Learning Objectives

Identify the main tissue types and discuss their roles in the human body.

By the end of this section, you will be able to:

  • Identify the four primary tissue types and discuss the structure and function of each
  • Describe the embryonic origin of tissue
  • Identify the various types of tissue membranes and the unique qualities of each

The term tissue is used to describe a group of cells that are similar in structure and perform a specific function.   Histology is the the field of study that involves the microscopic examination of tissue appearance, organization, and function.

Tissues are organized into four broad categories based on structural and functional similarities.  These categories are  epithelial, connective, muscle, and nervous.   The primary tissue types work together to contribute to the overall health and maintenance of the human body.   Thus, any disruption in the structure of a tissue can lead to injury or disease.

The Four Primary Tissue Types

Epithelial tissue refers to groups of cells that cover the exterior surfaces of the body, line internal cavities and passageways, and form certain glands. Connective tissue, as its name implies, binds the cells and organs of the body together. Muscle tissue contracts forcefully when excited, providing movement.  Nervous tissue is also excitable, allowing for the generation and propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body (Figure 4.1.1).

An understanding of the various primary tissue types present in the human body is essential for understanding the structure and function of organs which are composed of two or more primary tissue types.  This chapter will focus on examining epithelial and connective tissues.  Muscle and nervous tissue will be discussed in detail in future chapters.

This diagram shows the silhouette of a female surrounded by four micrographs of tissue. Each micrograph has arrows pointing to the organs where that tissue is found. The upper left micrograph shows nervous tissue that is whitish with several large, purple, irregularly-shaped neurons embedded throughout. Nervous tissue is found in the brain, spinal cord and nerves. The upper right micrograph shows muscle tissue that is red with elongated cells and prominent, purple nuclei. Cardiac muscle is found in the heart. Smooth muscle is found in muscular internal organs, such as the stomach. Skeletal muscle is found in parts that are moved voluntarily, such as the arms. The lower left micrograph shows epithelial tissue. This tissue is purple with many round, purple cells with dark purple nuclei. Epithelial tissue is found in the lining of GI tract organs and other hollow organs such as the small intestine. Epithelial tissue also composes the outer layer of the skin, known as the epidermis. Finally, the lower right micrograph shows connective tissue, which is composed of very loosely packed purple cells and fibers. There are large open spaces between clumps of cells and fibers. Connective tissue is found in the leg within fat and other soft padding tissue as well as bones and tendons.
Figure 4.1.1 – The Four Primary Tissue Types: Examples of nervous tissue, epithelial tissue, muscle tissue, and connective tissue found throughout the human body. Clockwise from nervous tissue, LM × 872, LM × 282, LM × 460, LM × 800. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)

 Embryonic Origin of Tissues

The cells composing a tissue share a common embryonic origin. The zygote, or fertilized egg, is a single cell formed by the fusion of an egg and sperm cell. After fertilization, the zygote gives rise many cells to form the embryo. The first embryonic cells generated have the ability to differentiate into any type of cell in the body and, as such, are called omnipotent, meaning each has the capacity to divide, differentiate, and develop into a new organism. As cell proliferation progresses, three major cell lines are established within the embryo. Each of these lines of embryonic cells forms the distinct germ layers from which all the tissues and organs of the human body eventually form. Each germ layer is identified by its relative position: ectoderm (ecto- = “outer”), mesoderm (meso- = “middle”), and endoderm (endo- = “inner”). Figure 4.1.2 shows the types of tissues and organs associated with each of the three germ layers. Note that epithelial tissue originates in all three layers, whereas nervous tissue derives primarily from the ectoderm and muscle tissue derives from the mesoderm.

This is a two column-table containing both text and illustrations. The left column is titled germ layer while the right column is titled “Gives rise to.” The germ layer in the first row is ectoderm. Ectoderm gives rise to epidermis, glands on the skin, some cranial bones, the pituitary and adrenal medulla, the nervous system, the tissue between the cheeks and gums, and the anus. This row contains three pictures. The leftmost picture illustrates several layers of yellow, oval-shaped skin cells with purple nuclei. The middle diagram shows a neuron, which is a yellow, star shaped cell with finger like branches at its corners. The neuron also has a purple nucleus and a yellow tube that connects to the bottom of the cell. The right image in this row shows a brown pigment cell embedded at the bottom layer of several skin cells. It is secreting dark-colored pigment into the skin cells from tentacle-like projections. The germ layer in the second row is mesoderm. Mesoderm gives rise to connective tissues, bone, cartilage, blood, the endothelium of blood vessels, muscle, synovial membranes, serous membranes that line body cavities, the kidneys, and the lining of the gonads. Five images are given in this row to illustrate. The leftmost image is cardiac muscle, which is cylindrical and curved. There are many open spaces between neighboring cardiac muscles. The next image shows skeletal muscle, which is a series of closely stacked cylinders with well defined horizontal striping. The middle image shows three tubule cells of the kidney, which are square shaped and contain a brown nucleus. The fourth image shows a series of red blood cells, which are red and saucer shaped with a slight depression at the center. The fifth image shows smooth muscles which are tightly packed, diamond shaped cells with oval-shaped nuclei. Endoderm gives rise to the lining of the airways and digestive system (except the mouth and distal part of digestive system). Also, the rectum and anal canal, digestive glands, endocrine glands, and adrenal cortex all develop from endoderm. The leftmost image in this row shows a lung cell, which is a large, purple, trapezoid-shaped cell. The middle image shows a pair of thyroid cells, which are rectangle-shaped with the upper edge of each cell having a row of finger like projections, similar in appearance to carpet. The rightmost image in this row shows a pancreatic cell, which is large and wedge-shaped. The pancreatic cell has small indentations throughout its cell membrane.
Figure 4.1.2 – Embryonic Origin of Tissues and Major Organs: Embryonic germ layers and the resulting primary tissue types formed by each.

 

Tissue Membranes

A tissue membrane is a thin layer or sheet of cells that either covers the outside of the body (e.g., skin), lines an internal body cavity (e.g., peritoneal cavity),  lines a vessel (e.g., blood vessel),  or lines a movable joint cavity (e.g., synovial joint).   Two basic types of tissue membranes are recognized based on the primary tissue type composing each: connective tissue membranes and epithelial membranes (Figure 4.1.3).

This illustrations shows the silhouette of a human female from an anterior view. Several organs are showing in her neck, thorax, abdomen left arm and right leg. Text boxes point out and describe the mucous membranes in several different organs. The topmost box points to the mouth and trachea. It states that mucous membranes line the digestive, respiratory, urinary and reproductive tracts. They are coated with the secretions of mucous glands. The second box points to the outside edge of the lungs as well as the large intestine and states that serous membranes line body cavities that are closed to the exterior of the body, including the peritoneal, pleural and pericardial cavities. The third box points to the skin of the hand. It states that cutaneous membrane, also known as the skin, covers the body surface. The fourth box points to the right knee. It states that synovial membranes line joint cavities and produce the fluid within the joint.
Figure 4.1.3 – Tissue Membranes: The two broad categories of tissue membranes in the body are (1) connective tissue membranes, which include synovial membranes, and (2) epithelial membranes, which include mucous membranes, serous membranes, and the cutaneous membrane, in other words, the skin.

Connective Tissue Membranes

A connective tissue membrane is built entirely of connective tissue. This type of membrane may be found encapsulating an organ, such as the kidney, or lining the cavity of a freely movable joint (e.g., shoulder).  When lining a joint, this membrane is referred to as a synovial membrane.  Cells in the inner layer of the synovial membrane release synovial fluid, a natural lubricant that enables the bones of a joint to move freely against one another with reduced friction.

 

Epithelial Membranes

An epithelial membrane is composed of an epithelial layer attached to a layer of connective tissue. A mucous membrane, sometimes called a mucosa, lines a body cavity or hollow passageway that is open to the external environment.  This type of membrane can be found lining portions of the digestive, respiratory, excretory, and reproductive tracts. Mucus, produced by  uniglandular cells and glandular tissue, coats the epithelial layer. The underlying connective tissue, called the lamina propria (literally “own layer”), helps support the epithelial layer.

A serous membrane lines the cavities of the body that do not open to the external environment.  Serous fluid secreted by the cells of the epithelium lubricates the membrane and reduces abrasion and friction between organs.  Serous membranes are identified according to location. Three serous membranes are found lining the thoracic cavity; two membranes that cover the lungs (pleura) and one membrane that covers the heart (pericardium). A fourth serous membrane, the peritoneum, lines the peritoneal cavity, covering the abdominal organs and forming double sheets of mesenteries that suspend many of the digestive organs.

A cutaneous membrane is a multi-layered membrane composed of epithelial and connective tissues.  The apical surface of this membrane exposed to the external environment and is covered with dead, keratinized cells that help protect the body from desiccation and pathogens.  The skin is an example of a cutaneous membrane.

Chapter Review

Aggregations of cells in the human body be classified into four types of tissues: epithelial, connective, muscle, and nervous. Epithelial tissues act as coverings, controlling the movement of materials across their surface. Connective tissue binds the various parts of the body together, providing support and protection. Muscle tissue allows the body to move and nervous tissues functions in communication.

All cells and tissues in the body derive from three germ layers: the ectoderm, mesoderm, and endoderm.

Membranes are layers of connective and epithelial tissues that line the external environment and internal body cavities of the body.  Synovial membranes are connective tissue membranes that protect and line the freely-movable joints. Epithelial membranes are composed of both epithelial tissue and connective tissue.  These membranes are found lining the external body surface (cutaneous membranes and mucous membranes) or lining the internal body cavities (serous membranes).

Review Questions

 

Critical Thinking Questions

Identify the four types of tissue in the body, and describe the major functions of each tissue.

The four types of tissues in the body are epithelial, connective, muscle, and nervous. Epithelial tissue is made of layers of cells that cover the surfaces of the body that come into contact with the exterior world, line internal cavities, and form glands. Connective tissue binds the cells and organs of the body together and performs many functions, especially in the protection, support, and integration of the body. Muscle tissue, which responds to stimulation and contracts to provide movement, is divided into three major types: skeletal (voluntary) muscles, smooth muscles, and the cardiac muscle in the heart. Nervous tissue allows the body to receive signals and transmit information as electric impulses from one region of the body to another.

 

What is the function of synovial membranes?

Synovial membranes are a type of connective tissue membrane that supports mobility in joints. The membrane lines the joint cavity and contains fibroblasts that produce hyaluronan, which leads to the production of synovial fluid, a natural lubricant that enables the bones of a joint to move freely against one another.

This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax, licensed under CC BY. This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images, from Anatomy & Physiology by OpenStax, are licensed under CC BY except where otherwise noted.

Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.

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Mohawk - PN Structure & Function of the Human Body Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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