Tissue and stem cell 1

Human Anatomy Overview

  • Cell Count and Types:

    • Approximately 30 trillion cells.

    • Around 250 differentiated cell types identified through histology.

  • Basic Tissue Types:

    • Four main types: Connective tissue, Epithelial tissue, Muscle tissue, Nervous tissue.

    • Definition of tissues = a group of cells that work together to carry out a specific function, typically organized from a specific stem cell type.

    • Many tissues types working together to carry out specific functions = organ. There are around 80 organs in 9 organ systems, each with specialized functions.

Diversity of Cell Types

  • Example Cell Types Identified:

    • Goblet cells, fibroblasts, enterocytes, pancreatic cells (exocrine & ductal), ß-cells, keratinocytes, neurons, and muscle cells (both cardiac and skeletal).

  • Advanced techniques like Single-Cell RNA-Seq are ongoing to identify additional cell types based on molecular signatures.

Major Tissue types

Types of Connective Tissue

  1. Loose Connective Tissue: Fibroblasts in loose fibers (collagen, elastin, reticulin). It connects organs and tissues and supports epithelia.

  2. Dense Connective Tissue: Rope-like structures like tendons and ligaments formed by fibroblast embedded in dense collagen fibers.

  3. Cartilage: Composed of chondrocytes embedded in matrix (collagen, elastin, proteoglycans); provides structural support and friction protection.

  4. Bone: Bone cells in dense, mineralized matrix (collagen). Initially formed as cartilage and replaced by bone via ossification.

  5. Adipose Tissue: Contains adipocytes specialized for fat storage.

  6. Blood: Considered a separate tissue; it includes various cell types (RBCs, granulocytes, monocytes, platelets and lymphocytes) suspended in plasma.

Epithelial Tissue

  • Characteristics

    • Sheets of cells covering surfaces or lining tubes with clear polarity (apical vs. basal surfaces).

    • Contain tight junctions for selective permeability, adherens junctions (composed of cadherins and linked to actin filament via catenins) for support, desmosomes (strong cell-cell adhesion junctions linked to intermediate filaments) and hemidesmosomes (which anchor the epithelial cells to the basement membrane).

  • Functions: Protection, absorption (e.g., in the gut and often apical surface have specialized structure).

  • Types of epithelial cells classified based on shape (simple = single layer of cells):

    1. Simple Squamous: Thin and flat cells.

    2. Simple Cuboidal: Cube-shaped cells.

    3. Simple Columnar: Tall, column-like cells.

    4. Stratified: Multiple layers of cells (squamous, cuboidal, columnar).

    5. Pseudostratified: Appears layered but is a single layer with cells of varying heights.

Types of Muscle Tissues

  1. Skeletal Muscle: Multinucleate myofibres, appear striated because sarcromere pattern; responsible for voluntary movement.

  2. Cardiac Muscle: Striated like skeletal; involuntary; mono or binucleate; individual cells joined with intercalated disc; contraction of the heart through gap junctions created by intercalated discs which allows fast electrical signal transmission.

  3. Smooth Muscle: Non-striated; composed of spindle-shaped cells; mononuclear; found in hollow organs.

Neural Tissues

  • Composed of neurons and glia (supporting cells).

  • Neurons: Consist of cell body, axons, and dendrites.

  • Nerve = bundles of axons

  • Glia: Various types, in CNS has 4 types: astrocytes (support and nutrients), oligodendrocytes (myelination), and microglia (immune responses) and ependymal cells (maintain homeostasis). In PNS, there are 2 types: Schwann cells and satellite cells.

  • Neuronal aggregations form nerves, and neuronal cell bodies aggregate in ganglia.

Cell Turnover and Lifespan

  • Intestinal cells have a rapid turnover cycle: They die and replace every 6-7 days from the proliferating stem cell population in intestinal crypts.

Stem Cells

  • Are undifferentiated cells that retain the capacity for continuous division and plasticity.

  • The function of stem cells is tissue maintenance through proliferation.

  • Stem cell characteristics:

    1. Undifferentiated

    2. Unlimited proliferation

    3. Self-renewal mechanism to produce more stem cells

    4. Differentiation to transit amplifying cells (progenitor cells)

  • Self-Renewal Mechanisms:

    • Asymmetric Division: Produces one stem cell and one transit amplifying cell.

    • Symmetric Division: May produce two identical daughter cells.

  • Niche Dependency: Stem cells typically reside in specific niches that maintain their stem cell behavior through signaling from surrounding cells.

    • For example, in Drosophila ovary, stem cells are exposed to Dpp, a BMP-like protein that keeps cells in mitosis. As Dpp is secreted by somatic cap cells, stem cells need to be surrounded by these cells, otherwise they begin differentiating.