Download Microanatomy of the Oral Cavity and Duodenum and more Study notes Histology in PDF only on Docsity!
LIP
Constitute the anterior wall of the vestibule of the mouth and are made of a
core of striated muscle (orbicularis oris) embedded in fibroelastic connective
tissue. In the lip we can distinguish 3 surfaces:
- Outer surface: covered with skin and a flat stratified squamous
keratinized epithelium, where we can distinguish:
o Hair follicles
o Sebaceous glands
o Sweat glands
- Red transition zone: between outer skin and inner mucous
membrane. Some of the characteristics of this part are:
o High papillae of the basal lamina (contain many vessels and
provide the red colour)
o Stratified squamous keratinized epithelium, is the place of transition of the outer skin
and mucosa. It is not as much keratinized as the outer surface o Sebaceous glands
without follicles
- Inner surface: composed of stratified squamous non-keratinized epithelium, we can find o
o Labial glands lying in the lamina propria
o Papilla of the basal lamina are not very prominent, and are somehow flat
The lips are mainly supplied by the superior and inferior labial branches of the facial artery. The
upper lip is innervated by superior labial branches of the infraorbital nerve and the lower lip is
innervated by the mental branch of the mandibular division of the trigeminal
TONGUE
It is divided into:
- Apex
- Dorsum
- Inferior surface
In the tongue we have the taste buds, which are structures specialized in
the recognition of flavours. They are located in papillae. We have the
following types of papillae:
- Filiform papillae: with a conic shape are the most numerous ones and the smallest. They are
located in the dorsal surface of the tongue and covered by stratified squamous keratinized
epithelium. These papillae are do not have taste buds and its main function is increase the
friction between the food and the tongue
- Fungiform papillae: mushroom-shaped and located in the anterior surface of the tongue. Are
bigger that the filiform papillae, covered by stratified squamous non-keratinized epithelium
and contain taste buds (epithelial localization) in the apical surface
- Foliate papillae: located bilaterally in the margins of the tongue. Are covered by stratified
squamous non-keratinized epithelium and contain taste buds
- Circumvallate papillae: 8-12 papillae located in the anterior surface of the terminal sulcus.
They are the largest ones and contain taste buds. The papillae are narrower at their base and
are covered by a stratified squamous non-keratinized epithelium.
MICROANATOMY
GLANDULA PAROTIS
The gland is surrounded by a fibrous capsule. From the inner
surface of the capsule, fibrous septa enter the gland, dividing it
into lobes and lobules. Large amounts of adipose tissue often
occur in the paratiroid gland and may be one of the distinguish
features. Delicate connective tissue containing blood vessels and
nerves, as well as myoepithelial cells are present around ducts and
acini. the secretory part of the parotid gland is composed entirely
of tubulo-alveolar serous acini, which cells are pyramidal in shape
with a big round nucleus located in the basal zone. The cells
contain a well-developed RER and Golgi apparatus, with
zymogen granules located in the apical portion of the cell. The
ducts of the parotid gland are formed first of intercalated ducts
(simple cuboidal epithelium) that collect the saliva directly from the
serous acini. After that some of these ducts unite and form the
intercalated ducts (simple columnar cells) which cells absorb Na
and Cl
and serete K
and HCO 3
. They are called striated ducts
because in the basal membrane they have many infoldings.
These striated ducts will unite to form intralobular ducts and later
interlobular ducts. The latter ones will also unite to form the
excretory ducts, which are lined with cuboidal epithelium. The cells
are called Serocites.
GLANDULA SUBLINGUALIS
One of the main differences of the sublingual
gland is that it is not surrounded by a fibrous
capsule. Conversely it has loose connective
tissue septa that divide the gland into lobules
The secretory portion of the sublingual gland is
composed of tubuloacinar mucous and serous
glands, but mainly mucous. The duct system is
similar to the one found in
the other glands. Secretions drain into the oral
cavity by minor sublingual ducts (of Rivinus), of
which there are 8-20 excretory ducts per gland,
each opening out onto the sublingual folds. It is
also possible to find an extra excretory duct that
connects with the submandibular duct and empty the salivary content in the sublingual caruncula.
striated fibers mixed with smooth fibers, and in the lower part we can only find smooth
fibers. In between the circular and longitudinal fibers we can find the Myenteric plexus
- Tunica adventitia or serous: is the outermost layer, and in the cervical and thoracic portions
we can find the tunica adventitia, connective tissue that connects the oesophagus with the
rest of the structures. In the abdominal part the serous (peritoneum) covers the lower portion
STOMACH
It is divided in:
- Cardia: surround the opening of the oesophagus into the
stomach
- Fundus: area above the level of the cardial orifice, close
related to the left part of the diaphragm
- Body: largest region of the stomach
- Pyloric part: divided into
o Pyloric antrum: extends from the insicura angularis
to the proximal limit of pyloric canal
o Pyloric canal: most tubular part of the stomach
- Cardial notch: superior angle created when the esophagus
enters the stomach
- Angular incisures: bend on the lesser curvature
- Greater curvature: extends from the left of the cardiac
orifice, over the dome of fundus and sweeps around until
the inferior part of pylorus
- Lesser curvature: forms the right border of the stomach and extends from the cardiac orifice
to the pylorus
The stomach wall possesses the usual 3 layers of alimentary
canal:
o Lamina epithelialis: lined with simple columnar
epithelium. The cells appear as having the apical
surface empty because of the extraction of the mucous
droplets during staining. The mucous secretion forms a
gel-like coat that adheres to the epithelial surface and
provides protection against the acid content secreted
by the glands. As a result of that, these cells also
contains high quantities of HCO 3
, to neutralize H
o Lamina propria: composed of loose connective
tissue, lymphatic nodules and the gastric glands
o Muscularis mucosae: composed of two relatively thin layers, the inner is circular and
the outer one is longitudinal. They play a role in facilitating the gastric secretion
o Submucosa: consists of dense connective tissue containing adipose tissue, lymph
vessels and Meissner’s plexus, the part of the enteric nervous system that plays an
important role controlling the secretion of the glands.
o Outer longitudinal layer
o Middle circular layer
o Inner oblique layer
DUODENUM
Functionally the small intestine is the principal site
for the digestion of food and absorption of products of
digestion. The chyme from the stomach is received by
the duodenum, where the enzymes from the pancreas
and the bile from the liver are also delivered.
Enzymes located in the glycocalyx and microvilli of
the enterocytes (intestinal absorptive cells) also
contribute to the digestion of the chyme.
o Lamina epithelialis: simple columnar
absorptive it has:
§ Plicae circulares: permanent transverse folds that contain a core of
submucosa. These folds appear 5-6cm after the pyloric sphincter and are
more numerous in the distal part of the duodenum.
§ Villi: from the plicae circulares emerge several projections with a core of
lamina propria and muscularis mucosae.
§ Microvilli: At the apical part of the enterocytes we can find a striated border,
the microvilli. The organelles of enterocytes are distributed in a polar way,
with the nucleus centrally located. Above the nucleus we can find the Golgi
complex, smooth endoplasmic reticulum and several vacuoles. Below the
nucleus we find the RER and several mitochondria.
§ Goblet cells
o Lamina propria: consists of loose connective tissue with several common structures:
§ Lymph nodules: the biggest ones are Payer’s patches, and extend through the
muscularis mucosae and can also reach the submucosa
§ (GALT)
§ Crypts of Lieberkühn: tubular glands, perpendicular to the lumen of the small
intestine with the aperture at the vase of the villi. These glands can be found
throughout the whole length of the small intestine and are composed of the
following cells:
§ Mucous cells: similar to goblet cells, are located in the upper portion of the
gland
§ Paneth cells: located in the deepest part of the glands, are related to the
secretion of lysozyme, an antibacterial substance
§ Stem cells: these are the most numerous cells, they occur in the middle part
of the crypts and provide the source of most of the cell types of the intestinal
epithelium. These cells have can be distinguished from the rest because they
have less microvilli and the nucleus is basal and poorly developed
§ Enteroendocrine cells: scattered among the walls of the intestinal glands.
Some types of these cells are:
§ K cell: secretes gastric inhibitory peptide
§ L cells: secrete glucagon-like peptide (GLP)
- Decreases glucagon secretion
- Increase insulin secretion
- Inhibits acid secretion
§ I cells: secrete cholecystokinin (CCK)
§ Lymph nodules: the biggest ones are Payer’s patches, and extend through the
muscularis mucosae and can also reach the submucosa
§ (GALT)
§ Crypts of Lieberkühn: tubular glands, perpendicular to the lumen of the small
intestine with the aperture at the vase of the villi. These glands can be found
throughout the whole length of the small intestine and are composed of the
following cells:
§ Mucous cells: similar to goblet cells, are located in the upper portion of the
gland
§ Paneth cells: located in the deepest part of the glands, are related to the
secretion of lysozyme, an antibacterial substance
§ Stem cells: these are the most numerous cells, they occur in the middle part
of the crypts and provide the source of most of the cell types of the intestinal
epithelium. These cells have can be distinguished from the rest because they
have less microvilli and the nucleus is basal and poorly developed
§ Enteroendocrine cells: scattered among the walls of the intestinal glands.
Some types of these cells are:
§ K cell: secretes gastric inhibitory peptide
§ L cells: secrete glucagon-like peptide (GLP)
- Decreases glucagon secretion
- Increase insulin secretion
- Inhibits acid secretion
§ I cells: secrete cholecystokinin (CCK)
- Inhibit gastric emptying
- Stimulates bile secretion
§ N cell: secrete neurotensin, which promotes smooth muscle contraction
§ S cell: secrete secretin, which stimulates the exocrine pancreas
o Muscularis mucosae: t forms the base of the mucosa with external longitudinal and
internal circular layers of smooth muscle cells. It follows the surface of the profiles
of the circular folds and sends strands into the core of villi.
o Lamina submucosa: It is composed of loose connective tissue with the usual
structures (vessels, lymph nodes...). It forms the core of plicae circulares and
contains the Meissner plexus, which controls the secretion of the Lyeberkühn crypts.
The main distinction between the duodenum and the other parts of the small intestine
are the Brunner glands, which are acinar glands located in the submucosa. These
glands composed of short columnar epithelial mucous cells secrete mucus and large
quantities of HCO 3
to neutralize the acid secreted from the stomach.
o Stratum circulare
o Stratum longitudinale
- Tunica serosa: It is the visceral peritoneum consisting of loose connective tissue covered by
mesothelium, a membrane composed of simple squamous epithelium
LARGE INTESTINE
o Lamina epithelialis: the luminal surface of the colon is
lined by a simple columnar epithelium, but in this case we
cannot see projections (villi) as in the small intestine, its
surface is smooth. It is composed of the following cells:
§ Columnar absorptive cells: its main function is
absorption, so in the apical surface they have
microvilli and secretory granules. They also secrete
IgA antibodies.
§ Mucous cells (goblet cells)
§ Microfold cells: located lying above the lymph follicles found in the lamina
propria. Have long microvilli
§ Stem cells: source of the other cells, are usually located at the bases of
Lieberkühn crypts
§ Enteroendocrine cells: located at the base of the Lieberkühn crypts
§ Brush cells: contains long microvilli
o Lamina propria: contains crypts of Lieberkühn with the habitual cells. The main
distinction with the small intestine is that the glands are more numerous, closer
between each other and reach deeper space in the lamina propria. We can also
distinguish lymphoid follicles, vessels, connective tissue...
o Muscularis mucosae: outer longitudinal and inner circular layers of muscle
o Lamina submucosa: composed of loose connective tissue and usual structures plus
the Meissner plexus for the innervation of glands and muscularis mucosae
o Outer longitudinal layer
o Inner circular layer
- Tunica serosa: Has outer longitudinal layer and inner circular layer of smooth muscle. The
longitudinal fibers form the three teniae coli (libera, mesocolica and omentalis). It also
contains the Myenteric plexus.
APPENDIX
- Lumen
- Tunica mucosa: simple columnar epithelium with high quantity of
mucous producing cells
o Lamina propria: apart from the normal constituents such as the
Lieberkühn cripts, it contains large quantities of lymphoid
tissue arranged into lymph nodules
o Muscularis mucosae: two layers, inner circular and outer
longitudinal. It is difficult to distinguish because of the
lymphatic tissue
o Lamina submucosa: loose connective tissue and other habitual
compounds with lymphoid follicles. In the central part we can
distinguish the germinal center (lighter)
- Muscular layer: two layers of muscle
- Serosa: thin connective tissue and peritoneum composed of mesothelial cells
GALL BLADDER
The wall of the gallbladder consists of:
- Mucosa: composed of simple columnar epithelium and
lamina propria, there is no muscularis mucosa or tunica
submucosa. The epithelial cells are rich in mitochondria
and have the nuclei in the basal third. All these cells are
able to secrete small amounts of mucus. Near the cystic
duct, the epithelium invaginates in the lamina propria and
forms mucous glands
- Tunica muscularia: the smooth muscle fibers run in
different directions and there are fibers of connective
tissue between them
- Connective tissue or peritoneum: the connective tissue is
locaed in the superior part of the gland, the one that is in
contact with the liver. The peritoneum is found in the
inferior surface
PANCREAS
The pancreas is covered by a thin capsule of connective tissue that
sends septa into it, separating the pancreas in several lobules. At the
same time, acini are surrounded by a basal lamina with a rich
capillary network.
The pancreas is composed of two different types of glandular tissue.
The main tissue mass is exocrine, in which pancreatic islets of
endocrine cells are embedded. The exocrine portion is the one in
charge of secreting digestive enzymes, so it is composed of
compound acinar glands with a similar structure to the parotid gland,
but with some important differences:
- Pancreas lack striated ducts
- Presence of centroacinar cells: intercalated ducts protrude a
little bit inside the acini, which forms the centroacinar cells.
Such cells are only found in the pancreatic tissue.
The exocrine pancreatic acinus is composed of several serous cells surrounding the lumen. These
cells are highly polarized, with a spherical nucleus and the typical characteristics of protein secretin
cells, with zymogen granules in the apical portion containing enzymes such as:
- Trypsinogen
- Chymotrypsinogen
- Carboxypeptidase
- Ribonuclease and deoxyribonuclease
- Lipase
NASAL CAVITY
The nose consists of the
- external nose: has a free tip and is attached to the
forehead by the root. The external orifices of the nose are
the two nostrils. The skin over the dorsum and sides of the
nose is thin and contains many sebaceous glands
- nasal cavity: extend from the nostrils in front to the
choanae behind. It is divided into right and left halves by
the nasal septum made of bone and hyaline cartilage.
The lateral wall of the n asal cavity is marked by 3 projections
called superior,middle and inferior conchae. The area below each
conca is referred as meatus (superior, middle, infgerior). The
chambers are divided into 3 regions:
- Vestibule: communicates with the external environment
anteriorly and is firstly lined with stratified squamous
keratinized epithelium, as a continuation of the skin. In
this region we can differentiate a considerable number of stiff hair, known as vibrissae,
which play an important role in filtering the air before it enters deep into the nasal cavity
and respiratory system. In order to trap small particles in this area there are many sebaceous
glands that secrete a sticky substance. The posterior part of the vestibule is lined with
stratified squamous non-keratinized epithelium, and is continuous posteriorly with the
pseudostratified ciliated columnar epithelium of the respiratory segment
- Respiratory segment: comprises the lower 2/3 of the nasal cavity and is lined with
pseudostratified ciliated columnar epithelium with many different types of cells:
o Ciliated cells: columnar cells with cilia that project into the mucus covering the
epithelium
o Goblet cells: secrete mucus
o Brush cells: contain microvilli and are specialized in the transduction of general
sensation
o Small granular cells: contain secretory granules
o Basal cells: stem cells that can become any other type of cells
In the lamina propria we can differentiate:
o Mucous glands with serous demilunes
o Venous plexus: important for warming the air inspired lymphatic nodules
- Olfactory segment: occupies the upper 1/3 of the nasal cavity and part of the lateral walls.
Like in the previous segment, the epithelium is pseudostratified ciliated columnar, but in
this case there are some special cells:
o Olfactory cells: are bipolar neurons with olfactory cilia hanging in the air, which
collect the olfactory information and send it to the nervous system, specially to the
enthorinal cortex and pirriform area
o Supporting cells: columnar cells that provide mechanical and metabolic support to
olfactory cells
o Basal cells: stem cells from which new olfactory cells and supporting cells
differentiate
forward facilitates the closure of the laryngeal inlet and opening of the oesophagus. Motor and
sensory innervation of the larynx is provided by the vagus nerve X.
Two pairs of mucosal folds, the vestibular and vocal folds, which project medially from the lateral
walls of the laryngeal cavity, constrict it and divide it into three major regions:
- Vestibule: from the inferior surface of the epiglottis to the vestibular folds. These folds don’t
have the intrinsic muscular investment of true vocal cords. Stratified squamous and
pseudostratified columnar ciliated epithelium line the larynx. The cilia beat upward toward
the pharynx moving the mucus and adherent particle to the laryngeal part where they are
swallowed. the lamina propria contains many serous and mucus glands that pour their
secretions onto the free surface. Lymph nodules are scattered throughout the lamina propria.
Over the vocal folds, where there is a considerable amount of wear and tear because of
vibration, the surface is covered with stratified non keratinized epithelium. The same
epithelium covers the anterior surface of epiglottis. Above the vocal folds, the lumen of the
larynx expand to the sinus. Extending superiorly from the sinus on each side is the laryngeal
saccule, a blind-ended tube lined with mucus membrane containing many goblet cells.
- Glottis: between the vestibular folds above and the vocal folds below. We can distinguish
two areas:
o Rima vestibuli: triangular opening between the two adj acent vestibular folds at the
entrance to the middle chamber of the laryngeal cavity
o Rima glottidis: triangular opening below the rima vestibuli formed between the vocal
folds
- Infraglottic space: from inferior border of the glottis to the inferior border of the cricoid
cartilage
Histologically the walls of the larynx has all the elements of those of the respiratory tract, mucus
membrane with pseudostratified ciliated columnar epithelium, cartilage plates, glands, lymphoid
follicles. The distinguishing feature is the presence of skeletal muscle between the cartilages.
TRACHEA
the trachea is a short flexible, air tube about 2.5 cm in
diameter and about 10 cm long. It is situated in the lower
part of the neck and the upper part of the thorax. It is
continuous with the inferior end of the larynx above, below,
at the level of the sternal angle it divides into right and left
main bronchi. The lumen of the trachea stays open because
of the arrangement of the series of U-shaped cartilaginous
bars. Located posteriorly, between the ends of each
horseshoe cartilage, are tracheal muscles. The walls of the
trachea consist of 4 layers:
- Mucosa: lined with ciliated, pseudostratified
columnar epithelium and an elastic fiber-rich lamina
propria. Mucous and basal cells are the principal cells
in the tracheal epithelium. Brush cells are also present but in small numbers.
- Submucosa: composed of a slightly denser connective tissue than the lamina propria with
embedded numerous seromucous glands.
- Cartilaginous layer: composed of C-shaped hyaline cartilage stacked one on top of each
other to form a supporting structure which might be described as a skeletal framework,
prevent collapse of tracheal lumen particularly during expiration. The middle layer of the
tracheal wall might be called tunica fibro-musculo-catilaginea
- Adventitia: composed of connective tissue that binds the trachea to adjacent structures,
contains blood vessels, lymphatic vessels and nerves
PULMO
Pulmonary acini are smaller units of structure that make
up the lobules. Each acinus consists of a terminal
bronchiole and respiratory bronchioles and alveoli that it
aerates. The terminal bronchioles divide and give rise to
respiratory bronchioles- the first part of bronchial tree
allows gas exchange to occur. The smallest functional
unit of pulmonary structure is thus the respiratory
bronchial unit. It consists of a single respiratory
bronchiole and the alveoli that supplies. The respiratory
bronchioles constitute a transitional zone in the
respiratory system concerned with both air conduction
and gas exchange between air and blood. They are lined
with cuboidal epithelium containing both ciliated and
Clara cells. The respiratory bronchioles end by
branching into alveolar ducts, its wall is lined with
cuboidal epithelium supported by connective tissue containing elastic fibers and a few smooth
muscle fibers. The alveolar sacs consists of several alveoli, each alveolus is surrounded by a rich
network of capillaries. Alveolar epithelium is composed of:
- Type I pneumocytes: cover about 95% of alveolar surface
- Type II pneumocytes: are secretory cells, cuboidal, they cover 5% of alveolar surface.
The apical cytoplasm of these cells is filled with lamellar bodies rich in phospholipids and
surfactant that reduces surface tension. Brush cells are also present in the alveolar wall but they are
very few. Another type of cell present is the alveolar phagocyte that remove debris such as carbon
particles.
RESPIRATORY MEMBRANE
Refers to the cells and cell products across which gas must diffuse between alveolar and capillary
compartment. It consist of:
- Thin molecular layer of surfactant
- Alveolar squamous epithelium
- Epithelial basement membrane
- A minute space of connective tissue
- Blood capillary basement membrane
PULMONARY CIRCULATION
The pulmonary artery enters the hilus of each lung, carrying oxygen-poor and carbon-rich blood to
the lungs for gaseous exchange. Having passed the network of capillaries the blood leaves the lungs
oxygen rich and carbon dioxide via the pulmonary veins.
BRONCHIAL CIRCULATION
The bronchial arteries supply all of the lung tissue except the alveoli. The bronchial and pulmonary
circulation anastomose at the level of the junction between the conducting and respiratory passages.
Most of the blood reaching the lungs via the bronchial arteries leaves the lungs via the pulmonary
veins.
EMBRYONAL LUNG
PERINATAL PUMONARY DEVELOPMENT
3 phases can be describes:
- Annuli fibrosi, formed around the base of the aorta, pulmonary artery and the
atrioventricular orifices
- Trigonum fibrosum., formed primarly in the vicinity of the cuspal area of the aortic valve
- Semptum membranaceum, constituting the upper portion of the interventricular septum
VESSELS
The vascular wall is composed of 3 basic structural constituents:
- Endothelium: special type of endothelium interposed as a
semipermeable barrier between two compartments of the
internal medium, the blood plasma and the interstitial
fluid. Endothelial cells perform several functions:
o Conversion of angiotensin I to angiotensin II
o Conversion of bradykin, serotonin, prostaglandins,
norepinephrine, thrombin etc.. to biologically inert
compounds
- Vascular smooth muscle: is present in all vessels except
capillaries and pericytic venules. Smooth muscle cells are
frequent and are arranged in helical layer in the tunica
media of blood vessels.
- Vascular connective tissue: components of connective
tissue are present in the walls of blood vessels in amounts
and proportions that vary based on local functional
requirements.
o Collagen fibers are found between muscle cells,
in adventitia and in some subendothelial layers
o Elastic fibers predominate in the arteries where
they are organized in parallel lamellae regularly
distributed between muscle cells
o Ground substances form a heterogenous gel in the
extracellular spaces of vessel wall. It contributes to the physical properties of the
walls of the vessels and probably affects the diffusion and permeability across the
wall.
VESSEL TUNICS
3 separate concentric layers of tissue or tunics make up the wall:
- Tunica intima: composed of a single layer of flattened squamous endothelial cells and the
underlying subendothelial connective tissue. The endothelial cells lining the lumen of the
blood vessel rest on a basal lamina. Subendothelial connective tissue lies immediately
beneath the endothelial cells and contains the basal membrane, and the internal elastic
lamina (fenestrated), which is found in muscular arteries and allows the diffusion of
nutrients for the cells in the tunica media
- Tunica media: composed of smooth muscle cells oriented concentrically around the lumen.
The tunica media is the thickest of all cells and contains smooth muscle fibers arranges in
layers, usually an internal circular and an external longitudinal. Interspersed between the
smooth muscle cells we can also find elastic fibers, proteoglycans, collagen type III. The
external elastic lamina is found in larger muscular arteries, it separates the tunica media
from the tunica adventitia
- Tunica adventitia: composed mainly of fibro elastic connective tissue arranged
longitudinally.
o Vasa vasorum: The tunica media and tunica adventitia of the thickest vessels cannot
receive direct nourishment from the diffusion that takes place through the internal
elastic lamina. As a result of that these vessels supply the deepest parts of these
layers. As blood in veins is less oxygenated, vasa vasorum is more prevalent in veins
than in arteries.
TONSILLA PALATINA (73) polipo
Tonsils form a discontinuous ring of lymphatic tissue around the entrance
of the mouth and nose into the pharynx. Palatine tonsils are located
between the palatoglossal are located between the palatoglossal and
palatopharyngeal arches. Are bilateral almond-shaped masses. The medial
surface of the palatine tonsils is covered by stratified squamous non-
keratinized epithelium. It is supported by connective tissue septa and a
network of finer fibers. Occasionally the mucosa forms the tonsillar
crypts. The lateral surface of the tonsil is lined by a layer of dense
connective tissue called capsule.
SPLEEN(57)HE tutto rosa con
macchiette- 57A AZAN
The spleen is the largest accumulation of lymphoid tissue in the
organism. Because of its abundance of phagocytic cells and the
close contact with the circulating blood, the spleen is the organ
for a generalized immune response against the infection. It
represents an important defence against any microorganism that
penetrate in the blood circulation. It is also the site of destruction
of RBC, which have an average life of 120 days. The surface of
the spleen, as stated, it is covered by peritoneum and after that
with a capsule of connective tissue that extend into the organ
forming trabeculae. The internal structure of the spleen is formed
of a stroma of reticular cells and fibers, and a parenchyma
divided into:
- White pulp: The basic structure of the white pulp is the
lymphatic splenic nodule, lymphatic splenic follicle or
Malpighian body, which is composed of:
o Accumulations of lymphatic tissue: we can
distinguish the germinal centers where lymphatic
cells proliferate, and the peripheral part, where
there are dividing B cells
o Artery: terminal branch of the splenic artery
located in the malpighian body. One of the main
characteristics of this small arteries is that they do
not have tunica adventitia, instead it is replaced by
a periarteriolar lymphatic sheath (considered
thymus dependent zone) made of T lymphocytes
o Marginal zone: located further away from the central arteriole (in proximity to red
pulp), it contains antigen presenting cells (APCs). This part is important for antigen
presentation
- Red pulp: is the major component of the spleen (75%) and is composed of:
o Splenic cords or Billroth’s cords: reticular tissue cells that occupy all the space
between the sinusoids. They contain:
§ Macrophages
§ Lymphocyte
§ Plasma cells
o Sinusoids: contain blood and its main primary function is to filter the blood of
antigens, microorganisms and worn-out RBC. All these structures pass through the
fenestratea and discontinuous endothelial cells and are phagocytised by the
macrophages present in the splenic cords.
THYMUS 58 formichine rosse
The thymus is fully formed t birth and persists as a
large organ until puberty, when T-cell differentiation
and proliferation is reduced. At this point most of the
lymphatic tissue of the thymus is replaced by adipose
tissue. In the thymus we can differentiate:
- Capsule: made of connective tissue it penetrates
into the parenchyma and divide the whole
thymus into two lobules. It also forms
trabeculae
- Stroma: made of epithelial reticular cells and
reticular fibers. They are important in the
development of the lymphocytes and in the
formation of the blood-thymus barrier. We can
also find nurse cells
- Cortex: densely packed with abundant small lymphocytes and macrophages
o Superficial or outer cortical region
o Deep or inner cortical region
o Medulla: there are fewer lymphocytes, dendritic cells and venules
STROMA AND CORTEX
These cells that migrate from the bone marrow are called thymocytes, which are the hematopoietic
progenitor cells present in the thymus and will eventually develop into mature T cells. In order to
provide the proper conditions for the development of thymocytes, we can find to important cells in
the stroma:
- Nurse cells: surround developing thymocytes in the peripheral part of the cortex. They are
important for the presentation of autoantigens through MHC-I and MHC-II, preventing
autoimmune diseases. They decide de development of thymocytes
- Epithelial reticular cells: by cell-to-cell contacts they create the microenvironment for the
proper development of thymocytes into T cells. According to its distribution in the cortex we
have the following types of epithelial reticular cells:
o Type 1: located surrounding the periphery of the cortex, just below the capsule and
forming the outer limit of the perivascular space. These cells secrete factors that
attract the stem cells
