Lamina (anatomy)

Lamina is a general anatomical term meaning a thin plate or layer, and it is applied to structures ranging from cartilage and bone to basement membrane and nuclear envelope.

The word is Latin for a thin slice or plate, and it is used both in gross anatomy, where a lamina can be held in the hand, and in microscopy, where it may be only nanometres thick.

Because the term is purely descriptive, a lamina in one organ has no necessary developmental or functional relationship to a lamina in another.

Definition

• A lamina is a flattened sheet of tissue that is thicker than a membrane but thinner than a block, and its two surfaces usually face different compartments.
• The term does not imply a specific tissue type, because laminae may be cartilaginous, osseous, fibrous, epithelial or membranous.
• In compound names lamina often specifies a named part, as in lamina cribrosa, lamina spiralis ossea or lamina propria.
• The plural is laminae, and the adjective laminar describes flow or structure arranged in layers.
• A lamina differs from a layer in that a lamina is usually a discrete anatomical part with a name, whereas a layer may be a histological description.

Examples

• The laminae of the thyroid cartilage are two quadrilateral plates that form the anterior and lateral walls of the larynx and fuse in the midline to make the laryngeal prominence.
• The lamina of the vertebral arch is the flat plate between the pedicle and the spinous process, and the two laminae enclose the vertebral foramen.
• The internal medullary lamina of the thalamus is a Y-shaped sheet of white matter that separates the thalamic nuclei into anterior, medial and lateral groups.^[1]
• The lamina propria is the layer of loose connective tissue immediately beneath an epithelium, and it carries capillaries, immune cells and glands.^[2]
• The nuclear lamina is a dense meshwork of intermediate filaments on the inner surface of the nuclear envelope, made mainly of lamins A, B and C.^[3]
• The lamina affixa is a layer of ependyma that covers the surface of the thalamus and lines the central part of the lateral ventricle.
• The cribriform plate, or lamina cribrosa of the ethmoid, is a perforated horizontal plate through which olfactory nerve filaments pass to the nasal cavity.
• The osseous spiral lamina is a bony shelf that projects into the cochlear canal and supports the basilar membrane.
• The lamina of the Drosophila optic lobe is the most peripheral neuropil of the insect visual system and receives photoreceptor axons.
• The basal lamina is a specialised extracellular matrix sheet beneath epithelial and endothelial cells, and it is distinct from the thicker basement membrane seen by light microscopy.

Structure

• A lamina is usually oriented so that its flat surfaces face two different spaces, which is why it can separate or connect compartments.
• The mechanical properties of a lamina depend on its fibre orientation, and many laminae are reinforced by collagen bundles running in a preferred direction.
• Bony laminae contain both compact and spongy bone, and their thickness reflects the load transmitted through the region.
• The nuclear lamina is anchored to the inner nuclear membrane by lamin B receptors and to chromatin through lamin-associated domains.
• The lamina propria varies in thickness along the gut, being thick in the stomach and thin in the small intestine, and it contains the mucosal immune system.

Function

• A lamina can act as a barrier, as in the nuclear lamina, or as a support, as in the osseous spiral lamina.
• The vertebral laminae protect the spinal cord and provide attachment for ligamenta flava and paraspinal muscles.
• The thyroid laminae shield the vocal folds and give the larynx its rigidity, while their angle determines the prominence of the neck.
• The lamina propria supports the epithelium, supplies its nutrition and hosts the immune cells that sample luminal antigens.
• The nuclear lamina maintains nuclear shape, organizes chromatin and participates in mechanotransduction.

Clinical significance

• Laminectomy is the surgical removal of part of the vertebral lamina to decompress the spinal cord or nerve roots.^[4]
• Laminoplasty enlarges the spinal canal while preserving the lamina, and it is used in cervical spondylotic myelopathy.
• Mutations in LMNA, the gene encoding lamins A and C, cause laminopathies such as Emery-Dreifuss muscular dystrophy and Hutchinson-Gilford progeria.
• Fibrosis of the lamina propria can narrow the gut lumen and is a feature of chronic inflammatory bowel disease.
• Fracture of the cribriform plate can produce cerebrospinal fluid rhinorrhoea and anosmia because olfactory filaments are torn.
• In cochlear implantation the osseous spiral lamina is a landmark for electrode placement and for preserving residual hearing.
• Thinning of the lamina cribrosa of the optic nerve head is associated with glaucoma and is imaged with optical coherence tomography.

Development

• Bony laminae form by endochondral ossification of the cartilaginous primordium, and their fusion in the midline creates the spinous process.
• The nuclear lamina disassembles and reassembles during mitosis, when lamin phosphorylation drives its breakdown and dephosphorylation allows reassembly.
• The lamina propria develops from the splanchnic mesenchyme that condenses beneath the developing epithelium.
• The cribriform plate ossifies late in fetal life, and its perforations form around the olfactory nerve bundles as they grow toward the brain.
• The lamina of the insect optic lobe develops from the inner proliferation centre and receives photoreceptor axons in a topographic order.

Comparative anatomy

• Laminae are widespread in vertebrates, and the vertebral laminae are a defining feature of the vertebral arch in amniotes.
• The thyroid cartilage laminae are largest in mammals with a long neck, and they are reduced or absent in some reptiles and birds.
• The nuclear lamina is conserved across eukaryotes, but the number of lamin genes differs between animals, plants and fungi.
• The lamina cribrosa of the ethmoid is present in most mammals, and its size correlates with the size of the olfactory bulb.

History

• Lamina entered anatomical Latin from classical Latin, where it meant a thin plate of metal or wood, and it was applied to any flat anatomical part.
• The nuclear lamina was described in the 1970s, and the lamin proteins were identified in the 1980s.
• Laminopathy was recognised as a disease category after LMNA mutations were linked to Emery-Dreifuss muscular dystrophy in the 1990s.
• Terminologia Anatomica uses lamina in many compound names but does not define it as a standalone organ term.

Research and controversies

• How the nuclear lamina senses mechanical force and transmits it to chromatin is an active area of cell biology.
• The boundary between lamina propria and submucosa is not always sharp in histology, which complicates fibrosis scoring in inflammatory bowel disease.
• Whether laminectomy or laminoplasty gives better long-term outcomes in cervical myelopathy is still debated, and the answer may depend on the number of levels involved.
• The term lamina is sometimes used for structures that are not true plates, such as the lamina muscularis mucosae, which is a thin muscle layer rather than a sheet.