Specializations of the Cell Surface (Cell Surface Modifications)

Epithelial cells are characterized by polarity, where structure and function differ across three distinct domains: apical, lateral, and basal.
Polarity is essential for functions such as asymmetric division, cell migration, and forming the immunological synapse.
Epithelial characteristics include cellularity (tightly packed cells), support by a basement membrane, and specialized junctions for attachment and communication.

Structure: Finger-like projections ( $0.1\,\mu m$ width; $1–3\,\mu m$ length) containing a core of actin filaments.
Proteins: Actin filaments are cross-linked by fimbrin and villin and attached to the plasma membrane by myosin I and calmodulin.
Function: Significantly increases surface area ( $15\times$ to $40\times$ ) for absorption and diffusion; forms the "brush border" in the kidneys and intestinal tract.

Cilia and flagella are microtubule-based projections anchored to a basal body (nine microtubule triplets).
Motile Cilia ( $9+2$ axoneme arrangement): Move fluids over surfaces. Located in the respiratory tract (mucus clearance), uterine tubes (ovum transport), and brain ventricles (CSF circulation).
Primary (Non-motile) Cilia ( $9+0$ axoneme arrangement): Act as sensory receptors for mechanical detection (kidney fluid flow) and signaling pathways like Wnt, Hedgehog, and PDGF.
Movement Mechanism: Axonemal dynein arms (with ATPase activity) generate sliding between microtubule doublets, while nexin links provide elasticity for the recovery stroke.
Flagella: Longer, single projections for propulsion, such as in mammalian sperm, which utilize a specific $9+9+2$ structure including dense fibers.

Ciliopathies: Diseases resulting from ciliary defects, leading to conditions like polycystic kidney disease (PKD), hydrocephalus, and retinal degeneration.
Primary Ciliary Dyskinesia (PCD): Characterized by chronic respiratory infections and male infertility (immotile sperm).
Kartagener Syndrome: A genetic disorder characterized by the absence of dynein arms, resulting in a triad of situs inversus (organ transposition), bronchiectasis, and chronic sinusitis.

Structure: Long, non-motile projections that containing actin filaments and lack an axoneme; structurally more similar to microvilli than cilia.
Locations and Function:
- Male Reproductive System: Located in the epididymis and vas deferens to facilitate fluid absorption.
- Inner Ear: Found on hair cells for mechanotransduction, essential for hearing and maintaining balance.

Lateral Domain: Features cell-to-cell junctions including occluding (tight junctions), anchoring (Zonulae adherentes, desmosomes, and Fasciae adherentes in cardiac muscle), and communicating (gap junctions).
Basal Domain: Includes the basement membrane and cell-to-ECM junctions like hemidesmosomes and focal adhesions.