A 58-year-old man from rural Maharashtra presents with a 6-month history of alternating constipation and diarrhoea, along with blood-stained stools. On examination, he is pale and has a palpable mass in the left lower abdomen. His haemoglobin is 9.2 g/dL. Colonoscopy reveals a stricturing lesion at the sigmoid colon with irregular, ulcerated mucosa. Biopsy shows infiltrating glands with marked desmoplastic reaction. Which of the following histological features is most predictive of poor prognosis in this patient?