Histology of Follicular Carcinoma of the Thyroid

Follicular carcinomas consist of a mixture of variably sized colloid- containing follicles, empty acini, and solid cords or alveoli of neoplastic cells (Fig. 16.8). They are surrounded by a well-defined capsule that is rich in arterioles, and also by venous sinusoids into which there are foci of capsular invasion extending into the venous sinusoid lumen. The less well-differentiated types of follicular carcinomas consist predominantly of trabeculae; they also show marked infiltration of  the capsule and prominent invasion, not only of capsular venous sinusoids, but also of veins outside the capsule of the thyroid gland. 

See later:
• Histology of the Oxyphil (HŸrthle or Askenazy) Tumour.