Chapter 1. (Epithelia)

▶ Epithelia separate the internal environment from the external environment by forming tightly cohesive sheets of polarized cells held together by specialized junctional complexes and cell adhesion molecules

▶ Epithelial cells participate in embryo morphogenesis and organ development

This chapter address the structural characteristics of epithelial cells General Characteristics of epithelia

1) tightly cohesive sheet of cells that covers or lines body surface (skin, intestine, secretory ducts) and forms functional units of secretory gland (salivary gland, liver)

2) basic function; protection (skin)

; absorption (small and large intestine)

; transport of material at the surface (mediated by cilia)

; secretion (gland)

; excretion (tubules of kidney)

; gas exchange (lung alveolus)

; gliding between surface (mesothelium, 중피세포)

3) Most epithelial cells renew continuously by mitosis (regeneration) 4) Epithelia lack direct blood and lymphatic supply (avascular; no blood supply). Nutrients are delivered by diffusion

5) Epithelial cells have no free intercellular substance (in contrast to connective tissues) (Control of permeability)

6) The cohesive nature of an epithelium is maintained by cell adhesion molecules and junctional complexes (attachment).

7) Epithelia are anchored to a (바닥판). The basal lamina and connective tissues components cooperate to form basement membrane (기저막)

8) Epithelia have structural and functional polarity

Epithelial Tissue - Characteristics & Functions https://www.youtube.com/watch?v=xI2hsH-ZHR4 ( 6 min)

■ Classification based on two parameters

1. shape of individual cells

1) squamous cells (편평상피세포): flattened (width > height)

2) cuboidal cells (입방상피세포): equal dimension (width = height)

3) columnar cells (원주상피세포) : taller than wider (height > width)

2. arrangement of the cells (the number of cell layers)

1) Simple epithelia (단층상피세포): one layer 1st name last name : subdivided into simple squamous epithelium (단층편평상피세포), simple cuboidal epithelium (단층입방상피세포), simple columnar epithelium (단층원주상피세포),

▶ The specific name endothelium (내피세포) is used for simple squamous epithelium lining blood and lymphatic vessel.

▶ Mesothelium (중피세포) is simple squamous epithelium lining all body cavities (peritoneum (복막), pericardium (심장막), and pleura (흉막) Figure 1-2. Simple epithelium

Hematoxylin (purple) and Eosin (rose pink) (H & E ) staining

2) Stratified epithelia (중층상피세포): more than one cell layer

- subclassified into stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium

- Stratified squamous is the epithelium most frequently found and can be subdivided into moderately keratinized (known as nonkeratinizing) and highly keratinized types

 consist of basal cell (undifferentiated , specialized for mitotic division, replace differentiating cell of upper layers) and outer layer cells (highly differentiated cells, contain keratin) ▶ Stratified squamous epithelium with moderately keratin: esophagus, vagina

; The cells of outlayer have nuclei

▶ Stratified squamous epithelium with abundant keratin: epidermis of skin The cells of outlayer lack nuclei.

성숙과정동안 각질화 (keratinization) 일어남 (keratin으로 구성된 거칠고 죽어있는 표면층을 형성)

; basal cell (바닥쪽의 세포)은 계속적을 분열하고 딸세포들은 지속적으로 표층으로 밀려올라가 그곳 에서 긍극적으로 떨어져 나감

Although rare, there are also stratified cuboidal epithelia (ovarian follicle) and stratified columnar epithelia (lining the intralobular ducts of salivary gland).

3. Special categories

1) Pseudostratified epithelium (거짓증층상피):

- consist of columnar cell and basal cell

i) Pseudostratified columnar ciliated epithelium (trachea, 기관)

- consist of columnar cell, basal cell, and Goblet cell(mucus-secreting cell) ii) Pseudostratified columnar epithelium with (부동섬모)(epididymis, 부고환)

2) Urothelium (transitional epithelium, urinary bladder): can stretch without breaking - Consist of basal cells, intermediated cells and columnar dome-shaped cells - An important feature is transitional height that varies with distention and contraction of the organ Empty bladder: Stratified cuboidal

Full bladder: Stratified squamous

Plaques of aggregated proteins are found on the apical plasma membrane of columnar cells Classification of Epithelia https://www.youtube.com/watch?v=mpH0DHHQ2cs (see after 50 sec) Essential concept

▶ Four basic tissues ▶ Classification into three major group

1. Epithelial tissues 1. The number of cell layer (simple, stratified)

2. Connective tissues 2. Shape of cells (squamous, cuboidal, columnar)

3. Muscle tissues 3. Shape of cells at the outmost layer (stratified columnar, stratified squamous, stratified cuboidal for 4. Nervous tisues pseudostratified epithelium)

▶ Property and function of Epithelium

▶subdivision of stratified squamous epithelium - moderately keratinized (nonkeratinized), highly keratinized types (Keratin: a kind of cytoskeletal protein)

▶ apical differentiation such as cilia, microvilli, stereocilia (respiratory tract, oviduct, nephron, small intestine, epididymis (부고환) and hair cells of inner ear) Epithelia cells polarity - An important aspect of epithelium is polarity which is essential to carry out specific function of various organ system

- Three geometrical domains represent polarity of epithelium.

1. Apical (uppermost) domain: exposed to lumen or external environment

2. Lateral domain: faces neighboring epithelial cell linked to each other by cell adhesion molecules and junctional complex :facilitates directional or vectorial transport function

3. Basal domain: associated with basal lamina (바닥판) that separate epithelium from underlying connective tissues.

- basement membrane (기저막): basal lamina-connective tissue complex ▶Apical domain: protection of epithelial surface (cilia in respiratory tract) : absorption of substance (microvilli in intestinal epithelium)

Apical differentiation

▶ Apical domain of some epithelial cells display three types of differentiation.

1. Cilia (섬모)

2. Microvilli (미세융모, 미세돌기)

3. Stereocilia (부동섬모) 1. Cilia ▶ Two types of cilia: multiple motile cilia and single or primary non-motile

▶ Ciliogenesis, the assembly process of both types of cilia, is initiated by the basal body, a structure originated from a basal body precursor located in centrosome (중심체).

▶Failing to dock to apical plasma membrane of basal body disrupts ciliogenesis and give rise to human respiratory disease and primary ciliary dyskinesia (운동장애)

▶Under nomal condition, basal body migrate to apical membrane and extend into axoneme (축사), a basic structure of cilium. (축사: 섬모 또는 편모의 가운데존재하는 축의 탄성섬유) Multiple motile cilia ▶ function to coordinate fluid or cargo flow on the surface of epithelium

▶ cell projections originating from basal body anchored by rootlets

▶ Basal body contains nine triplet microtubules (93 [triplets] + 0 ) in a helicoid array without a central microtubular component

▶ Cilium consists of axoneme formed by a central pair of microtubules (92 [doublets] + 2) surrounded by nine concentrically arranged microtubular pairs.

 This assembly is known as 9 + 2 microtubular doublet arrangement.

; Axoneme is also a component of sperm tail, or flagella. ▶ Trachea (기관) and oviduct (난관) are lined by ciliated epithelial cells,

In these epithelial cells, cilial activity is required for 1) the local defense of respiratory system, for 2) the transport of fertilized egg to uterine cavity

Single or primary non-motile cilia

▶ The importance of single cilium emerge from rare recessive human disorders known as ciliopathies caused by functional abnormality of cilia

▶ Significant aspects of primary cilium

1) function as a sensor that provides the cells with information about the surrounding external environment

2) Participate in the early stage of embryonic patterning leading to organogenesis

3) The position of single cilium, called kinocilium, of the hair cells of the in the inner ear, determine the correct polarity of adjacent stereocilia, essential for maintaining body balance and for hearing.

Cilia and flagella What is cilia? https://www.youtube.com/watch?v=ZRKUAG4IC2s ( 1 min)

Primary Cilia - the cells' ultimate antennae: https://www.youtube.com/watch?v=NFAO65d7M3A (11 min) 2. Microvilli (미세돌기, singular , )

▶ finger-like cell projection of apical epithelial surface containing microfilament (polymer of G monomers)

▶ The intestinal epithelium and portion of the nephron in kidney are lined by epithelial cells with microvili forming a brush border (absorptive function)

# brush border (솔가장자리) ; 세포표면이 분화된것으로 표면적증가시키는 둥근모양의 미세돌기로 구성

Structure of villi : https://www.youtube.com/watch?v=Onoq1Ok7_KM (1 .5 min) 3. Stereocilia (부동섬모, stereovilli)

▶ long and branching finger-like projection of apical epithelial cell surface

▶ contains actin microfilaments, similar to microvilli

▶ do not have axoneme

▶ typical of epithelial lining of epididymis and contributes to the process of sperm maturation in this organ

Epithelial polarity ; https://www.youtube.com/watch?v=toTTLCpFXOY (7.5 min) Essential concepts

▶ Apical domain of some epithelial cells display differentiation projecting into lumen. Apical differentiation can be motile (multiple cilia) and non-motile (primary cilium, microvilli and stereocilia/stereovilli)

▶ Cilia contain an axoneme, formed by a concentric array of nine microtubule doublets ( 9 + 2) surrounding a central pair.

▶ Basal body and centriole are formed by nine microtubule triplet in a helicoid arrangement. ( no central microtubule in basal body)

▶ Nonmotile microvilli and stereocilia contain actin microfilament core.