Epithelium and connective tissue
Introduced| English | Meaning |
|---|---|
| epithelium/ˌepɪˈθiːlɪəm/ | The boundary workforce: barrier, transport, secretion. |
| connective tissue | The infrastructure: support, a transport highway and the ground where immune cells work. |
| simple epithelium | simple epithelium |
| stratified epithelium | stratified epithelium |
| extracellular matrix | extracellular matrix |
| collagen/ˈkɒlədʒn/ | collagen |
| pseudostratified epithelium | pseudostratified epithelium |
Boundaries and infrastructure
- Two of the four tissue types do most of the body's division of labour. Epithelium is the boundary workforce: barrier, transport, secretion.
- Connective tissue is the infrastructure: support, a transport highway and the ground where immune cells work.
Naming an epithelium
- Epithelium 上皮 is named by layers and by the shape of its top cells. A simple epithelium 单层上皮 has one layer; a stratified epithelium 复层上皮 has many.
- The shapes are squamous (flat), cuboidal and columnar (tall). Layer × shape names every epithelium.
Match each epithelium to where it is found.
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Each place gets the layer and shape its job needs.
Shape follows job
- Exchange needs the shortest path, so it uses one flat layer, about 0.5 µm (capillaries, alveoli). Abrasion needs depth that can be replaced, so it uses stratified squamous (skin, oesophagus).
- Transport and secretion need machinery, so they use tall cells full of organelles: cuboidal (kidney tubules) or columnar (gut).
Why does the alveolus use simple squamous epithelium?
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One flat layer, about 0.5 µm, keeps the path for O₂ and CO₂ as short as possible.
Connective tissue
- Connective tissue 结缔组织 is cells in an extracellular matrix 细胞外基质 of fibres (collagen 胶原 and elastic) and ground substance.
- It runs from loose (areolar, under epithelia) to dense regular (tendon) to specialised forms: bone and blood.
Blood is a connective tissue.
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It has cells in a matrix, as every connective tissue does; its matrix is fluid plasma.
One layer that looks like many
- Pseudostratified epithelium 假复层上皮 has nuclei at different heights, so it looks layered. But every cell touches the basement membrane: it is one layer.
- It lines the airways, where its cilia sweep mucus upwards.
Airway epithelium has nuclei at several heights. How many layers does it have?
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It is pseudostratified, looking layered because the nuclei sit at different heights.
Worked example: reading a slide
- A section shows a free surface of tall cells with their nuclei in a row near the base, on a thin basement membrane, over loose tissue with small vessels.
- That is simple columnar epithelium on areolar connective tissue: an absorbing surface, the intestine pattern. Each clue is a coordinate in the grid.
Put the steps of naming an epithelium on a slide in order.
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The surface orients you, the layers and shape give the name, and the name predicts the function.
Worked example: the flattened surface
- (Fictional biopsy) Weight loss and fatty stools. The duodenal biopsy shows flattened villi: the absorbing simple columnar surface has lost most of its area.
- The poor absorption is not a mystery of function: the absorbing structure itself is gone, and the microscope shows why.
Flattened duodenal villi cause poor absorption mainly because —
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Villi multiply the area of the simple columnar surface; flatten them and absorption falls with the area.
Three common slips
- Basement membrane for the whole matrix. It is only the thin sheet that anchors the epithelium.
- "Blood is not connective tissue." It is: cells in a fluid matrix, plasma.
- Counting pseudostratified as many layers. Every cell touches the basement membrane.
Recap
- Layer × shape names every epithelium, and each name predicts a job.
- Connective tissue is cells plus matrix (collagen, elastic fibres, ground substance), from areolar to tendon, bone and blood.
- Pseudostratified looks layered but is one layer.