10.3 Internal Anatomy: Fascia, Glandular Tissue, and the TDLU

Why internal anatomy shows up on a positioning exam

You cannot critique a mammogram, explain density, or understand where calcifications belong without the internal map. The September 1, 2025 specs list fascial layers, the retroglandular fat space, fibrous tissues, glandular tissues (lobules, ductal sinus/acinus, and the terminal ductal lobular unit with its three parts), and adipose tissues. This section is that list, taught as one system.

Quick Answer: The breast sits between superficial and deep fascia. Behind the gland is the retroglandular fat space — proof that the posterior third is on the image. The functional unit is the TDLU: extralobular terminal duct + intralobular terminal duct + acini. Most breast cancers arise in the TDLU. Density is how much fibroglandular tissue there is relative to fat.

Fascial layers

Two fascial planes sandwich the gland:

• Superficial fascia — in the subcutaneous fat, just deep to the skin. Cooper ligaments pierce this layer to insert on the dermis.
• Deep fascia — the pectoral fascia covering pectoralis major. The breast is anterior to this fascia. It is not inside the muscle, except where the axillary tail angles toward the axilla.

Between skin and superficial fascia is subcutaneous fat. Between the posterior gland and the deep fascia is the retroglandular fat space. Implants, infection, and tumor spread respect or violate these planes. For this exam the practical question is: which fat plane proves you included the tissue?

Retroglandular space

The retroglandular fat space (retromammary fat) is a radiolucent band between posterior fibroglandular tissue and the pectoral muscle or chest wall. Both CC and MLO should show retroglandular fat. That stripe is how you know the posterior third was imaged. If glandular tissue runs off the posterior edge with no fat band, you missed the retroglandular space and you may have missed a posterior cancer.

On CC the fat stripe lies against the chest-wall edge of the image. On MLO it lies anterior to the pec. Do not confuse it with a poorly compressed dark band or with the anterior skin–fat interface.

Fibrous tissues

Fibrous stroma is the collagenous framework around ducts and lobules. It includes interlobular dense connective tissue, Cooper ligaments (physiology in 10.4), and periductal fibrous tissue. Fibrous tissue is radiopaque (white), like epithelium. A 2D mammogram cannot split fibrous from glandular — that is why composition language says fibroglandular density. With age and menopause, fibrous and glandular tissue usually involute and fat increases; some patients stay dense for life.

Glandular tissues: lobes, lobules, and ducts

The gland is organized as about 15–20 lobes, each draining through a lactiferous (collecting) duct to a nipple orifice. Each lobe contains many lobules. Hierarchy from nipple back to the milk-making unit:

ARRT's outline lists lobules, ductal sinus (acinus), and the TDLU with three named parts. Do not confuse the lactiferous sinus (subareolar ampulla of a major duct) with the acinus. If a stem says ductal sinus (acinus) in the glandular-tissue list, they mean the secretory end unit, not the subareolar ampulla.

A lobule is a cluster of acini plus intralobular terminal ducts in loose intralobular stroma. Lobules hypertrophy in pregnancy and the luteal phase and atrophy after menopause.

The terminal ductal lobular unit (TDLU)

The TDLU is the functional unit of the breast and the origin of most benign and malignant epithelial lesions. It includes:

1. Extralobular terminal duct
2. Intralobular terminal duct
3. Acini

flowchart LR
  ETD["Extralobular terminal duct"] --> ITD["Intralobular terminal duct"]
  ITD --> A1["Acini"]
  ITD --> A2["Acini"]
  subgraph TDLU["Terminal ductal lobular unit"]
    ETD
    ITD
    A1
    A2
  end

Most breast cancers arise in the TDLU. Ductal carcinoma in situ (DCIS) and invasive ductal carcinoma originate from TDLU epithelium even though the word ductal sounds like a large duct. Lobular carcinoma in situ (LCIS) and invasive lobular carcinoma arise from the same unit's lobular/acinar side. That is why a tiny cluster of calcifications in glandular tissue is treated as potential TDLU disease, whereas a lucent-centered calcification in the skin is not.

The extralobular terminal duct is a common path for DCIS to grow along. The acini and intralobular duct are the usual home of cysts, fibroadenomas, and lobular neoplasia. You do not need pathologist-level histology. You do need: TDLU = extralobular terminal duct + intralobular terminal duct + acini, and cancer starts here, not in Cooper ligaments and not in the lactiferous sinus unless disease has spread there.

Adipose tissues

Adipose tissue (fat) is radiolucent — dark gray to black. It occupies subcutaneous fat (skin to superficial fascia), fat mixed with fibroglandular tissue, retroglandular / retromammary fat, and intramammary fat including the fatty hilum of nodes.

Fat is not empty space. A fat-containing lesion (oil cyst, lipoma, hamartoma / fibroadenolipoma, intramammary node with hilum) has a different differential from a water-density mass. Complete fatty replacement can make cancers easier to see because they stand out white against black.

Why density equals more glandular and fibrous tissue relative to fat

Mammographic density is the proportion of white fibroglandular tissue versus dark fat on the image. It is not how the breast feels and not body-mass index. A small firm breast can be fatty; a large soft breast can be extremely dense.

BI-RADS composition categories (almost entirely fatty → scattered fibroglandular → heterogeneously dense → extremely dense) are the clinical language for this ratio. Density often falls after menopause as lobules involute and can rise with hormone therapy or lactation. For the exam: higher density = more glandular and fibrous tissue relative to fat, which both masks masses and is itself a risk marker. The technologist compresses adequately and includes all fibroglandular tissue. You do not fix density by clipping posterior tissue to make the image look blacker.

If glandular tissue extends to the posterior film edge on CC or MLO, you have not proved the retroglandular space is included — repeat the view rather than calling the breast too dense to image.