Cortex (anatomy)

In anatomy, the cortex is the outermost layer of an organ, as opposed to the inner medulla or core, and the word is used both in gross anatomy and in microscopy.

The Latin cortex means bark, rind or husk, and the metaphor of a outer shell around a softer centre runs through its use in the kidney, adrenal gland, brain and lymphoid organs.

Cortices are not merely geometric shells, because the outer layer usually has a different cell composition, blood supply and function from the tissue beneath it.

Definition and etymology

• Cortex is a general term for a superficial layer that can be separated from the underlying tissue by a capsule, a vascular boundary or a change in cell type.
• The plural is cortices, and the adjective cortical is used for anything belonging to a cortex, as in cortical bone or cortical neurons.
• The word entered anatomical Latin from classical Latin, where it referred to the bark of a tree, and the botanical sense is still visible in the naming of plant cortical tissue.
• Cortex is not synonymous with capsule, because a capsule is a discrete covering membrane while a cortex is a functional layer of tissue within the organ.
• In radiology the term cortical refers to the dense outer margin of an organ or bone, and loss of cortical definition is a common sign of aggressive disease.

General organization

• Most cortices are separated from the medulla by a vascular arcade, a connective tissue septum or a sharp transition in cell density.
• Cortical tissue is often the site of the organ's highest capillary density, because the outer layer receives the first branches of the supplying artery.
• The cortex of a solid organ is usually the part that is visible on the surface and is therefore the target of superficial biopsy.
• In tubular organs the cortex is less sharply defined, and the term may refer to the outer part of the wall rather than to a separate layer.
• Developmental origin varies, so cortical layers in different organs are not homologous merely because they share a name.
• Cortical thickness is a common morphometric variable, and it is measured by imaging, by histology or by direct caliper measurement on fixed specimens.

Notable examples

• The renal cortex lies between the renal capsule and the renal medulla and contains the glomeruli, proximal and distal convoluted tubules and cortical collecting ducts.^[1]
• The adrenal cortex surrounds the adrenal medulla and is divided into zona glomerulosa, zona fasciculata and zona reticularis, each producing a different class of steroid.
• The thymic cortex is densely packed with immature thymocytes and is the site of T-cell receptor rearrangement and positive selection.
• The cerebral cortex is the outer grey matter of the cerebrum and is the best-known cortex because of its role in perception, movement, language and consciousness.
• The cerebellar cortex has a highly regular three-layer structure, with a molecular layer, a single Purkinje cell layer and a granular layer.
• The ovarian cortex is the outer region of the ovary and contains the follicles, while the medulla contains the larger vessels and loose connective tissue.
• The lymph node cortex is the outer region beneath the capsule and contains lymphoid follicles and the subcapsular sinus.
• Cortical bone is the dense outer shell of a long bone, and it is distinguished from the trabecular bone of the medullary cavity.
• The cortical layer of the hair shaft is the middle layer of keratinocytes, between the cuticle and the medulla, and it carries the pigment that gives hair its colour.

Cerebral cortex

• The cerebral cortex is divided into sensory, motor and association areas, a scheme that remains the backbone of clinical neurology.
• Primary sensory areas receive and process information from vision, hearing and touch, and they include the primary visual, auditory and somatosensory cortices.
• The primary motor cortex executes voluntary movement, while the premotor cortex and supplementary motor area select and plan those movements.
• The posterior parietal cortex guides movement in space, and the dorsolateral prefrontal cortex chooses actions according to rules and internally generated goals.
• Association areas integrate sensory and motor information, and in humans they are expanded relative to other mammals, which is thought to support complex cognition.
• Association cortex is commonly divided into parasensory, frontal and paralimbic zones, each connected with a different set of upstream and downstream regions.^[2]
• The cortex is organised into six layers in most regions, although the motor cortex lacks a distinct granular layer IV and is therefore called agranular.
• Cortical folding increases surface area without increasing skull volume, and the major sulci are used as landmarks to divide the cortex into lobes.

Clinical significance

• Cortical atrophy is a common imaging finding in neurodegenerative disease and is quantified as thinning of the cortical ribbon on magnetic resonance images.
• Cortical infarcts damage the outer layer of the brain and can produce focal deficits that depend on the affected vascular territory.
• Adrenal cortical hyperplasia or adenoma causes hormone excess, such as primary aldosteronism, Cushing syndrome or virilisation.
• Renal cortical scarring follows reflux or infection and is seen as contour deformity and thinning of the cortex on imaging.
• Thymic cortical atrophy occurs with age and after stress, and it reduces the output of naive T cells.
• Cortical bone loss is the main determinant of fragility fractures in osteoporosis, and it is assessed by dual-energy x-ray absorptiometry and quantitative computed tomography.
• Ovarian cortical biopsy is used experimentally to harvest primordial follicles, but the procedure carries a risk of reducing ovarian reserve.

Comparative anatomy

• Cortical layers are ancient features of vertebrate organs, and a renal cortex is recognisable in fish, amphibians, reptiles and birds.
• The adrenal cortex of mammals is organised into zones that are less distinct in reptiles and birds, where the interrenal tissue is often mixed with medullary cells.
• Cerebral cortical thickness and folding differ markedly across mammals, and the relative expansion of association cortex is greatest in primates and cetaceans.
• The cerebellar cortex is remarkably conserved from fish to mammals, which is one reason its three-layer architecture is used as a model of cortical circuit design.

History

• Classical anatomists used cortex for the outer part of organs long before microscopy could resolve its cellular composition.
• The cellular layers of the cerebral cortex were mapped by Korbinian Brodmann in 1909, and his numbered areas are still used in clinical and imaging work.
• The renal cortex and adrenal cortex were named in the nineteenth century as staining and dissection revealed their distinct architecture.
• Terminologia Anatomica lists cortex as a general term and uses it in compounds such as cortex cerebri and cortex renis.

Research and controversies

• Whether cortical thinning in ageing is true tissue loss or partly a measurement artefact of imaging resolution is still debated.
• Cortical thickness measurements differ between surface-based and volume-based pipelines, and the choice of pipeline can change group comparisons.
• The functional boundaries of association cortex are less sharp than the classical map suggests, and resting-state connectivity often crosses cytoarchitectonic borders.
• Comparative studies continue to argue over whether cortical expansion is driven mainly by neuron number, by connectivity or by body size.