EM 004 Cell Structures

Plane of Section

Unlike the transverse section thru the middle of a nucleus seen in EM 007 Nucleus, this is a tangential section thru the edge of a nucleus as seen by transmission electron microscopy (TEM).

The surface of most nuclei are slightly irregular with invaginations. This micrograph is from a tangential section across one of these invaginations. Thus, the nucleus (blue) appears in cross-section on both the left and right sides of this image.

Nuclear Envelope

The nuclear envelope (cyan) is cut tangentially in the middle between the cross-sections of the nucleus. Examine this region for the circular profiles of nuclear pores (purple).

The center of some nuclear pores contain material being transported between the nucleus and the cytoplasm. The diameter of this opening (~9 µm) is much smaller than the diameter of the nuclear pore complex (~120 nm).

This section is through the outer membrane of the nuclear envelope because bound ribosomes are visible.

Polyribosomes

Polyribosomes (or polysomes) are multiple ribosomes bound to a single molecule of mRNA. They simultaneously translate a single molecule of mRNA into many copies of a protein.

Examine the cytoplasm (green) for polyribosomes (dark particles) in spiral and linear patterns.

Centrosome

The centrosome is the main microtubule organizing center (MOTC) of animal cells. A centrosome is pair of orthogonal centrioles surround by an amorphous mass of proteins, called pericentriolar material (PCM), which is responsible for microtubule nucleation and anchoring.

A centriole is a cylindrical structure (~200 nm in diameter) composed of a ring of nine triplets of microtubules. Next to the centriole (yellow) cut in cross-section is a small fragment of its orthogonal partner (yellow). Microtubules (25 nm in diameter; red) radiate from the PCM.