Interdigitating Cell Processes Research Articles

Meningioma with rhabdoid features: Pathologic findings in dogs.

Rhabdoid meningioma is a rare type of meningeal neoplasm in humans. This study reports the clinical, pathological, and ultrastructural features of 4 cases of canine meningioma with rhabdoid features. The cases were female and 8 to 12 years of age. Biopsies from complete surgical resections were examined for all cases. The whole brain with tumor recurrence was collected at necropsy in 2 dogs. Histologically, the tumors consisted of discohesive sheets of oval-polygonal cells with abundant eosinophilic cytoplasm and occasional paranuclear hyaline-like inclusions. Cells were intensely immunopositive for vimentin, negative for melan A and S100 protein in all cases, and showed variable immunolabeling for cytokeratin in 2 cases. Focal glial fibrillary acidic protein (GFAP)-immunopositive cells were present in 1 case. Ultrastructurally, the rhabdoid cells in case 1 contained prominent cytoplasmic whorls of intermediate filaments, recapitulating the ultrastructural features of rhabdoid meningioma in humans. In cases 2 and 3, the meningioma cells contained interdigitating cell processes folded in a maze-like fashion resembling rhabdoid-like meningioma in humans. In case 4, the voluminous cytoplasm contained many round-to-flattened mitochondria admixed with rough endoplasmic reticulum, indicating a predominant oncocytic differentiation and not the rhabdoid differentiation suggested by light microscopy. Thus, rhabdoid morphology occurs in different types of meningiomas, and ultrastructural findings are essential for a correct diagnosis.

Dynamic changes of podocytes caused by fibroblast growth factor 2 in culture

Fibroblast growth factor 2 (FGF2) augments podocyte injury, which induces glomerulosclerosis, although the mechanisms remain obscure. In this study, we investigated the effects of FGF2 on cultured podocytes with interdigitating cell processes in rats. After 48 h incubation with FGF2 dynamic changes in the shape of primary processes and cell bodies of podocytes resulted in the loss of interdigitation, which was clearly shown by time-lapse photography. FGF2 reduced the gene expressions of constituents of the slit diaphragm, inflections of intercellular junctions positive for nephrin, and the width of the intercellular space. Immunostaining for the proliferation marker Ki-67 was rarely seen and weakly stained in the control without FGF2, whereas intensely stained cells were frequently found in the presence of FGF2. Binucleation and cell division were also observed, although no significant increase in cell number was shown. An in vitro scratch assay revealed that FGF2 enhanced migration of podocytes. These findings show that FGF2 makes podocytes to transition from the quiescent state into the cell cycle and change their morphology due to enhanced motility, and that the culture system in this study is useful for analyzing the pathological changes of podocytes in vivo.

Induction of interdigitating cell processes in podocyte culture

Highly organized cell processes characterize glomerular podocytes invivo. However, podocytes in culture have a simple morphology lacking cell processes, especially upon reaching confluence. Here, we aimed to establish culture conditions under which cultured podocytes extend cell processes at confluence. Among various culture conditions that could possibly cause phenotypic changes in podocytes, we examined the effects of heparin, all-trans retinoic acid, fetal bovine serum, and extracellular matrices on the morphology of podocytes in rat primary culture. Consequently, long arborized cell processes were observed to radiate extensively from the cell body only when cells were cultured in the presence of heparin and all-trans retinoic acid on laminin-coated dishes with decreasing concentrations of fetal bovine serum. Primary processes branching repeatedly into terminal processes and cell process insertion under adjacent cell bodies were evident by electron microscopy-based analysis. Immunostaining for podocin showed conspicuous elongations of intercellular junctions. Under these conditions, the expression levels of podocyte-specific proteins and genes were markedly upregulated. Thus, we succeeded in establishing culture conditions in which the cultured podocytes exhibit phenotypes similar to those under invivo conditions.

Rhabdoid-like meningioma with inclusions consisting of accumulations of complex interdigitating cell processes rather than intermediate filaments

Rhabdoid-like meningioma with inclusions consisting of accumulations of complex interdigitating cell processes rather than intermediate filaments

Malignant Epithelioid Schwannoma of the Oral Cavity in a Cat

ABSTRACTA malignant epithelioid schwannoma of the oral cavity was diagnosed in an 8-year-old domestic short-hair cat. The mass was located in the gingiva of the upper left premolar to molar region and showed multinodular growth patterns. The mass comprised epithelioid cells arranged in densely packed sheets. Tumor cells had large, round to oval nuclei with prominent nucleoli and an abundant eosinophilic cytoplasm. Immunohistochemically, most of the tumor cells were positive for S-100 protein, glial fibrillary acidic protein and vimentin, but all lacked melanoma-associated antigen and muscle and neuroendocrine markers. Stains for type IV collagen showed linear immunoreactivity around single cells and groups of cells. Ultrastructurally, tumor cells were separated by a well-defined basement membrane, and interdigitating cell processes were observed. To our knowledge, this is the first report of feline malignant epithelioid schwannoma.

Necrotic rhabdoid meningiomas with aggressive clinical behavior

Rhabdoid meningioma (RM) is a rare, aggressive variant of meningioma classified as a WHO Grade III malignancy. RM exhibits a striking histological resemblance to other rhabdoid tumors and strong tendency towards local recurrences, CSF dissemination, and/or remote metastasis. The majority of reported cases are of secondary rhabdoid transformation in recurrent meningiomas. We present two unusual cases of rhabdoid meningiomas diagnosed as a primary intracranial lesion in adults that were associated with extensive necrosis and an aggressive clinical course. On histological examination, the majority of the tumor mass was composed of necrotic tissue with focal clusters of neoplastic cells, often localized around blood vessels. Most tumor cells exhibited typical rhabdoid morphology with large, vesicular, often eccentrically located nuclei with distinct nucleoli and abundant cytoplasm containing eosinophilic hyaline inclusions. Classical meningothelial features with focal whorl formation were scarce and seen only in one case; in the second case the tumor was entirely rhabdoid. The differential diagnosis with atypical teratoid/rhabdoid tumors (AT/RTs) and other neoplasms, particularly metastatic carcinoma, was considered. Immunohistochemical and electron microscopic study were critical for the accurate diagnosis of the rhabdoid subtype of meningiomas. Rhabdoid cells stained diffusely positive for vimentin and S-100 protein and showed focal but strong expression of epithelial membrane antigen and cytokeratins. The rhabdoid areas of the tumors exhibited high mitotic activity with a MIB-1 labeling index of 80 - 90%. The diagnosis of rhabdoid meningioma was supported by evidence of SNF5 (INI1) protein expression. Ultrastructural examination demonstrated the presence of interdigitating cell processes joined by numerous desmosomes and paranuclear whorls of intermediate filaments typical of the rhabdoid phenotype. Our two cases of rhabdoid meningiomas were associated with lethal outcome within a few months of initial diagnosis. Extensive necrosis in rhabdoid meningioma might be considered an additional predictor of aggressive clinical behavior.

Ultrastructural and immunohistochemical study of an adult case of chordoid meningioma

A 64-year-old male patient presented with generalized convulsions. Magnetic resonance imaging revealed a large meningeal tumor with some cysts in the right frontal region. Surgical resections were performed three times, and local radiation therapy was administered twice over a period of 8 years for the treatment of tumor recurrences. The tumor tended to recur in spite of the surgical and radiation therapies. The tumor was diagnosed as a chordoid meningioma, and the second surgical specimen showed increasing nuclear atypia and mitoses in tumor cells. An immunohistochemical study revealed the tumor cells were positive for vimentin, S-100 protein, and cytokeratin AE1/AE3. An electron microscopic study revealed intracytoplasmic vacuolar spaces, loosely connected interdigitating cell processes with intermediate junctions, and extracellular spaces which contained fluffy granular intercellular substances. The tumor cell surfaces displayed pseudopodia which extended into the intercellular spaces and the tumor cells had moderate quantities of cytoplasm containing abundant mitochondria and glycogen granules. According to the ultrastructural features in the past reports of chordoid meningiomas, these meningiomas are suspected to have a mixture of the characteristic ultrastructural features of meningothelial meningiomas and chordoid sarcomas.

Erythroblastic synartesis in a patient with small lymphocytic lymphoma

A 70-year-old patient was admitted with severe hypochromic microcytic anaemia with reticulocytopenia. The blood count showed a haemoglobin level of 49 g/l (MCV 73AE8 fl, MCH 22AE8 pg), white blood cell count of 6 · 10/l (normal differential) and platelet count of 440 · 10/l. The absolute reticulocyte count was 1AE5 · 10/l. The serum ferritin, vitamin B12 and folate levels were normal. A bone marrow aspiration and trephine biopsy were performed, including cytogenetic and flow cytometry analysis. The aspirate revealed striking erythroblastic hyperplasia and dyserythropoiesis with clusters of closely linked erythroid cells. At the intercellular junction of the erythroblasts, a clear non-basophilic zone was evident (Figs). The findings were diagnostic of erythroblastic synartesis. Furthermore, there were an increased number of lymphocytes in the marrow comprising 23% of nucleated cells. The trephine biopsy and flow cytometry confirmed the presence of a CD5+, CD19+, CD23+ low grade lymphoma (small lymphocytic lymphoma) infiltrating the marrow. Cytogenetic analysis was normal. Whole body computed tomography scans demonstrated intraabdominal lymphadenopathy and serum protein electrophoresis identified a monoclonal IgGj band (1AE5 g/l). Erythroblastic synartesis was first described in 1973 by Breton-Gorius et al in a patient with an acquired dyserythropoietic anaemia who died of haemochromatosis because of long-term transfusions. In that case, electron microscopy revealed junctions between erythroblasts formed by interdigitating cell processes. The new phenomenon was named ‘erythroblastic synartesis’, from the Greek words ‘rtm’ (with) and ‘ aqsgriV’ (link). This condition is a rare form of acquired autoimmune dyserythropoiesis that leads to ineffective erythropoiesis and accompanies lymphoproliferative and autoimmune disorders. Studies indicate that a monoclonal serum immunoglobulin (IgGj in our case) directed against an adhesive receptor of the erythroblast membrane is responsible for the dyserythropoiesis. Treatment of the underlying autoimmune or lymphoproliferative disease, as experienced with our patient, leads to correction of dyserythropoiesis and restoration of a normal haemoglobin level.

Papillary tumor of the spinal cord. Report of two cases

Tumors perhaps related to ependymoma include astroblastoma and papillary tumor of the pineal region (PTPR). We report two cases of spinal tumors with similar distinctive features. A 14‐year‐old female had a thoracic tumor in 2001, a lumbar recurrence in 2004, 2 cerebellar lesions in 2006, and in the temporal lobe in 2007, all with identical histology. A 22‐year‐old male presented at age 17 in 1999 with a cervical spine intramedullary tumor that recurred in 2002, 2005, and 2007. A 4th ventricular mass was identified in 2003. The 2003, 2005, and 2007 specimens showed similar histologic features. The tumors showed variable gadolinium enhancement by MRI. Microscopy showed papillary and solid cytoarchitecture with monomorphous epithelioid cells arranged around vascular papillae. IHC revealed: focal GFAP positivity in the 1st case and absence in the 2nd; strong diffuse cytoplasmic EMA positivity; and diffuse cytokeratin AE1/CAM5.2 expression. EM of the 2nd case demonstrated cytoplasmic lumina lined by microvilli, cilia at extracellular surfaces, tight junctions, and absence of interdigitating cell processes. These tumors may be most closely related to ependymomas, but share similarities with the PTPR, and may therefore represent a comparable entity of the spinal cord with a predilection for local recurrence or remote spread within the CNS.

Signet ring cell gastric schwannoma: report of a new distinctive morphological variant

An 89-year-old woman was seen for indigestion, light chest pain, and melanotic stools. Endoscopic examination revealed 2 submucosal gastric masses. A subtotal gastrectomy showed 2 submucosal masses in the stomach: one infiltrating through the muscularis propria into the serosa, the second one, a well-circumscribed submucosal nodule. Histologic examination showed large tumor cells infiltrating diffusely through the muscularis propria into the subserosa. On higher magnification, numerous signet ring cells were present against a myxoid stroma, in addition to large vacuolated epithelioid cells. There was no evidence of invasion, necrosis, nuclear pleomorphism, or mitotic activity. Initial diagnostic considerations based on the histology included signet ring cell carcinoma, malignant melanoma, and a myxoid mesenchymal tumor, including gastrointestinal stromal tumor. A panel of immunohistochemical stains showed diffuse strong positivity for S-100 protein and negative reaction for CD117, bcl-2, cytokeratin AE1/AE3, Melan-A, HMB45, smooth muscle antigen, and other differentiation markers. Electron microscopic examination revealed elongated, complex, and interdigitating cell processes covered by a thin layer of continuous basement membrane material characteristic of peripheral nerve sheath differentiation. The presentation of this tumor was significant in that it was multifocal and infiltrative, mimicking a malignant neoplasm. The extensive myxoid/signet ring cell change represents a heretofore-unreported histologic variant of gastric schwannoma.

Establishment of a benign meningioma cell line by hTERT-mediated immortalization

Meningioma represents the most common intracranial tumor, but well-characterized cell lines derived from benign meningiomas are not available. A major reason for the lack of benign tumor cell lines is senescence of nonmalignant cells in vitro, while malignant cells are often immortal. We have developed a meningioma cell line by retrovirally transducing primary cells derived from a human WHO grade I meningothelial meningioma with the human telomerase reverse transcriptase (hTERT) gene, which enables bypassing cellular senescence. Five clones have been cultured for more than 21 months so far, while corresponding nontransfected cells ceased proliferation within 3 months. Quantitative RT-PCR and a telomeric repeat amplification protocol (TRAP) assay revealed high hTERT mRNA levels and high telomerase activity in all transduced populations, while nontransduced cells were negative. The average telomere size of transduced cells was considerably longer than that of parental cells and the biopsy specimen. One clone, designated Ben-Men-1, was characterized in more detail, and exhibited typical cytological, immunocytochemical, ultrastructural and genetical features of meningioma, including whorl formation, expression of epithelial membrane antigen, desmosomes and interdigitating cell processes, as well as −22q. Following subdural transplantation into nude mice, tumor tissue with typical histological features of meningothelial meningioma was found. We conclude that Ben-Men-1 represents an immortalized yet differentiated cell line useful for biological and therapeutical studies on meningioma.

Primary histiocytic lymphoma of the central nervous system: a neoplasm frequently overshadowed by a prominent inflammatory component.

True histiocytic lymphoma, as defined by strict criteria, is a very rare neoplasm. We describe three cases occurring as primary tumors in the central nervous system. The patients, two females and one male, ranged in age from 11 to 69 years. The tumors involved the brain in two cases and spinal cord in one, with a size ranging from 7 to 17 mm. Two patients died at 4 months and 8 months, respectively, and one was alive with disease at 5 months. Pathologically, the tumors comprised groups and sheets of noncohesive large cells with pleomorphic vesicular nuclei, distinct nucleoli, and abundant eosinophilic cytoplasm. A dense inflammatory infiltrate consisting of neutrophils, lymphocytes, plasma cells, and histiocytes was present, with multiple foci of necrosis and abscess formation. All three cases demonstrated an identical immunophenotype: positive for CD68 and lysozyme; focally positive for S-100 protein, CD45RB, and CD4; and negative for CD3, CD20, CD21/CD35, CD1a, CD30, ALK1, myeloperoxidase, glial fibrillary acidic protein, and cytokeratin. The proliferative index ranged from 20% to 35%. Ultrastructural examination further confirmed the histiocytic nature of the tumor cells, characterized by irregularly folded or multisegmented nuclei and abundant cytoplasm containing lysosomes; Birbeck granules, interdigitating cell processes, and cell junctions were not found. Although the presence of abundant inflammatory cells could obscure the neoplastic histiocytes, making the distinction from inflammatory conditions difficult, awareness of this unusual histologic feature and the invariable finding of pleomorphic cells in some areas of the lesion permit the correct diagnosis to be made.

Reticulum Cell Sarcoma of Lymph Node with Mixed Dendritic and Fibroblastic Features

We report a case of clinically aggressive reticulum cell sarcoma with mixed follicular dendritic cell (FDC) and fibroblastic reticular cell (FRC) features. Histologically, the tumor was confined to lymph nodes occurring as a multifocal epithelioid and spindle cell proliferation with appreciable mitotic rate and numerous admixed non-neoplastic B-cells. Ultrastructural examination revealed elongated cells with prominent nucleoli, interdigitating cell processes and frequent desmosomes. These features are typical of FDC sarcoma. However, immunohistochemical stains showed no expression of antigens characteristic of FDCs, including CD21, CD23 and CD35. Cytogenetic characterization of this tumor, by conventional G-banding and multicolor spectral karyotyping, revealed multiple clonal chromosomal aberrations, including del(X)(p11.4) and add (21)(p11.2). Gene expression analysis by cDNA microarray of RNA obtained from short-term tumor cultures revealed high-level expression of a set of genes (including PDGF receptor-α and -β, certain metalloproteinases, and CD105) that were also highly expressed in cultures of nodal FRC cultured from non-neoplastic lymph nodes. We propose that this tumor represents a nodal sarcoma with intermediate differentiation between FDCs and FRCs. This case adds to the diversity of tumors that may arise from lymph node stroma and supports a possible relationship between the FDC and FRC lineages.

Clear Cell Meningioma of the Lumbo-sacral Spine With Chordoid Features

Clear cell meningioma (CCM) is a peculiar variant that differs from conventional meningioma in affecting younger patients, arising more often in spinal or cerebellopontine locations, and showing a higher recurrence rate. Classical meningothelial areas are scarce in these tumors and the differential diagnosis with other neoplasms, particularly metastatic carcinoma, is often difficult. We report a case of clear cell meningioma from the lumbosacral spine in which location, radiologic presentation, light microscopic appearance in initial sampling, and some of the ultrastructural findings were reminiscent of chordoma. The tumor cells were diffusely positive for vimentin and very focally positive for epithelial membrane antigen. Ultrastructural demonstration of interdigitating cell processes joined by numerous desmosomes confirmed the diagnosis of CCM.

Ultrastructural and Cytochemical Characteristics of Phagocytes in Kuruma Prawn.

Five types of phagocytes were observed in kuruma prawn Penaeus japonicus. Two of them were fixed phagocytes and three were hemocytes. Fixed phagocytes were detected in the heart and lymphoid organ. Phagocytes in the heart were observed on basal lamina covering sarcolemma of heart muscles and the cells had many lysosomal granules (0.1 μm in diameter). Phagocytes in the lymphoid organ was characterizedby numerous interdigitating cell processes and few cytoplasmic granules. The hemocytes were classified into three types, hyaline cell (HC), small granular cell (SGC) and large granular cell (LGC), according to the presence of cytoplasmic deposit and size of granules. The HC had cytoplasmic deposits and a few small granules (0.2 μm in diameter). The SGC had small granules (0.2 μm in diameter) but the LGC contained large granules (1 μm in diameter). Lysosomal enzymes, such as acid phosphatase, β-glucuronidase and α-naphthyl butylate esterase, were detected in all five phagocytic cell types. Esterase activity was higher in the LGC than in any other phagocytes. Peroxidase activity was also detected in all phagocytes, although it was weak. Prophenoloxidase activity was found only in the SGC and LGC. These results indicate that the fixed phagocytes are different from the hemocytes both in their ultrastructural and cytochemical characteristics.

Morphological and ultrastructural study of the histogenesis of meningeal granular cell tumors in rats.

To clarify the cell of origin of granular cell tumors in the rat brain, light and/or electron microscopic examinations were performed on 40 cases of spontaneous meningeal tumors in Wistar (Jcl: Wistar) rats. The meningeal tumors were histologically subclassified into 3 types: meningothelial meningiomas (MMs), 3 cases; granular cell tumors (GCTs), 28 cases; and mixed forms (MIXs) of GCTs and MMs, 9 cases. Of these tumors, 2 MMs, 2 GCTs, and 3 MIXs were examined by transmission electron microscopy. Tumor cells of MMs were characterized by cytoplasmic intermediate filaments and prominent interdigitating cell processes often connected with cellular junctions without surrounding basal laminae. GCTs were composed of 2 cell types: granular cells with many dense bodies and filamentous cells with fine intermediate filaments. MIXs consisted of granular cells, filamentous cells, and intermediate cells. The tumor cells in GCTs and MIXs were apposed to each other and connected with cellular junctions. MIXs displayed a spectrum of cellular differentiation in that 2 MIXs had a close morphological resemblance to GCTs and the other one to MMs. These findings indicate there might be a histogenetic sequence among MMs, GCTs, and MIXs. The present study supports the suggestion that GCTs and MIXs may be variants of MMs from the meningeal arachnoid cell.

Follicular Dendritic Cell Tumor of the Mesentery

Follicular dendritic cell (FDC) tumor is an exceedingly rare malignant neoplasm and occurs mainly in the cervical lymph nodes. We report a mesenteric FDC tumor occurring in a 66-year-old female, that manifested with intraabdominal multifocal recurrence 7 years after resection of the primary tumor. Histologically, both primary and recurrent tumors were composed of oval to spindle cells with paley eosinophilic cytoplasms, indistinct cell borders, round to elongated nuclei with clear or finely dispersed chromatin, and medium to large nucleoli. Characteristically, the tumor cells were growing in sheets, fascicles, and sometimes in whorls and a storiform pattern. In addition, focal necrosis, nuclear pleomorphism and abnormal mitoses were also observed. The neoplastic cells were intimately admixed with small lymphocytes. The diagnosis was confirmed by positive immunoreactivity with CD21 and CD35 antibodies and by ultrastructural demonstration of convoluted interdigitating cell processes connected by scattered desmosome-like junctions. Although our case showed a low proliferative activity evaluated by MIB-1, multifocal recurrence has occurred. The clinicopathologic features and differential diagnosis of FDC tumors are discussed with the review of the literature.

Morphological and immunohistochemical characteristics of axial structures in the transitory human tail

Ultrastructural relationships between the notochord and neighboring spinal cord were examined during the regression of the human tail. Also, the presence of certain extracellular matrix components in the notochord was immuno-histochemically analysed in the 4th to 12th week old embryos. At the early stages, a close apposition of the notochord to the spinal cord exists in the entire tail region. The external surface of both structures is covered with a continuous basal lamina. The narrow tissue interspace contains interdigitating cell processes and both amorphous and fibrillar extracellular matrix material. With advancing embryonic age, separation of the two structures occurs in craniocaudal direction and the widening interspace becomes occupied by mesenchymal cells. During tail regression and spinal cord retraction, the appearance of large intercellular spaces and cell degeneration takes place in both tissues. With age, the extracellular matrix of the notochord, predominantly the perinotochordal sheath, increases in amount and antigenic complexity. While the intensity of laminin, collagen type IV and type III expression rises continuously during the period examined, the expression of fibronectin begins first at later stages, after the separation of the notochord from the spinal cord. The possible developmental significance of the described phenomena in the regression of the posterior end of the human tail remains to be elucidated.

Follicular Dendritic Cell Tumors of the Oral Cavity

Follicular dendritic cell tumors are uncommon, and all the reported cases have occurred as primary lymph node tumors. We report two cases in the oral cavity, one in the soft palate and one in the tonsil. The tumors were characterized by sheets, whorls, and storiform arrays of spindly and syncytial-appearing cells with oval nuclei, fine chromatin, distinct nucleoli, and occasional nuclear pseudoinclusions. Multinucleated forms were present and were prominent in one case. An unusual feature was the presence of irregular pseudovascular spaces, which could raise a concern for vascular neoplasm. Because the tumors showed cohesive growth and a sharp interface with the fibrous stroma, they could also be mistaken for carcinoma, sarcoma, or melanoma. After radiation therapy, the palatal tumor showed a greater degree of nuclear pleomorphism, numerous nuclear pseudoinclusions, and striking nuclear grooving and foldings, mimicking interdigitating reticulum cell tumors. The diagnosis in both cases was confirmed by immunoreactivity with CD21 and CD35 and by ultrastructural demonstration of interdigitating cell processes with desmosomes. Both tumors also showed unexpected immunoreactivity with muscle-specific actin. Follicular dendritic cell tumor merits wider recognition of its possible extranodal occurrence as well as its full morphological spectrum in order to better define its behavior.

Ultrastructural Aspects of Spicule Formation in the Solitary Ascidian Herdmania momus (Urochordata, Ascidiacea)

AbstractThe solitary stolidobranch ascidian Herdmania momus contains numerous calcium carbonate spicules in its tunic and body tissues. The slender body spicules form inside complex sheaths in the body wall and branchial basket, where they remain for the life of the animal. The much smaller tunic spicules form inside the tunic blood vessels and then migrate to the tunic surface, where they become anchored by their spiny base. This paper is an ultrastructural investigation of the formation of the body spicules; the tunic spicules, which apparently form quite differently, will be the focus of a future study. The body spicules are composed of rows of closely packed acicular spines which form completely extracellularly. The spine tips are covered by flattened, highly pseudopodial sclerocytes bound together by tightly interdigitating cell processes. The basal regions of contiguous spines are covered by very thin sclerocyte cell processes. An organic matrix is present within the spines; its exact nature is not clear. A very dense extracellular inter‐spine matrix is located between the spine tips and the contiguous basal regions. Presclerocytes within the sheaths between the spicules are probably responsible for formation of the extracellular structures of the sheaths. The presclerocytes appear to aggregate and transform into sclerocytes at the apical end of the spicule. New spines are added at the apical end of the spicule as well as between larger spines. Comparisons are made between body spicule formation in H. momus and skeletogenesis in echinoids.