Sebaceous glands, especially the larger ones, are well endowed with blood vessels but no one has convincingly demonstrated a nervous supply. The many descriptions  of the  ultrastructure of sebaceous glands have been uneven, the quality of the illustrations has been erratic and interpretation of structure has been varied.

The sebaceous gland  consists  essentially of three major  cell types: undifferentiated,  differentiated  and mature cells.  Adjacent to  the  basement membrane are the undifferentiated cells; the basement membrane is recognizable,  separated from the piloerector muscle by a small amount of fibrous tissue.

  The undifferentiated cells as they are pushed towards the sebaceous duct become differentiated into lipidcontaining cells. As the lipid increases the mature cells disintegrate forming an acellular secretion sebum.

  In studies  with the electron microscope, the  undifferentiated cells   at the periphery of the gland can  be  seen  to  rest upon  the  basement membrane  and to be  connected  with each other by desmosomes  and  hemidesmosomes.9 The membranes of the granular  endoplasmic reticulum are coated with ribosomes and  there are in addition particles of ribonucleoprotein and glycogen scattered free in the cytoplasm.  All  the cells  have  numerous  mitochondria, usually appearing as short or wavy  rods. At this stage the Golgi zones  are usually inconspicuous; the nuclei are relatively  very large. 

Differentiation of the cell becomes evident  with the  appearance of one or more small lipid vacuoles within the cytoplasm. During the active phase  of lipid synthesis the cytoplasm becomes  packed with smooth-surface membranes  of the endoplasmic  reticulum. In  some partially differentiated cells a large Golgi zone becomes apparent ; typically it consists of parallel, smooth-surfaced thick membranes, slightly dilated cisterns and small vesicles.  The Golgi zone appears to be the centre where  lipid accumulates to  form sebum vacuoles. At an early stage of sebaceous transformation one of the cisterns in the Golgi zone becomes  more dilated  than the others and forms the  centre of the developing sebum vacuole.10 At a later stage the smooth membranes of the Golgi apparatus and the endoplasmic reticulum become orientated around the  edge of the developing vacuole, forming a sort of 'husk'.

The fully differentiated cells  come to contain very large sebum vacuoles which each compare in size to the nucleus; the cell may have a complement  of more than sixty vacuoles. In the final stage of differentiation the mitochondria become widely separated indicating that  their numbers have decreased; and  the  nucleus becomes  irregularly  shaped with  clumping  of the nucleochromatin and dispersal of the nucleolar  material.

   The contribution of hydrolytic enzymes to the events occurring in the secretion and discharge of sebaceous glands has received a considerable amount of attention in recent years. Acid hydrolases contained within lysosomes are known to play  an important  role in the physiological autolysis that occurs in the formation of a holocrine secretion.  Acid esterases and phosphatases have been demonstrated in sebaceous glands histochemically  at the light microscope level.

   The ultrastructural investigations of Brandes  et al are particularly illuminating.  They stress the role of acid phosphates and nonspecific esterases in the sequence of events occurring in the holocrine secretion of sebaceous glands.

   Not all cells in the centre part of the sebaceous gland have this appearance of being packed with lipid, for some cells resembling undifferentiated peripheral cells are occasionally  found at this site. Their surfaces have desmosomal attachments, abutting well-differentiated cells, and they contain tonofilaments.

   However, it has been  suggested that  the sebaceous gland secretion is not entirely holocrine.

   Lanthanum is a  useful marker to assess ultrastructurally movement  of small molecules.  When introduced intradermally into cattle, sheep  and rats it penetrates into the sebum through the intercellular spaces of the sebaceous gland. Recent work by Jackson et al  concluded that the sebaceous gland is permeable to the passage of small molecules outwards and probably in some  circumstances inwards. Their results,  support the conclusion  that sebum production may be a more complex mechanism than previously supposed involving the incorporation of substances directly from the dermis and active cells within the gland, as well as from lysis of superficial cells. It is  thus  doubtful if the sebaceous gland is a simple holocrine gland, at least in the species investigated. Similar studies are necessary in man.