Fundamentals of Epithelial Tissue and Axial Skeletal Anatomy

Fundamentals of Histology and Basic Tissues

Histology is centered on the study of tissues, which are defined as organized sets of cells that may be similar or different in nature but work in a coordinated manner to fulfill a specific biological function. These cellular organized units contain a variable amount of extracellular matrix, abbreviated as MEC. This matrix is not merely a filler; it provides critical structural support and participates actively in the physiological functioning of the tissue itself. In the biological study of the human body, tissues are categorized into four basic types: epithelial, nervous, muscular, and connective.

General Characteristics and Functions of Epithelial Tissue

Epithelial tissue is characterized by having cells that are tightly apposed to one another with very little extracellular matrix between them. A primary hallmark of this tissue is that it is avascular, meaning it does not contain blood vessels. Consequently, the cells must receive their nutrition via diffusion from the adjacent connective tissue. The basal surface of the epithelial cells is always adhered to a specialized structure known as the membrane basal or MB. Epithelial tissues exhibit a high mitotic index, allowing them to remain in a state of constant renewal. Functionally, epithelial tissue serves as a covering or lining for both the external and internal surfaces of the body, creating a selective barrier between the external environment and the underlying connective tissue. Additionally, these tissues are responsible for the synthesis and secretion of various compounds.

Cellular Polarity and the Basal Membrane

Cellular polarity is a defining characteristic of epithelial cells, which are organized into distinct domains: the apical membrane, the lateral membrane, and the basal membrane. This organization is supported by specific molecular complexes, including the Crumb complex, the PAR complex, and the Scribble complex. The apical domain faces the surface or lumen, while the lateral and basal domains (often referred to together as latero-basal) handle cell-to-cell and cell-to-matrix interactions. The basal membrane itself is composed of three primary components: proteoglycans, type IV collagen or Colágeno IV, and glycoproteins. It serves several vital functions, including anchoring the epithelium, delimiting tissue boundaries, and acting as a filter. Furthermore, it acts as a barrier against the invasion of tumor cells and facilitates signaling for processes such as morphogenesis, fetal development, and wound healing.

Classification of Lining Epithelia: Simple and Pseudostratified

Epithelia are classified based on their function into lining (revestimiento) and glandular types. Lining epithelia are categorized by the number of cell layers and the shape of the most superficial cells. Simple epithelia consist of a single layer of cells. Simple squamous or plano epithelium features flattened cells of low height arranged in a single row; examples include the capillary endothelium, the mesothelium of serous membranes, and the parietal layer of the Bowman’s capsule. Simple cuboidal or cúbico epithelium consists of cells with similar dimensions on all sides and central spherical nuclei. These are found in the excretory ducts of glands, thyroid follicles, and renal tubules. Simple columnar, prismático, or cilíndrico epithelium features cells taller than they are wide, with oval nuclei parallel to the major axis of the cell. These may have apical specializations like microvilli or cilia and are located in the internal surfaces of the stomach, small intestine, large intestine, uterus, and oviduct. Pseudostratified epithelium appears to have multiple layers because the nuclei are located at different heights, but it is actually a single layer because every cell remains in contact with the basal membrane. It consists of tall cylindrical cells and basal cuboidal cells and is typically found in the respiratory tract and the epididymis, often presenting cilia or stereocilia.

Stratified and Transitional Epithelia

Stratified epithelia consist of two or more layers of cells. Stratified squamous non-cornified or no cornificado epithelium consists of multiple layers where the superficial cells are flat. The basal layer contains proliferating cuboidal or cylindrical cells, the intermediate layer (stratum spinosum) contains polyhedral cells connected by desmosomes, and the superficial layer contains squamous cells. This type resists friction in moist environments such as the vagina, esophagus, and ocular mucosa. Stratified squamous cornified or cornificado epithelium is found in the skin and includes a layer of dead cells known as the stratum corneum or capa córnea which contains keratin. In thick skin, such as the palms and soles, an additional layer called the stratum lucidum is present between the stratum corneum and the stratum granulosum, which contains keratohyalin granules. Stratified cuboidal epithelium usually consists of two layers (biestratificado) and is found in gland ducts. Stratified columnar epithelium also has two or more layers, with cylindrical superficial cells, found in the major terminal ducts of glands. Transitional epithelium, also known as urotelio or polymorphous epithelium, changes its number of layers based on the state of the organ. It has convex superficial "umbrella" cells, intermediate polyhedral "racquet" cells, and basal cuboidal cells. This tissue lines the urinary tract.

Specialized Structures of the Apical Membrane

Apical specializations are structural modifications that enhance cellular function. Microvilli or microvellosidades contain an axis of actin filaments and measure $0.1$ micra in diameter and $1$ micra in length. They are supported by proteins such as villin (villina), fimbrin (fimbrina), and fascin (fascina), and are connected to a terminal web or velo terminal. Their function is to increase the surface area for absorption, appearing as a striated border (chapa estriada) in the small intestine or a brush border (ribete en cepillo) in renal tubules. Cilia or cilios contain an axoneme (axonema) composed of microtubules. They measure $0.2$ micra in diameter and $10$ micra in length, while flagella are longer than $15$ micra. Cilia function to move substances across the epithelial surface and are found in the respiratory tract and uterine tubes. Stereocilia or estereovellosidades are also formed by an actin filament axis like microvilli but lack independent mobility. They increase absorption surface and are specifically located in the epithelium of the epididymis and the ductus deferens.

Osteology of the Head: Neurocranium and Viscerocranium

The skeleton of the head is divided into the neurocranium and the viscerocranium. The neurocranium consists of $8$ bones providing a protective case for the brain: one frontal, two parietal, two temporal, one occipital, one ethmoid, and one sphenoid. The viscerocranium, or facial skeleton, consists of $14$ bones: two lacrimal, two nasal, two maxillary, two zygomatic (cigomático), two palatine, two inferior nasal conchae (concha inferior), one vomer, and one mandible. In the neonatal skull, the bones are connected by sutures and fontanelles (fontanelas), including the coronal, sagittal, lambdoid, and metopic (frontal) sutures, as well as the anterior, posterior, sphenoidal (anterolateral), and mastoid (posterolateral) fontanelles. Key landmarks include the frontal eminence and the parietal eminence.

Muscles of the Head and Neck

The muscles of the head are categorized by their function and location. The muscles of mastication include the temporal, masseter, lateral pterygoid (pterigoideo lateral), and medial pterygoid (pterigoideo medial). Facial muscles responsible for expression include the occipitofrontal (frontal belly), orbicularis oculi (orbicular del ojo), corrugator supercilii (corrugador de la ceja), levator labii superioris (elevador del labio superior), zygomaticus major (cigomático mayor), risorius, orbicularis oris (orbicular de la boca), and platysma. In the neck, muscles are divided into suprahyoid and infrahyoid groups. The suprahyoid muscles include the digastric (digástrico), mylohyoid (milohioideo), geniohyoid (genihioideo), and stylohyoid (estilohioideo). The infrahyoid muscles include the sternohyoid (esternohioideo), omohyoid (omohioideo), sternothyroid (esternotiroideo), and thyrohyoid (tirohioideo).

Anatomy of the Vertebral Column

The vertebral column constitutes the skeleton of the back and the main part of the axial skeleton. It has a length of approximately $73$ to $75$ cm and consists of $33$ irregular vertebrae, of which $24$ are mobile. The column is divided into five regions: the cervical region with $7$ vertebrae, the thoracic region with $12$ vertebrae, the lumbar region with $5$ vertebrae, the sacrum with $5$ fused vertebrae, and the coccyx with $4$ fused vertebrae. A typical vertebra consists of a vertebral body and a vertebral arch formed by two pedicles and two laminae. It has several processes: one spinous process, two transverse processes, and four articular processes. The opening within the vertebra is the vertebral foramen.

Regional Variations and Articulations of the Column

Vertebrae possess distinct characteristics based on their region. Cervical vertebrae feature a foramen transversario, a bituberculated transverse process, a bifid spinous process, and a triangular vertebral foramen. Thoracic vertebrae have costal facets (fositas costales) on the body and transverse processes, a long punctiform spinous process directed posteroinferiorly, and a circular vertebral foramen. Lumbar vertebrae have a rectangular spinous process and a triangular vertebral foramen. Atypical vertebrae include the atlas ($C1$), the axis ($C2$), and $C7$. The articulations of the column include synovial interfacet joints and secondary cartilaginous or symphysis intervertebral joints. The intervertebral discs consist of an outer fibrous ring (anillo fibroso) and a central gelatinous nucleus pulposus.

Superficial Muscles of the Back and Neck

The muscles of the anterolateral region of the neck include the sternocleidomastoid (sternocleidomastoideus), the anterior, middle, and posterior scalene muscles (escalenos), the longus colli, and the longus capitis. The superficial muscles of the back (dorso superficial) related to the axial skeleton and shoulder girdle include the trapezius (trapecio), latissimus dorsi (dorsal ancho), levator scapulae (elevador de la escápula), rhomboid minor (romboides menor), and rhomboid major (romboides mayor).

Questions & Discussion

In the study of the head and vertebral column, students must be able to identify specific anatomical landmarks. For a typical vertebra, these include the vertebral body, pedicles, laminae, spinous process, transverse processes, articular processes, and the vertebral foramen. Comparison between vertebral groups is essential; for instance, the cervical group is uniquely identified by the transverse foramen, the thoracic group by costal facets for rib articulation, and the lumbar group by their large size and rectangular spinous processes. In the skull, identification involves distinguishing between the eight bones of the neurocranium and the fourteen bones of the viscerocranium, as well as locating specific sutures and fontanelles in neonatal specimens.