Tissues and Cells

By the end of this lecture, you should appreciate 

–  From one cell, all others form 

–  Classifications of individual cell types 

–  Epithelial Cells 

–  Cell polarity in epithelia 

–  Epithelial organisation for specialised functions 

–  The components of the ECM 

Somatic cells  are cells of the body/adult cells

4 types:

Epithelial,

muscle,

nervous,

connective tissue.

Germ cells -gametes / egg and sperm

Germ cells generally cannot be changed in the body that bears them

Fertilized egg = Zygote

Blastocyst – contains cavity, 120-168h post fertilisation ( It possesses an inner cell mass (ICM) which subsequently forms the embryo)

Bilaminar disc refers to the epiblst and hypoblast, evolved from the embryoblast (ICM).

A trilaminar embryo (or trilaminar germ disk) is an early stage in the development of triploblastic organisms. It is an embryo which exists as three different germ layers - the ectoderm, the mesoderm and the endoderm.

Early embryo 

Early Embryo 

An embryo is a multicellular diploid eukaryote in its earliest stage of development,The development of the embryo is called embryogenesis.

Further development of the embryo: The trilaminar disc folds as it grows and develops, so that the ectoderm completely surrounds the body, and the endoderm becomes the tube of the primitive gut, 

 Ectoderm and Endoderm is epithelial 

 Mesoderm has the looser structure of primitive connective tissue (mesenchyme) 

• Epidermis of skin and its derivatives (including sweat glands and hair follicles)
• Epithelial linning of mouht and rectum
• Sense receptors in epidemis
• Cornea and lense of eye
• NS
• Adrenal medulla
• Tooth enamel

• Notochord
• Skeletal system
• Muscular and excretory system
• Circulatory and lymphatic systems
• Dermis of skin
• Lining of body cavity
• Adrenal cortex
• Reproductive system (except germ cells)

• Epithelial linning of digestive tract and respiratory system
• linning of urethra, urinary bladder and reproductive system
• Liver
• Pancreas
• Thymus
• Thyroid and parathyroid glands

Funtion: 

• Barrier
• Protection
• Maintenance 

specialised functions:

• Transport
• Secretion/lubrications
• Movement

Adjacent cells in an epithelium are connected by junctions, There are 3 types of junction:

Tight Junction: prevent movement of substances down the paracellular cleft (between adjacent cells) and separate 2 domains of the cell membrane 

Adherens junctions: Initiate cell-cell contacts and mediate the maturation and maintenance of the contact
Gap Junctions: allow movement of small molecules between adjacent cells so that they respond identically to intracellular signals

6 types:

Columnar e.g. goblet cells

Squamous e.g. Kidney

Stratified squamous e.g. Oesophageal tissue

Cuboidal e.g. uterine glandular epithelium

Pseudostratified columnar e.g. Pseudo-stratified trachea 

Secretory Epithelium e.g.Submaxillary gland 

Epithelial cells are polarised, i.e. they have distinct apical and basolateral regions 

This allows them to have specialised activities which are directional, e.g. secretion or absorption 

The basal surface is lined up on a specialised connective tissue layer called the basal lamina (or basement membrane) 

Maintenance of an epithelium requires cell division, and in some epithelia only a subset of cells retain this capacity 

Connective tissues have:

aqueous basis to allow free diffusion of O2, nutrients and waste 

 Components known as the extracellular matrix (ECM) 

 ECM has fibrous (collagen and elastin) elements to resist tensile forces 

 ECM has viscous elements (glycosaminoglycans) to resist compressive forces 

 The cells of connective tissues are fairly loosely associated, not attached by junctions 

Loose connective tissue 

• Contains fibroblasts (synthesise CT components) 

• Contains other cells including immune cells, fat cells, components of nervous and circulatory systems 

Dense connective tissue 

• Higher proportion of collagen fibres 

• Fewer cells other than fibroblasts 

• May be regular (e.g. tendons) or irregular (e.g. dermis)

Cartilage 

• Proteoglycans create stiffness 

• Maintaining cells (chondrocytes) are in spaces called lacunae 

• No blood vessels – cartilage depends on diffusion through aqueous phase 

• Most cartilage is hyaline but there are also special types of elastic and fibrous

Bone 

• Deposition of mineral (calcium phosphate) means high resistance to compressive forces 

• Differently oriented layers of collagen fibres mean high resistance to tensile forces 

• Maintaining cells (osteocytes) in lacunae 

• Remodelling by other cells (osteoblasts and osteoclasts) 

• Contains blood supply  

Summary of major classes of cell:

Epithelial 

- Cancers are known as carcinomas 

 Mesenchymal 

- Cancers are known as sarcomas 

 Haematopoietic 

- Cancers are known as leukaemias or lymphomas 

 Neural 

- Cancers are known as neuroblastomas or gliomas

Summary of major classes of cell:

Epithelial 

- Cancers are known as carcinomas 

 Mesenchymal 

- Cancers are known as sarcomas 

 Haematopoietic 

- Cancers are known as leukaemias or lymphomas 

 Neural 

- Cancers are known as neuroblastomas or gliomas