User:Kinkreet/SCRA/Alberts

In a multicellular organisms, individual cells have lost the ability for independent survival, and inter-depend on each other for survival. There are over 200 different cell types.

Stem cells can be defined as a cell which can divide indefinitely within the lifetime of the organism, into daughter cells which can differentiate or remain a stem cell. As an extension to this definition, a stem cell must also not be terminally differentiated.

Stem cells usually divide at a slow rate. The daughter cells derived from the division differentiate into a transit amplifying cells, which do divide rapidly, but giving rise to a limited number of progeny. Cell memory and signals from the rest of the body allows differentiated cells to remain in a specific lineage. For the pool of stem cells to be preserved, 50% of each division must result in a differentiating transit amplifying cell, and 50% remaining a stem cell. This asymmetrical division can be achieved through environmental asymmetry (the daughter cells are divided into different niches) and/or divisional asymmetry (the fate of each cell is decided at the time of division)

Epidermis and stem cells
Like any tissues, the epidermis is made up of many cell types, of note are the Langerhan cells (dendritic cells derived from the bone marrow), melanocytes (from neural crest) and Merkel cells which forms nerve endings. Appendages such as hairs, fingernails, sebaceous glands, and sweat glands, originate from specialized cells of the epidermis. The cells between hair folliles and other appendages are less specialized, and are called interfollicular epidermis. The interfollicular epidermis can be split into distinct layers. Basal cells reside next to the basal lamina next to the connective tissues of the dermis, these divide and the daughter cells can differentiate into prickle cells, which synthesizes more keratin filaments and forms desmosomes with other prickle cells. Prickle cells differentiate into granular cells, which secrete a substance that waterproofs the metabolically strata below, away from the outer layer of dead cells. The dead cells loses their internal organelles and nucleus through a process similar to partial apoptosis, and they are rigidified because involucin and other proteins form a ~12nm thick layer of proteins on the cytoplasmic plasma membrane surface, to reinforce squames and granular cells.

The cells of the basal layer are spidermal stem cells, as they divides some of the daughter cells differentiates into prickle cells, while some remain in the basal layer.

Stems cells reside in the bulge, and can migrate up or down to give rise to the interfollicular epidermis as well as the hair follicle.