TISSUES

By Oba Mike

NOTE: This note goes with the slides. There are clearer pictures in the slide for you to view and draw.

Introduction

A tissue is a group of cells that function together to carry out specialized activities. Cells of a particular tissue usually have a common embryonic origin.

The structure and functioning of a tissue are influenced by factors such as:

i.  The nature of the extracellular material that surrounds the tissue cells.

ii. The connections between the cells that compose the tissue.

Tissues exist in different forms, hard e.g bone, semisolid e.g fat, liquid e.g blood.

The branch of anatomy that deals exclusively with the study of tissues is termed histology. Pathology is the branch of medicine concerned with diseases of cells and tissues.

Types of tissues

Body tissues can be classified into four types according to their structure and function:

1. Epithelial tissues

2. Connective tissues

3. Muscular tissues

4. Nervous tissues

For now, we will focus only on epithelial and connective tissues. Muscular and nervous tissues will be discussed under muscular and nervous systems respectively.

Epithelial tissues

An epithelial tissue or epithelium consists of continuous sheet of cells arranged in either single or multiple layers. They cover body surfaces and line hollow organs, body cavities and ducts. Epithelial tissues also form glands. These tissues allow the body to interact with both internal and external environments. The cells that form epithelial tissues are closely packed and held tightly together by many cell junctions, there is little intercellular space between adjacent plasma membranes.

Functions of Epithelial Tissues

Epithelial tissues are found either at the outermost or innermost parts of the body and have five major functions:

a. Protection of underlying structures

b. Secretion

c. Absorption

d. Filtration

e. Excretion

Parts of an Epithelium

A typical epithelial tissue has three parts:

i. An Apical surface which is the outermost part and free.

ii. The lateral surfaces which connects with adjacent cells on either sides through cell junctions.

iii. The basal surface which adheres to the basement membrane.

Note: The basement membrane is a supporting structure to the epithelium.

Features of Epithelial Tissues

i. Epithelial tissues are avascular (lack blood vessels) and rely on neighbouring connective tissues for nutrients and waste removal through diffusion.

ii. Epithelial tissues are subjected to high stress and injury. They constantly renew through cell division.

Classification of Epithelial Tissues

Epithelial tissues are generally classified into two types:

1. Covering and Lining epithelium

2. Glandular epithelium.

Covering and Lining epithelium

This epithelium forms the outer covering of the skin and some internal organs. It also forms the inner lining of blood vessels, ducts and cavities as well as lining of the respiratory, digestive, urinary and reproductive systems.

Covering epithelium can be further classified based on two categories:

a. Number of cell layers

b. Cell shape

Number of Cell Layers

Based on this we have:

i. Simple epithelium: single layer of cells

ii. Pseudostratified epithelium: A simple epithelium that appears to have multiple layers because the cell nuclei lie at different levels and not all cells reach the apical surface.

iii. Stratified epithelium: Consists of two or more layers of cells.

Cell Shape

Based on this we have:

i. Squamous epithelium: Flat and thin cells

ii. Cuboidal epithelium: Consists of cube-shaped cells. These cells are as tall as they are wide.

iii. Columnar epithelium: Consist of cells that are taller than they are wide.

iv. Transitional epithelium: Consists of cells that change shape from squamous to cuboidal and back.

When we combine the two characteristics, we arrive at the following types of epithelial tissues:

Simple epithelium 

A.Simple squamous epithelium 

B.Simple cuboidal epithelium 

C.Simple columnar epithelium (ciliated and nonciliated)

 D.Pseudostratified columnar epithelium (ciliated and nonciliated.

Stratified epithelium

A. Stratified squamous epithelium (keratinized and nonkeratinized).

B.Stratified cuboidal epithelium

C.Stratified columnar epithelium

D.Transitional epithelium

The table in the PowerPoint shows details of these epithelia such as their location and functions. Copy it into your note.

Draw this diagram in your note.

Better picture in the PowerPoint (slides).

Glandular epithelium

The function of glandular epithelium is secretion.

A gland consists of a single cell or group of cells that secrete substances into ducts (tubes) or into the blood stream. Based on this, there are two types of glands:

Types of glands

1. Exocrine glands

2. Endocrine glands

Generally, exocrine glands secrete their products into ducts which convey them to their target sites. Endocrine glands secrete their products (called hormones) into the blood stream. Hence, exocrine glands have ducts while endocrine glands are ductless. Our focus now is on exocrine glands. Endocrine glands will be treated fully in endocrine system.

Exocrine glands

As already stated, exocrine glands have ducts that carry their secretions to target sites. Exocrine glands can be classified both structurally and functionally.

Structural Classification of Exocrine Glands

This is based on two criteria:

1. Shape of duct

If the duct is not branched, it is called simple, if it is branched, it is called compound .

2. Shape of secretory surface.

This can be tubular (straight), coiled or round (acinar).

Based on these two characteristics we have the following glands:

Simple glands

i. Simple tubular: Secretory surface is straight and attached to a single unbranched duct. E.g intestinal glands.

ii. Simple branched tubular: Secretory portion is branched and attaches to a single unbranched duct e.g gastric glands.

iii. Single coiled tubular: Secretory surface is coiled and attaches to a single unbranched duct e.g sweat glands.

iv. Simple acinar: Secretory portion is rounded and attached to a single unbranched duct. E.g penile urethral glands.

v. Simple branched acinar:

Secretory portion is branched, rounded and attached to a single unbranched duct. E.g sebaceous glands

Compound glands

i. Compound tubular: Secretory surface is straight (tubular) and attached to branched ducts.

ii. Compound acinar: Secretory surface is round and attached to branched ducts.

iii. Compound tubuloacinar: Secretory surface has both rounded and tubular portions attached to branched ducts.

Functional Classification of Exocrine Glands.

The functional of exocrine glands is based on how their secretions are released. Based on this we have:

1. Merocrine glands

2. Apocrine glands

3. Holocrine glands

Merocrine glands: These glands release their secretions without damage to any part of the cell. Majority of exocrine glands are merocrine. Examples are salivary glands and pancreas.

Apocrine glands: Apocrine glands release their secretions together with some parts of the secretory surface e.g mammary glands.

Holocrine glands: Holocrine glands release their secretions together with the entire secretory cell e.g sebaceous glands of the skin.