Laminitis: Hoof wall growth

The first thing you need to know about the hoof wall is that it is actually composed of three different sections:

• the distal hoof wall/pigmented wall/stratum medium, 
• the water line/unpigmented wall/stratum internum
• the white line/epidermal lamellae

To understand hoof wall growth we also need to focus on the lamellae, of which there are four types:

• Primary Epidermal Lamellae (PEL)
• Secondary Epidermal Lamellae (SEL)
• Primary Dermal Lamellae (PDL)
• Secondary Dermal Lamellae (SDL)

The lamellae from the epidermis (outside of the hoof) interlock with the lamellae from the dermis (inside of the hoof) by means of a basement membrane. "The basement membrane is a thin, unbroken sheet of extracellular material, partitioning the dermis from the epidermis (Pollitt)."

Each one of the SEL (the little fingers hanging off each PEL) is attached to the basement membrane by a hemidesmosome through anchoring filaments. Anchoring filaments are composed of a glycoprotein molecule called laminin-5 and a protein called BP-180.

Following along so far? Okay...

The coronet band is constantly creating new cells for the hoof wall, which means there needs to be a mechanism to move the already existing wall down towards the ground to make room for the new cells. To do that, lamellar remodeling enzymes called matrix metalloproteinases (MMPs) come through and pop the hemidesmosomes off the basement membrane (not all at once, mind you), tissue inhibitors called TIMPs then turn the MMPs off, and the loosened cells then move downward and reattach lower on the basement membrane by the anchoring filaments.

This process happens constantly in order to keep up with the growth from the coronet band. The MMPs, TIMPs, and anchoring filaments are always working in a very delicate balance to keep the hoof wall growing and replace material lost at ground level.

And that's hoof wall growth in a nutshell.

Upon further examination, Bowker's theory of hoof wall growth doesn't differ terribly from this, he just posits that the PELs actually contribute cells to the stratum internum to increase hoof wall thickness closer to the ground. For the purpose of understanding laminitis that isn't terribly important (just interesting).

All the info in this post I learned from reading the work of Chris Pollitt, I'll provide links later on. You'll notice that I've made a few words a little more "obvious." Try to remember those, they'll be important later.