Histology of Human Tissues: Ureter, Bladder, Urethra, Spleen, Tonsils, and Skin

Posted on Jan 29, 2025 in Biology

Ureter

Mucosa: Transitional epithelium and fibroelastic lamina propria. Smooth muscle: Two or three layers (longitudinal and circular, internal and external), with an outer longitudinal layer in the lower third. Adventitia.

Bladder

Transitional epithelium: Laminated epithelium consisting of cubic basal cells, polygonal intermediate cells, and superficial umbrella cells. Fibroelastic lamina propria. Smooth muscle. Adventitia.

Urethra

Mucosa, muscle, adventitia.

  • Male (20-25 cm): Transitional epithelium (prostatic urethra), pseudostratified cylindrical epithelium, stratified squamous non-keratinized epithelium (mucous). Connective tissue sheath, spongy erectile body, and mucous glands of Littre.
  • Female (4-5 cm): Non-keratinized stratified squamous epithelium. Lamina propria (loose, highly vascularized connective tissue). Paraurethral glands. Smooth muscle (inner longitudinal and outer circular). Skeletal muscle outside the smooth muscle (external urethral sphincter).

Spleen

Solid organ.

  • Capsule: Dense irregular connective tissue, elastic fibers, and isolated myocytes. No fat and trabeculae.
  • Red pulp: Pulp cords (Billroth cords) consisting of stellate reticulum fibers, plasma cells, macrophages, and blood cells. Splenic sinuses lined with fusiform endothelial cells surrounded by a discontinuous basal membrane.
  • Marginal zone: Loose clusters of B and T lymphocytes, macrophages, plasma cells, and interdigitating dendritic cells between the red and white pulp.
  • White pulp: Lymphoid nodules (B lymphocytes) and periarterial lymphoid sheaths (T lymphocytes) of a central follicle of Malpighi.
  • Vascularization: Splenic artery, trabecular arteries, central arteries, penicillar arteries, pulp arterioles, sheathed arterioles, and terminal arterial capillaries.

Tonsils

  • Palatine (2): Non-keratinized stratified squamous epithelium.
  • Pharyngeal (adenoids) (1): Respiratory epithelium and non-keratinized stratified squamous epithelium.
  • Lingual (multiple): All non-keratinized stratified squamous epithelium. High endothelial venules.

Skin

  • Epidermis (ectoderm): Keratinized stratified squamous epithelium.
  • Dermis (mesoderm): Irregular connective tissue.
  • Subcutis: Subcutaneous tissue or adipose tissue consisting of loose connective tissue rich in adipocytes.

Epidermal Layers

  • Basal or germinative layer: A layer of cubic or prismatic cells with round nuclei and basophilic cytoplasm. Joined to the basal membrane by hemidesmosomes.
  • Stratum spinosum: Layers of prismatic cells that flatten progressively, with round nuclei and eosinophilic cytoplasm (more eosinophilic and flatter towards the surface). Occasional mitosis. Cells have spines on their sides.
  • Stratum granulosum: Several layers (3-5) of flattened keratinocytes with elongated nuclei and abundant cytoplasm granules of keratohyalin. No mitosis.
  • Stratum lucidum: Only in thick skin (palms and soles). Anucleate keratinized cells with filaments and eleidin inside. Involucrin is present in the membrane (without keratin protein, function unknown).
  • Stratum corneum: Anucleate cells or eosinophilic scales. The innermost cells have desmosomes. Superficial cells lose their junctions and shed.

Epidermal Cells

  • Keratinocyte: Most numerous cell population of the epidermis.
  • Langerhans cell: Originates from the mononuclear-phagocytic system. Located in the stratum spinosum. Constitutes 2-4% of epidermal cells. Also found in mucosal epithelia. Presents irregular, dense nuclei and pale cytoplasm. Lacks junctions to keratinocytes. Contains Birbeck granules or vermiform granules.
  • Merkel cell: Most numerous in thick skin, oral mucosa, and hair follicle bases. Cytoplasmic extensions are introduced into keratinocytes by desmosomal junctions. Contains electron-dense granules.
  • Melanocyte: Located between keratinocytes (1/4-1/10). Dense chromatin, clear cytoplasm with extensions to keratinocytes, and isolated melanin pigment.

Thin Skin vs. Thick Skin

  • Thin skin: Contains piloerector muscles, sebaceous glands, hair follicles, and sweat glands.
  • Thick skin: Lacks piloerector muscles, sebaceous glands, and hair follicles. Contains a prominent stratum lucidum and more sweat glands.