of the mesonephric duct. Appendix of the testis is a derivative of the paramesonephric duct.

The organ of Rosenmuller, also known as epoophoron, is a remnant of the mesonephric tubule in females.

The mesonephros consists of a series of excretory tubules that develop in the thoracolumbar region. The mesonephric tubules drain into the mesonephric duct. Most of the mesonephric tubules disappear, but some mesonephric tubules take part in the formation of the efferent ductules in males.

The mesonephric tubules give rise to the following structures and remnants:

• Females: epoophoron and paroophoron
• Males: efferent ductules (ductuli efferentes or vasa efferentia), lobules of the head of the epididymis, and paradidymis.

The vagina has a dual origin: the upper portion is derived from the Mullerian duct

(paramesonephric duct) and the lower portion is derived from the cloaca (urogenital sinus).

The solid tips of the paramesonephric ducts come in contact with the urogenital sinus. Two solid evaginations, called the sinovaginal bulbs, develop from the pelvic part of the urogenital sinus. The sinovaginal bulbs proliferate to become a solid vaginal plate. This entire plate is canalized by the 5th month to form the vagina.

The upper part of the vagina is derived from the Mullerian duct, while the lower part is derived from the urogenital sinus. The vaginal fornices are of paramesonephric origin.

Agenesis of the clitoris occurs due to a defect in the development of the genital tubercle. The vestibule is derived from the urogenital groove.

The cytoplasm and nucleus together are referred to as protoplasm.

A cell is surrounded by a cell membrane (or plasma membrane), which encloses a complex material called protoplasm. It consists of a central, denser part called the nucleus, and an outer less dense part called the cytoplasm. The nucleus is surrounded by a nuclear membrane, which encloses chromatin, nucleolus, and nucleoplasm. The cytoplasm consists of

cell organelles and cytosol (also called hyaloplasm), which is the liquid component of the cytoplasm present outside the cell organelles. Vacuoplasm refers to the fluid-filled spaces enclosed within the membrane-bound cell organelles.

Absorption of the fluids into the cell occurs by a process called pinocytosis. The vesicles involved in the process are called pinocytotic vesicles.

Option A: Endocytosis is a process in which large molecules enter into the cells by invagination and formation of the vesicle.

Option B: Some cells use the process of endocytosis to engulf foreign matter (e.g., bacteria). The process is then referred to as phagocytosis.

Option D: The process of releasing the molecules that are produced within the cytoplasm to the exterior is called exocytosis or reverse pinocytosis.

The euchromatin in the nucleus of a cell has a loose appearance and stains lightly with hematoxylin and eosin (Hamp;E).

On staining with Hamp;E, the nucleus stains dark purple or blue, while the cytoplasm usually stains pink. However, in some cells, the nuclei are relatively large and stain lightly due to the presence of a delicate network of fibers called chromatin. At some places (in the nucleus), the chromatin is seen in the form of irregular dark masses called heterochromatin and some other places, the network is loose and stains lightly and is referred to as euchromatin.

The lumen of the rough endoplasmic reticulum is continuous with perinuclear space.

Rough endoplasmic reticulum (ER) arises from the outer nuclear membrane. Hence, its lumen is continuous with the perinuclear space (space between inner and outer nuclear membranes). The lumen is also continuous with the lumen of the smooth ER.

The ER is an organelle that consists of a network of flattened membrane-bound structures called cisternae. The ER is classified into rough ER and smooth ER, depending on whether the ribosomes are attached or not, respectively.

The seminiferous epithelium is a complex stratified epithelium, consisting of cells that form a lineage of spermatozoa (spermatogonia, spermatocytes, spermatids) and Sertoli cells.

Given below is a histological image showing the complex stratified epithelium of seminiferous ducts.

Simple epithelia and pseudostratified epithelium are both uni-layered epithelia.

Simple epithelium - typically present in sites at which absorption, secretion, and filtration occur. Depending on the shape of the cells, the simple epithelia are further classified as:

• Squamous epithelium
• Cuboidal epithelium
• Columnar epithelium

Pseudostratified epithelium - comprises only a single layer of cells but has its cell nuclei positioned at various levels in a manner falsely suggestive of stratified epithelia.

Given below is a histological image showing the pseudostratified columnar epithelium of the trachea.

Parietal peritoneum is lined by simple squamous epithelium. The simple squamous epithelium is seen at the following sites:

• Alveoli of the lungs
• Thin segment of the loop of Henle
• Lining of the blood and lymphatic vessels, where it is called endothelium
• Lining of the body cavities such as pleura, peritoneum, and pericardium, where it is called mesothelium.
• Lining of the inside of the heart, where it is known as endocardium.

Thyroid follicles are lined by cuboidal epithelium.

The cuboidal epithelium consists of a single layer of cells in which the height of the cells is about the same as their width. The nuclei are usually rounded.

Cuboidal cells are seen in:

• Follicles of the thyroid gland
• Surface of the ovary (where it is called the germinal epithelium).
• Choroid plexuses,
• Inner surface of the lens
• Pigment cell layer of the retina.
• Proximal convoluted tubules of the kidneys (brush border cuboidal)

The multilayered epithelium that consists of deeper columnar cells and a surface layer of flat cells is known as the stratified squamous epithelium.

Stratified (multi-layered) epithelia are of the following types:

1. Stratified squamous epithelium:

• The deepest layer is usually columnar, above which there are layers of cuboidal cells, and as it proceeds towards the surface of the epithelium the cells become increasingly flattened (or squamous).
• It is further classified into keratinized and non-keratinized epithelium.

The given histological images show keratinized stratified squamous epithelium of the oral mucosa and the non-keratinized stratified squamous epithelium of the esophagus.

2. Stratified cuboidal/columnar epithelium:

• It consists of two or more layers of cuboidal or columnar cells, respectively.

The given histological image shows stratified cuboidal epithelium of salivary glands.

3. Transitional epithelium:

• It is a specialized epithelium lining most of the urinary tract; hence, it is also called
• urothelium.
• The deepest layer consists of columnar or cuboidal cells, and the middle layer consists of polyhedral or pear-shaped cells.
• The surface layer consists of large, polypoid, dome-shaped or umbrella-shaped cells. As the epithelium gets stretched to accommodate urine, the surface cells become flattened.
• The cells are called transitional due to this apparent change (transition) from stratified cuboidal epithelium in a relaxed state to stratified squamous epithelium when it is stretched.

The given histological image shows transitional epithelium of the urinary bladder.

The cornea is lined by stratified squamous non–keratinized epithelium.

The cornea is made up of five layers. The outermost layer is made of non-keratinised stratified squamous epithelium (corneal epithelium) which can regenerate rapidly after damage.

Given below is a histological image of non-keratinised stratified squamous epithelium of the esophagus.