HISTO/CELL BIOLOGY - INTRO TO HISTOLOGY

What is histology?

Study of microscopic structure of tissues and organs

What is a tissue?

Group of cells of more or less similar structure; performs a particular function

What is an organ?

• Collection of tissues

• definite form and structure

• performs one or more specific functions

What are the four basic tissues in organs?

• epithelium

• connective tissue

• muscle

• nerves

What is an organ system?

Large functional unit made up of organs

What is differentiation?

Unspecialized structures become modified and specialized for specific functions

When does differentiation begin?

As soon as cells have formed by cell division

What changes occur during differentiation?

Changes in cell shape and physiology

How can differentiation be described?

“Division of labor”

Give an example of differentiation in epithelium.

Basal cells divide and differentiate

Give an example of differentiation in bone marrow.

Blood cells differentiate into WBCs, RBCs, etc.

Annotation — What develops during the 1st and 2nd weeks?

• 1st week: morula

• 2nd week: blastocyst

Diagram: What is the sequence from fertilization to implantation?

Fertilization → cleavage → morula → blastocyst → implantation

Diagram: What is the zona pellucida?

Outer covering around early embryo shown in the diagram - “cleavage state”

Diagram: What are the two main parts of the blastocyst?

Embryoblast (inner cells)

trophoblast (outer cells)

What is a zygote?

Fertilized egg cell

What are the three phases of embryonic development?

Cleavage; gastrulation; organogenesis

What happens during cleavage?

Zygote formed at fertilization undergoes repeated mitotic divisions

What is a morula?

Aggregation of cells formed by cleavage; week 1

What is a blastocyst?

• morula will CHANGE to sphere-like structure

• surface layer of cells and few cells inside; week 2

What are the inner cells of the blastocyst called?

Embryonic cells / embryoblast

Diagram: Identify the outer layer of the blastocyst.

Trophoblast

Diagram: Identify the inner cell group of the blastocyst.

Embryoblast

Diagram: Identify the cavity shown below the embryoblast.

Primary yolk sac

Diagram: What is the early sequence shown on the developmental timeline?

Fertilization → cleavage → blastocyst → implantation → bilaminar disc

How are embryoblast cells organized after blastocyst formation?

Into 2 layers:

epiblast and hypoblast (bilaminar embryo) → “disc stage”

What membranes are mentioned during week 2, what will the surface cells of blastocysts form?

Amniotic membrane and yolk sac

What happens during gastrulation?

Cells continue to divide, move and rearrange into germ layers

What is a trilaminar germ disc?

Embryo with three germ layers

Which three germ layers form during gastrulation?

Ectoderm; mesoderm; endoderm

According to the lecture, what does the epiblast become?

Ectoderm

According to the lecture, what does the hypoblast become?

Endoderm

According to the lecture, where does mesoderm form?

New middle layer formed by division of epiblast

What is invagination?

Inward folding of a cell layer

When does the epiblast surface begin to undergo invagination?

Near the end of the 2nd week of prenatal development.

What happens to the epiblast surface near the end of the 2nd week?

undergoes invagination (folding inward)

Where does invagination of the epiblast begin?

At one side of the epiblast — the future tail end.

In which direction does invagination proceed?

Another side of the epiblast - toward the head (cephalic) end

What is the structure formed by this invagination called in the lecture?

Neural fold (primitive streak)

What does the neural fold eventually form?

The neural tube

What does the neural tube develop into?

Central nervous system (CNS) - brain and spinal chord

Diagram: What are the two layers of the bilaminar disc?

Epiblast; hypoblast

Diagram: Identify the three germ layers in order.

1. Ectoderm (outer)

2. mesoderm (middle)

3. endoderm (inner)

What forms alongside the developing neural tube?

Neural crest

Where do neural crest cells migrate?

Into developing mesoderm; locate in both sides of neural tube

During which week does the lecture describe neural crest migration?

Week 3

• Annotation — What areas are noted in relation to neural crest cells?

Head and neck; teeth

What cells have an essential role in development of the human embryo?

• neural crest cells

Diagram: Identify the neural groove and neural fold.

neural groove: central depression

neural folds: raised sides

How does the neural tube form?

neural depression deepens → shoulders of this fold move together and fuse

What develops from the neural tube?

CNS: brain and spinal cord

What is the origin of neural crest cells?

Ectoderm at the shoulders/crest of neural folds

Where are neural crest cells positioned after migration?

Beside the neural tube

What happens to the embryo near the end of week 3?

Three embryonic layers fold laterally into curved tube-like structure with 2 different ends.

What are the two ends of the embryo called?

• cephalic (head)

• caudal (tail)

What are branchial processes / arches (pharyngeal arches)?

Processes on the ventral/cephalic side of the embryo inferior (below) the head fold

How many processes on each side are stated in the lecture?

Five branchial processes / arches

Diagram: Identify the three embryonic folds.

• head fold

• lateral fold

• tail fold

Where are neural crest cells located “relative to the neural tube”?

“between mesoderm cells on both sides of neural tube”

Which genes are active in neural crest cells?

Homeobox genes

What do homeobox genes control according to the lecture?

Surrounding mesoderm that forms:

• skull bones

• tooth tissues

• other tissues

• Annotation — Which homeobox gene families are highlighted? → aid in creating posterior and anterior teeth. **

• Msx

• Dlx

• Barx

** tooth tissues

What is ectomesenchyme according to the lecture?

Neural crest cells and surrounding mesodermal cells

Where does ectomesenchyme migrate?

Anteriorly to facial side with growth of cranial nerves

Diagram: What structures derive from surface ectoderm?

• epidermis

• hair

• nails

• glands

• enamel of teeth

Diagram: What structures derive from neural crest?

• ectomesencyhme

• bones and skull

• dentin

• periodontal ligaments

• alveolar bone

• pulp + cementum

What are ectoderm derivatives?

• nervous system; from neural tube and neural crest cells

• skin epithelium; hair, nails

• sensory epithelium; ear, nose, eyes

• pituitary; mammary and sweat glands

• enamel; from outer layer of embryo-ectoderm

Which tooth tissue derives from ectoderm?

Enamel

What are mesoderm derivatives?

• all connective tissue; muscles; vascular system

• urogenital system (not bladder); spleen; suprarenal cortex

What are endoderm derivatives?

• gastrointestinal tract and respiratory epithelial linings

• urinary bladder; liver; pancreas

What is organogenesis?

• germ layers undergo further cell division and specialization, and form the organs of the embryo

What characterizes the fetal period?

Maturation of tissues and organs; rapid body growth