There are three main types of cell junctions: occluding junctions, adhering junctions, and communicating junctions. Occluding junctions, also called tight junctions, form impermeable barriers between cells. Adhering junctions such as desmosomes anchor cells together through transmembrane proteins. Communicating junctions, including gap junctions, contain channels that allow small molecules to pass between cells and facilitate cell-cell communication.

2. 
 Intercellular space in closely packed tissue is about
20nm. The cells are bound together by the specific
adhesive glycoprotein.
 Epithelial cells adhere to each other through
glycoproteins called Cadherins
 Modified cell membranes contributing in cohesion
and communication are called Cell junctions
Cell Junctions

3. 
 There are three types of Cell Junctions
1. Occluding Junctions
2. Adhering Junctions
3. Communicating Junctions
Types of Cell Junctions

4. 
 Found in epithelial tissues
 Also known as “Tight Junctions”
 Do not allow passage of small molecules form
impermiable membrane.
Types:
 Zonula Occludens
 Fascia Occludens
Occluding Junctions

5. 
 Encircles the entire cell perimeter
 Occludes the intercellular space
 Series of focal fusions
 The adjacent cell membranes approach each other,
outer leaflets fuse, diverge again then fuse again
 At fusions sites specific trans membranous proteins
named (Occludins, and Claudins) perform the
binding function
 Less in PCT and more in the intestinal mucosa
Zonula Occludens

6. 

7. 
 A strip like tight junction of limited extent
 Found between the endothelial cells of the blood
vessels
Fascia Occludens

8. 
 Anchoring junctions
 Provide cell-cell or cell to basal lamina adherence
Types:
 Zonula adherens
 Fascia adherencs
 Macula adherens (Desmosomes)
 Hemidesmosomes
Adhering Junctions

9. 
 A belt like junction
 No fusion of cell membranes
 Trans membranous glycoprotein “E-cadherin”
occupies intercellular gap
 E-cadherin links to adherent proteins in cytoplasm
which are:
 Catenin
 Vinculin
Zonula Adherens

10. 
 Structurally it is similar to Zonula adherence
 But its cell junction is strip-like and (not ring-like or
belt-like)
 i.e. Cardiac muscle cells.
Fascia Adherens

11. 
 Macula adherins are commonly known as desmosomes
 “Spot-weld” like junctions
 Randomly distributed along lateral plasma membranes
of the cells in simple epithelium
 In stratified epithelium it is distributed throughout the
plasma membrane
 It is also found in cardiac muscle cells
Desmosomes

12. 
 Cell membrane in the region of junctions are seen
further apart (30mm) than the usual gap
 Electron dense attachment plaques are located
opposite to each other on the cytoplasmic aspects
 Intermediate filaments of the cytoskeleton are
anchored to the attachment plaques
 Two types of transmembranes glycoproteins named
Desmocolins and Desmogleins provide adherence
Desmosomes

13. 

14. 
 These junctions serve to anchor the epithelial cells to
the basal lamina
 A hemidesmosome is a spot like adhering junction
which gives appearance of a half desmosome
 In hemidesmosome transmembrane linker proteins
are integrins
 The cytoplasmic intermediate filaments of keratin
are inserted in to the attachment plaque
Hemidesmosomes

15. 

16. 
 Characterized by presence of minute tubular
passageways
 Provide direct cell to cell communication
 Tubular passages allow movement of ions and other
small molecules between adjacent cells
Communicating
Junctions

17. 
 Gap junction also called the “Nexus” which are
communication junctions, occur frequently between
the epithelial cells
 Also found in cardiac muscle cells, smooth muscles,
neurons, astrocytes, and osteocytes
 Plasma membrane of the adjoining cells are closely
opposed with a gap of only 2nm
 The gap junction contains closely packed numerous
tubular intercommunicating channels
Gap Junction

18. 
 The lumens of the channels of gap junction have an
average diameter of 1.5nm
 These channels permit free passage of ions, sugar
and amino acids
 In cardiac and smooth muscles the gap junction
provides electrical coupling of the adjacent cells
 Gap junctions are frequently found in embryonic
cells
Gap Junctions

19. 

20. 

21. 
 Static cell population
The cells incapable of mitotic division i.e.
 Neurons
 Cardiac Cells
 Stable Cell population
Division occurs on stimulation as a result of an
injury or a disease
 Renewing Cell population
Undergoes regular mitotic division i.e. epidermis of
skin and epithelial lining of GIT
Cell Renewal

22. 
 Cells belonging to renewing population undergoes a sequence of
events which are repeated over and over again
 The cycle is divided in to two parts
 M PHASE: in which mitosis occurs (30 to 60 minutes)
 INTERPHASE: it is intervening period between two cell divisions
consist of three sub phases
1- The G1 Phase (GAP-1)
- During this phase synthesis of RNA and proteins occur s
- Cell size is restored to normal
- The duration of G1 is about 8 hours
Cell Cycle

23. 
2- The S-Phase:
- During this synthesis of DNA takes place
- It results in preparation of exact replica of genetic material
and duplication of centrioles
- Duration is 8 hours
3- G2 (GAP-2) Phase:
- It is period between the end of S phase and beginning of
mitosis
- During this process production and accumulation of
energy for mitosis takes place
- Duration is 2 to 4 hours
Cell Cycle

24. 

25. 

27. 
Mitosis
• The process of cell division which results in the
production of two daughter cells from a single parent cell.
• The daughter cells are identical to one another and to the
original parent cell.

28. 
Mitosis can be divided into stages
1. Interphase
2. Prophase
3. Metaphase
4. Anaphase
5. Telophase

29. 
Interphase
The cell prepares for division
 DNA replicated
 Organelles replicated
 Cell increases in size

30. 
 Chromosomes become visible under LM
 Threads become shorter and thicker consist of two
chromatids joined by centromere
 Nucleoli disappears
 Centrioles separates and migrate
to each pole and starts giving out
mitotic spindle
Prophase
The cell prepares for nuclear division

31. 
 Chromosomes line up at the center of the cell
 Spindle fibers attach from daughter cells to
chromosomes at the centromere
 Equatorial plate is formed
 Microtubules of mitotic spindle
are attached at centromere
 Microtubules exert pull on
chromosomes
Metaphase
The cell prepares chromosomes for division

32. 
 Spindle fibers pull chromosomes apart
 ½ of each chromosome (called chromatid) moves to
each daughter cell
 Chromatids separate and move to respective poles as
an independent chromosome
 In human cell two identical sets of
46 chromosomes move to the
opposite poles
Anaphase
The chromosomes divide

33. 
 A constriction called cleavage furrow appears in
the middle of elongated cell
 Nuclear envelop is formed enclosing
chromosomes
 2 nuclei form
 Cell wall pinches in to form the
2 new daughter cells
Telophase
The cytoplasm divides

34. 

35. Mitosis -- Review
Interphase Prophase
Metaphase Anaphase
Telophase Interphase

37. 
• Meiosis is the type of cell division by which germ
cells (eggs and sperm) are produced.
• One parent cell produces four daughter cells.
• Daughter cells have half the number of chromosomes
found in the original parent cell
Meiosis

38. 
 During meiosis, DNA replicates once, but the
nucleus divides twice.
 Four stages can be described for each division of the
nucleus.
Meiosis

39. First division of meiosis

40. 
 Prophase is much longer consisting of five stages
1. Leptotene: Chromosomes becomes visible in the nucleus
2. Zygotene: Homologus chromosomes come together along
their entire length and synapses are formed
3. Pachytene: Chromosomes become thicker and shorter
Each chromosome pair is called bivalent
4. Diplotene: Chromosomes began to separate along their
length. Each bivalent consists of four chromatids
5. Diakinesis: Separation of chromosomes continue.
Nucleolus and the nuclear envelop disappears
Prophase

41. 
 A spindle of microtubules is produced by centrioles
 Equatorial plate is formed
 The bivalent chromosome pairs align in the centre of
the spindle
Metaphase

42. 
 Chromosomes of homologous pairs completely
separates and move to the opposite poles
 No division of centromere occurs and the whole
chromosomes move to opposite poles
Anaphase

43. 
 Nuclei are reconstructed
 The parent cell is divided in to two daughter cells
 Each daughter cell contains haploid (23)
chromosomes
 Each chromosome is double structured consisting of
two sister chromatids
Telophase

44. Second Division of Meiosis

45. 
Meiosis II

46. 
Differences in Mitosis & Meiosis
 Mitosis
 Asexual
 Cell divides once
 Two daughter cells
 Genetic information is
identical
 Meiosis
 Sexual
 Cell divides twice
 Four haploid daughter
cells
 Genetic information is
different