EPITHELIAL SYSTEM - Hypodermis

General description - Hypodermis - Embryonic Development of Hypodermis - Tail Hypodermis - Back to Contents

General Description

The external surface of C. elegans is formed by hypodermis which establishes the basic body form of the animal, acts in nutrient storage, secretes the cuticle and takes up apoptotic cell bodies by phagocytosis. Many of the hypodermal cells form multinucleate syncytia that are formed by cell fusions during development. Specialized variant hypodermal cells, “interfacial cells”, cover the major openings of the body to outside at the lips (arcade cells and buccal epithelium), the excretory pore (duct cell and pore cell), the vulva (vulval epithelium), the anus (rectal epithelium), as well as various sensory openings along the body (“socket” and “sheath” cells). All of these hypodermal cell compartments form a single epithelial cell layer and are tightly held together by zonula adherens on their lateral borders. Their apical surfaces are bound by the cuticle and their basal surfaces are covered by the basal lamina. Hypodermal cells can be described in 2 general categories:
1- Hypodermis
2- Specialized epithelial cells: Several types of supporting cells which might be considered to be of either hypodermal or glial character are included here. Some may act as potential guides for axon outgrowth, protectors of neuronal sensory receptors, or as linker cells to hold various portions of the hypodermis together. Many produce specialized adaptations of the cuticle, to permit the creation of holes or ridges in the cuticle. Some do not produce any cuticle products. These cells fall into 3 subcategories:
a- Seam cells
b- Interfacial epithelial cells: These include cells that are located at the junctions where hypodermis meets another type of tissue.

c- Other epithelial cells: These include tail spike cells and XXX cells of the nose.

Hypodermis

Hypodermis is formed by a main body syncytium, hyp7, and other hypodermal cells in the head and the tail, numbered from hyp 1 through hyp 11 (also hyp13 in male).

Cell Syncytium Nuclei  
Hyp 1 yes 3
Hyp 2 yes 2
Hyp 3 yes 2
Hyp 4 yes 3
Hyp 5 yes 2
Hyp 6 yes 6* * Hyp 6 fuses to hyp 7 during mid-L3 stage.
Hyp 7 yes

139 = 23 embryonic (including "hyp13") + 116 postembryonic [98 from seam blast divisions + 12 from P divisions (including hyp 12) + 6 from hyp 6*]

Contains 139 nuclei in the adult hermaphrodite (Shemer and Podbilewicz, 2000).

** Contains 144 nuclei in the adult male (Shemer and Podbilewicz, 2000).

Hyp 8** no 1
Hyp 9** no 1
Hyp 10** yes 2
Hyp 11** no 1
Hyp12   1(P12.pa) ventral hypodermal cell (aka preanal hypodermis) (Sulston and Horvitz 1977).
Hyp13 (in males) yes 2 (AB.pl/rappppa; i.e sisters of the T cells)

In hermaphrodites, fuses with hyp 7 syncytium during elongation of the embryo (Shemer and Podbilewicz, 2000). In males, fuses with hyp 7 at very late L4 (Nguyen et al, 1999).

** Until mid-L4 state hyp 8 - hyp 11 are arranged similarly between the sexes. In males, after mid-L4, hyp 8, hyp 11, hyp 9 and finally hyp 10 fuse (Nguyen et al, 1999).

In adult C. elegans, the lips anterior to the buccal cavity are covered by three narrow, concentric rings of hypodermal cells, hyp 1, hyp 2, and hyp3 (See HypFIG1 and HypFIG2). Hyp 1 forms the innermost ring, and connects to the arcade cells of the buccal cavity. Hyp 3 forms the outermost ring and connects to hyp 4. Thus, these 3 rostral rings serve to unite the outer hypodermis to the epithelial lining of the digestive tract.

Immediately behind hyp 3, there are three more rings of hypodermal syncytial cells, hyp 4, 5 and 6 that cover most of the head. Hyp 6 is separate from hyp 7 during the early larval stages but fuses to the main body hypodermal syncytium, hyp 7, at mid L3 stage (Yochem et al, 1998). Most of the C. elegans body is covered with hyp 7 which also surrounds the openings for the excretory canal and anus (See HypFIG3). There are 4 hypodermal cells at the tail, hyp8 through hyp 11. The two cells that make up hyp 10 fuse between 1.5 and 3-fold stages of the embryo. Hyp 10 fuses to hyp 8, 9, and 10 in the L4 stage in males but these cells stay separate in the hermaphrodite (see tail hypodermis; Nguyen et al, 1999).

Embryonic Development of the Hypodermis

In the embryo, the hypodermis is a monolayer of 78 cells which secrete the cuticle (Sulston et al, 1983). The smaller 23 cells are located in the head surrounding the opening of the buccal passage and the cuticular sensory organs, and the tail. The remaining larger 55 cells form two inner, two lateral and two outer rows with 17, 20, 18 cells, respectively, at about 250 min after the first cleavage (See HypFIG4 top, a: anus; e:excretory pore; d:deirid; short arrows indicate the direction of migration of cells during interdigitation of the two inner rows). By 390 min, the two inner rows migrate towards each other and interdigitate to make a single dorsal row of cells (See HypFIG4 bottom) (Podbilewicz and White, 1994; Williams-Masson, et al 1998).

At about the same time the external two rows migrate towards the ventral side and meet each other at the midline and form adherens junctions creating a cylindrical embryo (Williams-Masson et al, 1997). At this stage there is one dorsal, two lateral and two ventral rows of hypodermal cells (See HypFIG4). About 340 min after first cleavage, the first cell-to-cell fusion occurs between the two anterior ventral cells (# 18 and 19) of the epithelium to initiate formation of hyp 7 syncytium (Podbilewicz and White 1994). The second fusion in hyp 7 formation occurs in the dorsal side between two cells located anterior to the deirid sensilla (cells # 1 and 2). From then on fusion events progress towards the posterior part of the elongating embryo. The next stage of hypodermal syncytium formation occurs between the dorsal syncytium (of cells # 1-17) and ventral syncytium (of cells #18-23) under the lateral seam cells (Hedgecock et al, 1987; Singh and Sulston 1978; Priess and Hirsh 1986). Thus at hatching, a total of 23 cells have joined to make the hyp 7 syncytium that covers most of the dorsal surface and parts of the ventral surface of the head and the tail (See HypFIG5). The anterior ring of hyp 7 covers the area around the excretory canal and its posterior ring covers the anus. Between these two rings hyp 7 syncytium is not cylindyrical at this time. An additional 116 cells are added to hyp 7 syncytium during postembryonic development. Hyp 6 syncytium is initially formed by two separate fusions (between cells # I, II, III and IV and cells # V and VI) which then join to make hyp 6 during elongation phase of embryogenesis. At this time, hyp 6 is connected to hyp 5 and hyp 7 by adherens junctions (Sulston et al, 1983; Podbilewicz and White, 1994; Shemer and Podbilewicz, 2000).

250 min after cleavage There are a total of 78 cells; 23 are in the head .
55 cells arrange in 6 rows in the body of the spheroidal embryo.
250-390 min

Two inner rows interdigitate and make a single (dorsal) row.
Two outer rows of cells migrate towards each other and enclose the embryo on the ventral side.

340 min Cells #18 and 19 fuse to initiate hpy 7 syncytium formation, cells #1 and 2 follow. Elongation of the embryo starts to create a cylindrical embryo.
1.5-fold elongated embryo Cells #3-17 of the dorsal epithelium fuse. Ventral cell #20 fuses with 21 and #22 with 23. Later, dorsal syncytium of #1-2 fuses with that of #3-17.
1.5-fold to 3-fold elongated embryo Dorsal cell 1-17 fuses with ventral cells 18+19, 20+21 and 22+23. Fusion occurs under the seam cells without involvement of adherens junctions.
Two cells that make up hyp 10 fuse (I-IV and V-VI).
3-fold elongated embryo and L1 Hyp 7 syncytium has 23 nuclei (cells # 1-23) and covers the posterior of anus and excretory canal opening.
Post-hatch 116 more cells fuse to hyp7 from divisions of the seam cell progenitors (98 additional nuclei) and P1/2-P11/12 divisions (12 additional nuclei including hyp12).
At mid L3, hyp 6 (6 additional nuclei) fuses to hyp 7.

At hatching, the 12 unfused ventral epidermal cells (P1/2 through P11/12) are positioned as two parallel rows with each cell confronting its bilateral homolog along the ventral midline (See HypFIG5). In L1 these cells interdigitate to form a single row of cells on the ventral side (Sulston and Horvitz 1977). P1-P12 ventral cells divide soon after this and the anterior daughters detach from the epithelium and become neuroblasts (Sulston and Horvitz, 1977;Hedgecock et al, 1987).The posterior daughters of P1, P2 and P9-12 fuse with hyp7 at the end of the L1 stage. The posterior daughters of P3-8 divide at L3 stage to make 12 cells. Of these, the daughters of P3.p, P4.p, and P8.p fuse with hyp 7; the remaining become vulva precursor cells.

Below are a set of fluorescent micrographs of hypodermis from various sections of the body in adult hermaphrodite (HypFIG6-9). Note that, in areas where hypodermis becomes a thin layer around the circumference of the body due to impingement of muscle or pharyngeal tissue (refer to SW315lbop image seen below) , hypodermal GFP concentration becomes significantly lower giving the hypodermis a (false) striped appearance. As described above, hypodermal tissue (and hence hypodermally expressed GFP in these images) is actually only excluded from the regions where seam cells are located.

Tail Hypodermis

Tail hypodermal cells of an adult hermaphrodite is shown in HypFIG10. To see how tail hypodermis forms in hermaphrodite and male click here.


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