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The primary palate (premaxilla) develops from the intermaxillary segment that is formed by the fusion of the two medial nasal prominences.

The intermaxillary segment has the following components:

• Labial component - forms the philtrum of the upper lip
• Upper jaw component - forms the medial part of the maxillary bone with the four upper incisor teeth
• Palatal component - forms the triangular area in the front called the primary palate (premaxilla)

The order of the structures formed during eye development is Optic sulcus ° Optic vesicle °

Optic stalk ° Optic cup.

The musculature of the iris, including the sphincter pupillae and dilator pupillae, are derived from the ectoderm of the optic cup.

Most of the muscles of the body develop from mesoderm. The muscles which develop from ectoderm are:

• Muscles of the iris (sphincter and dilator pupillae)
• Arrectores pilorum of skin
• Myoepithelial cells lining the ducts of sweat glands

Anotia (complete absence of the ear pinna) occurs due to the defect in the development of 1st and 2nd pharyngeal arches.

The pinna develops from 6 tissue elevations called the auricular hillocks formed around the margins of the dorsal portion of the 1st pharyngeal cleft.

• 3 hillocks are from the 1st pharyngeal (mandibular) arch
• 3 hillocks are from the 2nd pharyngeal (hyoid) arch.

The mandibular arch contributes to the tragus of the pinna only, while the rest of the pinna is derived from the hyoid arch. Invagination of the first pharyngeal arch gives rise to the external auditory meatus.

Previously, it was believed that the external auditory meatus develops from the first pharyngeal cleft. But recent studies suggest that the first pharyngeal arch gives rise to the external auditory meatus.

In infantile progressive bulbar palsy, the affected structure is the medulla oblongata. It arises from the rhombocephalon (hindbrain).

The rhombencephalon is further subdivided into a cranial part (metencephalon) and a caudal part

(myelencephalon).

• Metencephalon - forms the pons and cerebellum
• Myelencephalon - forms the medulla oblongata

The flexure between the metencephalon and the myelencephalon is called as the pontine flexure.

The prosencephalon, mesencephalon, and rhombencephalon are initially arranged craniocaudally. Due to differential growth of different regions of the brain, various flexures form between them:

Note: A deep furrow called the rhombencephalic isthmus separates the mesencephalon from the metencephalon.

The pineal gland develops from the diencephalon. Hence, defect in its development can cause pineal agenesis.

The most caudal part of the roof plate of the diencephalon develops into the pineal gland. It is also known as the pineal body or epiphysis cerebri. The pineal body initially appears as an epithelial thickening in the midline. It begins to evaginate by the 7th week and becomes a solid organ.

The images given below show the pineal gland:

The zona glomerulosa and zona fasciculata of the adult cortex is present at birth.

The differentiation of the cortical zones of the adrenal gland begins during the late fetal period.

• Žona glomerulosa and zona fasciculata of the adult cortex are present at birth
• Žona reticularis is formed during the 3rd year of life

The correct order of the stages of development of the nephron from earlier to later is blastema stage --gt; vesicle stage --gt; comma 'C' stage --gt; S-shaped stage.

The tip of the ureteric bud, which is growing towards the metanephric blastema, becomes dilated to form an ampulla. The ampulla induces the cells of the metanephric blastema to develop into a nephron.

The cells of the metanephric blastema pass through the following stages when developing into a nephron:

The trigone on the posterior wall of the urinary bladder is formed by the incorporation of the lower ends of the mesonephric ducts.

After this incorporation, the ureters now open separately into the bladder, near the opening of the mesonephric duct. Due to the ascent of the kidneys, the ureteral openings move farther cranially, while the openings of the mesonephric ducts move close together. The triangular area between these ducts (trigone of the bladder) is thought to arise from the terminal portion of the mesonephric ducts.

Recent update: It was considered that the epithelium of the trigone is initially mesodermal and is later replaced by the endoderm-derived transitional epithelium. However, current research indicates that there is no mesodermal epithelial contribution to the bladder at the trigone (Gray's anatomy, 41st edition).

A palpable fluid-filled mass that is connected to the umbilicus superiorly and urinary bladder inferiorly is diagnostic of the urachal cyst.

During embryonic development, the upper end of the urogenital sinus communicates with the allantois, which lies in the umbilical cord. The allantois normally regresses to form a fibrous cord

called the urachus. In the adult, the urachus is seen as the median umbilical ligament. The remnants of the urachus that may be seen after birth are:

Bilateral cryptorchidism usually results in sterility.

In this condition, both testes fail to descend into the scrotum. Therefore, the higher temperature to which they are exposed in the abdominal cavity will inhibit spermatogenesis. The majority of men with untreated bilateral cryptorchidism have azoospermia.

The genital swellings in the male differentiate into the scrotum.

The genital swellings are a pair of elevations that develop on either side of the urethral folds. The genital swellings later become the scrotal swellings in the male, which give rise to the scrotum.

In females, they differentiate into the labia majora.

A unicornuate uterus is a Mullerian anomaly caused due to the defect in the formation or fusion of paired Mullerian ducts.

The uterus and the cervix develop from the paramesonephric ducts (Mullerian ducts).

The cytoplasm that is present within the channels of the endoplasmic reticulum is called vacuoplasm.

The presence of the endoplasmic reticulum divides the cytoplasm into two compartments, one within the channels called vaculoplasm and, that outside the channels called hyaloplasm or cytosol.

With respect to the mass ratio, the cell membrane predominantly consists of proteins.

The mass of proteins either equals or exceeds the mass of lipids in nearly all membranes. Most plasma membranes consist of approximately 50 lipid and 50 protein by weight. The lipid molecules in the cell membrane are predominantly phospholipids. The trilaminar structure of membranes is produced by the arrangement of lipid molecules. Each phospholipid molecule consists of an enlarged head with a phosphate portion and two thin tails. The head end is also called the polar end, while the tail end is the non-polar end. The head end is soluble in water and is said to be hydrophilic. The tail end is insoluble in water and is said to be hydrophobic.

The basal layer of cells present in the stratified squamous epithelium is composed of columnar cells.

The stratified squamous epithelium is made up of several layers of cells notably:

• Columnar cells - deepest layer
• Polygonal cells - middle layers
• Flattened squamous cells - superficial layers

Stratified squamous epithelium may be keratinized or non-keratinized.

Given below is a histological image of keratinized stratified squamous epithelium of the oral mucosa.

The zymogen granules are present in serous acini, whereas the mucous acini consists of mucoid material.

The following table shows a comparison between serous and mucous acini:

Serous acini

Triangular cells with rounded nucleus at the base

The cell boundaries are indist inct

Contains zymogen granules Darkly stained with HE

Thin watery secretion Example: Parotid gland

Mucous acini

Tall cells with flat nucleus at the base

The cell boundaries are distin ct

Contain mucoid material

Lightly stained and appear e mpty with HE

Thick mucoid secretion Example: Sublingual gland

The cavity in the shaft of the fetal long bone mainly contains red .

Bone is a soft, pulpy tissue found in the cavity and the spaces of spongy bone. It is mainly of two types:

• Yellow - made up predominantly of fat cells and may contain few islands of hemopoietic tissue. It is seen in the shaft of the long bones of an adult.
• Red - It is made up predominantly of hemopoietic tissue. It is seen throughout the skeleton of the fetus. It persists till about five years of age after which, the red in the
• shaft of long bones is gradually replaced by the yellow . By around 20-25 years of age, the sites where the red persists include:
• Vertebrae
• Sternum
• Ribs
• Clavicles
• Scapulae
• Pelvis
• Cranial bones
• Proximal ends of femur and humerus

The term gelatinous refers to the degenerated bone in the cranial bones, seen in old age. There is no white .

The osteoprogenitor cells give rise to osteoblasts. They are the stem cells of mesenchymal origin and microscopically resemble fibroblasts.

The image below shows light micrograph of osteoprogenitor cells forming a nidus.

The image below shows a light micrograph showing osteoblasts.