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Study Notes 2212
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ANHB 2212
1
Study Notes 2212
Embryological Development – HSD Essentials CH 3
nd
Maturation of the egg completes after fertilisation when the 2 meiotic division is initiated. Fertilisation
occurs normally at the ampulla, a section of the uterine tube. Sperm has little cytoplasm, and an acrosome
head, which allows for the reaction necessary for the sperm to enter the egg.
The acrosomal reaction penetrates through the clear zones of the zona pellucida. The zona pellucida then
denies entry to multiple, subsequent sperm.
One the sperm enters the egg meiosis II completes. Male and female pronuclei form and then mitosis
occurs. The zona pellucida remains until implantation. The embryo is at the Morulla, ‘mulberry’ stage
when cells start to protrude and make the surface of the zona pellucida uneven.
The Morulla then specialises into the blastocyst. This occurs when the embryo forms a cavity with an inner
cell mass on one side. Outside, surrounding cells are called the trophoblast. Only at the specialisation
stage can implantation occur. It is prevented by the zona pellucida until this stage, with the zona pellucida
containing the invasive trophoblast and preventing it from eating into other cell walls.
The embryo then implants itself within the endometrium of the uterus. At this stage the embryo is
bilaminar; it has an epiblast and a hypoblast. Also present is the amnion and the amniotic cavities.
The amnion (epiblast) shows the primitive streak when a cross section is taken. Streak at the tail end.
Epiblast then forms endoderm, mesoderm and ectoderm. The ‘beginning’ of this process occurs at the
primitive streak. Endoderm replaces the hypoblast.
The overall layering of the embryo occurs as follows;
 Initially epiblast and hypoblast are present. Then endoderm takes over the hypoblast. Mesoderm
forms in the middle and remaining epiblast becomes ectoderm. At the head end there is a section
where endoderm and ectoderm touch, with no mesoderm in between. This forms the mouth.
Ectoderm forms the nervous system, forming first the neural crest, then the neural tube. The
neural tube forms an axis within the embryo, separating it into right and left sides.
Folding then occurs; the ectoderm forms the interface with the exterior, is outside, forms the skin.
Endoderm is concerned with food intake, is inside. Mesoderm is everything else in between; bones,
cartilage etc.
This embryonic stage is also known as organogenisis.
The study of tissues; study of microscopic structures of the body. Two types – systemic histology (the
study of the organs and systems) and basic histology (the study of tissues).
Four tissues and sub groups. Each tissue is a group of cells with a common function. Think of size, shape,
nucleus and organelles in the context of function:
 Epithelium Tissue – covering of external surfaces, lining of cavities. Limiting structure, controlling
the passage of substances. Compact sheets of cells with a variety of other functions. Avascular
(supporting tissue needed) with little intercellular substance. Has a basement membrane.
 Muscle Tissue – movement. Sliding filaments of proteins. Classified into several categories (3).
Intimately associated with connective tissue in most places. Contraction and relaxation. Proteins
cannot be seen with a light microscope.
 Nervous Tissue – generation and conduction of nerve impulses.
 Connective Tissue – throughout the body, except for the CNS. Wide variety of structural features
and functions. Connects structures and tissues, binds, provides nutritional support, site of
defence reactions, transmits and resists forces and provides skeletal support.
Development
Layering and
Folding
Histology
Integrative
Example – the
Intestine
Muscle - movement
Supporting
Connective
Tissue
Neurons / nerve plexuses
2
Thin muscle layer
Submucosa – the
connective tissue
in between
Lining epithelium; provides a
barrier, secretes and absorbs.