HUMAN ANATOMY – VOLUME 1

Fig. 5. Stages of mitosis. Aggregations of chromatin, formation of chromosomes, formation of the spindle apparatus and regular positioning of chromosomes as well as centrioles within both cells are clearly evident.

A — interphase; B — prophase; C — metaphase; D — anaphase; E — telophase; F — late telophase (acc. to A.Ham, D. Kormack, 1982, with deviations). 1 — nucleolus, 2 — centrioles, 3 — mitotic spindle, 4 — aster; 5 — nucleolemme; 6 — kinetochore; 7 — contineous microtubules; 8,9 — chromosomes; 10 — chromosomal microtubules; 11 — formation of nucleus; 12 — fissure of granulation; 13 — bundle of actin fibers; 14 — residual corpuscle.

forms a division furrow perpendicular to the cell’s lengthwise axis. During telophase chromosomes, which have moved to the poles, become decondensed, transform into chromatin, and transcription (production) of

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RNA begins. The nuclear membrane, the nucleolus and membrane structures of future daughter cells are quickly formed. At the surface of the cell a groove along its equator divides it into two daughter cells. Through mitotic division daughter cells receive a set of chromosomes identical to that of the mother cell. Mitosis provides genetic stability, increase in number of cells and, consequently, growth of the organism as well as regeneration processes.

Meiosis takes place in sex cells. The results of this type of division are new cells with a single (haploid) set of chromosomes. This is important for passing on genetic information. During the fusion of two gametes of opposite sexes (during fertilization) the set of chromosomes becomes double (diploid). The diploid cell (the zygote), which forms as a result of fertilization, contains two complementary (homologous) sets of chromosomes. One chromosome of each homologous pair originates from the nucleus of the ovum, the other from the nucleus of the spermatozoon.

During meiosis of sex cells each daughter cell receives only one chromosome out of each homologous pair. This becomes possible due to two successive meiotic divisions. As a result, one diploid cell produces four haploid daughter cells. Each haploid cell contains twice as less chromosomes (23) as the nucleus of the mother cell (46). Also as a result of meiosis the haploid sex cells contain not only less chromosomes, but an altered combination of genes. Thus, the new organism carries not only a sum of the parents features, but its own individual characteristics as well.

Questions for revision and examination

1.Name the structural elements of the cell. Give their morpho-functional character-

istics.

2.List the membranous and non-membranous cell organelles. Name their functions.

3.What elements is the nucleus composed of? What functions does the nucleus per-

form?

4.Describe the structure of a DNA molecule.

5.Give the morphological characteristics of chromosomes; give their classification.

6.What is the cell cycle? What periods /phases/ can be distinguished in it?

7.What is meiosis? What are the differences between meiosis and mitosis?

TISSUES

Cells and their derivatives combine to form tissues. A tissue is a combination of cells and extracellular matrix united by a common embryological derivation, structure and function. The tissues in the human organism are subdivided into four types: epithelial, connective, muscular and nervous. Each type of tissue develops from a specific germ layer. Epithelial

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tissue derives from the endo-, ectoand mesoderm. Nervous tissue develops from the ectoderm.

EPITHELIAL TISSUE

Epithelial tissue covers the surface of the body and mucosas. Epithelial tissue also forms glands (glandular epithelium). Furthermore, there is sensory epithelium in the hearing, vestibular, smell and taste organs, the cells of which can perceive specific stimuli.

Classification of epithelium. Integumentary epithelium is subdivided into single layer (simple) and stratified (Fig. 6). In simple epithelium all cells lie on the basement membrane. In stratified epithelium cells lie in several layers, and only cells of the basal (deep) layer are in contact with the basement membrane. Simple epithelium is further subdivided into sin- gle-row (isomorphic) and multilayered (pseudostratified). In single-row epithelium the nuclei of all cells are situated on the same level and cells have the same height. In pseudostratified epithelium nuclei lie in several layers, while the cells may have different shapes. Depending on the shape of the cells and their ability to undergo keratinization epithelium can be stratified keratinous (squamous), non-keratinized (squamous, cuboidal, columnar) and transitional. All epithelial cells have certain common structural peculiarities. The apical part of epitheliocytes differs from the basal part. Cells of the integumentary epithelium form layers, which rest on the basement membrane and lack blood vessels. Epithelial cells contain all the general organelles. Cells, which secrete protein, are rich in granular endoplasmic reticulum. Cells, which produce steroids, have more smooth endoplasmic reticulum. Absorption cells posses an abundance of microvilli, and epitheliocytes, that cover the respiratory tract mucosa, have cilia.

The integumentary epithelium performs the barrier and protective functions as well as the functions of absorption (small intestine epithelium, peritoneum, pleura, nephron canals, etc.), secretion (amniotic epithelium, stria vascularis of the cochlear duct), ventilation (respiratory alveolocytes).

Simple epithelium. Simple epithelia include simple squamous, cuboidal, columnar and pseudostratified epithelium types. Simple squamous epithelium is a layer of flat cells, which lie on a basement membrane. Protrusions of cell surfaces can be found only above zones containing the nucleus. Epitheliocytes have a polygonal shape; they form the external wall of the renal glomerulus capsule; cover the posterior surface of the cornea; line blood and lymph vessels and chambers of the heart; cover the serous membranes (mesothelium).

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Fig. 6. Structure of the epithelial tissue.

A — simple ciliated epithelium (mesothelium); B — simple cuboidal epithelium; C — simple columnar epithelium; D — ciliar epithelium; E — transitional epithelium; F — stratified squamous nonkeratinized epithelium.

Endotheliocytes have an elongated or fusiform shape and a very thin layer of cytoplasm. The perinuclear part of the cell is thickened and protrudes into the lumen of the blood vessel. Microvilli are situated mostly above the nucleus. The cytoplasm contains micropinocytotic vesicles, solitary mitochondria, elements of the rough endoplasmic reticulum and the Golgi complex. Mesotheliocytes, which cover serous membranes (peri-

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Fig. 7. Structure of the columnar simple epithelium.

1 — microvilli; 2 — nucleus; 3 — basal membrane; 4 — connective tissue.

toneum, pleura, pericardium), partake of endotheliocytes. Their free surface is covered with a multitude of microvilli; some of the cells contain 2–3 nuclei. Mesotheliocytes facilitate the gliding of internal organs against each other and prevent the formation of adhesions between them. Respiratory epitheliocytes are large (50–100 mm), their cytoplasm is rich in micropinocytotic vesicles and ribosomes. Other organelles are not abundantly represented.

S i m p l e c u b o i d a l e p i t h e l i u m is formed by a single layer of cells of hexagonal shape. The nucleus is round and lies in the center of the cell. There are non-ciliary cuboidal epitheliocytes (the collecting ducts in kidneys, distal straight tubules, biliary ductules, the choroid plexus in the brain, the pigmental epithelium of the retina, etc.) and ciliary cuboidal epitheliocytes (terminal and respiratory bronchioles, ependymocytes which line the ventricles in the brain). The anterior surface of the crystalline lens is also covered with cuboidal epithelium. The surface of these cells is smooth.

S i m p l e c o l u m n a r (p r i s m a t i c) e p i t h e l i u m (Fig. 7) covers the mucosa of the gastrointestinal tract organs beginning at the entrance into the stomach up to the anal opening; it also covers the walls of the papillary ducts and collecting tubules in kidneys, striated ducts of the salivary glands, the bronchi, the uterus and uterine tubes. Columnar epitheliocytes are tall prismatic polygonal or round cells closely adjoined to

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lumen of the organ. This type of cells is subdivided

into three kinds. The highly differentiated surface epitheliocytes extend to the lumen of the organ. These cells have a round nucleus and well-devel- oped organelles, especially the Golgi complex and the endoplasmic reticulum. Their apical plasma membrane is covered with microvilli, stereocilia or cilia. Ciliary cells cover the mucosa of the nose, trachea and bronchi. Non-ciliary cells cover the mucosa of part of the male urethra, excretory ducts of various glands, the epididymal and deferent ducts. Interposed epitheliocytes are not well differentiated, do not have cilia or microvilli and do not extend to the lumen of the organ. They are situated between the surface cells. Basal epitheliocytes form the lowest (deep) cell layer. Basal epitheliocytes are the source of epithelium renewal (2% of the cell population is renewed daily).

Stratified epithelium. Stratified epithelia include non-keratinized and keratinized squamous epithelia and stratified cuboidal and columnar epithelia.

S t r a t i f i e d s q u a m o u s n o n - k e r a t i n i z e d e p i t h e l i u m (Fig. 8) covers the mucosa of the mouth, esophagus, the transition zone of the anal canal, the vocal folds /cords/, the vagina, female urethra and the

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Fig. 9. Structure of the stratified squamose keratinized epithelium (acc. to R. Krstic, with deviations).

1 — squamose flakes; 2 — keratoid layer; 3 — sparkling layer; 4 — granular layer; 5 — spinous layer; 6 — basal layer; 7 — melanocyte; 8 — intercellular fissures; 9 — basement membrane.

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external surface of the cornea. This type of epithelium is subdivided into basal, spinous (middle) and surface layers. The basal layer is formed by large prismatic or polyhedral cells, which lie on a basement membrane. The middle spinous layer is formed by large polygonal cells. These two layers form the germinal layer. Epitheliocytes divide through mitosis, move upwards, flatten out and substitute for cells, which are shed off the surface. The surface layer is formed by flattened cells, many of which have lost their nuclei. The cells closest to the surface become flat, dead, lose contact with each other and fall off.

S t r a t i f i e d s q u a m o u s k e r a t i n i z e d e p i t h e l i u m (Fig. 9) covers the entire surface of the skin forming the epidermis. The epidermis consists of five layers, namely, the basal layer, the spinous layer, stratum granulosum, stratum lucidum and stratum corneum (keratinized layer). The basal layer consists of prismatic cells, which have numerous processes and are surrounded by the basement membrane. The cytoplasm above the granular nucleus contains grains of melanin. In between basal epitheliocytes lie pigment cells called melanocytes. The spinous (prickle cell) layer is formed by several rows of large polygonal prickle cells. Both previous layers form the germinal layer, the cells of which undergo mitotic divisions and move towards the surface. Stratum granulosum consists of flat (squamous) epitheliocytes, which are rich in keratohyaline. As its amount increases, cells slowly degenerate. Stratum lucidum has a strong light-bending ability, which is due to eleidin containing flat epitheliocytes. The keratinized layer is formed by shedding flakes of keratin.

S t r a t i f i e d c u b o i d a l e p i t h e l i u m is formed by several (3 to 10) layers of cells. The surface layer consists of cuboidal cells. These cells have microvilli and contain grains of glycogen. Beneath the surface layer there are several layers of elongated fusiform cells. Directly on the basement there are polygonal and cuboidal cells. This type of epithelium is rare. It is found on small areas along short sectors between multinuclear columnar and stratified squamous non-keratinized epithelium posterior part of the nasal vestibule, excretory ducts of sweat glands.

S t r a t i f i e d c o l u m n a r e p i t h e l i u m also consists of several layers of cells (3–10). Surface epitheliocytes have a prismatic shape and may bear cilia. Epitheliocytes that lie beneath are polyhedral or cuboidal. This type of epithelium is found in some areas of the salivary and mammary glands excretory ducts, larynx and male urethra.

Transitional epithelium. In transitional epithelium the number of layers can change (decrease) when the mucosa of the organ is stretched. This type of epithelium lines the mucosa of the renal pelvis, ureters, bladder and beginning of the urethra. The plasma membrane of the surface layer

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is plicated, and its outer layer is thicker than the inner one. In an empty bladder the cells are tall, with 8–10 rows of cells seen on a section. In a full bladder cells are flattened with no more than 2–3 rows of cells, while the plasma membrane of the surface layer is smooth.

Glandular epithelium. Cells of the glandular epithelium (glandulocytes) form unicellular glands and the parenchyma of multicellular glands. Glands are subdivided into exocrine (have an excretory duct) and endocrine (no excretory ducts). Endocrine glands excrete their product directly into intercellular space, from which they enter the blood and lymph. Exocrine glands excrete their product through ducts onto the surface of the body or into lumens or organs (sweat, sebaceous, gastric and intestinal glands). Mixed glands contain an endocrine and exocrine parts (the pancreas).

The human organism contains many unicellular goblet exocrinocytes, which lie between other epithelial cells lining the mucosa of hollow organs of the digestive, respiratory, urinary and reproductive systems (Fig. 10). These exocrinocytes produce mucus, which consists

Fig. 10. Structure of a goblet cell.

1 — cellular microvilli; 2 — mucous (vesicle) granules; 3 — Golgi complex; 4 — mitochondrion; 5 — nucleus; 6 — rough endoplasmic reticulum.

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of glycoproteins and lines the mucosa with a thin layer. The structure of goblet cells depends on the phase of the secretory cycle they are in. The narrow, chromatin rich nucleus lies in the basal part of the cell, or its stalk. There is a well-developed Golgi complex located above the nucleus. The upper dilated part of the cell contains many secretory granules, which undergo merocrine secretion. Once the granules are secreted the cell becomes narrow.

Secretion of mucus from a cell onto the mucosa surface usually takes place through exocytosis.

E x o c r i n o c y t e s also form the beginning excretory portions of multicellular exocrine glands and their tubular ducts. The morphology of exocrinocytes depends on the type of secretion and the secretory phase. Glandular cells are structurally and functionally polarized. Their secretory vesicles and granules are concentrated in the apical (supranuclear) zone and are excreted through the apical microvilli-bearing cytolemma. These cells are rich in mitochondria, Golgi complex and endoplasmic reticulum structures. Rough endoplasmic reticulum prevails in protein synthesizing cells (e.g. granulocytes or the parotid gland). Smooth reticulum is found in cells, which synthesize lipids and carbohydrates (e.g. cortical endocrinocytes of adrenal glands).

Secretory processes in exocrinocytes take place in cycles, which are divided into four phases. During the first phase the substances necessary for synthesis enter the cell. The second phase is the synthesis of the product, which is carried in transport vesicles to the Golgi complex. In the Golgi complex substances, which are to be excreted, are collected in vacuoles. Vacuoles turn into secretory granules, which move in the apical direction. During the third phase the secretory granules are excreted from the cell. During the fourth phase the exocrinocytes are restored.

There are three types of secretion. By m e r o c r i n e (e c c r i n e) secretion the products are excreted by exocytosis. This type is found in serous (protein) glands. The cell structure is not impaired. A p o c r i n e s e - c r e t i o n (e.g. in lactocytes) is accompanied by destruction of the apical part of the cell (macroapocrine secretion) or only the apices of microvilli (microapocrine secretion). During h o l o c r i n e s e c r e t i o n the glandulocytes are completely destroyed and their protoplasm becomes part of the secretion (e.g. sebaceous glands).

Classifications of multicellular glands. According to the structure of the initial (secretory) portion glands are subdivided into tubular (in the form of a tube), acinar (shaped like a pear or an oblong grape) and alveolar (rounded, spherical). There are also tubuloacinar and tubuloalveolar glands (Fig. 11).

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