basic question

[aspiringmd1015]

going through pathoma, neoplasia chapter and said tumors usually arise from 4 main types of tissue contianed in our bodies, epithelium-lining of the surfaces of the body, mesenchyme- connective tissue, and then your blood cells. So i was wondering, i guess im lacking a bit in the basic cell bio department, but the parenchymal cells like myocytes, hepatocytes, etc etc. Are these classified as epithelium? for example, hepatic adenocarcioma, gland like tumor growth, it arises from hepatocytes no?

---

Members don't see this ad.

 

[SBR249]

You can't divide cells into parenchymal or not when you look at tumors to classify them because different organs arise from different germ layers. You must look at the embryological origins of the cells to decide whether they are epithelial (endoderm/ectoderm) or mesenchymal (mesoderm). Malignant epithelial tumors are always called carcinoma or adenocarcinoma (forms glands). Malignant mesenchymal tumors are always -sarcoma (prefixed with the tissue, eg. osteosarcoma).

So myocytes would be mesenchymal (not epithelial) because muscles arise from the mesoderm. So a benign myocyte tumor would be a myoma (eg. leiomyoma=smooth muscle, rhabdomyoma=striated muscle) and a malignant myocyte tumor is a myosarcoma (eg. leiomyosarcoma or rhabdomyosarcoma)

Hepatocytes, OTOH, are epithelial because the liver arises from the foregut which is endoderm. So hepatocytes are epithelial and because their tumors form glands (and not linings) they are called adenoma (benign) or adenocarcinoma (malignant). However, hepatocellular adenocarcinoma is often shortened to hepatocellular carcinoma because it is assumed that you know hepatocytes are not squamous and do not form linings.

Hematopoietic tumors are a bit different though they are mesenchymal in origin. Tumors that are mostly in circulation is called a leukemia and those that primarily involve lymph nodes are lymphomas though the distinction can be blurred since leukemia often have nodal involvement and lymphoma cells are often found in circulation. Tumors are further divided based on lineage into lymphoid or myeloid. Plasma cell tumors are called myelomas.

It sounds like you need to review cell bio/histology or embryology to make sure you understand the difference between germ layers.

 

[aspiringmd1015]

oh damn, didnt know that tumor classification didnt include parenchyma! yeah i did embryo from langmann during first and second year, so i have a good idea about gastrulation and all that, but thanks for the help! Really cleared my concept.

 

[aspiringmd1015]

any advice on a book that could go into this detail further?

 

[SBR249]

Going back on my notes, it seems that I was a little hasty to say that epithelia can only arise from endoderm/ectoderm. Mesoderm can also give rise to epithelial tissue such as mesothelium or renal tubular epithelium. So I guess if the cell of origin lines something (or forms from a lining) or forms glands then it's epithelial and epithelial cells can be from any of the 3 germ layers. If it's connective, vascular, or lymphatic tissue, it's mesenchymal which I believe only arises from the mesoderm. Just a quick correction on what I posted above.

In any case, Robbins neoplasia chapter has a section on how to name and classify tumors.

 

[aspiringmd1015]

i guess my main confusion was that i didnt know parenchyma can be of epithelial origin regardless of it being from any of the germ layers.

 

[thehundredthone]

All these are different families of terms entirely.

The germ cell layers (suffixed with -derm, and the neural crest cells) are the embryological origins of all tissue.

Epithelium, endothelium, mesothelium and glands are all epithelial tissue, i.e. they line all body surfaces/cavities. Epithelial tissue is the broad nomenclature, the other groups being connective, muscle and nervous tissue.

Parenchyma is the term that refers to the functional tissue of an organ. Its counterpart is the stroma (usually made up of connective tissue). As far as embyrological origins go, all stroma is mesodermal. In contrast, only spleen, heart and kidney parenchyma is mesodermal. Glandular organs will obviously have their parenchyma of an epithelial origin but this does not indicate ectodermal origin (e.g. hepatocytes are endodermal, as is most of the gut). In fact, there are very few organs whose parenchyma arises from a non epithelial origin - the brain, spleen and heart.